Distributed sound wave sensing system for multi-frequency pulse modulation

Abstract

The utility model discloses a distributed sound wave sensing system for multi-frequency pulse modulation, and relates to the technical field of distributed sound wave sensing. The system comprises a narrow linewidth laser, an optical fiber coupler, a polarization-maintaining optical isolator, an acousto-optic modulator, a polarization-maintaining delay optical fiber, an erbium-doped optical fiber amplifier, a polarization-maintaining dense wavelength division multiplexer, an optical fiber circulator, an optical fiber Bragg grating, a piezoelectric transducer, a polarization controller, a photoelectric balance detector, a data acquisition card, an arbitrary waveform generator and the like. According to the utility model, the multi-frequency delay ring is utilized to carry out multi-frequency modulation, the frequency number of light waves is increased along with the circulation times of the light waves in the delay ring, and under the condition that the utilization of a large-bandwidth electro-optical modulator is not increased, the flexible modulation of the frequency is realized by utilizing a low-bandwidth acousto-optical modulator without being influenced by carrier frequency and sidebands of each order; the low-bandwidth acousto-optic modulator is used for frequency modulation in the multi-frequency delay ring, and compared with an electro-optical modulation or multi-acousto-optic / multi-light source modulation scheme, the modulation cost is effectively saved.

Description

Technical Field

[0001] The utility model relates to the technical field of distributed acoustic wave sensing, in particular to a distributed acoustic wave sensing system with multi-frequency pulse modulation. Background Art

[0002] Distributed acoustic sensing (DAS) technology quantitatively analyzes vibration signals along the sensing fiber by demodulating the phase changes of the Rayleigh scattered light signal. Compared with other types of distributed optical fiber sensors (DOFS), it has unique advantages such as fast measurement speed and high detection sensitivity. It has broad application prospects in oil and gas pipeline monitoring, seismic wave monitoring, and perimeter intrusion detection.

[0003] However, due to the high coherence of optical signals in DAS systems, the optical signals within the half-width of the detection pulse light interfere with each other, and the position where the coherent signals cancel each other is called the coherent fading point. The signal-to-noise ratio of the optical signal is extremely low, resulting in demodulation distortion of the phase information at the corresponding position. The main means to solve coherent fading in DAS systems include using special optical fibers such as multi-mode, few-mode and multi-core optical fibers, scattering-enhanced point optical fibers as sensing units, or using multi-frequency, multi-wavelength, and multi-phase pulse modulation multiplexing. Traditional acousto-optic modulation devices have a good extinction ratio and are single-frequency modulation, but the modulation bandwidth is low and the number of modulated frequencies is small; while traditional electro-optic modulation devices have a high modulation bandwidth, but multi-frequency modulation is difficult and is affected by the carrier and various order sidebands. Therefore, it is necessary to achieve flexible multi-frequency modulation. Summary of the Invention

[0004] In view of the above problems, the present invention proposes a distributed acoustic wave sensing system with multi-frequency pulse modulation.

[0005] A multi-frequency pulse modulated distributed acoustic wave sensing system includes a narrow linewidth laser 1, a first fiber coupler 2, a first polarization-maintaining optical isolator 3, a second fiber coupler 4, a first acousto-optic modulator 5, a polarization-maintaining delay fiber 6, a first erbium-doped fiber amplifier 7, a polarization-maintaining dense wavelength division multiplexer 8, a second polarization-maintaining optical isolator 9, a second acousto-optic modulator 10, a second erbium-doped fiber amplifier 11, a first fiber circulator 12, a first fiber Bragg grating 13, a second fiber circulator 14, a piezoelectric transducer 15, a third erbium-doped fiber amplifier 16, a third fiber circulator 17, a second fiber Bragg grating 18, a polarization controller 19, a third fiber coupler 20, a photoelectric balance detector 21, a data acquisition card 22, and an arbitrary waveform generator 23.

