A distributed acoustic sensing device based on PDH frequency locking and a demodulation method

Abstract

The application discloses a kind of distributed acoustic wave sensing device and demodulation method based on PDH frequency locking, including narrow linewidth laser, first optical fiber coupler, heterodyne phase-sensitive optical time domain reflectometer and frequency stabilization feedback module;Frequency stabilization feedback module includes frequency stabilization modem module, reference cavity and server;Frequency stabilization modem module includes electro-optic modulator or acousto-optic modulator, photodetector, multiplier and low-pass filter;First road light is modulated after passing through electro-optic modulator or acousto-optic modulator, enters reference cavity and resonates, and resonant signal is reflected into photodetector by cavity, and the signal output of photodetector is demodulated after passing through multiplier and low-pass filter, as feedback input server and is proportionally integrated, and feedback output is modulated interface of narrow linewidth laser, and the frequency of laser is compensated, so that laser is locked on reference cavity;The application can simultaneously suppress the linewidth and phase drift of laser by frequency stabilization and homologous modem.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and more particularly to a distributed acoustic wave sensing device and demodulation method based on PDH frequency locking. Background Art

[0002] Distributed fiber optic sensing technology uses backscattered Rayleigh light in optical fibers to detect the acoustic field along the fiber. Optical fibers serve as both sensors and transmission methods, enabling ultra-high-density, wide-bandwidth, and long-distance vibration measurement. It is widely used in perimeter security, oil exploration, pipeline monitoring, and other fields, demonstrating enormous application value and prospects.

[0003] Distributed acoustic wave sensing is implemented based on phase-sensitive optical time-domain reflectometry, and its implementation methods include direct detection and heterodyne detection. In the heterodyne detection structure, the amplitude of the weak backscattered Rayleigh light is amplified by the stronger local oscillator light, and the signal-to-noise ratio is significantly higher than that of the direct detection structure, making it the mainstream implementation structure. However, under this structure, the linewidth and phase drift of the laser will have a significant impact on the detection results.

[0004] Since the backscattered Rayleigh light of the heterodyne detection structure needs to be coherent with the local oscillator light, the intensity of the coherent light is affected by the product of the laser linewidth and the flight time of the pulse light in the optical fiber. In long-distance optical fiber sensing applications, the linewidth significantly affects the system signal-to-noise ratio.

[0005] In addition, during the demodulation process, the low-frequency phase drift of the laser will be superimposed on the output signal phase, generating large low-frequency noise, affecting the low-frequency response of the system, and seriously affecting the detection performance of the equipment in the fields of underground structure inversion and hydroacoustic detection.

[0006] At present, in order to solve the problem of low-frequency phase drift of lasers, people have proposed two methods: using quadratic differential data processing and correcting frequency drift with the Mach-Zehnder interferometer structure. However, the true strain value of the optical fiber cannot be obtained after quadratic differentiation, which is not conducive to subsequent data analysis in fields such as geophysics. The signal-to-noise ratio of the Mach-Zehnder structure phase-sensitive optical time-domain reflectometer is much lower than that of the heterodyne detection structure phase-sensitive optical time-domain reflectometer, and it cannot take into account both low-frequency noise and system signal-to-noise ratio.

[0007] Therefore, how to overcome the laser linewidth and drift problems of heterodyne detection-type phase-sensitive optical time-domain reflectometers and provide a distributed acoustic wave sensing device that can simultaneously control the laser linewidth and phase drift, thereby improving the system's signal-to-noise ratio and low-frequency response, is an urgent problem that technicians in this field need to solve. Summary of the Invention

[0008] In view of this, the present invention provides a distributed acoustic wave sensing device and demodulation method based on PDH frequency locking to solve some of the technical problems mentioned in the background technology.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A distributed acoustic wave sensing device based on PDH frequency locking, comprising: a narrow linewidth laser, a first optical fiber coupler, a heterodyne phase-sensitive optical time domain reflectometer, and a frequency stabilization feedback module;

