Optical phase-locked loop DFB strain sensor based on injection locking

Abstract

The invention belongs to the technical field of optical fiber sensing, and particularly relates to an optical phase-locked loop (DFB) strain sensor based on injection locking, which comprises a PDH injection locking module, an optical phase-locked loop module and a strain sensing distributed feedback fiber laser module, the PDH injection locking module comprises a tunable narrow linewidth laser, a first optical fiber coupler, a phase modulator, an optical fiber circulator, a single-channel photoelectric detector, a lock-in amplifier, a first loop filter and a signal generator; the optical phase-locked loop module comprises an acousto-optic modulator, a second optical fiber coupler, a balanced photoelectric detector, a crystal filter, an amplitude detector unit, a second loop filter and a direct digital frequency synthesizer. According to the invention, high-resolution strain real-time measurement of the distributed feedback fiber laser can be realized, self free operation noise interference of a traditional strain sensing scheme based on the distributed feedback fiber laser is effectively solved, and higher strain measurement resolution is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic sensing, and particularly relates to an injection-locked optical phase-locked loop DFB strain sensor. Background Art

[0002] Different from passive fiber Bragg gratings, active fiber Bragg gratings - distributed feedback fiber lasers (DFB-FLs) have characteristics such as ultra-narrow linewidth and small size, and have great potential in sensing parameters such as high-precision and high-resolution fiber strain, acceleration, and magnetic field in recent years.

[0003] Traditional sensing demodulation schemes for distributed feedback fiber lasers mostly rely on open-loop phase demodulation schemes (Xu Dan, Song Wenzhang, Huang Junbin, etc. Demodulation interferometer and method for distributed feedback fiber laser hydrophone capable of noise reduction [P]. Hubei Province: CN202210832222.4, 2023-06-23.), and the central wavelength drift of the distributed feedback fiber laser is demodulated through an interferometer to obtain sensing information. This open-loop demodulation scheme does not solve the wavelength drift noise of the distributed feedback fiber laser under free running, which greatly reduces the sensing performance.

[0004] In 2020, Professor Liu Qingwen of Shanghai Jiao Tong University proposed a high-precision fiber optic strain sensing scheme based on injection locking (Liu Qingwen, He Zuyuan, Zhao Shuangxiang. Fiber Bragg grating temperature / strain sensing system and its demodulation method [P]. Shanghai City: CN201711458534.9, 2020-08-11. Liu Qingwen, He Zuyuan, Zhao Shuangxiang. High-precision fiber Bragg grating strain measurement system [P]. Shanghai City: CN201910130948.1, 2024-01-30.). By combining PDH demodulation technology and injection locking technology, a PDH error signal with gain is obtained by injecting a narrow linewidth tunable laser into the distributed feedback fiber laser to achieve higher-resolution strain sensing. However, this single-channel PDH injection locking technology is still affected by the wavelength drift noise of the distributed feedback fiber laser under free running. Therefore, it is particularly crucial to develop a high-resolution strain demodulation scheme that can effectively suppress the free running noise of the distributed feedback fiber laser. Summary of the Invention

[0005] The object of the present invention is to provide an injection-locked optical phase-locked loop DFB strain sensor, which can realize high-resolution real-time measurement of the strain of the distributed feedback fiber laser, effectively solve the interference of the self-free running noise of the traditional distributed feedback fiber laser strain sensing scheme, and has a higher strain measurement resolution.

[0006] The technical solution adopted by the present invention is specifically as follows:

[0007] An injection-locked optical phase-locked loop DFB strain sensor, comprising a PDH injection-locking module, an optical phase-locked loop module, and a strain-sensing distributed feedback fiber laser module;

[0008] The PDH injection-locking module includes a tunable narrow-linewidth laser, a first fiber coupler, a phase modulator, an optical fiber circulator, a single-channel photodetector, a lock-in amplifier, a first loop filter, and a signal generator;

[0009] The optical phase-locked loop module includes an acousto-optic modulator, a second fiber coupler, a balanced photodetector, a crystal filter, an amplitude detector unit, a second loop filter, and a direct digital frequency synthesizer;

[0010] The strain-sensing distributed feedback fiber laser module includes a pump source, a wavelength division multiplexer, a distributed feedback fiber laser unit, and a third fiber coupler;