[0006] The optical signal output end of the narrow linewidth laser 1 is connected to the optical signal input end of the first optical fiber coupler 2, and the optical signal output end of the first optical fiber coupler 2 is connected to the optical signal input end of the first polarization-maintaining optical isolator 3 and the optical signal input end of the polarization controller 19 respectively;

[0007] The optical signal output end of the first polarization-maintaining optical isolator 3 is connected to the first optical signal input end 4-1 of the second optical fiber coupler 4, and the first optical signal output end 4-2 of the second optical fiber coupler 4 is connected to the optical signal input end of the second acousto-optic modulator 10;

[0008] The optical signal output end of the second acousto-optic modulator 10 is connected to the optical signal input end of the second erbium-doped fiber amplifier 11, the optical signal output end of the second erbium-doped fiber amplifier 11 is connected to the first port 12-1 of the first fiber circulator 12, and the second port 12-2 of the first fiber circulator 12 is connected to the first fiber Bragg grating 13;

[0009] The second optical signal output terminal 4-3 of the second optical fiber coupler 4 is connected to the optical signal input terminal of the first acousto-optic modulator 5, the optical signal output terminal of the first acousto-optic modulator 5 is connected to one end of a polarization-maintaining delay optical fiber 6, the other end of the polarization-maintaining delay optical fiber 6 is connected to the optical signal input terminal of the first erbium-doped fiber amplifier 7, the optical signal output terminal of the first erbium-doped fiber amplifier 7 is connected to the optical signal input terminal of a polarization-maintaining dense wavelength division multiplexer 8, the optical signal output terminal of the polarization-maintaining dense wavelength division multiplexer 8 is connected to the optical signal input terminal of a second polarization-maintaining optical isolator 9, and the optical signal output terminal of the second polarization-maintaining optical isolator 9 is connected to the second optical signal input terminal 4-4 of the second optical fiber coupler 4;

[0010] Port No. 3 12-3 of the first fiber circulator 12 is in communication with port No. 14-1 of the second fiber circulator 14, port No. 2 14-2 of the second fiber circulator 14 is in communication with one end of a sensing fiber, the other end of the sensing fiber is in communication with a piezoelectric transducer 15, port No. 3 14-3 of the second fiber circulator 14 is in communication with an optical signal input end of a third erbium-doped fiber amplifier 16, an optical signal output end of the third erbium-doped fiber amplifier 16 is in communication with port No. 17-1 of a third fiber circulator 17, and port No. 2 17-2 of the third fiber circulator 17 is in communication with a second fiber Bragg grating 18;

[0011] The optical signal output end of the third port 17-3 of the third optical fiber circulator 17 and the polarization controller 19 are respectively connected to the optical signal input end of the third optical fiber coupler 20, the optical signal output end of the third optical fiber coupler 20 is connected to the optical signal input end of the photoelectric balance detector 21, and the electrical signal output end of the photoelectric balance detector 21 is connected to the data acquisition card 22;

[0012] The RF output terminal of the arbitrary waveform generator 23 is connected to the RF signal input terminal of the first acousto-optic modulator 5, the RF signal input terminal of the second acousto-optic modulator 10, the RF signal input terminal of the piezoelectric transducer 15 and the trigger signal input terminal of the data acquisition card 22 respectively.

[0013] In one possible implementation, the narrow linewidth laser 1 is a polarization-maintaining laser with an output power of 20 mW, a wavelength of 1550.12 nm, and a linewidth of 1 kHz.

[0014] In one possible implementation, the first fiber coupler 2 is a 1×2 polarization-maintaining coupler with a splitting ratio of 90:10; the second fiber coupler 4 is a 2×2 polarization-maintaining coupler with a splitting ratio of 50:50; and the third fiber coupler 20 is a 2×2 coupler with a splitting ratio of 50:50.

