Phi-otdr system based on 3*3 coupler double-end detection and phase demodulation method

Abstract

The application belongs to the technical field of distributed optical fiber sensing, and discloses a 3*3 coupler double-end detection phi-OTDR system and a phase demodulation method. The third output signal of the double-end detection module is used to construct an auxiliary signal, the two output signals of the 3*3 coupler double-end detection module are modified, the phase demodulation interference term caused by the interference in the pulse is eliminated, then the two output signals are used to calculate I component and Q component by using the relationship that the two output signals have a 2pi / 3 phase difference, and finally the phase demodulation is realized. The application introduces an auxiliary optical detection path, realizes OPD-independent spatial resolution, and only uses two signals to construct orthogonal terms in the phase demodulation algorithm, realizes high-quality phase demodulation, and solves the problem of the contradiction between spatial resolution and phase demodulation quality in the phi-OTDR system combined with the unbalanced interferometer.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology, specifically a φ-OTDR system based on a 3×3 coupler dual-end detection and a phase demodulation method. It solves the problem of the contradiction between spatial resolution and phase demodulation quality in a φ-OTDR system combined with an unbalanced interferometer, and achieves accurate positioning and high-quality phase demodulation. Background Technology

[0002] φ-OTDR, as an emerging distributed fiber optic vibration sensor, uses a narrow-linewidth pulsed laser as the sensing light source and injects it into the sensing fiber. It locates and reconstructs waveforms of external vibration events by monitoring changes in the intensity and phase of coherent Rayleigh backscattering (RBS) traces. φ-OTDR systems possess excellent electromagnetic interference resistance and real-time monitoring capabilities, and are widely used in fields such as seismic wave monitoring, perimeter security, and traffic flow monitoring.

[0003] Currently, there are two main types of φ-OTDR systems: one based on heterodyne coherent detection and the other based on direct detection. The φ-OTDR system using heterodyne coherent detection improves the system's signal-to-noise ratio by using frequency-shifted local light and RBS light for beat frequency modulation. By performing in-phase-quadrature demodulation or Hilbert-Huang transform on the output interference signal, it can quantitatively recover the vibration waveform information. However, this type of system requires precise control of the frequency shift and is sensitive to phase noise and environmental factors. The direct detection φ-OTDR system, on the other hand, utilizes the interference effect between RBS lights for demodulation, avoiding the shortcomings of the coherent detection φ-OTDR. For the direct detection φ-OTDR system, there are mainly two 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 interferometric structures: the Michelson interferometer (MI) or the Mach-Zehnder interferometer (MZI). Vibration information is extracted by detecting the phase of the output signal on the opposite side of the interferometer; this is called the single-end detection scheme. However, the optical path difference (OPD) introduced by the two unbalanced interferometer arms is positively correlated with the phase demodulation quality, while spatial resolution is negatively correlated with OPD, creating a contradiction. Currently, some solutions have been proposed to improve the spatial resolution of φ-OTDR combined with interferometers, such as using four-way detection or dual interferometers. However, these solutions increase system configuration complexity and introduce greater spontaneous emission noise, which reduces the quality of phase demodulation. In practical applications, to accurately recover vibration information, both high-quality recovery of the phase waveform and high-precision positioning are required; therefore, balancing these two requirements is crucial. Summary of the Invention

[0004] To address the contradiction between spatial resolution and phase demodulation quality in existing φ-OTDR systems combined with unbalanced interferometers, this invention provides a phase demodulation method for φ-OTDR systems based on dual-end detection using a 3×3 coupler, thereby improving the positioning and phase demodulation performance of φ-OTDR systems combined with interferometers.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a φ-OTDR system based on 3×3 coupler dual-end detection, comprising: a signal modulation module, a 3×3 coupler dual-end detection module, a signal acquisition module, and a signal processing module;

[0006] The signal modulation module is used to input pulse probe light into the sensing fiber and also to output the RBS sensing signal in the sensing fiber.

