Phi-OTDR (Optical Time Domain Reflectometer) system based on 3*3 coupler double-end detection and phase demodulation method

Abstract

The invention belongs to the technical field of distributed optical fiber sensing, and discloses a double-end detection phi-OTDR (optical time domain reflectometry) system based on a 3 * 3 coupler and a phase demodulation method. An auxiliary signal is constructed through a third output signal of a double-end detection module, and two output signals of the double-end detection module of the 3 * 3 coupler are corrected; a phase demodulation interference term caused by interference in the pulse is eliminated, an I component and a Q component are calculated according to the relation that two paths of output signals have the 2pi / 3 phase difference, and finally phase demodulation is achieved. According to the invention, by introducing the auxiliary optical detection path, the spatial resolution independent of OPD is realized, and only two paths of signals are adopted to construct an orthogonal term in a phase demodulation algorithm, so that high-quality phase demodulation is realized, and the problem of contradiction between the spatial resolution and the phase demodulation quality in a phi-OTDR system combined with an unbalanced interferometer is solved.

Description

Technical Field

[0001] The present invention belongs to the field of distributed optical fiber sensing technology, specifically a 3×3 coupler double-ended detection System and phase demodulation method, solve the problem of combining unbalanced interferometer The contradiction between spatial resolution and phase demodulation quality in the system is solved, and precise positioning and high-quality phase demodulation are achieved. Background Art

[0002] As an emerging distributed fiber optic vibration sensor, it uses narrow linewidth pulsed laser as the sensing light source and injects it into the sensing fiber. By monitoring the changes in the intensity and phase of the coherent Rayleigh Backscattering (RBS) trace, it achieves the positioning of external vibration events and waveform reconstruction. The system has good anti-electromagnetic interference capabilities and real-time monitoring characteristics, and is widely used in seismic wave monitoring, perimeter security, traffic flow monitoring and other fields.

[0003] at present, There are two main types of systems: one based on heterodyne coherent detection and the other based on direct detection. The system uses the frequency-shifted local light and RBS light to beat the frequency to improve the system signal-to-noise ratio. By performing in-phase-quadrature demodulation or Hilbert-Huang transform on the output interference signal, the vibration waveform information can be quantitatively restored. However, this type of system requires precise control of the frequency shift and is sensitive to phase noise and environmental factors. Direct detection The system uses the interference effect between RBS lights for demodulation, which can avoid the above-mentioned coherent detection For direct detection of There are mainly two types of phase demodulation schemes: phase generated carrier and 3×3 coupler. Among them, the 3×3 coupler phase demodulation scheme has received more attention because it does not require additional frequency modulation hardware. The 3×3 coupler phase demodulation scheme is mainly based on two basic interference structures: Michelson Interferometer (MI) or Mach-Zehnder Interferometer (MZI). The vibration information is extracted by detecting the phase of the output signal on the opposite side of the interferometer. We call it a single-ended detection scheme. However, the delayed optical path difference (OPD) introduced by the two unbalanced interference arms of the interferometer is positively correlated with the phase demodulation quality, while the spatial resolution is negatively correlated with the OPD, resulting in a contradiction between the two. At present, some methods have been proposed to improve the quality of phase demodulation combined with interferometers. There are solutions to spatial resolution, such as four-way detection or dual interferometers, but these solutions increase the complexity of system configuration and introduce greater spontaneous emission noise, which will reduce the quality of phase demodulation. In practical applications, in order to accurately recover vibration information, it is necessary to ensure high-quality recovery of the phase waveform and high-precision positioning. Therefore, it is critical to balance these two requirements at the same time. Summary of the invention

[0004] For the existing unbalanced interferometer The present invention provides a method for detecting the phase difference between the spatial resolution and the phase demodulation quality of the system based on a 3×3 coupler double-ended detection. Systematic phase demodulation method to improve the interferometer combined Positioning and phase demodulation performance of the system.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a 3×3 coupler double-ended detection The system includes: a signal modulation module, a 3×3 coupler double-end detection module, a signal acquisition module and a signal processing module;

