A fiber optic vibration sensing device operating in multiple modes in time and frequency domains

Abstract

The invention discloses a fiber vibration sensing device operating in time-frequency domain multi-mode, comprising a time-frequency domain modulation control module connected in a specific manner, a first polarization-maintaining narrow linewidth laser, a second polarization-maintaining narrow linewidth laser, a first 1×2 polarization-maintaining fiber coupler, a second 1×2 polarization-maintaining fiber coupler, a first acousto-optic switch, a first erbium-doped fiber amplifier, a first wavelength division multiplexer, an optical fiber circulator, a second wavelength division multiplexer, a first polarization diversity coherent detection module, a signal processing component, a third 1×2 polarization-maintaining fiber coupler, a fourth 1×2 polarization-maintaining fiber coupler, a second acousto-optic switch, a second erbium-doped fiber amplifier, a third 1×2 polarization-maintaining fiber coupler, a fourth 1×2 polarization-maintaining fiber coupler, a second acousto-optic switch, a second erbium-doped fiber amplifier, a second polarization diversity coherent detection module, a first sub-interferometer, and a second sub-interferometer. The device uses the same set of hardware to realize the φ-OTDR working mode or the OFDR working mode, and has high comprehensive performance.

Description

Technical Field

[0001] The present invention relates to distributed optical fiber sensing technology, in particular to an optical fiber sensing device suitable for ultra-long distance optical fiber vibration detection and supporting distributed optical fiber acoustic sensing, specifically an optical fiber vibration sensing device operating in multiple modes in time-frequency domain. Background Art

[0002] Typical distributed fiber optic vibration sensors include phase-sensitive optical time domain reflectometer (φ-OTDR) and optical frequency domain reflectometer (OFDR). Among them, φ-OTDR emits highly coherent pulsed laser and works by direct detection or heterodyne detection. It has the advantages of high sensitivity in vibration signal detection, support for voiceprint recognition and non-contact vibration signal spatial positioning. However, it also has the problems of limited detection dynamic range and short detection distance when distributed fiber optic amplification is not used. OFDR emits highly coherent continuous laser or frequency-modulated pulsed laser with linear frequency modulation of triangular wave or sawtooth wave and works by heterodyne detection. It has the advantages of large detection dynamic range, long detection distance, high resolution and high cascade amplification signal-to-noise ratio. However, it also has the problems of complex phase demodulation technology and difficulty in voiceprint recognition. Currently, there is no distributed fiber optic vibration sensing device that can simultaneously meet the requirements of long detection distance, high cascade amplification signal-to-noise ratio and support for voiceprint recognition. Summary of the invention

[0003] The purpose of the present invention is to provide a fiber optic vibration sensing device that works in multiple modes in the time-frequency domain in view of the shortcomings of the short detection distance of φ-OTDR and the lack of support for voiceprint recognition by OFDR. This device can support both the φ-OTDR working mode and the OFDR working mode through mode switching. This device can achieve both an ultra-long single-ended detection distance and support voiceprint recognition sensing detection by mode switching. This device uses the same set of hardware to achieve the φ-OTDR working mode or the OFDR working mode, and has high comprehensive performance.

[0004] The technical solution for achieving the purpose of the present invention is:

[0005] An optical fiber vibration sensing device operating in multiple modes in time and frequency domains, comprising:

[0006] A time-frequency domain modulation control module and a first polarization-maintaining narrow linewidth laser λ1 and a second polarization-maintaining narrow linewidth laser λ2 connected to the time-frequency domain modulation control module;

[0007] It also includes a first 1×2 polarization-maintaining fiber coupler A1, a second 1×2 polarization-maintaining fiber coupler A2, a first acousto-optic switch AOM-1, a first erbium-doped fiber amplifier EDFA-1, a first wavelength division multiplexer WDM-1, a fiber circulator CIR, a second wavelength division multiplexer WDM-2, a first polarization diversity coherent detection module PDI-1, and a signal processing component, to which a first polarization-maintaining narrow linewidth laser λ1 is sequentially connected, wherein the fiber circulator CIR is connected to a sensing optical cable to be tested, the first polarization diversity coherent detection module PDI-1 is connected to the second 1×2 polarization-maintaining fiber coupler A2, the first acousto-optic switch AOM-1 is also connected to a time-frequency domain modulation control module, and the signal processing component includes a data acquisition card and a computer, wherein the data acquisition card is used to amplify and convert the analog signal output by the photodetector into a digital signal, and transmit it to the computer for demodulation operation;

[0008] It also includes a third 1×2 polarization-maintaining fiber coupler B1, a fourth 1×2 polarization-maintaining fiber coupler B2, a second acousto-optic switch AOM-2, and a second erbium-doped fiber amplifier EDFA-2, to which the second polarization-maintaining narrow-linewidth laser λ2 is sequentially connected, wherein the second erbium-doped fiber amplifier EDFA-2 is sequentially connected to the first wavelength division multiplexer WDM-1 and the second polarization diversity coherent detection module PDI-2, the second polarization diversity coherent detection module PDI-2 is connected to the fourth 1×2 polarization-maintaining fiber coupler B2, and the second acousto-optic switch AOM-2 is also connected to the time-frequency domain modulation control module;

[0009] It also includes a first sub-interferometer OFI-1 connected to the first polarization-maintaining narrow linewidth laser λ1 and the time-frequency domain modulation control module;

[0010] The invention also includes a second interferometer OFI-2 connected to the second polarization-maintaining narrow-linewidth laser λ2 and the time-frequency domain modulation control module.