[0007] The 3×3 coupler dual-ended detection module includes a 3×3 coupler, a first Faraday rotation mirror, a second Faraday rotation mirror, a first avalanche photodetector, a second avalanche photodetector, and a third avalanche photodetector. The first input terminal of the 3×3 coupler is connected to the RBS sensing signal output terminal of the signal modulation module. The second and third input terminals are respectively connected to the first and second avalanche photodetectors. The first and second output terminals are respectively connected to the first and second Faraday rotation mirrors. The third output terminal is connected to the third avalanche photodetector. The output terminals of the first, second, and third avalanche photodetectors are connected to the signal acquisition module.

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

[0009] The signal modulation module includes: 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. Continuous light emitted by the narrow-linewidth laser is modulated into optical pulses by the acousto-optic modulator. The optical pulses are amplified by the first erbium-doped fiber amplifier and then enter the sensing fiber through the first optical circulator. The light is scattered at scattering points along the sensing fiber to generate RBS sensing signal light. After returning to the first optical circulator along the sensing fiber, the light is amplified by the second erbium-doped fiber amplifier and filtered to remove ASE noise before outputting the RBS sensing signal.

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

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

[0012] The signal processing module is used to obtain the auxiliary signal P based on 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 of the distances from the first and second avalanche photodetectors to the 3×3 coupler.

[0014] Furthermore, this invention also provides a phase demodulation method for a φ-OTDR system based on a 3×3 coupler dual-end detection, comprising the following steps:

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

[0016] Step S2: Connect the output signals of the first avalanche photodetector and the second avalanche photodetector with the auxiliary signal P respectively. X,z Subtracting (t) yields the first corrected signal P'. 2,z (t) and the second correction signal P' 3,z (t);

[0017] Step S3: Based on the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t), the I component is calculated. and Q component ;

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

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

[0020] ;

[0021] in, This represents the optical power corresponding to the pulse at position z within the sensing fiber. This represents the optical power corresponding to the pulse at position z-OPD within the sensing fiber, where OPD represents the optical path difference between the first and second Faraday rotating mirrors.

[0022] In step S3, the I component and Q component The calculation formulas are as follows:

[0023] ;

[0024] ;

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

[0026] ;

[0027] in, Indicates reverse cut, This indicates the phase unwinding operation.

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

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

[0030] (2) The present invention adopts a two-way phase demodulation algorithm, which uses only two signals to construct orthogonal terms for demodulation, suppresses low-frequency noise and phase dewinding errors, and achieves high-quality phase demodulation.

[0031] In summary, this invention proposes a φ-OTDR system and phase demodulation method based on a 3×3 coupler dual-end probe, introduces an auxiliary optical probe path, achieves OPD-independent spatial resolution and high-quality phase demodulation, and solves the problem of the contradiction between spatial resolution and phase demodulation quality in φ-OTDR systems combined with interferometers. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of a φ-OTDR system based on dual-end detection of a 3×3 coupler provided in Embodiment 1 of the present invention;

[0033] Figure 2 The flowchart shows the 3×3 coupler dual-end detection module and the phase demodulation algorithm used in the embodiments of the present invention.

[0034] Figure 3 This is a comparison diagram of the demodulation phase results of the dual-end detection scheme used in this invention and the traditional single-end detection scheme;

[0035] Figure 4 This is a comparison chart of the short-time energy localization curves of the dual-end detection scheme used in this invention and the traditional single-end detection scheme;

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

[0037] To make the objectives, 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 some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] like Figure 1 As shown, Embodiment 1 of the present invention provides a φ-OTDR system based on a 3×3 coupler dual-end detection, including: a signal modulation module, a 3×3 coupler dual-end detection module, a signal acquisition module, and a signal processing module.

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

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

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

[0043] Specifically, such as 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 optical pulses by the acousto-optic modulator 2. The optical pulses are amplified by the first erbium-doped fiber amplifier 3 and then enter the sensing fiber 5 through the first optical circulator 4. The light is scattered at the scattering point along the sensing fiber 5 to generate RBS sensing signal light. After returning to the first optical circulator 4 along the sensing fiber 5, the light is amplified by the second erbium-doped fiber amplifier 7 and filtered by the filter 8 to remove ASE noise before outputting the RBS sensing signal.

