CN115900787A - Implementation method and system of spectral domain reflectometer - Google Patents

Abstract

The invention provides a spectral domain reflectometer system and an implementation method, comprising the following steps: the light source module generates a local light signal and a probe light signal with comb spectrum, the local light signal is directly connected with the coherent detection module, a part of the probe light signal enters the optical fiber to be detected after passing through the passive optical path module, a backward scattering signal returned from the optical fiber to be detected enters the coherent detection module through the passive optical path module, and outputs a sensing electrical signal after being subjected to beat frequency with the local light; the other part directly enters a coherent detection module, and outputs a reference electric signal after being subjected to beat frequency with local light; the sensing electric signal and the reference electric signal are input into the signal processing module for data demodulation, and the strength and phase distribution signal of the backscattering signal on the optical fiber link is obtained.

Description

Implementation method and system of spectral domain reflectometer

technical field

The present invention relates to the technical field of optical fiber sensing, in particular, to a method and system for implementing a spectral domain reflectometer, and more specifically, to an optical reflectometer method for detecting backscattering in an optical fiber and determining its intensity and phase. system.

Background technique

Optical reflectometry is a technology that can detect and locate backscattered light (such as Rayleigh scattering) signals in optical fibers. By analyzing the intensity or phase information of the backscattered optical signal at different positions of the optical fiber, the detection of optical fiber link loss, the measurement of environmental strain and temperature and other physical quantities are realized.

According to the positioning principle of backscattered optical signals, the existing optical reflectometers mainly include three types: optical time domain reflectometer, optical frequency domain reflectometer and optical coherent domain reflectometer.

The optical time domain reflectometer transmits the detection pulse, receives the backscattered light generated by the detection pulse, and determines the location of the scattering according to the return time of the backscattered light. The shorter the duration of the detection pulse, the higher the spatial resolution, but the backward The worse the signal-to-noise ratio of scattered light, the contradiction between the spatial resolution and the detection signal-to-noise ratio makes the spatial resolution of this reflectometer not high, generally greater than 1 meter;

The optical frequency domain reflectometer uses a linearly tuned light source and coherent reception to beat the scattered light signal with the local light, and maps the time delay of the scattered light signal to the frequency of the beat signal. The spatial resolution of this scheme depends on the light source The frequency tuning range is no longer limited by the duration of the probe light, but this solution requires a high linearity, low phase noise swept source, which is very expensive.

Optical coherence domain reflectometers use low-coherence light as the light source. Interference signals can only be observed when the optical path of scattered light is the same as that of local light. High spatial resolution can be achieved, but the measurement distance is generally very short.

Patent document CN113014313A (application number: 201911328434.3) discloses an optical time domain reflectometer. The optical time domain reflectometer includes: a laser for emitting a detection light source; a modulator for converting the detection light source into a first pulse signal and a second pulse signal. The second pulse signal is used to send the first pulse signal to the optical fiber to be tested, so that the optical fiber to be tested generates a reflection signal; the coupler is used to couple the second pulse signal and the reflected signal of the optical fiber to be tested into an optical signal; the photoelectric conversion module uses It is used to convert the optical signal into an electrical signal; the programmable module is used to adjust the detection light source emitted by the laser according to the electrical signal.

Patent document CN114964329A (application number: 202210559130.3) discloses a double sideband optical frequency domain reflectometer, including: a modulation signal generating unit, a sensing signal receiving unit and a signal processing unit, the modulation signal generating unit generates double sideband through external modulation Detect the light wave, and divide the detection light wave into two paths, one path is used as the detection light input to the optical fiber to be tested, and the other path is used as the local light input to the sensing signal receiving unit, the optical fiber to be tested couples the change of the external physical quantity to the detection light wave, and transmits it The generated back Rayleigh scattering signal is sent back to the sensing signal receiving unit as signal light, and the sensing signal receiving unit uses the frequency shift method or IQ receiving method to divide the two sideband signal light from the two sideband frequency sweeps into two signals. Two different beat frequency signals, the signal processing unit performs Fourier transform on the two different beat frequency signals according to the frequency sweep range after aligning in the time domain, and obtains the intensity and phase information of Rayleigh scattering on the optical fiber.