[0011] The time-frequency domain modulation control module is provided with a signal processing chip and a first photoelectric sampling circuit 1, a second photoelectric sampling circuit 2, a first digital synthesis arbitrary waveform generating circuit DDS-1 and a second digital synthesis arbitrary waveform generating circuit DDS-2 connected to the signal processing chip. The signal processing chip outputs two TTL level pulse control signals, wherein the first photoelectric sampling circuit 1 is connected to the output end of the first sub-interferometer OFI-1, the second photoelectric sampling circuit 2 is connected to the output end of the second sub-interferometer OFI-2, the first digital synthesis arbitrary waveform generating circuit DDS-1 is connected to the first polarization-maintaining narrow The first polarization-maintaining narrow linewidth laser λ1 is connected to the first polarization-maintaining narrow linewidth laser λ2, the second digital synthesis arbitrary waveform generating circuit DDS-2 is connected to the second polarization-maintaining narrow linewidth laser λ2, the time-frequency domain modulation control module forms an independent closed-loop feedback control system with the first polarization-maintaining narrow linewidth laser λ1 and the first secondary interferometer OFI-1, the signal processing chip of the time-frequency domain modulation control module controls the first digital synthesis arbitrary waveform generating circuit DDS-1 to output a triangular wave frequency modulation signal, drives the first polarization-maintaining narrow linewidth laser λ1 to perform triangular wave frequency modulation, and the first polarization-maintaining narrow linewidth laser λ1 is connected to the first secondary interferometer OFI-1 through the first 1×2 polarization-maintaining fiber coupler A1. The first sub-interferometer OFI-1 outputs a detection signal to the first photoelectric sampling circuit 1, and the signal processing chip demodulates and analyzes the frequency modulation linearity of the first polarization-maintaining narrow linewidth laser λ1, and feedback controls the first digital synthesis arbitrary waveform generation circuit DDS-1 to correct the triangle wave frequency modulation signal, thereby completing the triangle wave linear frequency modulation negative feedback control; the time-frequency domain modulation control module, the second polarization-maintaining narrow linewidth laser λ2, and the second sub-interferometer OFI-2 form another independent closed-loop feedback control system, and the signal processing chip of the time-frequency domain modulation control module controls the second digital synthesis arbitrary waveform generation circuit DDS-1 to correct the triangle wave frequency modulation signal. The arbitrary waveform generating circuit DDS-2 outputs a triangular wave frequency modulation signal to drive the second polarization-maintaining narrow linewidth laser λ2 to perform triangular wave frequency modulation. The second polarization-maintaining narrow linewidth laser λ2 is connected to the second sub-interferometer OFI-2 via the third 1×2 polarization-maintaining fiber coupler B1. The second sub-interferometer OFI-2 outputs a detection signal to the second photoelectric sampling circuit 2. The signal processing chip demodulates and analyzes the frequency modulation linearity of the second polarization-maintaining narrow linewidth laser λ2, and feedback controls the second digital synthesis arbitrary waveform generating circuit DDS-2 to correct the triangular wave frequency modulation signal, thereby completing the triangular wave linear frequency modulation negative feedback control;The time-frequency domain modulation control module also controls the first polarization-maintaining narrow linewidth laser λ1 and / or the second polarization-maintaining narrow linewidth laser λ2 to work in a frequency-stabilizing mode, and the time-frequency domain modulation control module outputs a first TTL pulse control signal to control the first acousto-optic switch AOM-1 to modulate the laser output of the first polarization-maintaining narrow linewidth laser λ1 to directly continuously transmit and output or form a pulsed laser output, and the time-frequency domain modulation control module outputs a second TTL pulse control signal to control the second acousto-optic switch AOM-2 to modulate the laser output of the second polarization-maintaining narrow linewidth laser λ2 to directly continuously transmit and output or form a pulsed laser output, and the signal processing chip is any type of chip that supports logic operations, such as FPGA, ARM, DSP, or CPU. ;

[0012] The operating wavelengths of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 differ by at least 0.8 nm, and the frequency interval is greater than 100 GHz. The output laser polarization states of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 are both linearly polarized, and the pigtails are both polarization-maintaining fibers.

[0013] The first wavelength division multiplexer WDM-1 and the second wavelength division multiplexer WDM-2 are both provided with three optical fiber ports, namely, a common end, a transmission end and a reflection end, wherein the wavelength of the transmission end is consistent with the wavelength of the first polarization-maintaining narrow linewidth laser λ1, and the reflection end is used to connect the optical signal of the second polarization-maintaining narrow linewidth laser λ2; or the wavelength of the transmission end is consistent with the wavelength of the second polarization-maintaining narrow linewidth laser λ2, and the reflection end is used to connect the optical signal of the first polarization-maintaining narrow linewidth laser λ1.

[0014] The first interferometer OFI-1 and the second interferometer OFI-2 are both fiber interferometers, and are used to monitor and measure the frequency modulation linearity or laser center frequency drift of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2, respectively.

[0015] The first polarization diversity detection module PDI-1 and the second polarization diversity detection module PDI-2 are each composed of a 1×2 polarization-maintaining fiber coupler with a splitting ratio of 50:50, a polarization beam splitter, two 2×2 polarization-maintaining fiber couplers with a splitting ratio of 50:50, and two balanced photodetectors, wherein:

[0016] The components of the first polarization diversity detection module PDI-1 are connected as follows: the local oscillator light is connected to the input end of the fifth 1×2 polarization-maintaining fiber coupler A3, the output port 1 of the fifth 1×2 polarization-maintaining fiber coupler A3 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A4, the output port 2 of the fifth 1×2 polarization-maintaining fiber coupler A3 is connected to the input port 1 of the second 2×2 polarization-maintaining fiber coupler A5, the echo signal with a wavelength of λ1 of the sensor optical cable to be tested is connected to the input end of the first polarization beam splitter PBS-1, the output port 1 of the first polarization beam splitter PBS-1 is connected to the input port 2 of the first 2×2 polarization-maintaining fiber coupler A4, the output port 2 of the first polarization beam splitter PBS-1 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A5, and the output port 2 of the first polarization beam splitter PBS-1 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A5. The output end 2 of the polarization beam splitter PBS-1 is connected to the input port 2 of the second 2×2 polarization-maintaining fiber coupler A5, the output port 1 and the output port 2 of the first 2×2 polarization-maintaining fiber coupler A4 are respectively connected to the two input ends of the first balanced photodetector BPD-1, the output electrical signal S1 is connected to the signal processing component, the output port 1 and the output port 2 of the second 2×2 polarization-maintaining fiber coupler A5 are respectively connected to the two input ends of the second balanced photodetector BPD-2, the output electrical signal P1 is connected to the signal processing component, the electrical signal S1 is added to the electrical signal P1, and the sum is the coherent detection signal that is not affected by the change of the polarization state of the optical fiber to be measured;

[0017] The components of the second polarization diversity detection module PDI-2 are connected as follows: the local oscillator light is connected to the input end of the sixth 1×2 polarization-maintaining fiber coupler B3, the output port 1 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the third 2×2 polarization-maintaining fiber coupler B4, the output port 2 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the fourth 2×2 polarization-maintaining fiber coupler B5, the echo signal with a wavelength of λ2 of the sensor optical cable to be tested is connected to the input end of the second polarization beam splitter PBS-2, the output port 1 of the second polarization beam splitter PBS-2 is connected to the input port 2 of the third 2×2 polarization-maintaining fiber coupler B4, the output port 2 of the second polarization beam splitter PBS-2 is connected to the input port 1 of the third 2×2 polarization-maintaining fiber coupler B5, and the output port 2 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the fourth 2×2 polarization-maintaining fiber coupler B5. The output port 2 of the polarization beam splitter PBS-2 is connected to the input port 2 of the fourth 2×2 polarization-maintaining fiber coupler B5; the output port 1 and the output port 2 of the third 2×2 polarization-maintaining fiber coupler B4 are respectively connected to the two input ports of the third balanced photodetector BPD-3, and the output electrical signal S2 is connected to the signal processing component; the output port 1 and the output port 2 of the fourth 2×2 polarization-maintaining fiber coupler B5 are respectively connected to the two input ports of the fourth balanced photodetector BPD-4, and the output electrical signal P2 is connected to the signal processing component, and the electrical signal S2 is added to the electrical signal P2, and the sum is the coherent detection signal that is not affected by the change of the polarization state of the optical fiber to be measured.