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

[0045] Furthermore, such as Figure 1 As shown, the 3×3 coupler dual-end detection module also includes a second optical circulator 9, which is disposed 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 an auxiliary signal P based on 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 obtain the vibration phase by demodulating the two corrected signals.

[0047] Furthermore, in this embodiment, the distance between the third avalanche photodetector 15 and the 3×3 coupler 10 is equal to the smaller of the distances from the first avalanche photodetector 13 and the second avalanche photodetector 14 to the 3×3 coupler 10. That is, the optical path length of the auxiliary detection optical path corresponding to the third avalanche photodetector 15 is equal to the short arm of the MI interferometer.

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

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

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

[0051] ; (1)

[0052] ; (2)

[0053] in, Let z be the RBS electric field at position z in the sensing fiber. Let RBS electric field be the electric field at position z-OPD in the sensing fiber, where OPD represents the optical path difference between the first Faraday rotator mirror 11 and the second Faraday rotator mirror 12; its expressions are as follows:

[0054] ; (3)

[0055] ; (4)

[0056] Where S is the total number of scattering points, a i and a j Let be the Rayleigh scattering amplitudes of the i-th and j-th scattering points, respectively, and τ be the scattering amplitudes of the scattering points. i and τ j Let be their round-trip time. Here, the i-th and j-th scattering points originate from the scattering volume units corresponding to the short arm and delay arm, respectively. T is the pulse width, c is the speed of light in vacuum, α is the attenuation coefficient of the optical fiber, and n... f ν0 is the effective refractive index, ν0 is the optical frequency, OPD The round-trip time introduced by OPD is rect[·], which is the square wave pulse function. When 0 < [·] ≤ 1, rect[·] = 1, otherwise rect[·] = 0.

[0057] Therefore, substituting formulas (3) and (4) into formulas (1) and (2) respectively, 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:

[0058] ; (5)

[0059] ; (6)

[0060] In the formula, and Let be the electric fields of the output light intensities at ports b and c of the 3×3 coupler 10, respectively. and These represent the optical power P of the second channel, respectively. 2,z (t) and the third optical power P 3,z The intensity of the AC term generated by the interference between the RBS light at the i-th and j-th scattering points in (t); ϒ ij Λ represents the sum of the DC terms of the Rayleigh scattered light from the i-th scattering point within the scattering volume unit of the short arm of the interferometer and the j-th scattering point within the scattering volume unit of the delay arm of the interferometer. ip ,Λ jq Let the intensities of the AC terms generated by the interference between the RBS light from the i-th and p-th scattering points, and the RBS light from the j-th and q-th scattering points, be expressed as:

[0061] (7)

[0062] ； (8)

[0063] (9)

[0064] ,m=2,3; (10)

[0065] Where, φ ip and φ jq Λ represents the phase difference between the i-th and p-th scattering points, and between the j-th and q-th scattering points, respectively. The i-th and p-th scattering points originate from the scattering volume unit corresponding to the short arm of the interferometer, while the j-th and q-th scattering points originate from the scattering volume unit corresponding to the delay arm of the interferometer. m,ij The power P of the m-th optical path m,z The intensity of the AC term generated by the interference between the RBS light at the i-th and j-th scattering points in (t), φ ijLet be the phase difference between them, and 2π / 3 be the additional phase introduced due to the RBS light that produces the interference coming from the through and coupled interference arms.

[0066] Assuming the optical path length 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 represented as:

[0067] ; (11)

[0068] in, Represented as:

[0069] ； (12)

[0070] Construct an auxiliary signal P X,z (t), which is represented as:

[0071] ; (13)

[0072] Among them, P A,z (t) represents the optical power at position z in the sensing fiber, which is determined by the DC and AC quantities of the interfering electric field at position z in the sensing fiber, while P A,z-OPD (t) represents the optical power at the position z-OPD of the sensing fiber, which consists of the DC and AC quantities of the electric field interference within the pulse at the z-OPD position.

