Phase-Sensitive Optical Time Domain Reflectometer Based on a Wideband AOM for Suppressing Coherent Fading Effect and Its Working Method

Abstract

A phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing the coherent fading effect and its working method belong to the field of optics. To solve the problem of the coherent fading effect in existing phase-sensitive optical time domain reflectometers. The present invention includes a narrow linewidth laser, a first optical fiber coupler, a first acousto-optic modulator, a second acousto-optic modulator, an erbium-doped fiber amplifier, a circulator, a polarization controller, a sensing optical fiber, a second optical fiber coupler, a photoelectric balanced detector, an oscilloscope, a computer, and an arbitrary waveform generator. By using a broadband acousto-optic modulator in the system and simultaneously loading multiple microwave signals with different frequencies, multi-frequency detection pulsed light is generated, different backward Rayleigh scattering light intensity distributions are obtained, and based on the amplitude size as the judgment basis, the most accurate signal is always selected for phase reconstruction at any moment. The present invention has the advantages of simple and compact system structure, precise control of phase delay, and flexible control of frequency components in the system.

Description

Technical Field

[0001] The present invention belongs to the field of optics, and particularly relates to a phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing the coherent fading effect and a working method thereof. Background Art

[0002] A phase-sensitive optical time domain reflectometer (Ф-OTDR) is usually applied to detect weak vibration events. Its response speed is as high as the millisecond or even sub-millisecond level, and its sensitivity is as high as the nano-strain level, making it of great application value in the health monitoring of large engineering structures such as bridges, tunnels, and oil transportation pipelines. The Ф-OTDR system obtains the dynamic change process of the phase information in the vibration area over time by taking the difference of the phase information at two reference positions before and after the vibration area, so as to realize the quantitative analysis of the amplitude and frequency of the vibration event. However, the accuracy of the phase information completely depends on the signal intensity at the reference position. Due to the coherent fading effect in the Ф-OTDR system, the intensity of the backward Rayleigh scattered light at some positions in the optical fiber is extremely low, and the phase information at these positions is prone to distortion, resulting in the inability to correctly reflect the external vibration event and causing frequent false alarms in the actual application of the system such as structural health detection.

[0003] Therefore, it is of great significance to study the method for suppressing the coherent fading effect in the Ф-OTDR system. To solve this problem, researchers have done a lot of research work. For example, the research group of Zhang Xuping at Nanjing University (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.) proposed to simultaneously connect three acousto-optic modulators (AOMs) with different frequency shifts in parallel in the Ф-OTDR system to generate three different intensity curves of backward Rayleigh scattering light, and suppress the coherent fading effect through multiplexing. The research group of Sun Qizhen at Huazhong University of Science and Technology (Liu T, Li H, He T, et al. Fading Noise Free Distributed Acoustic Sensor Assisted with Double Wavelength lasers[C]. CLEO: Applications and Technology, 2020: JW2E.10.) used the wavelength division multiplexing scheme to suppress the coherent fading; Zhu Kun et al. at the Hong Kong Polytechnic University (Zhao Z, Wu H, Hu J, et al. Interference fading suppression in Φ-OTDR using space-division multiplexed probes[J]. Optics Express, 2021, 29(10): 15452-15462.) proposed to use a few-mode fiber as the sensing medium in the Ф-OTDR system, and multiplex the backward Rayleigh scattering light obtained under different modes to suppress the coherent fading effect.

[0004] The above coherent fading suppression schemes have greatly improved the problem of distorted demodulation phase information caused by the coherent fading effect in the Ф-OTDR system. However, the suppression scheme proposed by the research group of Zhang Xuping at Nanjing University has a complex system structure. When three AOMs with different frequency shifts are connected in parallel to the system, different phase delays are easily caused due to inconsistent fiber lengths, sacrificing the system response bandwidth; the suppression scheme proposed by the research group of Sun Qizhen at Huazhong University of Science and Technology introduces two lasers with different operating wavelengths in the Ф-OTDR system for wavelength division multiplexing, with a complex system structure and insufficient suppression of the coherent fading effect; in the suppression scheme proposed by Zhu Kun et al. at the Hong Kong Polytechnic University, the sensing medium is special. Summary of the Invention

[0005] A brief overview 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 overview is not an exhaustive overview 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 merely to present certain concepts in a simplified form as a prelude to the more detailed description to be discussed later.