[0018] The second 1×2 polarization-maintaining fiber coupler A2, the common end of the first wavelength division multiplexer WDM-1 and the corresponding input end of the first polarization-maintaining narrow linewidth laser λ1, the fiber circulator CIR, the common end of the second wavelength division multiplexer WDM-2 and the corresponding output end of the first polarization-maintaining narrow linewidth laser λ1 and the first polarization diversity coherent detection module PDI-1 together constitute the first main fiber interferometer OFI-3.

[0019] The fourth 1×2 polarization-maintaining fiber coupler B2, the common end of the first wavelength division multiplexer WDM-1 and the corresponding input end of the second polarization-maintaining narrow linewidth laser λ2, the fiber circulator CIR, the common end of the second wavelength division multiplexer WDM-2 and the corresponding output end of the second polarization-maintaining narrow linewidth laser λ2, and the second polarization diversity coherent detection module PDI-2 together constitute the second main fiber interferometer OFI-4.

[0020] The above-mentioned optical fiber vibration sensing device has three working modes:

[0021] 1) Time-frequency domain long-distance detection mode: The time-frequency domain modulation control module controls the first polarization-maintaining narrow-linewidth laser λ1 to work in the triangle wave continuous frequency modulation mode and the first acousto-optic switch AOM-1 to work in the continuous light transmission state, and the first main fiber interferometer OFI-3 performs detection to form the OFDR frequency domain detection mode; the time-frequency domain modulation control module controls the second polarization-maintaining narrow-linewidth laser λ2 to work in the stable frequency mode and the second acousto-optic switch AOM-2 to work in the pulse modulation state, and the second main fiber interferometer OFI-4 performs detection to form the φ-OTDR time domain detection mode; the OFDR frequency domain detection mode has a large dynamic range of 40dB, which is conducive to long-distance fiber vibration detection. The φ-OTDR time domain detection mode can support distributed fiber acoustic sensing demodulation, and can realize the spatial positioning and tracking of vibration sources. The time-frequency domain long-distance detection mode can have the advantages of both.

[0022] 2) Dual-wave time-domain enhanced sampling detection mode: The time-frequency domain modulation control module controls the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 to work in the stable frequency mode, and controls the first acousto-optic switch AOM-1 and the second acousto-optic switch AOM-2 to work in the pulse modulation state with the same pulse frequency. There is a delay difference of half a cycle width between the two wavelength laser pulses emitted in the same pulse period, and then they are injected into the detection optical cable together. The sensing signal of the first polarization-maintaining narrow linewidth laser λ1 is detected by the first main fiber interferometer OFI-3, and the sensing signal of the second polarization-maintaining narrow linewidth laser λ2 is detected by the second main fiber interferometer OFI-4, forming a φ-OTDR time-domain dual-pulse detection mode. The dual-wave time-domain enhanced sampling detection mode is equivalent to breaking through the limitation of the pulse period of the sensor optical cable to be tested, doubling the pulse frequency, that is, doubling the sampling frequency of distributed optical fiber acoustic sensing, which is beneficial to improving the demodulation frequency range of distributed optical fiber acoustic sensing;

[0023] 3) Dual-wave frequency domain quasi-continuous pulse cross-relay detection mode: The time-frequency domain modulation control module controls the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 to work in the triangle wave continuous frequency modulation mode, and controls the first acousto-optic switch AOM-1 and the second acousto-optic switch AOM-2 to work in the same pulse frequency and synchronously emitted millisecond-level quasi-continuous pulse modulation state. The sensor signal of the first polarization-maintaining narrow linewidth laser λ1 and the sensor signal of the second polarization-maintaining narrow linewidth laser λ2 after wavelength division are directly photoelectrically detected and demodulated by the signal processing component, forming OFDR dual-wave frequency domain pulse detection. Mode, among which, the dual-wave frequency domain quasi-continuous pulse detection mode is specially used for disturbance monitoring scenarios with relay submarine optical cables or land-based relay transmission optical cables. In this scenario, multiple fiber interferometers are connected in series behind each downstream optical fiber relay amplifier, and the output sensor signals of each adjacent fiber interferometer are cross-filtered at transmission wavelengths λ1 and λ2, and then coupled to the uplink optical fiber and relayed amplified and transmitted back. The laser pulse width of wavelengths λ1 and λ2 is equal to or slightly less than twice the round-trip delay of a single-segment relay transmission optical cable. The dual-wave frequency domain quasi-continuous pulse cross-relay detection mode can realize real-time vibration monitoring of thousands of kilometers of optical cables.

[0024] Compared with the prior art, the present technical solution adopts a multi-mode combination of time-domain φ-OTDR and frequency-domain OFDR, which can not only realize distributed optical fiber acoustic sensing recognition to improve pattern recognition capability, but also realize OFDR large dynamic range detection to improve detection distance, and can also realize acoustic sensing sampling frequency twice as high as that of ordinary φ-OTDR equipment, and can realize ultra-long distance cross-relay optical cable disturbance detection. In addition, the present technical solution adopts a solution based on polarization-maintaining light source beam splitting to output two equal-power linearly polarized light sources as local oscillator light, instead of a non-polarization-maintaining light source plus a polarization controller to adjust the polarization state to circular polarization and then use a polarization beam splitter to output two equal-power linearly polarized lights as local oscillator light. This allows the polarization diversity coherent detection module to maintain the best working state for a long time, avoiding the problem that the polarization diversity coherent detection module often needs to manually adjust the polarization controller to achieve the best working state.