[0073] According to formulas (5), (6), and (13), the signal P received by the first avalanche photodetector 13 will be... 2,z (t) and the signal P received by the second avalanche photodetector 14 3,z (t) and auxiliary signal P X,z Subtracting (t) can eliminate the phase demodulation interference term caused by intra-pulse interference in the signal. , , The component then yields the first corrected signal P'. 2,z (t) and the second correction signal P' 3,z (t) can be expressed as:

[0074] ;(14)

[0075] ; (15)

[0076] Furthermore, using P' 2,z (t) and P' 3,z The phase difference between (t) and (t) is 2π / 3, and the I component can be calculated. and Q component Their expressions are as follows:

[0077] ; (16)

[0078] ; (17)

[0079] Finally, the vibration phase φ can be extracted through arctangent calculation and phase unwinding. z (t), its calculation formula is:

[0080] ; (18)

[0081] in, Indicates reverse cut, This indicates the phase unwinding operation.

[0082] like Figure 1 As shown, in this embodiment, a piezoelectric transducer 6 is placed at the tail end of the sensing fiber 5 to simulate a vibration event, and the demodulation phase of the demodulation principle (dual-end detection scheme) proposed in this invention is compared with that of the traditional single-end detection scheme. Figure 3 As shown, (a) is the spatiotemporal waterfall plot of the demodulated phase of the single-ended detection scheme, (b) is the spatiotemporal waterfall plot of the demodulated phase of the dual-ended detection scheme, (c) is the spatial-frequency amplitude spectrum of the single-ended detection scheme, and (d) is the spatial-frequency amplitude spectrum of the dual-ended detection scheme. The comparison results show that within the PZT stretching region, the phase of the dual-ended detection scheme of the present invention remains consistent, while the demodulated phase obtained by the single-ended detection scheme is poor. The better consistency of the demodulated phase distribution indicates that the dual-ended detection scheme avoids a large number of false alarms or missed alarms.

[0083] A comparison of short-time energy localization curves for single-end and dual-end detection schemes was obtained under the condition of OPD of 50 m, as shown in the figure. 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 73 m and a peak value of 2366 m, indicating that OPD leads to the deterioration of spatial resolution and makes it impossible to distinguish two vibration events with an interval smaller than OPD. In contrast, the positioning curve of the dual-end detection scheme of the present invention shows two envelope peaks with ranges of 12 m and 13 m, and peak values ​​of 2334 m and 2346 m, respectively. The interval between the two peaks is 12 m, indicating that the present invention can better distinguish two vibration events with an interval of 10 m, and has higher spatial resolution.

[0084] Example 2

[0085] Embodiment 2 of the present invention provides a phase demodulation method for a φ-OTDR system based on a 3×3 coupler dual-end probe, comprising the following steps:

[0086] 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 based on the output signal of the third avalanche photodetector (15). X,z (t); In step S1, the auxiliary signal P X,z The formula for calculating (t) is:

[0087] ; (19)

[0088] in, This represents the optical power corresponding to the pulse at position z within the sensing fiber. The value represents the optical power corresponding to the pulse at the z-OPD position in the sensing fiber, where OPD represents the optical path difference between the first Faraday rotating mirror (11) and the second Faraday rotating mirror (12).

[0089] Step S2: The output signals of the first avalanche photodetector 13 and the second avalanche photodetector 14 are respectively compared with the auxiliary signal P. X,z Subtracting (t) yields the first corrected signal P'. 2,z (t) and the second correction signal P' 3,z (t);

[0090] Step S3: Based on the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t), the I component is calculated. and Q component ; I component and Q component The calculation formulas are (16) to (17).

[0091] Step S4: Through the I component and Q component Calculate the vibration phase φ z (t). Vibration phase φ z The formula for calculating (t) is (18).