[0006] The signal modulation module is used to input pulse detection light into the sensing optical fiber, and is also used to output the RBS sensing signal in the sensing optical fiber;

[0007] The 3×3 coupler double-end detection module includes a 3×3 coupler, a first Faraday rotating reflector, a second Faraday rotating reflector, a first avalanche photodetector, a second avalanche photodetector, and a third avalanche photodetector. The first input end of the 3×3 coupler is connected to the RBS sensing signal output end of the signal modulation module, the second input end and the third input end are respectively connected to the first avalanche photodetector and the second avalanche photodetector, the first output end and the second output end are respectively connected to the first Faraday rotating reflector and the second Faraday rotating reflector, and the third output end is connected to the third avalanche photodetector; the output ends of the first avalanche photodetector, the second avalanche photodetector, and the third avalanche photodetector are connected to the signal acquisition module;

[0008] The output end of the signal acquisition module is connected to the signal processing module, and the signal processing module is used to demodulate and obtain the vibration phase according to the signals output by the first avalanche photodetector, the second avalanche photodetector, and the third avalanche photodetector.

[0009] The signal modulation module comprises: a narrow linewidth laser, an acousto-optic modulator, a first erbium-doped fiber amplifier, a first optical circulator, a sensing fiber, a second erbium-doped fiber amplifier, and a filter; the continuous light emitted by the narrow linewidth laser is modulated into a light pulse by the acousto-optic modulator, the light pulse is amplified by the first erbium-doped fiber amplifier, enters the sensing fiber through the first optical circulator, is scattered at a scattering point along the sensing fiber to generate an RBS sensing signal light, returns to the first optical circulator along the sensing fiber, is amplified by the second erbium-doped fiber amplifier, and ASE noise is filtered out by the filter to output an RBS sensing signal.

[0010] The signal modulation module further includes an acousto-optic modulation driver, which is used to drive the acousto-optic modulator to modulate the continuous light emitted by the narrow linewidth laser into light pulses.

[0011] The 3×3 coupler double-end detection module further includes a second optical circulator, which is arranged between the output end of the signal modulation module and the first input end of the 3×3 coupler.

[0012] The signal processing module is used to obtain an auxiliary signal P according to the signal output by the third avalanche photodetector. X,z (t), and the output signals of the first avalanche photodetector and the second avalanche photodetector are corrected according to the auxiliary signal, and then the vibration phase is obtained by demodulating the two corrected signals.

[0013] The distance between the third avalanche photodetector and the 3×3 coupler is equal to the smaller value of the distance between the first avalanche photodetector and the second avalanche photodetector and the 3×3 coupler.

[0014] In addition, the present invention also provides a 3×3 coupler double-ended detection The phase demodulation method of the system comprises the following steps:

[0015] Step S1: Obtain the output signals of the first avalanche photodetector, the second avalanche photodetector, and the third avalanche photodetector, and obtain the auxiliary signal P according to the output signal of the third avalanche photodetector. X,z (t);

[0016] Step S2: Combine the output signals of the first avalanche photodetector and the second avalanche photodetector with the auxiliary signal P X,z (t) is subtracted to obtain the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t);

[0017] Step S3: According to the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t), calculate the I component Iz (t) and Q component Q z (t);

[0018] Step S4: Through the I component I z (t) and Q component Q z (t), calculate the vibration phase φ z (t).

[0019] In step S1, the auxiliary signal P X,z The calculation formula of (t) is:

[0020]

[0021] Among them, P A,z (t) represents the optical power corresponding to the pulse at position z in the sensing fiber, P A,z―OPD (t) represents the optical power corresponding to the pulse at the z-OPD position in the sensing optical fiber, where OPD represents the optical path difference corresponding to the first Faraday rotation mirror and the second Faraday rotation mirror.

[0022] In step S3, the I component I z (t) and Q component Q z The calculation formulas for (t) are:

[0023] I z (t) = -1 / 2P' 2,z (t)―P′ 3,z (t);

[0024]

[0025] In step S4, the vibration phase φ z The calculation formula of (t) is:

[0026]

[0027] Among them, tan ―1 represents the inverse tangent, and unwrap represents the phase unwrapping operation.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The present invention adopts a 3×3 coupler double-end detection structure and introduces a path output after 3×3 coupler splitting as an auxiliary light detection path. The output signal of the auxiliary light detection path avoids OPD interference because it does not enter the MI interferometer. It has an OPD-independent spatial resolution and can achieve high-precision positioning.