[0011] The light output from the narrow linewidth laser is divided into two paths through the first coupler, wherein the first path of light is input to the frequency stabilization feedback module, and the second path of light is used as the input light of the heterodyne phase-sensitive optical time domain reflectometer;

[0012] The frequency stabilization feedback module includes a frequency stabilization modulation and demodulation module, a reference cavity and a servo; the frequency stabilization modulation and demodulation module includes an electro-optic modulator or an acousto-optic modulator, a photodetector, a multiplier and a low-pass filter;

[0013] The first light path is modulated by an electro-optic modulator or an acousto-optic modulator and then enters the reference cavity for resonance. The resonant signal is reflected or transmitted through the cavity and then enters the photodetector. The output signal of the photodetector is demodulated by a multiplier and a low-pass filter and then used as feedback input to the servo for proportional integral processing. The feedback is then output to the modulation interface of the narrow linewidth laser to perform frequency compensation on the laser, thereby locking the laser to the reference cavity.

[0014] Preferably, the heterodyne phase-sensitive optical time domain reflectometer comprises a second optical fiber coupler, an acousto-optic modulator, an optical amplifier, a circulator, a sensing optical fiber, a third optical fiber coupler, a balanced detector, and a detection modulation and demodulation module;

[0015] The second optical input passes through the second fiber coupler, which splits the input light into two paths. One path is chopped and frequency modulated by an acousto-optic modulator. The modulated laser is amplified by an optical amplifier and injected into the sensing fiber through a circulator. The back-scattered Rayleigh light in the sensing fiber returns through the circulator and is input into the third fiber coupler. The third fiber coupler receives the back-scattered Rayleigh light and the other light from the second fiber coupler to form interference light. The interference light is converted into an electrical signal by a balanced detector and sent to the detection modulation and demodulation module. The detection modulation and demodulation module generates a radio frequency signal to drive the acousto-optic modulator, demodulates the electrical signal output by the balanced detector, and reconstructs the phase signal obtained after phase calculation of the demodulated signal into the acoustic field signal detected by the optical fiber.

[0016] Preferably, the modulation signal used by the detection modulation and demodulation module to drive the acousto-optic modulator and the local oscillator signal used to demodulate the output signal of the balanced detector are generated in the same clock domain, and the phase of each group of pulse modulation and demodulation signals remains unchanged.

[0017] Preferably, the modulation signal of the detection modulation and demodulation module is generated by cutting off the continuous local oscillator signal using a radio frequency switch or by using direct digital synthesis technology; the demodulation is generated by mixing and filtering the local oscillator signal with the balanced detector output signal, or by using a high-speed ADC to sample the signal, which is achieved by a digital down-conversion method or a direct intermediate frequency sampling method.

[0018] Preferably, the frequency stabilization feedback module also includes a signal source and a phase shifter. The signal source is used to generate a modulation signal in the radio frequency phase modulation of the electro-optical modulator or the acousto-optic modulator, and to generate a demodulation signal in the multiplier phase modulation, and adjust the phase of the signal to be demodulated through the phase shifter.

[0019] A distributed acoustic wave sensing demodulation method based on PDH frequency locking comprises the following steps:

[0020] S1. Use the first coupler to split the continuous laser output of the laser into two paths, one for frequency stabilization and the other as probe light;

[0021] S2. Use an electro-optic modulator or an acousto-optic modulator to perform radio frequency phase modulation on the frequency-stabilized laser. The modulated laser signal enters a reference cavity, adjusts the laser frequency, and resonates with the reference cavity. The resonant signal is reflected or transmitted through the cavity and enters a photodetector.

[0022] S3. The photodetector output signal is phase-demodulated by a multiplier and a low-pass filter, then fed back to a servo for proportional-integral processing. This feedback is then fed back to the modulation interface of the narrow-linewidth laser, where it is frequency-compensated and locked to the reference cavity.

