阅读说明：本技术 一种基于零差探测的分布式光纤声波传感装置及解调方法 (Distributed optical fiber acoustic wave sensing device based on homodyne detection and demodulation method ) 是由 王夏霄 马福 朱熔通 王艺臻 宋凝芳 于 2021-08-18 设计创作，主要内容包括：本发明公开了一种基于零差探测的分布式光纤声波传感装置及解调方法,包括窄线宽激光器、光纤隔离器、半导体光放大器、掺铒光纤放大器Ⅰ、光纤环形器、传感光纤、光纤耦合器Ⅰ、光纤耦合器Ⅱ、直波导相位调制器、第一法拉第旋转镜、第二法拉第旋转镜、掺铒光纤放大器Ⅱ、光电探测器Ⅰ、光电探测器Ⅱ、数据采集单元和数据处理单元。本发明中在原有基础上增加一个光纤耦合器Ⅰ、一个光电探测器Ⅰ和一个掺铒光纤放大器Ⅱ,结合提出的一种改进的信号解调方法,可解决现有技术中采用直波导相位调制器实现高频调制解调和准分布式传感,而不能实现全分布式传感的问题。(The invention discloses a distributed optical fiber sound wave sensing device and a demodulation method based on homodyne detection. The invention adds an optical fiber coupler I, a photoelectric detector I and an erbium-doped optical fiber amplifier II on the original basis, and combines with the proposed improved signal demodulation method, thereby solving the problem that the high-frequency modulation and demodulation and the quasi-distributed sensing can not be realized by adopting a straight waveguide phase modulator in the prior art.)

1. A distributed optical fiber acoustic wave sensing device based on homodyne detection is characterized by mainly comprising a pulse light generation module, a sensing optical fiber module, a carrier modulation module and a signal demodulation module;

the pulse light generation module comprises a narrow linewidth laser (1), an optical fiber isolator (2), a semiconductor optical amplifier (3), an erbium-doped optical fiber amplifier I (4), a four-port optical fiber circulator (5) and an optical fiber grating (6), wherein the tail fiber output end of the narrow linewidth laser (1) is connected with the input end of the optical fiber isolator (2); the output end of the optical fiber isolator (2) is connected with the input end of the semiconductor optical amplifier (3); the output end of the semiconductor optical amplifier (3) is connected with the input end of the erbium-doped optical fiber amplifier I (4); a first port (5-1) of the four-port optical fiber circulator is connected with the output end of an erbium-doped optical fiber amplifier I (4), a third port (5-3) of the four-port optical fiber circulator is connected with a first port (8-1) of an optical fiber coupler I, and a fourth port (5-4) of the four-port optical fiber circulator is connected with an optical fiber grating (6);

the sensing optical fiber module comprises a sensing optical fiber (7) which is connected with a second port (5-2) of the four-port optical fiber circulator;

the output port of the optical fiber coupler I (8) is respectively connected with the carrier modulation module and the signal demodulation module;

a second port (8-2) of the optical fiber coupler I is connected with a carrier modulation module, which comprises an optical fiber coupler II (9), a straight waveguide phase modulator (10), a first Faraday rotator mirror (11) and a second Faraday rotator mirror (12); the first port (9-1) of the optical fiber coupler II is connected with the second port (8-2) of the optical fiber coupler I, and the second port (9-2) of the optical fiber coupler II is connected with the input end of the straight waveguide phase modulator (10); the output end of the straight waveguide phase modulator (10) is connected with a first Faraday rotator mirror (11); a third port (9-3) of the optical fiber coupler II is connected with a second Faraday rotator mirror (12);

a third port (8-3) of the optical fiber coupler I is connected with a signal demodulation module which comprises an erbium-doped optical fiber amplifier II (13), a photoelectric detector I (14), a photoelectric detector II (15), a data acquisition unit (16) and a data processing unit (17); the input end of a photoelectric detector I (14) is connected with a third port (8-3) of an optical fiber coupler I, the input port of an erbium-doped optical fiber amplifier II (13) is connected with a fourth port (9-4) of the optical fiber coupler II, and the output port of the erbium-doped optical fiber amplifier II is connected with the input end of a photoelectric detector II (15); the output ends of the photoelectric detector I (14) and the photoelectric detector II (15) are connected with a data acquisition unit (16) and finally enter a data processing unit (17).