[0030] (2) The present invention adopts a two-path phase demodulation algorithm and uses only two signals to construct orthogonal terms for demodulation, thereby suppressing low-frequency noise and phase unwrapping errors and achieving high-quality phase demodulation.

[0031] In summary, the present invention proposes a 3×3 coupler double-ended detection method. The system and phase demodulation method introduce an auxiliary optical detection path to achieve OPD-independent spatial resolution and high-quality phase demodulation, solving the problem of combining with interferometer The problem of contradiction between the system's spatial resolution and phase demodulation quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A 3×3 coupler double-ended detection based on the embodiment of the present invention is provided Schematic diagram of the system structure;

[0033] Figure 2 A flow chart of a 3×3 coupler double-ended detection module and a phase demodulation algorithm used in an embodiment of the present invention;

[0034] Figure 3 A comparison diagram of demodulation phase results between the double-ended detection scheme adopted by the present invention and the traditional single-ended detection scheme;

[0035] Figure 4 A comparison diagram of short-time energy positioning curve results between the double-end detection scheme adopted by the present invention and the traditional single-end detection scheme;

[0036] Figure 1 In the figure: 1 is a narrow linewidth laser, 2 is an acousto-optic modulator, 3 is a first erbium-doped fiber amplifier, 4 is a first optical circulator, 5 is a single-mode optical fiber, 6 is a piezoelectric transducer, 7 is a second erbium-doped fiber amplifier, 8 is a filter, 9 is a second optical circulator, 10 is a 3×3 coupler, 11 is a first Faraday rotator mirror, 12 is a first Faraday rotator mirror, 13 is a first avalanche photodetector, 14 is a second avalanche photodetector, 15 is a third avalanche photodetector, 16 is a data acquisition card, and 17 is an acousto-optic modulation driver. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0038] Embodiment 1

[0039] like Figure 1 As shown, the first embodiment of the present invention provides a 3×3 coupler double-ended detection The system comprises: a signal modulation module, a 3×3 coupler double-end detection module, a signal acquisition module and a signal processing module.

[0040] The signal modulation module is used to input pulse detection light into the sensing optical fiber, and is also used to output the RBS sensing signal in the sensing optical fiber.

[0041] The 3×3 coupler double-end detection module includes a 3×3 coupler 10, a first Faraday rotating reflector 11, a second Faraday rotating reflector 12, a first avalanche photodetector 13, a second avalanche photodetector 14, and a third avalanche photodetector 15. The first input end of the 3×3 coupler 10 is connected to the RBS sensing signal output end of the signal modulation module, the second input end and the third input end are respectively connected to the first avalanche photodetector 13 and the second avalanche photodetector 14, the first output end and the second output end are respectively connected to the first Faraday rotating reflector 11 and the second Faraday rotating reflector 12, and the third output end is connected to the third avalanche photodetector 15; the output ends of the first avalanche photodetector 13, the second avalanche photodetector 14, and the third avalanche photodetector 15 are connected to the signal acquisition module. The first output end and the second output end of the 3×3 coupler 10 are respectively connected to the first Faraday rotating reflector 11 and the second Faraday rotating reflector 12 to form the short arm and the delay arm of the MI interferometer.

[0042] The output end of the signal acquisition module is connected to the signal processing module, and the signal processing module is used to demodulate and obtain the vibration phase according to the signals output by the first avalanche photodetector 13 , the second avalanche photodetector 14 , and the third avalanche photodetector 15 .