[0006] The problems to be solved by the present invention are the problems existing in the existing coherent fading suppression schemes, such as complex system structure, inconsistent phase delay, special sensing medium, and failure to fully suppress the coherent fading effect. The present invention proposes a phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing the coherent fading effect and its working method.

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

[0008] The phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing the coherent fading effect includes a narrow linewidth laser, a first optical fiber coupler, a first acousto-optic modulator, a second acousto-optic modulator, an erbium-doped fiber amplifier, a circulator, a polarization controller, a sensing optical fiber, a second optical fiber coupler, a photoelectric balanced detector, an oscilloscope, a computer, and an arbitrary waveform generator;

[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 first acousto-optic modulator and the input end of the polarization controller. The optical signal output end of the first acousto-optic modulator is simultaneously connected to the optical signal input end of the second acousto-optic modulator. The optical signal output end of the second 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 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 photoelectric balanced detector. The electrical signal output end of the photoelectric balanced detector 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 ends of the first acousto-optic modulator, the microwave signal loading ends of the second acousto-optic modulator, and the trigger signal input end of the oscilloscope.

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

[0011] Furthermore, the coupling ratio of the first fiber optic coupler is 90:10, and the coupling ratio of the second fiber optic coupler is 50:50.

[0012] Furthermore, the center frequency of the first acousto-optic modulator is 400 MHz, and the bandwidth is 100 MHz.

[0013] Furthermore, the frequency shift of the second acousto-optic modulator is -300 MHz, and the extinction ratio is 50 dB.

[0014] Furthermore, the detection bandwidth of the optoelectronic balanced detector is 300 MHz, and it has a high common-mode rejection ratio and sensitivity.

[0015] For the phase-sensitive optical time domain reflectometer based on broadband AOM to suppress coherent fading effect and its working method according to the present invention, the continuous light output by the narrow linewidth laser is divided into two upper and lower branches through the first fiber optic coupler; the continuous light in the upper branch is modulated into multi-frequency continuous light by the first acousto-optic modulator, and then modulated into multi-frequency pulsed light as the detection pulse by the second acousto-optic modulator, 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 multi-frequency modulation of the first acousto-optic modulator and the pulsed modulation of the second acousto-optic modulator are both realized by the arbitrary waveform generator; 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 optic coupler together with the backward Rayleigh scattered light generated in the sensing fiber for beat frequency detection, and is detected by the optoelectronic balanced detector. The output photocurrent is collected by the oscilloscope to obtain the multi-frequency backward Rayleigh scattered light. The computer is used for data processing, and based on the amplitude size as the judgment basis, the most accurate signal is always selected at any moment for phase reconstruction to suppress the phase distortion caused by the coherent fading effect and achieve high-fidelity phase extraction.

[0016] Furthermore, the modulation method of the first acousto-optic modulator is as follows:

[0017] Since the first acousto-optic modulator is a broadband AOM and has a certain working bandwidth, when the amplitude provided by the arbitrary waveform generator is A and the initial phase is the frequencies are f1, f2,..., f n n microwave signals E1(t), E2(t),..., En(t):

[0018]

[0019] The above n microwave signals E1(t), E2(t),..., En(t) are freely programmed on the arbitrary waveform generator and finally synthesized into the microwave signal E(t):

[0020] E(t) = E1(t) + E2(t) +... + E n(t) (2)

[0021] Set the single - frequency continuous light output by a narrow line - width laser, with a carrier frequency of f c , when the microwave signal E(t) is loaded on the first acousto - optic modulator, the single - frequency continuous light will be modulated by the first acousto - optic modulator into multi - frequency continuous light with frequencies of: f c +f1, f c +f2,..., f c +f n to achieve multi - frequency modulation;

[0022] Set the frequency shift of the second acousto - optic modulator to - Δf, then the frequency components of the n backward Rayleigh scattered lights with different intensity distributions detected by the phase - sensitive optical time - domain reflectometer based on broadband AOM to suppress the coherent fading effect are: f1 - Δf, f2 - Δf,..., f n - Δf, realizing flexible frequency control.

