CN114964329B - Double-sideband optical frequency domain reflectometer - Google Patents
Double-sideband optical frequency domain reflectometer Download PDFInfo
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- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35338—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using other arrangements than interferometer arrangements
- G01D5/35354—Sensor working in reflection
- G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
- G01D5/35361—Sensor working in reflection using backscattering to detect the measured quantity using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering
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- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35329—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using interferometer with two arms in transmission, e.g. Mach-Zender interferometer
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Abstract
A double sideband optical frequency domain reflectometer comprising: the device comprises a modulation signal generating unit, a sensing signal receiving unit and a signal processing unit, wherein the modulation signal generating unit generates double-sideband detection light waves in an external modulation mode, the detection light waves are divided into two paths, one path is used as detection light and input into an optical fiber to be detected, the other path is used as local light and input into the sensing signal receiving unit, the optical fiber to be detected couples the change of external physical quantity onto the detection light waves, backward Rayleigh scattering signals generated by the backward Rayleigh scattering signals are transmitted back to the sensing signal receiving unit as signal light, the sensing signal receiving unit divides two different beat frequency signals generated by two sideband frequency sweeps from the double-sideband signal light by using a frequency shift method or an IQ receiving method, and the signal processing unit performs Fourier transform on the two different beat frequency signals after aligning in a time domain according to a frequency sweep range to obtain the strength and phase information of Rayleigh scattering on the optical fiber. The invention adopts IQ demodulation or local optical frequency shift to demodulate the backscattering signals generated by the two sideband detection optical signals respectively, thereby realizing the purposes of expanding the frequency sweep range and improving the spatial resolution and the measurement range of the reflectometer.
Description
Technical Field
The invention relates to a technology in the field of optical sensing, in particular to a double-sideband optical frequency domain reflectometer for acquiring distribution information of a backscattering signal on an optical fiber in a mode of two frequency sweeping sidebands.
Background
The distributed optical fiber sensing technology adopts the optical reflectometer technology to realize the positioning of the back scattering signals. The Optical Frequency Domain Reflectometer (OFDR) system adopts an Optical signal of linear Frequency sweep as detection light and local light, a backward rayleigh scattering signal generated by an Optical fiber to be detected and the beat Frequency of the local light are proportional to the time delay of the backward scattering light, the position of the backward rayleigh scattering is determined by the size of the beat Frequency, the spatial resolution depends on the Frequency tuning range of a light source, and high spatial resolution can be realized.
The OFDR system needs a linear swept-frequency light source as detection light and local light, and the generation method of the swept-frequency light source can be divided into an internal modulation method and an external modulation method. The internal modulation scheme directly adopts a tunable laser to generate a frequency-swept optical signal, the frequency tuning range of a light source can reach dozens of THz, but the internal modulation laser generally has a wide line width and frequency-swept nonlinearity which is difficult to inhibit, so that the phase noise ratio of an OFDR system using the internal modulation scheme is larger, and the measurement distance is smaller. The OFDR system of the external modulation scheme uses a frequency-stabilized laser as seed light, and the frequency of the seed laser is changed through a modulator, so that linear frequency sweep is realized. The external modulation mode can obtain a sweep frequency light source with low phase noise and high linearity, and long-distance sensing is realized. The frequency modulation range of the common electro-optical modulator in the OFDR system is limited by the frequency bandwidth of the modulator and the driving electric signal; in addition, the existing electro-optical modulator scheme needs to adopt expensive single-sideband modulator, additional optical filter or injection locking scheme, etc., and can generate stable linear sweep light, so that the OFDR system becomes complicated and is not easy to work stably.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a double-sideband optical frequency domain reflectometer, which uses two positive and negative sidebands with different frequency sweeping directions generated by an electro-optical modulator as useful signals simultaneously through a seed light and the electro-optical modulator to replace a single linear frequency sweeping light in a classical OFDR system, and respectively demodulates back scattering signals generated by two sideband detection light signals by adopting an IQ demodulation or local optical frequency shifting mode, thereby realizing the purposes of expanding the frequency sweeping range and improving the spatial resolution and the measurement range of the reflectometer.