[0043] Specifically, Figure 1 As shown, in this embodiment, the signal modulation module includes: a narrow linewidth laser 1, an acousto-optic modulator 2, a first erbium-doped fiber amplifier 3, a first optical circulator 4, a sensing fiber 5, a second erbium-doped fiber amplifier 7, and a filter 8; the continuous light emitted by the narrow linewidth laser 1 is modulated into a light pulse by the acousto-optic modulator 2, the light pulse is amplified by the first erbium-doped fiber amplifier 3, and then enters the sensing fiber 5 through the first optical circulator 4, and the RBS sensing signal light is generated by scattering at the scattering points along the sensing fiber 5, and then returns to the first optical circulator 4 along the sensing fiber 5, and then is amplified by the second erbium-doped fiber amplifier 7, and the filter 8 filters out the ASE noise and outputs the RBS sensing signal.

[0044] Furthermore, if Figure 1As shown, the signal modulation module further includes an acousto-optic modulation driver 17, and the acousto-optic modulation driver 17 is used to drive the acousto-optic modulator 2 to modulate the continuous light emitted by the narrow linewidth laser 1 into light pulses.

[0045] Furthermore, if Figure 1 As shown, the 3×3 coupler double-end detection module further includes a second optical circulator 9 , which is arranged between the output end of the signal modulation module and the first input end of the 3×3 coupler 10 .

[0046] Specifically, in this embodiment, the signal processing module is used to obtain the auxiliary signal P according to the signal output by the third avalanche photodetector 15. X,z (t), and the output signals of the first avalanche photodetector 13 and the second avalanche photodetector 14 are corrected according to the auxiliary signal, and then the vibration phase is obtained by demodulating the two corrected signals.

[0047] Further, in this embodiment, the distance between the third avalanche photodetector 15 and the 3×3 coupler 10 is equal to the smaller value of the distance between the first avalanche photodetector 13 and the second avalanche photodetector 14 and the 3×3 coupler 10. In other words, the optical path of the auxiliary detection optical path corresponding to the third avalanche photodetector 15 is equal to the short arm of the MI interferometer.

[0048] Combine the following Figure 2 The phase demodulation principle of the embodiment of the present invention is introduced.

[0049] The RBS sensing signal output by the signal modulation module enters through the first input end (port a) of the 3×3 coupler 10, and is output from the first output end (port d), the second output end (port e), and the third output end (port f) of the 3×3 coupler 10. The sensing signals output from the first output end and the second output end are respectively reflected by the first Faraday rotating reflector 11 and the second Faraday rotating reflector 12, and then return to the 3×3 coupler 10. After entering the 3×3 coupler 10, the two signals are respectively output from the first input end (port a), the second input end (port b), and the third input end (port c). The signal output from the first input end (port a) will not return to the signal modulation module after passing through the second optical circulator 9. The signals output from the second input end (port b) and the third input end (port c) are respectively received by the first avalanche photodetector 13 and the second avalanche photodetector 14.

[0050] The electric fields of the two output light intensities of ports b and c of the 3×3 coupler 10 can be expressed as:

[0051] E 2,z (t) = 1 / 3·[E z (t)·exp(i·2π / 3)+Ez―OPD (t)·exp(i·2π / 3)]; (1)

[0052] E 3,z (t) = 1 / 3·[E z (t)·exp(i·2π / 3)+E z―OPD (t)·exp(i·4π / 3)]; (2)

[0053] Among them, E z (t) is the RBS electric field at position z in the sensing fiber, E z―OPD (t) is the RBS electric field at the position z-OPD in the sensing optical fiber, where OPD represents the optical path difference corresponding to the first Faraday rotation mirror 11 and the second Faraday rotation mirror 12; their expressions are respectively expressed as:

[0054]

[0055] Among them, S is the total number of scattering points, a i and a j are the Rayleigh scattering amplitudes of the i-th scattering point and the j-th scattering point, respectively, and τ i and τ j is their round trip time. The i-th scattering point and the j-th scattering point come from the scattering volume units corresponding to the short arm and the delayed arm respectively. T is the time width of the pulse, c is the speed of light in vacuum, α is the attenuation coefficient of the optical fiber, and n f is the effective refractive index, ν0 is the optical frequency, τ OPD is the round trip time introduced by OPD, rect[·] is the square wave pulse function, when 0<[·]≤1, rect[·]=1, otherwise rect[·]=0.

