A Ф-OTDR system based on phase-modulated optical frequency comb for suppressing coherent fading effect and its working method

Abstract

A Ф-OTDR system and its working method for suppressing coherent fading effect based on phase-modulated optical frequency comb belong to the field of optics. The technical problem to be solved by the present invention is the coherent fading effect in the Ф-OTDR system. The system of the present invention includes a narrow-linewidth laser, a first optical fiber coupler, a phase modulator, an acousto-optic modulator, an erbium-doped fiber amplifier, a circulator, an arbitrary waveform generator, a polarization controller, a sensing optical fiber, a second optical fiber coupler, a photodetector, an oscilloscope, and a computer; by utilizing the characteristic that the detection pulse lights with different center frequencies have different backward Rayleigh scattering light intensity distributions, continuous light in the form of a frequency comb is generated through phase modulation during the operation of the system and modulated into pulse light and simultaneously injected into the sensing optical fiber to obtain different backward Rayleigh scattering light intensity distributions, and the extracted phase information is continued in the way of amplitude evaluation, and the false phase information is eliminated, so as to realize the suppression of the coherent fading effect.

Description

Technical Field

[0001] The present invention belongs to the field of optics, and particularly relates to a Ф-OTDR system for suppressing coherent fading effect based on phase-modulated optical frequency comb and its working method. Background Art

[0002] The response speed of a Phase-sensitive Optical Time Domain Reflectometer (Ф-OTDR) is as high as the millisecond or even sub-millisecond level, and the sensitivity is as high as the nano-strain level. It is usually used for structural health monitoring of large-scale projects such as bridges, tunnels, and oil transportation pipelines. The accuracy of the demodulated phase information in the Ф-OTDR system completely depends on the signal intensity at the demodulation position. Due to the coherent fading effect, the signal intensity at some positions is extremely low, and the demodulated phase information is prone to distortion, resulting in the inability to correctly perform timing and quantitative analysis of the amplitude and frequency of vibration events. Therefore, it is of great significance to study the method for suppressing the coherent fading effect in the Ф-OTDR system.

[0003] In 1984, P. Healey et al. found that using a narrow linewidth light source would cause coherent fading in a Coherent Optical Time Domain Reflectometer (COTDR) system (P Healey. Fading in heterodyne OTDR[J]. Electronics Letters, 1984, 20(1): 30-32.). In 1993, H.F. Taylor et al. first regarded the coherent fading noise as a useful signal (H.F. Taylor, C.E. Lee. Apparatus and method for fiber optic intrusion sensing: US, US5194847 A[P], 1993.), and proposed the Ф-OTDR system; however, the coherent fading effect will cause demodulated phase distortion.

[0004] To effectively suppress the coherent fading effect in the Ф-OTDR system, scientific researchers have conducted a large number of studies. For example, the research group of Pan Zhengqing from the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences proposed (Pan Z, Liang K, Ye Q, et al. Phase-sensitive OTDR system based on digital coherent detection [C]. Asia Communications and Photonics Conference and Exhibition, IEEE, 2012: 1-6.) injecting two probe pulse lights with a π phase shift into the fiber under test for vibration information detection, and the influence of the coherent fading effect can be avoided through data processing. However, this technology will affect the spatial resolution and response bandwidth, and the suppression effect on the coherent fading effect is insufficient. The research group of Zhang Xuping from Nanjing University proposed (Zabihi M, Chen Y, Zhou T, et al. Continuous Fading Suppression Method for Ф-OTDR Systems Using Optimum Tracking over Multiple Probe Frequencies [J]. Journal of Lightwave Technology, 2019, 37(14): 3602-3610.) a frequency-division multiplexing Ф-OTDR system, injecting three probe pulse lights with different frequency shifts, and suppressing the coherent fading noise through data processing. However, the structure of this scheme is relatively complex, the phase delays are inconsistent, and the working bandwidth of the traditional acousto-optic modulator is limited. The Chinese invention patent, inventors: Wei Wei, Zhou Haomin (application publication number: CN109974756A) proposed a Φ-OTDR technology based on differential phase pulse emission and time-domain merging. By respectively performing phase modulation of 0, π / 3, 2π / 3, and π on the latter half pulses of the probe light pulses, four phase sequences can obtain four different measurement results, and then the time-domain merging technology is used to comprehensively utilize the four phase sequences to suppress the coherent fading effect. However, in this scheme, the phase shift modulation is executed within four consecutive periods. During the modulation period, the performance of the fiber must remain unchanged. Especially at the fading position, the frequency response decreases, sacrificing the response bandwidth of the system. Due to the complexity of the phase shift operation and performance loss, the differential phase shift pulse technology is difficult to adopt in practical applications. Summary of the Invention

