A BOTDR system based on an actively mode-locked dual-wavelength fiber laser

Abstract

This invention proposes a BOTDR system based on an actively mode-locked dual-wavelength fiber laser, comprising a dual-wavelength light source module, a coherent signal detection module, and a signal processing module. The coherent signal detection module is connected to both the dual-wavelength light source module and the signal processing module. The dual-wavelength light source module includes, in sequence, an electro-optic modulation unit, a fiber amplification unit, an optical frequency selection unit, and a first beam splitter. The fiber amplification unit generates fluorescence through spontaneous emission and emits the fluorescence to the optical frequency selection unit. The optical frequency selection unit selects the frequency of the fluorescence and emits the resulting first beam to the first beam splitter. The electro-optic modulation unit shifts the frequency of the first beam to form a second beam. The first beam splitter transmits the first and second beams at a preset splitting ratio to the electro-optic modulation unit and the coherent signal detection module, respectively. This application helps improve the system's signal-to-noise ratio and measurement accuracy.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, and in particular to a BOTDR system based on an actively mode-locked dual-wavelength fiber laser. Background Technology

[0002] In recent years, with the development of the Internet of Things (IoT) and smart cities, my country's demand for distributed fiber optic sensing technology has shown an exponential growth trend. Distributed Brillouin fiber optic sensing technology can measure the temperature or strain of the environment surrounding the fiber using the Brillouin scattering effect in standard single-mode fiber. It has been widely used in monitoring the status of large building structures, bridge health, petrochemical pipelines, military security, power cables, and coal mine safety. The Brillouin Optical Time Domain Reflectometer (BOTDR) is a typical distributed fiber optic sensing system with the advantage of single-end injection, and it has broad application prospects.

[0003] Chinese Patent Publication No. CN114608721A discloses a distributed optical fiber temperature strain sensing device, which includes a semiconductor laser, a 1*2 optical fiber coupler, a first electro-optic modulator, a pulse driver, a polarization scrambler, an optical fiber circulator, a Raman wavelength division multiplexer, a sensing optical fiber, a first avalanche photodiode detection module, a second avalanche photodiode detection module, a second electro-optic modulator, a microwave scanner, a 2*2 optical fiber coupler, a balanced detection module, and a multi-channel data acquisition system. This application utilizes a combination of coherent Brillouin scattering (BOTDR) and Raman scattering (DT)... The above method solves the temperature and strain cross-sensitivity effect of Brillouin scattering and realizes online monitoring of temperature and strain parameters of sensing fiber. However, the above method only uses semiconductor laser as light source. Due to the influence of the stimulated Brillouin threshold on the output power of a single wavelength beam, the pump power can only reach the stimulated Brillouin threshold of a single wavelength beam. Ultimately, this leads to the problem that the signal-to-noise ratio and measurement accuracy of the BOTDR system cannot meet the required accuracy. Therefore, it is necessary to provide a primary BOTDR system with higher incident pump power to improve the system signal-to-noise ratio and measurement accuracy. Summary of the Invention

[0004] In view of this, the present invention proposes a BOTDR system based on an actively mode-locked dual-wavelength fiber laser. By using the autofluorescence of the fiber amplification unit as a light source, and employing an optical frequency selection unit and an electro-optic modulation unit, two wavelengths are output, thereby improving the system's signal-to-noise ratio and measurement accuracy.

[0005] This invention provides a BOTDR system based on an actively mode-locked dual-wavelength fiber laser, comprising a dual-wavelength light source module, a coherent signal detection module, and a signal processing module. The coherent signal detection module is connected to both the dual-wavelength light source module and the signal processing module.

[0006] The dual-wavelength light source module includes an electro-optic modulation unit, an optical fiber amplification unit, an optical frequency selection unit, and a first beam splitting unit connected in sequence. The optical fiber amplification unit generates fluorescence through spontaneous emission and emits the fluorescence to the optical frequency selection unit. The optical frequency selection unit selects the frequency of the fluorescence and emits the selected first beam to the first beam splitting unit. The electro-optic modulation unit shifts the frequency of the first beam to form a second beam. The first beam splitting unit transmits the first beam and the second beam to the electro-optic modulation unit and the coherent signal detection module respectively with a preset splitting ratio.