[0015] In one possible implementation, the first AOM 5 is a broadband polarization-maintaining modulator with a 3dB bandwidth of 100 MHz, an operating range of 30 MHz to 130 MHz, and an extinction ratio of 50 dB; the second AOM 10 is a polarization-maintaining modulator with a carrier frequency of 200 MHz and an extinction ratio of 50 dB.

[0016] In one possible implementation, the input end and the output end of the first erbium-doped fiber amplifier 7 are polarization-maintaining fibers.

[0017] In one possible implementation, the central transmission wavelength of the polarization-maintaining dense wavelength division multiplexer 8 is 1550.12 nm, and the 3 dB transmission bandwidth is 0.20 nm.

[0018] In one possible implementation, the central reflection wavelength of the first fiber Bragg grating 13 and the second fiber Bragg grating 18 is 1550.12 nm, and the 3 dB reflection bandwidth is 0.08 nm.

[0019] In one possible implementation, the 3dB operating bandwidth of the photoelectric balanced detector 21 is 2 GHz.

[0020] In one possible implementation, the data acquisition card 22 has a sampling rate of 5 GS / s and a sampling resolution of 14 bits.

[0021] The beneficial technical effects of the utility model are:

[0022] This utility model proposes a distributed acoustic wave sensing system with multi-frequency pulse modulation, which has the following advantages: 1) Flexible frequency modulation: Utilizing a multi-frequency delay loop for multi-frequency modulation, the frequency of the light wave increases with the number of times the light wave circulates within the delay loop. This flexible frequency modulation is achieved using a low-bandwidth acousto-optic modulator (AOM), without requiring the use of a high-bandwidth electro-optic modulator. This modulation is unaffected by the carrier frequency and various order sidebands. 2) Low modulation cost: Utilizing a low-bandwidth AOM for frequency modulation within the multi-frequency delay loop significantly reduces modulation costs compared to electro-optic modulation or multi-AO / multi-light source modulation schemes. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood by reading the detailed description below with reference to the accompanying drawings. In the accompanying drawings, several embodiments of the present invention are shown in an exemplary and non-limiting manner, in which:

[0024] Figure 1 This is a structural diagram of a distributed acoustic wave sensing system with multi-frequency pulse modulation according to an embodiment of the present utility model;

[0025] Figure 2 This is an example diagram of the original beat frequency signal measured in an embodiment of the present utility model;

[0026] Figure 3 This is an example diagram of frequency information of the original beat signal in an embodiment of the present utility model;

[0027] Figure 4 Schematic diagram of the relationship between the number of multiplexed frequencies and fading probability in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are provided solely to enable those skilled in the art to better understand and implement the present invention, and are not intended to limit the scope of the present invention in any way. Rather, these embodiments are provided to make this disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0029] The present invention provides a distributed acoustic wave sensing system with multi-frequency pulse modulation. Figure 1 As shown, the system includes: a narrow linewidth laser 1, a first fiber coupler 2, a first polarization-maintaining optical isolator 3, a second fiber coupler 4, a first acousto-optic modulator 5, a polarization-maintaining delay fiber 6, a first erbium-doped fiber amplifier 7, a polarization-maintaining dense wavelength division multiplexer 8, a second polarization-maintaining optical isolator 9, a second acousto-optic modulator 10, a second erbium-doped fiber amplifier 11, a first fiber circulator 12, a first fiber Bragg grating 13, a second fiber circulator 14, a piezoelectric transducer 15, a third erbium-doped fiber amplifier 16, a third fiber circulator 17, a second fiber Bragg grating 18, a polarization controller 19, a third fiber coupler 20, a photoelectric balanced detector 21, a data acquisition card 22, and an arbitrary waveform generator 23;

[0030] The optical signal output end of the narrow linewidth laser 1 is connected to the optical signal input end of the first optical fiber coupler 2, and the optical signal output end of the first optical fiber coupler 2 is connected to the optical signal input end of the first polarization-maintaining optical isolator 3 and the optical signal input end of the polarization controller 19 respectively;