[0005] A brief summary of the present invention is given below to provide a basic understanding of certain aspects of the present invention. It should be understood that this summary is not an exhaustive summary of the present invention. It is not intended to identify the key or important parts of the present invention, nor is it intended to limit the scope of the present invention. Its purpose is only to present certain concepts in a simplified form as a prelude to the more detailed description discussed later.

[0006] The problem to be solved by the present invention is to propose a Ф-OTDR system and its working method for suppressing coherent fading effects based on a phase-modulated optical frequency comb in view of the deficiencies of the coherent fading effect suppression scheme in the existing Ф-OTDR system.

[0007] To achieve the above object, the present invention is realized by the following technical solutions:

[0008] A Ф-OTDR system for suppressing coherent fading effects based on a phase-modulated optical frequency comb includes a narrow-linewidth laser, a first optical fiber coupler, a phase modulator, an acousto-optic modulator, an erbium-doped fiber amplifier, a circulator, an arbitrary waveform generator, a polarization controller, a sensing optical fiber, a second optical fiber coupler, a photodetector, an oscilloscope, and a computer;

[0009] The optical signal output end of the narrow-linewidth laser is connected to the optical signal input end of the first optical fiber coupler. The two optical signal output ends of the first optical fiber coupler are respectively and simultaneously connected to the optical signal input end of the phase modulator and the input end of the polarization controller. The optical signal output end of the phase modulator is simultaneously connected to the optical signal input end of the acousto-optic modulator. The optical signal output end of the acousto-optic modulator is simultaneously connected to the optical signal input end of the erbium-doped fiber amplifier. The optical signal output end of the erbium-doped fiber amplifier is simultaneously connected to the first optical signal port of the circulator. The second optical signal port of the circulator is connected to the sensing optical fiber. The optical signal input end of the second optical fiber coupler is simultaneously connected to the optical signal output end of the polarization controller and the third optical signal port of the circulator. The optical signal output end of the second optical fiber coupler is simultaneously connected to the optical signal input end of the photodetector. The electrical signal output end of the photodetector is simultaneously connected to the electrical signal input end of the oscilloscope. The oscilloscope is simultaneously connected to the computer. The microwave signal output ends of the arbitrary waveform generator are respectively and simultaneously connected to the microwave signal loading end of the phase modulator, the microwave signal loading end of the acousto-optic modulator, and the trigger signal input end of the oscilloscope.

[0010] Further, the narrow-linewidth laser uses a single-frequency narrow-linewidth fiber laser with an output power of 10 mW and a wavelength of 1550 nm.

[0011] Further, the coupling ratio of the first fiber coupler is 90:10, and the coupling ratio of the second fiber coupler is 50:50. Further, the working wavelength of the phase modulator (3) is 1525 nm - 1605 nm, the modulation bandwidth is 10 GHz, and the half-wave voltage is 7 V.

[0012] Further, the frequency shift of the acousto-optic modulator is -300 MHz, and the extinction ratio is 50 dB.

[0013] Further, the detection bandwidth of the photodetector is 1 GHz.