[0007] Based on the above technical solutions, preferably, the electro-optic modulation unit includes a first polarization controller, a first microwave source, and a first electro-optic modulator. The first polarization controller is connected to the first beam splitting unit, and the first electro-optic modulator is connected to the fiber amplification unit. The first electro-optic modulator is connected to both the first polarization controller and the first microwave source. The first polarization controller adjusts the polarization state of the light input to the first electro-optic modulator. The first microwave source drives the first electro-optic modulator so that the first electro-optic modulator performs harmonic frequency locking based on the fundamental frequency light. The first polarization controller shifts the frequency of the first beam to form the second beam.

[0008] Based on the above technical solutions, preferably, the fiber amplification unit includes a second polarization controller and a first erbium-doped fiber amplifier connected in sequence. The second polarization controller is connected to the electro-optic modulation unit, and the first erbium-doped fiber amplifier is connected to the optical frequency selection unit. The second polarization controller adjusts the polarization state of the light input to the first erbium-doped fiber amplifier and emits the adjusted light to the first erbium-doped fiber amplifier. The first erbium-doped fiber amplifier emits the fluorescence and amplifies the power of the light passing through the first erbium-doped fiber amplifier.

[0009] More preferably, the optical frequency selection unit includes a first optical circulator, an apodized fiber grating, and an optical isolator. The first port of the first optical circulator is connected to the fiber amplification unit, the second port of the first optical circulator is connected to the apodized fiber grating, the third port of the first optical circulator is connected to the optical isolator, and the optical isolator is connected to the first beam splitter. The first optical circulator transmits the fluorescence, the first beam, and the second beam in a preset transmission direction. The apodized fiber grating reflects the first beam that satisfies the Bragg condition back into the first optical circulator, and the optical isolator isolates the beam reflected by the apodized fiber grating.

[0010] More preferably, the coherent signal detection module includes a second beam splitting unit, a pump detection path, a local oscillation signal path, and a beat frequency unit. The input terminals of the pump detection path and the local oscillation signal path are both connected to the second beam splitting unit, and the output terminals of the pump detection path and the local oscillation signal path are both connected to the beat frequency unit. The beat frequency unit is connected to the signal processing module. The second beam splitting unit is used to transmit the first beam and the second beam to the pump detection path and the local oscillation signal path respectively at a preset splitting ratio. The pump detection path is used to generate back-stokes light through spontaneous Pryoulance scattering within the optical fiber. The local oscillation signal path is used to generate Stokes sidebands through a modulator. The beat frequency unit is used to perform optical beat frequency mixing between the back-stokes light and the Stokes sidebands.

[0011] More preferably, both the first beam splitting unit and the second beam splitting unit include a Y-type fiber coupler, and the beam splitting ratio of the Y-type fiber coupler is 90 / 10.

[0012] More preferably, the pump-probe path includes a third polarization controller, an acousto-optic modulator, a pulse generator, a second erbium-doped fiber amplifier, a bandpass filter, a second optical circulator, and a single-mode fiber connected in sequence. The third polarization controller is connected to the second beam splitter, and the second optical circulator is connected to the beat frequency unit. The third polarization controller adjusts the polarization state of the light input to the acousto-optic modulator. The pulse generator drives the acousto-optic modulator so that the acousto-optic modulator modulates the input beam into a probe light pulse. The second erbium-doped fiber amplifier amplifies the probe light pulse and then filters out the spontaneous amplification noise through the bandpass filter. The probe light pulse enters the second optical circulator as backscattered Stokes light in the single-mode fiber.

[0013] More preferably, the local oscillation signal path includes a fourth polarization controller, a second electro-optic modulator, a second microwave source, and a polarization scrambler connected in sequence. The fourth polarization controller is connected to the second beam splitting unit, and the polarization scrambler is connected to the beat frequency unit. The fourth polarization controller adjusts the polarization state of the light input to the second electro-optic modulator, and the second microwave source drives the second electro-optic modulator so that the second electro-optic modulator outputs the Stokes sideband. The polarization scrambler suppresses the polarization noise generated in the single-mode fiber.

[0014] More preferably, the beat frequency unit is an X-type fiber optic coupler.

[0015] More preferably, the signal processing module includes a balanced detector and a data acquisition and processing unit, wherein the balanced detector is connected to the data acquisition and processing unit.