[0025] This device can support both φ-OTDR working mode and OFDR working mode through mode switching. This device uses mode switching to achieve both ultra-long single-ended detection distance and support voiceprint recognition sensor detection. This device uses the same set of hardware to achieve φ-OTDR working mode or OFDR working mode, and has high comprehensive performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of an embodiment;

[0027] Figure 2 is a structural diagram of a time-frequency domain modulation control module in an embodiment;

[0028] Figure 3 is a schematic structural diagram of a first polarization diversity coherent detection module in an embodiment;

[0029] Figure 4 Schematic diagram of the structure of the second polarization diversity coherent detection module in the embodiment. DETAILED DESCRIPTION

[0030] The content of the present invention is further described below in conjunction with the drawings and embodiments, but the present invention is not limited thereto.

[0031] Example:

[0032] Reference Figure 1 , a fiber optic vibration sensing device operating in multiple modes in time and frequency domains, comprising:

[0033] A time-frequency domain modulation control module and a first polarization-maintaining narrow linewidth laser λ1 and a second polarization-maintaining narrow linewidth laser λ2 connected to the time-frequency domain modulation control module;

[0034] It also includes a first 1×2 polarization-maintaining fiber coupler A1, a second 1×2 polarization-maintaining fiber coupler A2, a first acousto-optic switch AOM-1, a first erbium-doped fiber amplifier EDFA-1, a first wavelength division multiplexer WDM-1, a fiber circulator CIR, a second wavelength division multiplexer WDM-2, a first polarization diversity coherent detection module PDI-1, and a signal processing component, to which a first polarization-maintaining narrow linewidth laser λ1 is sequentially connected, wherein the fiber circulator CIR is connected to a sensing optical cable to be tested, the first polarization diversity coherent detection module PDI-1 is connected to the second 1×2 polarization-maintaining fiber coupler A2, the first acousto-optic switch AOM-1 is also connected to a time-frequency domain modulation control module, and the signal processing component includes a data acquisition card and a computer, wherein the data acquisition card is used to amplify and convert the analog signal output by the photodetector into a digital signal, and transmit it to the computer for demodulation operation;

[0035] It also includes a third 1×2 polarization-maintaining fiber coupler B1, a fourth 1×2 polarization-maintaining fiber coupler B2, a second acousto-optic switch AOM-2, and a second erbium-doped fiber amplifier EDFA-2, to which the second polarization-maintaining narrow-linewidth laser λ2 is sequentially connected, wherein the second erbium-doped fiber amplifier EDFA-2 is sequentially connected to the first wavelength division multiplexer WDM-1 and the second polarization diversity coherent detection module PDI-2, the second polarization diversity coherent detection module PDI-2 is connected to the fourth 1×2 polarization-maintaining fiber coupler B2, and the second acousto-optic switch AOM-2 is also connected to the time-frequency domain modulation control module;

[0036] It also includes a first sub-interferometer OFI-1 connected to the first polarization-maintaining narrow linewidth laser λ1 and the time-frequency domain modulation control module;

[0037] The second interferometer OFI-2 is also included, which is connected to the second polarization-maintaining narrow linewidth laser λ2 and the time-frequency domain modulation control module. Figure 2As shown, the time-frequency domain modulation control module is provided with a signal processing chip and a first photoelectric sampling circuit 1, a second photoelectric sampling circuit 2, a first digital synthesis arbitrary waveform generating circuit DDS-1 and a second digital synthesis arbitrary waveform generating circuit DDS-2 connected to the signal processing chip. The signal processing chip outputs two TTL level pulse control signals, wherein the first photoelectric sampling circuit 1 is connected to the output end of the first sub-interferometer OFI-1, the second photoelectric sampling circuit 2 is connected to the output end of the second sub-interferometer OFI-2, the first digital synthesis arbitrary waveform generating circuit DDS-1 is connected to the first protection The first polarization-maintaining narrow linewidth laser λ1 is connected to the second polarization-maintaining narrow linewidth laser λ2, the time-frequency domain modulation control module forms an independent closed-loop feedback control system with the first polarization-maintaining narrow linewidth laser λ1 and the first sub-interferometer OFI-1, the signal processing chip of the time-frequency domain modulation control module controls the first digital synthesis arbitrary waveform generation circuit DDS-1 to output a triangular wave frequency modulation signal, drives the first polarization-maintaining narrow linewidth laser λ1 to perform triangular wave frequency modulation, and the first polarization-maintaining narrow linewidth laser λ1 is connected to the first polarization-maintaining narrow linewidth laser λ2 via the first 1×2 polarization-maintaining fiber coupler A1. The first sub-interferometer OFI-1 outputs a detection signal to the first photoelectric sampling circuit 1, and the signal processing chip demodulates and analyzes the frequency modulation linearity of the first polarization-maintaining narrow linewidth laser λ1, and feedback controls the first digital synthesis arbitrary waveform generation circuit DDS-1 to correct the triangular wave frequency modulation signal, thereby completing the triangular wave linear frequency modulation negative feedback control; the time-frequency domain modulation control module and the second polarization-maintaining narrow linewidth laser λ2 and the second sub-interferometer OFI-2 form another independent closed-loop feedback control system, and the signal processing chip of the time-frequency domain modulation control module controls the second digital synthesis The arbitrary waveform generating circuit DDS-2 outputs a triangular wave frequency modulation signal to drive the second polarization-maintaining narrow linewidth laser λ2 to perform triangular wave frequency modulation. The second polarization-maintaining narrow linewidth laser λ2 is connected to the second sub-interferometer OFI-2 via the third 1×2 polarization-maintaining fiber coupler B1. The second sub-interferometer OFI-2 outputs a detection signal to the second photoelectric sampling circuit 2. The signal processing chip demodulates and analyzes the frequency modulation linearity of the second polarization-maintaining narrow linewidth laser λ2, and feedback controls the second digital synthesis arbitrary waveform generating circuit DDS-2 to correct the triangular wave frequency modulation signal, thereby completing the triangular wave linear frequency modulation negative feedback control;The time-frequency domain modulation control module also controls the first polarization-maintaining narrow linewidth laser λ1 and / or the second polarization-maintaining narrow linewidth laser λ2 to work in a frequency-stabilizing mode. The time-frequency domain modulation control module outputs a first TTL pulse control signal to control the first acousto-optic switch AOM-1 to modulate the laser output of the first polarization-maintaining narrow linewidth laser λ1 to directly continuously transmit and output or form a pulsed laser output. The time-frequency domain modulation control module outputs a second TTL pulse control signal to control the second acousto-optic switch AOM-2 to modulate the laser output of the second polarization-maintaining narrow linewidth laser λ2 to directly continuously transmit and output or form a pulsed laser output. The signal processing chip is any type of chip that supports logic operations, such as FPGA, ARM, DSP, or CPU. In this example, the signal processing chip is a KINTEX-7 series FPGA chip of XILINX. ;