Patent document CN203617996U (application number: 201320800522.0) discloses a coherent optical time domain reflectometer device, which is characterized in that it includes a laser, an optical beam splitter, a pair of pulse generators, an optical circulator, and a photoelectric detector. A coherent optical time-domain reflectometry method, characterized in that: the test pulse light is used to obtain the scattering/reflection information of the device under test, the local pulse light is used to interfere with the scattered/reflected optical signal, and the test pulse light and the local The respective repetition frequency of the pulsed light can completely construct the reflected/scattered light signal, and then obtain the position information of the scattered/reflected light through photoelectric conversion and analog-to-digital conversion; the pulse width and the required space of the test pulsed light and local pulsed light The resolution is suitable, and the distance between adjacent pulses is much larger than the pulse width.

Contents of the invention

Aiming at the defects in the prior art, the object of the present invention is to provide a spectral domain reflectometer system and its implementation method.

A spectral domain reflectometer system provided according to the present invention includes: a light source module generates a local optical signal and a detection optical signal with a comb spectrum, the local optical signal is directly connected to the coherent detection module, and the detection optical signal passes through the passive optical path module Part of it enters the optical fiber to be tested, and the backscattering signal returned from the optical fiber to be tested enters the coherent detection module through the passive optical circuit module, and outputs the sensing electrical signal after beating with the local light; the other part directly enters the coherent detection module, and beats with the local light The reference electrical signal is output after the frequency; the sensing electrical signal and the reference electrical signal are input to the signal processing module for data demodulation, and the intensity and phase distribution signal of the backscattering signal on the optical fiber link are obtained.

Preferably, the light source module is a single optical comb light source module, and the light source module includes: a narrow linewidth laser light source or an optical frequency comb light source, a signal generator, a phase or intensity modulator, an acousto-optic modulator, a first coupler and an optical filter;

The narrow-linewidth laser light source is divided into a local optical path and a detection optical path through the first coupler; the local optical path is input into the acousto-optic modulator to cause a frequency shift of the narrow-band laser to obtain a local optical signal; the detection optical path corresponds to The narrow-band laser light in the phase or modulator is modulated by the periodic signal generated by the signal generator to generate a probe light signal with a comb spectrum.

The optical frequency comb light source is divided into a local optical path and a detection optical path through the first coupler; the local optical path is input into the optical filter to filter to obtain a single-frequency comb, and a local optical signal is obtained; the detection optical path is The optical frequency comb light source generates an optical signal with a comb spectrum.

Preferably, the light source module is a dual-comb light source module, which simultaneously generates two optical frequency combs with slightly different repetition frequencies as local light and probe light respectively.

Preferably, the passive optical circuit module includes a first coupler, a third coupler and an optical circulator;

The detection optical signal with a comb-like spectrum enters the first coupler and is divided into two paths, one path of optical signal with greater energy enters the optical circulator to connect to the optical fiber to be tested, and the optical signal with smaller energy enters the fourth coupling The backscattered signal from the optical fiber to be tested enters the second coupler; the local optical signal enters the third coupler, the optical signal with a relatively high beam split enters the second coupler, and the optical signal with a relatively small beam split into the fourth coupler.

Preferably, the coherent detection module includes: a second coupler, a fourth coupler, a first balanced photodetector, and a second balanced photodetector;

The local light and detection light signals output by the passive optical path module are respectively beat at the second coupler and the fourth coupler, and the generated beat frequency signals are input to the first balanced photodetector and the first balanced photodetector respectively. The second balanced photodetector; the first balanced photodetector and the second balanced photodetector convert the beat-frequency optical signal into an electrical signal, and output a sensing electrical signal and a reference electrical signal respectively.

Preferably, the signal processing module includes: a data acquisition card and a digital signal processor;

The input sensing electrical signal and reference electrical signal are collected and stored through the data acquisition card, and the collected sensing electrical signal and reference electrical signal are input into a digital signal processor for signal demodulation.

