WO2014083800A1 - Polarization-multiplexed light transmitter and polarity determination method - Google Patents

Polarization-multiplexed light transmitter and polarity determination method Download PDF

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Publication number
WO2014083800A1
WO2014083800A1 PCT/JP2013/006788 JP2013006788W WO2014083800A1 WO 2014083800 A1 WO2014083800 A1 WO 2014083800A1 JP 2013006788 W JP2013006788 W JP 2013006788W WO 2014083800 A1 WO2014083800 A1 WO 2014083800A1
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Prior art keywords
optical
signal
polarization
polarity
modulated
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PCT/JP2013/006788
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French (fr)
Japanese (ja)
Inventor
慎介 藤澤
俊治 伊東
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日本電気株式会社
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Priority to JP2014549797A priority Critical patent/JP6222105B2/en
Publication of WO2014083800A1 publication Critical patent/WO2014083800A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/06—Polarisation multiplex systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5053—Laser transmitters using external modulation using a parallel, i.e. shunt, combination of modulators
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
    • H04B10/50575—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output to control the modulator DC bias

Definitions

  • the present invention relates to a polarization multiplexed optical transmitter and a polarity determination method used in an optical digital coherent optical transmission system, and to an optical digital coherent polarization multiplexed optical transmitter that modulates an optical signal using a Mach-Zehnder optical modulator. And a polarity determination method.
  • each of four optical phases for example, 45 degrees, 135 degrees, 225 degrees, and 315 degrees
  • a polarization multiplexed QPSK (QuadraturerPhase Shift Keying) optical signal 2 bits for example, 00, 01, 11, 10) are allocated (symbol mapping).
  • An example of the constellation of the QPSK optical signal is shown in FIGS. 11 (a) and 11 (b).
  • An optical digital coherent optical transmission device is generally one of optical quadrature modulators when optically modulating an input optical signal branched into two by a polarization maintaining optical splitter or the like based on a drive signal voltage.
  • a Mach-Zehnder type optical modulator is used.
  • the Mach-Zehnder optical modulator changes the intensity of an optical signal by using interference between the two optical paths by separating the input optical signal into two parts, passing the two optical paths, and recombining them.
  • the Mach-Zehnder optical modulator changes the phase of an optical signal passing through the optical path by applying a drive signal voltage to the two optical paths, and controls the intensity of the modulated light to be output.
  • FIG. 12 shows the relationship between the drive signal voltage of the Mach-Zehnder optical modulator and the characteristics of the modulated light to be output.
  • an optical quadrature modulator is driven using a drive electrical signal generated by a digital-analog converter.
  • Transmission end digital signal processing is pre-equalization processing such as pre-wavelength dispersion imparting processing, spectrum shaping processing, signal amplitude amplification processing, etc., applied to the drive signal voltage by digital signal processing, and then an analog signal by a digital / analog converter It is processing to convert to.
  • pre-equalization processing such as pre-wavelength dispersion imparting processing, spectrum shaping processing, signal amplitude amplification processing, etc.
  • the pre-equalization processing is processing for preliminarily calculating the waveform distortion generated in the optical transmission path on the optical transmitter side, and adding reverse distortion to cancel the generated distortion to the transmission signal.
  • the first is a method (method 1) in which a time offset of different optical phases is given to two independent optical signals by giving a frequency offset to the drive signal.
  • the second is a method (method 2) in which the pre-chromatic dispersion imparting process is performed on the drive signal by digital signal processing using the transmission end digital signal processing technology.
  • Non-Patent Document 1 is disclosed in Non-Patent Document 1, for example. As shown in FIG. 13A, method 1 gives temporal variations of different optical phases to two independent optical signals that are polarization multiplexed. In this case, polarization multiplexed signals having different carrier frequencies between polarization components orthogonal to each other are generated, and the quality is improved.
  • the chromatic dispersion is an optical phase rotation in which the chromatic dispersion is proportional to the square of the frequency deviation from the carrier frequency of the optical signal in the frequency domain
  • the sign of chromatic dispersion given by the conversion processing is inverted.
  • the sign of chromatic dispersion is inverted, the temporal variation cannot be canceled as described above, and the optical signal cannot be demodulated. Therefore, the quality is not improved.
  • the optical spectrum of the output signal light of the polarization multiplexed optical transmitter is measured with an optical spectrum analyzer to detect the sign of temporal variation in the optical phase of the polarization components orthogonal to each other.
  • Technology has been proposed.
  • a polarization multiplexed optical transmitter equipped with an optical spectrum analyzer is very expensive and slow in detecting the sign of temporal variation in optical phase.
  • the received signal quality when the output signal light of the optical transmitter is demodulated in the optical receiver is measured, and the bias of the optical quadrature modulator is based on the measurement result It has been proposed to determine the polarity by detecting the correctness of the setting.
  • the bias of the optical quadrature modulator of the optical transmitter based on the quality of the signal demodulated in the optical receiver, it takes time for feedback. Further, it is necessary to prepare a communication path between the optical transmitter and the optical receiver, which increases the system scale and the system cost.
  • An object of the present invention is made in view of the above problems, and is a polarization multiplexed optical transmitter and a polarity determination capable of quickly detecting the polarity of an optical quadrature modulator in its own apparatus without arranging expensive equipment. It is to provide a method.
  • a polarization multiplexed optical transmitter generates a drive signal for adding a predetermined frequency shift to a polarization component of an optical signal and outputs the drive signal, and two optical signals, respectively.
  • Two light modulation means that modulates two polarization components using a drive signal, adds a phase difference, combines them, and outputs a modulated combined light, and two modulated combined lights in a state where polarizations are orthogonal to each other
  • a polarization multiplexing unit that multiplexes and outputs a polarization multiplexed optical signal, a polarization unit that changes a polarization state of the polarization multiplexed optical signal to a state in which each polarization component is mixed, and a mixing unit that outputs a mixed optical signal;
  • Polarity determining means for determining the relationship between the polarities of the two modulated combined lights based on the frequency distribution.
  • the polarity determination method generates and outputs a drive signal that adds a predetermined frequency shift to the polarization component of an optical signal, and outputs two polarization components for each of the two optical signals.
  • Modulates using a drive signal adds a phase difference, combines and outputs two modulated combined lights, multiplexes the two modulated combined lights with their polarizations orthogonal, and outputs a polarization multiplexed optical signal
  • the polarization state of the polarization multiplexed optical signal is changed to a state where each polarization component is mixed, and the mixed optical signal is output, and the relationship between the polarities of the two modulated combined lights is determined based on the frequency distribution of the intensity of the mixed optical signal. .
  • the polarization multiplexed optical transmitter and the polarity determination method according to the present invention can quickly detect the polarity of the optical quadrature phase modulator in its own apparatus without arranging expensive equipment.
  • FIG. 1 is a block configuration diagram of a polarization multiplexed optical transmitter 10 according to a first embodiment of the present invention. It is a block block diagram of the polarization multiplexed optical transmitter 100 which concerns on the 2nd Embodiment of this invention. It is a block block diagram of the optical quadrature phase modulator 250 which concerns on the 2nd Embodiment of this invention.
  • the (a) polarity of the polarization multiplexed optical signal output from the polarization multiplexed optical transmitter 100 according to the second embodiment of the present invention is as set, (b) when one polarity is inverted, (c) both It is an optical spectrum when the polarity of is reversed.
  • FIG. 1 A block diagram of a polarization multiplexed optical transmitter according to the present embodiment is shown in FIG.
  • the polarization multiplexed optical transmitter 10 includes a drive unit 20, two light modulation units 31 and 32, a polarization multiplexing unit 40, a mixing unit 50, and a polarity determination unit 60.
  • the driving means 20 generates a driving signal for modulating two polarization components of the optical signal.
  • the driving means 20 performs pre-equalization processing such as pre-wavelength dispersion imparting processing and spectrum shaping processing on the generated drive signal by digital signal processing (transmission end digital signal processing), and converts it to an analog signal by a digital / analog converter. After the conversion, the light is output to the light modulation means 31 and 32.
  • the driving unit 20 when determining the relationship between the polarities of the light modulators 31 and 32, temporally has a constant speed predetermined for each optical phase of the two polarization components of the optical signal. A drive signal that gives a variation is generated. Here, imparting a constant speed temporal variation to the optical phase of the optical signal is equivalent to adding a frequency shift.
  • the drive unit 20 When determining the relationship between the polarities of the light modulators 31 and 32, the drive unit 20 generates a drive signal that adds a predetermined frequency shift to each of the two polarization components of the optical signal and outputs the drive signal to the light modulators 31 and 32. To do.
  • Each of the optical modulation means 31 and 32 modulates the two polarization components of the input optical signal by using the drive signal input from the drive means 20, adds a phase difference to the two polarization components, and combines the two polarization components.
  • the modulated combined light is output to the polarization multiplexing means 40.
  • the polarization multiplexing unit 40 multiplexes the modulated combined light input from the light modulation units 31 and 32 in a state where the polarizations are orthogonal to each other, and outputs a polarization multiplexed optical signal.
  • the polarization multiplexed optical signal output from the polarization multiplexing unit 40 is output from the polarization multiplexed optical transmitter 10 as a transmission signal, and a part thereof is output to the mixing unit 50.
  • the mixing unit 50 changes the polarization state of the polarization multiplexed optical signal input from the polarization multiplexing unit 40 from a polarization state orthogonal to each other to a state in which each polarization component is mixed, and outputs the mixed optical signal to the polarity determination unit 60.
  • the polarization axes orthogonal to each other may be temporally varied at a constant speed using a polarization scrambler or the like.
  • the polarity determination unit 60 determines the relationship between the polarities of the two light modulation units 31 and 32 based on the frequency distribution of the intensity of the mixed optical signal input from the mixing unit 50.
  • the polarity determination means 60 separates the mixed optical signal input from the mixing means 50 into a high frequency component and a low frequency component using a separation means (not shown).
  • the polarity determination unit 60 further determines the relationship between the polarities of the two light modulation units 31 and 32 based on the magnitude relationship between the intensity of the high frequency component and the intensity of the low frequency component. For example, the driving unit 20 adds frequency shifts having the same sign to the two polarization components of the optical signal. And when the intensity
  • the polarity determination unit 60 determines whether or not a component corresponding to the frequency shift given to the extracted component is included, and determines the relationship between the polarities of the two light modulation units 31 and 32. In this case, the driving unit 20 adds frequency shifts having different signs to the two polarization components of the optical signal. When no beat component is detected from the extracted components, the polarity determination unit 60 determines that the polarities of the two light modulation units 31 and 32 are as set. On the other hand, when a beat component corresponding to the deviation of the frequency shift added from the extracted component is detected, the polarity determination unit 60 determines that the polarity of one of the light modulation units 31 and 32 is inverted.
  • the polarization multiplexed optical transmitter 10 can quickly detect the polarity of the optical quadrature modulator in its own apparatus without arranging expensive equipment.
  • the polarization multiplexed optical transmitter 10 controls the driving unit 20 to modulate one polarization component.
  • the polarity of the light modulation means 31 and 32 can be corrected.
  • a bias control means for controlling the bias of the light modulation means may be disposed in the polarization multiplexed optical transmitter 10.
  • the bias controller corrects the bias points of the light modulators 31 and 32 to the adjacent bias points, and corrects the polarities of the light modulators 31 and 32.
  • the bias control unit controls the bias points of the light modulation units 31 and 32 so that the intensity of the modulated combined light output from the light modulation units 31 and 32 when the drive signal is not input is minimized.
  • FIG. 2 shows a block diagram of the polarization multiplexed optical transmitter according to the present embodiment.
  • the polarization multiplexed light transmitter 100 includes a polarization multiplexed light generation unit 200 and a polarity detection unit 300.
  • the polarization multiplexed light generation unit 200 is a part that functions as a general polarization multiplexed light transmitter.
  • the polarization multiplexed light generator 200 includes a laser oscillator 210, a drive signal generator 220, drive signal transmitters 230a and 230b, bias controllers 240a and 240b, optical quadrature modulators 250a and 250b, a polarization rotation plate 260, and a polarization multiplexer. 270.
  • the laser oscillator 210 generates continuous light having a predetermined optical frequency and outputs it as an input optical signal.
  • the continuous light output from the laser oscillator 210 is branched into two and input to the optical quadrature modulators 250a and 250b, respectively.
  • the drive signal generator 220 generates a drive signal for driving the optical quadrature modulators 250a and 250b, and outputs the generated drive signal to the drive signal transmitters 230a and 230b.
  • the drive signal generation unit 220 generates a drive signal corresponding to a modulation scheme used in an optical communication system such as a multilevel optical phase modulation scheme such as QPSK, based on one or a plurality of transmission bit strings.
  • the drive signal generation unit 220 generates a training signal for determining the polarities of the optical quadrature modulators 250a and 250b when the polarization multiplexed optical transmitter 100 is started up and when the drive signal is interrupted. It outputs to the signal transmission parts 230a and 230b.
  • the training signal is complex signal data that adds a frequency shift of the same sign of a predetermined frequency to the modulated multiplexed light output from the optical quadrature modulators 250a and 250b.
  • the frequency of the modulated combined light is shifted by inputting a high-frequency clock signal having a predetermined speed that temporally varies the phase of the drive signal to the optical quadrature modulators 250a and 250b. You can also.
  • the drive signal generation unit 220 shifts the frequency of the modulated combined light output from the optical quadrature phase modulator 250a to “fc + fx”, and the frequency of the modulated combined light output from the optical quadrature phase modulator 250b. Is shifted to “fc + fy” to generate a training signal.
  • fc is a carrier frequency.
  • the absolute value of the frequency shift amount to be applied is within the device band range of the drive signal transmission units 230a and 230b, and it is desirable that the Mach-Zehnder type optical modulators 252a and 252b can separate the frequency components. . For example, it is desirable to set to several tens of GHz for a drive signal transmission unit for a high-speed optical communication system.
  • the drive signal generation unit 220 when the polarity inversion of the optical quadrature modulators 250a and 250b is detected, the drive signal generation unit 220 generates and outputs a training signal for inverting the sign of the modulated light in order to correct the polarity. To do. A method of correcting the polarity of the optical quadrature modulators 250a and 250b will be described later.
  • the drive signal transmission units 230a and 230b perform transmission end digital signal processing necessary for driving the optical quadrature phase modulators 250a and 250b on the drive signal or the training signal input from the drive signal generation unit 220, respectively. Output to optical quadrature modulators 250a and 250b.
  • the drive signal transmission units 230a and 230b perform pre-equalization processing such as pre-chromatic dispersion applying processing and spectrum shaping processing by digital signal processing as the transmission end digital signal processing.
  • the drive signal transmission units 230a and 230b further convert the digital signal processed signal into an analog signal by a digital / analog converter, and output the analog signal to the optical quadrature modulators 250a and 250b.
  • the bias controllers 240a and 240b control the bias points of the optical quadrature modulators 250a and 250b.
  • the bias controllers 240a and 240b are optical orthogonal so that the light intensity of the modulated combined light output from the optical quadrature modulators 250a and 250b is minimized when the input voltage of the drive signal is zero.
  • the bias points of the phase modulators 250a and 250b are controlled.
  • the optical quadrature modulators 250a and 250b optically modulate the input optical signal input from the laser oscillator 210 using the drive signals input from the drive signal transmitters 230a and 230b, respectively, and output modulated combined light. Further, in the present embodiment, the optical quadrature modulators 250a and 250b are input from the drive signal transmitters 230a and 230b, respectively, when the polarization multiplexed optical transmitter 100 is started and when the input optical signal is stopped.
  • the training signal is optically modulated and output.
  • FIG. 3 shows a block diagram of the optical quadrature modulator 250 according to this embodiment.
  • the optical quadrature modulator 250 includes a polarization maintaining optical splitter 251 and Mach-Zehnder optical modulators 252a and 252b.
  • the optical quadrature modulator 250 further includes optical splitters 253a and 253b, an optical phase shifter 254, an optical coupler 255, an optical splitter 256, and optical signal detectors 257a, 257b, and 258.
  • the input optical signal input from the laser oscillator 210 to the optical quadrature modulator 250 is branched into two by the polarization maintaining optical splitter 251 while maintaining the polarization state, and is output to the Mach-Zehnder optical modulators 252a and 252b, respectively. .
  • the Mach-Zehnder optical modulators 252a and 252b optically modulate the input optical signal input from the polarization maintaining optical splitter 251 using the drive signal input from the drive signal transmission units 230a and 230b, and transmit the optical signal to the optical splitters 253a and 253b. Output.
  • the Mach-Zehnder optical modulators 252a and 252b optically modulate the training signals input from the drive signal transmitters 230a and 230b when the polarization multiplexed optical transmitter 100 is started and when the input optical signal is stopped. And output to the optical splitters 253a and 253b.
  • the optical splitters 253a and 253b output part of the modulated light input from the Mach-Zehnder optical modulators 252a and 252b to the optical signal detectors 257a and 257b, and output most of the modulated light to the optical coupler 255 side.
  • the optical phase shifter 254 is disposed between the optical splitter 253b and the optical coupler 255, changes the phase of the modulated light input from the optical splitter 253b by ⁇ / 2, and outputs the result to the optical coupler 255.
  • the modulated light output from the Mach-Zehnder optical modulator 252a is defined as I phase
  • the modulated light output from the Mach-Zehnder optical modulator 252b via the optical phase shifter 254 is defined as Q-phase.
  • the optical coupler 255 includes modulated light (phase I) output from the Mach-Zehnder optical modulator 252a and modulated light in which the phase output from the Mach-Zehnder optical modulator 252b via the optical phase shifter 254 changes by ⁇ / 2. (Q phase) is multiplexed and modulated multiplexed light is output.
  • a part of the modulated / multiplexed light output from the optical coupler 255 is output to the optical signal detector 258 in the optical splitter 256 and most is output to the polarization multiplexing unit 270 side.
  • the optical signal detectors 257a, 257b, and 258 measure the optical intensities of the modulated light and the modulated combined light output from the Mach-Zehnder optical modulators 252a and 252b and the optical coupler 255 via the optical splitters 253a and 253b and the optical splitter 256, respectively.
  • the measurement result is output to the bias control unit 240.
  • the bias controller 240 controls the bias point of the optical quadrature modulator 250 based on the measurement results input from the optical signal detectors 257a, 257b, and 258.
  • the polarization rotation plate 260 is disposed between the optical quadrature modulator 250 b and the polarization multiplexing unit 270.
  • the polarization rotation plate 260 performs optical quadrature phase modulation so that the polarization state of the modulated combined light output from the optical quadrature modulator 250b is orthogonal to the polarization state of the modulated combined light output from the drive signal transmission unit 230a.
  • the modulated combined light output from the detector 250 b is polarized and output to the polarization multiplexing unit 270.
  • the polarization multiplexing unit 270 multiplexes the modulated combined light input from the optical quadrature modulator 250a and the modulated combined light input from the optical quadrature modulator 250b via the polarization rotation plate 260, and a polarization multiplexed optical signal. Is output as a transmission optical signal to the optical transmission line.
  • the polarity detector 300 is a part that detects the polarities of the optical quadrature modulators 250a and 250b.
  • the polarity detection unit 300 includes an optical splitter 310, a polarizing plate 320, a Mach-Zehnder optical interferometer 330, optical signal detectors 340a and 340b, a signal detection unit 350, and a signal adjustment unit 360.
  • the optical splitter 310 branches a part of the polarization multiplexed optical signal output from the polarization multiplexing unit 270 to the polarizing plate 320.
  • the polarizing plate 320 changes the polarization state of the polarization multiplexed optical signal output from the polarization multiplexing unit 270 so that two optical signals having a controlled polarization axis and polarization states orthogonal to each other are mixed, Output mixed optical signal. If the polarization rotation of the polarization rotation plate 260 of the polarization multiplexed light generation unit 200 can be changed to zero only during the period when the training signal is applied, the polarizing plate 320 can be omitted.
  • the Mach-Zehnder optical interferometer 330 is set so that the 3 dB band coincides with the ITU (International Telecommunication Union) grid frequency, and the mixed optical signal input from the polarizing plate 320 is converted into a high frequency component and a low frequency based on the carrier frequency fc.
  • the frequency components are separated and output to the optical signal detectors 340a and 340b, respectively.
  • the free spectral region (FSR: “Free Spectral Range”) of the Mach-Zehnder type optical interferometer 330 needs to be at least larger than half of the absolute value of the applied frequency offset amount.
  • the larger the FSR the easier the device size of the Mach-Zehnder interferometer can be reduced, and the size of the optical transmitter can be reduced.
  • an FSR of about several tens of GHz is desirable, and management is facilitated by matching the frequency interval of the ITU grid.
  • An optical interleaver can be used instead of the Mach-Zehnder type optical interferometer.
  • Each of the optical signal detectors 340a and 340b converts the high frequency component and the low frequency component input from the Mach-Zehnder optical interferometer 330 into an electric intensity signal and outputs it to the signal detection unit 350.
  • the signal detector 350 compares the magnitude of the electrical intensity signal corresponding to the high frequency component input from the optical signal detector 340a and the electrical intensity signal corresponding to the low frequency component input from the optical signal detector 340b. . And the signal detection part 350 discriminate
  • the signal adjustment unit 360 controls the sign of the drive signal output from the drive signal transmission units 230a and 230b based on the determination result of the polarities of the optical quadrature phase modulators 250a and 250b, whereby the optical quadrature phase modulator 250a and The polarity of 250b is corrected.