[0038] The time-frequency domain modulation control module controls the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 to operate in a linear frequency modulation mode, or the time-frequency domain modulation control module controls the first acousto-optic switch AOM-1 and the second acousto-optic switch AOM-2 to operate in a pulse modulation mode; the first polarization-maintaining narrow linewidth laser λ1 outputs a narrow linewidth laser signal connected to the input end of the first 1×2 polarization-maintaining fiber coupler A1, the first 1×2 polarization-maintaining fiber coupler A1 has two outputs, namely a through arm and a coupling arm, the coupling arm of the first 1×2 polarization-maintaining fiber coupler A1 is connected to the first sub-interferometer OFI-1, and the output end of the first sub-interferometer OFI-1 is connected to A time-frequency domain modulation control module is input, which is used to monitor and feedback control the frequency modulation linearity of the first polarization-maintaining narrow-linewidth laser λ1; the straight-through arm of the first 1×2 polarization-maintaining fiber coupler A1 is connected to the input end of the second 1×2 polarization-maintaining fiber coupler A2, and the second 1×2 polarization-maintaining fiber coupler A2 has two outputs, namely, a straight-through arm and a coupling arm. The coupling arm of the second 1×2 polarization-maintaining fiber coupler A2 is connected to the local oscillator light input end of the first polarization diversity detection module PDI-1, the straight-through arm of the second 1×2 polarization-maintaining fiber coupler A2 is connected to the optical fiber input end of the first acousto-optic switch AOM-1, and the optical fiber output end of the first acousto-optic switch AOM-1 is connected to the first erbium-doped fiber amplifier The output end of the first erbium-doped fiber amplifier EDFA-1 is connected to the λ1 end corresponding to the first wavelength division multiplexer WDM-1, the common end of the first wavelength division multiplexer WDM-1 is connected to the 1 port of the optical fiber circulator CIR, the 2 port of the optical fiber circulator CIR is connected to the sensor cable to be tested, the echo signal of the sensor cable to be tested is input by the 2 port of the optical fiber circulator CIR, output by the 3 port of the optical fiber circulator CIR, connected to the common end of the second wavelength division multiplexer WDM-2, and output by the λ1 end corresponding to the second wavelength division multiplexer WDM-2, and connected to the optical input end to be tested of the first polarization diversity detection module PDI-1, It is coherent with the local oscillator light connected to the coupling arm of the second 1×2 polarization-maintaining fiber coupler A2, and outputs two electrical signals P1 and S1 to the signal processing component for sampling and demodulation processing; the second polarization-maintaining narrow-linewidth laser λ2 outputs a narrow-linewidth laser signal connected to the input end of the third 1×2 polarization-maintaining fiber coupler B1, and the third 1×2 polarization-maintaining fiber coupler B1 has two outputs, a straight-through arm and a coupling arm. The coupling arm of the third 1×2 polarization-maintaining fiber coupler B1 is connected to the second sub-interferometer OFI-2, and the output end of the second sub-interferometer OFI-2 is connected to the time-frequency domain modulation control module, which is used to monitor and feedback-control the frequency modulation linearity of the second narrow-linewidth laser λ2;The through arm of the third 1×2 polarization-maintaining fiber coupler B1 is connected to the input end of the fourth 1×2 polarization-maintaining fiber coupler B2. The fourth 1×2 polarization-maintaining fiber coupler B2 has two outputs, namely, a through arm and a coupling arm. The coupling arm of the fourth 1×2 polarization-maintaining fiber coupler B2 is connected to the local oscillator light input end of the second polarization diversity detection module PDI-2. The through arm of the fourth 1×2 polarization-maintaining fiber coupler B2 is connected to the fiber input end of the second acousto-optic switch AOM-2. The fiber output end of the second acousto-optic switch AOM-2 is connected to the input end of the second erbium-doped fiber amplifier EDFA-2. The output end of the second erbium-doped fiber amplifier EDFA-2 is connected to the λ2 end corresponding to the first wavelength division multiplexer WDM-1. , outputted from the common end of the first wavelength division multiplexer WDM-1 and connected to port 1 of the optical fiber circulator CIR, port 2 of the optical fiber circulator CIR is connected to the sensor cable to be tested, the echo signal of the sensor cable to be tested is inputted from port 2 of the optical fiber circulator CIR and outputted from port 3 of the optical fiber circulator CIR, connected to the common end of the second wavelength division multiplexer WDM-2, and outputted from the λ2 end corresponding to the second wavelength division multiplexer WDM-2, connected to the optical input end to be tested of the second polarization diversity detection module PDI-2, and coherent with the local oscillator light connected to the coupling arm of the fourth 1×2 polarization-maintaining optical fiber coupler B2, and outputs two electrical signals P2 and S2 to the signal processing component for sampling and demodulation processing. ;

[0039] In this example, the operating wavelengths of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 differ by at least 0.8 nm, and the frequency interval is greater than 100 GHz. The output laser polarization states of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 are both linearly polarized, and the pigtails are both polarization-maintaining fibers.

[0040] In this example, the first wavelength division multiplexer WDM-1 and the second wavelength division multiplexer WDM-2 are both provided with three optical fiber ports, namely, a common end, a transmission end and a reflection end, wherein the wavelength of the transmission end is consistent with the wavelength of the first polarization-maintaining narrow linewidth laser λ1, and the reflection end is used to connect the optical signal of the second polarization-maintaining narrow linewidth laser λ2; or the wavelength of the transmission end is consistent with the wavelength of the second polarization-maintaining narrow linewidth laser λ2, and the reflection end is used to connect the optical signal of the first polarization-maintaining narrow linewidth laser λ1.

[0041] In this example, the first interferometer OFI-1 and the second interferometer OFI-2 are both fiber interferometers, and the first interferometer OFI-1 and the second interferometer OFI-2 are used to monitor and measure the frequency modulation linearity or laser center frequency drift of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2, respectively.