[0014] A Ф-OTDR system based on phase-modulated optical frequency comb for suppressing coherent fading effect and its working method include the following steps:

[0015] The continuous light output by the narrow-linewidth laser is divided into two upper and lower branches by the first fiber coupler; the continuous light in the upper branch is modulated by the phase modulator to generate continuous light in the form of a frequency comb, and is modulated by the acousto-optic modulator into pulsed light, while generating a certain amount of frequency shift. After being amplified in optical power by the erbium-doped fiber amplifier, it is injected into the sensing fiber through the circulator; the continuous light in the lower branch is used as the intrinsic light. After the polarization state is adjusted by the polarization controller, it is injected into the second fiber coupler together with the backward Rayleigh scattering light generated due to the uneven distribution of the refractive index of the medium in the sensing fiber for beat frequency detection, and is detected by the photodetector. The output photocurrent is collected by the oscilloscope to obtain the multi-frequency backward Rayleigh scattering light, and the computer is used for data processing. The driving pulses of the phase modulator and the acousto-optic modulator are both provided by an arbitrary waveform generator.

[0016] Further, it includes the following steps:

[0017] S1. For the beat frequency signal I(t) collected by the oscilloscope, use the processing program in the computer to perform digital filtering processing to extract the backward Rayleigh scattering light signals with frequencies of ω i (i = 1, 2, 3,..., n);

[0018] S2. Demodulate the multi-frequency backward Rayleigh scattering light signals extracted in step S1 respectively to obtain the intensity information A i and the phase information

[0019] S3. For the phase reconstruction interval [a, b], respectively obtain the minimum signal intensities A min_i (i = 1, 2, 3,..., n) of each backward Rayleigh scattering light signal at positions a and b, and then obtain the maximum value among the minimum values A min_i , and the phase difference

[0020] S4. Always use the phase difference obtained in step S3 at any time. Perform connection, eliminate the distorted phase information, and achieve the suppression of the coherent fading effect.

[0021] Furthermore, the method for the phase modulator to generate an equal-amplitude optical frequency comb is as follows:

[0022] Define f c as the optical carrier frequency; f m as the modulation frequency; V m as the modulation voltage amplitude; V π as the half-wave voltage of the phase modulator; γ is the modulation index:

[0023] γ = V m / V π .π

[0024] For any periodic modulation signal m(t) with a fundamental frequency of f m , perform Fourier series expansion on it to obtain:

[0025]

[0026] where φ k is the phase of the modulation signal; k is the harmonic number; t is the time.

[0027] Then, under the modulation of m(t), continuous light in the form of a frequency comb is generated, and its time-domain expression E m (t) is:

[0028]

[0029] The beneficial effects of the present invention are as follows:

[0030] The Ф-OTDR system and its working method for suppressing the coherent fading effect based on phase-modulated optical frequency comb according to the present invention have a larger modulation bandwidth and more frequency components, which are convenient for fully suppressing the coherent fading. Compared with the traditional acousto-optic modulation, the method of generating an optical frequency comb by phase modulation has a larger working bandwidth (usually greater than several tens of GHz), can modulate more frequency components, and is convenient for more effectively eliminating the distorted phase information.

[0031] The Ф-OTDR system and its working method for suppressing the coherent fading effect based on phase-modulated optical frequency comb according to the present invention do not sacrifice the response bandwidth of the system. Multiple pulsed lights containing multiple frequency components are simultaneously injected into the sensing optical fiber in the system without sacrificing the response bandwidth of the system.

[0032] An Ф-OTDR system based on a phase-modulated optical frequency comb for suppressing coherent fading effects and its working method according to the present invention have a simple device structure and precisely controllable phase delay. Compared with Ф-OTDR systems using frequency division and wavelength division multiplexing, this method uses the phase modulation method to generate an optical frequency comb containing multiple frequency components at one time, with a simple structure and the phase delay between each frequency component can be precisely controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 FIG. is a schematic diagram of an Ф-OTDR system based on a phase-modulated optical frequency comb for suppressing coherent fading effects according to the present invention;

[0034] Figure 2 FIG. is a result diagram of the phase-modulated optical frequency comb measured during the working process of an Ф-OTDR system based on a phase-modulated optical frequency comb for suppressing coherent fading effects according to the second specific embodiment;