The invention is realized by the following technical scheme:
the invention relates to a double-sideband optical frequency domain reflectometer, which comprises: modulation signal produces unit, sensing signal receiving element and signal processing unit, wherein: the modulation signal generation unit generates double-sideband detection light waves in an external modulation mode, the detection light waves are divided into two paths, one path is used as detection light to be input into an optical fiber to be detected, the other path is used as local light to be input into a sensing signal receiving unit, the optical fiber to be detected couples the change of external physical quantity to the detection light waves, backward Rayleigh scattering signals generated by the backward Rayleigh scattering signals are used as signal light to be transmitted back to the sensing signal receiving unit, the sensing signal receiving unit distinguishes beat frequency signals generated by the frequency sweeps of the two sidebands from the double-sideband signal light by using a frequency shift method or an IQ (in phase and quadrature) receiving method, and the signal processing unit performs Fourier transformation on the beat frequency signals generated by the two sidebands after aligning in a time domain according to a frequency sweep range to obtain the intensity and phase information of Rayleigh scattering on the optical fiber.
The modulation signal generating unit includes: electro-optical intensity modulator and single frequency laser, signal generator and fiber coupler that link to each other respectively to and erbium-doped fiber amplifier and the fiber circulator that link to each other in proper order, wherein: the single-frequency laser generates a beam of seed light with fixed frequency, and the signal generator generates a radio frequency sweep frequency signal with linearly changing frequency to drive the electro-optical intensity modulator. After the seed light is modulated by the optical intensity modulator, a double-sideband frequency-sweeping optical wave signal with the frequency symmetrical on two sides of the seed optical frequency and the difference value of the frequency of the seed optical frequency equal to the radio frequency-sweeping signal is generated, the double-sideband frequency-sweeping optical wave is divided into two beams of light through the optical fiber coupler, one beam of light is input into the erbium-doped optical fiber amplifier, the amplified light is guided by the optical fiber circulator to be input into the optical fiber to be detected as detection light, and the other beam of light is input into the sensing signal receiving unit as local reference light. The physical quantity to be measured influences the transmission of the light waves in the optical fiber to be measured through influencing the physical parameters of the optical fiber to be measured, so that the phase and the intensity of a backward Rayleigh scattering signal generated on the optical fiber to be measured can be changed. The backward Rayleigh scattering signal is guided by the optical fiber circulator and is input into the sensing signal receiving unit as signal light.
The single-frequency laser is preferably a narrow linewidth laser.
The electro-optic intensity modulator is preferably a mach-zehnder modulator (MZM).
The sensing signal receiving unit has any one of the following structures:
(1) the sensing signal receiving unit includes: the acousto-optic frequency shifter and the polarization collector are connected with the modulation signal generating unit, and the balanced photoelectric detector and the multi-channel analog-to-digital conversion equipment are connected with the polarization collector; when the sensing signal receiving unit separates positive and negative sidebands from the double-sideband signal light by using a frequency shift method, a modulation signal with fixed frequency is loaded on the acousto-optic frequency shifter, and the frequency of the reference light input into the sensing signal receiving unit is increased or decreased after passing through the acousto-optic frequency shifter and is changed into the frequency of the modulation signal on the acousto-optic frequency shifter. The signal light is input into the polarization splitter and split into a first polarization state and a second polarization state, so that the signal light with different polarization states can respectively generate interference with the reference light with the same polarization state. After the reference light after frequency shift is subjected to polarization state adjustment, the reference light respectively interferes with signal light in a first polarization state and a second polarization state in a polarization diversity device to obtain two groups of beat frequency signals, the two groups of beat frequency signals respectively represent two different linear polarizations of the light signals and are respectively output to two balanced photoelectric detectors, and the beat frequency signals in different polarization states are obtained by square detection and common mode component filtering. The electric signal output by the photoelectric detector is input into a multi-channel analog-to-digital conversion device, converted into a digital signal and sent to a signal processing unit through a data transmission medium; the reference light after frequency shift and the signal light returned by Rayleigh scattering spectrum are respectively mixed in the polarization splitter according to different polarization states. The balanced photoelectric detector completes square detection to obtain the result of mutual beat frequency of two sidebands in the reference light and the signal light in pairs, wherein beat frequency signals of the positive first-order signal light and the negative first-order reference light and beat frequency signals of the positive first-order reference light and the negative first-order signal light are filtered by a pre-filter in the multichannel analog-to-digital conversion equipment, namely digital signals collected by the analog-to-digital conversion equipment only comprise beat frequency signals of the positive first-order signal light and the positive first-order reference light and beat frequency signals of the negative first-order reference light and the negative first-order signal light.