  • the polarity correction method will be described later.
  • the drive signal generation unit 220 shifts the frequency of the modulated combined light (X-polarized light) output from the optical quadrature modulator 250a to “fc + fx” and performs optical quadrature modulation when the carrier frequency is fc.
  • a training signal for shifting the frequency of the modulated combined light (Y-polarized light) output from the detector 250b to “fc + fy” is generated and output.
  • the modulated combined light of “fc + fx” and the modulated combined light of “fc + fy” are polarization multiplexed and input to the polarizing plate 320.
  • the polarization-combined light of “fc + fx” and the modulated light of “fc + fy” are mixed in the polarization state in the polarizing plate 320 and separated into a high frequency component and a low frequency component based on the carrier frequency fc in the Mach-Zehnder interferometer 330. Is done.
  • the frequency offset to be applied to the polarization multiplexed optical signal is set so that the optical frequency shifts (+ fx, + fy) are output from the high frequency component port of the Mach-Zehnder interferometer 330, respectively. Accordingly, when the polarities of the optical quadrature modulators 250a and 250b are as set, the magnitude of the high frequency component is significantly larger than the magnitude of the low frequency component. On the other hand, for example, when the polarity of the optical quadrature modulator 250a is inverted, the sign of the frequency shift is inverted and changed from “+ fx” to “ ⁇ fx” and separated to the low frequency component side. The magnitude of the high frequency component is equal to the magnitude of the low frequency component. Furthermore, when the polarities of the optical quadrature modulators 250a and 250b are reversed, the magnitude of the low frequency component is significantly larger than the magnitude of the high frequency component.
  • the inversion of the polarities of the optical quadrature modulators 250a and 250b can be detected by monitoring the magnitude relationship of the intensity of the electric intensity signals output from the two optical detectors 340a and 340b in the optical signal detector 350.
  • . 4A shows a case where the polarities of the optical quadrature phase modulators 250a and 250b are both set
  • FIG. 4B shows a case where the polarity of one of the optical quadrature phase modulators 250a and 250b is inverted
  • FIG. 4C shows the case where the polarities of the optical quadrature modulators 250a and 250b are both reversed.
  • the unit of the vertical axis in FIG. 4 is a power spectral density (PSD: Power spectral density).
  • PSD Power spectral density
  • the Mach-Zehnder optical interferometer 330 separates the high frequency component and the low frequency component based on the carrier frequency fc, so that the size of the high frequency component is compared with the size of the low frequency component. Significantly larger. Therefore, as shown in FIG. 4A, the mixed optical signal output from the polarizing plate 320 has a peak in the frequency component of +5 GHz with respect to fc. This coincides with the amount of frequency shift given to each of the X-polarized signal light and the Y-polarized signal light.
  • the mixed light signal output from the polarizing plate 320 has peaks in the frequency components of +5 GHz and ⁇ 5 GHz.
  • the magnitude of the low frequency component is significantly larger than the magnitude of the high frequency component. Therefore, by appropriately setting the magnitude of the frequency offset for the polarization multiplexed optical signal, the mixed optical signal output from the polarizing plate 320 has a frequency smaller than the carrier frequency fc, as shown in FIG. A peak appears in the component.
  • the polarities of the optical quadrature modulators 250a and 250b are reversed. Contrary to the setting, the electric signal intensity of the optical detector 340b is larger than the electric signal intensity of the optical detector 340a.
  • the magnitude relationship between the electric signal intensity corresponding to the high frequency component output from the optical detector 340a and the electric signal intensity corresponding to the low frequency component output from the optical detector 340b. Can be detected to detect the presence or absence of polarity reversal of the optical quadrature modulators 250a and 250b.
  • the setting of the polarizing plate 320 is changed with time so that there is no change in the magnitude relationship of the electric signal intensity of the photodetectors 340a and 340b. It is desirable to confirm.
  • the signal adjustment unit 360 is driven, for example
  • the setting of the signal transmission unit 230a is changed to invert the sign of the I-phase drive signal of the optical quadrature modulator 250a.
  • the sign of the Q-phase drive signal of the optical quadrature modulator 250a can be inverted, or the setting of the drive signal transmitter 230b can be changed to drive the I-phase or Q-phase of the optical quadrature modulator 250b. It is also possible to invert the sign of the signal.
  • the magnitude relationship of the electric signal strengths of the photodetectors 340a and 340b is monitored again.
  • the polarities of the optical quadrature modulators 250a and 250b have been corrected as set (optical quadrature phase modulation).
  • the polarity of the vessel 250a was reversed).
  • the optical quadrature modulator 250a when the magnitude relationship opposite to the result of the frequency shift provided by the drive signal generation unit 220 is realized, the optical orthogonal It can be seen that the polarity of the phase modulator 250b has been reversed. In this case, the polarity of the optical quadrature modulators 250a and 250b is corrected as set by returning the setting for the optical quadrature modulator 250a and changing the setting for the optical quadrature modulator 250b.
  • the signal adjustment unit 360 The setting of the drive signal transmission units 230a and 230b is changed to invert the sign of the I-phase drive signal of the optical quadrature modulators 250a and 250b. Note that the sign of the driving signal of the Q phase can be inverted instead of the I phase, and the sign of the driving signal of a different phase can be inverted.
  • the drive signal generation unit 220 when the polarization multiplexed optical transmitter 100 is activated and when a drive signal is interrupted, the drive signal generation unit 220 generates a modulation signal output from the optical quadrature modulator 250a.
  • a training signal for shifting the frequency of the wave light to “fc + fx” and shifting the frequency of the modulated multiplexed light output from the optical quadrature modulator 250b to “fc + fy” is generated and output to the drive signal transmission units 230a and 230b ( S101).
  • the drive signal transmitters 230a and 230b perform transmission-end digital signal processing on the input training signals and apply them to the optical quadrature modulators 250a and 250b.
  • the optical quadrature modulators 250a and 250b optically modulate and multiplex the applied training signals, and output modulated combined light (S102).
  • the modulated combined light output from the optical quadrature modulator 250 a and the modulated combined light output from the optical quadrature modulator 250 b and the polarization rotation plate 260 are multiplexed and output by the polarization multiplexer 270.
  • a part of the polarization multiplexed optical signal output from the polarization multiplexing unit 270 is input to the polarizing plate 320 (S103).
  • the polarizing plate 320 mixes the polarization state of the input polarization multiplexed optical signal and outputs it to the Mach-Zehnder interferometer 330.
  • the Mach-Zehnder optical interferometer 330 converts the input mixed optical signal into a high frequency component and a low frequency. The components are separated and output to the optical signal detectors 340a and 340b, respectively (S104).
  • the optical signal detectors 340a and 340b convert the input high frequency components and low frequency components into electric intensity signals and output them to the signal detection unit 350.
  • the signal detection unit 350 monitors the magnitude relationship of the signals input from the photodetectors 340a and 340b.
  • the polarization multiplexed optical transmitter 100 determines that the polarities of the optical quadrature modulators 250a and 250b are as set, and ends the generation of the training signal. Then, the normal transmission operation is restored (S107).
  • the polarization multiplexed optical transmitter 100 changes the setting of the drive signal transmitters 230a and 230b to invert the sign of the I-phase or Q-phase drive signal of the optical quadrature modulators 250a and 250b, thereby changing the polarity. Correction is performed (S108). When the polarities of the optical quadrature modulators 250a and 250b become appropriate, the polarization multiplexed optical transmitter 100 returns to the normal transmission operation.
  • one optical quadrature phase modulator shifts the carrier frequency by fx, and the other optical quadrature phase modulator has fy.
  • the modulated / multiplexed training signal is separated into a high frequency component and a low frequency component by using the polarizing plate 320 and the Mach-Zehnder optical interferometer 330.
  • the polarization multiplexed optical transmitter 100 determines the polarities of the optical quadrature modulators 250a and 250b based on the comparison result between the intensity of the high frequency component and the intensity of the low frequency component.
  • the polarization multiplexed optical transmitter 100 reverses the sign of the I-phase or Q-phase drive signal of the optical quadrature modulators 250a and 250b. Correct.
  • the polarization multiplexed optical transmitter 100 can quickly detect the polarity of the optical quadrature phase modulator in its own device without arranging an expensive device such as an optical spectrum analyzer and the optical quadrature phase modulation.
  • the polarity of the vessel can be corrected.
  • the polarity is corrected by inverting the sign of the drive signal. It is not limited to.
  • the bias control units 240a and 240b are controlled to correct either the I-phase or Q-phase bias of the optical quadrature modulators 250a and 250b to the adjacent bias point among the applied bias values. But you can correct the polarity.
  • the polarization multiplexed optical transmitter 100 applies temporal variation of the optical phase at a predetermined speed to the training signal, and determines the bias point by the bias control units 240a and 240b. Thereafter, the polarization multiplexed optical transmitter 100 detects the sign of temporal variation of the optical phase of the output polarization multiplexed optical signal, and detects the polarities of the optical quadrature modulators 250a and 250b. The polarization multiplexed optical transmitter 100 further corrects the bias points of the optical quadrature modulators 250a and 250b according to the detection result.
  • a training signal that shifts the frequency of the two modulated multiplexed lights is generated, and the training signal is transmitted as an optical signal. Modulated and combined.
  • a frequency shift can be applied to the input optical signal (actual transmission data signal) input from the laser oscillator 210.
  • the drive signal generation unit 220 has a function of giving a frequency shift of a predetermined frequency to each polarization component of the input optical signal to the drive signal generated based on the input optical signal input from the laser oscillator 210. Is added.
  • the optical quadrature modulators 250a and 250b optically modulate the input optical signal input from the laser oscillator 210 using a drive signal to which a function for giving a frequency shift is added. Note that when the drive signal transmission units 230a and 230b perform pre-equalization or the like using transmission end digital signal processing on the drive signal, a function of giving a frequency shift of a predetermined frequency to each polarization component of the input optical signal Can also be added to the drive signal.
  • FIG. 6 shows a numerical simulation result when the drive signal generation unit 220 adds a function of giving a frequency shift of 5 GHz to each polarization component of the 50 Gbps polarization multiplexed QPSK signal to the drive signal.
  • 6A shows a case where the polarities of the optical quadrature phase modulators 250a and 250b are as set
  • FIG. 6B shows a case where the polarity of one of the optical quadrature phase modulators 250a and 250b is inverted. .
  • the optical quadrature phase modulators 250a and 250b are added in the normal transmission operation by adding a function of giving a frequency shift to the drive signal corresponding to the input optical signal (actual transmission data signal) input from the laser oscillator 210. Can be determined. Note that when the polarity inversion of the optical quadrature modulators 250a and 250b is detected, the polarity correction method described in the above embodiment can be applied.
  • the optical quadrature modulators 250a and 250b are driven by adding a function of giving a frequency shift to the drive signal subjected to data modulation, switching to a normal transmission operation is performed at high speed. Can be done. This is because the deviation of the operating points of the bias controllers 240a and 240b when switching to transmission data is very small.
  • the polarities of the optical quadrature modulators 250a and 250b are dynamically changed by adding the function of periodically adding the above-described frequency shift to the drive signal generated based on the actual transmission data signal.
  • the polarity can be corrected by detecting the inversion of the polarity.
  • the polarization multiplexed optical transmitter 100 can also be applied to detection of polarity reversal of an optical quadrature modulator in a single polarization optical communication system. Further, it can be applied to an arbitrary optical phase modulation system, and can be applied to the configuration of an optical transmitter specialized for each optical phase modulation system other than the optical quadrature phase modulator.
  • FIG. 7 shows a block diagram of the polarization multiplexed optical transmitter according to the present embodiment.
  • the polarization multiplexed light transmitter 100B according to the present embodiment includes a polarization multiplexed light generation unit 200B and a polarity detection unit 300B.
  • the polarization multiplexed light generator 200B includes a laser oscillator 210B, a drive signal generator 220B, drive signal transmitters 230aB and 230bB, bias controllers 240aB and 240bB, optical quadrature modulators 250aB and 250bB, a polarization rotation plate 260B, and a polarization multiplexer. 270B.
  • the polarity detection unit 300B includes an optical splitter 310B, a polarizing plate 320B, an optical signal detector 370B, a beat signal separation unit 380B, a signal detection unit 350B, and a signal adjustment unit 360B. In the following, the description will focus on the differences from the polarization multiplexed optical transmitter 100 of FIG. 2 described in the second embodiment.
  • the drive signal generator 220B When determining the polarities of the optical quadrature modulators 250aB and 250bB, the drive signal generator 220B according to the present embodiment outputs modulated light to which frequency shifts of different frequencies are given from the optical quadrature modulators 250aB and 250bB. A training signal to be generated is generated.
  • the difference in the amount of frequency shift to be applied within the device band of the optical signal detector 370B.
  • the frequency shift amount to be applied is within the device band range of the drive signal transmission units 230aB and 230bB and outside the device band range of the optical signal detector 370B.
  • the frequency shift amount to be applied can be changed every time the polarities of the optical quadrature modulators 250aB and 250bB are detected in consideration of the temporal change of the frequency characteristics due to the aging of the optical signal detector 370B.
  • the drive signal generation unit 220B can also apply a frequency shift to the modulated light by inputting high-frequency clock signals with different speeds, at which the phase of the drive signal fluctuates with time, to the optical quadrature modulators 250aB and 250bB.
  • the optical splitter 310B branches a part of the polarization multiplexed optical signal output from the polarization multiplexing unit 270B to the polarizing plate 320B in which the polarization axis is managed.
  • the polarizing plate 320B changes the polarization state of the polarization multiplexed optical signal so that optical signals in the polarization state orthogonal to each other are mixed, and outputs it to the optical signal detector 370B.
  • the mixed optical signal output from the polarizing plate 320B includes a signal component whose optical phase varies with time according to the difference in speed of temporal variation of the optical phase applied between the polarization components of the polarization multiplexed signal. Is included.
  • the optical signal detector 370B converts the light intensity of the mixed optical signal into an electrical intensity signal and outputs it to the beat signal separation unit 380B.
  • the beat signal separation unit 380B extracts only the temporally varying component from the electrical intensity signal.
  • the signal is output to the signal detector 350B.
  • the signal detection unit 350B investigates whether or not the output from the beat signal separation unit 380B includes a beat component corresponding to the frequency shift of the frequency given in the drive signal generation unit 220B, and based on the investigation result, optical orthogonality
  • the polarities of the phase modulators 250aB and 250bB are determined.
  • the drive signal generation unit 220B When the carrier frequency is fc, the drive signal generation unit 220B generates and outputs a training signal that shifts the frequency of X-polarized light to “fc + fx” and shifts the frequency of Y-polarized light to “fc + fy”.
  • the modulated light output from the optical quadrature modulators 250aB and 250bB is given by Expression (1).
  • ⁇ is a relative phase between the phase of the X-polarized light signal and the phase of the Y-polarized light signal.
  • Eox and Eoy are the complex amplitudes of the X-polarized light signal and the Y-polarized light signal, respectively.
  • the output from the optical signal detector 370B includes the frequency shift amount given to the X-polarized optical signal by the drive signal generation unit 220B and the frequency shift amount given to the Y-polarized optical signal. And a beat signal having a frequency different from the above.
  • the frequency of the beat signal observed in the optical signal detector 370B is the sum of two frequency shift amounts. That is, the frequency of the beat signal is the sum of the frequency shift amount given to the X-polarized light signal and the frequency shift amount given to the Y-polarized light signal by the drive signal generation unit 220B.
  • a training signal is generated in the drive signal generation unit 220B so that a frequency shift of ⁇ 1.4 GHz is applied to the X-polarized optical signal and a frequency shift of +1.5 GHz is applied to the Y-polarized optical signal.
  • the numerical simulation result in this case is shown in FIG. FIG. 8A shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are the same, and FIG. 8B shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are different from each other.
  • the device band of the optical signal detector 370B is 1 GHz.
  • FIG. 8B when the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, a sharp frequency peak of 100 MHz appears in the signal spectrum of the output signal of the optical signal detector 370B. This coincides with the sum of the frequency shifts applied to the X-polarized light signal and the Y-polarized light signal. This frequency peak (about 115 dB) is detected as a beat signal by the optical signal detector 370B because it is about 50 dB larger than the intensity of the signal corresponding to the actual transmission data (about 65 dB).
  • the optical quadrature phase modulators 250aB and 250bB have the same polarity. It can be determined whether or not it has.
  • the frequency shift amount is set so that the beat signal is detected when the polarities of the optical quadrature modulators 250aB and 250bB are the same, and the beat signal is not detected when the polarities of the quadrature modulators 250aB and 250bB are different from each other. It can also be set.
  • the polarization multiplexed optical transmitter 100B operates in the same manner as in the second embodiment when the signal detection unit 350B determines that the polarities of the optical quadrature phase modulators 250aB and 250bB are different from each other. That is, the polarization multiplexed optical transmitter 100B changes the setting of the drive signal transmission unit 230aB so as to invert the sign of the I-phase drive signal of the optical quadrature modulator 250aB.
  • the sign of the Q-phase drive signal of the optical quadrature modulator 250aB can be inverted, or the setting of the drive signal transmitter 230bB can be changed to drive the I-phase or Q-phase of the optical quadrature modulator 250bB. It is also possible to invert the sign of the signal.
  • the polarization multiplexed optical transmitter 100B controls the bias control units 240aB and 240bB, and among the bias values to be applied, either the I-phase or Q-phase bias of the optical quadrature modulators 250aB and 250bB is adjacent. Correct to the bias point.
  • FIG. 9 shows an operation flow of polarity detection and polarity correction of the polarization multiplexed optical transmitter 100B.
  • the drive signal generator 220B generates training signals for driving the optical quadrature modulators 250aB and 250bB so as to impart different frequency shifts to the modulated light at the time of start-up and when a signal break occurs. Generate.
  • the drive signal generation unit 220B outputs the generated training signal to the drive signal transmission units 230aB and 230bB (S201).
  • the drive signal transmission units 230aB and 230bB perform transmission end digital signal processing on the input training signals and apply them to the optical quadrature modulators 250aB and 250bB.
  • the training signals input to the optical quadrature modulators 250aB and 250bB are optically modulated and combined (S202), and after being multiplexed by the polarization multiplexing unit 270, a part thereof is input to the polarizing plate 320B (S203). ).
  • the polarization multiplexed optical signal is mixed in the polarization state in the polarizing plate 320B, converted into an electric intensity signal in the optical signal detector 370B, and then only a component that temporally varies is extracted in the beat signal separation unit 380B, and the signal detection unit It is output to 350B (S204).
  • the signal detection unit 350B investigates whether or not a beat signal at a frequency position corresponding to the sum of the frequency shift amounts given by the drive signal generation unit 220B is detected from the input electric intensity signal, and based on the investigation result, the light detection unit 350B
  • the polarities of the quadrature modulators 250aB and 250bB are determined (S205).
  • the signal detection unit 350B further changes the setting of the polarization state of the polarizing plate 320B in time to detect the frequency component of the beat signal. It is confirmed whether or not (S206).
  • the polarization multiplexed optical transmitter 100B uses the optical quadrature phase modulators 250a and 250b. Are determined to have the same polarity. In this case, the polarization multiplexed light transmitter 100B ends the generation of the training signal and returns to the normal transmission operation (S207).
  • the polarization multiplexed optical transmitter 100B determines that the polarities of the optical quadrature phase modulators 250a and 250b are not the same. The same applies to the case where the setting changes with time by changing the setting of the polarization state of the polarizing plate 320B (YES in S206). In either case, the signal adjustment unit 360B corrects the polarity by inverting the sign of one drive signal (S208). When the polarities of the optical quadrature modulators 250aB and 250bB become the same, the polarization multiplexed optical transmitter 100B returns to the normal transmission operation.
  • the polarization multiplexed optical transmitter 100B drives the optical quadrature modulators 250aB and 250bB so that the drive signal generation unit 220B imparts frequency shifts of different frequencies to the modulated light.
  • the signal detection unit 350B beats having a frequency corresponding to the difference in speed of temporal variation of the optical phase given by the drive signal generation unit 220B to the electrical intensity signal obtained by modulating / multiplexing the training signal. Investigate whether the signal is included. Then, the polarization multiplexed optical transmitter 100B determines whether or not the polarities of the optical quadrature modulators 250aB and 250bB are the same based on the investigation result of the signal detector 350B.
  • the polarization multiplexed optical transmitter 100B can quickly detect the polarities of the optical quadrature modulators 250aB and 250bB in its own device without disposing an expensive device such as an optical spectrum analyzer, and can also perform optical orthogonality.
  • the polarities of the phase modulators 250aB and 250bB can be corrected.
  • a training signal for driving the optical quadrature modulators 250aB and 250bB is generated so as to impart a frequency shift of a different frequency to the modulated light, and the training signal is optically modulated / combined.
  • the present invention is not limited to this.
  • the drive signal generation unit 220B can add the following function to the drive signal generated based on the optical signal input from the laser oscillator 210B in the normal transmission operation state. That is, a function is added to two optical signals having polarization states orthogonal to each other, and a frequency shift having a different frequency is added to the modulated signal based on the same transmission data signal between the polarization components.
  • a function is added to two optical signals having polarization states orthogonal to each other, and a frequency shift having a different frequency is added to the modulated signal based on the same transmission data signal between the polarization components.