[0011] The distributed feedback fiber laser unit serves as a strain-sensing element; by feeding back and tracking the central zero point of the PDH error signal amplified by the injection-locking technology, the wavelength of the tunable narrow-linewidth laser serving as the reference channel is fed back and tracked to the distributed feedback fiber laser unit at a low frequency, realizing free-running noise compensation of the distributed feedback fiber laser unit;

[0012] The acousto-optic modulator generates a single-sideband optical signal as the strain-sensing channel, the radio frequency signal for sideband modulation is provided by the direct digital frequency synthesizer, and the single-sideband optical signal is locked to the external crystal filter reference at a high frequency by using the optical phase-locked loop, and the external strain-sensing quantity is output as the signal fed back to the direct digital frequency synthesizer.

[0013] Further, locking the single-sideband optical signal to the external crystal filter reference at a high frequency by using the optical phase-locked loop is achieved by locking to the hypotenuse of the AMD error signal through feedback, and the beat signal between the single-sideband optical signal generated by the acousto-optic modulator and the distributed feedback fiber narrow-linewidth laser generated by the distributed feedback fiber laser unit after feedback tracking of the narrow-linewidth laser is locked to the external crystal filter reference.

[0014] Further, the narrow-linewidth laser generated by the tunable narrow-linewidth laser in the PDH injection-locking module is output to the first fiber coupler and then divided into two beams. One of the beams is phase-modulated by the phase modulator, and the modulation signal is provided by the signal generator, and then enters the optical fiber circulator through the "1" port of the optical fiber circulator and exits from the "2" port of the optical fiber circulator to the strain-sensing distributed feedback fiber laser module;

[0015] After the pump source generates pump laser, it is injected into the distributed feedback fiber laser unit through a wavelength division multiplexer, and then the distributed feedback fiber narrow linewidth laser is generated and radiated to both ends. One end is split by a wavelength division multiplexer and then injected into a third fiber coupler, and then respectively incident into a PDH injection locking module and a phase-locked loop module;

[0016] When the narrow linewidth laser wavelength generated by the tunable narrow linewidth laser matches the distributed feedback fiber narrow linewidth laser wavelength generated by the distributed feedback fiber laser unit, the two narrow linewidth lasers will produce an injection locking effect and enter through the "2" port of the fiber optic circulator together, and exit through the "3" port of the fiber optic circulator to a single-channel photodetector. After demodulation by a lock-in amplifier, a PDH error signal amplified by the injection locking technology is obtained. Through the first loop filter, the low-frequency feedback of the tunable narrow linewidth laser is tracked at the central zero point of the injection locking PDH error signal. At this time, the central wavelength of the tunable narrow linewidth laser is tracked at the central wavelength of the distributed feedback fiber laser unit to compensate for the free-running noise of the distributed feedback fiber laser unit;

[0017] Another beam of laser from the tunable narrow linewidth laser after the first fiber coupler passes through an acousto-optic modulator to generate a single-sideband optical signal, and its single-sideband frequency shift amount is determined by the radio frequency signal provided by a direct digital frequency synthesizer. Another beam of laser from the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit after the third fiber coupler and the single-sideband optical signal are incident into a second fiber coupler together, and then the two outputs of the second fiber coupler are incident into a balanced photodetector to generate a beat signal. The beat signal passes through a crystal filter and then generates an AMD error signal through an amplitude detector unit. Through the second loop filter, the direct digital frequency synthesizer is high-frequency regulated to feedback-lock the single-sideband optical signal generated by the acousto-optic modulator on the slope of the AMD error signal. At this time, the beat signal between the single-sideband optical signal and the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit is beat-locked on the external crystal filter reference through the phase-locked loop technology.

[0018] Further, the tunable narrow linewidth laser has an external voltage tunable function, and its central wavelength tuning range covers the central wavelength of the distributed feedback fiber laser unit.

[0019] Further, the first fiber coupler and the third fiber coupler can be 1x2 fiber couplers or 2x2 fiber couplers with different splitting ratios.

[0020] Further, the second fiber coupler is a 50:50 2x2 fiber coupler.

[0021] Further, the pump source is a 980nm pump laser or a 1480nm pump laser.

[0022] Further, the distributed feedback fiber laser unit is a π-phase shift fiber grating directly written on erbium-doped fiber.