The mixing means that: the optical field of the signal light in the input optical mixer is E S Reference light field is E L Two paths of output light fields obtained according to the input-output relation of the mixer are respectively as follows:
the square detection refers to: the balanced photoelectric detector adopts a built-in photodiode to carry out square detection, and obtains photocurrent signals corresponding to two input optical fields as follows:
the beat signals obtained by mutually beating the two beat signals refer to: balancing the differential mode components between the square-detected photocurrent signals in the photodetector:
(2) the sensing signal receiving unit includes: a polarization diversity IQ receiver, a balanced photoelectric detector and a multi-channel analog-to-digital conversion device which are connected with the modulation signal generating unit; when the sensing signal receiving unit separates positive and negative sidebands from the double-sideband signal light by using an IQ (in-phase quadrature) receiving method, the signal light is input into a polarization diversity IQ receiver and then is separated by a polarization beam splitter according to a first polarization state and a second polarization state, two groups of beat frequency signals are obtained and respectively represent two different linear polarizations of the optical signal, and the two groups of beat frequency signals are respectively output to different 90-degree optical mixers and are respectively mixed with local light. The output light of each 90-degree optical mixer is input into two different balanced photoelectric detectors, and beat frequency complex signals in different polarization states are obtained through square detection and common-mode component filtering. The electrical signal output by the photoelectric detector is input into a multi-channel analog-to-digital conversion device, converted into a digital signal and sent to a signal processing unit through a data transmission medium; the reference light and the signal light returned by the Rayleigh scattering spectrum are respectively subjected to IQ mixing in different polarization states in a polarization diversity IQ receiver. The balanced photoelectric detector completes square detection of IQ signals to obtain results of mutual beat frequency of two sidebands in the reference light and the signal light respectively; the beat frequency signals of the positive first-order signal light and the negative first-order reference light and the beat frequency signals of the positive first-order reference light and the negative first-order signal light are filtered by a pre-filter in the multichannel analog-to-digital conversion equipment, and the digital signals collected by the analog-to-digital conversion equipment only comprise the beat frequency signals of the positive first-order signal light and the positive first-order reference light and the beat frequency signals of the negative first-order reference light and the negative first-order signal light.
The IQ mixing refers to: the optical field of the signal light input into the 90-degree optical mixer is E S Reference light field is E L According to IQ mixThe four output light fields obtained from the input-output relationship of the frequency converter are respectively as follows:
the square detection of the IQ signal refers to: the balanced photoelectric detector adopts a built-in photodiode to carry out square detection, and obtains photocurrent signals corresponding to four input light fields as follows:
the beat signals obtained by mutually beating two pairs of the signals refer to: balancing the differential mode component between the square-detected photocurrent signals in the photodetector:
i.e. the real and imaginary parts, respectively, of the same complex signal.
The signal processing unit includes: a signal processing device. The signal processing equipment can be computing equipment with high performance, such as a server, a personal computer, an industrial personal computer, a DSP board card or an FPGA board card.
The spatial resolution of the double-sideband optical frequency domain reflectometer not only depends on the frequency coverage range of the two sidebands, but also depends on the maximum frequency difference of the two sidebands. In order to avoid the influence of the beat frequency between the two sidebands, the frequency coverage ranges of the two sidebands are discontinuous, which is equivalent to that a window function of an optical frequency domain (time domain) is superposed on a sweep frequency signal formed by the maximum frequency difference in the sweep frequency process of the two sidebands, the window function can further improve the spatial resolution on the premise of broadening the frequency spectrum, so that the improvement of the final spatial resolution is more than twice of the original single sideband sweep frequency effect, namely the width of the frequency spectrum of a beat frequency spectrum generated by a certain scattering point in a distance domain is reduced by 3 dB.
Technical effects
The invention adopts the positive and negative double side bands to sweep frequency simultaneously at the transmitting end, adopts the frequency shift receiving or IQ receiving scheme technology at the receiving end to realize the identification of the positive and negative side band sweep frequency signals, and carries out Fourier transform after arranging the positive and negative sweep frequency signals according to the frequency at the signal processing end.
Compared with the prior art, the distributed OFDR sensing system is realized by using the double-sideband swept-frequency light source generated by external modulation and providing the corresponding double-sideband demodulation method. The invention can expand the equivalent sweep frequency range of the optical frequency domain to more than two times of the sweep frequency range of the radio frequency signal and realize that the spatial resolution is improved to more than two times of the prior art.