[0016] The BOTDR system based on an actively mode-locked dual-wavelength fiber laser of the present invention has the following advantages over the prior art:

[0017] The fluorescence generated by the spontaneous emission of the first erbium-doped fiber amplifier enters the apodized fiber grating for wavelength selection. An electro-optic modulator is used to actively mode-lock and frequency-shift the selected beam, thereby forming a dual-wavelength output. The dual-wavelength beam passes through an optical circulator and the apodized fiber grating back to the first erbium-doped fiber amplifier for amplification. After multiple cycles of amplification, a laser output is formed, providing a light source for the coherent signal detection module, thereby improving the system signal-to-noise ratio and extending the sensing distance.

[0018] By keeping the power corresponding to a single wavelength below the stimulated Brillouin scattering threshold, dual-wavelength pumping can greatly improve the total pump power of the BOTDR system.

[0019] Compared to tunable single-wavelength lasers, the frequency uncertainty obtained by actively mode-locked dual-wavelength fiber lasers is reduced by half, thereby improving the measurement accuracy of the system. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the BOTDR system provided in an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the structure of the BOTDR system provided in an embodiment of the present invention;

[0023] Figure 3 The output spectrum of the dual-wavelength light source module provided in this embodiment of the invention;

[0024] Figure 4 A three-dimensional curve of the Brillouin gain spectrum provided in an embodiment of the present invention;

[0025] Figure 5 A temperature test result diagram provided for an embodiment of the present invention;

[0026] Figure 6 The figure shows the comparison results of system measurement accuracy provided in the embodiments of the present invention.

[0027] Figure reference numerals: 1. Dual-wavelength light source module; 11. Electro-optic modulation unit; 111. First polarization controller; 112. First microwave source; 113. First electro-optic modulator; 12. Fiber amplification unit; 121. Second polarization controller; 122. First erbium-doped fiber amplifier; 13. Optical frequency selection unit; 131. First optical circulator; 132. Apodized fiber grating; 133. Optical isolator; 14. First beam splitting unit; 141. First Y-coupler; 2. Coherent signal detection module; 21. Second beam splitting unit; 211. Third erbium-doped fiber amplifier; 2 12. Second Y-coupler; 22. Pump detection path; 221. Third polarization controller; 222. Pulse generator; 223. Acousto-optic modulator; 224. Second erbium-doped fiber amplifier; 225. Bandpass filter; 226. Second optical circulator; 227. Single-mode fiber; 23. Local oscillation signal path; 231. Fourth polarization controller; 232. Second microwave source; 233. Second electro-optic modulator; 234. Polarization scrambler; 24. Beat frequency unit; 241. X-coupler; 3. Signal processing module; 31. Balance detector; 32. Data acquisition and processing unit. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Before introducing the embodiments of the present invention, some terms involved in the embodiments of the present invention will be defined and explained.

[0030] Apodized grating: If there is a transition process at the beginning and end of the refractive index modulation of the grating, and its refractive index modulation envelope is not uniform but presents a certain functional form, the spectrum of the grating will be greatly improved. This is called apodization of the grating, and such a grating is called an apodized grating.

[0031] Spatial hole burning effect: When two counter-propagating quasi-monochromatic light waves are superimposed, they form a so-called standing-wave interference pattern with a period half the wavelength. When this occurs in a laser gain medium, the gain preferentially saturates at the inverse node of the pattern. Stimulated emission keeps the excitation of laser-active ions at a low level, thus creating a periodic modulation pattern. The resulting amplification of light with a certain wavelength (possibly deviating from the laser wavelength) depends on how well its own standing-wave mode adapts to the modulation excitation of the gain medium. The total (single-pass or double-pass) gain is most strongly saturated at the laser wavelength itself, and the node of the light is precisely in the region of strongest saturation. Light of other wavelengths experiences lower gain saturation, which may lead to a distortion of the gain spectrum shape, i.e., a non-uniform gain saturation.

[0032] This invention discloses a BOTDR system based on an actively mode-locked dual-wavelength fiber laser, with reference to... Figure 1 The aforementioned BOTDR system includes a dual-wavelength light source module 1, a coherent signal detection module 2, and a signal processing module 3. The coherent signal detection module 2 is connected to both the dual-wavelength light source module 1 and the signal processing module 3.