[0042] In this example, the first polarization diversity detection module PDI-1 and the second polarization diversity detection module PDI-2 are each composed of a 1×2 polarization-maintaining fiber coupler with a splitting ratio of 50:50, a polarization beam splitter, two 2×2 polarization-maintaining fiber couplers with a splitting ratio of 50:50, and two balanced photodetectors, wherein:

[0043] The components of the first polarization diversity detection module PDI-1 are connected as follows: Figure 2 As shown, the local oscillator light is connected to the input end of the fifth 1×2 polarization-maintaining fiber coupler A3, the output port 1 of the fifth 1×2 polarization-maintaining fiber coupler A3 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A4, and the output port 2 of the fifth 1×2 polarization-maintaining fiber coupler A3 is connected to the input port 1 of the second 2×2 polarization-maintaining fiber coupler A5. The echo signal with a wavelength of λ1 of the sensor optical cable to be tested is connected to the input end of the first polarization beam splitter PBS-1, and the output end 1 of the first polarization beam splitter PBS-1 is connected to the input port 2 of the first 2×2 polarization-maintaining fiber coupler A4 and the output end 2 of the first polarization beam splitter PBS-1. The output end 2 is connected to the input port 2 of the second 2×2 polarization-maintaining fiber coupler A5, the output port 1 and the output port 2 of the first 2×2 polarization-maintaining fiber coupler A4 are respectively connected to the two input ends of the first balanced photodetector BPD-1, the output electrical signal S1 is connected to the signal processing component, the output port 1 and the output port 2 of the second 2×2 polarization-maintaining fiber coupler A5 are respectively connected to the two input ends of the second balanced photodetector BPD-2, the output electrical signal P1 is connected to the signal processing component, the electrical signal S1 is added to the electrical signal P1, and the sum is the coherent detection signal that is not affected by the change of the polarization state of the optical fiber to be measured;

[0044] The components of the second polarization diversity detection module PDI-2 are connected as follows: Figure 3As shown, the local oscillator light is connected to the input end of the sixth 1×2 polarization-maintaining fiber coupler B3, the output port 1 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the third 2×2 polarization-maintaining fiber coupler B4, the output port 2 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the fourth 2×2 polarization-maintaining fiber coupler B5, the echo signal with a wavelength of λ2 of the sensing optical cable to be tested is connected to the input end of the second polarization beam splitter PBS-2, the output end 1 of the second polarization beam splitter PBS-2 is connected to the input port 2 of the third 2×2 polarization-maintaining fiber coupler B4, the output end 2 of the second polarization beam splitter PBS-2 The output end 2 is connected to the input port 2 of the fourth 2×2 polarization-maintaining fiber coupler B5; the output port 1 and the output port 2 of the third 2×2 polarization-maintaining fiber coupler B4 are respectively connected to the two input ends of the third balanced photodetector BPD-3, and the output electrical signal S2 is connected to the signal processing component; the output port 1 and the output port 2 of the fourth 2×2 polarization-maintaining fiber coupler B5 are respectively connected to the two input ends of the fourth balanced photodetector BPD-4, and the output electrical signal P2 is connected to the signal processing component, and the electrical signal S2 is added to the electrical signal P2, and the sum is the coherent detection signal that is not affected by the change of the polarization state of the optical fiber to be measured.

[0045] In this example, the second 1×2 polarization-maintaining fiber coupler A2, the common end of the first wavelength division multiplexer WDM-1 and the corresponding input end of the first polarization-maintaining narrow linewidth laser λ1, the fiber circulator CIR, the common end of the second wavelength division multiplexer WDM-2 and the corresponding output end of the first polarization-maintaining narrow linewidth laser λ1 and the first polarization diversity coherent detection module PDI-1 together constitute the first main fiber interferometer OFI-3.

[0046] In this example, the fourth 1×2 polarization-maintaining fiber coupler B2, the common end of the first wavelength division multiplexer WDM-1 and the corresponding input end of the second polarization-maintaining narrow linewidth laser λ2, the fiber circulator CIR, the common end of the second wavelength division multiplexer WDM-2 and the corresponding output end of the second polarization-maintaining narrow linewidth laser λ2, and the second polarization diversity coherent detection module PDI-2 together constitute the second main fiber interferometer OFI-4.

[0047] In this example, the optical fiber vibration sensing device has three working modes:

[0048] 1) Time-frequency domain long-distance detection mode: The time-frequency domain modulation control module controls the first polarization-maintaining narrow-linewidth laser λ1 to work in the triangle wave continuous frequency modulation mode and the first acousto-optic switch AOM-1 to work in the continuous light transmission state, and the first main fiber interferometer OFI-3 performs detection to form the OFDR frequency domain detection mode; the time-frequency domain modulation control module controls the second polarization-maintaining narrow-linewidth laser λ2 to work in the stable frequency mode and the second acousto-optic switch AOM-2 to work in the pulse modulation state, and the second main fiber interferometer OFI-4 performs detection to form the φ-OTDR time domain detection mode. The OFDR frequency domain detection mode has a large dynamic range of 40dB, which is conducive to long-distance fiber vibration detection. The φ-OTDR time domain detection mode can support distributed fiber acoustic sensing demodulation, and can realize the spatial positioning and tracking of vibration sources. The time-frequency domain long-distance detection mode can have the advantages of both.

[0049] 2) Dual-wave time-domain enhanced sampling detection mode: The time-frequency domain modulation control module controls the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 to work in the stable frequency mode, and controls the first acousto-optic switch AOM-1 and the second acousto-optic switch AOM-2 to work in the pulse modulation state with the same pulse frequency. There is a delay difference of half a cycle width between the two wavelength laser pulses emitted in the same pulse period, and then they are injected into the detection optical cable together. The sensing signal of the first polarization-maintaining narrow linewidth laser λ1 is detected by the first main fiber interferometer OFI-3, and the sensing signal of the second polarization-maintaining narrow linewidth laser λ2 is detected by the second main fiber interferometer OFI-4, forming a φ-OTDR time-domain dual-pulse detection mode. The dual-wave time-domain enhanced sampling detection mode is equivalent to breaking through the limitation of the pulse period of the sensor optical cable to be tested, doubling the pulse frequency, that is, doubling the sampling frequency of distributed optical fiber acoustic sensing, which is beneficial to improving the demodulation frequency range of distributed optical fiber acoustic sensing;

[0050] 3) Dual-wave frequency domain quasi-continuous pulse cross-relay detection mode: The time-frequency domain modulation control module controls the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 to work in the triangle wave continuous frequency modulation mode, and controls the first acousto-optic switch AOM-1 and the second acousto-optic switch AOM-2 to work in the same pulse frequency and synchronously emitted millisecond-level quasi-continuous pulse modulation state. The sensor signal of the first polarization-maintaining narrow linewidth laser λ1 and the sensor signal of the second polarization-maintaining narrow linewidth laser λ2 after wavelength division are directly photoelectrically detected and demodulated by the signal processing component, forming OFDR dual-wave frequency domain pulse detection. Mode, among which, the dual-wave frequency domain quasi-continuous pulse detection mode is specially used for disturbance monitoring scenarios with relay submarine optical cables or land-based relay transmission optical cables. In this scenario, multiple fiber interferometers are connected in series behind each downstream optical fiber relay amplifier, and the output sensor signals of each adjacent fiber interferometer are cross-filtered at transmission wavelengths λ1 and λ2, and then coupled to the uplink optical fiber and relayed amplified and transmitted back. The laser pulse width of wavelengths λ1 and λ2 is equal to or slightly less than twice the round-trip delay of a single-segment relay transmission optical cable. The dual-wave frequency domain quasi-continuous pulse cross-relay detection mode can realize real-time vibration monitoring of thousands of kilometers of optical cables.