[0023] Advantages of the present invention:

[0024] The phase - sensitive optical time - domain reflectometer based on broadband AOM to suppress the coherent fading effect and its working method of the present invention can effectively suppress the coherent fading effect: According to the characteristic that the backward Rayleigh scattered lights of the detection pulse lights with different frequency shifts in the Ф - OTDR system have different intensity distributions, the most accurate signal is always preferentially selected in time for phase reconstruction, so as to effectively suppress the coherent fading effect and achieve high - fidelity phase extraction. Its advantage is that the most accurate phase information can always be accurately selected for any phase reconstruction interval.

[0025] The phase - sensitive optical time - domain reflectometer based on broadband AOM to suppress the coherent fading effect and its working method of the present invention have flexible controllability of frequency components: A broadband acousto - optic modulator is used for multi - frequency modulation in the system, and the microwave modulation signal is provided by an arbitrary waveform generator, with flexible controllability of the number and interval of frequencies, which is conducive to simultaneously obtaining multiple backward Rayleigh scattered lights with inconsistent intensity distributions to fully suppress the coherent fading effect.

[0026] The phase - sensitive optical time - domain reflectometer based on broadband AOM to suppress the coherent fading effect and its working method of the present invention: In the Ф - OTDR system, to avoid the aliasing of the backward Rayleigh scattered lights generated by the detection optical pulses injected into the sensing fiber at two adjacent times, it is required that the repetition period of the detection pulses injected into the sensing fiber must be greater than the time required for the detection pulse to travel back and forth in the sensing fiber; secondly, according to the Nyquist sampling theorem, the highest frequency range of the vibration signals that the Ф - OTDR system can detect does not exceed half of the repetition frequency of the detection pulses injected into the sensing fiber; compared with the traditional single - pulse system, this system does not sacrifice the detection response bandwidth.

[0027] The phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing coherent fading effects and its working method thereof. Precise phase delay control: In a wavelength-division or frequency-division multiplexed Ф-OTDR system, due to the existence of fiber delay, the phase delays of each frequency component are usually inconsistent. In this method, the frequency components of the probing pulsed light are simultaneously modulated and generated by a broadband acousto-optic modulator, and the phase delays are controllable and consistent.

[0028] The phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects and its working method thereof. The system structure is simple and compact: A typical frequency-division multiplexed system or wavelength-division multiplexed system usually has a relatively complex structure and high cost. This method loads multiple microwave signals with different frequencies onto a single broadband acousto-optic modulator to implement a multi-frequency Ф-OTDR system, and the structure is simple. Description of the Drawings

[0029] Figure 1 is a schematic structural diagram of the phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects according to the present invention;

[0030] Figure 2 is a time-domain diagram of multi-frequency signals measured during the operation of the phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in the second specific embodiment;

[0031] Figure 3 is a frequency-domain diagram of multi-frequency signals measured during the operation of the phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in the second specific embodiment;

[0032] Figure 4 is a curve diagram showing the change of distorted phase information with time measured during the operation of the phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in the second specific embodiment;

[0033] Figure 5 is a curve diagram showing the change of high-fidelity phase information with time measured during the operation of the phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in the second specific embodiment. Specific Embodiments

[0034] In the following, 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 in the process of developing any such actual specific embodiment in order to achieve the specific goals of the developer, for example, to comply with those restrictions related to the system and business, and such restrictions may vary with different embodiments. In addition, it should also be understood that although the development work may be very complex and time-consuming, such development work is only a routine task for those skilled in the art who benefit from the disclosure of the present invention.

[0035] 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.