  • two independent polarization states orthogonal to each other are reduced in order to reduce transmission quality degradation due to nonlinear optical effects in an optical fiber transmission line. This applies to the case where a frequency shift is applied to the optical signal.
  • the frequency shift amount imparted to the X-polarized optical signal by the drive signal generation unit 220B and the frequency shift amount imparted to the Y-polarized optical signal A beat signal having a frequency corresponding to the difference between the two is generated.
  • the frequency of the beat signal observed is the frequency shift amount given to the X-polarized optical signal by the drive signal generation unit 220B and the Y-polarized optical signal. Is the sum of the amount of frequency shift given to.
  • FIG. 10 shows a numerical simulation result for a specific drive signal generated by the drive signal generator 220B.
  • This drive signal gives a frequency shift of ⁇ 1.4 GHz to the X-polarized optical signal in addition to the normal data modulation, and in addition to the data modulation based on the same transmission data as the Y-polarized optical signal, +1. It was assumed to give a frequency shift of 5 GHz.
  • FIG. 10A shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are the same, and FIG. 10B shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are different from each other.
  • FIG. 10B when the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, a frequency peak of 100 MHz appears in the signal spectrum of the output signal of the optical signal detector 370B. This coincides with the sum of the frequency shifts applied to each of the X-polarized light signal and the Y-polarized light signal.
  • the drive signal generation unit 220B adds a function of giving a predetermined frequency shift to the drive signal, so that the optical quadrature modulator 250aB is used using the input optical signal (actual transmission data signal) input from the laser oscillator 210B. , 250 bB polarity relationship can be detected.
  • the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, the polarity correction method described in the second embodiment can be applied.
  • the polarity relationship between the optical quadrature modulators 250aB and 250bB can also be determined by adding a function of giving a frequency shift to the drive signal generated based on the actual transmission data signal.
  • the switching to the normal transmission operation is performed at high speed. It can be carried out. This is because the deviation of the operating points of the bias controllers 240aB and 240bB when switching to transmission data is very small.
  • the polarity of the optical quadrature modulators 250aB and 250bB is dynamically changed by adding the function of periodically giving the above-described frequency shift to the drive signal generated based on the actual transmission data signal.
  • the polarity can be corrected by detecting the inversion of the polarity.
  • Polarization multiplexing optical transmitter 20
  • Light modulation means 40
  • Polarization multiplexing means 50
  • Mixing means 60
  • Polarity determination means 100, 100B Polarization multiplexing optical transmitter 200, 200B Polarization multiplexing light generation unit 210, 210B Laser oscillator 220, 220B Drive signal generator 230a, 230b, 230aB, 230bB Drive signal transmitter 240a, 240b, 240aB, 240bB Bias controller 250a, 250b, 250aB, 250bB
  • Optical quadrature modulator 251 Polarization maintaining optical splitter 252a, 252b Mach-Zehnder optical modulation 253a, 253b
  • Optical splitter 254 Optical phase shifter 255
  • Optical coupler 256 Optical splitter 257a, 257b, 258

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Abstract

This polarization-multiplexed light transmitter is equipped with: a driving means that generates and outputs driving signals for applying predetermined frequency shift to polarization components of a light signal; two light modulation means that each modulates two polarization components of each of two light signals by using the driving signals, combines the modulated polarization components with the addition of a phase difference, and outputs a modulated composite light beam; a polarization-multiplexing means that multiplexes the two modulated composite light beams with the light beams being orthogonally polarized to each other and outputs a polarization-multiplexed light signal; a mixing means that changes the polarization state of the polarization-multiplexed signal into a state in which the respective polarization components are mixed together and outputs a mixed light signal; and a polarity determination means that determines the polarity relationship between the two modulated composite light beams on the basis of frequency distribution of the intensity of the mixed light signal.

Description

偏光多重光送信機および極性判定方法Polarized multiple optical transmitter and polarity determination method
　本発明は、光デジタルコヒーレント方式の光伝送システムに使用される偏光多重光送信機および極性判定方法に関し、マッハツェンダ型光変調器を用いて光信号を変調する光デジタルコヒーレント方式の偏光多重光送信機および極性判定方法に関する。 The present invention relates to a polarization multiplexed optical transmitter and a polarity determination method used in an optical digital coherent optical transmission system, and to an optical digital coherent polarization multiplexed optical transmitter that modulates an optical signal using a Mach-Zehnder optical modulator. And a polarity determination method.
　インターネット等の普及により、基幹ネットワークを通過するトラフィックが急増している。これに対応するために、１波長あたり１００Ｇｂｐｓを超える超高速の波長多重長距離光通信システムの実現が強く望まれている。それを実現するための技術の一つに、光デジタルコヒーレント方式がある。光デジタルコヒーレント方式が適用された光伝送装置は、例えば、特許文献１に開示されている。 The traffic that passes through the backbone network is increasing rapidly due to the spread of the Internet. In order to cope with this, it is strongly desired to realize an ultrahigh-speed wavelength multiplexing long-distance optical communication system exceeding 100 Gbps per wavelength. One of the technologies for realizing this is an optical digital coherent method. An optical transmission device to which an optical digital coherent method is applied is disclosed in Patent Document 1, for example.
　光デジタルコヒーレント方式が適用された光伝送装置においては、偏光多重されたＱＰＳＫ（Quadrature Phase Shift Keying）光信号の４つの光位相(例えば、４５度、１３５度、２２５度、３１５度) に、それぞれ２ビット（例えば００、０１、１１、１０）を割り当てる（シンボル・マッピング）。ＱＰＳＫ光信号のコンスタレーションの一例が図１１（ａ）、（ｂ）に示される。 In an optical transmission apparatus to which an optical digital coherent method is applied, each of four optical phases (for example, 45 degrees, 135 degrees, 225 degrees, and 315 degrees) of a polarization multiplexed QPSK (QuadraturerPhase Shift Keying) optical signal 2 bits (for example, 00, 01, 11, 10) are allocated (symbol mapping). An example of the constellation of the QPSK optical signal is shown in FIGS. 11 (a) and 11 (b).
　光デジタルコヒーレント方式の光伝送装置においては、偏波保持光スプリッタ等によって２分岐した入力光信号を駆動信号電圧に基づいて光変調する時に、一般的に、光直交位相変調器の一つであるマッハツェンダ型光変調器が用いられる。マッハツェンダ型光変調器は、入力光信号を２つに分離して２つの光路を通過させ、再度合波することによって、２つの光路の干渉を利用して光信号の強度を変化させる。マッハツェンダ型光変調器は、２つの光路に駆動信号電圧を加えることによって、光路を通過する光信号の位相を変化させ、出力される変調光の強度を制御する。 An optical digital coherent optical transmission device is generally one of optical quadrature modulators when optically modulating an input optical signal branched into two by a polarization maintaining optical splitter or the like based on a drive signal voltage. A Mach-Zehnder type optical modulator is used. The Mach-Zehnder optical modulator changes the intensity of an optical signal by using interference between the two optical paths by separating the input optical signal into two parts, passing the two optical paths, and recombining them. The Mach-Zehnder optical modulator changes the phase of an optical signal passing through the optical path by applying a drive signal voltage to the two optical paths, and controls the intensity of the modulated light to be output.
　ここで、マッハツェンダ型光変調器は、起点となるバイアス点とそれに隣接するバイアス点とで、駆動信号電圧を掃引したときの出力信号光の位相の変化が異なる。マッハツェンダ型光変調器の駆動信号電圧と出力される変調光の特性との関係を図１２に示す。 Here, in the Mach-Zehnder type optical modulator, the change in the phase of the output signal light when the drive signal voltage is swept differs between the bias point as the starting point and the bias point adjacent thereto. FIG. 12 shows the relationship between the drive signal voltage of the Mach-Zehnder optical modulator and the characteristics of the modulated light to be output.
　図１２に示されるように、駆動信号電圧を出力信号光の強度が最大となる２点間で掃引した場合、図１２の位置Ａのバイアス点では出力信号光の位相が「π→０」に変化するのに対して、図１２の位置Ｂのバイアス点では出力信号光の位相が「０→π」に変化する。従って、起点となるマッハツェンダ型光変調器のバイアス点においてＱＰＳＫ信号のシンボル・マッピングを定める時、互いに隣接するバイアス点においては、図１１（ａ）、（ｂ）に示すように、０１、１０の割り当てが異なる。これを極性問題と呼ぶ。 As shown in FIG. 12, when the drive signal voltage is swept between two points where the intensity of the output signal light is maximum, the phase of the output signal light becomes “π → 0” at the bias point at position A in FIG. In contrast to this, at the bias point at position B in FIG. 12, the phase of the output signal light changes from “0 → π”. Therefore, when the symbol mapping of the QPSK signal is determined at the bias point of the Mach-Zehnder type optical modulator as the starting point, the bias points adjacent to each other are 01, 10 as shown in FIGS. 11 (a) and 11 (b). Assignment is different. This is called a polarity problem.
　一方、光伝送の更なる高速化を図るために、ＱＡＭ (Quadrature Amplitude Modulation)等の多値度の大きな変調方式を適用することが提案されている。多値度の大きな変調方式を適用することにより、電気部品の駆動速度を高速化する際の波形歪みの影響を低減できる。多値度の大きな変調方式を採用した光デジタルコヒーレント方式の光伝送装置においては、デジタル・アナログ変換器によって生成した駆動電気信号を用いて光直交位相変調器を駆動する。 On the other hand, in order to further increase the speed of optical transmission, it has been proposed to apply a modulation method having a large multilevel such as QAM (Quadrature Amplitude Modulation). By applying a modulation method having a large multi-value degree, it is possible to reduce the influence of waveform distortion when increasing the driving speed of electrical components. In an optical digital coherent optical transmission device that employs a modulation method having a large multilevel, an optical quadrature modulator is driven using a drive electrical signal generated by a digital-analog converter.
　また、無線通信で広く知られている直交周波数分割多重方式（ＯＦＤＭ： Orthogonal Frequency Division Multiplexing）方式のような複雑な送信光信号において、送信端デジタル信号処理技術を適用することが提案されている。送信端デジタル信号処理とは、駆動信号電圧に前置波長分散付与処理、スペクトル整形処理、信号振幅増幅処理などの予等化処理をデジタル信号処理によって施した後、デジタル・アナログ変換器によりアナログ信号に変換する処理である。 Also, it has been proposed to apply a transmission-end digital signal processing technique to a complex transmission optical signal such as an orthogonal frequency division multiplexing (OFDM) method that is widely known in wireless communication. Transmission end digital signal processing is pre-equalization processing such as pre-wavelength dispersion imparting processing, spectrum shaping processing, signal amplitude amplification processing, etc., applied to the drive signal voltage by digital signal processing, and then an analog signal by a digital / analog converter It is processing to convert to.
　ここで、予等化処理とは、光伝送路で発生する波形歪みを光送信機側で予め計算し、発生する歪みを打ち消す逆の歪みを送信信号へ付与するための処理である。予等化方式には主に、以下の２つの方式がある。一つ目は、駆動信号に周波数オフセットを付与することにより２個の独立した光信号に対して異なる光位相の時間的変動を付与する方式（方式１）である。二つ目は、送信端デジタル信号処理技術を活用して駆動信号に対して前置波長分散付与処理をデジタル信号処理により実施する方式（方式２）である。 Here, the pre-equalization processing is processing for preliminarily calculating the waveform distortion generated in the optical transmission path on the optical transmitter side, and adding reverse distortion to cancel the generated distortion to the transmission signal. There are mainly the following two types of pre-equalization methods. The first is a method (method 1) in which a time offset of different optical phases is given to two independent optical signals by giving a frequency offset to the drive signal. The second is a method (method 2) in which the pre-chromatic dispersion imparting process is performed on the drive signal by digital signal processing using the transmission end digital signal processing technology.
　方式１については、例えば、非特許文献１に開示されている。方式１は、図１３（ａ）に示すように、偏光多重される２つの独立な光信号に対し、異なる光位相の時間変動を付与する。この場合、互いに直交する偏光成分間で搬送波周波数が異なる偏光多重信号が生成され、品質が改善される。 System 1 is disclosed in Non-Patent Document 1, for example. As shown in FIG. 13A, method 1 gives temporal variations of different optical phases to two independent optical signals that are polarization multiplexed. In this case, polarization multiplexed signals having different carrier frequencies between polarization components orthogonal to each other are generated, and the quality is improved.
　しかし、光信号に対して光位相の時間的変動を付与する時にマッハツェンダ型変調器の極性問題により光位相の時間的変動の符号が変化した場合、図１３（ｂ）に示すように、変調光の光スペクトルの周波数シフトの符号が反転する。この場合、光送信機が付与した光位相の時間的変動を相殺するために光受信機が光位相の時間的変動を加えても、時間的変動の符号が異なるため、時間的変動を相殺することができず、光信号の復調ができない。従って、品質が改善されない。 However, when the sign of the temporal variation of the optical phase changes due to the polarity problem of the Mach-Zehnder type modulator when the temporal variation of the optical phase is applied to the optical signal, as shown in FIG. The sign of the frequency shift of the optical spectrum is inverted. In this case, even if the optical receiver adds the temporal variation of the optical phase to cancel the temporal variation of the optical phase applied by the optical transmitter, the temporal variation is different, so the temporal variation is canceled. Cannot be demodulated. Therefore, the quality is not improved.
　一方、方式２は、波長分散が周波数領域上の光信号の搬送波周波数からの周波数偏差の２乗に比例する光位相回転であることから、光直交位相変調器の極性が反転した場合、予等化処理により付与する波長分散の符号が反転する。波長分散の符号が反転した場合、上述のように時間的変動を相殺することができず、光信号の復調ができない。従って、品質が改善されない。 On the other hand, since the chromatic dispersion is an optical phase rotation in which the chromatic dispersion is proportional to the square of the frequency deviation from the carrier frequency of the optical signal in the frequency domain, when the polarity of the optical quadrature modulator is reversed, The sign of chromatic dispersion given by the conversion processing is inverted. When the sign of chromatic dispersion is inverted, the temporal variation cannot be canceled as described above, and the optical signal cannot be demodulated. Therefore, the quality is not improved.
特開２００７－０４３６３８号公報JP 2007-043638 A
　そこで、上述の方式１に対して、偏光多重光送信機の出力信号光の光スペクトルを光スペクトル・アナライザで測定することにより、互いに直交する偏光成分の光位相の時間的変動の符号を検出する技術が提案されている。しかし、光スペクトル・アナライザを具備した偏光多重光送信機は非常に高価であると共に、光位相の時間的変動の符号を検出する速度が遅い。 Therefore, in contrast to the above-described method 1, the optical spectrum of the output signal light of the polarization multiplexed optical transmitter is measured with an optical spectrum analyzer to detect the sign of temporal variation in the optical phase of the polarization components orthogonal to each other. Technology has been proposed. However, a polarization multiplexed optical transmitter equipped with an optical spectrum analyzer is very expensive and slow in detecting the sign of temporal variation in optical phase.
　また、マッハツェンダ型光変調器の極性を確定する技術として、光送信機の出力信号光を光受信機において復調した際の受信信号品質を測定し、測定結果に基づいて光直交位相変調器のバイアス設定の正誤を検出して極性を判別することが提案されている。しかし、光受信機において復調した信号の品質に基づいて、光送信機の光直交位相変調器のバイアスを設定する場合、フィードバックのための時間がかかる。さらに、光送信機と光受信機の間の通信路を用意する必要があり、システム規模が大きくなり、システムのコストが高くなる。 Also, as a technique for determining the polarity of the Mach-Zehnder optical modulator, the received signal quality when the output signal light of the optical transmitter is demodulated in the optical receiver is measured, and the bias of the optical quadrature modulator is based on the measurement result It has been proposed to determine the polarity by detecting the correctness of the setting. However, when setting the bias of the optical quadrature modulator of the optical transmitter based on the quality of the signal demodulated in the optical receiver, it takes time for feedback. Further, it is necessary to prepare a communication path between the optical transmitter and the optical receiver, which increases the system scale and the system cost.
　本発明の目的は、上記の課題に鑑みてなされたものであり、高価な機器を配置することなく自機内において光直交位相変調器の極性を速やかに検出できる、偏光多重光送信機および極性判定方法を提供することにある。 An object of the present invention is made in view of the above problems, and is a polarization multiplexed optical transmitter and a polarity determination capable of quickly detecting the polarity of an optical quadrature modulator in its own apparatus without arranging expensive equipment. It is to provide a method.
　上記目的を達成するために本発明に係る偏光多重光送信機は、光信号の偏光成分に所定の周波数シフトを付加する駆動信号を生成して出力する駆動手段と、２つの光信号についてそれぞれ、２つの偏光成分を駆動信号を用いて変調し、位相差を付加して合波し、変調合波光を出力する、２つの光変調手段と、２つの変調合波光を偏光を直交させた状態で多重化し、偏光多重光信号を出力する偏光多重手段と、偏光多重光信号の偏光状態を各偏光成分が混合した状態に変化させ、混合光信号を出力する混合手段と、混合光信号の強度の周波数分布に基づいて２つの変調合波光の極性の関係を判定する極性判定手段と、を備える。 In order to achieve the above object, a polarization multiplexed optical transmitter according to the present invention generates a drive signal for adding a predetermined frequency shift to a polarization component of an optical signal and outputs the drive signal, and two optical signals, respectively. Two light modulation means that modulates two polarization components using a drive signal, adds a phase difference, combines them, and outputs a modulated combined light, and two modulated combined lights in a state where polarizations are orthogonal to each other A polarization multiplexing unit that multiplexes and outputs a polarization multiplexed optical signal, a polarization unit that changes a polarization state of the polarization multiplexed optical signal to a state in which each polarization component is mixed, and a mixing unit that outputs a mixed optical signal; Polarity determining means for determining the relationship between the polarities of the two modulated combined lights based on the frequency distribution.
　上記目的を達成するために本発明に係る極性判定方法は、光信号の偏光成分に所定の周波数シフトを付加する駆動信号を生成して出力し、２つの光信号についてそれぞれ、２つの偏光成分を駆動信号を用いて変調し、位相差を付加して合波して２つの変調合波光を出力し、２つの変調合波光を偏光を直交させた状態で多重化して偏光多重光信号を出力し、偏光多重光信号の偏光状態を各偏光成分が混合した状態に変化させて混合光信号を出力し、混合光信号の強度の周波数分布に基づいて２つの変調合波光の極性の関係を判定する。 In order to achieve the above object, the polarity determination method according to the present invention generates and outputs a drive signal that adds a predetermined frequency shift to the polarization component of an optical signal, and outputs two polarization components for each of the two optical signals. Modulates using a drive signal, adds a phase difference, combines and outputs two modulated combined lights, multiplexes the two modulated combined lights with their polarizations orthogonal, and outputs a polarization multiplexed optical signal The polarization state of the polarization multiplexed optical signal is changed to a state where each polarization component is mixed, and the mixed optical signal is output, and the relationship between the polarities of the two modulated combined lights is determined based on the frequency distribution of the intensity of the mixed optical signal. .
　本発明に係る偏光多重光送信機および極性判定方法は、高価な機器を配置することなく自機内において光直交位相変調器の極性を速やかに検出できる。 The polarization multiplexed optical transmitter and the polarity determination method according to the present invention can quickly detect the polarity of the optical quadrature phase modulator in its own apparatus without arranging expensive equipment.
本発明の第１の実施形態に係る偏光多重光送信機１０のブロック構成図である。1 is a block configuration diagram of a polarization multiplexed optical transmitter 10 according to a first embodiment of the present invention. 本発明の第２の実施形態に係る偏光多重光送信機１００のブロック構成図である。It is a block block diagram of the polarization multiplexed optical transmitter 100 which concerns on the 2nd Embodiment of this invention. 本発明の第２の実施形態に係る光直交位相変調器２５０のブロック構成図である。It is a block block diagram of the optical quadrature phase modulator 250 which concerns on the 2nd Embodiment of this invention. 本発明の第２の実施形態に係る偏光多重光送信機１００から出力された偏光多重光信号の（ａ）極性が設定通りの場合、（ｂ）一方の極性が反転した場合、（ｃ）両方の極性が反転した場合の光スペクトルである。When the (a) polarity of the polarization multiplexed optical signal output from the polarization multiplexed optical transmitter 100 according to the second embodiment of the present invention is as set, (b) when one polarity is inverted, (c) both It is an optical spectrum when the polarity of is reversed. 本発明の第２の実施形態に係る偏光多重光送信機１００の動作フロー図である。It is an operation | movement flowchart of the polarization multiplexed optical transmitter 100 which concerns on the 2nd Embodiment of this invention. 本発明の第２の実施形態に係る偏光多重光送信機１００から出力された別の偏光多重光信号の（ａ）極性が同一の場合、（ｂ）一方の極性が反転した場合の光スペクトルである。When (a) polarity of another polarization multiplexed optical signal output from the polarization multiplexed optical transmitter 100 according to the second embodiment of the present invention is the same, (b) an optical spectrum when one polarity is inverted. is there. 本発明の第３の実施形態に係る偏光多重光送信機１００Ｂのブロック構成図である。It is a block block diagram of the polarization multiplexed optical transmitter 100B which concerns on the 3rd Embodiment of this invention. 本発明の第３の実施形態に係る偏光多重光送信機１００Ｂから出力された偏光多重光信号の（ａ）極性が同一の場合、（ｂ）極性が異なる場合の光スペクトルである。It is an optical spectrum in case the (a) polarity of the polarization multiplexed optical signal output from the polarization multiplexed optical transmitter 100B according to the third embodiment of the present invention is the same, and (b) the polarity is different. 本発明の第３の実施形態に係る偏光多重光送信機１００Ｂの動作フロー図である。It is an operation | movement flowchart of the polarization multiplexed optical transmitter 100B which concerns on the 3rd Embodiment of this invention. 本発明の第３の実施形態に係る偏光多重光送信機１００Ｂから出力された別の偏光多重光信号の（ａ）極性が同一の場合、（ｂ）極性が異なる場合の光スペクトルである。It is an optical spectrum when the (a) polarity of another polarization multiplexed optical signal output from the polarization multiplexed optical transmitter 100B according to the third embodiment of the present invention is the same, and (b) the polarity is different. ＱＰＳＫ光信号のコンスタレーションの一例を示す図である。It is a figure which shows an example of the constellation of a QPSK optical signal. マッハツェンダ型光変調器から出力される変調光の特性と駆動信号電圧との関係を示す図である。It is a figure which shows the relationship between the characteristic of the modulated light output from a Mach-Zehnder type optical modulator, and a drive signal voltage. （ａ）極性が設定通りの場合、（ｂ）極性が反転した場合の、光位相の時間的変動を付与した偏光多重光信号の光スペクトルの図である。(A) It is a figure of the optical spectrum of the polarization multiplexed optical signal to which the temporal variation of the optical phase is given when the polarity is as set and (b) the polarity is reversed.