[0033] The dual-wavelength light source module 1 includes an electro-optic modulation unit 11, an optical fiber amplification unit 12, an optical frequency selection unit 13, and a first beam splitting unit 14 connected in sequence, such as... Figure 2 As shown, where,

[0034] The fiber optic amplification unit 12 is used to generate fluorescence through spontaneous emission and emit the fluorescence to the optical frequency selection unit 13. The fiber optic amplification unit 12 includes a second polarization controller 121 and a first erbium-doped fiber amplifier 122 connected in sequence. The second polarization controller 121 is connected to the electro-optic modulation unit 11, and the first erbium-doped fiber amplifier 122 is connected to the optical frequency selection unit 13. The second polarization controller 121 can be a TH-PFC030 three-ring polarization controller, and the first erbium-doped fiber amplifier 122 can be an AEDFA-30 erbium-doped fiber amplifier. The first erbium-doped fiber amplifier 122 has a built-in 980 nm pump source with a pump current of 50 mA. The second polarization controller 121 adjusts the polarization state of the light input to the first erbium-doped fiber amplifier 122 and emits the adjusted light to the first erbium-doped fiber amplifier 122. The first erbium-doped fiber amplifier 122 emits fluorescence and amplifies the power of the light passing through it.

[0035] The optical frequency selection unit 13 is used to select the frequency of fluorescence and emit the first beam obtained by frequency selection to the first beam splitting unit 14. The optical frequency selection unit 13 includes a first optical circulator 131, an apodized fiber grating 132 and an optical isolator 133. The first port of the first circulator is connected to the fiber amplification unit 12, the second port of the first optical circulator 131 is connected to the apodized fiber grating 132, the third port of the first circulator is connected to the optical isolator 133, and the optical isolator 133 is connected to the first beam splitting unit 14.

[0036] The first circulator transmits fluorescence, a first beam, and a second beam in a preset transmission direction. The preset transmission direction can be first port – second port – third port – first port, so that the beam input to the first optical circulator 131 can only propagate in the preset transmission direction.

[0037] The apodized fiber grating 132 reflects a first beam satisfying the Bragg condition into a first circulator. In one example, the apodized fiber grating 132 has a 3 dB bandwidth of 0.15 nm and its temperature can be controlled at 23°C by a temperature controller electrically connected to the apodized fiber grating 132, resulting in a center wavelength of 1550.126 nm for the reflected beam. Furthermore, the first circulator is used in conjunction with the apodized fiber grating 132 to prevent the backlight generated in the apodized fiber grating 132 from returning to the first erbium-doped fiber amplifier 122, thereby suppressing the spatial hole burning effect.

[0038] Optical isolator 133 uses the Faraday effect of magneto-optical crystal to isolate the beam reflected by the apodized fiber grating 132, so that the beam can only be transmitted in one direction. The light reflected by the fiber optic echo can be well isolated by optical isolator 133, thereby improving the light transmission efficiency.

[0039] The electro-optic modulation unit 11 is used to shift the frequency of the first beam to form a second beam. The electro-optic modulation unit 11 includes a first polarization controller 111, a first microwave source 112, and a first electro-optic modulator 113. The first polarization controller 111 is connected to the first beam splitting unit 14, and the first electro-optic modulator 113 is connected to the fiber amplification unit 12. The first electro-optic modulator 113 is connected to the first polarization controller 111 and the first microwave source 112 respectively. The first electro-optic modulator can be a lithium niobate intensity modulator.

[0040] The first polarization controller 111 adjusts the polarization state of the light input to the first electro-optic modulator. The first microwave source 112 drives the first electro-optic modulator 113 to perform harmonic mode locking based on the fundamental frequency light. The first polarization controller 111 shifts the frequency of the first beam to form a second beam. The first polarization controller 111 can be a TH-PFC030 three-ring polarization controller. The first microwave source 112 outputs a 4.99 GHz microwave signal to perform harmonic mode locking on the first electro-optic modulator 113 based on the fundamental frequency, causing the dual-wavelength light source module 1 to output two lasers with a wavelength interval of 0.04 nm. Simultaneously, the first polarization controller 111 and the second polarization controller 121 adjust the polarization states of the input and output light of the first electro-optic modulator 113 to maximize both input and output power, thereby maximizing the output power of the dual-wavelength light source module 1.