Preferably, in the signal processing module, an analog-to-digital converter is used to digitize the sensing electrical signal and the reference electrical signal, and then Fourier transform is used to obtain the spectrum of the sensing electrical signal and the reference electrical signal respectively, and the reference electrical signal The phase of the frequency spectrum is conjugated, the modulus value is reciprocal, and then multiplied by the spectrum of the sensing electrical signal, and the result is inversely Fourier transformed to obtain the intensity and phase information of the backscattered signal on the optical fiber to be tested.

A method for implementing a spectral domain reflectometer according to the present invention includes: the light source module generates a local optical signal and a detection optical signal with a comb spectrum, the local optical signal is directly connected to the coherent detection module, and the detection optical signal passes through a passive optical path The latter part of the module enters the optical fiber to be tested, and the backscattering signal returned from the optical fiber to be tested enters the coherent detection module through the passive optical circuit module, and outputs the sensing electrical signal after beating with the local light; the other part directly enters the coherent detection module, and the local The reference electrical signal is output after the optical beat frequency; the sensing electrical signal and the reference electrical signal are both input to the signal processing module for data demodulation, and the intensity and phase distribution signal of the backscattering signal on the optical fiber link are obtained.

Preferably, when the light source module is a single optical comb light source module, the light source module includes: a narrow linewidth laser light source, a pulse generator, a phase modulator, an acousto-optic modulator, and a first coupler;

The narrow-linewidth laser light source is divided into a local optical path and a detection optical path through the first coupler; the local optical path is input into the acousto-optic modulator to cause a frequency shift of the narrow-band laser to obtain a local optical signal; the detection optical path corresponds to The narrow-band laser is modulated by the pulse generated by the pulse generator in the phase modulator to generate a detection optical signal with a comb spectrum;

When the light source module is a dual optical comb light source module, the light source module includes: a narrow linewidth laser light source, a pulse generator, a first phase modulator, a second phase modulator, a first coupler, and a first optical filter and a second optical filter;

The narrow linewidth laser light source is equally divided into a local optical path and a detection optical path through the first coupler; the local optical path and the detection optical path are respectively input into the first phase modulator and the second phase modulator, And the first phase modulator and the second phase modulator are respectively modulated in different sequences by the pulse generator; the output optical signal of the first phase modulator enters the first optical filter for filtering processing , to obtain the optical comb signal in the required frequency range, and output it as a detection optical signal; the output light of the second phase modulator enters the second optical filter for filtering processing, and obtain the optical comb signal in the required frequency range signal and output it as a local optical signal.

Preferably, the passive optical circuit module includes a second coupler, a fourth coupler and an optical circulator;

The detection optical signal with a comb-like spectrum enters the second coupler to obtain an optical signal with 90% energy and an optical signal with 10% energy; the optical signal with 90% energy enters the optical circulator and connects to the optical fiber to be tested, with The optical signal with 10% energy enters the fifth coupler; the backscattered signal from the optical fiber to be tested enters the third coupler; the local optical signal with a comb spectrum enters the fourth coupler to obtain 90% energy respectively The optical signal and the optical signal with 10% energy; the optical signal with 90% energy enters the third coupler, and the optical signal with 10% energy enters the fifth coupler;

The coherent detection module includes: a third coupler, a fifth coupler, a first balanced photodetector, and a second balanced photodetector;

The local light and detection light signals output by the passive optical path module are respectively beat at the third coupler and the fifth coupler, and the generated beat frequency signals are input to the first balanced photodetector and the first balanced photodetector respectively. The second balanced photodetector; the first balanced photodetector and the second balanced photodetector convert the beat frequency optical signal into an electrical signal, and respectively output a sensing electrical signal and a reference electrical signal;

The signal processing module includes: a data acquisition card and a digital signal processor;

Collect and store the input sensing electrical signal and reference electrical signal through the data acquisition card, and input the collected sensing electrical signal and reference electrical signal into the digital signal processor for signal demodulation;

In the signal processing module, an analog-to-digital converter is used to digitize the sensing electrical signal and the reference electrical signal, and then Fourier transform is used to respectively obtain the spectrum of the sensing electrical signal and the reference electrical signal, and the spectrum of the reference electrical signal is The phase is conjugated, the modulus is reciprocal, and then multiplied by the spectrum of the sensing electrical signal, and the result is inversely Fourier transformed to obtain the intensity and phase information of the backscattered signal on the optical fiber to be tested.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention uses the technical features of continuous light and spectrum sampling to break through the contradiction between spatial resolution and detection signal-to-noise ratio, and realize the technical effect of higher spatial resolution and dynamic range;