Drawings
FIG. 1 is a schematic diagram of a double sideband optical frequency domain reflectometer scheme (1);
FIG. 2 is a schematic diagram of demodulation of the double sideband optical frequency domain reflectometer of FIG. 1;
FIG. 3 is a schematic diagram of a double sideband optical frequency domain reflectometer scheme (2);
FIG. 4 is a schematic diagram of an IQ receiver formed by a 90 ° optical mixer and a Balanced Photodetector (BPD) of the double-sideband optical frequency domain reflectometer of FIG. 3;
FIG. 5 is a schematic diagram of demodulation of the double sideband optical frequency domain reflectometer of FIG. 3;
in the figure: the device comprises a modulation signal generating unit 1, a sensing signal receiving unit 2, a signal processing unit 3, an optical fiber to be tested 4, a single-frequency laser 10, an electro-optical intensity modulator 11, a signal generator 12, an optical fiber coupler 13, an erbium-doped optical fiber amplifier 14, an optical fiber circulator 15, a balanced photoelectric detector 20, a multi-channel analog-to-digital conversion device 21, a signal processing device 30, an acousto-optic frequency shifter 40, a polarization divider 41 and a polarization diversity receiver 42;
fig. 6 is a power spectral density of a beat signal for which embodiment 1 uses scheme (1) to achieve double sideband signal reception;
FIG. 7 is a diagram showing the power spectral density of a beat signal for double sideband signal reception using scheme (2) of example 2;
FIG. 8 is a schematic diagram of a Rayleigh scattering spectrum of an optical fiber to be measured after being processed by a signal processing unit;
fig. 9 is a graphical representation of the spatial resolution comparison of the rayleigh scatter spectrum using only a single sideband data with the rayleigh scatter spectrum using both sideband data.
Detailed Description
Example 1
As shown in fig. 1, an OFDR system using an external modulation scheme for implementing double-sideband signal demodulation by using a frequency shift method includes a modulation signal generating unit 1, a sensing signal receiving unit 2 and a signal processing unit 3, which are connected in sequence, wherein: the modulation signal generating unit 1 outputs modulation signals to the sensing signal receiving unit 2 and the signal processing unit 3 respectively, the sensing signal receiving unit 2 receives sensing information transmitted back by the optical fiber 4 to be tested and completes separation and analog-to-digital conversion of positive and negative sideband signals, the digital signals are transmitted to the signal processing unit 3, and the signal processing unit 3 demodulates the distribution information of the backscattering signals on the optical fiber according to the modulation signals and the sensing signals.
The modulation signal generating unit 1 includes: a single-frequency fiber laser 10, a signal generator 12, a fiber coupler 13, an erbium-doped fiber amplifier 14 and a fiber circulator 15 which are respectively connected with an electro-optical intensity modulator 11, wherein: the optical fiber circulator 15 outputs a detection signal to the optical fiber 4 to be detected, and the output light of the single-frequency optical fiber laser 10 is output to the electro-optical intensity modulator 11 and divided into two sub-branches: the phase of one path is directly controlled by the input radio frequency signal, and the phase of the other path is controlled by the reverse radio frequency signal and a direct current signal in series; the direct current signal is adjusted to keep the phase difference of the two sub-branches pi, so that the intensity modulator can restrain carrier waves and even harmonics.
The power of the radio frequency signal input to the electro-optical intensity modulator 11 is adjusted to 20dBm, so that the output light of the electro-optical intensity modulator 11 mainly consists of two first-order sidebands, and the power of the high-order sidebands and the carrier wave can be ignored.
Preferably, when the radio frequency signal loaded onto the electro-optic intensity modulator 11 is a swept frequency signal, the two first-order sidebands of the output light are swept frequency light having the same sweep frequency rate and sweep frequency range, except that the sweep direction of the negative first-order sidebands is opposite to that of the positive first-order sidebands.
The sensing signal receiving unit 2 comprises: an acousto-optic frequency shifter 40 and a polarization splitter 41 connected to the modulation signal generation unit 1, a balanced photodetector 20 connected to the polarization splitter 41, and a multi-channel analog-to-digital conversion device 21.