2. The distributed optical fiber acoustic wave sensing device based on homodyne detection according to claim 1, wherein the optical fiber coupler II (9), the straight waveguide phase modulator (10), the first Faraday rotator mirror (11) and the second Faraday rotator mirror (12) together form a polarization-maintaining unbalanced Michelson interferometer structure.

3. The distributed optical fiber acoustic wave sensing device based on homodyne detection as claimed in any one of claims 1-2, wherein the center wavelength of the narrow linewidth laser (1) is in 1550nm band, the linewidth is less than 3kHz, the laser power is up to 20mW, and the optical fiber isolator (2) adopts a common single-mode optical fiber with the working band of 1550 nm.

4. The distributed optical fiber acoustic wave sensing device based on homodyne detection as claimed in any one of claims 1-2, wherein the four-port optical fiber circulator (5) is in one-way conduction; the optical fiber grating (6) adopts an optical fiber Bragg grating with the working waveband of 1550 nm.

5. The distributed optical fiber acoustic wave sensing device based on homodyne detection as claimed in any one of claims 1-2, wherein the sensing optical fiber (7) is a common single-mode optical fiber, the diameter of the fiber core is 8-10 μm, the diameter of the cladding is 125 μm, the optical fiber coupler I (8) is a 1 x 2 single-mode optical fiber coupler, and the splitting ratio is 50: 50, the fiber coupler II (9) is a polarization-maintaining fiber coupler of 2 multiplied by 2, the splitting ratio is 50: and 50, the working waveband of the polarization-maintaining straight waveguide phase modulator (10) is 1550 nm.

6. The distributed optical fiber acoustic wave sensing device based on homodyne detection as claimed in any one of claims 1-2, wherein the operating waveband of the polarization-maintaining first Faraday rotator mirror (11) and the second Faraday rotator mirror (12) is 1550nm, and the performance parameters of the two Faraday rotators and the mirrors are consistent and both consist of Faraday rotators and mirrors with a rotation angle of 45 degrees.

7. The distributed optical fiber acoustic wave sensing device based on homodyne detection as claimed in any one of claims 1-2, wherein the photodetector I (14) and the photodetector II (15) are selected from detectors capable of detecting nW-level optical power.

8. The sensing method of the distributed optical fiber acoustic wave sensing device based on homodyne detection as claimed in any one of claims 1 to 7, characterized by comprising the following steps:

continuous laser emitted by a narrow-linewidth laser (1) is injected into a semiconductor optical amplifier (3) after passing through an optical fiber isolator (2), the semiconductor optical amplifier (3) modulates the continuous light emitted by the laser into pulse light with certain pulse interval and pulse width, an erbium-doped optical fiber amplifier I (4) amplifies the pulse light and then injects the amplified pulse light into a four-port optical fiber circulator (5) and injects the amplified pulse light into a sensing optical fiber (7) through a second port (5-2) of the four-port optical fiber circulator, backward Rayleigh scattered light which is transmitted and returned by the sensing optical fiber (7) and carries the self-interference sound wave signal to be detected is injected into a first port (8-1) of the optical fiber coupler I (8) through a third port (5-3) of the four-port optical fiber circulator, and a fourth port (5-4) of the four-port optical fiber circulator is connected with the optical fiber grating (6); rayleigh scattered light signals carrying sound wave information to be detected are divided into two paths through an optical fiber coupler I (8), and are respectively injected into a carrier modulation module and a signal demodulation module through a second port (8-2) of the optical fiber coupler I and a third port (8-3) of the optical fiber coupler I; the first path directly enters a photoelectric detector I (14); the second path enters an optical fiber coupler II (9) and needs to be modulated by a rear-end unbalanced Michelson interferometer, the unbalanced Michelson interferometer has a fixed big-small arm difference of 5-10 m, and the spatial resolution of the system is determined by the width of pulse light; a second port (9-2) of the optical fiber coupler II (9) is subjected to carrier modulation by a straight waveguide phase modulator (10) and then is reflected back by a first Faraday rotary mirror (11), a third port (9-3) of the optical fiber coupler II (9) is reflected back by a second Faraday rotary mirror (12), two returned beams of light are interfered at the optical fiber coupler II (9), and the interference light enters a photoelectric detector II (15) after being amplified by an erbium-doped optical fiber amplifier II (13); finally, the two paths of signal light are subjected to photoelectric conversion and then enter the data acquisition unit (16), and finally the frequency and the amplitude of the acoustic wave signal to be detected can be obtained after the signal light is processed by the data processing unit (17).