[0035] Figure 3 FIG. is an accumulated distribution diagram of the backward Rayleigh scattering light intensity information measured during the working process of an Ф-OTDR system based on a phase-modulated optical frequency comb for suppressing coherent fading effects according to the second specific embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] Hereinafter, exemplary specific embodiments of the present invention will be described in conjunction with the accompanying drawings. For clarity and conciseness, not all features of the actual embodiments are described in the specification. However, it should be understood that many embodiment-specific decisions must be made during the development of any such actual specific embodiment in order to achieve the developer's specific goals, for example, to comply with those system- and business-related constraints, and such constraints may vary with different embodiments. In addition, it should also be understood that although the development work may be very complex and time-consuming, for those skilled in the art who benefit from the disclosure of the present invention, such development work is only a routine task.

[0037] Here, it should also be noted that in order to avoid obscuring the present invention with unnecessary details, only the device structures and / or processing steps closely related to the solution according to the present invention are shown in the drawings, while other details less related to the present invention are omitted.

[0038] In order to further understand the content, features and effects of the present invention, the following specific embodiments are exemplified and described in detail in conjunction with the accompanying drawings: Specific Embodiment 1:

[0040] Refer to the appendix Figure 1: A Ф-OTDR system based on phase-modulated optical frequency comb to suppress coherent fading effect, comprising a narrow-linewidth laser 1, a first optical fiber coupler 2, a phase modulator 3, an acousto-optic modulator 4, an erbium-doped fiber amplifier 5, a circulator 6, an arbitrary waveform generator 7, a polarization controller 8, a sensing optical fiber 9, a second optical fiber coupler 10, a photodetector 11, an oscilloscope 12, and a computer 13;

[0041] The optical signal output end of the narrow-linewidth laser 1 is connected to the optical signal input end of the first optical fiber coupler 2. The two optical signal output ends of the first optical fiber coupler 2 are respectively and simultaneously connected to the optical signal input end of the phase modulator 3 and the input end of the polarization controller 8. The optical signal output end of the phase modulator 3 is simultaneously connected to the optical signal input end of the acousto-optic modulator 4. The optical signal output end of the acousto-optic modulator 4 is simultaneously connected to the optical signal input end of the erbium-doped fiber amplifier 5. The optical signal output end of the erbium-doped fiber amplifier 5 is simultaneously connected to the first optical signal port 6-1 of the circulator 6. The second optical signal port 6-2 of the circulator 6 is connected to the sensing optical fiber 9. The optical signal input end of the second optical fiber coupler 10 is simultaneously connected to the output end of the polarization controller 8 and the third optical signal port 6-3 of the circulator 6. The optical signal output end of the second optical fiber coupler 10 is simultaneously connected to the optical signal input end of the photodetector 11. The electrical signal output end of the photodetector 11 is simultaneously connected to the electrical signal input end of the oscilloscope 12. The oscilloscope 12 is simultaneously connected to the computer 13. The microwave signal output ends of the arbitrary waveform generator 7 are respectively and simultaneously connected to the microwave signal loading end of the phase modulator 3, the microwave signal loading end of the acousto-optic modulator 4, and the trigger signal input end of the oscilloscope 12.

[0042] Preferably, the narrow-linewidth laser 1 is a single-frequency narrow-linewidth fiber laser with an output power of 10 mW and a wavelength of 1550 nm.

[0043] Preferably, the coupling ratio of the first optical fiber coupler 2 is 90:10, and the coupling ratio of the second optical fiber coupler 10 is 50:50.

[0044] Preferably, the operating wavelength of the phase modulator 3 is 1525 nm - 1605 nm, the modulation bandwidth is 10 GHz, and the half-wave voltage is 7 V.

[0045] Preferably, the frequency shift of the acousto-optic modulator 4 is -300 MHz, and the extinction ratio is 50 dB.

[0046] Preferably, the detection bandwidth of the photodetector 11 is 1 GHz.

[0047] The Ф-OTDR system based on phase-modulated optical frequency comb to suppress coherent fading effect described in this embodiment does not sacrifice the response bandwidth of the system. Multiple-frequency-component pulsed light in the system is simultaneously injected into the sensing optical fiber without sacrificing the response bandwidth of the system.