[0023] S4. The detection light signal is split into two paths by a second fiber coupler, one path is sent into the optical fiber for detection, and the other path is used as the local oscillator light;

[0024] S5. The probe light is modulated by an acousto-optic modulator and then sent into the optical fiber through a circulator. Part of the Rayleigh scattered light in the sensing fiber is returned to the fiber incident end through the circulator and input into a third fiber coupler. The backscattered Rayleigh light coheres with the local oscillator light in the coupler and outputs two sets of light fields. These are converted into current signals by a balanced detector, which outputs the real part of the coherent signal current to obtain the final output current signal:

[0025] S6. Demodulate the disturbance signal of the output current signal using a digital orthogonal demodulation method, calculate the phase value of the backscattered Rayleigh light, and reproduce the vibration signal.

[0026] Preferably, the modulation signal driving the acousto-optic modulator and the local oscillator signal used to demodulate the output signal of the balanced detector are generated in the same clock domain, and the phase of each group of pulse modulation and demodulation signals remains unchanged.

[0027] Preferably, in step S2, the light field of the laser modulated by the electro-optic modulator or the acousto-optic modulator is:

[0028] ;

[0029] Where E0 is the intensity of the light field after beam splitting, ω is the laser angular frequency, β is the modulation depth, Ω is the modulation frequency, i is an imaginary number, and t is time;

[0030] The light field reflected back from the cavity is:

[0031] ;

[0032] Where F(·) is the reflectivity of the corresponding frequency, J is the Bessel function, J0 is the zero-order Bessel curve, and J1 is the first-order Bessel curve;

[0033] In step S3, the photodetector output signal is:

[0034] ;

[0035] Among them, P c is the carrier power, P s is the sideband power, F(·) * is the conjugate of the corresponding frequency reflectivity, 2Ωterms is the interference term between the two sidebands and can be ignored, and Im is an imaginary number;

[0036] The signal is demodulated with Ω as the local oscillator frequency and low-pass filtered to obtain the error signal:

[0037] .

[0038] Preferably, in step S5, the light field after the acousto-optic modulator modulates the detection light is:

[0039] ;

[0040] Where E0 is the intensity of the probe light, Δω is the modulation frequency;

[0041] The light field of part of the Rayleigh scattered light in the sensing fiber returning to the fiber incident end through the circulator is:

[0042] ;

[0043] The two sets of light fields output by the third fiber coupler are:

[0044] ;

[0045] ;

[0046] in, For the local oscillator light;

[0047] The current signal finally output by the balanced detector is:

[0048] ;

[0049] Among them, ϕ is the angle between the light field signals of the two groups of light, is the disturbance signal to be demodulated, To detect the intensity of the optical signal, is the local oscillation light intensity, is the initial phase of the local oscillator light.

[0050] Preferably, the specific content of step S6 is:

[0051] The signal output by the balanced detector is sampled by the acquisition card and can be abbreviated as:

[0052] ;

[0053] in, is the signal strength after digitization, is the phase to be demodulated, n is the discrete sampling point;

[0054] Construct a pair of co-directional and quadrature reference signals with a frequency of Δω in the digital domain:

[0055] ;

[0056] ;

[0057] in, and For the constructed co-directional-orthogonal signals;

[0058] Multiply the sampled signal with the co-directional and orthogonal signals respectively. The time domain multiplication corresponds to the frequency domain convolution. The signal is moved to the baseband and double frequency position in the frequency domain, that is:

[0059] ;

[0060] ;

[0061] in, and is the same direction orthogonal signal after mixing;

[0062] After filtering out high-frequency terms through low-pass filtering, the phase value of the backscattered Rayleigh light can be obtained by solving the inverse tangent function:

[0063] ;

[0064] ;

[0065] ;

[0066] in, and is the filtered orthogonal signal.

