WO2011101919A1 - 光送信機 - Google Patents
光送信機 Download PDFInfo
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- WO2011101919A1 WO2011101919A1 PCT/JP2010/001135 JP2010001135W WO2011101919A1 WO 2011101919 A1 WO2011101919 A1 WO 2011101919A1 JP 2010001135 W JP2010001135 W JP 2010001135W WO 2011101919 A1 WO2011101919 A1 WO 2011101919A1
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- 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
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- 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
- H04B10/50577—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output to control the phase of the modulating signal
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- 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
- H04B10/5051—Laser transmitters using external modulation using a series, i.e. cascade, combination of modulators
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- 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
- H04B10/516—Details of coding or modulation
Definitions
- the present invention relates to, for example, an optical transmitter including a plurality of optical modulators and having a function of controlling a phase of a modulation signal in the plurality of optical modulators.
- Optical submarine cable transmission systems can be broadly classified into non-relay systems that are applied to strait crossings and long-distance relay systems that include submarine repeaters intended for transoceanic crossings.
- An optical submarine cable relay transmission system that requires a long-distance repeater system is composed of a submarine repeater transmission line and land coastal station devices at both ends of the submarine repeater system. It is common.
- wavelength division multiplexing optical transmission technique As a technique for efficiently transmitting a plurality of information using such an optical cable, there is a wavelength division multiplexing optical transmission technique (WDM).
- WDM wavelength division multiplexing optical transmission technique
- a plurality of signals are assigned (divided) to optical signals having different wavelengths, and these signals are multiplexed and transmitted bidirectionally through two optical fibers.
- optical signals from light sources having different wavelengths are combined by an optical multiplexer, and on the receiving side, optical signals of respective wavelengths are demultiplexed by an optical demultiplexer, and these are converted into electrical signals by a light receiving element.
- This technology has the advantage of enabling large-capacity information transmission from a small amount of cable resources.
- a plurality of optical transmitters generate transmission signals using laser beams having different wavelengths.
- a plurality of transmission signals generated by the optical transmitter are multiplexed by an optical wavelength demultiplexing device and then transmitted through an optical submarine cable.
- each signal is received by a plurality of optical receivers.
- the transmitted light intensity increases. For example, if the transmission light intensity of one optical transceiver is +10 dBm and the multiplexing number is 64, the total transmission light intensity reaches +28 dBm.
- the intensity of the transmitted light input to the optical fiber is increased, the nonlinear effect of the optical fiber becomes noticeable and deteriorates the transmission characteristics, so that the total transmitted light intensity cannot be increased too much. For this reason, it is necessary to reduce the transmission light intensity per wave.
- reducing the transmission light intensity directly leads to deterioration of the S / N ratio (Signal to Noise), which also causes deterioration of transmission characteristics.
- the optical transmitter disclosed in Patent Document 1 includes a phase shifter that can control the phase of the modulation signal input to the optical intensity modulator, and inputs the modulation signal input to the optical phase modulator and the optical intensity modulator.
- the modulation signal is mixed by a mixer, and the phase shifter is feedback-controlled so that the phase relationship between the two modulation signals always becomes a constant value. According to this method, the phases of the modulation signals input to the two optical modulators can always be matched.
- the patent document discloses a method of receiving a transmission signal generated by modulation by two optical modulators by using a photodiode (PD) and converting it to an electric signal, and extracting a signal component modulated by the optical intensity modulator. It is disclosed. According to this method, since the delay of the optical fiber can be compensated, the phases of the two modulation signals can always be matched even when the length of the optical fiber changes or the optical path length changes with temperature. Can do.
- PD photodiode
- Patent Document 2 discloses a method for more easily matching the phases of a plurality of modulation signals.
- the phase difference between the plurality of modulation signals is not always constant but varies with temperature. This is because the refractive index of the optical fiber that transmits the optical signal changes depending on the temperature, the optical path length changes, the delay amount inside the IC changes, and the like.
- the phase of a plurality of modulation signals is matched by arranging a temperature monitor unit and controlling the delay amount in the phase shifter according to the temperature.
- the optical transmitter disclosed in Patent Document 1 can compensate for the delay due to the optical fiber by feedback control.
- it is essential to demultiplex the optical signal from the optical modulator and extract the modulated signal by PD.
- it is necessary to use a large number of expensive parts such as a high-speed PD that can receive the frequency of the modulation signal input to the optical modulator and a high-speed mixer that processes the high-speed signal.
- a high-speed PD that can receive the frequency of the modulation signal input to the optical modulator and a high-speed mixer that processes the high-speed signal.
- the phases of the modulation signals input to the plurality of optical modulators can be easily matched.
- the amount of phase that varies with temperature changes also changes. Therefore, when the optical fiber length between the optical modulators changes due to, for example, disconnection of the optical fiber between the optical modulators and re-fusion, an optimum phase control amount corresponding to the optical fiber length is measured each time. There is a problem that it needs to be done and takes a very long time. In addition, the optimum value is only an approximation from the measurement result, and there is a problem that variation occurs in delay compensation accuracy.
- the present invention has been made to solve the above-described problems, and can realize a function of automatically matching the phases of modulation signals input to two optical modulators with an inexpensive and simple configuration.
- the object is to provide an optical transmitter.
- An optical transmitter includes a data generation unit that generates a plurality of modulation signals, a driver that amplifies the plurality of modulation signals generated by the data generation unit, and at least one of a plurality of modulation signals input to the driver
- a phase shifter that performs phase control on one or more modulation signals, a plurality of optical modulators connected in series that modulate an optical signal based on each modulation signal amplified by a driver, and a last-stage light
- An optical coupler that demultiplexes the optical signal modulated by the modulator, a photodiode that detects and photoelectrically converts the optical signal demultiplexed by the optical coupler, and the intensity of the signal photoelectrically converted by the photodiode is maximized.
- a phase control unit for controlling the phase control amount of the phase shifter.