As shown in fig. 2, the abscissa represents time, the ordinate represents frequencies of optical signals in different states, s1 is local reference light emitted from the second branch of the optical fiber coupler 13, s2 is a process in which the local reference light is frequency-shifted by the acousto-optic frequency shifter 40 to change its frequency, s3 is a process in which probe light emitted from the first branch of the optical fiber coupler 13 reaches the rayleigh scattering point P through the erbium-doped optical fiber amplifier 14 and the optical fiber circulator 15, and s4 is a process in which backward rayleigh scattered light generated at the rayleigh scattering point P reaches the polarization splitter 41 as signal light; v is 0 Frequency, f, of the output light wave of the single-frequency laser 10 0 Omega is the frequency shift amount of the acousto-optic frequency shifter 40 for the starting frequency of the radio frequency swept frequency signal generated by the signal generator 12. Tau is p The time for the detection light emitted from the fiber circulator 15 to reach a certain Rayleigh scattering point P on the fiber to be measured and the time for the backward Rayleigh scattering light generated at the point to reach the polarization splitter 41 through the fiber circulator 15 are also tau p . Since the time of the light wave emitted from the second branch of the fiber coupler 13 reaching the polarization splitter 41 through the acousto-optic frequency shifter 40 is negligible, the difference between the backward rayleigh scattering light generated by the rayleigh scattering point P and the time delay of the reference light reaching the polarization splitter 41 is 2 τ p 。Δf 1 The frequency, Δ f, obtained by beating between backward Rayleigh scattering light generated when the forward first order braid probe light reaches the Rayleigh scattering point P and the forward first order sideband of the local light 2 The frequency obtained by the mutual beat frequency between the negative first-order sidebands;
the acousto-optic frequency shifter 40 in this embodiment is configured to shift 60MHz positively.
The multi-channel analog-to-digital conversion device 21 in the present embodiment is configured in a two-channel mode at a sampling rate of 256 MHz.
In the present embodiment, the signal processing device 30 is a personal computer.
In this embodiment, the output wavelength of the single-frequency laser 10 is 1550.12nm, i.e., the frequency is 193.40THz. The frequency of the start frequency of the sweep signal generated by the signal generator 12 is 4GHz, the frequency of the end frequency of the sweep is 20GHz, and the sweep time is set to 1.4ms, so that the sweep rate is 11.43THz/s.
In this embodiment, the optical fiber 4 to be measured uses a common single mode optical fiber having a total length of about 470 m. The corresponding maximum beat frequency is about 52.60MHz.
Fig. 6 is a power spectral density of a beat frequency signal obtained by the double-sideband OFDR system implementing double-sideband signal reception by using the frequency shift method according to embodiment 1, wherein the horizontal axis in the figure is the frequency of the beat frequency signal, the vertical axis is the power spectral density of the beat frequency signal, and the vertical axis uses a logarithmic axis. The beat frequency signal received by the scheme (1) is a real signal, so the frequency range is 0Hz to (Fs/2) Hz, and Fs is the sampling rate used by the multichannel analog-to-digital conversion of the embodiment 1.
In the figure, the power spectral density profile of the beat frequency signal is symmetrical with the frequency loaded by the acousto-optic frequency shifter, namely 60MHz, the left side of the beat frequency spectrum generated by the negative first-order sideband frequency sweeping signal, and the right side of the beat frequency spectrum generated by the positive first-order sideband frequency sweeping signal. Since the beat spectrum different on the left and right sides includes information from probe light in different frequency ranges, the beat spectrum includes information that does not overlap with each other.
The signal processing unit 3 extracts different beat signals from the beat domain, aligns them in the optical frequency domain (time domain) according to the sweep range, and then calculates their fourier transform. And obtaining the Rayleigh scattering spectrum with improved resolution.
As shown in fig. 8, in the rayleigh scattering spectrum processed by the signal processing unit 3, for better comparison of spatial resolution, a fresnel reflection peak with a good signal-to-noise ratio of about 101m in the scattering spectrum is selected.
As shown in fig. 9, the discrimination capability of the spatial resolution is improved by using fourier interpolation. According to the experimental result, the spatial resolution of the ordinary OFDR system is 5.74mm according to the frequency sweep range set by the embodiment, and the spatial resolution of 2.12mm can be realized by setting the double-sideband OFDR system using the frequency shift method according to the same frequency sweep range. The improvement of spatial resolution of the double-sideband OFDR system is more than twice that of the ordinary OFDR system.
Example 2
As shown in fig. 3, compared with embodiment 1, the present embodiment relates to an OFDR system that implements double sideband signal demodulation by using IQ receiving method, and a sensing signal receiving unit 2 thereof includes: a polarization diversity IQ receiver 42 connected to the modulation signal generation unit 1, a balanced photodetector 20 and a multi-channel analog-to-digital conversion device 21.