[0067] It can be seen from the above technical solution that, compared with the prior art, the present invention discloses a distributed acoustic wave sensing device and demodulation method based on PDH frequency locking, which locks the system's laser to a reference cavity, such as a Fabry-Perot resonant cavity (FP cavity), to obtain extremely high frequency stability and at the same time narrow the output linewidth of the laser; by coordinating with synchronous modulation and demodulation, phase drift is suppressed and an extremely high signal-to-noise ratio is obtained; since the phase drift of the laser and the drift introduced by modulation and demodulation are eliminated, the measurement results of the present invention can be used to measure the true strain value of the optical fiber rather than the relative size. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0069] Figure 1 A schematic diagram of a distributed acoustic wave sensing device based on PDH frequency locking provided by the present invention;

[0070] Figure 2 A schematic diagram of a distributed acoustic wave sensing demodulation method based on PDH frequency locking provided by the present invention. DETAILED DESCRIPTION

[0071] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0072] The embodiment of the present invention discloses a distributed acoustic wave sensing device based on PDH frequency locking, such as Figure 1 , including: a narrow linewidth laser, a first fiber coupler, a heterodyne phase-sensitive optical time domain reflectometer and a frequency stabilization feedback module;

[0073] The light output from the narrow linewidth laser is divided into two paths through the first coupler, wherein the first path of light is input to the frequency stabilization feedback module, and the second path of light is used as the input light of the heterodyne phase-sensitive optical time domain reflectometer;

[0074] The frequency stabilization feedback module includes a frequency stabilization modulation and demodulation module, a reference cavity and a servo; the frequency stabilization modulation and demodulation module includes an electro-optic modulator or an acousto-optic modulator, a photodetector, a multiplier and a low-pass filter;

[0075] The first light path is modulated by an electro-optic modulator or an acousto-optic modulator and then enters the reference cavity for resonance. The resonant signal is reflected or transmitted through the cavity and then enters the photodetector. The output signal of the photodetector is demodulated by a multiplier and a low-pass filter and then used as feedback input to the servo for proportional integral processing. The feedback is then output to the modulation interface of the narrow linewidth laser to perform frequency compensation on the laser, thereby locking the laser to the reference cavity.

[0076] In this embodiment, a narrow linewidth laser is used to output monochromatic DC laser. The laser has an external modulation interface, and the intensity and phase control of the laser signal are achieved through piezoelectric modulation, magneto-optical modulation, acousto-optic modulation, current modulation and other methods; preferably, a distributed feedback laser (DFB laser) based on current modulation is selected.

[0077] The frequency stabilization modulation and demodulation module further includes a polarization beam splitter prism and a quarter wave plate; the reference cavity includes a free-space Fabry-Perot cavity, a fiber Fabry-Perot cavity, a fiber ring cavity, or an optical microcavity;

[0078] The laser signal modulated by the electro-optic modulator or acousto-optic modulator enters the reference cavity through a polarization beam splitter prism and a quarter-wave plate. After resonance occurs, the cavity reflected light is returned to the photodetector through the cooperation of the quarter-wave plate and the polarization beam splitter prism.

[0079] To further implement the above technical solution, a heterodyne phase-sensitive optical time domain reflectometer includes a second fiber coupler, an acousto-optic modulator, an optical amplifier, a circulator, a sensing fiber, a third fiber coupler, a balanced detector, and a detection modulation and demodulation module;

[0080] The second optical input passes through the second fiber coupler, which splits the input light into two paths. One path is chopped and frequency modulated by an acousto-optic modulator. The modulated laser is amplified by an optical amplifier and injected into the sensing fiber through a circulator. The back-scattered Rayleigh light in the sensing fiber returns through the circulator and is input into the third fiber coupler. The third fiber coupler receives the back-scattered Rayleigh light and the other light from the second fiber coupler to form interference light. The interference light is converted into an electrical signal by a balanced detector and sent to the detection modulation and demodulation module. The detection modulation and demodulation module generates a radio frequency signal to drive the acousto-optic modulator, demodulates the electrical signal output by the balanced detector, and reconstructs the phase signal obtained after phase calculation of the demodulated signal into the acoustic field signal detected by the optical fiber.

[0081] To further implement the above technical solution, the modulation signal used by the detection modulation and demodulation module to drive the acousto-optic modulator and the local oscillator signal used to demodulate the output signal of the balanced detector are generated in the same clock domain, and the phase of each set of pulse modulation and demodulation signals remains unchanged.