[0031] The optical signal output end of the first polarization-maintaining optical isolator 3 is connected to the first optical signal input end 4-1 of the second optical fiber coupler 4, and the first optical signal output end 4-2 of the second optical fiber coupler 4 is connected to the optical signal input end of the second acousto-optic modulator 10;

[0032] The optical signal output end of the second acousto-optic modulator 10 is connected to the optical signal input end of the second erbium-doped fiber amplifier 11, the optical signal output end of the second erbium-doped fiber amplifier 11 is connected to the first port 12-1 of the first fiber circulator 12, and the second port 12-2 of the first fiber circulator 12 is connected to the first fiber Bragg grating 13;

[0033] The second optical signal output terminal 4-3 of the second optical fiber coupler 4 is connected to the optical signal input terminal of the first acousto-optic modulator 5, the optical signal output terminal of the first acousto-optic modulator 5 is connected to one end of a polarization-maintaining delay optical fiber 6, the other end of the polarization-maintaining delay optical fiber 6 is connected to the optical signal input terminal of the first erbium-doped fiber amplifier 7, the optical signal output terminal of the first erbium-doped fiber amplifier 7 is connected to the optical signal input terminal of a polarization-maintaining dense wavelength division multiplexer 8, the optical signal output terminal of the polarization-maintaining dense wavelength division multiplexer 8 is connected to the optical signal input terminal of a second polarization-maintaining optical isolator 9, and the optical signal output terminal of the second polarization-maintaining optical isolator 9 is connected to the second optical signal input terminal 4-4 of the second optical fiber coupler 4;

[0034] Port No. 3 12-3 of the first fiber circulator 12 is in communication with port No. 14-1 of the second fiber circulator 14, port No. 2 14-2 of the second fiber circulator 14 is in communication with one end of a sensing fiber, the other end of the sensing fiber is in communication with a piezoelectric transducer 15, port No. 3 14-3 of the second fiber circulator 14 is in communication with an optical signal input end of a third erbium-doped fiber amplifier 16, an optical signal output end of the third erbium-doped fiber amplifier 16 is in communication with port No. 17-1 of a third fiber circulator 17, and port No. 2 17-2 of the third fiber circulator 17 is in communication with a second fiber Bragg grating 18;

[0035] The optical signal output end of the third port 17-3 of the third optical fiber circulator 17 and the polarization controller 19 are respectively connected to the optical signal input end of the third optical fiber coupler 20, the optical signal output end of the third optical fiber coupler 20 is connected to the optical signal input end of the photoelectric balance detector 21, and the electrical signal output end of the photoelectric balance detector 21 is connected to the data acquisition card 22;

[0036] The RF output terminal of the arbitrary waveform generator 23 is connected to the RF signal input terminal of the first acousto-optic modulator 5, the RF signal input terminal of the second acousto-optic modulator 10, the RF signal input terminal of the piezoelectric transducer 15 and the trigger signal input terminal of the data acquisition card 22 respectively.

[0037] In this embodiment, preferably, the narrow linewidth laser 1 is a polarization-maintaining laser with an output power of 20 mW, a wavelength of 1550.12 nm, and a linewidth of 1 kHz.

[0038] In this embodiment, preferably, the first optical fiber coupler 2 is a 1×2 polarization-maintaining coupler with a splitting ratio of 90:10; the second optical fiber coupler 4 is a 2×2 polarization-maintaining coupler with a splitting ratio of 50:50; and the third optical fiber coupler 20 is a 2×2 coupler with a splitting ratio of 50:50.

[0039] In this embodiment, preferably, the first acousto-optic modulator 5 is a broadband polarization-maintaining modulator with a 3dB bandwidth of 100 MHz, an operating range of 30 MHz-130 MHz, and an extinction ratio of 50 dB; the second acousto-optic modulator 10 is a polarization-maintaining modulator with a carrier frequency of 200 MHz and an extinction ratio of 50 dB.