2. Through the technical characteristics of the non-sweeping light source (that is, the optical comb), the present invention realizes the technical effect of avoiding the nonlinearity of frequency sweeping, greatly simplifying the system complexity and ensuring the stability and accuracy of the system performance;

3. By using the technical features of coherent light sources, the present invention realizes the technology that signals can be observed without adjusting the coherence length as long as the optical path difference between the detection path and the reference path does not exceed the coherence length of the light source without phase noise compensation Effect.

4. The present invention avoids a series of problems caused by the sweeping nonlinearity of the sweeping light source, and reduces the cost and system complexity of optical frequency domain reflection; compared with optical time domain reflectometer and optical coherence domain reflectometer, it has High positioning accuracy and signal-to-noise ratio and long measurable distance.

Description of drawings

Other characteristics, objects and advantages of the present invention will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings:

Figure 1 is a system block diagram of the spectral domain reflectometer solution.

Figure 2 is a block diagram of a single-frequency laser source as a single-comb light source.

Fig. 3 is a block diagram of an optical frequency comb as a single optical comb light source.

Figure 4 is a dual-comb light source module.

In Figure 1: 1-light source module, 2-optical coupler, 3-optical circulator, 4-second coupler, 5-third coupler, 6-fourth coupler, 7-first balance detector, 8-second balance detector, 9-data acquisition card, 10-signal processing unit.

In Fig. 2: 1-narrow linewidth laser source, 2-optical coupler, 3-signal generator, 4-modulator, 5-modulator.

In Fig. 3: 1-optical frequency comb laser source, 2-optical coupler, 3-optical filter.

In Fig. 4: 1-dual optical comb laser source.

Detailed ways

The present invention will be described in detail below in conjunction with specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

Example 1

Aiming at the deficiencies of existing reflectometers in terms of performance index and device cost, the present invention proposes a spectral domain reflectometer method and system, which can realize the detection of the sensor to be tested by using the detection optical signal with a comb-like spectrum without frequency sweeping. The distributed measurement of the components can achieve high spatial resolution, avoid the adverse effects caused by the nonlinearity of the frequency sweep, and simplify the system structure.

As shown in Figure 1, the present invention discloses a spectral domain reflectometer system using a single optical comb, including: a light source module, a passive optical path module, a coherent detection module and a signal processing module, wherein: the output of the light source module is divided into a detection optical path and the local light path, and input to the first coupler 2 and the third coupler 5 respectively; the first coupler 2 divides the incident detection light path into two parts, and the part with larger energy enters the No. 1 port of the optical circulator 3 And output to the optical fiber to be tested through 2 ports, the Rayleigh backscattered light produced by the optical fiber to be tested is input to the optical circulator 3 through the 2 port of the optical circulator 3 and then input to the second coupler 4 through the 3 port; the lower part of the energy Enter the fourth coupler 6. The local light is divided into two parts with equal energy by the third coupler 5 and input into the second coupler 4 and the fourth coupler 6 respectively. The local light and the detection light signal beat at the second coupler 4 and the fourth coupler 6, and the generated beat frequency light signals are respectively input into the first balanced photodetector 7 and the second balanced photodetector 8; the balanced photodetector The beat-frequency optical signal is converted into an electrical signal and input to the data acquisition card 9 for subsequent signal processing and data demodulation to obtain the intensity and phase of backward astigmatism.

The passive optical module includes: a first coupler 2, a third coupler 5 and an optical circulator 3, wherein the first coupler 2 is a 90:10 fiber coupler, and 90% of the output ports are 1 port, 10% of the output ports are 2 ports and the third coupler 5 is a 50:50 coupler.

The coherent detection module includes a second coupler 4 , a fourth coupler 6 , a first balanced detector 7 and a second balanced detector 8 . Both the second coupler 4 and the fourth coupler 6 are 50:50 couplers.

The bandwidths of the first balanced detector 7 and the second balanced detector 8 are both 1.6 GHz.