9. The sensing method of claim 8, wherein:

a first path of signal light in the signal demodulation module, namely a light signal detected by the photoelectric detector I (14), is only subjected to differential processing to determine the position of the occurrence of the sound wave event; after the second path of signal light, namely the optical signal detected by the photoelectric detector II (15), is subjected to carrier modulation, phase demodulation can be carried out, and phase information can be effectively extracted; the light intensity signal I detected by the photoelectric detector II (15)₂Can be expressed as:

wherein A is the amplitude of the DC signal component contained in the signal; b is the amplitude of the AC signal component, Ccos (w)₀t) is a carrier modulation signal;the phase change is caused by the signal to be detected and the environmental drift together;can be divided into frequency w_sThe acoustic wave signal to be measured and the phase change caused by external interference such as environmental drift; d is the amplitude of the acoustic signal to be measured, w_sIs the frequency of the acoustic signal to be measured, t is the time,phase changes caused by environmental noise.

10. A demodulation method of the distributed optical fiber acoustic wave sensing device based on homodyne detection according to any one of claims 1 to 7, characterized by comprising the following steps:

s1: the first path of signals obtained by the photoelectric detector I (14) is subjected to differential operation processing, and the positions Z of all the sound wave events on the sensing optical fiber (7) are calculated₁，Z₂，…，Z_n；

S2: for the second path of signals obtained by the photoelectric detector II (15), the sensing optical fiber (7) is selected to be divided by Z₁，Z₂，…，Z_nSampling points in a distance without acoustic wave events or sampling points in a reference optical fiber in a preset distance before the sensing optical fiber (7) at least comprise sampling points in a pulse width;

s3: demodulating the acoustic event occurrence location Z₁The information of the second path is divided by Z₁The sample points at other positions are all replaced by the points found in S2;

s4: the processed second path of signal obtained after S3 is subjected to global PGC-Arctan phase demodulation processing to obtain the signal Z₁Acoustic information at the location;

s5: likewise, for Z₂，…，Z_nThe locations are processed according to S3 and S4 to obtain the amplitude and frequency information at the location of each acoustic event.

Technical Field

The invention belongs to the field of optical fiber acoustic wave sensors, and particularly relates to a distributed optical fiber acoustic wave sensing device based on homodyne detection and a demodulation method.

Background

The distributed optical fiber acoustic wave sensor has the advantages of electromagnetic interference resistance, corrosion resistance, insulation, high adaptability to various complex terrains, no need of external field power supply and the like, and can be widely applied to many national defense and military fields such as perimeter security, pipeline monitoring, communication line detection, underwater detection, earthquake early warning and the like.

Phase-sensitive optical time domain reflectometer based on homodyne detectionThe distributed optical fiber acoustic wave sensor realizes distributed measurement and reduction of acoustic wave signals by detecting phase information carried by optical fiber backward Rayleigh scattering light. In order to realize the quantitative measurement of the acoustic wave signal to be measured, the amplitude and the frequency of the signal to be measured can be obtained only by the data acquisition unit after the detected backward Rayleigh scattering optical signal needs to be subjected to carrier modulation and then is subjected to phase demodulation by the data processing unit.

At present, a phase generation carrier modulation and demodulation scheme in a distributed fiber acoustic wave sensing system based on homodyne detection includes a michelson interferometer structure formed by a piezoelectric ceramic (PZT) phase modulator or a straight waveguide phase modulator. Although the PZT phase modulator is simple to manufacture, only optical fibers are directly wound on the PZT, the working frequency of the PZT phase modulator is generally only dozens of kHz, the modulation frequency is low, high-frequency modulation cannot be realized, the long-term stability performance is poor, and the PZT phase modulator is not easy to integrate. The modulation frequency of the straight waveguide phase modulator is as high as MHz, so that the detection frequency of the system is not limited by the carrier frequency of the phase modulator and is only related to the pulse repetition frequency, and after the straight waveguide phase modulator is replaced, high-frequency modulation and detection of higher sound wave signal frequency can be realized, but the subsequent phase demodulation method can only carry out global demodulation, only can realize quasi-distributed sensing, and is difficult to realize fully-distributed sensing.