[0082] In order to further implement the above technical solution, the modulation signal of the detection modulation and demodulation module is generated by cutting off the continuous local oscillator signal using an RF switch or using direct digital synthesis technology; the demodulation is generated by mixing and filtering the local oscillator signal with the balanced detector output signal, or by using a high-speed ADC to sample the signal, which is achieved by a digital down-conversion method or a direct intermediate frequency sampling method.

[0083] In order to further implement the above technical solution, the frequency stabilization feedback module also includes a signal source and a phase shifter. The signal source is used to generate a modulation signal in the radio frequency phase modulation of the electro-optical modulator or the acousto-optic modulator, and to generate a demodulation signal in the multiplier phase modulation, and adjust the phase of the signal to be demodulated through the phase shifter.

[0084] A distributed acoustic wave sensing demodulation method based on PDH frequency locking, such as Figure 2 , including the following steps:

[0085] S1. Use the first coupler to split the continuous laser output of the laser into two paths, one for frequency stabilization and the other as probe light;

[0086] S2. Use an electro-optic modulator or an acousto-optic modulator to perform radio frequency phase modulation on the frequency-stabilized laser. The modulated laser signal enters a reference cavity, adjusts the laser frequency, and resonates with the reference cavity. The resonant signal is reflected from the cavity and enters a photodetector.

[0087] S3. The photodetector output signal is phase-demodulated by a multiplier and a low-pass filter, then fed back to a servo for proportional-integral processing. This feedback is then fed back to the modulation interface of the narrow-linewidth laser, where it is frequency-compensated and locked to the reference cavity.

[0088] S4. The detection light signal is split into two paths by a second fiber coupler, one path is sent into the optical fiber for detection, and the other path is used as the local oscillator light;

[0089] S5. The probe light is modulated by an acousto-optic modulator and then sent into the optical fiber through a circulator. Part of the Rayleigh scattered light in the sensing fiber is returned to the fiber incident end through the circulator and input into a third fiber coupler. The backscattered Rayleigh light coheres with the local oscillator light in the coupler and outputs two sets of light fields. These are converted into current signals by a balanced detector, which outputs the real part of the coherent signal current to obtain the final output current signal:

[0090] S6. Demodulate the disturbance signal of the output current signal using a digital orthogonal demodulation method, calculate the phase value of the backscattered Rayleigh light, and reproduce the vibration signal.

[0091] In this embodiment,

[0092] To further implement the above technical solution, the modulation signal driving the acousto-optic modulator and the local oscillator signal used to demodulate the output signal of the balanced detector are generated in the same clock domain, and the phase of each set of pulse modulation and demodulation signals remains unchanged.

[0093] In order to further implement the above technical solution, in step S2,

[0094] The light field of a single-frequency laser is:

[0095] ;

[0096] The light field of the laser modulated by the electro-optic modulator is:

[0097] ;

[0098] Where E0 is the intensity of the light field after beam splitting, ω is the laser angular frequency, β is the modulation depth, Ω is the modulation frequency, i is an imaginary number, and t is time;

[0099] The light field reflected back from the cavity is:

[0100] ;

[0101] Where F(·) is the reflectivity of the corresponding frequency, J is the Bessel function, J0 is the zero-order Bessel curve, and J1 is the first-order Bessel curve;

[0102] In step S3, the photodetector output signal is:

[0103] ;

[0104] Among them, P c is the carrier power, P s is the sideband power, F(·) * is the conjugate of the corresponding frequency reflectivity, 2Ωterms is the interference term between the two sidebands and can be ignored, and Im is an imaginary number;

[0105] When the carrier and the cavity resonate, the sidebands are almost completely reflected. , which can be ignored at this time The signal is demodulated with Ω as the local oscillator frequency and low-pass filtered to obtain the error signal:

[0106] .

[0107] In this embodiment, when the laser frequency is consistent with the resonant frequency of a reference cavity (such as a Fabry-Perot resonant cavity), the error signal passes through zero. When the laser frequency deviates from the resonant frequency, the demodulated signal starts to fluctuate from zero. Therefore, this error signal can be used to feedback adjust the laser frequency, generally using a PID algorithm for adjustment.