- an expensive component such as a high-speed PD or a high-speed mixer is used by controlling the phase control amount of the phase shifter so that the intensity of the signal from the photodiode is maximized.
- the function of automatically matching the phases of the modulation signals input to the two optical modulators can be realized with an inexpensive and simple configuration. Further, it is not necessary to actually measure the optimum phase control amount for the length and temperature of the optical fiber.
- FIG. 5 is a vector diagram showing a phase transition when an optical phase modulator is used for the data modulation unit in Embodiment 1 of the present invention.
- FIG. 1 is a diagram showing a configuration of an optical transmitter according to Embodiment 1 of the present invention.
- the optical transmitter has a configuration applicable to an optical transmitter using a plurality of optical modulators.
- FIG. 1 shows an example in which an RZ-DPSK (Return to Zero-Differential Phase Shift Keying) modulation method is applied. The configuration is shown. As shown in FIG.
- RZ-DPSK Return to Zero-Differential Phase Shift Keying
- the optical transmitter includes a data modulation unit (optical modulator) 1, a clock modulation unit (optical modulator) 2, an optical coupler 3, a photodiode (PD) 4, a phase control unit 5a, and data generation.
- the unit 6 includes a phase shifter 7 and first and second drivers 8 and 9.
- the data modulation unit 1 performs phase modulation on an optical signal input from a light source (not shown) based on the data signal input from the data generation unit 6 via the first driver 8.
- the optical signal phase-modulated by the data modulator 1 is output to the clock modulator 2.
- the clock modulation unit 2 performs intensity modulation on the optical signal phase-modulated by the data modulation unit 1 based on the clock signal input from the data generation unit 6 via the phase shifter 7 and the second driver 9. It is.
- the optical signal intensity-modulated by the clock modulation unit 2 is output to the optical coupler 3.
- the optical coupler 3 demultiplexes the optical signal intensity-modulated by the clock modulation unit 2 into two.
- One optical signal demultiplexed by the optical coupler 3 is output to an optical wavelength demultiplexing device (not shown), multiplexed with a plurality of optical signals of other wavelengths, and then transmitted to the receiving side.
- the other optical signal is output to PD4.
- the PD 4 detects the optical signal demultiplexed by the optical coupler 3 and performs photoelectric conversion.
- the signal photoelectrically converted by the PD 4 is output to the phase controller 5a.
- the phase control unit 5a performs feedback control of the phase control amount of the phase shifter 7 by the hill-climbing method based on the signal photoelectrically converted by the PD 4 so that the intensity of this signal is maximized.
- the data generation unit 6 generates a data signal and a clock signal.
- the data signal generated by the data generator 6 is output to the first driver 8, and the clock signal is output to the phase shifter 7.
- the phase shifter 7 performs phase control of the clock signal generated by the data generation unit 6 based on the phase control amount by the phase control unit 5a.
- the clock signal whose phase is controlled by the phase shifter 7 is output to the second driver 9.
- the first driver 8 optically amplifies the data signal generated by the data generation unit 6.
- the data signal optically amplified by the first driver 8 is output to the data modulator 1.
- the second driver 9 optically amplifies the clock signal whose phase is controlled by the phase shifter 7.
- the clock signal optically amplified by the second driver 9 is output to the clock modulation unit 2.
- phase modulation of an optical signal by the data modulation unit 1 will be described.
- the data modulation unit 1 there are two cases where a light intensity modulator is used as an optical modulator and an optical phase modulator is used.
- FIG. 2 is a conceptual diagram showing an eye pattern when an optical signal is phase-modulated using a light intensity modulator in the data modulation unit 1 according to Embodiment 1 of the present invention.
- FIG. 3 is a vector diagram showing a phase transition in the case where a light intensity modulator is used for the data modulation unit 1 in Embodiment 1 of the present invention.
- V ⁇ drive voltage
- the light is once extinguished in the middle of the transition between 0 and ⁇ , and then it is again in the light emitting state.
- the shape of the eye pattern greatly depends on the waveform of the data signal input to the data modulation unit 1, and if it is an ideal rectangular wave, there is almost no extinction state, and it always approaches a light emitting state.
- the rise time and fall time of the data signal are slow, since the transition from the light emitting state to the extinguished state and from the extinguished state to the light emitting state is slow, the extinguished state becomes long and an eye pattern as shown in FIG.
- FIG. 4 is a conceptual diagram showing an eye pattern when an optical signal is phase-modulated using an optical phase modulator in the data modulation unit 1 according to Embodiment 1 of the present invention.
- FIG. 5 is a vector diagram showing the state of phase transition when an optical phase modulator is used for the data modulation unit 1 in the first embodiment of the present invention.
- the phase of the optical signal can be directly modulated. 1 is assigned to the light phases 0 and ⁇ .
- the amplitude is always constant even during the phase transition between 0 and ⁇ .
- the light does not enter the extinction state in the middle of the transition between 0 and ⁇ , and the light emission state always occurs, and the shape of the eye pattern does not depend on the waveform of the input data signal.
- phase modulation of an optical signal can be performed by using one of two types of optical modulators, but phase modulation is often performed using a light intensity modulator.
- phase modulation is often performed using a light intensity modulator.
- the reason for this is that when phase modulation is performed using an optical phase modulator, the phase transition between 0 and ⁇ is a transition that has passed through the arc of the vector diagram, and therefore a phase other than 0 and ⁇ is used in the phase transition process.
- a modulation amount is generated. This is because it is known that, as the rise time and fall time become gentle, a bright line spectrum is generated in the transmission rate period, and waveform deterioration occurs in adjacent bits where phase transition occurs.
- phase modulation is performed using a light intensity modulator
- there is no phase component other than 0 and ⁇ so that no bright line spectrum is generated and reception waveform deterioration does not occur.
- the present invention also assumes that a light intensity modulator is used for the data modulation unit 1.
- FIG. 6 is a conceptual diagram showing an eye pattern when the optical signal is RZ (Return to Zero) modulated by the clock modulation unit 2 according to the first embodiment of the present invention.