[0036] In order to further understand the content, features and effects of the present invention, the following specific embodiments are exemplified and described in detail with reference to the accompanying drawings as follows: Specific Embodiment 1:

[0038] Referring to the attached Figure 1 , a phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing coherent fading effects, comprising a narrow linewidth laser 1, a first optical fiber coupler 2, a first acousto-optic modulator 3, a second acousto-optic modulator 4, an erbium-doped fiber amplifier 5, a circulator 6, a polarization controller 7, a sensing optical fiber 8, a second optical fiber coupler 9, a photoelectric balanced detector 10, an oscilloscope 11, a computer 12 and an arbitrary waveform generator 13;

[0039] 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 first acousto-optic modulator 3 and the input end of the polarization controller 7. The optical signal output end of the first acousto-optic modulator 3 is simultaneously connected to the optical signal input end of the second acousto-optic modulator 4. The optical signal output end of the second 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 8. The optical signal input end of the second optical fiber coupler 9 is simultaneously connected to the output end of the polarization controller 7 and the third optical signal port 6-3 of the circulator 6. The optical signal output end of the second optical fiber coupler 9 is simultaneously connected to the optical signal input end of the optoelectronic balanced detector 10. The electrical signal output end of the optoelectronic balanced detector 10 is simultaneously connected to the electrical signal input end of the oscilloscope 11. The oscilloscope 11 is simultaneously connected to the computer 12. The microwave signal output ends of the arbitrary waveform generator 13 are respectively and simultaneously connected to the microwave signal loading end of the first acousto-optic modulator 3, the microwave signal loading end of the second acousto-optic modulator 4, and the trigger signal input end of the oscilloscope 11.

[0040] Preferably, the narrow linewidth laser 1 uses a single-frequency narrow linewidth fiber laser, with an output power of 10 mW and an output wavelength of 1550 nm.

[0041] Preferably, the coupling ratio of the first optical fiber coupler 2 is 90:10, and the coupling ratio of the second optical fiber coupler 9 is 50:50.

[0042] Preferably, the center frequency of the first acousto-optic modulator 3 is 400 MHz, and the bandwidth is 100 MHz.

[0043] Preferably, the frequency shift of the second acousto-optic modulator 4 is -300 MHz, and the extinction ratio is 50 dB.

[0044] Preferably, the detection bandwidth of the optoelectronic balanced detector 10 is 300 MHz.

[0045] The phase-sensitive optical time domain reflectometer based on broadband AOM to suppress the coherent fading effect in this embodiment effectively suppresses the coherent fading effect: According to the characteristic that the backward Rayleigh scattered light intensity distributions of the detection pulse lights with different frequency shifts in the Ф-OTDR system are different, the most accurate signal is always preferentially selected in time for phase reconstruction, so as to effectively suppress the coherent fading effect and achieve high-fidelity phase extraction. Its advantage lies in that the most accurate phase information can always be accurately selected for any phase reconstruction interval.

[0046] The phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing coherent fading effects described in this embodiment has a flexibly controllable frequency component: a broadband acousto-optic modulator is used for multi-frequency modulation in the system, and the microwave modulation signal is provided by an arbitrary waveform generator. The number and interval of frequencies are flexibly controllable, which is beneficial to simultaneously obtaining multiple backward Rayleigh scattered lights with inconsistent intensity distributions to fully suppress the coherent fading effects.

[0047] The phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in this embodiment does not sacrifice the response bandwidth of the system: in a Ф-OTDR system, to avoid aliasing of the backward Rayleigh scattered light generated by the probe light pulses injected into the sensing fiber at two adjacent times, it is required that the repetition period of the probe pulses injected into the sensing fiber must be greater than the time required for the probe pulses to travel back and forth in the sensing fiber; secondly, according to the Nyquist sampling theorem, the maximum frequency range of the vibration signals that the Ф-OTDR system can detect does not exceed half of the repetition frequency of the probe pulses injected into the sensing fiber; compared with the traditional single-pulse system, this system does not sacrifice the detection response bandwidth of the system.

[0048] The phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in this embodiment has precise control of phase delay: in a wavelength-division or frequency-division multiplexed Ф-OTDR system, due to the existence of fiber delay, the phase delays of each frequency component are usually inconsistent; in this method, the frequency components of the probe pulse light are simultaneously modulated by a broadband acousto-optic modulator, and the phase delays are controllable and consistent.