[0040] In this embodiment, preferably, the central transmission wavelength of the polarization-maintaining dense wavelength division multiplexer 8 is 1550.12 nm, and the 3dB transmission bandwidth is 0.20 nm; the central reflection wavelength of the first fiber Bragg grating 13 and the second fiber Bragg grating 18 is 1550.12 nm, and the 3dB reflection bandwidth is 0.08 nm.

[0041] In this embodiment, preferably, the input end and the output end of the first erbium-doped fiber amplifier 7 are polarization-maintaining fibers.

[0042] In this embodiment, preferably, the 3dB operating bandwidth of the photoelectric balance detector 21 is 2 GHz.

[0043] In this embodiment, preferably, the sampling rate of the data acquisition card 22 is 5 GS / s and the sampling resolution is 14 bits.

[0044] In this embodiment, preferably, the working process of a distributed acoustic wave sensing system with multi-frequency pulse modulation includes:

[0045] The single-frequency continuous light output by the narrow linewidth laser 1 is divided into two paths, upper and lower, by the first fiber coupler 2; the upper path is used as the detection pulse light modulation of the DAS system; the lower path continuous light is used as the reference light of the DAS system after the polarization state is adjusted by the polarization controller 19;

[0046] The uplink continuous light passes through the first polarization-maintaining optical isolator 3 and is divided into two upper and lower branches by the second optical fiber coupler 4: a first upper branch and a first lower branch. The first lower branch continuous light sequentially enters the multi-frequency delay loop composed of the first acousto-optic modulator 5, the polarization-maintaining delay optical fiber 6, the first erbium-doped fiber amplifier 7, the polarization-maintaining dense wavelength division multiplexer 8, and the second polarization-maintaining optical isolator 9 for multi-frequency modulation. The modulated optical signal is further divided into two upper and lower branches by the second optical fiber coupler 4: a second upper branch and a second lower branch. The first upper branch continuous light and the second upper branch continuous light are periodically modulated into pulsed light by the second acousto-optic modulator 10 and then transmitted to the second erbium-doped fiber amplifier 11. The power is pre-amplified and then injected into the first fiber Bragg grating 13 through the first fiber circulator 12 to filter out ASE noise. The pulsed light after ASE noise filtering is injected into the sensing fiber and the piezoelectric transducer 15 in sequence through the second fiber circulator 14. The second lower branch continuous light is further modulated by the multi-frequency delay loop. Among them, in the multi-frequency delay loop, the first acousto-optic modulator 5 is used for frequency shift modulation, the polarization-maintaining delay fiber 6 is used to extend the delay of the delay loop, the first erbium-doped fiber amplifier 7 is used to compensate for the loss of the delay loop, the polarization-maintaining dense wavelength division multiplexer 8 is used to filter out ASE noise, and the second polarization-maintaining optical isolator 9 is used to prevent reverse transmission of the optical signal.

[0047] The backscattered Rayleigh light signal returned by the sensing fiber is injected into the third erbium-doped fiber amplifier 16 for power pre-amplification through the second fiber circulator 14. It is then injected into the second fiber Bragg grating 18 by the third fiber circulator 17 to filter out ASE noise. After that, it is injected into the third fiber coupler 20 together with the reference light of the downlink DAS system for coherent frequency beat. It is then converted into electrical and electronic signals by the photoelectric balanced detector 21 and recorded by the data acquisition card 22.

[0048] The arbitrary waveform generator 23 is used to control the frequency shift of the first AOM 5 and the second AOM 10 and the on-off switching of the optical signal, and to control the data acquisition cycle of the data acquisition card 22 .

[0049] The collected multi-frequency beat signals are filtered to produce multiple single-frequency beat signals, and the amplitude and phase information of each single-frequency beat signal are demodulated separately through Hilbert transform. The phase information of each single-frequency beat signal is weighted and averaged using the amplitude information as the weight, so as to suppress the phase demodulation distortion caused by the fading point due to the coherent fading effect in the DAS system.