[0092] In summary, this invention discloses a φ-OTDR system and phase demodulation method based on a 3×3 coupler dual-end detection. The system consists of three parts: a signal modulation module, a 3×3 coupler dual-end detection module, and a signal acquisition module. By introducing an auxiliary optical detection path and a simple two-path phase demodulation algorithm, the positioning and phase demodulation performance of the φ-OTDR system combined with an interferometer is improved, resolving the contradiction between spatial resolution and phase demodulation quality in the φ-OTDR system combined with an interferometer.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions 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 φ-OTDR system based on dual-end detection using a 3×3 coupler, 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 probe light into the sensing fiber and also to output the RBS sensing signal in the sensing fiber. The 3×3 coupler dual-end detection module includes a 3×3 coupler (10), a first Faraday rotating mirror (11), a second Faraday rotating mirror (12), a first avalanche photodetector (13), a second avalanche photodetector (14), and a third avalanche photodetector (15). The first input terminal of the 3×3 coupler (10) is connected to the RBS sensing signal output terminal of the signal modulation module. The second and third input terminals are respectively connected to the first avalanche photodetector (13) and the second avalanche photodetector (14). The first and second output terminals are respectively connected to the first Faraday rotating mirror (11) and the second Faraday rotating mirror (12). The third output terminal is connected to the third avalanche photodetector (15). The output terminals 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 of the signal acquisition module is connected to the signal processing module. The signal processing module is used to demodulate the vibration phase based on the signals output by the first avalanche photodetector (13), the second avalanche photodetector (14), and the third avalanche photodetector (15). The signal processing module is also used to obtain the auxiliary signal P based on 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 obtain the vibration phase by demodulating the two corrected signals.

2. The φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 1, characterized in that, 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 light pulses by the acousto-optic modulator (2). The light pulses are amplified by the first erbium-doped fiber amplifier (3) and then enter the sensing fiber (5) through the first optical circulator (4). The RBS sensing signal light is generated by scattering at the scattering point along the sensing fiber (5). After returning to the first optical circulator (4) along the sensing fiber (5), it is amplified by the second erbium-doped fiber amplifier (7), and the ASE noise is filtered out by the filter (8) before the RBS sensing signal is output.

3. The φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 2, characterized in that, The signal modulation module further includes an acousto-optic modulation driver (17), which is used to drive the acousto-optic modulator (2) to modulate the continuous light emitted by the narrow linewidth laser (1) into optical pulses.

4. The φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 1, characterized in that, The 3×3 coupler dual-end detection module also includes a second optical circulator (9), which is located between the output end of the signal modulation module and the first input end of the 3×3 coupler (10).

5. The φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 1, characterized in that, The distance between the third avalanche photodetector (15) and the 3×3 coupler (10) is equal to the smaller of the distances between the first avalanche photodetector (13) and the second avalanche photodetector (14) and the 3×3 coupler (10).

6. A phase demodulation method for a φ-OTDR system based on dual-end detection of a 3×3 coupler according to any one of claims 1 to 5, characterized in that, Includes the following steps: 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 based on 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 compared with the auxiliary signal P. X,z Subtracting (t) yields the first corrected signal P'. 2,z (t) and the second correction signal P' 3,z (t); Step S3: Based on the first correction signal P' 2,z (t) and the second correction signal P' 3,z (t), the I component is calculated. and Q component ; Step S4: Through the I component and Q component Calculate the vibration phase φ z (t).

7. The phase demodulation method for a φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 6, characterized in that, In step S1, the auxiliary signal P X,z The formula for calculating (t) is: ； in, This represents the optical power corresponding to the pulse at position z within the sensing fiber. The value represents the optical power corresponding to the pulse at the z-OPD position in the sensing fiber, where OPD represents the optical path difference between the first Faraday rotating mirror (11) and the second Faraday rotating mirror (12).

8. The phase demodulation method for a φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 6, characterized in that, In step S3, the I component and Q component The calculation formulas are as follows: ； 。 9. A phase demodulation method for a φ-OTDR system based on dual-end detection of a 3×3 coupler according to claim 6, characterized in that, In step S4, the vibration phase φ z The formula for calculating (t) is: ； in, Indicates reverse cut, This indicates the phase unwinding operation.

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