[0049] The phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects described in this embodiment has a simple and compact system structure: a typical frequency-division multiplexed system or wavelength-division multiplexed system usually has a relatively complex structure and high cost. This method loads multiple microwave signals with different frequencies onto a single broadband acousto-optic modulator to implement a multi-frequency Ф-OTDR system, and the structure is simple. Specific Embodiment 2:

[0051] The phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing the coherent fading effect and its working method according to Embodiment 1. The continuous light output by the narrow linewidth laser 1 is divided into two upper and lower branches by the first fiber coupler 2. The continuous light in the upper branch is modulated by the first acousto-optic modulator 3 into multi-frequency continuous light, and then modulated by the second acousto-optic modulator 4 into multi-frequency pulsed light as the detection pulse, 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 fiber 8 through the circulator 6. The multi-frequency modulation of the first acousto-optic modulator 3 and the pulsed modulation of the second acousto-optic modulator 4 are both realized by the arbitrary waveform generator 13. The continuous light in the lower branch serves as the eigenlight. After the polarization state is adjusted by the polarization controller 7, it is injected into the second fiber coupler 9 together with the backward Rayleigh scattered light generated in the sensing fiber 8 for beat frequency detection, and detected by the photoelectric balanced detector 10. The output photocurrent is collected by the oscilloscope 11 to obtain the multi-frequency backward Rayleigh scattered light as Figure 2 and Figure 3 shown, Figure 2 which is the distribution of the multi-frequency backward Rayleigh scattered light in the time domain with respect to the sensing distance, Figure 3 and Figure 4 which is the frequency domain information of the multi-frequency backward Rayleigh scattered light. It can be clearly seen that there are multiple frequency information after modulation by the first acousto-optic modulator 3 and frequency shift by the second acousto-optic modulator 4. The computer 12 is used for data processing. Based on the amplitude size as the judgment basis, the most accurate signal is always selected at any time for phase reconstruction to suppress the phase distortion caused by the coherent fading effect and achieve high-fidelity phase extraction. When a triangular vibration event with a frequency of 25 Hz and an amplitude of 10 V is applied in the system, the distorted phase information extracted by the system is as Figure 5 shown. It can be seen that the phase information demodulated before suppressing the coherent fading effect has serious distortion and fails to accurately extract the amplitude and frequency information of the vibration event. The high-fidelity phase information extracted after suppressing the coherent fading effect is as

[0052] shown, accurately extracting the frequency and amplitude information of the vibration event without information distortion.

[0053] Furthermore, the modulation method of the first acousto-optic modulator 3 is as follows: Since the first acousto-optic modulator 3 is a broadband AOM with a certain working bandwidth, when n microwave signals E1(t), E2(t),..., En(t) with amplitudes of A and initial phases of n and frequencies of f1, f2,..., f

[0054]

[0055] The above n microwave signals E1(t), E2(t),..., En(t) are freely written on an arbitrary waveform generator and finally synthesized into a microwave signal E(t):

[0056] E(t) = E1(t) + E2(t) +... + E n (t) (2)

[0057] Set the single-frequency continuous light output by the narrow linewidth laser 1, with a carrier frequency of f c , when the microwave signal E(t) is loaded on the first acousto-optic modulator 3, the single-frequency continuous light will be modulated by the first acousto-optic modulator 3 to frequencies of: f c + f1, f c + f2,..., f c + f n of multi-frequency continuous light, thus realizing multi-frequency modulation;

[0058] Set the frequency shift of the second acousto-optic modulator 4 to -Δf, then the frequency components of the n backward Rayleigh scattering lights with different intensity distributions detected by the phase-sensitive optical time domain reflectometer based on broadband AOM to suppress the coherent fading effect are: f1 - Δf, f2 - Δf,..., f n - Δf, realizing flexible frequency control.