As shown in fig. 5, the abscissa represents time, the ordinate represents the frequency of the optical signal in different states, s1 is the local reference light emitted from the second branch of the optical fiber coupler 13, s3 is the process of the signal probe light emitted from the first branch of the optical fiber coupler 13 reaching the rayleigh scattering point P, and s4 is the process of the backward rayleigh scattered light generated at the rayleigh scattering point P reaching the polarization splitter IQ receiver 42 as the signal light; v is 0 Frequency, f, of the output light wave of the single-frequency laser 10 0 For the starting frequency, τ, of the radio frequency swept frequency signal generated by the signal generator 12 p The time of the detection light emitted from the optical fiber circulator 15 reaching a certain Rayleigh scattering point P on the optical fiber to be measured, and the time delay difference of the backward Rayleigh scattering light generated on the point reaching the polarization diversity IQ receiver 42 through the optical fiber circulator 15 is 2 tau p 。Δf 3 The frequency, Δ f, obtained by beating between backward Rayleigh scattering light generated when the forward first order braid probe light reaches the Rayleigh scattering point P and the forward first order sideband of the local light 4 The frequency obtained by the mutual beat frequency between the negative first-order sidebands;
in this embodiment, the parameters of the modulation signal generating unit 1 are completely the same as those of embodiment 1.
In this embodiment, the signal processing unit 3 is completely the same as in embodiment 1.
In the present embodiment, the multi-channel analog-to-digital conversion device 21 is configured in a four-channel mode at a sampling rate of 125 MHz. Since the sampling rate is halved with an increased number of channels, the performance requirements for most freely configurable four-channel analog-to-digital conversion devices do not change compared to embodiment 1.
Fig. 7 shows the power spectral density of the beat signal obtained by the dual-sideband OFDR system implementing the IQ method for the dual-sideband OFDR system of embodiment 2. In the figure, the horizontal axis represents the frequency of the beat frequency signal, the vertical axis represents the power spectral density of the beat frequency signal, and the vertical axis represents the logarithmic axis. The received beat signal is (2) used as a complex signal, so the frequency range is (-Fs/2) Hz to (Fs/2) Hz, and Fs is the sampling rate used by the multi-channel analog-to-digital conversion of embodiment 2.
In the embodiment, the received signal is a complex signal, so that the positive frequency and the negative frequency are not influenced by each other. In the figure, the outline of the power spectral density of the beat frequency signal is symmetrical according to the frequency of 0MHz, the left side of the beat frequency signal is a beat frequency spectrum generated by a negative-order sideband frequency sweep signal, and the right side of the beat frequency spectrum is a beat frequency spectrum generated by a positive-order sideband frequency sweep signal. Since the beat spectrum different on the left and right sides includes information from probe light in different frequency ranges, the beat spectrum includes information that does not overlap with each other.
This embodiment requires the use of more photodetectors than embodiment 1, but also omits the frequency shift operation.
The frequency shift method and the IQ receiving method can easily filter out the Rayleigh scattering spectra of the positive sideband and the negative sideband respectively from the Rayleigh scattering spectra containing the information of the two sidebands by using band-pass filtering. After splicing the Rayleigh signals of the positive sideband and the negative sideband, the equivalent sweep frequency range can be improved so as to improve the spatial resolution and the dynamic range of measurement.
In this embodiment, the parameters of the optical fiber 4 to be measured are completely the same as those in embodiment 1, and thus the obtained rayleigh scattering spectrum data are also completely the same as those in embodiment 1, as shown in fig. 8 and 9.
In fig. 8, the horizontal axis is distance, the distance from a certain position on the optical fiber to the input end thereof corresponds to, the vertical axis is power spectral density of the rayleigh scattering spectrum of the optical fiber to be measured, and the vertical axis uses logarithmic coordinate axes, corresponding to intensity of backward rayleigh scattering at a certain position of the optical fiber to be measured.
In fig. 8, two curves are plotted for comparison with the prior art: the first thin solid line is a demodulation method which only uses single sideband data and corresponds to OFDR in the prior art; the second thin dashed line is the demodulation method using both sideband data, corresponding to the double sideband OFDR of the present invention. The thin solid line and the thin dashed line have the same outline, which indicates that the double-sideband OFDR has the consistency with the Rayleigh scattering spectrum of the optical fiber acquired by the OFDR in the prior art.
In order to better observe the difference of the spatial resolution in fig. 8, fig. 9 selects the fresnel reflection peak generated by the APC plug in the optical fiber to be measured, which is located at about 101m in fig. 8, and respectively amplifies the fresnel reflection peaks and places the amplified fresnel reflection peaks in two sub-graphs.
In fig. 9, the horizontal axis is distance, the vertical axis is power spectral density of the rayleigh scattering spectrum of the optical fiber to be measured, the intensity of backward rayleigh scattering corresponding to a certain position of the optical fiber to be measured is normalized by using the maximum value thereof, and the vertical axis uses a linear coordinate axis for better observing the change of spatial resolution.