[0108] In order to further implement the above technical solution, in step S5, the light field of the laser before modulation is: ;

[0109] The light field after the acousto-optic modulator modulates the detection light is:

[0110] ;

[0111] Where E0 is the intensity of the probe light, Δω is the modulation frequency;

[0112] After the probe light is sent into the optical fiber, due to the inhomogeneity of the optical fiber's refractive index, part of the Rayleigh scattered light in the sensing fiber returns to the optical fiber incident end through the circulator. The light field can be approximated as:

[0113] ;

[0114] The backscattered Rayleigh light is coherent with the local oscillator light in the coupler, and the light fields of the two groups of light output by the third fiber coupler are:

[0115] ;

[0116] ;

[0117] in, For the local oscillator light;

[0118] The optical signal is converted into a current signal by a balanced detector. The detector output signal is the real part of the coherent signal, and the output current is:

[0119] ;

[0120] ;

[0121] in, is the conjugate of the local oscillator light, It is the conjugate of the light field of the part of Rayleigh scattered light in the sensing fiber that returns to the incident end of the fiber through the circulator;

[0122] The current signal of the balanced detector finally output by subtracting the two outputs is:

[0123] ;

[0124] Among them, ϕ is the angle between the light field signals of the two groups of light, is the disturbance signal to be demodulated, To detect the intensity of the optical signal, is the local oscillation light intensity, is the initial phase of the local oscillator light.

[0125] In order to further implement the above technical solution, the specific content of step S6 is:

[0126] The signal output by the balanced detector is sampled by the acquisition card and can be abbreviated as:

[0127] ;

[0128] in, is the signal strength after digitization, is the phase to be demodulated, n is the discrete sampling point;

[0129] Construct a pair of co-directional and quadrature reference signals with a frequency of Δω in the digital domain:

[0130] ;

[0131] ;

[0132] in, and For the constructed co-directional-orthogonal signals;

[0133] Multiply the sampled signal with the co-directional and orthogonal signals respectively. The time domain multiplication corresponds to the frequency domain convolution. The signal is moved to the baseband and double frequency position in the frequency domain, that is:

[0134] ;

[0135] ;

[0136] in, and is the same direction orthogonal signal after mixing;

[0137] After filtering out high-frequency terms through low-pass filtering, the phase value of the backscattered Rayleigh light can be obtained by solving the inverse tangent function:

[0138] ;

[0139] ;

[0140] ;

[0141] in, and is the filtered orthogonal signal in the same direction;

[0142] From the above derivation, it can be seen that when there is a deviation between the constructed in-phase and orthogonal reference signal frequency and the detector output signal, the demodulated signal will be offset by the difference between the two frequencies, affecting the final detection result. Therefore, the system's modulation and demodulation systems need to be implemented in the same clock domain.

[0143] As an optimization scheme of the phase-sensitive optical time domain reflectometer based on heterodyne detection, the present invention has a much better signal-to-noise ratio than other distributed optical fiber sensing schemes. For the heterodyne detection phase-sensitive optical time domain reflectometer, the system signal-to-noise ratio is , where Δω is the laser line width, τ d is the flight time of light in the optical fiber, corresponding to the detection distance. For a 40-kilometer detection optical fiber, the signal-to-noise ratio of the system of the present invention is more than 10 times higher than that of a phase-sensitive optical time-domain reflectometer system built with a 1kHz narrow-linewidth laser. By suppressing the laser drift to <30MHz@10h, the system's low-frequency response can reach 10mHz.