- the drive voltage is V ⁇ and the same frequency as that of the data signal.
- the waveform when the above signal is applied is shown.
- the clock modulation unit 2 performs intensity modulation on the optical signal using an optical intensity modulator.
- the clock modulation unit 2 since the clock modulation unit 2 performs intensity modulation based on the clock signal, the optical signal repeats light emission and extinction.
- This clock modulation by the clock modulation unit 2 is called RZ conversion, and the signal quality is improved by always turning off the bit between the bits once for the optical signal phase-modulated to 0 and ⁇ by the data modulation unit 1. Can be improved.
- FIG. 7 is a conceptual diagram showing an eye pattern when the optical signal is CSRZ modulated by the clock modulation unit 2 according to the first embodiment of the present invention.
- a CSRZ eye pattern is characterized by a high duty ratio when compared to an RZ eye pattern.
- FIG. 8 is a conceptual diagram showing an eye pattern when modulated by the data modulation unit 1 and the clock modulation unit 2 when the modulation signals in the first embodiment of the present invention are phase matched.
- FIG. 9 is a conceptual diagram showing an eye pattern when modulated by the data modulation unit 1 and the clock modulation unit 2 when the modulation signals in the first embodiment of the present invention are not phase matched.
- the optical signal modulated by the data modulation unit 1 and the clock modulation unit 2 is output as a transmission signal. At this time, it is very important to match the phases of the data signal and the clock signal.
- the data modulation unit 1 When the phases of the data signal and the clock signal are matched, the data modulation unit 1 performs phase modulation as shown in FIG. 8A on the optical signal, and then the clock modulation unit 2 performs FIG. By performing intensity modulation as shown in FIG. 8C, a waveform as shown in FIG. 8C can be obtained.
- the data modulator 1 performs phase modulation as shown in FIG. 9A on the optical signal, and then the clock modulator 2
- intensity modulation as shown in FIG. 9B is performed, the light is extinguished at the points of 0 and ⁇ , and the transition state from 0 to ⁇ , or the transition state from ⁇ to 0 is the light emitting state.
- FIG. 9C the data portion is quenched and does not have a correct waveform, and cannot be demodulated even if it is output to the optical receiver.
- the phase shifter 7 is arranged between the data generation unit 6 and the second driver 9 to control the phase of the clock signal.
- the phases of the two modulation signals are matched.
- the phase matching between the two modulation signals is not limited to the above example, and may be configured to control the phase of the data signal, or may be configured to control both phases. good.
- FIG. 10 is a flowchart showing the operation of the optical transmitter according to the first embodiment of the present invention.
- the data modulation unit 1 is input from a light source (not shown) based on the data signal input from the data generation unit 6 via the first driver 8.
- Phase modulation is performed on the obtained optical signal (step ST101).
- the optical signal phase-modulated by the data modulator 1 is output to the clock modulator 2.
- the clock modulation unit 2 performs intensity modulation on the optical signal phase-modulated by the data modulation unit 1 based on the clock signal input from the data generation unit 6 via the phase shifter 7 and the second driver 9. Perform (step ST102).
- the optical signal intensity-modulated by the clock modulation unit 2 is output to the optical coupler 3.
- the optical coupler 3 demultiplexes the optical signal intensity-modulated by the clock modulation unit 2 into two (step ST103).
- One optical signal demultiplexed by the optical coupler 3 is output to an optical wavelength demultiplexing device (not shown), multiplexed with a plurality of optical signals of other wavelengths, and then transmitted to the receiving side.
- the other optical signal is output to PD4.
- the PD 4 performs photoelectric conversion by detecting the optical signal demultiplexed by the optical coupler 3 (step ST104).
- the signal photoelectrically converted by the PD 4 is output to the phase controller 5a.
- the phase controller 5a controls the phase control amount of the phase shifter 7 by the hill-climbing method based on the signal photoelectrically converted by the PD 4 so that the intensity of this signal is maximized (step ST105).
- FIG. 11 is a conceptual diagram showing the relationship between the phase difference between the data signal and the clock signal and the intensity of the optical signal output from the optical transmitter (equal to the intensity of the signal output from the PD 4).
- the change in the light intensity with respect to the phase shift when the phase difference is 0 is shown.
- the intensity of the optical signal is maximized when the phases of the two modulation signals are matched (the intensity of the signal from the PD 4 is maximized), and gradually decreases as the phase shifts. Therefore, by controlling the phase control amount of the phase shifter 7 using the hill-climbing method in the phase control unit 5a, the phase of the two modulation signals is matched by controlling to the point where the intensity of the signal from the PD 4 is maximized. It becomes possible.
- the initial phase of the clock signal is A
- the intensity of the signal from the PD 4 at this phase A is measured.
- the phase shifter 7 is controlled so that the phase A + ⁇ and the phase A ⁇ are shifted from the phase A by an arbitrary phase control amount ⁇ , and the signal intensity is measured for each of the three points.
- the phase having the highest signal strength among the three points is set as B, and the phase B, the phase B + ⁇ , and the phase B ⁇ are repeatedly measured to control the phase with the maximum signal strength. It becomes possible.
- the hill climbing method is not a control method limited to the above.
- the intensity of signals at two points of phase A and phase A + ⁇ is compared to determine which is higher. If the intensity of the signal at phase A is high, then the intensity of the signals at phase A and phase A- ⁇ are compared.
- control of comparing the intensity of the signal at the phase A + ⁇ and the phase A + 2 ⁇ may be considered.
- the intensity of the signal from the PD 4 corresponding to the optical intensity of the optical signal from the optical transmitter is detected, and the hill climbing method is performed so that the intensity of this signal is maximized. Since it is configured to control the phase control amount of the phase shifter 7, it is possible to match the phases of the two modulation signals.
- the present invention is not limited to this modulation method, and may be applied to, for example, the RZ-OOK modulation method. Is possible.