Disclosure of Invention

In view of this, in order to solve the problems that the PZT phase modulator in the prior art cannot realize high-frequency modulation, has poor long-term stability and is not easy to integrate, and the distributed optical fiber acoustic wave sensing system based on the straight waveguide phase modulator cannot realize fully distributed sensing, the invention provides a distributed optical fiber acoustic wave sensing device based on homodyne detection, which adds an optical fiber coupler, a photoelectric detector and an erbium-doped optical fiber amplifier on the basis of adopting the scheme of the straight waveguide phase modulator, and correspondingly provides an improved phase demodulation method to solve the problem that the PZT phase modulator cannot realize fully distributed sensing.

The technical scheme of the invention is as follows:

a distributed optical fiber acoustic wave sensing device based on homodyne detection mainly comprises a pulse light generation module, a sensing optical fiber module, a carrier modulation module and a signal demodulation module;

the pulse light generation module comprises a narrow linewidth laser 1, an optical fiber isolator 2, a semiconductor optical amplifier 3, an erbium-doped optical fiber amplifier I4, a four-port optical fiber circulator 5 and an optical fiber grating 6, wherein the tail fiber output end of the narrow linewidth laser 1 is connected with the input end of the optical fiber isolator 2; the output end of the optical fiber isolator 2 is connected with the input end of the semiconductor optical amplifier 3; the output end of the semiconductor optical amplifier 3 is connected with the input end of the erbium-doped optical fiber amplifier I4; a first port 5-1 of the four-port optical fiber circulator is connected with an output end of an erbium-doped optical fiber amplifier I4, a third port 5-3 of the four-port optical fiber circulator is connected with a first port 8-1 of an optical fiber coupler I, and a fourth port 5-4 of the four-port optical fiber circulator is connected with an optical fiber grating 6;

the sensing optical fiber module comprises a sensing optical fiber 7 which is connected with a second port 5-2 of the four-port optical fiber circulator;

the output port of the optical fiber coupler I8 is respectively connected with a carrier modulation module and a signal demodulation module;

a second port 8-2 of the optical fiber coupler I is connected with a carrier modulation module, which comprises an optical fiber coupler II 9, a straight waveguide phase modulator 10, a first Faraday rotator mirror 11 and a second Faraday rotator mirror 12; the first port 9-1 of the optical fiber coupler II is connected with the second port 8-2 of the optical fiber coupler I, and the second port 9-2 of the optical fiber coupler II is connected with the input end of the straight waveguide phase modulator 10; the output end of the straight waveguide phase modulator 10 is connected with a first Faraday rotator mirror 11; a third port 9-3 of the optical fiber coupler II is connected with a second Faraday rotator mirror 12;

a third port 8-3 of the optical fiber coupler I is connected with a signal demodulation module which comprises an erbium-doped optical fiber amplifier II 13, a photoelectric detector I14, a photoelectric detector II 15, a data acquisition unit 16 and a data processing unit 17; the input end of the photoelectric detector I14 is connected with a third port 8-3 of the optical fiber coupler I, the input port of the erbium-doped optical fiber amplifier II 13 is connected with a fourth port 9-4 of the optical fiber coupler II, and the output port of the erbium-doped optical fiber amplifier II is connected with the input end of the photoelectric detector II 15; the output ends of the photoelectric detectors I14 and II 15 are connected with a data acquisition unit 16 and finally enter a data processing unit 17.

Preferably, the optical fiber coupler ii 9, the straight waveguide phase modulator 10, the first faraday rotator mirror 11, and the second faraday rotator mirror 12 together form a polarization-maintaining unbalanced michelson interferometer structure.

Preferably, the narrow linewidth laser 1 has a center wavelength in a 1550nm band, a linewidth of less than 3kHz, and a laser power of 20 mW.

Preferably, the narrow linewidth laser 1 has a linewidth of less than 10kHz and a power of 10mW or more.

Preferably, the optical fiber isolator 2 uses a common single mode optical fiber having an operating band of 1550 nm.

Preferably, the four-port optical fiber circulator 5 is in one-way conduction; the optical fiber grating 6 adopts an optical fiber Bragg grating with the working wave band of 1550 nm.

Preferably, the sensing fiber 7 is a common single-mode fiber, the diameter of the fiber core is 8-10 μm, and the diameter of the cladding is 125 μm.