[0144] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0145] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A distributed acoustic wave sensing device based on PDH frequency locking, characterized in that: include: Narrow linewidth laser, first fiber coupler, heterodyne phase-sensitive optical time domain reflectometer and frequency stabilization feedback module; The light output from the narrow linewidth laser is divided into two paths through the first coupler, wherein the first path of light is input to the frequency stabilization feedback module, and the second path of light is used as the input light of the heterodyne phase-sensitive optical time domain reflectometer; The frequency stabilization feedback module includes a frequency stabilization modulation and demodulation module, a reference cavity and a servo; the frequency stabilization modulation and demodulation module includes an electro-optic modulator or an acousto-optic modulator, a photodetector, a multiplier and a low-pass filter; The first light path is modulated by an electro-optic modulator or an acousto-optic modulator and then enters the reference cavity for resonance. The resonant signal is reflected or transmitted through the cavity and then enters the photodetector. The output signal of the photodetector is demodulated by a multiplier and a low-pass filter and then used as feedback input to the servo for proportional integral processing. The feedback is then output to the modulation interface of the narrow linewidth laser to perform frequency compensation on the laser, thereby locking the laser to the reference cavity.

2. A distributed acoustic wave sensing device based on PDH frequency locking according to claim 1, characterized in that: The heterodyne phase-sensitive optical time domain reflectometer includes a second optical fiber coupler, an acousto-optic modulator, an optical amplifier, a circulator, a sensing optical fiber, a third optical fiber coupler, a balanced detector, and a detection modulation and demodulation module; The second optical input is split into two paths through a second fiber coupler, one of which is chopped and frequency modulated by an acousto-optic modulator. The modulated laser is amplified by an optical amplifier and injected into the sensing fiber through a circulator. The backscattered Rayleigh light in the sensing fiber returns through the circulator and is input into a third fiber coupler. The third fiber coupler receives the backscattered Rayleigh light and the other light from the second fiber coupler to form interference light. The interference light is converted into an electrical signal by a balanced detector and sent to the detection modulation and demodulation module. The detection modulation and demodulation module generates a radio frequency signal to drive the acousto-optic modulator, demodulates the electrical signal output by the balanced detector, and reconstructs the phase signal obtained after phase calculation of the demodulated signal into the acoustic field signal detected by the optical fiber.

3. A distributed acoustic wave sensing device based on PDH frequency locking according to claim 2, characterized in that: The modulation signal driven by the detection modulation and demodulation module and the local oscillator signal used to demodulate the output signal of the balanced detector are generated in the same clock domain, and the phase of each group of pulse modulation and demodulation signals remains unchanged.

4. A distributed acoustic wave sensing device based on PDH frequency locking according to claim 2, characterized in that: The modulation signal of the detection modulation and demodulation module is generated by cutting off the continuous local oscillator signal using an RF switch or by using direct digital synthesis technology; the demodulation is generated by mixing and filtering the local oscillator signal with the output signal of the balanced detector, or by sampling the signal using a high-speed ADC, and is achieved by a digital down-conversion method or a direct intermediate frequency sampling method.

5. A distributed acoustic wave sensing device based on PDH frequency locking according to claim 1, characterized in that: The frequency stabilization feedback module also includes a signal source and a phase shifter. The signal source is used to generate a modulation signal in the radio frequency phase modulation of the electro-optical modulator or the acousto-optic modulator, and to generate a demodulation signal in the multiplier phase modulation, and to adjust the phase of the demodulated signal through the phase shifter.

6. A distributed acoustic wave sensing demodulation method based on PDH frequency locking, characterized in that: The following steps are involved: S1. Use a first coupler to split the continuous laser light output by the narrow-linewidth laser into two paths, one for frequency stabilization and the other as probe light; S2. Use an electro-optic modulator or an acousto-optic modulator to perform radio frequency phase modulation on the frequency-stabilized laser. The modulated laser signal enters a reference cavity, adjusts the frequency of the narrow-linewidth laser, and resonates with the reference cavity. The resonant signal is reflected or transmitted through the cavity and enters a photodetector. S3. The photodetector output signal is phase-demodulated by a multiplier and a low-pass filter, then fed back to a servo for proportional-integral processing. This feedback is then fed back to the modulation interface of the narrow-linewidth laser, where it is frequency-compensated and locked to the reference cavity. S4. The detection light signal is split into two paths by a second fiber coupler, one path is sent into the optical fiber for detection, and the other path is used as the local oscillator light; S5. The probe light is modulated by an acousto-optic modulator and then fed into an optical fiber via a circulator. A portion of the Rayleigh scattered light within the sensing fiber is returned to the fiber input end via the circulator and fed into a third fiber coupler. The backscattered Rayleigh light coheres with the local oscillator light in the coupler, generating two sets of light fields. These are then converted into current signals by a balanced detector, which outputs the real part of the coherent signal current to produce the final output current signal. S6. Demodulate the disturbance signal of the output current signal using a digital orthogonal demodulation method, calculate the phase value of the backscattered Rayleigh light, and reproduce the vibration signal.