In fig. 9, a solid line with a triangular mark is the scaled rayleigh scattering spectrum, a frequency sampling point of the rayleigh scattering spectrum by the discrete fourier transform is at the triangular mark, and a dotted line is a fourier interpolation curve calculated by using the frequency sampling value of the existing rayleigh scattering spectrum, so as to better calculate the change of the spatial resolution. The Fourier interpolation can be matched with the discrete time Fourier transformation of actual data, and the 3dB spectral width can more accurately represent the spatial resolution. The spatial resolution calculated from the 3dB spectral width is noted in fig. 9.
According to the experimental result, according to the parameters set in this embodiment, the spatial resolution of the rayleigh scattering spectrum obtained by using the double-sideband OFDR of the IQ method is 2.12mm, which is 2.7 times of the spatial resolution 5.74mm obtained by using the ordinary OFDR, which indicates that the improvement of the spatial resolution of the double-sideband OFDR system is more than twice that of the ordinary OFDR system.
Compared with the prior art, the device has the advantages that the measurement range is expanded to more than two times of the original measurement range, and the spatial resolution is improved to more than two times of the original measurement range.
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (6)
1. A double-sideband optical frequency domain reflectometer, comprising: modulation signal produces unit, sensing signal receiving element and signal processing unit, wherein: the modulation signal generation unit generates double-sideband detection light waves in an external modulation mode, the detection light waves are divided into two paths, one path is used as detection light to be input into an optical fiber to be detected, the other path is used as local light to be input into the sensing signal receiving unit, the optical fiber to be detected couples the change of external physical quantity to the detection light waves, backward Rayleigh scattering signals generated by the optical fiber to be detected are transmitted back to the sensing signal receiving unit as signal light, the sensing signal receiving unit divides two different beat frequency signals generated by frequency sweeping of two sidebands from the double-sideband signal light by using a frequency shift method or an IQ (in phase and Quadrature) receiving method, the signal processing unit performs Fourier transformation on the two different beat frequency signals after aligning the two beat frequency signals in a time domain according to a frequency sweeping range, and strength and phase information of Rayleigh scattering on the optical fiber are obtained;
the sensing signal receiving unit has any one of the following structures:
(1) the sensing signal receiving unit includes: acousto-optic frequency shifter and polarization splitter connected to modulation signal generating unit, balanced photodetector connected to polarization splitter and multi-channel analog-to-digital conversion device, or
(2) The sensing signal receiving unit includes: a polarization diversity IQ receiver, a balanced photoelectric detector and a multi-channel analog-to-digital conversion device which are connected with the modulation signal generating unit;
when the sensing signal receiving unit separates positive and negative sidebands from the double-sideband signal light by using a frequency shift method, a modulation signal with fixed frequency is loaded on the acousto-optic frequency shifter, and the frequency of the reference light input into the sensing signal receiving unit is increased or decreased after passing through the acousto-optic frequency shifter and is changed into the frequency of the modulation signal on the acousto-optic frequency shifter; the signal light is input into the polarization splitter and is split into a first polarization state and a second polarization state, so that the signal light with different polarization states can respectively generate interference with the reference light with the same polarization state; after the polarization state of the reference light after frequency shift is adjusted, the reference light respectively interferes with signal light in a first polarization state and a second polarization state in a polarization diversity device to obtain two groups of beat frequency signals which respectively represent two different linear polarizations of the light signals and are respectively output to two balanced photoelectric detectors, and the beat frequency signals in different polarization states are obtained by square detection and common mode component filtering; the electric signal output by the photoelectric detector is input into a multi-channel analog-to-digital conversion device, converted into a digital signal and sent to a signal processing unit through a data transmission medium; the reference light after frequency shift and the signal light returned by the Rayleigh scattering spectrum are respectively mixed in a polarization splitter according to different polarization states; the balanced photoelectric detector completes square detection to obtain the results of two mutual beat frequencies of the reference light and the signal light, wherein beat frequency signals of the positive first-order signal light and the negative first-order reference light and beat frequency signals of the positive first-order reference light and the negative first-order signal light are filtered by a pre-filter in the multichannel analog-to-digital conversion equipment, namely digital signals collected by the analog-to-digital conversion equipment only comprise beat frequency signals of the positive first-order signal light and the positive first-order reference light and beat frequency signals of the negative first-order reference light and the negative first-order signal light.