[0023] The technical effects achieved by the present invention are as follows:

[0024] A PLL DFB strain sensor based on injection locking of the present invention utilizes the PDH technology and the optical injection locking technology. By feedback tracking the central zero point of the PDH error signal amplified by the injection locking technology, the free-running noise of the distributed feedback fiber laser is compensated; by using the phase-locked loop technology and feedback locking to the hypotenuse of the AMD error signal, high-resolution real-time strain measurement of the distributed feedback fiber laser is achieved, effectively solving the interference of the free-running noise of the traditional distributed feedback fiber laser-based strain sensing scheme, and having a higher strain measurement resolution. Description of the Drawings

[0025] Figure 1 is a schematic diagram of the solution of the present invention;

[0026] Figure 2 is a schematic diagram of the principle of the PLL (a) DFB strain sensing based on injection locking (b) of the present invention;

[0027] Figure 3 is a schematic diagram of the injection-locked PDH error signal (a) and the AMD error signal (b) of the present invention.

[0028] In the drawings, the list of components represented by each reference numeral is as follows:

[0029] 1. PDH injection locking module; 2. Phase-locked loop module; 3. Strain sensing distributed feedback fiber laser module; 101. Tunable narrow linewidth laser; 102. First fiber coupler; 103. Phase modulator; 104. Fiber circulator; 105. Single-channel photodetector; 106. Lock-in amplifier; 107. First loop filter; 108. Signal generator; 201. Acousto-optic modulator; 202. Second fiber coupler; 203. Balanced photodetector; 204. Crystal filter; 205. Amplitude detector unit; 206. Second loop filter; 207. Direct digital frequency synthesizer; 301. Pump source; 302. Wavelength division multiplexer; 303. Distributed feedback fiber laser unit; 304. Third fiber coupler. Detailed Embodiments

[0030] In order to make the objectives and advantages of the present invention clearer, the present invention will be specifically described below in conjunction with embodiments. It should be understood that the following text is only used to describe one or several specific implementation manners of the present invention, and does not strictly limit the specific protection scope claimed by the present invention.

[0031] As Figures 1-3 shown, a DFB strain sensor based on injection locking optical phase-locked loop includes a PDH injection locking module 1, an optical phase-locked loop module 2, and a strain sensing distributed feedback fiber laser module 3;

[0032] The PDH injection locking module 1 includes a tunable narrow linewidth laser 101, a first fiber coupler 102, a phase modulator 103, a fiber circulator 104, a single-channel photodetector 105, a lock-in amplifier 106, a first loop filter 107, and a signal generator 108;

[0033] The optical phase-locked loop module 2 includes an acousto-optic modulator 201, a second fiber coupler 202, a balanced photodetector 203, a crystal filter 204, an amplitude detector unit 205, a second loop filter 206, and a direct digital frequency synthesizer 207;

[0034] The strain sensing distributed feedback fiber laser module 3 includes a pump source 301, a wavelength division multiplexer 302, a distributed feedback fiber laser unit 303, and a third fiber coupler 304;

[0035] Among them, the core of this technical solution is to use the distributed feedback fiber laser unit 303 as a strain sensing element; combining the PDH technology and the injection locking technology, by feeding back and tracking the center zero point of the PDH error signal amplified by the feedback tracking injection locking technology, the wavelength of the tunable narrow linewidth laser 101 as a reference channel is fed back and tracked to the distributed feedback fiber laser unit 303 at a low frequency to achieve free-running noise compensation of the distributed feedback fiber laser unit 303;

[0036] The acousto-optic modulator 201 is used to generate a single-sideband optical signal as a strain sensing channel, the direct digital frequency synthesizer 207 provides a radio frequency signal for sideband modulation, and the single-sideband optical signal is locked to the external crystal filter 204 reference at a high frequency by using the optical phase-locked loop. At this time, the external strain sensing quantity is output as the signal fed back to the direct digital frequency synthesizer 207.

[0037] Locking the single-sideband optical signal to the external crystal filter 204 reference at a high frequency by using the optical phase-locked loop specifically means locking the beat frequency signal between the single-sideband optical signal generated by the acousto-optic modulator 201 after the narrow linewidth laser is fed back and tracked and the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit 303 to the external crystal filter 204 reference by feeding back and locking to the hypotenuse of the AMD error signal, so as to realize high-resolution strain real-time measurement of the distributed feedback fiber laser unit 303.