[0050] Figure 2 The time domain distribution of the beat frequency signal measured for the system; Figure 3 The frequency domain distribution of the beat signal measured by the system shows the 20 frequency information modulated by the multi-frequency delay loop; Figure 4 This is the relationship between the number of frequencies reused and the fading probability of the multi-frequency delay loop modulation. The more frequencies there are, the lower the fading probability.

[0051] This utility model proposes a distributed acoustic wave sensing system with multi-frequency pulse modulation, which has the following advantages: 1) Flexible frequency modulation: Utilizing a multi-frequency delay loop for multi-frequency modulation, the frequency of the light wave increases with the number of times the light wave circulates within the delay loop. This flexible frequency modulation is achieved using a low-bandwidth acousto-optic modulator (AOM), without requiring the use of a high-bandwidth electro-optic modulator. This modulation is unaffected by the carrier frequency and various order sidebands. 2) Low modulation cost: Utilizing a low-bandwidth AOM for frequency modulation within the multi-frequency delay loop significantly reduces modulation costs compared to electro-optic modulation or multi-AO / multi-light source modulation schemes.

[0052] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division into various aspects does not mean that the features of these aspects cannot be combined to benefit. Such division is merely for the convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A distributed acoustic wave sensing system with multi-frequency pulse modulation, characterized in that: The invention comprises a narrow linewidth laser (1), a first optical fiber coupler (2), a first polarization-maintaining optical isolator (3), a second optical fiber coupler (4), a first acousto-optic modulator (5), a polarization-maintaining delay optical fiber (6), a first erbium-doped optical fiber amplifier (7), a polarization-maintaining dense wavelength division multiplexer (8), a second polarization-maintaining optical isolator (9), a second acousto-optic modulator (10), a second erbium-doped optical fiber amplifier (11), a first optical fiber circulator (12), a first optical fiber Bragg grating (13), a second optical fiber circulator (14), a piezoelectric transducer (15), a third erbium-doped optical fiber amplifier (16), a third optical fiber circulator (17), a second optical fiber Bragg grating (18), a polarization controller (19), a third optical fiber coupler (20), a photoelectric balance detector (21), a data acquisition card (22), and an arbitrary waveform generator (23); The optical signal output end of the narrow linewidth laser (1) is connected to the optical signal input end of the first optical fiber coupler (2), and the optical signal output end of the first optical fiber coupler (2) is respectively connected to the optical signal input end of the first polarization-maintaining optical isolator (3) and the optical signal input end of the polarization controller (19); The optical signal output end of the first polarization-maintaining optical isolator (3) is connected to the first optical signal input end (4-1) of the second optical fiber coupler (4), and the first optical signal output end (4-2) of the second optical fiber coupler (4) is connected to the optical signal input end of the second acousto-optic modulator (10); The optical signal output end of the second acousto-optic modulator (10) is connected to the optical signal input end of the second erbium-doped fiber amplifier (11), the optical signal output end of the second erbium-doped fiber amplifier (11) is connected to the first port (12-1) of the first optical fiber circulator (12), and the second port (12-2) of the first optical fiber circulator (12) is connected to the first fiber Bragg grating (13); The second optical signal output end (4-3) of the second optical fiber coupler (4) is connected to the optical signal input end of the first acousto-optic modulator (5), the optical signal output end of the first acousto-optic modulator (5) is connected to one end of the polarization-maintaining delay optical fiber (6), the other end of the polarization-maintaining delay optical fiber (6) is connected to the optical signal input end of the first erbium-doped optical fiber amplifier (7), the optical signal output end of the first erbium-doped optical fiber amplifier (7) is connected to the optical signal input end of the polarization-maintaining dense wavelength division multiplexer (8), the optical signal output end of the polarization-maintaining dense wavelength division multiplexer (8) is connected to the optical signal input end of the second polarization-maintaining optical isolator (9), and the optical signal output end of the second polarization-maintaining optical isolator (9) is connected to the second optical signal input end (4-4) of the second optical fiber coupler (4); The third port (12-3) of the first optical fiber circulator (12) is in communication with the first port (14-1) of the second optical fiber circulator (14), the second port (14-2) of the second optical fiber circulator (14) is in communication with one end of the sensing optical fiber, the other end of the sensing optical fiber is in communication with the piezoelectric transducer (15), the third port (14-3) of the second optical fiber circulator (14) is in communication with the optical signal input end of the third erbium-doped optical fiber amplifier (16), the optical signal output end of the third erbium-doped optical fiber amplifier (16) is in communication with the first port (17-1) of the third optical fiber circulator (17), and the second port (17-2) of the third optical fiber circulator (17) is in communication with the second optical fiber Bragg grating (18); The third port (17-3) of the third optical fiber circulator (17) and the optical signal output end of the polarization controller (19) are respectively connected to the optical signal input end of the third optical fiber coupler (20), the optical signal output end of the third optical fiber coupler (20) is connected to the optical signal input end of the photoelectric balance detector (21), and the electrical signal output end of the photoelectric balance detector (21) is connected to the data acquisition card (22); The radio frequency output end of the arbitrary waveform generator (23) is respectively connected to the radio frequency signal input end of the first acousto-optic modulator (5), the radio frequency signal input end of the second acousto-optic modulator (10), the radio frequency signal input end of the piezoelectric transducer (15), and the trigger signal input end of the data acquisition card (22).