（第１の実施形態）
　第１の実施形態に係る偏光多重光送信機について説明する。本実施形態に係る偏光多重光送信機のブロック構成図を図１に示す。図１において、偏光多重光送信機１０は、駆動手段２０、２つの光変調手段３１、３２、偏光多重手段４０、混合手段５０および極性判定手段６０を備える。 (First embodiment)
A polarization multiplexed optical transmitter according to the first embodiment will be described. A block diagram of a polarization multiplexed optical transmitter according to the present embodiment is shown in FIG. In FIG. 1, the polarization multiplexed optical transmitter 10 includes a drive unit 20, two light modulation units 31 and 32, a polarization multiplexing unit 40, a mixing unit 50, and a polarity determination unit 60.
　駆動手段２０は、光信号の２つの偏光成分を変調するための駆動信号を生成する。駆動手段２０は、生成した駆動信号に、前置波長分散付与処理、スペクトル整形処理などの予等化処理をデジタル信号処理によって施し（送信端デジタル信号処理）、デジタル・アナログ変換器によりアナログ信号に変換した後、光変調手段３１、３２へ出力する。 The driving means 20 generates a driving signal for modulating two polarization components of the optical signal. The driving means 20 performs pre-equalization processing such as pre-wavelength dispersion imparting processing and spectrum shaping processing on the generated drive signal by digital signal processing (transmission end digital signal processing), and converts it to an analog signal by a digital / analog converter. After the conversion, the light is output to the light modulation means 31 and 32.
　また、本実施形態に係る駆動手段２０は、光変調手段３１、３２の極性の関係を判定する時、光信号の２つの偏光成分のそれぞれの光位相に予め定められた一定の速度の時間的変動を付与する駆動信号を生成する。ここで、光信号の光位相に一定の速度の時間的変動を付与することは、周波数シフトを付加することと等価である。駆動手段２０は、光変調手段３１、３２の極性の関係を判定する時に、光信号の２つの偏光成分にそれぞれ所定の周波数シフトを付加する駆動信号を生成して光変調手段３１、３２へ出力する。 In addition, when determining the relationship between the polarities of the light modulators 31 and 32, the driving unit 20 according to the present embodiment temporally has a constant speed predetermined for each optical phase of the two polarization components of the optical signal. A drive signal that gives a variation is generated. Here, imparting a constant speed temporal variation to the optical phase of the optical signal is equivalent to adding a frequency shift. When determining the relationship between the polarities of the light modulators 31 and 32, the drive unit 20 generates a drive signal that adds a predetermined frequency shift to each of the two polarization components of the optical signal and outputs the drive signal to the light modulators 31 and 32. To do.
　光変調手段３１、３２はそれぞれ、入力された光信号の２つの偏光成分を駆動手段２０から入力された駆動信号を用いて変調し、２つの偏光成分に位相差を付加した後で合波し、変調合波光を偏光多重手段４０へ出力する。 Each of the optical modulation means 31 and 32 modulates the two polarization components of the input optical signal by using the drive signal input from the drive means 20, adds a phase difference to the two polarization components, and combines the two polarization components. The modulated combined light is output to the polarization multiplexing means 40.
　偏光多重手段４０は、光変調手段３１、３２から入力された変調合波光を、互いに偏光が直交した状態で多重化し、偏光多重光信号を出力する。偏光多重手段４０から出力された偏光多重光信号は送信信号として偏光多重光送信機１０から出力されると共に、一部が混合手段５０へ出力される。 The polarization multiplexing unit 40 multiplexes the modulated combined light input from the light modulation units 31 and 32 in a state where the polarizations are orthogonal to each other, and outputs a polarization multiplexed optical signal. The polarization multiplexed optical signal output from the polarization multiplexing unit 40 is output from the polarization multiplexed optical transmitter 10 as a transmission signal, and a part thereof is output to the mixing unit 50.
　混合手段５０は、偏光多重手段４０から入力された偏光多重光信号の偏光状態を、互いに直交する偏光状態から各偏光成分が混合した状態に変化させ、混合光信号を極性判定手段６０へ出力する。なお、偏波スクランブラ等を用いて、互いに直交する偏光軸を一定の速度で時間的に変動してもよい。 The mixing unit 50 changes the polarization state of the polarization multiplexed optical signal input from the polarization multiplexing unit 40 from a polarization state orthogonal to each other to a state in which each polarization component is mixed, and outputs the mixed optical signal to the polarity determination unit 60. . Note that the polarization axes orthogonal to each other may be temporally varied at a constant speed using a polarization scrambler or the like.
　極性判定手段６０は、混合手段５０から入力された混合光信号の強度の周波数分布に基づいて２つの光変調手段３１、３２の極性の関係を判定する。 The polarity determination unit 60 determines the relationship between the polarities of the two light modulation units 31 and 32 based on the frequency distribution of the intensity of the mixed optical signal input from the mixing unit 50.
　本実施形態において、極性判定手段６０は図示しない分離手段を用いて混合手段５０から入力された混合光信号を高周波数成分と低周波数成分とに分離する。極性判定手段６０はさらに、高周波数成分の強度と低周波数成分の強度との大小関係に基づいて２つの光変調手段３１、３２の極性の関係を判定する。例えば、駆動手段２０が光信号の２つの偏光成分にそれぞれ同じ符号の周波数シフトを付加する。そして、高周波数成分の強度が低周波数成分の強度より有意に大きい場合、極性判定手段６０は、２つの光変調手段３１、３２の極性が設定通りであると判定する。一方、高周波数成分の強度と低周波数成分の強度が同等の場合、極性判定手段６０は、光変調手段３１、３２のいずれか一方の極性が反転したと判定する。 In the present embodiment, the polarity determination means 60 separates the mixed optical signal input from the mixing means 50 into a high frequency component and a low frequency component using a separation means (not shown). The polarity determination unit 60 further determines the relationship between the polarities of the two light modulation units 31 and 32 based on the magnitude relationship between the intensity of the high frequency component and the intensity of the low frequency component. For example, the driving unit 20 adds frequency shifts having the same sign to the two polarization components of the optical signal. And when the intensity | strength of a high frequency component is significantly larger than the intensity | strength of a low frequency component, the polarity determination means 60 determines with the polarity of the two light modulation means 31 and 32 being as setting. On the other hand, when the intensity of the high frequency component is the same as the intensity of the low frequency component, the polarity determination unit 60 determines that the polarity of one of the light modulation units 31 and 32 is inverted.
　なお、図示しない抽出手段を用いて混合光信号から時間的に変動する成分のみを抽出することもできる。極性判定手段６０は、抽出した成分に付与した周波数シフトに対応する成分が含まれているか否か判断し、２つの光変調手段３１、３２の極性の関係を判定する。この場合、駆動手段２０が光信号の２つの偏光成分にそれぞれ異なる符号の周波数シフトを付加する。そして、抽出された成分からビート成分が検出されない場合、極性判定手段６０は、２つの光変調手段３１、３２の極性が設定通りであると判定する。一方、抽出された成分から付加された周波数シフトの偏差に相当するビート成分が検出された場合、極性判定手段６０は、光変調手段３１、３２のいずれか一方の極性が反転したと判定する。 It should be noted that it is possible to extract only a component that varies with time from the mixed optical signal by using an extracting means (not shown). The polarity determination unit 60 determines whether or not a component corresponding to the frequency shift given to the extracted component is included, and determines the relationship between the polarities of the two light modulation units 31 and 32. In this case, the driving unit 20 adds frequency shifts having different signs to the two polarization components of the optical signal. When no beat component is detected from the extracted components, the polarity determination unit 60 determines that the polarities of the two light modulation units 31 and 32 are as set. On the other hand, when a beat component corresponding to the deviation of the frequency shift added from the extracted component is detected, the polarity determination unit 60 determines that the polarity of one of the light modulation units 31 and 32 is inverted.
　以上のように、本実施形態に係る偏光多重光送信機１０は、高価な機器を配置することなく自機内において光直交位相変調器の極性を速やかに検出できる。 As described above, the polarization multiplexed optical transmitter 10 according to the present embodiment can quickly detect the polarity of the optical quadrature modulator in its own apparatus without arranging expensive equipment.
　ここで、偏光多重光送信機１０は、極性判定手段６０において光変調手段３１、３２の極性が反転したと判定した場合、駆動手段２０を制御して一方の偏光成分を変調するための駆動信号の符号を反転させて、光変調手段３１、３２の極性を補正することができる。 Here, when the polarization determining optical transmitter 10 determines that the polarities of the light modulating units 31 and 32 are inverted in the polarity determining unit 60, the polarization multiplexed optical transmitter 10 controls the driving unit 20 to modulate one polarization component. The polarity of the light modulation means 31 and 32 can be corrected.
　なお、偏光多重光送信機１０に、光変調手段のバイアスを制御するバイアス制御手段を配置することもできる。そして、光変調手段３１、３２の極性が反転した場合、バイアス制御手段が光変調手段３１、３２のバイアス点を隣接するバイアス点に補正し、光変調手段３１、３２の極性を補正する。なお、バイアス制御手段は、駆動信号が入力していない時に光変調手段３１、３２から出力される変調合波光の強度が最小になるように、光変調手段３１、３２のバイアス点を制御する。 It should be noted that a bias control means for controlling the bias of the light modulation means may be disposed in the polarization multiplexed optical transmitter 10. When the polarities of the light modulators 31 and 32 are inverted, the bias controller corrects the bias points of the light modulators 31 and 32 to the adjacent bias points, and corrects the polarities of the light modulators 31 and 32. The bias control unit controls the bias points of the light modulation units 31 and 32 so that the intensity of the modulated combined light output from the light modulation units 31 and 32 when the drive signal is not input is minimized.
　（第２の実施形態）
　第２の実施形態について説明する。本実施形態に係る偏光多重光送信機のブロック構成図を図２に示す。図２において、偏光多重光送信機１００は、偏光多重光生成部２００および極性検出部３００を備える。 (Second Embodiment)
A second embodiment will be described. FIG. 2 shows a block diagram of the polarization multiplexed optical transmitter according to the present embodiment. In FIG. 2, the polarization multiplexed light transmitter 100 includes a polarization multiplexed light generation unit 200 and a polarity detection unit 300.
　先ず、偏光多重光生成部２００について説明する。偏光多重光生成部２００は、一般的な偏光多重光送信機の機能を担う部分である。偏光多重光生成部２００は、レーザ発振器２１０、駆動信号生成部２２０、駆動信号送信部２３０ａ、２３０ｂ、バイアス制御部２４０ａ、２４０ｂ、光直交位相変調器２５０ａ、２５０ｂ、偏光回転板２６０および偏光多重部２７０を備える。 First, the polarization multiplexed light generation unit 200 will be described. The polarization multiplexed light generation unit 200 is a part that functions as a general polarization multiplexed light transmitter. The polarization multiplexed light generator 200 includes a laser oscillator 210, a drive signal generator 220, drive signal transmitters 230a and 230b, bias controllers 240a and 240b, optical quadrature modulators 250a and 250b, a polarization rotation plate 260, and a polarization multiplexer. 270.
　レーザ発振器２１０は、所定の光周波数の連続光を生成して入力光信号として出力する。レーザ発振器２１０から出力された連続光は、２分岐されて光直交位相変調器２５０ａ、２５０ｂにそれぞれ入力する。 The laser oscillator 210 generates continuous light having a predetermined optical frequency and outputs it as an input optical signal. The continuous light output from the laser oscillator 210 is branched into two and input to the optical quadrature modulators 250a and 250b, respectively.
　駆動信号生成部２２０は、光直交位相変調器２５０ａ、２５０ｂを駆動するための駆動信号を生成し、生成した駆動信号を駆動信号送信部２３０ａ、２３０ｂへ出力する。駆動信号生成部２２０は、１つまたは複数の送信ビット列に基づいて、ＱＰＳＫ等の多値光位相変調方式などの光通信システムにおいて使用する変調方式に応じた駆動信号を生成する。 The drive signal generator 220 generates a drive signal for driving the optical quadrature modulators 250a and 250b, and outputs the generated drive signal to the drive signal transmitters 230a and 230b. The drive signal generation unit 220 generates a drive signal corresponding to a modulation scheme used in an optical communication system such as a multilevel optical phase modulation scheme such as QPSK, based on one or a plurality of transmission bit strings.
　また、駆動信号生成部２２０は、偏光多重光送信機１００の起動時および駆動信号断等が発生した時に、光直交位相変調器２５０ａ、２５０ｂの極性を判断するためのトレーニング信号を生成し、駆動信号送信部２３０ａ、２３０ｂへ出力する。ここで、トレーニング信号は、光直交位相変調器２５０ａ、２５０ｂから出力される変調合波光に、予め定められた周波数の同符号の周波数シフトを付与する複素信号データである。なお、トレーニング信号を用いる代わりに、駆動信号の位相を時間的に変動する所定の速度の高周波クロック信号を光直交位相変調器２５０ａ、２５０ｂへ入力させることによって、変調合波光の周波数をシフトさせることもできる。 Further, the drive signal generation unit 220 generates a training signal for determining the polarities of the optical quadrature modulators 250a and 250b when the polarization multiplexed optical transmitter 100 is started up and when the drive signal is interrupted. It outputs to the signal transmission parts 230a and 230b. Here, the training signal is complex signal data that adds a frequency shift of the same sign of a predetermined frequency to the modulated multiplexed light output from the optical quadrature modulators 250a and 250b. Instead of using the training signal, the frequency of the modulated combined light is shifted by inputting a high-frequency clock signal having a predetermined speed that temporally varies the phase of the drive signal to the optical quadrature modulators 250a and 250b. You can also.
　本実施形態に係る駆動信号生成部２２０は、光直交位相変調器２５０ａから出力される変調合波光の周波数を「ｆｃ＋ｆｘ」にシフトさせ、光直交位相変調器２５０ｂから出力される変調合波光の周波数を「ｆｃ＋ｆｙ」にシフトさせる、トレーニング信号を生成する。ここで、ｆｃは搬送波周波数である。ここで、付与する周波数シフト量の絶対値は、駆動信号送信部２３０ａ、２３０ｂのデバイス帯域の範囲内であり、マッハツェンダ型光変調器２５２ａ、２５２ｂが周波数成分を分離できる大きさであることが望ましい。例えば、高速光通信システム用の駆動信号送信部に対しては数十ＧＨｚに設定することが望ましい。 The drive signal generation unit 220 according to the present embodiment shifts the frequency of the modulated combined light output from the optical quadrature phase modulator 250a to “fc + fx”, and the frequency of the modulated combined light output from the optical quadrature phase modulator 250b. Is shifted to “fc + fy” to generate a training signal. Here, fc is a carrier frequency. Here, the absolute value of the frequency shift amount to be applied is within the device band range of the drive signal transmission units 230a and 230b, and it is desirable that the Mach-Zehnder type optical modulators 252a and 252b can separate the frequency components. . For example, it is desirable to set to several tens of GHz for a drive signal transmission unit for a high-speed optical communication system.
　さらに、駆動信号生成部２２０は、光直交位相変調器２５０ａ、２５０ｂの極性の反転が検出された場合、極性を補正するために、変調光の符号を反転させるためのトレーニング信号を生成して出力する。光直交位相変調器２５０ａ、２５０ｂの極性の補正方法については後述する。 Further, when the polarity inversion of the optical quadrature modulators 250a and 250b is detected, the drive signal generation unit 220 generates and outputs a training signal for inverting the sign of the modulated light in order to correct the polarity. To do. A method of correcting the polarity of the optical quadrature modulators 250a and 250b will be described later.
　駆動信号送信部２３０ａ、２３０ｂはそれぞれ、駆動信号生成部２２０から入力された駆動信号またはトレーニング信号に、光直交位相変調器２５０ａ、２５０ｂを駆動するのに必要となる送信端デジタル信号処理を行い、光直交位相変調器２５０ａ、２５０ｂへ出力する。本実施形態において、駆動信号送信部２３０ａ、２３０ｂは、送信端デジタル信号処理として、前置波長分散付与処理、スペクトル整形処理などの予等化処理をデジタル信号処理によって施す。駆動信号送信部２３０ａ、２３０ｂはさらに、デジタル信号処理した信号をデジタル・アナログ変換器によってアナログ信号に変換し、光直交位相変調器２５０ａ、２５０ｂへ出力する。 The drive signal transmission units 230a and 230b perform transmission end digital signal processing necessary for driving the optical quadrature phase modulators 250a and 250b on the drive signal or the training signal input from the drive signal generation unit 220, respectively. Output to optical quadrature modulators 250a and 250b. In the present embodiment, the drive signal transmission units 230a and 230b perform pre-equalization processing such as pre-chromatic dispersion applying processing and spectrum shaping processing by digital signal processing as the transmission end digital signal processing. The drive signal transmission units 230a and 230b further convert the digital signal processed signal into an analog signal by a digital / analog converter, and output the analog signal to the optical quadrature modulators 250a and 250b.
　バイアス制御部２４０ａ、２４０ｂは、光直交位相変調器２５０ａ、２５０ｂのバイアス点を制御する。本実施形態において、バイアス制御部２４０ａ、２４０ｂは、駆動信号の入力電圧がゼロの時に、光直交位相変調器２５０ａ、２５０ｂから出力される変調合波光の光強度が最小になるように、光直交位相変調器２５０ａ、２５０ｂのバイアス点を制御する。 The bias controllers 240a and 240b control the bias points of the optical quadrature modulators 250a and 250b. In the present embodiment, the bias controllers 240a and 240b are optical orthogonal so that the light intensity of the modulated combined light output from the optical quadrature modulators 250a and 250b is minimized when the input voltage of the drive signal is zero. The bias points of the phase modulators 250a and 250b are controlled.
　光直交位相変調器２５０ａ、２５０ｂはそれぞれ、駆動信号送信部２３０ａ、２３０ｂから入力された駆動信号を用いてレーザ発振器２１０から入力された入力光信号を光変調し、変調合波光を出力する。さらに、本実施形態において、光直交位相変調器２５０ａ、２５０ｂはそれぞれ、偏光多重光送信機１００の起動時および入力光信号の入力が停止している時に、駆動信号送信部２３０ａ、２３０ｂから入力されたトレーニング信号を光変調して出力する。 The optical quadrature modulators 250a and 250b optically modulate the input optical signal input from the laser oscillator 210 using the drive signals input from the drive signal transmitters 230a and 230b, respectively, and output modulated combined light. Further, in the present embodiment, the optical quadrature modulators 250a and 250b are input from the drive signal transmitters 230a and 230b, respectively, when the polarization multiplexed optical transmitter 100 is started and when the input optical signal is stopped. The training signal is optically modulated and output.
　光直交位相変調器２５０ａ、２５０ｂについて詳細に説明する。本実施形態に係る光直交位相変調器２５０のブロック構成図を図３に示す。図３において、光直交位相変調器２５０は、偏波保持光スプリッタ２５１、マッハツェンダ型光変調器２５２ａ、２５２ｂを備える。光直交位相変調器２５０は、さらに、光スプリッタ２５３ａ、２５３ｂ、光移相器２５４、光カプラ２５５、光スプリッタ２５６および光信号ディテクタ２５７ａ、２５７ｂ、２５８を備える。 The optical quadrature modulators 250a and 250b will be described in detail. FIG. 3 shows a block diagram of the optical quadrature modulator 250 according to this embodiment. In FIG. 3, the optical quadrature modulator 250 includes a polarization maintaining optical splitter 251 and Mach-Zehnder optical modulators 252a and 252b. The optical quadrature modulator 250 further includes optical splitters 253a and 253b, an optical phase shifter 254, an optical coupler 255, an optical splitter 256, and optical signal detectors 257a, 257b, and 258.