[0048] A Ф-OTDR system based on phase-modulated optical frequency comb to suppress coherent fading effect described in this embodiment has a simple structure and precise and controllable phase delay. Compared with the Ф-OTDR systems using frequency division and wavelength division multiplexing, this method uses the phase modulation method to generate an optical frequency comb containing multiple frequency components at one time, with a simple structure, and the phase delay between each frequency component can be precisely controlled. Specific Embodiment 2:

[0050] A Ф-OTDR system based on phase-modulated optical frequency comb to suppress coherent fading effect and its working method described in Specific Embodiment 1 include the following steps:

[0051] The continuous light output by the narrow-linewidth laser 1 is divided into two upper and lower branches through the first optical fiber coupler 2; the continuous light in the upper branch is modulated by the phase modulator 3 to generate continuous light in the form of a frequency comb, and is modulated into pulsed light by the acousto-optic modulator 4, while generating a certain amount of frequency shift, and then amplified in optical power by the erbium-doped fiber amplifier 5 and injected into the sensing fiber 9 through the circulator 6; the continuous light in the lower branch is used as the eigenlight, and after the polarization state is adjusted by the polarization controller 8, it is injected into the second optical fiber coupler 10 together with the backward Rayleigh scattering light generated due to the uneven distribution of the refractive index of the medium in the sensing fiber 9 for beat frequency, and is detected by the photodetector 11. The output photocurrent is collected by the oscilloscope 12 to obtain multi-frequency backward Rayleigh scattering light. The computer 13 is used for data processing, and the driving pulses of the phase modulator 3 and the acousto-optic modulator 4 are both provided by the arbitrary waveform generator 7.

[0052] Furthermore, it includes the following steps:

[0053] S1. For the beat frequency signal I(t) collected by the oscilloscope 12, digital filtering processing is performed using the processing program in the computer 13 to extract the backward Rayleigh scattering light signals with frequencies of ω i (i = 1, 2, 3,..., n);

[0054] S2. Demodulate the multi-frequency backward Rayleigh scattering light signals extracted in step S1 respectively to obtain the intensity information A i and the phase information (i = 1, 2, 3,..., n);

[0055] S3. For the phase reconstruction interval [a, b], respectively obtain the minimum signal intensities A min_i (i = 1, 2, 3,..., n) of each backward Rayleigh scattering light signal at positions a and b, and then obtain the maximum value among the minimum values A min_i , and the phase difference between the backward Rayleigh scattering light signal corresponding to this maximum value between positions a and b

[0056] S4. Always use the phase difference obtained in step S3 at any time. Perform connection, eliminate the distorted phase information, and realize the suppression of the coherent fading effect.

[0057] During the operation of the Ф-OTDR system based on phase-modulated optical frequency comb for suppressing the coherent fading effect described in this embodiment, the cumulative distribution diagram of the backward Rayleigh scattering light intensity information measured is as Figure 3 shown. Taking 10% of the normalized intensity as the threshold for defining the coherent fading effect, it can be seen from Figure 3 that the phase information distortion rate caused by the coherent fading effect is reduced from 15 - 19% to 0.25%.

[0058] Furthermore, the method for the phase modulator 3 to generate an equal-amplitude optical frequency comb:

[0059] Define f c as the optical carrier frequency; f m as the modulation frequency; V m as the modulation voltage amplitude; V π as the half-wave voltage of the phase modulator; γ is the modulation index:

[0060] γ = V m / V π .π

[0061] For any periodic modulation signal m(t) with a fundamental frequency of f m , perform a Fourier series expansion on it to obtain:

[0062]

[0063] where φ k is the phase of the modulation signal; k is the harmonic number; t is the time.