Claims

1. An optical fiber vibration sensing device operating in multiple modes in time and frequency domains, characterized in that: include: A time-frequency domain modulation control module and a first polarization-maintaining narrow linewidth laser λ1 and a second polarization-maintaining narrow linewidth laser λ2 connected to the time-frequency domain modulation control module; It also includes a first 1×2 polarization-maintaining fiber coupler A1, a second 1×2 polarization-maintaining fiber coupler A2, a first acousto-optic switch AOM-1, a first erbium-doped fiber amplifier EDFA-1, a first wavelength division multiplexer WDM-1, a fiber circulator CIR, a second wavelength division multiplexer WDM-2, a first polarization diversity coherent detection module PDI-1, and a signal processing component, to which a first polarization-maintaining narrow linewidth laser λ1 is sequentially connected, wherein the fiber circulator CIR is connected to a sensing optical cable to be tested, the first polarization diversity coherent detection module PDI-1 is connected to the second 1×2 polarization-maintaining fiber coupler A2, the first acousto-optic switch AOM-1 is also connected to a time-frequency domain modulation control module, and the signal processing component includes a data acquisition card and a computer, wherein the data acquisition card is used to amplify and convert the analog signal output by the photodetector into a digital signal, and transmit it to the computer for demodulation operation; It also includes a third 1×2 polarization-maintaining fiber coupler B1, a fourth 1×2 polarization-maintaining fiber coupler B2, a second acousto-optic switch AOM-2, and a second erbium-doped fiber amplifier EDFA-2, to which the second polarization-maintaining narrow-linewidth laser λ2 is sequentially connected, wherein the second erbium-doped fiber amplifier EDFA-2 is sequentially connected to the first wavelength division multiplexer WDM-1 and the second polarization diversity coherent detection module PDI-2, the second polarization diversity coherent detection module PDI-2 is connected to the fourth 1×2 polarization-maintaining fiber coupler B2, and the second acousto-optic switch AOM-2 is also connected to the time-frequency domain modulation control module; It also includes a first sub-interferometer OFI-1 connected to the first polarization-maintaining narrow linewidth laser λ1 and the time-frequency domain modulation control module; Also included is a second sub-interferometer OFI-2 connected to both the second polarization-maintaining narrow linewidth laser λ2 and the time-frequency domain modulation control module; The first interferometer OFI-1 and the second interferometer OFI-2 are both fiber interferometers, and the first interferometer OFI-1 and the second interferometer OFI-2 are used to monitor and measure the frequency modulation linearity or laser center frequency drift of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 respectively; The second 1×2 polarization-maintaining fiber coupler A2, the common end of the first wavelength division multiplexer WDM-1 and the corresponding first polarization-maintaining narrow linewidth laser λ1 input end, the fiber circulator CIR, the common end of the second wavelength division multiplexer WDM-2 and the corresponding first polarization-maintaining narrow linewidth laser λ1 output end and the first polarization diversity coherent detection module PDI-1 together constitute a first main fiber interferometer OFI-3; The fourth 1×2 polarization-maintaining fiber coupler B2, the common end of the first wavelength division multiplexer WDM-1 and the corresponding input end of the second polarization-maintaining narrow linewidth laser λ2, the fiber circulator CIR, the common end of the second wavelength division multiplexer WDM-2 and the corresponding output end of the second polarization-maintaining narrow linewidth laser λ2, and the second polarization diversity coherent detection module PDI-2 together constitute the second main fiber interferometer OFI-4.

2. The optical fiber vibration sensing device operating in time-frequency domain multi-mode according to claim 1, characterized in that: The time-frequency domain modulation control module is provided with a signal processing chip and a first photoelectric sampling circuit 1, a second photoelectric sampling circuit 2, a first digital synthesis arbitrary waveform generating circuit DDS-1 and a second digital synthesis arbitrary waveform generating circuit DDS-2 connected to the signal processing chip. The signal processing chip outputs two TTL level pulse control signals, wherein the first photoelectric sampling circuit 1 is connected to the output end of the first sub-interferometer OFI-1, the second photoelectric sampling circuit 2 is connected to the output end of the second sub-interferometer OFI-2, the first digital synthesis arbitrary waveform generating circuit DDS-1 is connected to the first polarization-maintaining narrow linewidth laser λ1, the second digital synthesis arbitrary waveform generating circuit DDS-2 is connected to the second polarization-maintaining narrow linewidth laser λ2, and the time-frequency domain modulation control module is provided with a signal processing chip and a first photoelectric sampling circuit 1, a second photoelectric sampling circuit 2, a first digital synthesis arbitrary waveform generating circuit DDS-1 and a second digital synthesis arbitrary waveform generating circuit DDS-2 are connected to the second polarization-maintaining narrow linewidth laser λ2. The control module, the first polarization-maintaining narrow linewidth laser λ1, and the first sub-interferometer OFI-1 form an independent closed-loop feedback control system. The signal processing chip of the time-frequency domain modulation control module controls the first digital synthesis arbitrary waveform generation circuit DDS-1 to output a triangular wave frequency modulation signal, drives the first polarization-maintaining narrow linewidth laser λ1 to perform triangular wave frequency modulation, and the first polarization-maintaining narrow linewidth laser λ1 is connected to the first sub-interferometer OFI-1 via the first 1×2 polarization-maintaining fiber coupler A1. The first sub-interferometer OFI-1 outputs a detection signal to the first photoelectric sampling circuit 1, and the signal processing chip demodulates and analyzes the frequency modulation linearity of the first polarization-maintaining narrow linewidth laser λ1, and feedback controls the first digital synthesis arbitrary waveform generation circuit DDS-1 to process the triangular wave frequency modulation signal. The signal is corrected to complete the triangular wave linear frequency modulation negative feedback control; the time-frequency domain modulation control module, the second polarization-maintaining narrow linewidth laser λ2, and the second sub-interferometer OFI-2 form another independent closed-loop feedback control system. The signal processing chip of the time-frequency domain modulation control module controls the second digital synthesis arbitrary waveform generation circuit DDS-2 to output a triangular wave frequency modulation signal, drives the second polarization-maintaining narrow linewidth laser λ2 to perform triangular wave frequency modulation, and the second polarization-maintaining narrow linewidth laser λ2 is connected to the second sub-interferometer OFI-2 via the third 1×2 polarization-maintaining fiber coupler B1. The second sub-interferometer OFI-2 outputs a detection signal to the second photoelectric sampling circuit 2, and the signal processing chip demodulates and analyzes the frequency modulation linearity of the second polarization-maintaining narrow linewidth laser λ2, and feeds back The second digital synthesis arbitrary waveform generating circuit DDS-2 is controlled to correct the triangle wave frequency modulation signal, thereby completing the triangle wave linear frequency modulation negative feedback control; the time-frequency domain modulation control module also controls the first polarization-maintaining narrow linewidth laser λ1 and / or the second polarization-maintaining narrow linewidth laser λ2 to operate in a stable frequency mode, the time-frequency domain modulation control module outputs a first TTL pulse control signal to control the first acousto-optic switch AOM-1 to modulate the laser output of the first polarization-maintaining narrow linewidth laser λ1 to directly continuously transmit and output or form a pulsed laser output, and the time-frequency domain modulation control module outputs a second TTL pulse control signal to control the second acousto-optic switch AOM-2 to modulate the laser output of the second polarization-maintaining narrow linewidth laser λ2 to directly continuously transmit and output or form a pulsed laser output.