[0056] Therefore, by substituting formula (3) and formula (4) into formula (1) and formula (2), we can obtain the signal P received by the first avalanche photodetector 13: 2,z (t) and the signal P received by the second avalanche photodetector 14 3,z (t) can be expressed as:

[0057]

[0058] In the formula, E 2,z and E 3,z are the electric fields of the output light intensities of ports b and c of the 3×3 coupler 10, respectively, where Λ 2,ij and Λ 3,ij Respectively represent the second optical power P 2,z (t) and the third optical power P 3,zThe intensity of the AC term generated by the interference between the RBS light of the i-th and j-th scattering points in (t); γ ij represents the sum of the DC intensities of the Rayleigh scattered light from the i-th scattering point in the scattering volume unit of the short arm of the interferometer and the j-th scattering point in the scattering volume unit of the delay arm of the interferometer, Λ ip ,Λ jq are the AC terms generated by the interference between the RBS lights of the i-th and p-th scattering points, and the RBS lights of the j-th and q-th scattering points, respectively, expressed as:

[0059]

[0060] Among them, φ ip and φ jq are the phase differences between the i-th scattering point and the p-th scattering point, and between the j-th scattering point and the q-th scattering point, respectively. The i-th scattering point and the p-th scattering point come from the scattering volume unit corresponding to the short arm of the interferometer, and the j-th scattering point and the q-th scattering point come from the scattering volume unit corresponding to the delay arm of the interferometer. m,ij is the optical power P of the mth channel m,z The intensity of the AC term generated by the interference between the RBS light of the i-th and j-th scattering points in (t), φ ij is the phase difference between them, and 2π / 3 is the additional phase introduced because the interfering RBS light comes from the straight-through and coupled interferometer arms.

[0061] Assuming that the optical path of the auxiliary detection path is the same as the short arm of the MI interferometer, the optical power P detected by the third avalanche photodetector 15 is A,z (t) can be expressed as:

[0062]

[0063] Among them, E A,z (t) is expressed as:

[0064]

[0065] Construct an auxiliary signal P X,z (t), which is expressed as:

[0066]

[0067] Among them, P A,z (t) represents the optical power at the sensing fiber position z, which is determined by the DC and AC quantities of the interfering electric field at the sensing fiber position z, and P A,z-OPD(t) represents the optical power at the sensing fiber position z-OPD, which is composed of the DC and AC quantities of the electric field interference within the pulse at the z-OPD position.

[0068] According to formulas (5), (6), and (13), the signal P received by the first avalanche photodetector 13 is 2,z (t) and the signal P received by the second avalanche photodetector 14 3,z (t) and the auxiliary signal P X,z After subtracting (t), the phase demodulation interference term γ caused by the intra-pulse interference in the signal can be eliminated. ij ,Λ ij ,Λ jq Component, the first corrected signal P' 2,z (t) and the second correction signal P' 3,z (t) can be expressed as:

[0069]

[0070] Furthermore, using P' 2,z (t) and P' 3,z (t) has a phase difference of 2π / 3, and the I component I can be calculated. z (t) and Q component Q z (t), their expressions are:

[0071] I z (t) = -1 / 2P' 2,z (t)―P′ 3,z (t); (16)

[0072]

[0073] Finally, the vibration phase φ can be extracted by inverse tangent operation and phase unwrapping. z (t), which is calculated as:

[0074]

[0075] Among them, tan ―1 represents the inverse tangent, and unwrap represents the phase unwrapping operation.

[0076] like Figure 1 As shown, in this embodiment, a piezoelectric transducer 6 is placed at the end of the sensing optical fiber 5 to simulate a vibration event, and the demodulation phase of the demodulation principle (double-end detection scheme) proposed by the present invention is compared with the traditional single-end detection scheme. Figure 3As shown, (a) is the time-space waterfall diagram of the demodulation phase of the single-ended detection scheme, (b) is the time-space waterfall diagram of the demodulation phase of the dual-ended detection scheme, (c) is the space-frequency amplitude spectrum of the single-ended detection scheme, and (d) is the space-frequency amplitude spectrum of the dual-ended detection scheme. The comparison results show that within the PZT stretching area, the phase of the dual-ended detection scheme of the present invention remains consistent, while the demodulation phase obtained by the single-ended detection scheme performs poorly. The better consistency of the demodulation phase distribution indicates that the dual-ended detection scheme avoids a large number of false alarms or missed alarms.