Preferably, the fiber coupler i 8 is a 1 × 2 single-mode fiber coupler, and the splitting ratio is 50: 50.

preferably, the fiber coupler ii 9 is a 2 × 2 polarization maintaining fiber coupler, and the splitting ratio is 50: 50.

preferably, the polarization maintaining straight waveguide phase modulator 10 has an operating band of 1550 nm.

Preferably, the working waveband of the polarization-maintaining first Faraday rotator mirror 11 and the second Faraday rotator mirror 12 is 1550nm, and the performance parameters of the two are consistent and both consist of a Faraday rotator and a reflector with a rotation angle of 45 degrees.

Preferably, the photodetector I14 and the photodetector II 15 are selected from detectors capable of detecting optical power in nW order.

According to the sensing method of the distributed optical fiber acoustic wave sensing device based on homodyne detection, the steps are as follows:

continuous laser emitted by a narrow-linewidth laser 1 is injected into a semiconductor optical amplifier 3 after passing through an optical fiber isolator 2, the semiconductor optical amplifier 3 modulates the continuous light emitted by the laser into pulse light with certain pulse interval and pulse width, an erbium-doped optical fiber amplifier I4 amplifies the pulse light, then injects the amplified pulse light into a four-port optical fiber circulator 5 and injects the amplified pulse light into a sensing optical fiber 7 through a second port 5-2 of the four-port optical fiber circulator, backward Rayleigh scattering light which is transmitted and returned by the sensing optical fiber 7, self-interference occurs and carries a sound wave signal to be detected, and is injected into a first port 8-1 of an optical fiber coupler I8 through a third port 5-3 of the four-port optical fiber circulator, and a fourth port 5-4 of the four-port optical fiber circulator is connected with an optical fiber grating 6; rayleigh scattered light signals carrying sound wave information to be detected are divided into two paths through the optical fiber coupler I8, and are respectively injected into the carrier modulation module and the signal demodulation module through the second port 8-2 of the optical fiber coupler I and the third port 8-3 of the optical fiber coupler I. The first path directly enters the photoelectric detector I14; the second path enters an optical fiber coupler II 9 and needs to be modulated by a rear-end unbalanced Michelson interferometer, and the unbalanced Michelson interferometer has a fixed arm difference of 5-10 m and determines the spatial resolution of the system together with the pulse light width. A second port 9-2 of the optical fiber coupler II 9 is subjected to carrier modulation by a straight waveguide phase modulator 10 and then reflected back by a first Faraday rotator mirror 11, a third port 9-3 of the optical fiber coupler II 9 is reflected back by a second Faraday rotator mirror 12, two returned beams of light interfere at the optical fiber coupler II 9, and the interference light enters a photoelectric detector II 15 after being amplified by an erbium-doped optical fiber amplifier II 13; finally, the two paths of signal light are subjected to photoelectric conversion and then enter the data acquisition unit 16, and finally the frequency and the amplitude of the acoustic wave signal to be detected can be obtained after the signal light is processed by the data processing unit 17.

Preferably, the first path of signal light in the signal demodulation module, namely the light signal detected by the photoelectric detector I14, is only subjected to differential processing to determine the position of the occurrence of the acoustic wave event; after the second path of signal light, namely the optical signal detected by the photoelectric detector II 15, is subjected to carrier modulation, phase demodulation can be carried out, and phase information can be effectively extracted; the light intensity signal I detected by the photoelectric detector II 15₂Can be expressed as:

wherein A is the amplitude of the DC signal component contained in the signal; b is the amplitude of the AC signal component, Ccos (w)₀t) is a carrier modulation signal;the phase change caused by the signal to be measured and the environmental drift together.Can be divided into frequency w_sThe acoustic wave signal to be measured and the phase change caused by external interference such as environmental drift. D is the amplitude of the acoustic signal to be measured, w_sIs the frequency of the acoustic signal to be measured, t is the time,phase changes caused by environmental noise.