　レーザ発振器２１０から光直交位相変調器２５０へ入力された入力光信号は、偏波保持光スプリッタ２５１によって偏光状態が維持されたまま２分岐され、それぞれマッハツェンダ型光変調器２５２ａ、２５２ｂへ出力される。 The input optical signal input from the laser oscillator 210 to the optical quadrature modulator 250 is branched into two by the polarization maintaining optical splitter 251 while maintaining the polarization state, and is output to the Mach-Zehnder optical modulators 252a and 252b, respectively. .
　マッハツェンダ型光変調器２５２ａ、２５２ｂは、偏波保持光スプリッタ２５１から入力された入力光信号を駆動信号送信部２３０ａ、２３０ｂから入力された駆動信号を用いて光変調し、光スプリッタ２５３ａ、２５３ｂへ出力する。また、マッハツェンダ型光変調器２５２ａ、２５２ｂは、偏光多重光送信機１００の起動時および入力光信号の入力が停止している時に、駆動信号送信部２３０ａ、２３０ｂから入力されたトレーニング信号を光変調し、光スプリッタ２５３ａ、２５３ｂへ出力する。 The Mach-Zehnder optical modulators 252a and 252b optically modulate the input optical signal input from the polarization maintaining optical splitter 251 using the drive signal input from the drive signal transmission units 230a and 230b, and transmit the optical signal to the optical splitters 253a and 253b. Output. The Mach-Zehnder optical modulators 252a and 252b optically modulate the training signals input from the drive signal transmitters 230a and 230b when the polarization multiplexed optical transmitter 100 is started and when the input optical signal is stopped. And output to the optical splitters 253a and 253b.
　光スプリッタ２５３ａ、２５３ｂは、マッハツェンダ型光変調器２５２ａ、２５２ｂから入力された変調光の一部を光信号ディテクタ２５７ａ、２５７ｂへ出力すると共に、変調光の大部分を光カプラ２５５側へ出力する。 The optical splitters 253a and 253b output part of the modulated light input from the Mach-Zehnder optical modulators 252a and 252b to the optical signal detectors 257a and 257b, and output most of the modulated light to the optical coupler 255 side.
　光移相器２５４は、光スプリッタ２５３ｂと光カプラ２５５との間に配置され、光スプリッタ２５３ｂから入力された変調光の位相をπ／２変化させて光カプラ２５５へ出力する。以下、マッハツェンダ型光変調器２５２ａから出力された変調光をＩ相、光移相器２５４を介してマッハツェンダ型光変調器２５２ｂから出力された変調光をＱ相と定義する。 The optical phase shifter 254 is disposed between the optical splitter 253b and the optical coupler 255, changes the phase of the modulated light input from the optical splitter 253b by π / 2, and outputs the result to the optical coupler 255. Hereinafter, the modulated light output from the Mach-Zehnder optical modulator 252a is defined as I phase, and the modulated light output from the Mach-Zehnder optical modulator 252b via the optical phase shifter 254 is defined as Q-phase.
　光カプラ２５５は、マッハツェンダ型光変調器２５２ａから出力された変調光（Ｉ相）と、光移相器２５４を介してマッハツェンダ型光変調器２５２ｂから出力された位相がπ／２変化した変調光（Ｑ相）とを合波して、変調合波光を出力する。 The optical coupler 255 includes modulated light (phase I) output from the Mach-Zehnder optical modulator 252a and modulated light in which the phase output from the Mach-Zehnder optical modulator 252b via the optical phase shifter 254 changes by π / 2. (Q phase) is multiplexed and modulated multiplexed light is output.
　光カプラ２５５から出力された変調合波光は、光スプリッタ２５６において一部が光信号ディテクタ２５８に出力されると共に大部分は偏光多重部２７０側へ出力される。 A part of the modulated / multiplexed light output from the optical coupler 255 is output to the optical signal detector 258 in the optical splitter 256 and most is output to the polarization multiplexing unit 270 side.
　光信号ディテクタ２５７ａ、２５７ｂ、２５８はそれぞれ、光スプリッタ２５３ａ、２５３ｂ、光スプリッタ２５６を介してマッハツェンダ型光変調器２５２ａ、２５２ｂ、光カプラ２５５から出力された変調光および変調合波光の光強度を計測し、計測結果をバイアス制御部２４０へ出力する。バイアス制御部２４０は、光信号ディテクタ２５７ａ、２５７ｂ、２５８から入力された計測結果に基づいて光直交位相変調器２５０のバイアス点を制御する。 The optical signal detectors 257a, 257b, and 258 measure the optical intensities of the modulated light and the modulated combined light output from the Mach-Zehnder optical modulators 252a and 252b and the optical coupler 255 via the optical splitters 253a and 253b and the optical splitter 256, respectively. The measurement result is output to the bias control unit 240. The bias controller 240 controls the bias point of the optical quadrature modulator 250 based on the measurement results input from the optical signal detectors 257a, 257b, and 258.
　図２の説明に戻る。偏光回転板２６０は、光直交位相変調器２５０ｂと偏光多重部２７０との間に配置される。偏光回転板２６０は、光直交位相変調器２５０ｂから出力された変調合波光の偏光状態が駆動信号送信部２３０ａから出力された変調合波光の偏光状態に対して直交するように、光直交位相変調器２５０ｂから出力された変調合波光を偏光して偏光多重部２７０へ出力する。 Returning to the explanation of FIG. The polarization rotation plate 260 is disposed between the optical quadrature modulator 250 b and the polarization multiplexing unit 270. The polarization rotation plate 260 performs optical quadrature phase modulation so that the polarization state of the modulated combined light output from the optical quadrature modulator 250b is orthogonal to the polarization state of the modulated combined light output from the drive signal transmission unit 230a. The modulated combined light output from the detector 250 b is polarized and output to the polarization multiplexing unit 270.
　偏光多重部２７０は、光直交位相変調器２５０ａから入力された変調合波光と、偏光回転板２６０を介して光直交位相変調器２５０ｂから入力された変調合波光とを多重化し、偏光多重光信号を送信光信号として光伝送路に出力する。 The polarization multiplexing unit 270 multiplexes the modulated combined light input from the optical quadrature modulator 250a and the modulated combined light input from the optical quadrature modulator 250b via the polarization rotation plate 260, and a polarization multiplexed optical signal. Is output as a transmission optical signal to the optical transmission line.
　次に、極性検出部３００について説明する。極性検出部３００は、光直交位相変調器２５０ａ、２５０ｂの極性を検出する部分である。極性検出部３００は、光スプリッタ３１０、偏光板３２０、マッハツェンダ型光干渉計３３０、光信号ディテクタ３４０ａ、３４０ｂ、信号検出部３５０および信号調整部３６０を備える。 Next, the polarity detection unit 300 will be described. The polarity detector 300 is a part that detects the polarities of the optical quadrature modulators 250a and 250b. The polarity detection unit 300 includes an optical splitter 310, a polarizing plate 320, a Mach-Zehnder optical interferometer 330, optical signal detectors 340a and 340b, a signal detection unit 350, and a signal adjustment unit 360.
　光スプリッタ３１０は、偏光多重部２７０から出力された偏光多重光信号の一部を偏光板３２０へ分岐する。 The optical splitter 310 branches a part of the polarization multiplexed optical signal output from the polarization multiplexing unit 270 to the polarizing plate 320.
　偏光板３２０は、管理された偏光軸を有し、偏光状態が互いに直交する２つの光信号が混合されるように、偏光多重部２７０から出力された偏光多重光信号の偏光状態を変化させ、混合光信号を出力する。なお、トレーニング信号が印加されている期間だけ偏光多重光生成部２００の偏光回転板２６０の偏光回転をゼロに変更できる場合は偏光板３２０を省略することができる。 The polarizing plate 320 changes the polarization state of the polarization multiplexed optical signal output from the polarization multiplexing unit 270 so that two optical signals having a controlled polarization axis and polarization states orthogonal to each other are mixed, Output mixed optical signal. If the polarization rotation of the polarization rotation plate 260 of the polarization multiplexed light generation unit 200 can be changed to zero only during the period when the training signal is applied, the polarizing plate 320 can be omitted.
　マッハツェンダ型光干渉計３３０は、３ｄＢ帯域がＩＴＵ（International Telecommunication Union）グリッド周波数に一致するように設定され、偏光板３２０から入力された混合光信号を、搬送波周波数ｆｃを基準に高周波数成分と低周波数成分とに分離してそれぞれ光信号ディテクタ３４０ａ、３４０ｂへ出力する。 The Mach-Zehnder optical interferometer 330 is set so that the 3 dB band coincides with the ITU (International Telecommunication Union) grid frequency, and the mixed optical signal input from the polarizing plate 320 is converted into a high frequency component and a low frequency based on the carrier frequency fc. The frequency components are separated and output to the optical signal detectors 340a and 340b, respectively.
　ここで、マッハツェンダ型光干渉計３３０の自由スペクトル領域（ＦＳＲ： Free Spectral Range）は、少なくとも印加する周波数オフセット量の絶対値の半分より大きい必要がある。ＦＳＲが大きいほどマッハツェンダ型干渉計のデバイスサイズの小型化が容易であり、光送信機のサイズを小型化できる。一般的に、数十ＧＨｚ程度のＦＳＲが望ましく、更にＩＴＵグリッドの周波数間隔に合わせることにより管理が容易となる。マッハツェンダ型光干渉計の代わりに、光インターリーバーを用いることもできる。 Here, the free spectral region (FSR: “Free Spectral Range”) of the Mach-Zehnder type optical interferometer 330 needs to be at least larger than half of the absolute value of the applied frequency offset amount. The larger the FSR, the easier the device size of the Mach-Zehnder interferometer can be reduced, and the size of the optical transmitter can be reduced. In general, an FSR of about several tens of GHz is desirable, and management is facilitated by matching the frequency interval of the ITU grid. An optical interleaver can be used instead of the Mach-Zehnder type optical interferometer.
　光信号ディテクタ３４０ａ、３４０ｂはそれぞれ、マッハツェンダ型光干渉計３３０から入力された高周波数成分と低周波数成分を電気強度信号に変換して信号検出部３５０へ出力する。 Each of the optical signal detectors 340a and 340b converts the high frequency component and the low frequency component input from the Mach-Zehnder optical interferometer 330 into an electric intensity signal and outputs it to the signal detection unit 350.
　信号検出部３５０は、光信号ディテクタ３４０ａから入力された高周波数成分に対応する電気強度信号と、光信号ディテクタ３４０ｂから入力された低周波数成分に対応する電気強度信号と、の大きさを比較する。そして、信号検出部３５０は、電気強度信号の大小関係から光直交位相変調器２５０ａ、２５０ｂの極性を判別する。光信号ディテクタ３４０ａ、３４０ｂから出力された電気強度信号の大小関係に基づいて光直交位相変調器２５０ａ、２５０ｂの極性が判別できる理由については後述する。 The signal detector 350 compares the magnitude of the electrical intensity signal corresponding to the high frequency component input from the optical signal detector 340a and the electrical intensity signal corresponding to the low frequency component input from the optical signal detector 340b. . And the signal detection part 350 discriminate | determines the polarity of the optical quadrature phase modulator 250a, 250b from the magnitude relationship of an electrical intensity signal. The reason why the polarities of the optical quadrature modulators 250a and 250b can be determined based on the magnitude relationship between the electrical intensity signals output from the optical signal detectors 340a and 340b will be described later.
　信号調整部３６０は、光直交位相変調器２５０ａ、２５０ｂの極性の判別結果に基づいて駆動信号送信部２３０ａ、２３０ｂから出力される駆動信号の符号を制御することによって、光直交位相変調器２５０ａ、２５０ｂの極性を補正する。極性の補正方法については後述する。 The signal adjustment unit 360 controls the sign of the drive signal output from the drive signal transmission units 230a and 230b based on the determination result of the polarities of the optical quadrature phase modulators 250a and 250b, whereby the optical quadrature phase modulator 250a and The polarity of 250b is corrected. The polarity correction method will be described later.
　次に、高周波数成分の大きさと低周波数成分の大きさを比較することによって光直交位相変調器２５０ａ、２５０ｂの極性が判別できる理由について説明する。 Next, the reason why the polarities of the optical quadrature modulators 250a and 250b can be determined by comparing the magnitudes of the high frequency component and the low frequency component will be described.
　本実施形態に係る駆動信号生成部２２０は、搬送波周波数がｆｃの時に、光直交位相変調器２５０ａから出力される変調合波光（Ｘ偏光）の周波数を「ｆｃ＋ｆｘ」にシフトさせると共に光直交位相変調器２５０ｂから出力される変調合波光（Ｙ偏光）の周波数を「ｆｃ＋ｆｙ」にシフトさせるトレーニング信号を生成して出力する。この場合、「ｆｃ＋ｆｘ」の変調合波光と「ｆｃ＋ｆｙ」の変調合波光とが偏光多重化されて偏光板３２０に入力する。 The drive signal generation unit 220 according to the present embodiment shifts the frequency of the modulated combined light (X-polarized light) output from the optical quadrature modulator 250a to “fc + fx” and performs optical quadrature modulation when the carrier frequency is fc. A training signal for shifting the frequency of the modulated combined light (Y-polarized light) output from the detector 250b to “fc + fy” is generated and output. In this case, the modulated combined light of “fc + fx” and the modulated combined light of “fc + fy” are polarization multiplexed and input to the polarizing plate 320.
　「ｆｃ＋ｆｘ」の変調合波光と「ｆｃ＋ｆｙ」の変調合波光は、偏光板３２０において偏光状態が混合され、マッハツェンダ型光干渉計３３０において搬送波周波数ｆｃを基準に高周波数成分と低周波数成分とに分離される。 The polarization-combined light of “fc + fx” and the modulated light of “fc + fy” are mixed in the polarization state in the polarizing plate 320 and separated into a high frequency component and a low frequency component based on the carrier frequency fc in the Mach-Zehnder interferometer 330. Is done.
　本実施形態では、偏光多重光信号に対して印加する周波数オフセットを、光周波数シフト（＋ｆｘ、＋ｆｙ）がそれぞれマッハツェンダ型干渉計３３０の高周波成分ポートから出力されるように設定する。従って、光直交位相変調器２５０ａ、２５０ｂの極性が設定通りの場合、高周波数成分の大きさが低周波数成分の大きさと比較して有意に大きくなる。一方、例えば、光直交位相変調器２５０ａの極性が反転している場合、周波数シフトの符号が反転して「＋ｆｘ」から「－ｆｘ」に変化して低周波数成分側に分離されることから、高周波数成分の大きさと低周波数成分の大きさとが同等になる。さらに、光直交位相変調器２５０ａ、２５０ｂの極性が共に反転した場合、低周波数成分の大きさが高周波数成分の大きさと比較して有意に大きくなる。 In this embodiment, the frequency offset to be applied to the polarization multiplexed optical signal is set so that the optical frequency shifts (+ fx, + fy) are output from the high frequency component port of the Mach-Zehnder interferometer 330, respectively. Accordingly, when the polarities of the optical quadrature modulators 250a and 250b are as set, the magnitude of the high frequency component is significantly larger than the magnitude of the low frequency component. On the other hand, for example, when the polarity of the optical quadrature modulator 250a is inverted, the sign of the frequency shift is inverted and changed from “+ fx” to “−fx” and separated to the low frequency component side. The magnitude of the high frequency component is equal to the magnitude of the low frequency component. Furthermore, when the polarities of the optical quadrature modulators 250a and 250b are reversed, the magnitude of the low frequency component is significantly larger than the magnitude of the high frequency component.
　従って、２個の光ディテクタ３４０ａ、３４０ｂから出力される電気強度信号の強度の大小関係を光信号検出部３５０においてモニターすることにより、光直交位相変調器２５０ａ、２５０ｂの極性の反転を検出できる。 Therefore, the inversion of the polarities of the optical quadrature modulators 250a and 250b can be detected by monitoring the magnitude relationship of the intensity of the electric intensity signals output from the two optical detectors 340a and 340b in the optical signal detector 350.
　具体例として、駆動信号生成部２２０において偏光多重光信号の各偏光成分に対してそれぞれ＋５ＧＨｚ（＝ｆｘ＝ｆｙ）の周波数シフトを付与するトレーニング信号を生成した場合の数値シミュレーション結果を図４に示す。図４（ａ）は、光直交位相変調器２５０ａ、２５０ｂの極性が共に設定通りの場合、図４（ｂ）は、光直交位相変調器２５０ａ、２５０ｂのどちらか一方の極性が反転した場合、図４（ｃ）は光直交位相変調器２５０ａ、２５０ｂの極性が共に反転した場合である。なお、図４の縦軸の単位はパワースペクトル密度（ＰＳＤ:Power spectral density）である。図４に示すように、付与した周波数シフト量に対応する周波数成分は７０ｄＢ程度あるため、信号検出部３５０において十分検出できる。 As a specific example, FIG. 4 shows a numerical simulation result when the driving signal generation unit 220 generates a training signal that gives a frequency shift of +5 GHz (= fx = fy) to each polarization component of the polarization multiplexed optical signal. . 4A shows a case where the polarities of the optical quadrature phase modulators 250a and 250b are both set, and FIG. 4B shows a case where the polarity of one of the optical quadrature phase modulators 250a and 250b is inverted. FIG. 4C shows the case where the polarities of the optical quadrature modulators 250a and 250b are both reversed. The unit of the vertical axis in FIG. 4 is a power spectral density (PSD: Power spectral density). As shown in FIG. 4, since the frequency component corresponding to the applied frequency shift amount is about 70 dB, the signal detector 350 can sufficiently detect the frequency component.
　極性が設定通りの場合、マッハツェンダ型光干渉計３３０において搬送波周波数ｆｃを基準に高周波数成分と低周波数成分とに分離することにより、高周波数成分の大きさが低周波数成分の大きさと比較して有意に大きくなる。従って、図４（ａ）に示すように、偏光板３２０から出力された混合光信号はｆｃを基準に＋５ＧＨｚの周波数成分にピークが現れる。これは、Ｘ偏光の信号光とＹ偏光の信号光のそれぞれに付与した周波数シフト量と一致する。 When the polarity is as set, the Mach-Zehnder optical interferometer 330 separates the high frequency component and the low frequency component based on the carrier frequency fc, so that the size of the high frequency component is compared with the size of the low frequency component. Significantly larger. Therefore, as shown in FIG. 4A, the mixed optical signal output from the polarizing plate 320 has a peak in the frequency component of +5 GHz with respect to fc. This coincides with the amount of frequency shift given to each of the X-polarized signal light and the Y-polarized signal light.
　また、光直交位相変調器２５０ａ、２５０ｂのどちらか一方の極性が反転した場合、高周波数成分の大きさと低周波数成分の大きさが同等となる。従って、図４（ｂ）に示すように、偏光板３２０から出力された混合光信号は＋５ＧＨｚと－５ＧＨｚの周波数成分にピークが現れる。 Also, when the polarity of one of the optical quadrature modulators 250a and 250b is inverted, the magnitude of the high frequency component and the magnitude of the low frequency component are equal. Therefore, as shown in FIG. 4B, the mixed light signal output from the polarizing plate 320 has peaks in the frequency components of +5 GHz and −5 GHz.
　さらに、光直交位相変調器２５０ａ、２５０ｂの極性が共に反転した場合、低周波数成分の大きさが高周波数成分の大きさと比較して有意に大きくなる。従って、偏光多重光信号に対して周波数オフセットの大きさを適切に設定することにより、図４（ｃ）に示すように、偏光板３２０から出力された混合光信号は搬送波周波数ｆｃよりも小さい周波数成分にピークが現れる。 Furthermore, when the polarities of the optical quadrature modulators 250a and 250b are reversed, the magnitude of the low frequency component is significantly larger than the magnitude of the high frequency component. Therefore, by appropriately setting the magnitude of the frequency offset for the polarization multiplexed optical signal, the mixed optical signal output from the polarizing plate 320 has a frequency smaller than the carrier frequency fc, as shown in FIG. A peak appears in the component.
　なお、光ディテクタ３４０ａの電気信号強度が光ディテクタ３４０ｂの電気信号強度と比較して大きくなるように印加する周波数シフトを設定する場合、光直交位相変調器２５０ａ、２５０ｂの極性が共に反転することによって、設定とは反対に光ディテクタ３４０ｂの電気信号強度が光ディテクタ３４０ａの電気信号強度よりも大きくなる。 In addition, when setting the frequency shift to apply so that the electric signal intensity of the optical detector 340a becomes larger than the electric signal intensity of the optical detector 340b, the polarities of the optical quadrature modulators 250a and 250b are reversed. Contrary to the setting, the electric signal intensity of the optical detector 340b is larger than the electric signal intensity of the optical detector 340a.
　以上のように、信号検出部３５０において、光ディテクタ３４０ａから出力された高周波数成分に対応する電気信号強度と、光ディテクタ３４０ｂから出力された低周波数成分に対応する電気信号強度と、の大小関係をモニターすることにより、光直交位相変調器２５０ａ、２５０ｂの極性反転の有無を検出することができる。 As described above, in the signal detection unit 350, the magnitude relationship between the electric signal intensity corresponding to the high frequency component output from the optical detector 340a and the electric signal intensity corresponding to the low frequency component output from the optical detector 340b. Can be detected to detect the presence or absence of polarity reversal of the optical quadrature modulators 250a and 250b.