[0077] Under the condition of OPD of 50m, the short-time energy positioning curve comparison of the single-end detection scheme and the dual-end detection scheme is obtained, as shown in Figure 4 As shown, (a) is the positioning curve of the single-end detection scheme, and (b) is the positioning curve of the dual-end detection scheme. The comparison results show that the positioning curve of the single-end detection scheme shows an envelope peak with a range of 73m and a peak value of 2366m, indicating that OPD causes the spatial resolution to deteriorate and cannot distinguish two vibration events with an interval less than OPD. The positioning curve of the dual-end detection scheme of the present invention shows two envelope peaks with ranges of 12m and 13m, respectively, and peak values ​​of 2334m and 2346m, respectively, wherein the interval between the two peaks is 12m, indicating that the present invention better realizes the distinction between two vibration events with an interval of 10m, and has a higher spatial resolution.

[0078] Embodiment 2

[0079] A method for detecting a 3×3 coupler double-ended device is provided in the second embodiment of the present invention. The phase demodulation method of the system comprises the following steps:

[0080] Step S1: Obtain the output signals of the first avalanche photodetector (13), the second avalanche photodetector (14), and the third avalanche photodetector (15), and obtain the auxiliary signal P according to the output signal of the third avalanche photodetector (15). X,z (t); In step S1, the auxiliary signal P X,z The calculation formula of (t) is:

[0081]

[0082] Among them, P A,z (t) represents the optical power corresponding to the pulse at position z in the sensing fiber, P A,z―OPD (t) represents the optical power corresponding to the pulse at the z-OPD position in the sensing optical fiber, wherein OPD represents the optical path difference corresponding to the first Faraday rotation mirror (11) and the second Faraday rotation mirror (12).

[0083] Step S2: The output signals of the first avalanche photodetector 13 and the second avalanche photodetector 14 are respectively combined with the auxiliary signal P X,z (t) is subtracted to obtain the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t);

[0084] Step S3: According to the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t), calculate the I component I z (t) and Q component Q z (t); I component I z (t) and Q component Q z The calculation formula of (t) is as follows: (16) to (17).

[0085] Step S4: Through the I component I z (t) and Q component Q z (t), calculate the vibration phase φ z (t). Vibration phase φ z The calculation formula of (t) is (18).

[0086] In summary, the present invention discloses a 3×3 coupler double-ended detection method. The system is composed of a signal modulation module, a 3×3 coupler double-end detection module and a signal acquisition module. By introducing an auxiliary optical detection path and a simple two-path phase demodulation algorithm, the system improves the interferometer Positioning and phase demodulation performance in the system solves the problem of combining interferometer The problem of contradiction between the system's spatial resolution and phase demodulation quality.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A double-ended detection based on 3×3 coupler The system is characterized in that include: Signal modulation module, 3×3 coupler double-ended detection module, signal acquisition module and signal processing module; The signal modulation module is used to input pulse detection light into the sensing optical fiber, and is also used to output the RBS sensing signal in the sensing optical fiber; The 3×3 coupler double-end detection module comprises a 3×3 coupler (10), a first Faraday rotating reflector (11), a second Faraday rotating reflector (12), a first avalanche photodetector (13), a second avalanche photodetector (14), and a third avalanche photodetector (15); the first input end of the 3×3 coupler (10) is connected to the RBS sensing signal output end of the signal modulation module, the second input end and the third input end are respectively connected to the first avalanche photodetector (13) and the second avalanche photodetector (14); the first output end and the second output end are respectively connected to the first Faraday rotating reflector (11) and the second Faraday rotating reflector (12), and the third output end is connected to the third avalanche photodetector (15); the output ends of the first avalanche photodetector (13), the second avalanche photodetector (14), and the third avalanche photodetector (15) are connected to the signal acquisition module; The output end of the signal acquisition module is connected to a signal processing module, and the signal processing module is used to demodulate and obtain a vibration phase according to the signals output by the first avalanche photodetector (13), the second avalanche photodetector (14), and the third avalanche photodetector (15).