A demodulation method based on the distributed optical fiber acoustic wave sensing device based on homodyne detection comprises the following steps:

s1: the first path of signals obtained by the photoelectric detector I14 is subjected to differential operation processing, and the positions Z of all sound wave events on the sensing optical fiber 7 are calculated₁，Z₂，…，Z_n；

S2: to the meridian lightThe second path of signal obtained by the electric detector II 15 is selected from the sensing optical fiber 7 to be divided by Z₁，Z₂，…，Z_nThe sampling points in a distance without the occurrence of the acoustic wave event outside or the sampling points in a reference optical fiber preset in a distance before the premise of the sensing optical fiber 7 at least comprise the number of sampling points in a pulse width;

s3: demodulating the acoustic event occurrence location Z₁The information of the second path is divided by Z₁The sample points at other positions are all replaced by the points found in S2;

s4: the processed second path of signal obtained after S3 is subjected to global PGC-Arctan phase demodulation processing to obtain the signal Z₁Acoustic information at the location;

s5: likewise, for Z₂，…，Z_nThe locations are processed according to S3 and S4 to obtain the amplitude and frequency information at the location of each acoustic event.

Compared with the prior art, the invention adopting the technical scheme has the following beneficial effects:

(1) according to the distributed optical fiber acoustic wave sensing device and the demodulation method based on the homodyne detection, the positioning of the acoustic wave event is firstly realized by adding the optical fiber coupler and introducing one path of signal by the photoelectric detector, and then the detection and the restoration of the signal to be detected on the whole sensing optical fiber are realized by combining the other path of signal with the positioning position and adopting an improved demodulation method.

(2) The invention is not damagedThe original structure does not influence the integrity of the system, does not influence the acquisition of signals to be detected, does not increase unknown interference, and realizes fully distributed sensing while realizing high-frequency modulation and broadband detection.

Drawings

FIG. 1 is a schematic diagram of a distributed optical fiber acoustic wave sensing device based on homodyne detection according to an embodiment of the present invention;

fig. 2 is a flowchart of a demodulation method of a distributed optical fiber acoustic wave sensing system according to an embodiment of the present invention.

In the drawings:

1-Narrow Linewidth Laser (NLL), 2-optical fiber Isolator (ISO), 3-Semiconductor Optical Amplifier (SOA), 4-erbium-doped fiber amplifier I (EDFA I), 5-four-port fiber circulator, first port of 5-1-four-port fiber circulator, second port of 5-2-four-port fiber circulator, third port of 5-3-four-port fiber circulator, fourth port of 5-4-four-port fiber circulator, 6-fiber grating, 7-sensing fiber, 8-fiber coupler I, first port of 8-1-fiber coupler I, second port of 8-2-fiber coupler I, third port of 8-3-fiber coupler I, 9-fiber coupler II, 9-1-a first port of the optical fiber coupler II, 9-2-a second port of the optical fiber coupler II, 9-3-a third port of the optical fiber coupler II, 9-4-a fourth port of the optical fiber coupler II, 10-a straight waveguide phase modulator, 11-a first Faraday rotator mirror, 12-a second Faraday rotator mirror, 13-an erbium-doped optical fiber amplifier II (EDFA II), 14-a photoelectric detector I (PD I), 15-a photoelectric detector II (PD II), 16-a data acquisition unit (DAQ) and 17-a data processing unit (DP).

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

The distributed optical fiber acoustic wave sensing device based on homodyne detection, as shown in fig. 1, mainly comprises a pulse light generation module, a sensing optical fiber module, a carrier modulation module and a signal demodulation module.

Specifically, the pulse light generation module comprises a narrow linewidth laser 1, an optical fiber isolator 2, a semiconductor optical amplifier 3, an erbium-doped optical fiber amplifier I4, a four-port optical fiber circulator 5 and an optical fiber grating 6, wherein the tail fiber output end of the narrow linewidth laser 1 is connected with the input end of the optical fiber isolator 2; the output end of the optical fiber isolator 2 is connected with the input end of the semiconductor optical amplifier 3; the output end of the semiconductor optical amplifier 3 is connected with the input end of the erbium-doped optical fiber amplifier I4; the first port 5-1 of the four-port optical fiber circulator is connected with the output end of the erbium-doped optical fiber amplifier I4, the third port 5-3 of the four-port optical fiber circulator is connected with the first port 8-1 of the optical fiber coupler I, and the fourth port 5-4 of the four-port optical fiber circulator is connected with the optical fiber grating 6. The sensing fiber module includes a sensing fiber 7 that is connected to the second port 5-2 of the four-port fiber optic circulator. And the output port of the optical fiber coupler I8 is respectively connected with the carrier modulation module and the signal demodulation module. And a second port 8-2 of the optical fiber coupler I is connected with a carrier modulation module, and the carrier modulation module comprises an optical fiber coupler II 9, a straight waveguide phase modulator 10, a first Faraday rotator mirror 11 and a second Faraday rotator mirror 12. The first port 9-1 of the optical fiber coupler II is connected with the second port 8-2 of the optical fiber coupler I, and the second port 9-2 of the optical fiber coupler II is connected with the input end of the straight waveguide phase modulator 10; the output end of the straight waveguide phase modulator 10 is connected with a first Faraday rotator mirror 11; and the third port 9-3 of the optical fiber coupler II is connected with a second Faraday rotator mirror 12. And a third port 8-3 of the optical fiber coupler I is connected with a signal demodulation module which comprises an erbium-doped optical fiber amplifier II 13, a photoelectric detector I14, a photoelectric detector II 15, a data acquisition unit 16 and a data processing unit 17. The input end of the photoelectric detector I14 is connected with a third port 8-3 of the optical fiber coupler I, the input port of the erbium-doped optical fiber amplifier II 13 is connected with a fourth port 9-4 of the optical fiber coupler II, and the output port of the erbium-doped optical fiber amplifier II is connected with the input end of the photoelectric detector II 15; the output ends of the photoelectric detectors I14 and II 15 are connected with a data acquisition unit 16 and finally enter a data processing unit 17.