　なお、偏光板３２０の偏光軸の管理が不十分である場合を考慮して、偏光板３２０の設定を時間的に変動させて光ディテクタ３４０ａ、３４０ｂの電気信号強度の大小関係に変化がないことを確認することが望ましい。 In consideration of the case where the polarization axis of the polarizing plate 320 is not sufficiently managed, the setting of the polarizing plate 320 is changed with time so that there is no change in the magnitude relationship of the electric signal intensity of the photodetectors 340a and 340b. It is desirable to confirm.
　次に、光直交位相変調器２５０ａ、２５０ｂの極性が反転した場合に極性を補正する方法について説明する。光直交位相変調器２５０ａ、２５０ｂのどちらか一方の極性が反転していると判定された場合（高周波数成分の大きさと低周波数成分の大きさが同等）、信号調整部３６０は、例えば、駆動信号送信部２３０ａの設定を変更して、光直交位相変調器２５０ａのＩ相の駆動信号の符号を反転させる。なお、光直交位相変調器２５０ａのＱ相の駆動信号の符号を反転させることもできるし、駆動信号送信部２３０ｂの設定を変更して、光直交位相変調器２５０ｂのＩ相またはＱ相の駆動信号の符号を反転させることもできる。 Next, a method for correcting the polarity when the polarities of the optical quadrature modulators 250a and 250b are inverted will be described. When it is determined that the polarity of one of the optical quadrature modulators 250a and 250b is inverted (the magnitude of the high frequency component is the same as the magnitude of the low frequency component), the signal adjustment unit 360 is driven, for example The setting of the signal transmission unit 230a is changed to invert the sign of the I-phase drive signal of the optical quadrature modulator 250a. The sign of the Q-phase drive signal of the optical quadrature modulator 250a can be inverted, or the setting of the drive signal transmitter 230b can be changed to drive the I-phase or Q-phase of the optical quadrature modulator 250b. It is also possible to invert the sign of the signal.
　そして、上述の設定変更を行った後に再度、光ディテクタ３４０ａ、３４０ｂの電気信号強度の大小関係をモニターする。そして、駆動信号生成部２２０にて付与する周波数シフトの結果と同一の大小関係が実現された場合、光直交位相変調器２５０ａ、２５０ｂの極性が設定通りに補正されたと判断する（光直交位相変調器２５０ａの極性が反転していた）。 Then, after the above setting change, the magnitude relationship of the electric signal strengths of the photodetectors 340a and 340b is monitored again. When the same magnitude relationship as the result of the frequency shift given by the drive signal generation unit 220 is realized, it is determined that the polarities of the optical quadrature modulators 250a and 250b have been corrected as set (optical quadrature phase modulation). The polarity of the vessel 250a was reversed).
　一方、上述の設定変更を光直交位相変調器２５０ａに対して行った後の再モニターにおいて、駆動信号生成部２２０にて付与する周波数シフトの結果と反対の大小関係が実現された場合、光直交位相変調器２５０ｂの極性が反転していたと分かる。この場合、光直交位相変調器２５０ａに対する設定を元に戻し、光直交位相変調器２５０ｂに対して設定変更を行うことにより、光直交位相変調器２５０ａ、２５０ｂの極性が設定通りに補正される。 On the other hand, in the re-monitoring after the above-described setting change is performed on the optical quadrature modulator 250a, when the magnitude relationship opposite to the result of the frequency shift provided by the drive signal generation unit 220 is realized, the optical orthogonal It can be seen that the polarity of the phase modulator 250b has been reversed. In this case, the polarity of the optical quadrature modulators 250a and 250b is corrected as set by returning the setting for the optical quadrature modulator 250a and changing the setting for the optical quadrature modulator 250b.
　さらに、信号検出部３５０が光直交位相変調器２５０ａ、２５０ｂの極性が共に反転している場合（低周波数成分の大きさが高周波数成分の大きさよりも有意に大きい）、信号調整部３６０は、駆動信号送信部２３０ａ、２３０ｂの設定を変更して、光直交位相変調器２５０ａ、２５０ｂのＩ相の駆動信号の符号を反転させる。なお、Ｉ相の代わりにＱ相の駆動信号の符号を反転させることもできるし、互いに異なる相の駆動信号の符号を反転させることもできる。 Further, when the signal detection unit 350 has both polarities of the optical quadrature modulators 250a and 250b reversed (the magnitude of the low frequency component is significantly larger than the magnitude of the high frequency component), the signal adjustment unit 360 The setting of the drive signal transmission units 230a and 230b is changed to invert the sign of the I-phase drive signal of the optical quadrature modulators 250a and 250b. Note that the sign of the driving signal of the Q phase can be inverted instead of the I phase, and the sign of the driving signal of a different phase can be inverted.
　次に、本実施形態に係る偏光多重光送信機１００の極性検出手順および極性補正手順について説明する。偏光多重光送信機１００の極性検出および極性補正の動作フローを図５に示す。 Next, the polarity detection procedure and the polarity correction procedure of the polarization multiplexed light transmitter 100 according to this embodiment will be described. An operation flow of polarity detection and polarity correction of the polarization multiplexed optical transmitter 100 is shown in FIG.
　本実施形態に係る偏光多重光送信機１００において、偏光多重光送信機１００の起動時および駆動信号断等の発生時、駆動信号生成部２２０は、光直交位相変調器２５０ａから出力される変調合波光の周波数を「ｆｃ＋ｆｘ」にシフトさせ、光直交位相変調器２５０ｂから出力される変調合波光の周波数を「ｆｃ＋ｆｙ」にシフトさせるトレーニング信号を生成して駆動信号送信部２３０ａ、２３０ｂへ出力する（Ｓ１０１）。 In the polarization multiplexed optical transmitter 100 according to the present embodiment, when the polarization multiplexed optical transmitter 100 is activated and when a drive signal is interrupted, the drive signal generation unit 220 generates a modulation signal output from the optical quadrature modulator 250a. A training signal for shifting the frequency of the wave light to “fc + fx” and shifting the frequency of the modulated multiplexed light output from the optical quadrature modulator 250b to “fc + fy” is generated and output to the drive signal transmission units 230a and 230b ( S101).
　駆動信号送信部２３０ａ、２３０ｂは、入力されたトレーニング信号に送信端デジタル信号処理を行い、光直交位相変調器２５０ａ、２５０ｂへ印加する。光直交位相変調器２５０ａ、２５０ｂは、印加されたトレーニング信号を光変調して合波し、変調合波光を出力する（Ｓ１０２）。光直交位相変調器２５０ａから出力された変調合波光と、光直交位相変調器２５０ｂおよび偏光回転板２６０から出力された変調合波光とは、偏光多重部２７０において多重化されて出力される。偏光多重部２７０から出力された偏光多重光信号の一部が、偏光板３２０に入力する（Ｓ１０３）。 The drive signal transmitters 230a and 230b perform transmission-end digital signal processing on the input training signals and apply them to the optical quadrature modulators 250a and 250b. The optical quadrature modulators 250a and 250b optically modulate and multiplex the applied training signals, and output modulated combined light (S102). The modulated combined light output from the optical quadrature modulator 250 a and the modulated combined light output from the optical quadrature modulator 250 b and the polarization rotation plate 260 are multiplexed and output by the polarization multiplexer 270. A part of the polarization multiplexed optical signal output from the polarization multiplexing unit 270 is input to the polarizing plate 320 (S103).
　偏光板３２０は、入力された偏光多重光信号の偏光状態を混合してマッハツェンダ型光干渉計３３０へ出力し、マッハツェンダ型光干渉計３３０は、入力された混合光信号を高周波数成分と低周波数成分とに分離してそれぞれ光信号ディテクタ３４０ａ、３４０ｂへ出力する（Ｓ１０４）。 The polarizing plate 320 mixes the polarization state of the input polarization multiplexed optical signal and outputs it to the Mach-Zehnder interferometer 330. The Mach-Zehnder optical interferometer 330 converts the input mixed optical signal into a high frequency component and a low frequency. The components are separated and output to the optical signal detectors 340a and 340b, respectively (S104).
　光信号ディテクタ３４０ａ、３４０ｂは、入力された高周波数成分、低周波数成分を電気強度信号に変換して信号検出部３５０へ出力する。信号検出部３５０は、光ディテクタ３４０ａ、３４０ｂから入力された信号の大小関係をモニターする。 The optical signal detectors 340a and 340b convert the input high frequency components and low frequency components into electric intensity signals and output them to the signal detection unit 350. The signal detection unit 350 monitors the magnitude relationship of the signals input from the photodetectors 340a and 340b.
　光ディテクタ３４０ａ、３４０ｂからの出力信号の大小関係が、駆動信号生成部２２０での設定通りであれば（Ｓ１０５のＹＥＳ）、偏光板３２０の偏光状態の設定を時間的に変更させる。出力信号の大小関係が維持されている場合（Ｓ１０６のＹＥＳ）、偏光多重光送信機１００は、光直交位相変調器２５０ａ、２５０ｂの極性が設定通りであると判断し、トレーニング信号の生成を終了し、通常の送信動作に復帰する（Ｓ１０７）。 If the magnitude relationship between the output signals from the photodetectors 340a and 340b is as set in the drive signal generator 220 (YES in S105), the setting of the polarization state of the polarizing plate 320 is changed over time. When the magnitude relationship between the output signals is maintained (YES in S106), the polarization multiplexed optical transmitter 100 determines that the polarities of the optical quadrature modulators 250a and 250b are as set, and ends the generation of the training signal. Then, the normal transmission operation is restored (S107).
　一方、光ディテクタ３４０ａ、３４０ｂの出力信号の大小関係が駆動信号生成部２２０での設定と異なる場合（Ｓ１０５のＮＯ）や光ディテクタ３４０ａ、３４０ｂの出力信号の大小関係が時間的に変化した場合（Ｓ１０６のＮＯ）、次のように動作する。つまり、偏光多重光送信機１００は、駆動信号送信部２３０ａ、２３０ｂの設定を変更して光直交位相変調器２５０ａ、２５０ｂのＩ相またはＱ相の駆動信号の符号を反転させることによって、極性を補正する（Ｓ１０８）。光直交位相変調器２５０ａ、２５０ｂの極性が適正になった場合、偏光多重光送信機１００は通常の送信動作に復帰する。 On the other hand, when the magnitude relationship between the output signals of the photodetectors 340a and 340b is different from the setting in the drive signal generator 220 (NO in S105), or when the magnitude relationship between the output signals of the photodetectors 340a and 340b changes with time ( The operation is as follows. That is, the polarization multiplexed optical transmitter 100 changes the setting of the drive signal transmitters 230a and 230b to invert the sign of the I-phase or Q-phase drive signal of the optical quadrature modulators 250a and 250b, thereby changing the polarity. Correction is performed (S108). When the polarities of the optical quadrature modulators 250a and 250b become appropriate, the polarization multiplexed optical transmitter 100 returns to the normal transmission operation.
　以上のように、本実施形態に係る偏光多重光送信機１００は、駆動信号生成部２２０において、一方の光直交位相変調器にｆｘだけ搬送波周波数をシフトさせ、他方の光直交位相変調器にｆｙだけ搬送波周波数をシフトさせるトレーニング信号を生成する。変調・多重化したトレーニング信号は、偏光板３２０およびマッハツェンダ型光干渉計３３０を用いて高周波数成分と低周波数成分とに分離される。そして、偏光多重光送信機１００は、高周波数成分の強度と低周波数成分の強度との比較結果に基づいて、光直交位相変調器２５０ａ、２５０ｂの極性を判定する。 As described above, in the polarization multiplexed optical transmitter 100 according to the present embodiment, in the drive signal generation unit 220, one optical quadrature phase modulator shifts the carrier frequency by fx, and the other optical quadrature phase modulator has fy. Generate a training signal that shifts the carrier frequency only. The modulated / multiplexed training signal is separated into a high frequency component and a low frequency component by using the polarizing plate 320 and the Mach-Zehnder optical interferometer 330. Then, the polarization multiplexed optical transmitter 100 determines the polarities of the optical quadrature modulators 250a and 250b based on the comparison result between the intensity of the high frequency component and the intensity of the low frequency component.
　偏光多重光送信機１００は、光直交位相変調器２５０ａ、２５０ｂの極性が反転している場合、光直交位相変調器２５０ａ、２５０ｂのＩ相またはＱ相の駆動信号の符号を反転させることによって極性を補正する。 When the polarities of the optical quadrature modulators 250a and 250b are reversed, the polarization multiplexed optical transmitter 100 reverses the sign of the I-phase or Q-phase drive signal of the optical quadrature modulators 250a and 250b. Correct.
　従って、本実施形態に係る偏光多重光送信機１００は、光スペクトル・アナライザ等の高価な装置を配置することなく、自機内において光直交位相変調器の極性を速やかに検出できると共に光直交位相変調器の極性を補正することができる。 Therefore, the polarization multiplexed optical transmitter 100 according to the present embodiment can quickly detect the polarity of the optical quadrature phase modulator in its own device without arranging an expensive device such as an optical spectrum analyzer and the optical quadrature phase modulation. The polarity of the vessel can be corrected.
　ここで、本実施形態に係る偏光多重光送信機１００においては、光直交位相変調器２５０ａ、２５０ｂの極性が反転している場合に駆動信号の符号を反転させることによって極性を補正したが、これに限定されない。例えば、バイアス制御部２４０ａ、２４０ｂを制御して、印加するバイアスの値のうち、光直交位相変調器２５０ａ、２５０ｂのＩ相またはＱ相のいずれか一方のバイアスを隣接のバイアス点に補正することでも、極性を補正できる。 Here, in the polarization multiplexed optical transmitter 100 according to this embodiment, when the polarities of the optical quadrature modulators 250a and 250b are inverted, the polarity is corrected by inverting the sign of the drive signal. It is not limited to. For example, the bias control units 240a and 240b are controlled to correct either the I-phase or Q-phase bias of the optical quadrature modulators 250a and 250b to the adjacent bias point among the applied bias values. But you can correct the polarity.
　この場合、偏光多重光送信機１００は、トレーニング信号に予め定められた速度の光位相の時間的変動を付与し、バイアス制御部２４０ａ、２４０ｂによってバイアス点を確定する。その後、偏光多重光送信機１００は、出力された偏光多重光信号の光位相の時間的変動の符号を検知し、光直交位相変調器２５０ａ、２５０ｂの極性を検出する。偏光多重光送信機１００はさらに、検出結果に応じて、光直交位相変調器２５０ａ、２５０ｂのバイアス点を補正する。 In this case, the polarization multiplexed optical transmitter 100 applies temporal variation of the optical phase at a predetermined speed to the training signal, and determines the bias point by the bias control units 240a and 240b. Thereafter, the polarization multiplexed optical transmitter 100 detects the sign of temporal variation of the optical phase of the output polarization multiplexed optical signal, and detects the polarities of the optical quadrature modulators 250a and 250b. The polarization multiplexed optical transmitter 100 further corrects the bias points of the optical quadrature modulators 250a and 250b according to the detection result.
　ここで、本実施形態では、偏光多重光送信機１００の起動時および駆動信号の生成が停止している時に、２つの変調合波光の周波数をシフトさせるトレーニング信号を生成し、該トレーニング信号を光変調・合波等した。しかし、周波数シフトが付与されたトレーニング信号を光変調・合波等する代わりに、レーザ発振器２１０から入力した入力光信号（実送信データ信号）に周波数シフトを付与することもできる。 Here, in the present embodiment, when the polarization multiplexed optical transmitter 100 is started and when the generation of the drive signal is stopped, a training signal that shifts the frequency of the two modulated multiplexed lights is generated, and the training signal is transmitted as an optical signal. Modulated and combined. However, instead of optically modulating / combining the training signal to which the frequency shift has been applied, a frequency shift can be applied to the input optical signal (actual transmission data signal) input from the laser oscillator 210.
　この場合、駆動信号生成部２２０は、レーザ発振器２１０から入力された入力光信号に基づいて生成した駆動信号に、入力光信号の各偏光成分にそれぞれ予め定められた周波数の周波数シフトを付与する機能を付加する。光直交位相変調器２５０ａ、２５０ｂは、レーザ発振器２１０から入力された入力光信号を、周波数シフトを付与する機能が付加された駆動信号を用いて光変調する。なお、駆動信号送信部２３０ａ、２３０ｂが、駆動信号に送信端デジタル信号処理を活用した予等化等を施す時に、入力光信号の各偏光成分に予め定められた周波数の周波数シフトを付与する機能を駆動信号に付加することもできる。 In this case, the drive signal generation unit 220 has a function of giving a frequency shift of a predetermined frequency to each polarization component of the input optical signal to the drive signal generated based on the input optical signal input from the laser oscillator 210. Is added. The optical quadrature modulators 250a and 250b optically modulate the input optical signal input from the laser oscillator 210 using a drive signal to which a function for giving a frequency shift is added. Note that when the drive signal transmission units 230a and 230b perform pre-equalization or the like using transmission end digital signal processing on the drive signal, a function of giving a frequency shift of a predetermined frequency to each polarization component of the input optical signal Can also be added to the drive signal.
　具体例として、駆動信号生成部２２０が、５０Ｇｂｐｓ偏光多重ＱＰＳＫ信号の各偏光成分にそれぞれ５ＧＨｚの周波数シフトを付与する機能を駆動信号に付加した場合の数値シミュレーション結果を図６に示す。図６（ａ）は、光直交位相変調器２５０ａ、２５０ｂの極性が設定通りの場合、図６（ｂ）は、光直交位相変調器２５０ａ、２５０ｂのどちらか一方の極性が反転した場合である。 As a specific example, FIG. 6 shows a numerical simulation result when the drive signal generation unit 220 adds a function of giving a frequency shift of 5 GHz to each polarization component of the 50 Gbps polarization multiplexed QPSK signal to the drive signal. 6A shows a case where the polarities of the optical quadrature phase modulators 250a and 250b are as set, and FIG. 6B shows a case where the polarity of one of the optical quadrature phase modulators 250a and 250b is inverted. .
　図６（ａ）において、光直交位相変調器２５０ａ、２５０ｂの極性が設定通りの場合、各偏光成分にそれぞれ高周波方向に５ＧＨｚの周波数シフトが付与される。これにより、混合光信号の光スペクトルは、搬送波周波数ｆｃを基準として高周波数成分が大きく、低周波数成分が小さくなる。従って、高周波数成分と低周波数成分とに分離した場合に高周波数成分の大きさが低周波数成分の大きさと比較して大きくなる。 6A, when the polarities of the optical quadrature modulators 250a and 250b are as set, a frequency shift of 5 GHz is given to each polarization component in the high frequency direction. As a result, the optical spectrum of the mixed optical signal has a large high frequency component and a small low frequency component with respect to the carrier frequency fc. Therefore, when the high frequency component and the low frequency component are separated, the size of the high frequency component becomes larger than the size of the low frequency component.
　図６（ｂ）において、光直交位相変調器２５０ａ、２５０ｂのどちらか一方の極性が反転した場合、偏光板３２０からの混合光信号の光スペクトルが搬送波周波数ｆｃに関して対称となる。従って、高周波数成分と低周波数成分とに分離した場合に高周波数成分の大きさと低周波数成分の大きさが同等となる。 6B, when the polarity of one of the optical quadrature modulators 250a and 250b is inverted, the optical spectrum of the mixed optical signal from the polarizing plate 320 is symmetric with respect to the carrier frequency fc. Therefore, when the high frequency component and the low frequency component are separated, the size of the high frequency component is equal to the size of the low frequency component.
　従って、レーザ発振器２１０から入力された入力光信号（実送信データ信号）に対応する駆動信号に周波数シフトを付与する機能を付加することにより、通常の送信動作において、光直交位相変調器２５０ａ、２５０ｂの極性を判定することができる。なお、光直交位相変調器２５０ａ、２５０ｂの極性の反転を検出した場合は、前述の実施形態で説明した極性の補正方法を適用することができる。 Therefore, the optical quadrature phase modulators 250a and 250b are added in the normal transmission operation by adding a function of giving a frequency shift to the drive signal corresponding to the input optical signal (actual transmission data signal) input from the laser oscillator 210. Can be determined. Note that when the polarity inversion of the optical quadrature modulators 250a and 250b is detected, the polarity correction method described in the above embodiment can be applied.
　ここで、駆動信号生成部２２０において、データ変調を施した駆動信号に周波数シフトを付与する機能を付加して光直交位相変調器２５０ａ、２５０ｂを駆動する場合、通常の送信動作への切り替えを高速に行うことができる。その理由は、送信データに切り替えた際のバイアス制御部２４０ａ、２４０ｂの動作点のずれが微小だからである。 Here, in the drive signal generation unit 220, when the optical quadrature modulators 250a and 250b are driven by adding a function of giving a frequency shift to the drive signal subjected to data modulation, switching to a normal transmission operation is performed at high speed. Can be done. This is because the deviation of the operating points of the bias controllers 240a and 240b when switching to transmission data is very small.
　また、実送信データ信号に基づいて生成した駆動信号に、定期的に上述の周波数シフトを付与する機能を付加することにより、光直交位相変調器２５０ａ、２５０ｂの極性が動的に変化する場合でも、極性の反転を検知して極性を補正することができる。 Further, even when the polarities of the optical quadrature modulators 250a and 250b are dynamically changed by adding the function of periodically adding the above-described frequency shift to the drive signal generated based on the actual transmission data signal. The polarity can be corrected by detecting the inversion of the polarity.