2. A 3×3 coupler double-ended detection method according to claim 1 The system is characterized in that The signal modulation module comprises: a narrow linewidth laser (1), an acousto-optic modulator (2), a first erbium-doped fiber amplifier (3), a first optical circulator (4), a sensing fiber (5), a second erbium-doped fiber amplifier (7), and a filter (8); the continuous light emitted by the narrow linewidth laser (1) is modulated into a light pulse by the acousto-optic modulator (2); the light pulse is amplified by the first erbium-doped fiber amplifier (3) and then enters the sensing fiber (5) through the first optical circulator (4); the RBS sensing signal light is generated by scattering at a scattering point along the sensing fiber (5), returns to the first optical circulator (4) along the sensing fiber (5), is amplified by the second erbium-doped fiber amplifier (7), and ASE noise is filtered out by the filter (8) before outputting the RBS sensing signal.

3. A 3×3 coupler double-ended detection method according to claim 2 The system is characterized in that The signal modulation module further comprises an acousto-optic modulation driver (17), wherein the acousto-optic modulation driver (17) is used to drive the acousto-optic modulator (2) to modulate the continuous light emitted by the narrow linewidth laser (1) into light pulses.

4. A 3×3 coupler double-ended detection method according to claim 1 The system is characterized in that The 3×3 coupler double-end detection module further comprises a second optical circulator (9), wherein the second optical circulator (9) is arranged between the output end of the signal modulation module and the first input end of the 3×3 coupler (10).

5. A 3×3 coupler double-ended detection method according to claim 1 The system is characterized in that The signal processing module is used to obtain an auxiliary signal P according to the signal output by the third avalanche photodetector (15). X,z (t), and correct the output signals of the first avalanche photodetector (13) and the second avalanche photodetector (14) according to the auxiliary signal, and then demodulate the two corrected signals to obtain the vibration phase.

6. A 3×3 coupler double-ended detection method according to claim 1 The system is characterized in that The distance between the third avalanche photodetector (15) and the 3×3 coupler (10) is equal to the smaller value of the distance between the first avalanche photodetector (13) and the second avalanche photodetector (14) and the 3×3 coupler (10).

7. A 3×3 coupler double-ended detection device according to any one of claims 1 to 6 The phase demodulation method of the system is characterized in that: The following steps are involved: Step S1: Obtain the output signals of the first avalanche photodetector (13), the second avalanche photodetector (14), and the third avalanche photodetector (15), and obtain the auxiliary signal P according to the output signal of the third avalanche photodetector (15). X,z (t); Step S2: The output signals of the first avalanche photodetector (13) and the second avalanche photodetector (14) are respectively combined with the auxiliary signal P X,z (t) is subtracted to obtain the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t); Step S3: According to the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t), calculate the I component I z (t) and Q component Q z (t); Step S4: Through the I component I z (t) and Q component Q z (t), calculate the vibration phase φ z (t).

8. A 3×3 coupler double-ended detection method according to claim 7 The phase demodulation method of the system is characterized in that: In step S1, the auxiliary signal P X,z The calculation formula of (t) is: Among them, P A,z (t) represents the optical power corresponding to the pulse at position z in the sensing fiber, P A,z―OPD (t) represents the optical power corresponding to the pulse at the z-OPD position in the sensing optical fiber, wherein OPD represents the optical path difference corresponding to the first Faraday rotation mirror (11) and the second Faraday rotation mirror (12).

9. A 3×3 coupler double-ended detection method according to claim 7 The phase demodulation method of the system is characterized in that: In step S3, the I component I z (t) and Q component Q z The calculation formulas for (t) are: I z (t)=―1 / 2P′ 2,z (t)―P′ 3,z (t); 10. A 3×3 coupler double-ended detection method according to claim 7 The phase demodulation method of the system is characterized in that: In step S4, the vibration phase φ z The calculation formula of (t) is: Among them, tan ―1 represents the inverse tangent, and unwrap represents the phase unwrapping operation.

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