[0059] The phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing the coherent fading effect and its working method in this embodiment can effectively suppress the coherent fading effect. According to the characteristic that the backward Rayleigh scattered light intensity distributions of the detection pulse lights with different frequency shifts in the Ф-OTDR system are different, the most accurate signal is always preferentially selected in time for phase reconstruction, so as to effectively suppress the coherent fading effect and achieve high-fidelity phase extraction. Its advantages are as follows: the most accurate phase information can always be accurately selected for any phase reconstruction interval; the frequency components are flexibly controllable. A broadband acousto-optic modulator is used for multi-frequency modulation in the system, and the microwave modulation signal is provided by an arbitrary waveform generator, and the number and interval of frequencies are flexibly controllable, which is beneficial to obtaining multiple backward Rayleigh scattered lights with inconsistent intensity distributions at the same time to fully suppress the coherent fading effect; the response bandwidth of the system is not sacrificed. In the Ф-OTDR system, to avoid the aliasing of the backward Rayleigh scattered lights generated by the detection light pulses injected into the sensing fiber at two adjacent times, the repetition period of the detection pulses injected into the sensing fiber must be greater than the time required for the detection pulses to travel back and forth in the sensing fiber once. Then, according to the Nyquist sampling theorem, the highest frequency range of the vibration signals that the Ф-OTDR system can detect does not exceed half of the repetition frequency of the detection pulses injected into the sensing fiber. Compared with the traditional single-pulse system, this method does not sacrifice the detection response bandwidth of the system; the phase delay is accurately controlled. In a wavelength-division or frequency-division multiplexed Ф-OTDR system, due to the existence of fiber delay, the phase delays of the respective frequency components are inconsistent. In the system, the frequency components of the detection pulse light are simultaneously modulated and generated by a broadband acousto-optic modulator, and the phase delay is controllable and consistent; the system structure is simple and compact. A typical frequency-division multiplexed system or wavelength-division multiplexed system usually has a relatively complex structure and high cost. This method loads multiple microwave signals with different frequencies onto a single broadband acousto-optic modulator to implement a multi-frequency Ф-OTDR system, and the structure is simple.

[0060] 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 phrase "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element.

[0061] Although the present application has been described above with reference to specific embodiments, various modifications can be made thereto and components thereof 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 features in the specific embodiments disclosed in the present application can be combined with each other in any way, and the exhaustive description of these combinations is not given in this specification only for the consideration of saving space and resources. Therefore, the present application is not limited to the specific embodiments disclosed herein, but includes those falling within the scope of the claims.

Claims

1. A phase-sensitive optical time domain reflectometer based on a broadband acousto-optic modulator (AOM) for suppressing coherent fading effects, characterized in that: It includes a narrow linewidth laser (1), a first optical fiber coupler (2), a first acousto-optic modulator (3), a second acousto-optic modulator (4), an erbium-doped fiber amplifier (5), a circulator (6), a polarization controller (7), a sensing optical fiber (8), a second optical fiber coupler (9), a photoelectric balanced detector (10), an oscilloscope (11), a computer (12), and an arbitrary waveform generator (13); 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 first acousto-optic modulator (3) and the input end of the polarization controller (7). The optical signal output end of the first acousto-optic modulator (3) is simultaneously connected to the optical signal input end of the second acousto-optic modulator (4). The optical signal output end of the second 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 (8). The optical signal input end of the second optical fiber coupler (9) is simultaneously connected to the output end of the polarization controller (7) and the third optical signal port (6-3) of the circulator (6). The optical signal output end of the second optical fiber coupler (9) is simultaneously connected to the optical signal input end of the photoelectric balanced detector (10). The electrical signal output end of the photoelectric balanced detector (10) is simultaneously connected to the electrical signal input end of the oscilloscope (11). The oscilloscope (11) is simultaneously connected to the computer (12). The microwave signal output ends of the arbitrary waveform generator (13) are respectively and simultaneously connected to the microwave signal loading end of the first acousto-optic modulator (3), the microwave signal loading end of the second acousto-optic modulator (4), and the trigger signal input end of the oscilloscope (11); The continuous light output by the narrow linewidth laser (1) is divided into two upper and lower branches by the first optical fiber coupler (2). The continuous light in the upper branch is modulated into multi-frequency continuous light by the first acousto-optic modulator (3), and is modulated into multi-frequency pulsed light as a detection pulse by the second acousto-optic modulator (4), and at the same time a certain amount of frequency shift is generated. Then, after being amplified in optical power by the erbium-doped fiber amplifier (5), it is injected into the sensing optical fiber (8) through the circulator (6). The multi-frequency modulation of the first acousto-optic modulator (3) and the pulsed modulation of the second acousto-optic modulator (4) are both realized by the arbitrary waveform generator (13). The continuous light in the lower branch is used as the intrinsic light. After the polarization state is adjusted by the polarization controller (7), it is injected into the second optical fiber coupler (9) together with the backward Rayleigh scattered light generated in the sensing optical fiber (8) for beat frequency, and is detected by the photoelectric balanced detector (10). The output photocurrent is collected by the oscilloscope (11) to obtain multi-frequency backward Rayleigh scattered light. The computer (12) is used for data processing. Based on the amplitude size as the judgment basis, the most accurate signal is always selected at any time for phase reconstruction to suppress the phase distortion caused by the coherent fading effect and achieve high-fidelity phase extraction.