- FIG. 12 is a conceptual diagram showing an eye pattern when OOK modulation is performed in the optical transmitter according to Embodiment 1 of the present invention.
- FIG. 13 is a conceptual diagram showing an eye pattern when RZ-OOK modulation is performed in the optical transmitter according to the first embodiment of the present invention.
- the optical transmitter when the RZ-OOK modulation scheme is applied two optical modulators are used similarly to the configuration of the optical transmitter shown in FIG. 1, and OOK modulation is performed with one optical modulator.
- RZ modulation is performed by the optical modulator.
- the waveform of an optical signal output from an optical transmitter to which this RZ-OOK modulation method is applied is generally called an NRZ (Non Return to Zero) waveform.
- This optical transmitter performs OZ modulation as shown in FIG. 12 on an optical signal input from a light source (not shown), and then performs RZ modulation as shown in FIG. Such a waveform can be obtained, and the present invention can be applied to the present modulation method.
- the number of optical modulators connected in series is two.
- the present invention is not limited to this.
- the phase shifter 7 that controls the phase of the modulation signal is used in FIG. 1, as shown in FIG. 14, at least one of the optical signals input to the optical modulators 1 and 2 is detected.
- the phase shifter 10 that performs phase control may be used. At this time, various types of optical phase shifters and optical delay lines are used as the phase shifter 10.
- Embodiment 2 shows a configuration in which the control time is greatly shortened by controlling the phase shifter 7 by synchronous detection instead of the hill-climbing method.
- FIG. 15 is a diagram showing a configuration of an optical transmitter according to the second embodiment of the present invention.
- the optical transmitter according to the second embodiment shown in FIG. 15 adds a minute signal generator 11, a mixer 12, and a synchronous detector 13 to the optical transmitter according to the first embodiment shown in FIG. 5a is changed to the phase controller 5b.
- Other configurations are the same, and the same reference numerals are given and the description thereof is omitted.
- the minute signal generator 11 generates a low frequency minute signal (dither signal).
- the low frequency is a frequency at which synchronous detection can be performed without greatly affecting the data signal.
- the dither signal generated by the minute signal generator 11 is output to the mixer 12 and the synchronous detector 13.
- the mixer 12 superimposes the dither signal generated by the minute signal generator 11 on the signal indicating the amount of phase control by the phase controller 5b.
- a signal indicating the phase control amount on which the dither signal is superimposed by the mixer 12 is output to the phase shifter 7.
- the synchronous detection unit 13 performs synchronous detection on the dither signal included in the signal photoelectrically converted by the PD 4 and the dither signal generated by the minute signal generation unit 11 to generate an error signal.
- the error signal generated by the synchronous detector 13 is output to the phase controller 5b.
- the phase control unit 5b Based on the error signal generated by the synchronous detection unit 13, the phase control unit 5b performs feedback control of the phase control amount of the phase shifter 7 so that the error signal is minimized.
- FIG. 16 is a flowchart showing the operation of the optical transmitter according to the second embodiment of the present invention.
- the operation of the optical transmitter according to the second embodiment shown in FIG. 16 the description of the same operation as that of the optical transmitter according to the first embodiment shown in FIG. 10 is simplified.
- the operation of the optical transmitter is as follows. First, the optical signal modulated by the data modulation unit 1 and the clock modulation unit 2 is branched by the optical coupler 3 and photoelectrically converted by the PD 4 (steps ST161 to 164). ).
- the phase shifter 7 performs the phase control of the clock signal by adding the dither signal generated by the minute signal generation unit 11 in addition to the phase control amount (for example, voltage) by the phase control unit 5b. Therefore, a dither signal also appears in the signal after photoelectric conversion by PD4.
- FIG. 17 is a conceptual diagram showing the relationship between the applied dither signal and the output dither signal when the phase control amount in the second embodiment of the present invention deviates from the optimum value.
- FIG. 18 is a conceptual diagram showing the relationship between the applied dither signal and the output dither signal when the phase control amount is the optimum value in the second embodiment of the present invention.
- the frequency of the dither signal generated and applied by the minute signal generation unit 11 is assumed to be f and is output from the PD4.
- the frequency of the dither signal is also f.
- phase control amount by the phase control unit 5 b when the phase control amount by the phase control unit 5 b is an optimum value, it is output from the PD 4 with respect to the frequency f of the dither signal generated and applied by the minute signal generation unit 11.
- the frequency of the dither signal is 2f.
- a signal including the dither signal photoelectrically converted by the PD 4 is output to the synchronous detector 13.
- the synchronous detector 13 performs synchronous detection on the dither signal generated by the minute signal generator 11 and the dither signal included in the signal photoelectrically converted by the PD 4 to generate an error signal (step ST165).
- the error signal generated by the synchronous detector 13 is output to the phase controller 5b.
- the phase controller 5b controls the phase control amount of the phase shifter 7 so that the error signal is minimized (step ST166).
- FIG. 19 is a conceptual diagram showing the relationship between the phase control amount and the error signal in the second embodiment of the present invention.
- the error signal is positive when the phase control amount is lower than the optimum value, and when the phase control amount is higher than the optimum value as shown in FIG. 19C.
- the error signal is negative. Therefore, the phase control unit 5b controls the phase control amount based on the value and polarity of the error signal so that the error signal becomes 0 as shown in FIG.
- synchronous detection is performed on the applied dither signal and the dither signal output from the PD 4 to generate an error signal, and the phase shifter so that the error signal is minimized.
- the phases of the two modulation signals can be matched with high speed and high accuracy.
- the number of optical modulators connected in series is two.
- the present invention is not limited to this.
- the phase shifter 7 that controls the phase of the modulation signal is used in FIG. 15, the phase shifter that performs phase control on at least one of the optical signals input to the optical modulators 1 and 2. 10 may be used.
- Embodiment 3 FIG.
- the phase control unit 5a always controls the phase shifter 7.