[0064] Then, under the modulation of m(t), a continuous light in the form of a frequency comb is generated, and its time-domain expression E m (t) is:

[0065]

[0066] Furthermore, assume that the modulation signal m(t) contains the fundamental frequency, second harmonic, and third harmonic. When γ1 = 1.092, γ2 = 1.073, φ2 = 0.5π, γ3 = 0.962, φ3 = 0, the single-frequency continuous light is modulated by the phase modulator into a continuous light in the form of an equal-amplitude optical frequency comb, containing seven frequency components:

[0067] f c ±nf m , where n = 0, 1, 2, 3;

[0068] The detected multi-frequency backward Rayleigh scattering optical signal is as follows Figure 2 shown, including seven frequency components: f c ±nf m -300 MHz, where n = 0, 1, 2, 3

[0069] The specific number and interval of modulation frequencies can be flexibly controlled.

[0070] A Ф-OTDR system and its working method based on phase-modulated optical frequency comb to suppress coherent fading effect described in this embodiment have a larger modulation bandwidth and more frequency components, which can fully suppress coherent fading. Compared with traditional acousto-optic modulation, the method of generating optical frequency comb by phase modulation has a larger working bandwidth, can modulate more frequency components, is convenient for more effectively removing distorted phase information and realizing high-fidelity phase extraction; without sacrificing the response bandwidth of the system, multiple pulsed lights containing multiple frequency components in the system are simultaneously injected into the sensing optical fiber without sacrificing the detection bandwidth of the system; the system structure is simple and the phase delay is precisely controllable. Compared with the Ф-OTDR systems of frequency division multiplexing and wavelength division multiplexing, this method uses the phase modulation method to generate an optical frequency comb containing multiple frequency components at one time, with a simple structure, and the phase delay between each frequency component can be precisely controlled.

[0071] It should be noted that relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the element.

[0072] Although the present application has been described above with reference to specific embodiments, various improvements can be made to it and components therein can be replaced with equivalents without departing from the scope of the present application. In particular, as long as there is no structural conflict, the various features in the specific embodiments disclosed in the present application can be combined with each other in any way. The exhaustive description of these combinations is omitted in this specification only for the sake of saving space and resources. Therefore, the present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A Ф-OTDR system based on phase-modulated optical frequency comb for suppressing coherent fading effect, characterized in that: It includes a narrow linewidth laser (1), a first optical fiber coupler (2), a phase modulator (3), an acousto-optic modulator (4), an erbium-doped fiber amplifier (5), a circulator (6), an arbitrary waveform generator (7), a polarization controller (8), a sensing optical fiber (9), a second optical fiber coupler (10), a photodetector (11), an oscilloscope (12), and a computer (13); The optical signal output terminal of the narrow linewidth laser (1) is connected to the optical signal input terminal of the first optical fiber coupler (2). The two optical signal output terminals of the first optical fiber coupler (2) are respectively and simultaneously connected to the optical signal input terminal of the phase modulator (3) and the input terminal of the polarization controller (8). The optical signal output terminal of the phase modulator (3) is simultaneously connected to the optical signal input terminal of the acousto-optic modulator (4). The optical signal output terminal of the acousto-optic modulator (4) is simultaneously connected to the optical signal input terminal of the erbium-doped fiber amplifier (5). The optical signal output terminal of the erbium-doped fiber amplifier (5) is simultaneously connected to the first optical signal port (6-1) of the circulator (6). The second optical signal port (6-2) of the circulator (6) is connected to the sensing optical fiber (9). The optical signal input terminal of the second optical fiber coupler (10) is simultaneously connected to the output terminal of the polarization controller (8) and the third optical signal port (6-3) of the circulator (6). The optical signal output terminal of the second optical fiber coupler (10) is simultaneously connected to the optical signal input terminal of the photodetector (11). The electrical signal output terminal of the photodetector (11) is simultaneously connected to the electrical signal input terminal of the oscilloscope (12). The oscilloscope (12) is simultaneously connected to the computer (13). The microwave signal output terminals of the arbitrary waveform generator (7) are respectively and simultaneously connected to the microwave signal loading terminal of the phase modulator (3), the microwave signal loading terminal of the acousto-optic modulator (4), and the trigger signal input terminal of the oscilloscope (12).

2. The Ф-OTDR system based on phase-modulated optical frequency comb for suppressing coherent fading effect according to claim 1, wherein: The narrow linewidth laser (1) uses a single-frequency narrow linewidth fiber laser with an output power of 10 mW and a wavelength of 1550 nm.