The signal processing module includes a data acquisition card 9 and a signal processing unit 10 .

The data acquisition card 9 samples the input electrical signal, and inputs the original data into the signal processing unit 10 for data demodulation.

The sampling rate of the data acquisition card 9 is f _c =1 GHz, and the resolution is 8 bits.

The concrete steps of this embodiment are as follows:

Step 1: Turn on the laser, make the light source module in working state, record the optical power signals detected by the two balanced detectors, and obtain the detected light receiving signal I ₁ (t) from the output electrical signal of the first balanced detector. It performs Fourier transform to obtain the frequency domain information I ₁ (f) of the detection circuit signal.

再除以I₁(f)与I₂(f)幅值的乘积，即

其中*表示共轭。Step 2: After obtaining the signals received by the first and second balanced detectors, perform Fourier transform on the output signal I ₂ (t) of the second balanced detector to obtain frequency domain information I ₂ (f), and convert I ₁ (f) multiplied by

Then divide by the product of the magnitudes of I ₁ (f) and I ₂ (f), that is

where * indicates conjugation.

其中：c为真空中的光速，n为光纤的有效折射率，f_c为系统采样率，单位为赫兹(Hz)，k为该点对应的索引值。Step 3: Then perform inverse Fourier transform on R(f) to obtain the Rayleigh reflectivity curve, ie {R(t)=IFFT(R(f))}. The position of a certain point on the fiber to be tested is

Where: c is the speed of light in vacuum, n is the effective refractive index of the fiber, f _c is the system sampling rate in Hertz (Hz), and k is the index value corresponding to this point.

其中B为光梳频率覆盖带宽，实际空间分辨率由反射峰的半高全宽定义。Step 4: Zoom in to obtain spatial resolution at the reflection peak position. The spatial resolution of this embodiment is determined by the frequency coverage of the detection optical frequency comb, namely

where B is the frequency coverage bandwidth of the optical comb, and the actual spatial resolution is defined by the full width at half maximum of the reflection peak.

Example 2

Embodiment 2 is an implementation of a single optical comb system light source module. As shown in FIG. 2 , this embodiment includes: a narrow linewidth laser source 1 , a first coupler 2 , a signal generator 3 , a modulator 4 , and an acousto-optic modulator 5 .

Preferably, the linewidth of the narrow linewidth laser source is less than 1kHz;

Described coupler is a 50:50 fiber optic coupler;

The modulator is a phase or intensity modulator,

The concrete steps of this embodiment are as follows:

Step 1: First load the pre-written data into the signal generator 3, connect the output signal to the phase or intensity modulator 4, turn on the laser 1, and the output optical signal is divided into two parts with equal energy by the first coupler 2, One part enters the phase or intensity modulator 4 and the other part enters the acousto-optic modulator 5 . The outputs of the phase or intensity modulator 4 and the acousto-optic modulator 5 are used as detection path and local path optical signals respectively, and are connected to the subsequent optical system.

Example 3

Embodiment 3 is an implementation of a single optical comb system light source module. As shown in FIG. 3 , this embodiment includes: an optical frequency comb laser source 1 , a first coupler 2 , and an optical filter 3 .

The optical filter is an adjustable optical filter.

The concrete steps of this embodiment are as follows:

Turn on the laser 1, and the output optical signal passes through the first coupler 2 and is divided into two parts with equal energy, one part is directly output as the detection path signal and connected to the subsequent optical path, and the other part enters the optical filter 3, and the center frequency of the optical filter 3 is adjusted. The comb tooth at a frequency is selected as the local optical signal and connected to the subsequent optical system.

Example 4

Embodiment 4 is an implementation of a dual optical comb system light source module. As shown in FIG. 4 , this embodiment includes: a dual-comb light source 1 .

The dual-comb light source can be generated by electro-optic modulation or mode-locked lasers;

The concrete steps of this embodiment are as follows:

Make the dual-comb light source in working condition and connect it to the follow-up optical system.