[0038] This optical phase-locked loop distributed feedback fiber laser strain sensing scheme based on PDH injection locking combines PDH technology, optical injection locking technology, and optical phase-locked loop technology to achieve high-resolution real-time strain measurement of the distributed feedback fiber laser unit 303, effectively solving the self-free-running noise interference of traditional strain sensing schemes based on the distributed feedback fiber laser unit 303 and having a higher strain measurement resolution.

[0039] Among them, the narrow-linewidth laser generated by the tunable narrow-linewidth laser 101 in the PDH injection locking module 1 is output to the first fiber coupler 102 and then divided into two beams. One beam is phase-modulated by the phase modulator 103, and the modulation signal is provided by the signal generator 108. Then it is incident into the fiber circulator 104 through the "1" port of the fiber circulator 104 and exits from the "2" port of the fiber circulator 104 to the strain sensing distributed feedback fiber laser module 3; the pump source 301 generates pump laser, which is incident into the distributed feedback fiber laser unit 303 through the wavelength division multiplexer 302, and then generates distributed feedback fiber narrow-linewidth laser that radiates to both ends. One end is incident into the third fiber coupler 304 through the wavelength division multiplexer 302 for splitting light and is respectively incident into the PDH injection locking module 1 and the optical phase-locked loop module 2;

[0040] When the wavelength of the narrow-linewidth laser generated by the tunable narrow-linewidth laser 101 matches the wavelength of the distributed feedback fiber narrow-linewidth laser generated by the distributed feedback fiber laser unit 303, the two narrow-linewidth lasers will produce an injection locking effect and are incident together through the "2" port of the fiber circulator 104 and exit from the "3" port of the fiber circulator 104 to the single-channel photodetector 105. The PDH error signal amplified by the injection locking technology is obtained through demodulation by the lock-in amplifier 106. The tunable narrow-linewidth laser 101 is low-frequency feedback tracked at the center zero point of the injection locking PDH error signal through the first loop filter 107. At this time, the center wavelength of the tunable narrow-linewidth laser 101 is tracked at the center wavelength of the distributed feedback fiber laser unit 303 to compensate for the free-running noise of the distributed feedback fiber laser unit 303;

[0041] Another beam of laser from the tunable narrow-linewidth laser 101 after the first fiber coupler 102 passes through the acousto-optic modulator 201 to generate a single-sideband optical signal, and the single-sideband frequency shift amount thereof is determined by the radio frequency signal provided by the direct digital frequency synthesizer 207. Another beam of laser from the distributed feedback fiber narrow-linewidth laser generated by the distributed feedback fiber laser unit 303 after the third fiber coupler 304 and the single-sideband optical signal are incident into the second fiber coupler 202 together, and then the two ends of the second fiber coupler 202 output and are incident into the balanced photodetector 203 to generate a beat signal. The beat signal passes through the crystal filter 204 and then generates an AMD error signal through the amplitude detector unit 205. The direct digital frequency synthesizer 207 is regulated at high frequency through the second loop filter 206 to feedback-lock the single-sideband optical signal generated through the acousto-optic modulator 201 on the slope of the AMD error signal. At this time, the beat signal between the single-sideband optical signal and the distributed feedback fiber narrow-linewidth laser generated by the distributed feedback fiber laser unit 303 is beat-locked on the external crystal filter 204 reference through the optical phase-locked loop technology, so as to realize the real-time measurement of the strain signal loaded on the distributed feedback fiber laser unit 303 in the external environment.

[0042] The tunable narrow-linewidth laser 101 has an external voltage tunable function, and its central wavelength tuning range covers the central wavelength of the distributed feedback fiber laser unit 303;

[0043] The functions of the first fiber coupler 102 and the third fiber coupler 304 are optical energy distribution, and they can be replaced by 1×2 fiber couplers or 2×2 fiber couplers with different splitting ratios;

[0044] The function of the phase modulator 103 is to modulate the phase of the optical signal, and it can be replaced by other devices with the function of modulating the phase of the optical signal;

[0045] The lock-in amplifier 106 has an IQ demodulation function and can be replaced by other IQ demodulation modules;

[0046] The functions of the first loop filter 107 and the second loop filter 206 are feedback control, and they can be replaced by other feedback control function modules;