2. The double sideband optical frequency domain reflectometer as in claim 1, wherein said modulation signal generating unit comprises: electro-optical intensity modulator and single frequency laser, signal generator and fiber coupler that link to each other respectively to and erbium-doped fiber amplifier and the fiber circulator that link to each other in proper order, wherein: the single-frequency laser generates a beam of seed light with fixed frequency, and the signal generator generates a radio frequency sweep frequency signal with linearly changing frequency to drive the electro-optical intensity modulator; after the seed light is modulated by the optical intensity modulator, a double-sideband frequency-sweeping optical wave signal with the frequency symmetrical on two sides of the seed optical frequency and the difference value of the frequency of the seed optical frequency equal to the radio frequency-sweeping signal is generated, the double-sideband frequency-sweeping optical wave is divided into two beams of light by the optical fiber coupler, wherein one beam of light is input into the erbium-doped optical fiber amplifier, the amplified light is guided by the optical fiber circulator to be input into an optical fiber to be detected as detection light, and the other beam of light is input into the sensing signal receiving unit as local reference light; the physical quantity to be measured influences the transmission of the light waves in the optical fiber to be measured through influencing the physical parameters of the optical fiber to be measured, so that the phase and the intensity of a backward Rayleigh scattering signal generated on the optical fiber to be measured can be changed; the backward Rayleigh scattering signal is guided by the optical fiber circulator and is input into the sensing signal receiving unit as signal light.
3. The double sideband optical frequency domain reflectometer as in claim 1, wherein mixing means: the optical field of the signal light in the input optical mixer is E S The reference light field is E L The two output optical fields obtained according to the input-output relationship of the mixer are respectively as follows:
the square detection refers to: the balanced photoelectric detector adopts a built-in photodiode to carry out square detection, and photocurrent signals corresponding to two input optical fields are obtained as follows:
the beat signals obtained by mutually beating the two beat signals refer to: balancing the differential mode components between the square-detected photocurrent signals in the photodetector:
4. the double-sideband optical frequency domain reflectometer as in claim 1, wherein when the sensing signal receiving unit separates the positive and negative sidebands from the double-sideband signal light by the IQ receiving method, the signal light is input to the polarization diversity IQ receiver and then separated by the polarization beam splitter according to the first polarization state and the second polarization state, so that two groups of beat signals are obtained, which represent two different linear polarizations of the optical signal, respectively, and are output to different 90 ° optical mixers, respectively, for mixing with the local light; the output light of each 90-degree optical mixer is input into two different balanced photoelectric detectors, and beat frequency complex signals in different polarization states are obtained through square detection and common-mode component filtering; the electrical signal output by the photoelectric detector is input into a multi-channel analog-to-digital conversion device, converted into a digital signal and sent to a signal processing unit through a data transmission medium; IQ mixing is respectively carried out on the reference light and the signal light returned by the Rayleigh scattering spectrum in a polarization diversity IQ receiver according to different polarization states; the balanced photoelectric detector completes square detection of IQ signals to obtain results of mutual beat frequency of two sidebands in the reference light and the signal light respectively; the beat frequency signals of the positive first-order signal light and the negative first-order reference light and the beat frequency signals of the positive first-order reference light and the negative first-order signal light are filtered by a pre-filter in the multichannel analog-to-digital conversion equipment, and the digital signals collected by the analog-to-digital conversion equipment only comprise the beat frequency signals of the positive first-order signal light and the positive first-order reference light and the beat frequency signals of the negative first-order reference light and the negative first-order signal light.
5. The double sideband optical frequency domain reflectometer as in claim 4, wherein said IQ mixing is: the optical field of the signal light input into the 90-degree optical mixer is E S Reference light field is E L The four output light fields obtained according to the input-output relationship of the IQ mixer are respectively:
the square detection of the IQ signal refers to: the balanced photoelectric detector adopts a built-in photodiode to carry out square detection, and obtains photocurrent signals corresponding to four input light fields as follows:
the beat signals obtained by mutually beating two pairs of the signals refer to: balancing the differential mode component between the square-detected photocurrent signals in the photodetector:
i.e. the real and imaginary parts, respectively, of the same complex signal.
6. The double side-band optical-frequency domain reflectometer as in claim 1 wherein the spatial resolution of the double side-band optical-frequency domain reflectometer depends not only on the frequency coverage of the two side-bands themselves, but also on the maximum frequency difference between the two side-bands; in order to avoid the influence of the mutual beat frequency between the two sidebands, the frequency coverage ranges of the two sidebands are discontinuous, which is equivalent to that a window function of an optical frequency domain (time domain) is superimposed on a frequency sweep signal formed by the maximum frequency difference in the frequency sweep process of the two sidebands, and the window function can further improve the spatial resolution on the premise of broadening the frequency spectrum, so that the improvement of the final spatial resolution is more than twice of the frequency sweep effect of the original single sideband.
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