3. The optical fiber vibration sensing device operating in time-frequency domain multi-mode according to claim 1, characterized in that: The operating wavelengths of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 differ by at least 0.8 nm, and the frequency interval is greater than 100 GHz. The output laser polarization states of the first polarization-maintaining narrow linewidth laser λ1 and the second polarization-maintaining narrow linewidth laser λ2 are both linearly polarized, and the pigtails are both polarization-maintaining fibers.

4. The optical fiber vibration sensing device operating in time-frequency domain multi-mode according to claim 1, characterized in that: The first wavelength division multiplexer WDM-1 and the second wavelength division multiplexer WDM-2 are both provided with three optical fiber ports, namely, a common end, a transmission end and a reflection end, wherein the wavelength of the transmission end is consistent with the wavelength of the first polarization-maintaining narrow linewidth laser λ1, and the reflection end is used to connect the optical signal of the second polarization-maintaining narrow linewidth laser λ2; or the wavelength of the transmission end is consistent with the wavelength of the second polarization-maintaining narrow linewidth laser λ2, and the reflection end is used to connect the optical signal of the first polarization-maintaining narrow linewidth laser λ1.

5. The optical fiber vibration sensing device operating in time-frequency domain multi-mode according to claim 1, characterized in that: The first polarization diversity detection module PDI-1 and the second polarization diversity detection module PDI-2 are both provided with a 1×2 polarization-maintaining fiber coupler with a splitting ratio of 50:50, a polarization beam splitter, two 2×2 polarization-maintaining fiber couplers with a splitting ratio of 50:50, and two balanced photodetectors, wherein: The components of the first polarization diversity detection module PDI-1 are connected as follows: the local oscillator light is connected to the input end of the fifth 1×2 polarization-maintaining fiber coupler A3, the output port 1 of the fifth 1×2 polarization-maintaining fiber coupler A3 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A4, the output port 2 of the fifth 1×2 polarization-maintaining fiber coupler A3 is connected to the input port 1 of the second 2×2 polarization-maintaining fiber coupler A5, the echo signal with a wavelength of λ1 of the sensor optical cable to be tested is connected to the input end of the first polarization beam splitter PBS-1, the output port 1 of the first polarization beam splitter PBS-1 is connected to the input port 2 of the first 2×2 polarization-maintaining fiber coupler A4, the output port 2 of the first polarization beam splitter PBS-1 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A5, and the output port 2 of the first polarization beam splitter PBS-1 is connected to the input port 1 of the first 2×2 polarization-maintaining fiber coupler A5. The output end 2 of the polarization beam splitter PBS-1 is connected to the input port 2 of the second 2×2 polarization-maintaining fiber coupler A5, the output port 1 and the output port 2 of the first 2×2 polarization-maintaining fiber coupler A4 are respectively connected to the two input ends of the first balanced photodetector BPD-1, the output electrical signal S1 is connected to the signal processing component, the output port 1 and the output port 2 of the second 2×2 polarization-maintaining fiber coupler A5 are respectively connected to the two input ends of the second balanced photodetector BPD-2, the output electrical signal P1 is connected to the signal processing component, the electrical signal S1 is added to the electrical signal P1, and the sum is the coherent detection signal that is not affected by the change of the polarization state of the optical fiber to be measured; The components of the second polarization diversity detection module PDI-2 are connected as follows: the local oscillator light is connected to the input end of the sixth 1×2 polarization-maintaining fiber coupler B3, the output port 1 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the third 2×2 polarization-maintaining fiber coupler B4, the output port 2 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the fourth 2×2 polarization-maintaining fiber coupler B5, the echo signal with a wavelength of λ2 of the sensor optical cable to be tested is connected to the input end of the second polarization beam splitter PBS-2, the output port 1 of the second polarization beam splitter PBS-2 is connected to the input port 2 of the third 2×2 polarization-maintaining fiber coupler B4, the output port 2 of the second polarization beam splitter PBS-2 is connected to the input port 1 of the third 2×2 polarization-maintaining fiber coupler B5, and the output port 2 of the sixth 1×2 polarization-maintaining fiber coupler B3 is connected to the input port 1 of the fourth 2×2 polarization-maintaining fiber coupler B5. The output port 2 of the polarization beam splitter PBS-2 is connected to the input port 2 of the fourth 2×2 polarization-maintaining fiber coupler B5; the output port 1 and the output port 2 of the third 2×2 polarization-maintaining fiber coupler B4 are respectively connected to the two input ports of the third balanced photodetector BPD-3, and the output electrical signal S2 is connected to the signal processing component; the output port 1 and the output port 2 of the fourth 2×2 polarization-maintaining fiber coupler B5 are respectively connected to the two input ports of the fourth balanced photodetector BPD-4, and the output electrical signal P2 is connected to the signal processing component, and the electrical signal S2 is added to the electrical signal P2, and the sum is the coherent detection signal that is not affected by the change of the polarization state of the optical fiber to be measured.

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