　本実施形態に係る偏光多重光送信機１００は、単一偏光光通信システムにおける光直交位相変調器の極性の反転の検出に適用することもできる。また、任意の光位相変調方式に対応可能であり、光直交位相変調器以外の各光位相変調方式に特化した光送信機の構成に適用することもできる。 The polarization multiplexed optical transmitter 100 according to the present embodiment can also be applied to detection of polarity reversal of an optical quadrature modulator in a single polarization optical communication system. Further, it can be applied to an arbitrary optical phase modulation system, and can be applied to the configuration of an optical transmitter specialized for each optical phase modulation system other than the optical quadrature phase modulator.
　（第３の実施形態）
　第３の実施形態について説明する。本実施形態に係る偏光多重光送信機のブロック構成図を図７に示す。図７において、本実施形態に係る偏光多重光送信機１００Ｂは、偏光多重光生成部２００Ｂおよび極性検出部３００Ｂを備える。 (Third embodiment)
A third embodiment will be described. FIG. 7 shows a block diagram of the polarization multiplexed optical transmitter according to the present embodiment. In FIG. 7, the polarization multiplexed light transmitter 100B according to the present embodiment includes a polarization multiplexed light generation unit 200B and a polarity detection unit 300B.
　偏光多重光生成部２００Ｂは、レーザ発振器２１０Ｂ、駆動信号生成部２２０Ｂ、駆動信号送信部２３０ａＢ、２３０ｂＢ、バイアス制御部２４０ａＢ、２４０ｂＢ、光直交位相変調器２５０ａＢ、２５０ｂＢ、偏光回転板２６０Ｂおよび偏光多重部２７０Ｂを備える。極性検出部３００Ｂは、光スプリッタ３１０Ｂ、偏光板３２０Ｂ、光信号ディテクタ３７０Ｂ、ビート信号分離部３８０Ｂ、信号検出部３５０Ｂおよび信号調整部３６０Ｂを備える。以下、第２の実施形態で説明した図２の偏光多重光送信機１００と異なる部分を中心に説明する。 The polarization multiplexed light generator 200B includes a laser oscillator 210B, a drive signal generator 220B, drive signal transmitters 230aB and 230bB, bias controllers 240aB and 240bB, optical quadrature modulators 250aB and 250bB, a polarization rotation plate 260B, and a polarization multiplexer. 270B. The polarity detection unit 300B includes an optical splitter 310B, a polarizing plate 320B, an optical signal detector 370B, a beat signal separation unit 380B, a signal detection unit 350B, and a signal adjustment unit 360B. In the following, the description will focus on the differences from the polarization multiplexed optical transmitter 100 of FIG. 2 described in the second embodiment.
　本実施形態に係る駆動信号生成部２２０Ｂは、光直交位相変調器２５０ａＢ、２５０ｂＢの極性を判断する場合、互いに異なる周波数の周波数シフトが付与された変調光を光直交位相変調器２５０ａＢ、２５０ｂＢから出力させるトレーニング信号を生成する。 When determining the polarities of the optical quadrature modulators 250aB and 250bB, the drive signal generator 220B according to the present embodiment outputs modulated light to which frequency shifts of different frequencies are given from the optical quadrature modulators 250aB and 250bB. A training signal to be generated is generated.
　ここで、付与する周波数シフト量の差分は、光信号ディテクタ３７０Ｂのデバイス帯域内に設定することが望ましい。例えば、光通信システム用の光ディテクタに対しては数十～数百ＭＨｚに設定することが望ましい。また、付与する周波数シフト量は、駆動信号送信部２３０ａＢ、２３０ｂＢのデバイス帯域の範囲内であり、かつ、光信号ディテクタ３７０Ｂのデバイス帯域の範囲外であることが望ましい。例えば、高速光通信システム用の駆動信号送信部に対しては数ＧＨｚに設定することが望ましい。なお、付与する周波数シフト量は、光信号ディテクタ３７０Ｂの経年劣化による周波数特性の時間的変化を考慮し、光直交位相変調器２５０ａＢ、２５０ｂＢの極性を検出する度に変更することができる。駆動信号生成部２２０Ｂは、駆動信号の位相が時間的変動する異なる速度の高周波クロック信号を光直交位相変調器２５０ａＢ、２５０ｂＢに入力することによっても、周波数シフトを変調光に付与することができる。 Here, it is desirable to set the difference in the amount of frequency shift to be applied within the device band of the optical signal detector 370B. For example, it is desirable to set it to several tens to several hundreds of MHz for an optical detector for an optical communication system. Further, it is desirable that the frequency shift amount to be applied is within the device band range of the drive signal transmission units 230aB and 230bB and outside the device band range of the optical signal detector 370B. For example, it is desirable to set to several GHz for a drive signal transmission unit for a high-speed optical communication system. The frequency shift amount to be applied can be changed every time the polarities of the optical quadrature modulators 250aB and 250bB are detected in consideration of the temporal change of the frequency characteristics due to the aging of the optical signal detector 370B. The drive signal generation unit 220B can also apply a frequency shift to the modulated light by inputting high-frequency clock signals with different speeds, at which the phase of the drive signal fluctuates with time, to the optical quadrature modulators 250aB and 250bB.
　光スプリッタ３１０Ｂは、偏光多重部２７０Ｂから出力された偏光多重光信号の一部を、偏光軸が管理された偏光板３２０Ｂへ分岐する。偏光板３２０Ｂは、互いに直交する偏光状態の光信号が混合するように偏光多重光信号の偏光状態を変化させて光信号ディテクタ３７０Ｂへ出力する。偏光板３２０Ｂから出力された混合光信号には、偏光多重信号の各偏光成分の間に付与された光位相の時間的変動の速度の差に応じた、光位相が時間的に変動する信号成分が含まれる。 The optical splitter 310B branches a part of the polarization multiplexed optical signal output from the polarization multiplexing unit 270B to the polarizing plate 320B in which the polarization axis is managed. The polarizing plate 320B changes the polarization state of the polarization multiplexed optical signal so that optical signals in the polarization state orthogonal to each other are mixed, and outputs it to the optical signal detector 370B. The mixed optical signal output from the polarizing plate 320B includes a signal component whose optical phase varies with time according to the difference in speed of temporal variation of the optical phase applied between the polarization components of the polarization multiplexed signal. Is included.
　光信号ディテクタ３７０Ｂは、混合光信号の光強度を電気強度信号に変換してビート信号分離部３８０Ｂへ出力し、ビート信号分離部３８０Ｂは、電気強度信号から時間的に変動する成分のみを取り出して信号検出部３５０Ｂへ出力する。信号検出部３５０Ｂは、ビート信号分離部３８０Ｂからの出力に、駆動信号生成部２２０Ｂにおいて付与した周波数の周波数シフトに相当するビート成分が含まれているか否か調査し、調査結果に基づいて光直交位相変調器２５０ａＢ、２５０ｂＢの極性を判別する。 The optical signal detector 370B converts the light intensity of the mixed optical signal into an electrical intensity signal and outputs it to the beat signal separation unit 380B. The beat signal separation unit 380B extracts only the temporally varying component from the electrical intensity signal. The signal is output to the signal detector 350B. The signal detection unit 350B investigates whether or not the output from the beat signal separation unit 380B includes a beat component corresponding to the frequency shift of the frequency given in the drive signal generation unit 220B, and based on the investigation result, optical orthogonality The polarities of the phase modulators 250aB and 250bB are determined.
　次に、互いに直交する偏光状態の２つの連続光に周波数シフトを付与し、光信号ディテクタ３７０Ｂから出力された信号に付与した周波数シフトに対応するビート成分が含まれているか調査することで、極性の関係を判別できる理由について説明する。 Next, by applying a frequency shift to two continuous lights having polarization states orthogonal to each other and investigating whether or not a beat component corresponding to the frequency shift given to the signal output from the optical signal detector 370B is included, The reason why the relationship can be determined will be described.
　駆動信号生成部２２０Ｂは、搬送波周波数がｆｃの時に、Ｘ偏光の周波数を「ｆｃ＋ｆｘ」にシフトさせると共にＹ偏光の周波数を「ｆｃ＋ｆｙ」にシフトさせるトレーニング信号を生成して出力する。この時に光直交位相変調器２５０ａＢ、２５０ｂＢから出力される変調光は式（１）で与えられる。ここで、Δθは、Ｘ偏光の光信号の位相とＹ偏光の光信号の位相との相対位相である。
Figure JPOXMLDOC01-appb-I000001
When the carrier frequency is fc, the drive signal generation unit 220B generates and outputs a training signal that shifts the frequency of X-polarized light to “fc + fx” and shifts the frequency of Y-polarized light to “fc + fy”. At this time, the modulated light output from the optical quadrature modulators 250aB and 250bB is given by Expression (1). Here, Δθ is a relative phase between the phase of the X-polarized light signal and the phase of the Y-polarized light signal.
Figure JPOXMLDOC01-appb-I000001
　一方、偏光板３２０において偏光状態を混合した場合、光信号ディテクタ３７０Ｂから出力される信号は式（２）で与えられる。
Figure JPOXMLDOC01-appb-I000002
On the other hand, when the polarization state is mixed in the polarizing plate 320, the signal output from the optical signal detector 370B is given by Expression (2).
Figure JPOXMLDOC01-appb-I000002
　ここで、ＥoxおよびＥoyはそれぞれ、Ｘ偏光の光信号とＹ偏光の光信号の複素振幅である。式（２）に示すように、光信号ディテクタ３７０Ｂからの出力には、駆動信号生成部２２０ＢによってＸ偏光の光信号に付与された周波数シフト量とＹ偏光の光信号に付与された周波数シフト量との差分の周波数のビート信号が含まれる。 Here, Eox and Eoy are the complex amplitudes of the X-polarized light signal and the Y-polarized light signal, respectively. As shown in Expression (2), the output from the optical signal detector 370B includes the frequency shift amount given to the X-polarized optical signal by the drive signal generation unit 220B and the frequency shift amount given to the Y-polarized optical signal. And a beat signal having a frequency different from the above.
　光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合、光信号ディテクタ３７０Ｂにおいて観測されるビート信号の周波数は２つの周波数シフト量の和になる。つまり、ビート信号の周波数は、駆動信号生成部２２０ＢにてＸ偏光の光信号に対して付与した周波数シフト量とＹ偏光の光信号に対して付与した周波数シフト量との和になる。 When the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, the frequency of the beat signal observed in the optical signal detector 370B is the sum of two frequency shift amounts. That is, the frequency of the beat signal is the sum of the frequency shift amount given to the X-polarized light signal and the frequency shift amount given to the Y-polarized light signal by the drive signal generation unit 220B.
　従って、信号検出部３５０Ｂによって、周波数シフト量の和と同一位置に周波数ピークが検出された場合、光直交位相変調器２５０ａＢ、２５０ｂＢのどちらか一方の極性が反転していると分かる。 Therefore, when the frequency peak is detected at the same position as the sum of the frequency shift amounts by the signal detection unit 350B, it can be understood that the polarity of one of the optical quadrature phase modulators 250aB and 250bB is inverted.
　具体例として、Ｘ偏光の光信号に－１．４ＧＨｚの周波数シフトを付与し、Ｙ偏光の光信号に＋１．５ＧＨｚの周波数シフトを付与するように、駆動信号生成部２２０Ｂにおいてトレーニング信号を生成した場合の数値シミュレーション結果を図８に示す。図８（ａ）は光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一の場合、図８（ｂ）は光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合である。なお、光信号ディテクタ３７０Ｂのデバイス帯域を１ＧＨｚとした。 As a specific example, a training signal is generated in the drive signal generation unit 220B so that a frequency shift of −1.4 GHz is applied to the X-polarized optical signal and a frequency shift of +1.5 GHz is applied to the Y-polarized optical signal. The numerical simulation result in this case is shown in FIG. FIG. 8A shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are the same, and FIG. 8B shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are different from each other. The device band of the optical signal detector 370B is 1 GHz.
　図８（ａ）において、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一である場合、光信号ディテクタ３７０Ｂからの出力信号の信号スペクトルに２．９ＧＨｚの小さい周波数ピークが現れる。これは、Ｘ偏光の光信号およびＹ偏光の光信号にそれぞれ付与した周波数シフトの差分と一致する。図８（ａ）の周波数ピークは光信号ディテクタ３７０Ｂにおいてビート信号として検出されない。 8A, when the polarities of the optical quadrature modulators 250aB and 250bB are the same, a small frequency peak of 2.9 GHz appears in the signal spectrum of the output signal from the optical signal detector 370B. This coincides with the difference between the frequency shifts applied to the X-polarized light signal and the Y-polarized light signal, respectively. The frequency peak in FIG. 8A is not detected as a beat signal in the optical signal detector 370B.
　一方、図８（ｂ）において、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合、光信号ディテクタ３７０Ｂの出力信号の信号スペクトルに１００ＭＨｚの鋭い周波数ピークが現れる。これは、Ｘ偏光の光信号およびＹ偏光の光信号に付与した周波数シフトの和と一致する。この周波数ピーク（約１１５ｄB）は、実送信データに対応する信号の強度（６５ｄB程度）と比較して５０ｄB程度大きいため、光信号ディテクタ３７０Ｂによりビート信号として検出される。 On the other hand, in FIG. 8B, when the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, a sharp frequency peak of 100 MHz appears in the signal spectrum of the output signal of the optical signal detector 370B. This coincides with the sum of the frequency shifts applied to the X-polarized light signal and the Y-polarized light signal. This frequency peak (about 115 dB) is detected as a beat signal by the optical signal detector 370B because it is about 50 dB larger than the intensity of the signal corresponding to the actual transmission data (about 65 dB).
　従って、光信号ディテクタ３７０Ｂからの出力信号の信号スペクトルに、付与した周波数シフトに対応するビート信号が含まれているか否かを調査することによって、光直交位相変調器２５０ａＢ、２５０ｂＢが互いに同一の極性を有するか否か判別できる。 Therefore, by examining whether the signal spectrum of the output signal from the optical signal detector 370B includes a beat signal corresponding to the applied frequency shift, the optical quadrature phase modulators 250aB and 250bB have the same polarity. It can be determined whether or not it has.
　なお、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一である場合にビート信号が検出され、直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合にビート信号が検出されないように、周波数シフト量を設定することもできる。 The frequency shift amount is set so that the beat signal is detected when the polarities of the optical quadrature modulators 250aB and 250bB are the same, and the beat signal is not detected when the polarities of the quadrature modulators 250aB and 250bB are different from each other. It can also be set.
　そして、本実施形態に係る偏光多重光送信機１００Ｂは、信号検出部３５０Ｂが光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なると判定した場合、第２の実施形態と同様に動作する。すなわち、偏光多重光送信機１００Ｂは、光直交位相変調器２５０ａＢのＩ相の駆動信号の符号を反転させるように、駆動信号送信部２３０ａＢの設定を変更する。なお、光直交位相変調器２５０ａＢのＱ相の駆動信号の符号を反転させることもできるし、駆動信号送信部２３０ｂＢの設定を変更して、光直交位相変調器２５０ｂＢのＩ相またはＱ相の駆動信号の符号を反転させることもできる。 The polarization multiplexed optical transmitter 100B according to the present embodiment operates in the same manner as in the second embodiment when the signal detection unit 350B determines that the polarities of the optical quadrature phase modulators 250aB and 250bB are different from each other. That is, the polarization multiplexed optical transmitter 100B changes the setting of the drive signal transmission unit 230aB so as to invert the sign of the I-phase drive signal of the optical quadrature modulator 250aB. The sign of the Q-phase drive signal of the optical quadrature modulator 250aB can be inverted, or the setting of the drive signal transmitter 230bB can be changed to drive the I-phase or Q-phase of the optical quadrature modulator 250bB. It is also possible to invert the sign of the signal.
　さらに、駆動信号送信部２３０ａＢ、２３０ｂＢの設定を変更する代わりに次のように動作することもできる。すなわち、偏光多重光送信機１００Ｂは、バイアス制御部２４０ａＢ、２４０ｂＢを制御して、印加するバイアスの値のうち、光直交位相変調器２５０ａＢ、２５０ｂＢのＩ相またはＱ相のいずれかのバイアスを隣接のバイアス点に補正する。 Furthermore, instead of changing the settings of the drive signal transmission units 230aB and 230bB, the following operation can be performed. That is, the polarization multiplexed optical transmitter 100B controls the bias control units 240aB and 240bB, and among the bias values to be applied, either the I-phase or Q-phase bias of the optical quadrature modulators 250aB and 250bB is adjacent. Correct to the bias point.
　次に、本実施形態に係る偏光多重光送信機１００Ｂの極性検出手順および極性補正手順について説明する。偏光多重光送信機１００Ｂの極性検出および極性補正の動作フローを図９に示す。 Next, the polarity detection procedure and the polarity correction procedure of the polarization multiplexed light transmitter 100B according to this embodiment will be described. FIG. 9 shows an operation flow of polarity detection and polarity correction of the polarization multiplexed optical transmitter 100B.
　図９において、駆動信号生成部２２０Ｂは、起動時および信号断発生時等に、それぞれ互いに異なる周波数の周波数シフトを変調光に付与するように光直交位相変調器２５０ａＢ、２５０ｂＢを駆動するトレーニング信号を生成する。駆動信号生成部２２０Ｂは、生成したトレーニング信号を駆動信号送信部２３０ａＢ、２３０ｂＢへ出力する（Ｓ２０１）。 In FIG. 9, the drive signal generator 220B generates training signals for driving the optical quadrature modulators 250aB and 250bB so as to impart different frequency shifts to the modulated light at the time of start-up and when a signal break occurs. Generate. The drive signal generation unit 220B outputs the generated training signal to the drive signal transmission units 230aB and 230bB (S201).
　駆動信号送信部２３０ａＢ、２３０ｂＢは、入力されたトレーニング信号に送信端デジタル信号処理を行い、光直交位相変調器２５０ａＢ、２５０ｂＢへ印加する。光直交位相変調器２５０ａＢ、２５０ｂＢに入力されたトレーニング信号は、光変調および合波され（Ｓ２０２）、さらに偏光多重部２７０において多重化された後、一部が偏光板３２０Ｂに入力される（Ｓ２０３）。 The drive signal transmission units 230aB and 230bB perform transmission end digital signal processing on the input training signals and apply them to the optical quadrature modulators 250aB and 250bB. The training signals input to the optical quadrature modulators 250aB and 250bB are optically modulated and combined (S202), and after being multiplexed by the polarization multiplexing unit 270, a part thereof is input to the polarizing plate 320B (S203). ).
　偏光多重光信号は、偏光板３２０Ｂにおいて偏光状態が混合され、光信号ディテクタ３７０Ｂにおいて電気強度信号に変換された後、ビート信号分離部３８０Ｂにおいて時間的に変動する成分のみが取り出されて信号検出部３５０Ｂへ出力される（Ｓ２０４）。 The polarization multiplexed optical signal is mixed in the polarization state in the polarizing plate 320B, converted into an electric intensity signal in the optical signal detector 370B, and then only a component that temporally varies is extracted in the beat signal separation unit 380B, and the signal detection unit It is output to 350B (S204).
　信号検出部３５０Ｂは、駆動信号生成部２２０Ｂによって付与した周波数シフト量の和に相当する周波数位置のビート信号が入力された電気強度信号から検出されるか否か調査し、調査結果に基づいて光直交位相変調器２５０ａＢ、２５０ｂＢの極性を判別する（Ｓ２０５）。 The signal detection unit 350B investigates whether or not a beat signal at a frequency position corresponding to the sum of the frequency shift amounts given by the drive signal generation unit 220B is detected from the input electric intensity signal, and based on the investigation result, the light detection unit 350B The polarities of the quadrature modulators 250aB and 250bB are determined (S205).
　所定の周波数位置においてビート信号が検出されなかった場合（Ｓ２０５のＮＯ）、信号検出部３５０Ｂはさらに、偏光板３２０Ｂの偏光状態の設定を時間的に変更させてビート信号の周波数成分が検出されるか否か確認する（Ｓ２０６）。偏光板３２０の偏光状態の設定を変化させても変化しない場合、すなわち、ビート信号の周波数成分が検出されない場合（Ｓ２０６のＮＯ）、偏光多重光送信機１００Ｂは、光直交位相変調器２５０ａ、２５０ｂの極性が同一であると判断する。この場合、偏光多重光送信機１００Ｂは、トレーニング信号の生成を終了して通常の送信動作に復帰する（Ｓ２０７）。 If a beat signal is not detected at a predetermined frequency position (NO in S205), the signal detection unit 350B further changes the setting of the polarization state of the polarizing plate 320B in time to detect the frequency component of the beat signal. It is confirmed whether or not (S206). When the setting of the polarization state of the polarizing plate 320 does not change, that is, when the frequency component of the beat signal is not detected (NO in S206), the polarization multiplexed optical transmitter 100B uses the optical quadrature phase modulators 250a and 250b. Are determined to have the same polarity. In this case, the polarization multiplexed light transmitter 100B ends the generation of the training signal and returns to the normal transmission operation (S207).