Particularly, the invention adds a path of signal consisting of an optical fiber coupler I8 and a photoelectric detector I14, and the signal is used for positioning the position of the sound wave occurrence event; meanwhile, in consideration of the problem of large loss in the carrier modulation module, an erbium-doped fiber amplifier II 13 is introduced for amplifying signals. In addition, a reference optical fiber with a distance can be preset at the front end of the sensing optical fiber in advance, and the sampling point in the section of optical fiber is used for subsequent demodulation. Further, based on two paths of signals, the invention provides an improved phase demodulation method, which not only can realize high-frequency modulation and broadband detection, but also can realize the fully-distributed sensing of the system.

The present invention is further illustrated by the specific sensing process of the present invention. In this embodiment, the narrow linewidth laser 1 has a central wavelength in a 1550nm band, a linewidth less than 3kHz, and a laser power of 20 mW. Other types of narrow linewidth lasers may be used in other embodiments, but are required to have linewidths less than 10kHz and powers greater than 10 mW. The optical fiber isolator 2 adopts a common single mode optical fiber with the working waveband of 1550nm to prevent backward scattering light from influencing the stability of the laser light source. The optical fiber circulator 5 is a four-port optical fiber circulator and is in one-way conduction. The optical fiber grating 6 adopts an optical fiber Bragg grating with the working wave band of 1550nm to suppress the spontaneous radiation noise of the erbium-doped optical fiber amplifier I4. The sensing fiber 7 is a common single-mode fiber, the diameter of a fiber core is 8-10 mu m, and the diameter of a cladding is 125 mu m. The optical fiber coupler I8 is a 1 × 2 single-mode optical fiber coupler, and the splitting ratio is 50: 50. the optical fiber coupler II 9 is a polarization-maintaining optical fiber coupler of 2 multiplied by 2, and the splitting ratio is 50: 50, it should be noted here that the fiber coupler II 9 can set the actual splitting ratio according to the optical power in the specific implementation. The polarization maintaining straight waveguide phase modulator 10 has an operating band of 1550 nm. The working waveband of the first Faraday rotator mirror 11 and the second Faraday rotator mirror 12 with the polarization maintaining function is 1550nm, the performance parameters of the first Faraday rotator mirror and the second Faraday rotator mirror are consistent, and the first Faraday rotator mirror and the second Faraday rotator mirror are both composed of a Faraday rotator and a reflector with a rotation angle of 45 degrees and are used for realizing the rotation of the polarization state of light and the reflection of the light. The photoelectric detectors I14 and II 15 are selected from detectors capable of detecting nW-level optical power. The data acquisition unit 16 and the data processing unit 17 are used for data signal acquisition and processing of the whole system, and are used for realizing distributed real-time sound wave monitoring and restoration.