3. The Ф-OTDR system based on phase-modulated optical frequency comb for suppressing coherent fading effect according to claim 2, wherein: The coupling ratio of the first optical fiber coupler (2) is 90:10, and the coupling ratio of the second optical fiber coupler (10) is 50:

50.

4. A Ф-OTDR system based on a phase-modulated optical frequency comb for suppressing the coherent fading effect according to claim 2, characterized in that: The working wavelength of the phase modulator (3) is 1525 nm - 1605 nm, the modulation bandwidth is 10 GHz, and the half-wave voltage is 7 V.

5. The Ф-OTDR system based on phase-modulated optical frequency comb for suppressing coherent fading effect according to claim 4, wherein: The frequency shift of the acousto-optic modulator (4) is -300 MHz, and the extinction ratio is 50 dB.

6. The Ф-OTDR system based on phase modulation optical frequency comb for suppressing coherent fading effect according to claim 5, wherein: The detection bandwidth of the photodetector (11) is 1 GHz.

7. A Ф-OTDR system and its working method for suppressing coherent fading effect based on phase-modulated optical frequency comb according to any one of claims 1-6, characterized in that: It includes the following steps: The continuous light output by the narrow linewidth laser (1) is divided into upper and lower branches by the first optical fiber coupler (2); the continuous light in the upper branch is modulated by the phase modulator (3) to generate continuous light in the form of an optical frequency comb, and is modulated into pulsed light by the acousto-optic modulator (4), while generating a certain amount of frequency shift. After being amplified in optical power by the erbium-doped fiber amplifier (5), it is injected into the sensing optical fiber (9) through the circulator (6); the continuous light in the lower branch serves as the intrinsic light. After the polarization state is adjusted by the polarization controller (8), it is injected into the second optical fiber coupler (10) together with the backward Rayleigh scattered light generated due to the uneven distribution of the refractive index of the medium in the sensing optical fiber (9) for beat frequency detection, and is detected by the photodetector (11). The output photocurrent is collected by the oscilloscope (12) to obtain multi-frequency backward Rayleigh scattered light. The computer (13) is used for data processing. The driving pulses of the phase modulator (3) and the acousto-optic modulator (4) are both provided by the arbitrary waveform generator (7).

8. A Ф-OTDR system and its working method for suppressing coherent fading effect based on phase-modulated optical frequency comb according to claim 7, characterized in that: It includes the following steps: S1. For the beat signal I(t) collected by the oscilloscope 12, perform digital filtering processing using the processing program in the computer 13 to extract the backward Rayleigh scattering optical signal with a frequency of ω i (i = 1, 2, 3,..., n); S2. Demodulate the multi-frequency backward Rayleigh scattering optical signals extracted in step S1 respectively to obtain intensity information A i and phase information S3. For the phase reconstruction interval [a, b], respectively obtain the minimum signal intensity A of each backward Rayleigh scattering optical signal at positions a and b min_i (i = 1, 2, 3, …, n), and then obtain the minimum value A min_i of the maximum value in, and the phase difference between the backward Rayleigh scattering optical signals corresponding to the maximum value between positions a and b S4. Always use the phase difference obtained in step S3 at any time for connection, eliminate the distorted phase information, and achieve the suppression of the coherent fading effect.

9. A Ф-OTDR system and its working method for suppressing coherent fading effect based on phase-modulated optical frequency comb according to claim 7, characterized in that: The method for the phase modulator (3) to generate an equal-amplitude optical frequency comb is: Define f c as the optical carrier frequency; f m as the modulation frequency; V m as the modulation voltage amplitude; V π as the half-wave voltage of the phase modulator; γ is the modulation index: γ = V m / V π .π For any periodic modulation signal m(t) with a fundamental frequency of f m , performing a Fourier series expansion on it yields: where φ k is the phase of the modulation signal; k is the harmonic order; t is time; Then, under the modulation of m(t), continuous light in the form of a frequency comb is generated, and its time-domain expression E m (t) is as follows:

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