- the waveform always fluctuates on the time axis. This is called jitter or wander, and the reception characteristics. Will contribute to deterioration.
- the phase shift between the two modulation signals is divided into an initial amount and a fluctuation amount.
- the fluctuation amount varies depending on the temperature, the refractive index of the optical fiber changes, the optical path length changes, or the delay amount inside the IC changes. Because of this, the amount of fluctuation is small if the temperature does not change. Therefore, Embodiment 3 shows a configuration to which a function for determining start / stop of control according to temperature is added.
- FIG. 20 is a diagram showing a configuration of an optical transmitter according to the third embodiment of the present invention.
- the optical transmitter according to the third embodiment shown in FIG. 20 adds the temperature monitor unit 14 to the optical transmitter according to the first embodiment shown in FIG. 1, and changes the phase control unit 5a to the phase control unit 5c. Is.
- Other configurations are the same, and the same reference numerals are given and the description thereof is omitted.
- the temperature monitor unit 14 detects the temperature of the optical fiber through which the optical signal is transmitted.
- the optical fiber temperature detected by the temperature monitor unit 14 is output to the phase control unit 5c.
- the phase controller 5c controls the phase shifter 7 according to the optical fiber temperature detected by the temperature monitor unit 14.
- the phase control unit 5c stops the control of the phase shifter 7 and continues to output the optimum phase control amount, which is detected by the temperature monitor unit 14 at this time.
- the optical fiber temperature is stored. Thereafter, the temperature of the optical fiber detected by the temperature monitor unit 14 is monitored, and when the temperature difference becomes greater than a preset temperature with reference to the stored temperature, the control of the phase shifter 7 is resumed.
- FIG. 21 is a flowchart showing the operation of the optical transmitter according to the third embodiment of the present invention.
- the operation of the optical transmitter is as follows. First, at the start of control, the phase control unit 5c controls the phase shifter 7 by the hill-climbing method in the same manner as the optical transmitter according to the first embodiment. The phase control amount is controlled so that the phases of the two modulation signals are matched (step ST211).
- the phase control unit 5c determines that the optimum phase control amount has been reached when the intensity of the signal from the PD 4 exceeds a preset threshold (step ST212).
- This threshold value is not limited to the signal intensity, and may be a difference in signal intensity between two phase control amounts. Further, instead of the threshold value, the signal intensity at the three phase control amounts may be measured, and it may be determined that the optimum phase control amount has been reached when the central value becomes the highest value.
- the phase control unit 5c stops the control of the phase shifter 7, continues to output the optimum phase control amount, and stores the optical fiber temperature detected by the temperature monitoring unit 14 (step ST213). As a result, the control of the phase shifter 7 is stopped once the optimum phase control amount is reached. Therefore, there is no influence on the jitter characteristics.
- the phase control unit 5c compares the temperature of the optical fiber detected by the temperature monitor unit 14 with the stored temperature, and resumes the control of the phase shifter 7 when the temperature difference exceeds a preset value. (Step ST214).
- the phase control unit 5c starts control, the jitter characteristic deteriorates. However, since the optimum phase control amount is reached immediately, the control of the phase shifter 7 is stopped and the jitter characteristic is not affected.
- FIG. 22 is a diagram showing another configuration of the optical transmitter according to Embodiment 3 of the present invention.
- the optical transmitter according to the third embodiment shown in FIG. 22 adds the temperature monitor unit 14 and the lock detection unit 15 to the optical transmitter according to the second embodiment shown in FIG. 15, and controls the phase control unit 5b. It is changed to the part 5d.
- Other configurations are the same, and the same reference numerals are given and the description thereof is omitted.
- the lock detection unit 15 determines that the control loop is in a locked state when the error signal generated by the synchronous detection unit 13 is equal to or less than a preset threshold value, and controls the phase shifter 7 by the phase control unit 5d. Is output to the phase controller 5d.
- the phase control unit 5d controls the phase shifter 7 in accordance with the optical fiber temperature by the temperature monitor unit 14 and the presence / absence of a lock signal from the lock detection unit 15.
- the phase controller 5 d stops the control of the phase shifter 7 and continues to output the optimum phase control amount.
- Other processes are as described above, and the description thereof is omitted.
- the control of the phase shifter 7 is stopped once the optimum phase control amount is reached, and the control of the phase shifter 7 is restarted when the optical fiber temperature changes. Since the phase shifter 7 is not controlled while the optical fiber temperature is stable and the optimum phase control amount is obtained, the phases of the two modulation signals can be matched without affecting the jitter characteristics. Can do.
- Embodiment 4 FIG.
- the control of the phase shifter 7 is stopped while the optimum phase control amount is reached and the temperature is stable, and the control of the phase shifter 7 is resumed when the optical fiber temperature changes, thereby providing jitter characteristics. Realization of improvement.
- the optimum phase control amount and the temperature at that time are stored, and when the control of the phase shifter 7 is resumed, A configuration in which the stored phase control amount is read and the phases of the two modulation signals are matched without performing the hill climbing method or the synchronous detection will be described.
- FIG. 23 is a diagram showing a configuration of an optical transmitter according to the fourth embodiment of the present invention.
- the optical transmitter according to the fourth embodiment shown in FIG. 23 adds the phase control amount storage unit 16 to the optical transmitter according to the third embodiment shown in FIG. 20, and the phase control unit 5c is added to the phase control unit 5e. It has been changed.
- Other configurations are the same, and the same reference numerals are given and the description thereof is omitted.
- the phase control amount storage unit 16 stores the phase control amount and the optical fiber temperature detected by the temperature monitor unit 14 when the phase control unit 5e determines that the optimum phase control amount has been reached.
- the phase control unit 5e controls the phase shifter 7 based on the optical fiber temperature stored in the phase control amount storage unit 16 in addition to the function of the phase control unit 5c.
- the optical fiber temperature detected by the temperature monitor unit 14 is stored in the phase control amount storage unit 16 when the phase control unit 5e resumes the control of the phase shifter 7, the optical fiber is detected.