The sensing process of the distributed optical fiber acoustic wave sensing system based on homodyne detection is as follows:

continuous laser emitted by a narrow-linewidth laser 1 is injected into a semiconductor optical amplifier 3 after passing through an optical fiber isolator 2, the semiconductor optical amplifier 3 modulates the continuous light emitted by the laser into pulse light with certain pulse interval and pulse width, an erbium-doped optical fiber amplifier I4 amplifies the pulse light, then injects the amplified pulse light into a four-port optical fiber circulator 5 and injects the amplified pulse light into a sensing optical fiber 7 through a second port 5-2 of the four-port optical fiber circulator, backward Rayleigh scattering light which is transmitted and returned by the sensing optical fiber 7, self-interference occurs and carries a sound wave signal to be detected, and is injected into a first port 8-1 of an optical fiber coupler I8 through a third port 5-3 of the four-port optical fiber circulator, and a fourth port 5-4 of the four-port optical fiber circulator is connected with an optical fiber grating 6; rayleigh scattered light signals carrying sound wave information to be detected are divided into two paths through the optical fiber coupler I8, and are respectively injected into the carrier modulation module and the signal demodulation module through the second port 8-2 of the optical fiber coupler I and the third port 8-3 of the optical fiber coupler I. The first path directly enters the photoelectric detector I14; the second path enters an optical fiber coupler II 9 and needs to be modulated by a rear-end unbalanced Michelson interferometer, and the unbalanced Michelson interferometer has a fixed arm difference of 5-10 m and determines the spatial resolution of the system together with the pulse light width. The second port 9-2 of the optical fiber coupler II 9 is subjected to carrier modulation by a straight waveguide phase modulator 10 and then reflected back by a first Faraday rotary mirror 11, the third port 9-3 of the optical fiber coupler II 9 is reflected back by a second Faraday rotary mirror 12, two returned beams of light interfere at the optical fiber coupler II 9, and the interference light enters a photoelectric detector II 15 after being amplified by an erbium-doped optical fiber amplifier II 13. Finally, the two paths of signal light are subjected to photoelectric conversion and then enter the data acquisition unit 16, and finally the frequency and the amplitude of the acoustic wave signal to be detected can be obtained after the signal light is processed by the data processing unit 17.

Particularly, the first path of signal light in the signal demodulation module, namely the light signal detected by the photoelectric detector I14, is not modulated, the phase information is easily submerged by noise, the change of the real phase cannot be reflected, the extraction is inconvenient, and generally only difference processing is carried out to determine the position of the sound wave event; after the second path of signal light, namely the optical signal detected by the photoelectric detector II 15, is subjected to carrier modulation, phase demodulation can be performed, and phase information can be effectively extracted. The light intensity signal I detected by the photoelectric detector II 15₂Can be expressed as:

wherein A is the amplitude of the DC signal component contained in the signal; b is the amplitude of the AC signal component, Ccos (w)₀t) is carrier modulationSignal preparation;the phase change caused by the signal to be measured and the environmental drift together.Can be divided into frequency w_sThe acoustic wave signal to be measured and the phase change caused by external interference such as environmental drift. D is the amplitude of the acoustic signal to be measured, w_sIs the frequency of the acoustic signal to be measured, t is the time,phase changes caused by environmental noise. In order to improve the signal-to-noise ratio and the demodulation accuracy, a phase generation carrier-arc tangent PGC-Arctan algorithm is usually selected to restore the signal to be detected.

Therefore, further, the present invention combines the features of two signals to provide an improved signal demodulation method, which comprises the following steps:

s1: carrying out differential operation processing on the first path of signals obtained by the photoelectric detector I, and calculating the positions Z of all sound wave events on the sensing optical fiber₁，Z₂，…，Z_n；

S2: selecting a sensing optical fiber to remove Z from the second path of signals obtained by the photoelectric detector II₁，Z₂，…，Z_nSampling points in a distance without acoustic wave events or sampling points in a reference optical fiber preset in a distance before the premise of the sensing optical fiber at least comprise a sampling point number in a pulse width;

s3: demodulating the acoustic event occurrence location Z₁The information of the second path is divided by Z₁The sample points at other positions are all replaced by the points found in S2;

s4: the processed second path of signal obtained after S3 is subjected to global PGC-Arctan phase demodulation processing to obtain the signal Z₁Acoustic information at the location;

s5: likewise, for Z₂，…，Z_nThe locations are processed according to S3 and S4 to obtain the amplitude and frequency information at the location of each acoustic event.

Finally, the invention can provide a distributed optical fiber acoustic wave sensing system based on homodyne detection, which comprises the device and the demodulation method provided by the invention.

It will be apparent to those skilled in the art that various modifications and improvements can be made to the embodiments of the present invention without departing from the inventive concept of the present application, which falls within the scope of the present application.