2. The multi-frequency pulse modulated distributed acoustic wave sensing system according to claim 1, characterized in that: The narrow linewidth laser (1) is a polarization-maintaining laser with an output power of 20 mW, a wavelength of 1550.12 nm, and a linewidth of 1 kHz.

3. The multi-frequency pulse modulated distributed acoustic wave sensing system according to claim 1, characterized in that: The first optical fiber coupler (2) is a 1×2 polarization-maintaining coupler with a splitting ratio of 90:10; the second optical fiber coupler (4) is a 2×2 polarization-maintaining coupler with a splitting ratio of 50:50; and the third optical fiber coupler (20) is a 2×2 coupler with a splitting ratio of 50:

50.

4. The multi-frequency pulse modulated distributed acoustic wave sensing system according to claim 1, characterized in that: The first acousto-optic modulator (5) is a broadband polarization-maintaining modulator with a 3dB bandwidth of 100MHz, an operating range of 30MHz-130MHz, and an extinction ratio of 50dB; the second acousto-optic modulator (10) is a polarization-maintaining modulator with a carrier frequency of 200MHz and an extinction ratio of 50dB.

5. The distributed acoustic wave sensing system with multi-frequency pulse modulation according to claim 1, characterized in that: The input end and the output end of the first erbium-doped fiber amplifier (7) are polarization-maintaining fibers.

6. The distributed acoustic wave sensing system with multi-frequency pulse modulation according to claim 1, characterized in that: The central transmission wavelength of the polarization-maintaining dense wavelength division multiplexer (8) is 1550.12 nm, and the 3dB transmission bandwidth is 0.20 nm.

7. The multi-frequency pulse modulated distributed acoustic wave sensing system according to claim 1, characterized in that: The central reflection wavelength of the first fiber Bragg grating (13) and the second fiber Bragg grating (18) is 1550.12 nm, and the 3dB reflection bandwidth is 0.08 nm.

8. The multi-frequency pulse modulated distributed acoustic wave sensing system according to claim 1, characterized in that: The 3dB operating bandwidth of the photoelectric balance detector (21) is 2 GHz.

9. The multi-frequency pulse modulated distributed acoustic wave sensing system according to claim 1, characterized in that: The data acquisition card (22) has a sampling rate of 5 GS / s and a sampling resolution of 14 bits.

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