　一方、所定の周波数位置においてビート信号が検出された場合（Ｓ２０５のＹＥＳ）、偏光多重光送信機１００Ｂは、光直交位相変調器２５０ａ、２５０ｂの極性が同一ではないと判断する。偏光板３２０Ｂの偏光状態の設定を変更することによって時間的に変化した場合（Ｓ２０６のＹＥＳ）も同様である。いずれの場合も、信号調整部３６０Ｂによって、一方の駆動信号の符号を反転させて極性を補正する（Ｓ２０８）。そして、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一になった場合、偏光多重光送信機１００Ｂは、通常の送信動作に復帰する。 On the other hand, when a beat signal is detected at a predetermined frequency position (YES in S205), the polarization multiplexed optical transmitter 100B determines that the polarities of the optical quadrature phase modulators 250a and 250b are not the same. The same applies to the case where the setting changes with time by changing the setting of the polarization state of the polarizing plate 320B (YES in S206). In either case, the signal adjustment unit 360B corrects the polarity by inverting the sign of one drive signal (S208). When the polarities of the optical quadrature modulators 250aB and 250bB become the same, the polarization multiplexed optical transmitter 100B returns to the normal transmission operation.
　以上のように、本実施形態に係る偏光多重光送信機１００Ｂは、駆動信号生成部２２０Ｂにおいて、それぞれ互いに異なる周波数の周波数シフトを変調光に付与するように光直交位相変調器２５０ａＢ、２５０ｂＢを駆動するトレーニング信号を生成する。一方、信号検出部３５０Ｂにおいて、トレーニング信号を変調・多重等して得られた電気強度信号に、駆動信号生成部２２０Ｂが付与した光位相の時間的変動の速度の差に相当する周波数を有するビート信号が含まれるかどうか調査する。そして、偏光多重光送信機１００Ｂは、信号検出部３５０Ｂの調査結果に基づいて光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一か否か判別する。 As described above, the polarization multiplexed optical transmitter 100B according to the present embodiment drives the optical quadrature modulators 250aB and 250bB so that the drive signal generation unit 220B imparts frequency shifts of different frequencies to the modulated light. To generate a training signal. On the other hand, in the signal detection unit 350B, beats having a frequency corresponding to the difference in speed of temporal variation of the optical phase given by the drive signal generation unit 220B to the electrical intensity signal obtained by modulating / multiplexing the training signal. Investigate whether the signal is included. Then, the polarization multiplexed optical transmitter 100B determines whether or not the polarities of the optical quadrature modulators 250aB and 250bB are the same based on the investigation result of the signal detector 350B.
　従って、本実施形態に係る偏光多重光送信機１００Ｂは、光スペクトル・アナライザ等の高価な装置を配置することなく自機内において光直交位相変調器２５０ａＢ、２５０ｂＢの極性を速やかに検出できると共に光直交位相変調器２５０ａＢ、２５０ｂＢの極性を補正することができる。 Therefore, the polarization multiplexed optical transmitter 100B according to the present embodiment can quickly detect the polarities of the optical quadrature modulators 250aB and 250bB in its own device without disposing an expensive device such as an optical spectrum analyzer, and can also perform optical orthogonality. The polarities of the phase modulators 250aB and 250bB can be corrected.
　なお、本実施形態では、互いに異なる周波数の周波数シフトを変調光に付与するように光直交位相変調器２５０ａＢ、２５０ｂＢを駆動するトレーニング信号を生成し、このトレーニング信号を光変調・合波等したが、これに限定されない。 In the present embodiment, a training signal for driving the optical quadrature modulators 250aB and 250bB is generated so as to impart a frequency shift of a different frequency to the modulated light, and the training signal is optically modulated / combined. However, the present invention is not limited to this.
　駆動信号生成部２２０Ｂは、通常の送信動作状態において、レーザ発振器２１０Ｂから入力された光信号に基づいて生成した駆動信号に下記の機能を付加することもできる。すなわち、互いに直交する偏光状態の２つの光信号に、各偏光成分間で同一の送信データ信号に基づいた変調信号に周波数が異なる周波数シフトをそれぞれ付与させる機能を付加する。特に、非特許文献１に記されているように、偏光多重光通信システムにおいて、光ファイバ伝送路中の非線形光学効果による伝送品質の劣化を低減するために、互いに直交する偏光状態の２つの独立した光信号に対して、周波数シフトを付与する場合が該当する。 The drive signal generation unit 220B can add the following function to the drive signal generated based on the optical signal input from the laser oscillator 210B in the normal transmission operation state. That is, a function is added to two optical signals having polarization states orthogonal to each other, and a frequency shift having a different frequency is added to the modulated signal based on the same transmission data signal between the polarization components. In particular, as described in Non-Patent Document 1, in a polarization multiplexed optical communication system, two independent polarization states orthogonal to each other are reduced in order to reduce transmission quality degradation due to nonlinear optical effects in an optical fiber transmission line. This applies to the case where a frequency shift is applied to the optical signal.
　この場合も、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一である場合に駆動信号生成部２２０ＢにてＸ偏光の光信号に付与した周波数シフト量とＹ偏光の光信号に付与した周波数シフト量との差分に相当する周波数のビート信号が発生する。 Also in this case, when the polarities of the optical quadrature modulators 250aB and 250bB are the same, the frequency shift amount imparted to the X-polarized optical signal by the drive signal generation unit 220B and the frequency shift amount imparted to the Y-polarized optical signal A beat signal having a frequency corresponding to the difference between the two is generated.
　一方、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合は、観測されるビート信号の周波数は駆動信号生成部２２０ＢにてＸ偏光の光信号に付与した周波数シフト量とＹ偏光の光信号に付与した周波数シフト量の和となる。 On the other hand, when the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, the frequency of the beat signal observed is the frequency shift amount given to the X-polarized optical signal by the drive signal generation unit 220B and the Y-polarized optical signal. Is the sum of the amount of frequency shift given to.
　駆動信号生成部２２０Ｂの生成する具体的な駆動信号に対する数値シミュレーション結果を図１０に示す。この駆動信号は、Ｘ偏光の光信号に対して通常のデータ変調に加えて－１．４ＧＨｚの周波数シフトを付与し、Ｙ偏光の光信号に同一の送信データに基づくデータ変調に加えて＋１．５ＧＨｚの周波数シフトを付与すると仮定した。図１０（ａ）は、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一の場合、図１０（ｂ）は、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合である。 FIG. 10 shows a numerical simulation result for a specific drive signal generated by the drive signal generator 220B. This drive signal gives a frequency shift of −1.4 GHz to the X-polarized optical signal in addition to the normal data modulation, and in addition to the data modulation based on the same transmission data as the Y-polarized optical signal, +1. It was assumed to give a frequency shift of 5 GHz. FIG. 10A shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are the same, and FIG. 10B shows a case where the polarities of the optical quadrature phase modulators 250aB and 250bB are different from each other.
　図１０（ａ）において、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が同一の場合、信号ディテクタ３７０Ｂの出力信号の信号スペクトルに２．９ＧＨｚの小さい周波数ピークが出現する。これは、Ｘ偏光の光信号とＹ偏光の光信号のそれぞれに付与した周波数シフトの差分と一致する。 10A, when the polarities of the optical quadrature modulators 250aB and 250bB are the same, a small frequency peak of 2.9 GHz appears in the signal spectrum of the output signal of the signal detector 370B. This coincides with the difference in frequency shift applied to each of the X-polarized light signal and the Y-polarized light signal.
　一方、図１０（ｂ）において、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なる場合、光信号ディテクタ３７０Ｂの出力信号の信号スペクトルに１００ＭＨｚの周波数ピークが現れる。これは、Ｘ偏光の光信号とＹ偏光の光信号のそれぞれに付与した周波数シフトの和と一致する。 On the other hand, in FIG. 10B, when the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, a frequency peak of 100 MHz appears in the signal spectrum of the output signal of the optical signal detector 370B. This coincides with the sum of the frequency shifts applied to each of the X-polarized light signal and the Y-polarized light signal.
　従って、駆動信号生成部２２０Ｂが駆動信号に所定の周波数シフトを付与する機能を付加することにより、レーザ発振器２１０Ｂから入力した入力光信号（実送信データ信号）を用いて、光直交位相変調器２５０ａＢ、２５０ｂＢの極性の関係を検出できる。なお、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が互いに異なっている場合、第２の実施形態で説明した極性の補正方法を適用することができる。 Accordingly, the drive signal generation unit 220B adds a function of giving a predetermined frequency shift to the drive signal, so that the optical quadrature modulator 250aB is used using the input optical signal (actual transmission data signal) input from the laser oscillator 210B. , 250 bB polarity relationship can be detected. When the polarities of the optical quadrature modulators 250aB and 250bB are different from each other, the polarity correction method described in the second embodiment can be applied.
　以上のように、実送信データ信号に基づいて生成した駆動信号に周波数シフトを付与する機能を付加することでも、光直交位相変調器２５０ａＢ、２５０ｂＢの極性の関係を判別できる。 As described above, the polarity relationship between the optical quadrature modulators 250aB and 250bB can also be determined by adding a function of giving a frequency shift to the drive signal generated based on the actual transmission data signal.
　なお、駆動信号生成部２２０Ｂにおいて、データ変調を施した駆動信号に周波数シフトを付与する機能を付加して光直交位相変調器２５０ａＢ、２５０ｂＢを駆動する場合、通常の送信動作への切り替えを高速に行うことができる。これは、送信データに切り替えた際のバイアス制御部２４０ａＢ、２４０ｂＢの動作点のずれが微小だからである。 In addition, in the drive signal generation unit 220B, when the optical quadrature modulators 250aB and 250bB are driven by adding a function of giving a frequency shift to the drive signal subjected to data modulation, the switching to the normal transmission operation is performed at high speed. It can be carried out. This is because the deviation of the operating points of the bias controllers 240aB and 240bB when switching to transmission data is very small.
　また、実送信データ信号に基づいて生成した駆動信号に、定期的に上述の周波数シフトを付与する機能を付加することにより、光直交位相変調器２５０ａＢ、２５０ｂＢの極性が動的に変化する場合でも、極性の反転を検知して極性を補正することができる。 Further, even when the polarity of the optical quadrature modulators 250aB and 250bB is dynamically changed by adding the function of periodically giving the above-described frequency shift to the drive signal generated based on the actual transmission data signal. The polarity can be corrected by detecting the inversion of the polarity.
　本願発明は上記実施形態に限定されるものではなく、この発明の要旨を逸脱しない範囲の設計の変更等があってもこの発明に含まれる。 The invention of the present application is not limited to the above-described embodiment, and any design change or the like within a range not departing from the gist of the invention is included in the invention.
　本願発明は上記実施形態に限定されるものではなく、この発明の要旨を逸脱しない範囲の設計の変更等があってもこの発明に含まれる。また、本願発明は、２０１２年１１月２７日に出願された日本出願特願２０１２－２５８５９１を基礎とする優先権を主張し、その開示の全てをここに取り込む。 The invention of the present application is not limited to the above-described embodiment, and any design change or the like within a range not departing from the gist of the invention is included in the invention. The present invention claims priority based on Japanese Patent Application No. 2012-258591 filed on November 27, 2012, the entire disclosure of which is incorporated herein.
　冗長構成をとったネットワークシステムに広く適用することができる。 Can be widely applied to redundant network systems.
　１０　　偏光多重光送信機
　２０　　駆動手段
　３１、３２　　光変調手段
　４０　　偏光多重手段
　５０　　混合手段
　６０　　極性判定手段
　１００、１００Ｂ　　偏光多重光送信機
　２００、２００Ｂ　　偏光多重光生成部
　２１０、２１０Ｂ　　レーザ発振器
　２２０、２２０Ｂ　　駆動信号生成部
　２３０ａ、２３０ｂ、２３０ａＢ、２３０ｂＢ　　駆動信号送信部
　２４０ａ、２４０ｂ、２４０ａＢ、２４０ｂＢ　　バイアス制御部
　２５０ａ、２５０ｂ、２５０ａＢ、２５０ｂＢ　　光直交位相変調器
　２５１　偏波保持光スプリッタ
　２５２ａ、２５２ｂ　　マッハツェンダ型光変調器
　２５３ａ、２５３ｂ　　光スプリッタ
　２５４　　光移相器
　２５５　　光カプラ
　２５６　　光スプリッタ
　２５７ａ、２５７ｂ、２５８　　光信号ディテクタ
　２６０、２６０Ｂ　　偏光回転板
　２７０、２７０Ｂ　　偏光多重部
　３００、３００Ｂ　　極性検出部
　３１０、３１０Ｂ　　光スプリッタ
　３２０、３２０Ｂ　　偏光板
　３３０　　マッハツェンダ型光干渉計
　３４０ａ、３４０ｂ　　光信号ディテクタ
　３５０、３５０Ｂ　　信号検出部
　３６０、３６０Ｂ　　信号調整部
　３７０Ｂ　　光信号ディテクタ
　３８０Ｂ　　ビート信号分離部 DESCRIPTION OF SYMBOLS 10 Polarization multiplexing optical transmitter 20 Driving means 31, 32 Light modulation means 40 Polarization multiplexing means 50 Mixing means 60 Polarity determination means 100, 100B Polarization multiplexing optical transmitter 200, 200B Polarization multiplexing light generation unit 210, 210B Laser oscillator 220, 220B Drive signal generator 230a, 230b, 230aB, 230bB Drive signal transmitter 240a, 240b, 240aB, 240bB Bias controller 250a, 250b, 250aB, 250bB Optical quadrature modulator 251 Polarization maintaining optical splitter 252a, 252b Mach-Zehnder optical modulation 253a, 253b Optical splitter 254 Optical phase shifter 255 Optical coupler 256 Optical splitter 257a, 257b, 258 Optical signal detector 260, 260B Polarization rotating plate 270, 270B Polarization multiplexing Unit 300, 300B Polarity detection unit 310, 310B Optical splitter 320, 320B Polarizer 330 Mach-Zehnder type optical interferometer 340a, 340b Optical signal detector 350, 350B Signal detection unit 360, 360B Signal adjustment unit 370B Optical signal detector 380B Beat signal separation unit

Claims (10)

  1. 光信号の偏光成分に所定の周波数シフトを付加する駆動信号を生成して出力する駆動手段と、
    ２つの光信号についてそれぞれ、２つの偏光成分を前記駆動信号を用いて変調し、位相差を付加して合波し、変調合波光を出力する、２つの光変調手段と、
    前記２つの変調合波光を偏光を直交させた状態で多重化し、偏光多重光信号を出力する偏光多重手段と、
    前記偏光多重光信号の偏光状態を各偏光成分が混合した状態に変化させ、混合光信号を出力する混合手段と、
    前記混合光信号の強度の周波数分布に基づいて前記２つの変調合波光の極性の関係を判定する極性判定手段と、
    を備える偏光多重光送信機。     Drive means for generating and outputting a drive signal for adding a predetermined frequency shift to the polarization component of the optical signal;
    Two optical modulation means for modulating two polarization components for each of the two optical signals using the drive signal, adding a phase difference and combining the two optical components, and outputting modulated combined light;
    Polarization multiplexing means for multiplexing the two modulated combined lights in a state in which the polarizations are orthogonal to each other and outputting a polarization multiplexed optical signal;
    Mixing means for changing the polarization state of the polarization multiplexed optical signal to a state in which each polarization component is mixed, and outputting a mixed optical signal;
    Polarity determining means for determining the relationship between the polarities of the two modulated combined lights based on the frequency distribution of the intensity of the mixed optical signal;
    A polarization multiplexed optical transmitter.
  2. 前記極性判定手段は、前記混合光信号を高周波数成分と低周波数成分とに分離して出力する分離手段を備え、
    前記極性判定手段は、前記高周波数成分の強度と前記低周波数成分の強度との大小関係に基づいて、前記２つの変調合波光の極性の関係を判定する、
    請求項１記載の偏光多重光送信機。     The polarity determination means includes separation means for separating the mixed optical signal into a high frequency component and a low frequency component and outputting the separated light signal,
    The polarity determination means determines the relationship between the polarities of the two modulated combined lights based on the magnitude relationship between the intensity of the high frequency component and the intensity of the low frequency component.
    The polarization multiplexed optical transmitter according to claim 1.
  3. 前記駆動手段は、２つの偏光成分に対して同じ符号の周波数シフトを付加し、
    前記極性判定手段は、高周波数成分と低周波数成分の大きさが同等の場合、前記２つの変調合波光の極性が互いに異なると判定する、請求項２記載の偏光多重光送信機。     The driving means adds a frequency shift of the same sign to the two polarization components,
    The polarization multiplexed optical transmitter according to claim 2, wherein the polarity determination unit determines that the polarities of the two modulated multiplexed lights are different from each other when the magnitudes of the high frequency component and the low frequency component are equal.
  4. 前記極性判定手段は、前記混合光信号から時間的に変動する成分のみを抽出して出力する抽出手段を備え、
    前記極性判定手段は、前記抽出された成分に前記付与した周波数シフトに対応する成分が含まれているか否か判断することによって、前記２つの変調合波光の極性の関係を判定する、
    請求項１記載の偏光多重光送信機。     The polarity determination unit includes an extraction unit that extracts and outputs only a temporally varying component from the mixed light signal,
    The polarity determination means determines the relationship between the polarities of the two modulated combined lights by determining whether or not the extracted component includes a component corresponding to the applied frequency shift.
    The polarization multiplexed optical transmitter according to claim 1.
  5. 前記駆動手段は、２つの偏光成分に対して異なる符号の周波数シフトを付加し、
    前記極性判定手段は、前記抽出された成分に前記周波数シフトの偏差に相当するビート成分が含まれていると判断した場合、前記２つの変調合波光の極性が互いに異なると判定する、請求項４記載の偏光多重光送信機。     The driving means adds frequency shifts of different signs to the two polarization components,
    The polarity determination unit determines that the polarities of the two modulated combined lights are different from each other when it is determined that the extracted component includes a beat component corresponding to the deviation of the frequency shift. The polarization multiplexed optical transmitter described.
  6. 前記光変調手段は、
    　前記光信号を２分岐して出力するスプリッタと、
    　前記２分岐された光信号をそれぞれ前記駆動信号を用いて変調して変調光を出力する２つのマッハツェンダ型光変調器と、
    　２つの前記変調光の一方の位相を遅延させる移相器と、
    　位相差がついた２つの変調光を合波し、変調合波光を出力する光カプラと、
    を備える請求項１乃至５のいずれか１項記載の偏光多重光送信機。     The light modulating means includes
    A splitter for branching and outputting the optical signal;
    Two Mach-Zehnder optical modulators that respectively modulate the two-branched optical signals using the drive signals and output modulated light;
    A phase shifter for delaying the phase of one of the two modulated lights;
    An optical coupler that multiplexes two modulated lights having a phase difference and outputs the modulated multiplexed light;
    The polarization multiplexed optical transmitter according to claim 1, further comprising:
  7. 前記２つの変調合波光の極性の関係の判定結果に基づいて、前記変調合波光の極性を補正する極性補正手段をさらに備える請求項１乃至６のいずれか１項記載の偏光多重光送信機。 The polarization multiplexed optical transmitter according to any one of claims 1 to 6, further comprising a polarity correction unit that corrects the polarity of the modulated multiplexed light based on a determination result of a polarity relationship between the two modulated multiplexed lights.
  8. 前記極性補正手段は、前記駆動手段を制御して前記駆動信号の符号を反転させることによって前記変調合波光の極性を補正する、請求項７記載の偏光多重光送信機。 8. The polarization multiplexed optical transmitter according to claim 7, wherein the polarity correcting unit corrects the polarity of the modulated multiplexed light by controlling the driving unit to invert the sign of the driving signal.
  9. 前記光変調手段のバイアスを制御するバイアス制御手段をさらに備え、
    前記極性補正手段は、前記バイアス制御手段を制御してバイアスを隣接のバイアス点に補正することによって前記変調合波光の極性を補正する、請求項７記載の偏光多重光送信機。     Bias control means for controlling the bias of the light modulation means;
    8. The polarization multiplexed optical transmitter according to claim 7, wherein the polarity correction unit corrects the polarity of the modulated multiplexed light by controlling the bias control unit to correct the bias to an adjacent bias point.
  10. 光信号の偏光成分に所定の周波数シフトを付加する駆動信号を生成して出力し、
    ２つの光信号についてそれぞれ、２つの偏光成分を前記駆動信号を用いて変調し、位相差を付加して合波し、２つの変調合波光を出力し、
    前記２つの変調合波光を偏光を直交させた状態で多重化し、偏光多重光信号を出力し、
    前記偏光多重光信号の偏光状態を各偏光成分が混合した状態に変化させ、混合光信号を出力し、
    前記混合光信号の強度の周波数分布に基づいて前記２つの変調合波光の極性の関係を判定する、
    極性判定方法。     Generate and output a drive signal that adds a predetermined frequency shift to the polarization component of the optical signal,
    For each of the two optical signals, two polarization components are modulated using the driving signal, combined with a phase difference, and two modulated combined light is output.
    The two modulated combined lights are multiplexed with the polarizations orthogonal to each other, and a polarization multiplexed optical signal is output,
    Change the polarization state of the polarization multiplexed optical signal to a state where each polarization component is mixed, and output a mixed optical signal,
    Determining the relationship between the polarities of the two modulated combined lights based on the frequency distribution of the intensity of the mixed optical signal;
    Polarity judgment method.
PCT/JP2013/006788 2012-11-27 2013-11-19 Polarization-multiplexed light transmitter and polarity determination method WO2014083800A1 (en)

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