- the phase control amount corresponding to the temperature is read from the phase control amount storage unit 16 and the phase shifter 7 is controlled.
- the present invention can also be applied to the optical transmitter that controls the phase shifter 7 by performing the synchronous detection of the second embodiment.
- the optimum phase control amount and the optical fiber temperature at that time are stored, and the optical fiber temperature when the control of the phase shifter 7 is resumed is already stored. Since the phase control amount corresponding to the temperature of the optical fiber is read and the phase shifter 7 is controlled, the phases of the two modulation signals can be matched without affecting the jitter characteristics.
- the optical transmitter according to the present invention can automatically match the phase of the modulation signal by a simple method, and includes a plurality of optical modulators, and has a function of controlling the phase of the modulation signal in the plurality of optical modulators. It is suitable for use in an existing optical transmitter.
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Abstract
Description
この方式では、光送信機により光信号の位相変化に情報を載せ、光受信機では1シンボル前の位相と干渉させて位相変化を強度変化に変換しツインフォトダイオード(ツインPD)で光電変換することで信号を受信する。これにより、一般的に用いられている変調方式であるOOK(On Off Keying)変調方式と比較して、理論上3dBの受信感度の改善が可能となる。
実施の形態1.
図1はこの発明の実施の形態1に係る光送信機の構成を示す図である。当該光送信機は、複数の光変調器を使用する光送信機に適用可能な構成であり、図1では例としてRZ-DPSK(Return to Zero-Differential Phase Shift Keying)変調方式を適用した場合の構成を示している。
光送信機は、図1に示すように、データ変調部(光変調器)1、クロック変調部(光変調器)2、光カプラ3、フォトダイオード(PD)4、位相制御部5a、データ生成部6、位相器7及び第1,2ドライバ8,9から構成される。
なお、一般的な光変調器では、光カプラ3とPD4の機能が内蔵されているものが多く、その場合は光変調器内部のそれらを使用すればよい。
位相制御部5aは、PD4により光電変換された信号に基づいて、この信号の強度が最大となるように山登り法で位相器7の位相制御量をフィードバック制御するものである。
第2ドライバ9は、位相器7により位相が制御されたクロック信号を光増幅するものである。この第2ドライバ9により光増幅されたクロック信号はクロック変調部2に出力される。
データ変調部1では光変調器として、光強度変調器を使用する場合と光位相変調器を使用する場合の2つの場合がある。
図2はこの発明の実施の形態1におけるデータ変調部1に光強度変調器を用いて光信号を位相変調した場合のアイパターンを示す概念図である。また、図3はこの発明の実施の形態1におけるデータ変調部1に光強度変調器を用いた場合の位相遷移の様子を示したベクトル図である。
図2に示すように、データ変調部1に光強度変調器を用いた場合では、強度変調を行う場合に必要な駆動電圧(Vπ)の2倍の駆動電圧を印加して、データ信号の0と1を光の位相の0とπに割り付ける。この場合、図3に示すように、0とπの位相遷移の途中では振幅が0になる瞬間が発生する。
図4はこの発明の実施の形態1におけるデータ変調部1に光位相変調器を用いて光信号を位相変調した場合のアイパターンを示す概念図である。また、図5はこの発明の実施の形態1におけるデータ変調部1に光位相変調器を用いた場合の位相遷移の様子を示したベクトル図である。
図4に示すように、データ変調部1に光位相変調器を用いた場合では、光信号の位相を直接変調できるため、駆動に必要な電圧であるVπを印加して、データ信号の0と1を光の位相の0とπに割り付ける。この場合、図5に示すように、0とπの位相遷移の途中においても常に振幅は一定である。
図6はこの発明の実施の形態1におけるクロック変調部2により光信号をRZ(Return to Zero)変調した場合のアイパターンを示す概念図であり、駆動電圧はVπであり、データ信号と同じ周波数の信号を印加したときの波形を示している。
クロック変調部2では、光強度変調器を用いて光信号に対して強度変調を行う。図6に示すように、クロック変調部2では、クロック信号に基づいて強度変調を行っているため、光信号は発光と消光を繰り返している。このクロック変調部2によるクロック変調はRZ化と呼ばれ、データ変調部1で0とπに位相変調された光信号に対し、ビット間を常に一度消光状態(Zero)にすることで信号品質を向上させることができる。
図7はこの発明の実施の形態1におけるクロック変調部2により光信号をCSRZ変調した場合のアイパターンを示す概念図である。
駆動電圧を2Vπとすることで0とπを常に反転できるため、キャリア成分を抑圧することができる。図7に示すように、CSRZ化したアイパターンでは、RZ化したアイパターンと比較した際に、Duty比が高くなるのが特徴である。
図8はこの発明の実施の形態1における変調信号が位相整合している場合にデータ変調部1及びクロック変調部2により変調された際のアイパターンを示す概念図である。また、図9はこの発明の実施の形態1における変調信号が位相整合していない場合にデータ変調部1及びクロック変調部2により変調された際のアイパターンを示す概念図である。
光送信機では、データ変調部1及びクロック変調部2により変調された光信号が送信信号として出力されるが、この際、データ信号及びクロック信号の位相を整合させることが非常に重要となる。
図10はこの発明の実施の形態1に係る光送信機の動作を示すフローチャートである。