Those skilled in the art know that, in addition to realizing the system, device and each module thereof provided by the present invention in a purely computer-readable program code mode, the system, device and each module thereof provided by the present invention can be completely programmed by logically programming the method steps. The same program is implemented in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers, and embedded microcontrollers, among others. Therefore, the system, device and each module provided by the present invention can be regarded as a hardware component, and the modules included in it for realizing various programs can also be regarded as the structure in the hardware component; A module for realizing various functions can be regarded as either a software program realizing a method or a structure within a hardware component.

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. In the case of no conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims (10)

1. A spectral domain reflectometry system, comprising: the light source module generates a local light signal and a probe light signal with comb spectrum, the local light signal is directly connected with the coherent detection module, a part of the probe light signal enters the optical fiber to be detected after passing through the passive optical path module, a backward scattering signal returned from the optical fiber to be detected enters the coherent detection module through the passive optical path module, and outputs a sensing electrical signal after being subjected to beat frequency with the local light; the other part directly enters a coherent detection module, and outputs a reference electric signal after beat frequency with local light; the sensing electric signal and the reference electric signal are input into the signal processing module for data demodulation, and the strength and phase distribution signal of the backscattering signal on the optical fiber link is obtained.

2. The spectro-domain reflectometry system of claim 1 wherein the light source module is a single comb light source module, said light source module comprising: the device comprises a narrow-linewidth laser light source or light frequency comb light source, a signal generator, a phase or intensity modulator, an acousto-optic modulator, a first coupler and a light filter;

the narrow linewidth laser light source is divided into a local light path and a detection light path through the first coupler; the local light path is input into the acousto-optic modulator to enable narrow-band laser to generate frequency shift, and a local optical signal is obtained; the narrow-band laser corresponding to the detection optical path is modulated by the periodic signal generated by the signal generator in the phase or modulator to generate a detection optical signal with a comb-shaped spectrum.

The optical frequency comb light source is divided into a local light path and a detection light path through the first coupler; the local optical path is input into the optical filter to be filtered to obtain a single-frequency comb tooth, and a local optical signal is obtained; the detection optical path is used for generating an optical signal with a comb spectrum for the optical frequency comb light source.

3. The spectral domain reflectometry system of claim 1 wherein the light source module is a dual optical comb light source module that simultaneously generates two optical frequency combs with slightly different repetition frequencies as the local light and the probe light, respectively.

4. A spectral domain reflectometry system as in claims 1 to 3, wherein the passive optical module comprises a first coupler, a third coupler and an optical circulator;

the detection optical signal with the comb spectrum enters the first coupler and is divided into two paths, one path of optical signal with larger energy enters the optical circulator to be connected with the optical fiber to be detected, and the optical signal with smaller energy enters the fourth coupler; returning a back scattering signal from the optical fiber to be tested to enter a second coupler; and the local optical signal enters the third coupler, the optical signal with higher beam splitting ratio enters the second coupler, and the optical signal with smaller beam splitting ratio enters the fourth coupler.

5. The spectral domain reflectometry system of claim 1, wherein the coherent detection module comprises: a second coupler, a fourth coupler, a first balanced photodetector, and a second balanced photodetector;

the local light and the detection light signals output by the passive optical path module are respectively subjected to beat frequency at the second coupler and the fourth coupler, and generated beat frequency light signals are respectively input into the first balanced photoelectric detector and the second balanced photoelectric detector; the first balanced photoelectric detector and the second balanced photoelectric detector convert beat frequency optical signals into electric signals and respectively output sensing electric signals and reference electric signals.

6. The spectral domain reflectometry system of claim 1, wherein the signal processing module comprises: a data acquisition card and a digital signal processor;

the data acquisition card is used for acquiring and storing the input sensing electric signal and the reference electric signal, and inputting the acquired sensing electric signal and the reference electric signal into the digital signal processor for signal demodulation.

7. The spectral domain reflectometry system of claim 6, wherein in the signal processing module, the sensing electrical signal and the reference electrical signal are digitized by an analog-to-digital converter, then the frequency spectrums of the sensing electrical signal and the reference electrical signal are obtained by fourier transform, the frequency spectrum of the reference electrical signal is subjected to phase conjugation, the modulus is subjected to reciprocal, then the reciprocal is multiplied by the frequency spectrum of the sensing electrical signal, and the result is subjected to inverse fourier transform to obtain the strength and phase information of the backscattered signal on the optical fiber to be measured.