[0041] The first beam splitting unit 14 is used to transmit the first beam and the second beam to the electro-optic modulation unit 11 and the coherent signal detection module 2 respectively at a preset splitting ratio. The first beam splitting unit 14 includes a first Y-type fiber coupler. Since it is necessary to ensure that the amplification gain in the resonant cavity, which includes the first polarization controller 111, the first electro-optic modulator 113, the second polarization controller 121, the first optical circulator 131, the apodized fiber grating 132, the optical isolator 133, the first Y-type coupler 141, and the first microwave source 112, is greater than the loss, the splitting ratio of the first Y-type fiber coupler is preferably set to 90 / 10. 90% of the output of the first Y-type coupler 141 is fed back to the resonant cavity, and 10% of the output of the first Y-type coupler 141 is input to the coherent signal detection module 2. The cavity length of the resonant cavity is 16.6 mm. m, with a longitudinal mode spacing of 12.409 MHz, Figure 3 The output spectrum of the dual-wavelength laser from the dual-wavelength light source module is shown.

[0042] The fluorescence generated by the spontaneous emission of the first erbium-doped fiber amplifier 122 enters the apodized fiber grating 132 for wavelength selection. An electro-optic modulator is used to actively mode-lock and frequency-shift the selected beam, thereby forming a dual-wavelength output. The dual-wavelength beam passes through an optical circulator and the apodized fiber grating 132 back to the first erbium-doped fiber amplifier 122 for amplification. After multiple cycles of amplification, a laser output is formed, providing a light source for the coherent signal detection module 2, thereby improving the system signal-to-noise ratio and extending the sensing distance.

[0043] In one possible example, the first polarization controller 111, the first electro-optic modulator 113, and the second polarization controller 121 can be regarded as a frequency shifting branch of the dual-wavelength light source module 1. That is, the purpose of outputting multi-wavelength laser can be achieved by increasing the number of frequency shifting branches connected to the dual-wavelength light source module 1.

[0044] In this embodiment, as Figure 2 As shown, the coherent signal detection module 2 includes a second beam splitting unit 21, a pump detection path 22, a local oscillation signal path 23, and a beat frequency unit 24. The input terminals of the pump detection path 22 and the local oscillation signal path 23 are both connected to the second beam splitting unit 21, and the output terminals of the pump detection path 22 and the local oscillation signal path 23 are both connected to the beat frequency unit 24. The beat frequency unit 24 is connected to the signal processing module 3.

[0045] The second beam splitting unit 21 is used to transmit the first beam and the second beam to the pump detection path 22 and the local oscillation signal path 23 respectively with a preset beam splitting ratio. The second beam splitting unit 21 includes a third erbium-doped fiber amplifier 211 and a second Y-coupler 212. The dual-wavelength beam emitted from the dual-wavelength light source module 1 is amplified by the third erbium-doped fiber amplifier 211 and used as the light source of the coherent signal detection module 2. At the same time, the optical power of the coherent signal detection module 2 can be controlled by setting the pump current of the third erbium-doped fiber amplifier 211. The beam splitting ratio of the second Y-coupler 212 can be set to 90 / 10 or 70 / 30.

[0046] Because the differential of the acousto-optic modulator 223 in the pump-detector path 22 is too large, and continuous light needs to be converted into pulsed light, the power loss of the pump-detector path 22 is large. Therefore, 90% of the output of the second Y-coupler 212 needs to be input to the pump-detector path 22, and 10% of the output of the second Y-coupler 212 needs to be input to the local oscillation signal path 23, or 70% of the output of the second Y-coupler 212 needs to be input to the pump-detector path 22, and 30% of the output of the second Y-coupler 212 needs to be input to the local oscillation signal path 23. Preferably, the splitting ratio of the second Y-coupler 212 is set to 90 / 10.

[0047] The pump-probe path 22 is used to transmit back-stokes light generated by spontaneous Purlillon scattering within the single-mode fiber 227. The pump-probe path 22 includes, in sequence, a third polarization controller 221, an acousto-optic modulator 223, a pulse generator 222, a second erbium-doped fiber amplifier 224, a bandpass filter 225, a second optical circulator 226, and the single-mode fiber 227. The third polarization controller 221 is connected to the second beam splitter 21, and the second optical circulator 226 is connected to the beat frequency unit 24. The single-mode fiber 227 has a length of 48.6 km. The third polarization controller 221 adjusts the polarization state of the light input to the acousto-optic modulator 223. The pulse generator 222 drives the acousto-optic modulator 223 so that the acousto-optic modulator 223 modulates the input beam into a probe light pulse with adjustable pulse width, thereby enabling measurements with different spatial resolutions. The second erbium-doped fiber amplifier 224 adjusts the probe pulse light power and then filters out the spontaneous amplification noise through the bandpass filter 225. The probe light pulse enters the second optical circulator 226 after being backscattered by Brillouin scattering in the single-mode fiber 227. The third polarization controller 221 can be a three-ring polarization controller of model TH-PFC030, and the acousto-optic modulator 223 can be a Fiber-Q acousto-optic modulator 223.