光送信機の動作では、図10に示すように、まず、データ変調部1は、データ生成部6から第1ドライバ8を介して入力されたデータ信号に基づいて、不図示の光源から入力された光信号に対して位相変調を行う(ステップST101)。このデータ変調部1により位相変調された光信号はクロック変調部2に出力される。
次いで、位相制御部5aは、PD4により光電変換された信号に基づいて、この信号の強度が最大となるように山登り法で位相器7の位相制御量を制御する(ステップST105)。
図11に示すように、光信号の強度は、2つの変調信号の位相が整合したときが最大となり(PD4からの信号の強度が最大となる)、位相がずれていくに従い徐々に低下する。そのため、位相制御部5aにおいて山登り法を用いて位相器7の位相制御量を制御することにより、このPD4からの信号の強度が最大となる点に制御して2つの変調信号の位相を整合させることが可能となる。
RZ-OOK変調方式を適用した場合の光送信機では、図1に示す光送信機の構成と同様に2つの光変調器を用いて、1つの光変調器でOOK変調を行い、もう1つの光変調器でRZ変調を行う。このRZ-OOK変調方式を適用した光送信機により出力される光信号の波形は一般にNRZ(Non Return to Zero)波形と呼ばれる。
実施の形態1では、山登り法を用いて位相器7を制御している。この方法では、容易に最適な位相に制御することが可能であるが、検索するステップ幅(実施の形態1に記載のα)を細かくすると、測定する点数が増加し、それに比例して最適点に到達するまでの制御時間が増加する。一方、このステップ幅を荒くすると最適点の精度が劣化するため、あまり荒くすることはできない。そこで、実施の形態2では、山登り法ではなく、同期検波によって位相器7を制御することによって制御時間を大幅に短縮するように構成したものについて示す。
位相制御部5bは、同期検波部13により生成された誤差信号に基づいて、この誤差信号が最小となるように位相器7の位相制御量をフィードバック制御するものである。
図16はこの発明の実施の形態2に係る光送信機の動作を示すフローチャートである。図16に示す実施の形態2に係る光送信機の動作において、図10に示す実施の形態1に係る光送信機と同様の動作についてはその説明を簡略化する。
光送信機の動作は、図16に示すように、まず、データ変調部1及びクロック変調部2によって変調された光信号は、光カプラ3によって分岐されPD4により光電変換される(ステップST161~164)。ここで、位相器7は、位相制御部5bによる位相制御量(例えば電圧)に加えて、微小信号生成部11によって生成されたディザ信号も付加してクロック信号の位相制御を行う。そのため、PD4による光電変換後の信号にもディザ信号が現れる。
図17に示すように、位相制御部5bによる位相制御量が最適値からずれている場合には、微小信号生成部11により生成され印加されたディザ信号の周波数をfとすると、PD4から出力されたディザ信号の周波数もfとなる。
このPD4により光電変換されたディザ信号を含む信号は同期検波部13に出力される。
次いで、位相制御部5bは、同期検波部13により生成された誤差信号に基づいて、この誤差信号が最小となるように位相器7の位相制御量を制御する(ステップST166)。
図19(a)に示すように、位相制御量が最適値よりも低い場合には誤差信号がプラスとなり、図19(c)に示すように、位相制御量が最適値よりも高い場合には誤差信号がマイナスとなる。そのため、位相制御部5bは、誤差信号の値と極性から、図19(b)に示すように、誤差信号が0になるように位相制御量を制御する。
さらに、図15では変調信号を位相制御する位相器7を用いたが、各光変調器1,2に入力される光信号のうち少なくとも1つ以上の光信号に対して位相制御を行う位相器10を用いてもよい。
実施の形態1では、位相制御部5aは位相器7を常に制御しているが、この場合、常に時間軸上で波形が揺れていることになり、これはジッタやワンダと呼ばれ、受信特性が劣化する一因となる。一方、2つの変調信号の位相ずれは、初期量と変動量に分かれるが、変動量は温度によって光ファイバの屈折率が変化し光路長が変化することやIC内部の遅延量が変化することなどが原因であるため、温度が変化しなければ変動量は僅かである。そこで、実施の形態3では、温度によって制御の開始/停止を決める機能を付加した構成について示す。
位相制御部5cは、位相制御部5aの機能に加えて、温度モニタ部14により検出された光ファイバ温度に応じて位相器7の制御を行うものである。ここで、位相制御部5cは、位相制御量が最適値に達した場合に、位相器7の制御を停止して最適な位相制御量を出力し続け、このときの温度モニタ部14により検出された光ファイバ温度を記憶する。その後、温度モニタ部14により検出された光ファイバ温度を監視し、記憶した温度を基準にして予め設定した温度差以上になった場合には、位相器7の制御を再開する。
図21はこの発明の実施の形態3に係る光送信機の動作を示すフローチャートである。
光送信機の動作は、図21に示すように、まず、制御開始時においては、実施の形態1に係る光送信機と同様に、位相制御部5cは山登り法により位相器7の制御を行い、2つの変調信号の位相が整合するように位相制御量を制御する(ステップST211)。
図22はこの発明の実施の形態3に係る光送信機の別の構成を示す図である。図22に示す実施の形態3に係る光送信機は、図15に示す実施の形態2に係る光送信機に温度モニタ部14及びロック検出部15を追加して、位相制御部5bを位相制御部5dに変更したものである。その他の構成は同様であり同一の符号を付しその説明を省略する。
その他の処理は上述した通りでありその説明を省略する。
実施の形態3では、最適な位相制御量となり温度が安定している間は位相器7の制御を停止し、光ファイバ温度が変化した場合に位相器7の制御を再開することで、ジッタ特性の改善を実現している。それに対して、実施の形態4では、上記の制御に加え、最適な位相制御量とそのときの温度を記憶し、位相器7の制御を再開する際に、一度記憶した温度に対しては、山登り法や、同期検波を行わず、記憶した位相制御量を読み出して2つの変調信号の位相を整合させる構成について示す。
なお、図24に示すように、実施の形態2の同期検波を行い位相器7を制御する光送信機に対しても適応することが可能である。
Claims (8)
- 複数の変調信号を生成するデータ生成部と、
前記データ生成部により生成された複数の変調信号を増幅するドライバと、
前記ドライバに入力される複数の変調信号のうち少なくとも1つ以上の変調信号に対して位相制御を行う位相器と、
前記ドライバにより増幅された各変調信号に基づいて光信号の変調を行う、直列接続された複数の光変調器と、
最後段の前記光変調器により変調された光信号を分波する光カプラと、
前記光カプラにより分波された光信号を検出して光電変換するフォトダイオードと、
前記フォトダイオードにより光電変換された信号の強度が最大となるように前記位相器の位相制御量を制御する位相制御部と
を備える光送信機。 - 前記位相器の代わりに、前記各光変調器に入力される光信号のうち少なくとも1つ以上の光信号に対して位相制御を行う位相器を備える