7. The distributed acoustic wave sensing demodulation method based on PDH frequency locking according to claim 6, characterized in that: The modulation signal driving the acousto-optic modulator and the local oscillator signal used to demodulate the output signal of the balanced detector are generated in the same clock domain, and the phase of each group of pulse modulation and demodulation signals remains unchanged.

8. The distributed acoustic wave sensing demodulation method based on PDH frequency locking according to claim 6, characterized in that: In step S2, the light field of the laser modulated by the electro-optic modulator is: E=E0e i(ωt+βsinΩt) Where E0 is the intensity of the light field after beam splitting, ω is the laser angular frequency, β is the modulation depth, Ω is the modulation frequency, i is an imaginary number, and t is time; The light field reflected back from the cavity is: E r =E0[F(ω)J0(β)e i(ωt) +F(ω+Ω)J1e i(ω+Ω)t -F(ω-Ω)J1e i(ω-Ω)t ] Where F(·) is the reflectivity of the corresponding frequency, J is the Bessel function, J0 is the zero-order Bessel curve, and J1 is the first-order Bessel curve; In step S3, the photodetector output signal is: Among them, P c is the carrier power, P s is the sideband power, F(·) * is the conjugate of the corresponding frequency reflectivity, 2Ωterms is the interference term between the two sidebands, and Im is an imaginary number; The signal is demodulated with Ω as the local oscillator frequency and low-pass filtered to obtain the error signal:

9. The distributed acoustic wave sensing demodulation method based on PDH frequency locking according to claim 8, characterized in that: In step S5, the light field after the acousto-optic modulator modulates the detection light is: E=E0e i(ω+△ω)t Where E0 is the intensity of the probe light, Δω is the modulation frequency; The light field of part of the Rayleigh scattered light in the sensing fiber returning to the fiber incident end through the circulator is: The two sets of light fields output by the third fiber coupler are: E out1 (t)=E r (t)+E lo (t) E out2 (t)=E r (t)-E lo (t) Among them, E lo (t) is the local oscillator light; The current signal finally output by the balanced detector is: Among them, φ is the angle between the light field signals of the two groups of light, is the disturbance signal to be demodulated, I r To detect the intensity of the optical signal, I lo is the local oscillation light intensity, is the initial phase of the local oscillator light.

10. The distributed acoustic wave sensing demodulation method based on PDH frequency locking according to claim 9, characterized in that: The specific content of step S6 is: The signal output by the balanced detector is sampled by the acquisition card and can be abbreviated as: Among them, A[n] is the signal strength after digitization, is the phase to be demodulated, n is the discrete sampling point; Construct a pair of co-directional and quadrature reference signals with a frequency of Δω in the digital domain: Q0[n]=sin[Δωn] I0[n]=cos[Δωn] Among them, Q0[n] and I0[n] are constructed co-directional-orthogonal signals; Multiply the sampled signal with the co-directional and orthogonal signals respectively. The time domain multiplication corresponds to the frequency domain convolution. The signal is moved to the baseband and double frequency position in the frequency domain, that is: Among them, Q1[n] and I1[n] are the same-direction orthogonal signals after mixing; After filtering out high-frequency terms through low-pass filtering, the phase value of the backscattered Rayleigh light can be obtained by solving the inverse tangent function: Among them, Q2[n] and I2[n] are filtered orthogonal signals in the same direction.

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