[0047] The function of the acousto-optic modulator 201 is to generate a single-sideband optical signal, and its modulation radio frequency source is provided by the direct digital frequency synthesizer 207. Among them, the acousto-optic modulator 201 can be replaced by other devices with the function of generating a single-sideband optical signal, and the direct digital frequency synthesizer 207 can be replaced by other radio frequency sources with voltage control and numerical control;

[0048] The function of the second fiber coupler 202 is to combine the laser energies of the two incident channels and achieve equal split output, which is a 50:50 2×2 fiber coupler;

[0049] The balanced photodetector 203 is configured to receive the outputs of the two output ports of the second fiber coupler 202, obtaining the single-sideband optical signal and the beat signal of the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit 303. The frequency of this beat signal is related to the frequency source provided by the direct digital frequency synthesizer 207 to the acousto-optic modulator 201;

[0050] The crystal filter 204 is configured to provide band-pass filtering within the frequency variation range of the beat signal output by the balanced photodetector 203, and can be replaced by a band-pass filter or a band-stop filter of other structures;

[0051] The amplitude detector unit 205 is configured to perform amplitude detection on the beat signal after passing through the crystal filter 204 to generate an AMD error signal, and can be replaced by an active or passive amplitude detector;

[0052] The pump source 301 is configured to provide pump laser, and can be replaced by a 980nm pump laser or a 1480nm pump laser;

[0053] The wavelength division multiplexer 302 is configured to couple the pump laser provided by the pump source 301 into the distributed feedback fiber laser unit 303, and separate the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit 303 from the pump laser. Its parameters should be matched with the central wavelength of the pump source 301 and the central wavelength of the distributed feedback fiber laser unit 303;

[0054] The structure of the distributed feedback fiber laser unit 303 is a π-phase shift fiber grating directly written on erbium-doped fiber. It is equivalent to a π-phase shift fiber grating under the condition that there is no incident pump laser from the pump source 301. Under the condition that there is incident pump laser from the pump source 301, a resonant cavity and frequency selection mode locking are formed through the π-phase shift grating, thereby realizing the output of narrow linewidth laser;

[0055] An optical phase-locked loop distributed feedback fiber laser strain sensing scheme based on PDH injection locking uses PDH technology and optical injection locking technology. By tracking the center zero point of the PDH error signal amplified by the feedback tracking injection locking technology, the tunable narrow linewidth laser 101 is locked on the distributed feedback fiber laser unit 303 to compensate for the free-running noise of the distributed feedback fiber laser unit 303; Using the optical phase-locked loop technology, by locking to the hypotenuse of the AMD error signal through feedback, the single-sideband optical signal generated by the narrow linewidth laser after feedback tracking via the acousto-optic modulator 201 and the beat signal of the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit 303 are locked on the external crystal filter 204 reference to achieve high-resolution strain real-time measurement of the distributed feedback fiber laser unit 303.

[0056] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. The structures, devices, and operation methods not specifically described and explained in the present invention are implemented by conventional means in the art without special description and limitation.

Claims

1. An optical phase-locked loop (DFB) strain sensor based on injection locking, characterized in that: It comprises a PDH injection locking module (1), an optical phase-locked loop module (2), and a strain sensing distributed feedback fiber laser module (3); The PDH injection locking module (1) comprises a tunable narrow linewidth laser (101), a first optical fiber coupler (102), a phase modulator (103), an optical fiber circulator (104), a single-channel photodetector (105), a phase-locked amplifier (106), a first loop filter (107) and a signal generator (108); The optical phase-locked loop module (2) comprises an acousto-optic modulator (201), a second optical fiber coupler (202), a balanced photodetector (203), a crystal filter (204), an amplitude detector unit (205), a second loop filter (206) and a direct digital frequency synthesizer (207); The strain sensing distributed feedback fiber laser module (3) comprises a pump source (301), a wavelength division multiplexer (302), a distributed feedback fiber laser unit (303) and a third fiber coupler (304); The distributed feedback fiber laser unit (303) is used as a strain sensing element; the wavelength of the tunable narrow linewidth laser (101) as a reference channel is tracked to the distributed feedback fiber laser unit (303) by feedback tracking the central zero point of the PDH error signal amplified by the injection locking technology, and low-frequency feedback is used to track the wavelength to the distributed feedback fiber laser unit (303), thereby realizing free-running noise compensation of the distributed feedback fiber laser unit (303); The acousto-optic modulator (201) generates a single-sideband optical signal as a strain sensing channel, a direct digital frequency synthesizer (207) provides a sideband modulated radio frequency signal, and an optical phase-locked loop is used to lock the single-sideband light to an external crystal filter (204) reference with high-frequency feedback, and the external strain sensing quantity is fed back to the direct digital frequency synthesizer (207) as a signal output.

2. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The optical phase-locked loop is used to lock the single-sideband light to the external crystal filter (204) with high-frequency feedback. The reference is: By feedback locking to the hypotenuse of the AMD error signal, the single sideband optical signal generated by the narrow linewidth laser after feedback tracking via the acousto-optic modulator (201) and the beat frequency signal of the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit (303) are locked to the external crystal filter (204) reference.

3. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The narrow linewidth laser light generated by the tunable narrow linewidth laser (101) in the PDH injection locking module (1) is output to the first optical fiber coupler (102) and then divided into two beams, one of which is phase modulated by a phase modulator (103), the modulation signal is provided by a signal generator (108), and then injected into the optical fiber circulator (104) through the "1" port of the optical fiber circulator (104), and then emitted to the strain sensing distributed feedback optical fiber laser module (3) through the "2" port of the optical fiber circulator (104); The pump source (301) generates pump laser light which is injected into the distributed feedback fiber laser unit (303) via the wavelength division multiplexer (302), and generates distributed feedback fiber narrow line width laser light which is radiated to both ends, one end of which passes through the wavelength division multiplexer (302) and is injected into the third fiber coupler (304) for splitting, and is respectively injected into the PDH injection locking module (1) and the optical phase-locked loop module (2); When the wavelength of the narrow linewidth laser generated by the tunable narrow linewidth laser (101) matches the wavelength of the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit (303), the two narrow linewidth laser beams will produce an injection locking effect, and are incident from the "2" port of the fiber circulator (104) together, and are emitted from the "3" port of the fiber circulator (104) to the single-channel photodetector (105), and are demodulated by the phase-locked amplifier (106) to obtain a PDH error signal amplified by the injection locking technology, and the low-frequency feedback of the tunable narrow linewidth laser (101) is tracked at the central zero point of the injection-locked PDH error signal through the first loop filter (107). At this time, the central wavelength of the tunable narrow linewidth laser (101) is tracked on the central wavelength of the distributed feedback fiber laser unit (303) to compensate for the free-running noise of the distributed feedback fiber laser unit (303); The tunable narrow linewidth laser (101) passes through another laser beam after the first optical fiber coupler (102), and passes through an acousto-optic modulator (201) to generate a single sideband optical signal, wherein the frequency shift of the single sideband is determined by a radio frequency signal provided by a direct digital frequency synthesizer (207). The distributed feedback optical fiber narrow linewidth laser generated by the distributed feedback optical fiber laser unit (303) passes through another laser beam after the third optical fiber coupler (304), and is incident on the second optical fiber coupler (202) together with the single sideband optical signal, and then the outputs at both ends of the second optical fiber coupler (202) are incident on a balanced photodetector. (203) to generate a beat signal, the beat signal passes through a crystal filter (204) and then generates an AMD error signal through an amplitude detector unit (205), and the single-sideband optical signal generated by the acousto-optic modulator (201) is feedback-locked to the hypotenuse of the AMD error signal through a second loop filter (206) by high-frequency control of a direct digital frequency synthesizer (207), at which time the single-sideband optical signal and the beat signal of the distributed feedback fiber narrow linewidth laser generated by the distributed feedback fiber laser unit (303) are beat-locked to the reference of the external crystal filter (204) through an optical phase-locked loop technology.

4. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The tunable narrow linewidth laser (101) has an external voltage tunable function, and its central wavelength tuning range covers the central wavelength of the distributed feedback fiber laser unit (303).

5. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The first optical fiber coupler (102) and the third optical fiber coupler (304) can be 1-to-2 optical fiber couplers or 2-to-2 optical fiber couplers with different splitting ratios.

6. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The second optical fiber coupler (202) is a 50:50 2-to-2 optical fiber coupler.

7. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The pump source (301) is a 980nm pump laser or a 1480nm pump laser.

8. The optical phase-locked loop (DFB) strain sensor based on injection locking according to claim 1, characterized in that: The distributed feedback fiber laser unit (303) is a π-phase-shifted fiber grating directly written on an erbium-doped fiber.

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