ことを特徴とする請求項1記載の光送信機。 - 前記光信号が伝送される光ファイバの温度を検出する温度モニタ部を備え、
前記位相制御部は、前記温度モニタ部により検出された温度に応じて前記位相器を制御する
ことを特徴とする請求項1記載の光送信機。 - 前記光信号が伝送される光ファイバの温度を検出する温度モニタ部と、
前記フォトダイオードにより光電変換された信号の強度が最大となった際の前記位相制御部による位相制御量及び前記温度モニタ部により検出された温度を記憶する位相制御量記憶部とを備え、
前記位相制御部は、前記位相器を制御する際に、前記温度モニタにより検出された温度が前記位相制御量記憶部に記憶されている場合には、前記位相制御量記憶部に記憶されている当該温度に対応する位相制御量を読み出して前記位相器の制御を行う
ことを特徴とする請求項1記載の光送信機。 - 微小信号を生成する微小信号生成部と、
複数の変調信号を生成するデータ生成部と、
前記データ生成部により生成された複数の変調信号を増幅するドライバと、
前記ドライバに入力される複数の変調信号のうち少なくとも1つ以上の変調信号に対して、前記微小信号生成部により生成された微小信号を付加した位相制御を行う位相器と、
前記ドライバにより増幅された各変調信号に基づいて光信号の変調を行う、直列接続された複数の光変調器と、
最後段の前記光変調器により変調された光信号を分波する光カプラと、
前記光カプラにより分波された光信号を検出して光電変換するフォトダイオードと、
前記フォトダイオードにより光電変換された信号に含まれる微小信号と前記微小信号生成部により生成された微小信号とに基づいて誤差信号を生成する同期検波部と、
前記同期検波部により生成された誤差信号が最小となるように前記位相器の位相制御量を制御する位相制御部と
を備える光送信機。 - 前記位相器の代わりに、前記各光変調器に入力される光信号のうち少なくとも1つ以上の光信号に対して、前記微小信号生成部により生成された微小信号を付加した位相制御を行う位相器を備える
ことを特徴とする請求項5記載の光送信機。 - 前記光信号が伝送される光ファイバの温度を検出する温度モニタ部と、
前記同期検波部により生成された誤差信号が閾値以下である場合に、前記位相制御部による前記位相器の制御を停止させることを示す信号を出力するロック検出部とを備え、
前記位相制御部は、前記温度モニタ部により検出された温度及び前記ロック検出部により出力された信号に応じて前記位相器を制御する
ことを特徴とする請求項5記載の光送信機。 - 前記光信号が伝送される光ファイバの温度を検出する温度モニタ部と、
前記同期検波部により生成された誤差信号が最小となった際の前記位相制御部による位相制御量及び前記温度モニタ部により検出された温度を記憶する位相制御量記憶部とを備え、
前記位相制御部は、前記位相器を制御する際に、前記温度モニタにより検出された温度が前記位相制御量記憶部に記憶されている場合には、前記位相制御量記憶部に記憶されている当該温度に対応する位相制御量を読み出して前記位相器の制御を行う
ことを特徴とする請求項5記載の光送信機。
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WO2013031786A1 (ja) * | 2011-09-01 | 2013-03-07 | 日本電気株式会社 | 位相調整回路、光送信装置および位相調整方法 |
JP2020053858A (ja) * | 2018-09-27 | 2020-04-02 | 富士通株式会社 | 伝送システム、伝送装置、及び伝送方法 |
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US10404371B2 (en) * | 2013-05-01 | 2019-09-03 | The United States Of America As Represented By The Secretary Of The Army | Opto-electronic oscillator with stimulated Brillouin scattering feedback |
US10382159B2 (en) * | 2015-04-27 | 2019-08-13 | Mitsubishi Electric Corporation | Dummy-light generating device, optical transmission apparatus, and dummy-light generating method |
KR102146051B1 (ko) * | 2020-03-31 | 2020-08-19 | 국방과학연구소 | 광위상배열 기반의 ladar 시스템 및 이의 제어 방법 |
KR20220125558A (ko) * | 2021-03-05 | 2022-09-14 | 삼성전자주식회사 | 호스트 인터페이스 및 이를 포함하는 시스템-온-칩 |
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- 2010-02-22 US US13/508,548 patent/US20120230679A1/en not_active Abandoned
- 2010-02-22 EP EP10846061.9A patent/EP2541805B1/en active Active
- 2010-02-22 WO PCT/JP2010/001135 patent/WO2011101919A1/ja active Application Filing
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WO2013031786A1 (ja) * | 2011-09-01 | 2013-03-07 | 日本電気株式会社 | 位相調整回路、光送信装置および位相調整方法 |
JP2020053858A (ja) * | 2018-09-27 | 2020-04-02 | 富士通株式会社 | 伝送システム、伝送装置、及び伝送方法 |
JP7230401B2 (ja) | 2018-09-27 | 2023-03-01 | 富士通株式会社 | 伝送システム、伝送装置、及び伝送方法 |
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EP2541805A4 (en) | 2015-06-03 |
CN102763349A (zh) | 2012-10-31 |
KR20120104369A (ko) | 2012-09-20 |
US20120230679A1 (en) | 2012-09-13 |
JP5484556B2 (ja) | 2014-05-07 |
EP2541805A1 (en) | 2013-01-02 |
JPWO2011101919A1 (ja) | 2013-06-17 |
EP2541805B1 (en) | 2019-04-10 |
KR101389810B1 (ko) | 2014-04-29 |
CN102763349B (zh) | 2016-04-13 |
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