[0048] The local oscillation signal path 23 is used to generate Stokes sidebands through the modulator. The local oscillation signal path 23 includes a fourth polarization controller 231, a second electro-optic modulator 233, a second microwave source 232, and a polarization scrambler 234 connected in sequence. The fourth polarization controller 231 is connected to the second beam splitting unit 21, and the polarization scrambler 234 is connected to the beat frequency unit 24. The fourth polarization controller 231 adjusts the polarization state of the input light of the second electro-optic modulator 233. The second microwave source 232 drives the second electro-optic modulator 233 so that the second electro-optic modulator 233 outputs Stokes sidebands. The second electro-optic modulator operates in a carrier suppression state. The optical sidebands output by the second electro-optic modulator change near the Brillouin frequency shift to obtain the Brillouin gain spectrum. The polarization scrambler 234 with a polarization scrambling frequency of 700 MHz suppresses the polarization noise generated in the single-mode fiber 227. The fourth polarization controller 231 can be a three-ring polarization controller of model TH-PFC030, and the second electro-optic modulator 233 can be a lithium niobate intensity modulator.

[0049] The beat frequency unit 24 is used to optically beat the backward Stokes light and the Stokes sideband. The beat frequency unit 24 is an X-type fiber coupler. The backward Stokes light output from the pump detection circuit 22 and the Stokes sideband output from the local oscillation signal circuit 23 are optically beaten in the X-type fiber coupler, so that the phase difference and sum of the Stokes light and the Stokes sideband are achieved, thereby reducing the bandwidth requirements of the detection equipment.

[0050] In this embodiment, the signal processing module 3 includes a balanced detector 31 and a data acquisition and processing unit 32. The balanced detector 31 is connected to the data acquisition and processing unit 32. The local oscillation optical signal output from the local oscillation signal path 23 and the probe optical signal output from the pump probe path 22 are coupled by an X-type fiber coupler and then coherently detected by the balanced photodetector. The intermediate frequency electrical signal output by the balanced detector 31 is acquired and processed by the data acquisition and processing module to obtain the Brillouin frequency shift distribution along the single-mode fiber 227. Then, based on the demodulation relationship between the Brillouin frequency shift and temperature and strain, fiber-optic distributed temperature or strain sensing is realized. The balanced detector 31 can be selected as the MBD-300-A model. The data acquisition and processing unit 32 includes a host computer and a software program for data processing.

[0051] Working principle:

[0052] First, the fluorescence generated by the first erbium-doped fiber amplifier 122 enters the first optical circulator 131 and the apodized fiber grating 132. The beams that satisfy the Bragg condition of the apodized fiber grating 132 are reflected back to the first optical circulator 131, while the beams that do not satisfy the Bragg condition are transmitted out of the apodized fiber grating 132. The first beam reflected by the apodized fiber grating 132 enters the optical isolator 133 and the first Y-coupler 141. The first beam output from the 90% output end of the first Y-coupler 141 enters the first electro-optic modulator 113 through the first polarization controller 111. At the same time, the first electro-optic modulator 113 performs frequency shifting to form a second beam, which then enters the input end of the first erbium-doped fiber amplifier 122 through the second polarization controller 121.

[0053] The first beam, the second beam, and the fluorescence resonate in the resonant cavity to form a laser, which is output from the 10% output end of the first Y-coupler 141. After being amplified by the third erbium-doped fiber amplifier 211, it serves as the light source for the coherent signal detection module 2. When the laser enters the second Y-coupler 212, the light is split into two paths: a pump-probe path 22 and a local oscillation signal path 23. In the pump-probe path 22, the beam input from the 90% output port of the second Y-coupler 212 is modulated into a probe pulse by the acousto-optic modulator 223 through the third polarization controller 221. The probe pulse is amplified by the third erbium-doped fiber amplifier 211 and then filtered by the bandpass filter 225 to remove amplified spontaneous emission noise from the probe signal. The filtered probe pulse enters the first port of the second optical circulator 226, and then enters the 48.6 km long standard single-mode fiber 227 through the second port. In single-mode fiber 227, the backward Stokes light formed by spontaneous Pryoyuan scattering enters the third port of the second optical circulator 226, and then enters the input port of the X-coupler 241.