2. The phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing coherent fading effect according to claim 1, wherein: The narrow linewidth laser (1) adopts a single-frequency narrow linewidth fiber laser, with an output power of 10 mW and an output wavelength of 1550 nm.

3. The phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing coherent fading effect according to claim 1 or 2, characterized in that: The coupling ratio of the first fiber coupler (2) is 90:10, and the coupling ratio of the second fiber coupler (9) is 50:

50.

4. The phase-sensitive optical time domain reflectometer based on a broadband acou- optic modulator (AOM) for suppressing coherent fading effects according to claim 3, characterized in that: The center frequency of the first acousto-optic modulator (3) is 400 MHz, and the bandwidth is 100 MHz.

5. The phase-sensitive optical time domain reflectometer based on a broadband AOM for suppressing coherent fading effects according to claim 4, characterized in that: The frequency shift of the second acousto-optic modulator (4) is -300 MHz, and the extinction ratio is 50 dB.

6. The phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing coherent fading effect according to claim 4, wherein: The detection bandwidth of the optoelectronic balanced detector (10) is 300 MHz.

7. A working method of the phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing coherent fading effect according to any one of claims 1-6, characterized in that: The continuous light output by the narrow linewidth laser (1) is divided into two upper and lower branches through the first fiber coupler (2); the continuous light in the upper branch is modulated into multi-frequency continuous light by the first acousto-optic modulator (3), and is modulated into multi-frequency pulsed light as a detection pulse by the second 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 fiber (8) through the circulator (6). The multi-frequency modulation of the first acousto-optic modulator (3) and the pulse modulation of the second acousto-optic modulator (4) are both realized by the arbitrary waveform generator (13); the continuous light in the lower branch is used as the intrinsic light. After the polarization state is adjusted by the polarization controller (7), it is injected into the second fiber coupler (9) together with the backward Rayleigh scattered light generated in the sensing fiber (8) for beat frequency, and is detected by the optoelectronic balanced detector (10). The output photocurrent is collected by the oscilloscope (11) to obtain the multi-frequency backward Rayleigh scattered light. The computer (12) is used for data processing. Based on the magnitude as the judgment basis, the most accurate signal is always selected at any moment for phase reconstruction to suppress the phase distortion caused by the coherent fading effect and achieve high-fidelity phase extraction.

8. The working method of the phase-sensitive optical time domain reflectometer based on broadband AOM for suppressing coherent fading effect according to claim 7, characterized in that: The modulation method of the first acousto-optic modulator (3) is as follows: Since the first acousto-optic modulator (3) is a bandwidth AOM with a certain operating bandwidth, when the amplitude provided by the arbitrary waveform generator (13) is A and the initial phase is the frequencies are f1, f2,..., f n of the n microwave signals E1(t), E2(t),..., En(t): The above n microwave signals E1(t), E2(t) …… En(t) are freely programmed on the arbitrary waveform generator (13), and finally synthesized into the microwave signal E(t): E(t) = E1(t) + E2(t) +... + E n (t) (2) Set the single-frequency continuous light output by the narrow linewidth laser (1), with the carrier frequency being f c , when the microwave signal E(t) is loaded on the first acousto-optic modulator (3), the single-frequency continuous light is modulated by the first acousto-optic modulator (3) to a frequency of: f c + f1, f c + f2,... f c + f n of multi-frequency continuous light, thereby realizing multi-frequency modulation; Set the frequency shift of the second acousto-optic modulator (4) to -Δf. Then, the frequency components of the n backward Rayleigh scattering lights with different intensity distributions detected by the phase-sensitive optical time domain reflectometer based on broadband AOM to suppress the coherent fading effect are: f1 - Δf, f2 - Δf,..., f n - Δf, realizing flexible frequency control.

Images