8. A method for implementing a spectral domain reflectometer, comprising: the light source module generates a local light signal and a probe light signal with comb spectrum, the local light signal is directly connected with the coherent detection module, a part of the probe light signal enters the optical fiber to be detected after passing through the passive optical path module, a backward scattering signal returned from the optical fiber to be detected enters the coherent detection module through the passive optical path module, and outputs a sensing electrical signal after being subjected to beat frequency with the local light; the other part directly enters a coherent detection module, and outputs a reference electric signal after beat frequency with local light; the sensing electric signal and the reference electric signal are input into the signal processing module for data demodulation, and the strength and phase distribution signal of the backscattering signal on the optical fiber link is obtained.

9. The method of claim 8, wherein when the light source module is a single optical comb light source module, the light source module comprises: the device comprises a narrow-linewidth laser light source, a pulse generator, a phase modulator, an acousto-optic modulator and a first coupler;

the narrow linewidth laser light source is divided into a local light path and a detection light path through the first coupler; the local light path is input into the acousto-optic modulator to enable narrow-band laser to generate frequency shift, and a local optical signal is obtained; narrow-band laser corresponding to the detection optical path is modulated by the pulse generated by the pulse generator in the phase modulator to generate a detection optical signal with a comb spectrum;

when the light source module is a dual optical comb light source module, the light source module includes: the device comprises a narrow linewidth laser light source, a pulse generator, a first phase modulator, a second phase modulator, a first coupler, a first optical filter and a second optical filter;

the narrow-linewidth laser light source is divided into a local light path and a detection light path through the first coupler; the local optical path and the detection optical path are respectively input into the first phase modulator and the second phase modulator, and the first phase modulator and the second phase modulator are respectively modulated by different sequences generated by the pulse generator; the output optical signal of the first phase modulator enters the first optical filter for filtering processing to obtain an optical comb signal in a required frequency range, and the optical comb signal is output as a detection optical signal; and the output light of the second phase modulator enters the second optical filter for filtering processing to obtain an optical comb signal in a required frequency range, and the optical comb signal is output as a local optical signal.

10. The method of claim 8, wherein the passive optical module comprises a second coupler, a fourth coupler, and an optical circulator;

the detection optical signal with the comb spectrum enters the second coupler to respectively obtain an optical signal with 90% of energy and an optical signal with 10% of energy; the optical signal with the energy of 90% enters the optical circulator to be connected with the optical fiber to be tested, and the optical signal with the energy of 10% enters the fifth coupler; returning a back scattering signal from the optical fiber to be tested to enter a third coupler; the local optical signal with the comb spectrum enters the fourth coupler to obtain an optical signal with 90% of energy and an optical signal with 10% of energy respectively; an optical signal with energy of 90% enters the third coupler, and an optical signal with energy of 10% enters the fifth coupler;

the coherent detection module includes: a third coupler, a fifth coupler, a first balanced photodetector, and a second balanced photodetector;

the local light and the detection light signals output by the passive optical path module are respectively subjected to beat frequency at the third coupler and the fifth coupler, and generated beat frequency light signals are respectively input into the first balanced photoelectric detector and the second balanced photoelectric detector; the first balance photoelectric detector and the second balance photoelectric detector convert beat frequency optical signals into electric signals and respectively output sensing electric signals and reference electric signals;

the signal processing module includes: a data acquisition card and a digital signal processor;

the method comprises the following steps of collecting and storing input sensing electric signals and reference electric signals through a data collection card, and inputting the collected sensing electric signals and reference electric signals into a digital signal processor for signal demodulation;

in the signal processing module, the sensing electrical signal and the reference electrical signal are digitized by an analog-to-digital converter, frequency spectrums of the sensing electrical signal and the reference electrical signal are respectively obtained by Fourier transform, the phase of the frequency spectrum of the reference electrical signal is conjugated, the modulus is inverted, the frequency spectrum of the sensing electrical signal is multiplied, the result is subjected to inverse Fourier transform, and the strength and the phase information of the backscattering signal on the optical fiber to be measured are obtained.

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