[0054] In the local oscillation signal path 23, the laser light input from the 10% output port of the second Y-coupler 212 enters the fourth polarization controller 231, and then enters the second electro-optic modulator 233 to form a Stokes sideband as the local oscillation signal light. The local oscillation signal light enters the other input port of the X-coupler 241 after passing through the polarization scrambler 234. At this time, the probe pulse light and the local oscillation signal light input from the two ports of the X-coupler 241 enter the balanced detector 31 for frequency beats. The differential electrical signal output by the balanced detector 31 is transmitted to the data acquisition and processing unit 32. By sweeping the frequency, the Brillouin gain spectrum of the sensing fiber can be obtained. The Brillouin frequency shift at each point on the fiber can be obtained from the spectrum. Based on the relationship between the Brillouin frequency shift and temperature / strain, the temperature / strain information on the fiber can be analyzed, realizing the measurement of temperature / strain along the fiber.

[0055] In one example, the pump currents of the first erbium-doped fiber amplifier 122 and the second erbium-doped fiber amplifier 224 are set through the system modulation window of the host computer MATLAB software. The parameters can be set through the system sweep frequency range and sweep frequency interval settings, as well as the system measurement progress monitoring interface. The sweep frequency interval is set to 10.83 GHz to 11.08 GHz, and the sweep frequency interval is 10 MHz. A 200 m section of a 48.6 km standard single-mode fiber 227 is placed in a 60°C constant temperature chamber, while the remaining fibers are kept at room temperature and in a relaxed state to obtain a three-dimensional Brillouin gain spectrum. Figure 4 As shown, the Brillouin gain spectrum exhibits a Lorentz shape along the sensing fiber.

[0056] The Brillouin frequency shift was obtained by fitting the electric domain Brillouin gain spectrum obtained from the BOTDR system to a Lorentz curve, and then converted into temperature. The results are as follows: Figure 5 As shown.

[0057] The system's measurement accuracy was evaluated using the standard deviation of the extracted Brillouin frequency shift and the frequency uncertainty, with the results as follows: Figure 6 As shown. From Figure 6 As can be seen, the system tail frequency uncertainty obtained using the dual-wavelength light source module is 3.08 MHz, while the frequency uncertainty at the tail of the sensing fiber using the tunable single-wavelength laser is 6.18 MHz, indicating that the system standard deviation is doubled. Overall, the frequency uncertainty obtained by the actively mode-locked dual-wavelength fiber laser is reduced by half compared to that of the single laser, thus improving the system measurement accuracy.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A BOTDR system based on an actively mode-locked dual-wavelength fiber laser, characterized in that, It includes a dual-wavelength light source module (1), a coherent signal detection module (2), and a signal processing module (3). The coherent signal detection module (2) is connected to both the dual-wavelength light source module (1) and the signal processing module (3). The dual-wavelength light source module (1) includes an electro-optic modulation unit (11), an optical fiber amplification unit (12), an optical frequency selection unit (13), and a first beam splitting unit (14) connected in sequence. The optical fiber amplification unit (12) generates fluorescence through spontaneous emission and emits the fluorescence to the optical frequency selection unit (13). The optical frequency selection unit (13) selects the frequency of the fluorescence and emits the first beam obtained by frequency selection to the first beam splitting unit (14). The electro-optic modulation unit (11) shifts the frequency of the first beam to form a second beam. The first beam splitting unit (14) transmits the first beam and the second beam to the electro-optic modulation unit (11) and the coherent signal detection module (2) respectively with a preset splitting ratio.

2. The system as described in claim 1, characterized in that, The electro-optic modulation unit (11) includes a first polarization controller (111), a first microwave source (112), and a first electro-optic modulator (113). The first polarization controller (111) is connected to the first beam splitting unit (14), and the first electro-optic modulator (113) is connected to the fiber amplification unit (12). The first electro-optic modulator (113) is connected to the first polarization controller (111) and the first microwave source (112) respectively. The first polarization controller (111) adjusts the polarization state of the light input to the first electro-optic modulator (113). The first microwave source (112) drives the first electro-optic modulator (113) so that the first electro-optic modulator (113) performs harmonic frequency locking based on the fundamental frequency light. The first beam enters the first electro-optic modulator (113) through the first polarization controller (111), and at the same time, the second beam is formed after frequency shifting in the first electro-optic modulator (113).

3. The system as described in claim 1, characterized in that, The fiber amplification unit (12) includes a second polarization controller (121) and a first erbium-doped fiber amplifier (122) connected in sequence. The second polarization controller (121) is connected to the electro-optic modulation unit (11), and the first erbium-doped fiber amplifier (122) is connected to the optical frequency selection unit (13). The second polarization controller (121) adjusts the polarization state of the light input to the first erbium-doped fiber amplifier (122) and emits the adjusted light to the first erbium-doped fiber amplifier (122). The first erbium-doped fiber amplifier (122) emits the fluorescence and amplifies the power of the light passing through the first erbium-doped fiber amplifier (122).

4. The system as described in claim 1, characterized in that, The optical frequency selection unit (13) includes a first optical circulator (131), an apodized fiber grating (132), and an optical isolator (133). The first port of the first optical circulator (131) is connected to the optical fiber amplification unit (12), the second port of the first optical circulator (131) is connected to the apodized fiber grating (132), and the third port of the first optical circulator (131) is connected to the optical isolator (133). The optical isolator (133) is connected to the first beam splitting unit (14). The first optical circulator (131) transmits the fluorescence, the first beam, and the second beam in a preset transmission direction. The apodized fiber grating (132) reflects the first beam that satisfies the Bragg condition back into the first optical circulator (131). The optical isolator (133) isolates the beam reflected back by the apodized fiber grating (132).

5. The system as described in claim 1, characterized in that, The coherent signal detection module (2) includes a second beam splitting unit (21), a pump detection path (22), a local oscillation signal path (23), and a beat frequency unit (24). The input terminals of the pump detection path (22) and the local oscillation signal path (23) are both connected to the second beam splitting unit (21), and the output terminals of the pump detection path (22) and the local oscillation signal path (23) are both connected to the beat frequency unit (24). The beat frequency unit (24) is connected to the signal processing module (3). The second beam splitting unit (21) is used to transmit the first beam and the second beam to the pump detection path (22) and the local oscillation signal path (23) respectively with a preset beam splitting ratio. The pump detection path (22) is used to generate back-stokes light through spontaneous Pryoul scattering in the optical fiber. The local oscillation signal path (23) is used to generate Stokes sidebands through a modulator. The beat frequency unit (24) is used to perform optical beat frequency on the back-stokes light and the Stokes sidebands.

6. The system as described in claim 5, characterized in that, Both the first beam splitting unit (14) and the second beam splitting unit (21) include a Y-type fiber coupler, and the beam splitting ratio of the Y-type fiber coupler is 90 / 10.

7. The system as described in claim 5, characterized in that, The pump detection circuit (22) includes a third polarization controller (221), an acousto-optic modulator (223), a pulse generator (222), a second erbium-doped fiber amplifier (224), a bandpass filter (225), a second optical circulator (226), and a single-mode fiber (227) connected in sequence. The third polarization controller (221) is connected to the second beam splitter (21), and the second optical circulator (226) is connected to the beat frequency unit (24). The third polarization controller (221) adjusts the input... The acousto-optic modulator (223) modulates the polarization state of the light, and the pulse generator (222) drives the acousto-optic modulator (223) to modulate the input beam into a probe light pulse. The second erbium-doped fiber amplifier (224) amplifies the probe light pulse and then filters out the spontaneous amplification noise through the bandpass filter (225). The probe light pulse enters the second optical circulator (226) after being backscattered by Brillouin in the single-mode fiber (227).

8. The system as described in claim 7, characterized in that, The local oscillation signal path (23) includes a fourth polarization controller (231), a second electro-optic modulator (233), a second microwave source (232), and a polarization scrambler (234) connected in sequence. The fourth polarization controller (231) is connected to the second beam splitting unit (21), and the polarization scrambler (234) is connected to the beat frequency unit (24). The fourth polarization controller (231) adjusts the polarization state of the light input to the second electro-optic modulator (233), and the second microwave source (232) drives the second electro-optic modulator (233) so that the second electro-optic modulator (233) outputs the Stokes sideband. The polarization scrambler (234) suppresses the polarization noise generated in the single-mode fiber (227).

9. The system as described in claim 5, characterized in that, The beat frequency unit (24) is an X-type fiber optic coupler.

10. The system as claimed in claim 1, characterized in that, The signal processing module (3) includes a balance detector (31) and a data acquisition and processing unit (32), wherein the balance detector (31) is connected to the data acquisition and processing unit (32).

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