BOTDR (Brillouin Optical Time Domain Reflectometer) system fused with Stokes-anti-Stokes light and demodulation method

Abstract

The invention provides a BOTDR (Brillouin optical time domain reflectometry) system fusing Stokes-anti-Stokes light and a demodulation method, and belongs to the technical field of Brillouin distributed optical fiber sensing. The narrow linewidth laser is used for generating continuous optical signals; the input end of the coupler is connected with the laser, and the output end is connected with the pulse light branch and the local oscillator light branch. The pulsed light branch comprises a semiconductor optical amplifier, a random scrambler, a first erbium-doped optical fiber amplifier and a circulator which are connected in sequence; the local oscillator light branch comprises an electro-optical modulator, and the electro-optical modulator regulates and controls sideband frequency through a microwave module; the second erbium-doped optical fiber amplifier is used for extracting the Stokes light and the anti-Stokes light; the single-mode coupler mixes the double-sideband local oscillator light with the anti-Stokes light and the Stokes light; and the electrical beat frequency signals are acquired, filtered and enveloped and demodulated through an acquisition card. The detection capability of the Brillouin scattering signal is enhanced, the system structure is simplified, and the hardware cost and the maintenance difficulty are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of Brillouin distributed optical fiber sensing, and particularly relates to a BOTDR system and a demodulation method integrating Stokes-anti-Stokes light. Background Technique

[0002] The statements in this part only provide background technical information related to the present invention, and do not necessarily constitute prior art.

[0003] As one of the typical application solutions of distributed optical fiber sensing, Brillouin Optical Time Domain Reflectometer (BOTDR) is widely used in the temperature and strain monitoring of facilities such as power, transportation, and oil and gas pipelines because of its characteristics of long-distance detection and single-end access. The BOTDR system senses based on the linear relationship between the Brillouin frequency shift of the self-generated Brillouin scattering signal and temperature and strain. Compared with the stimulated Brillouin scattering signal, the self-generated Brillouin scattering signal is very weak and attenuates with the increase of the fiber distance. The signal intensity affects the measurement accuracy and measurement distance of distributed optical fiber sensing. Therefore, enhancing the signal intensity of the BOTDR system and improving the signal-to-noise ratio play a crucial role.

[0004] There are two schemes for obtaining the Brillouin frequency shift in the BOTDR system. One is the electrical domain mixing technology, whose basic principle is that the Brillouin scattering signal is beat with the local oscillator light, and after being converted into an electrical signal by a photodetector, it is down-converted to the baseband by an electronic mixer. The other is the optical domain mixing technology, which directly coherently mixes the Brillouin scattering signal and the local oscillator light in the optical domain, and the bandwidth of the generated beat signal is reduced to several hundred MHz, and a low-bandwidth balanced detector is used to output the electrical beat signal. Both schemes use the MWG swept local oscillator light, band-pass filter narrow-band filtering, and envelope detector to obtain the beat amplitude, so as to obtain the three-dimensional self-generated Brillouin gain spectrum, and calculate the corresponding Brillouin frequency shift through curve fitting. However, the introduction of the band-pass filter and envelope detector increases the complexity of the system. In addition, in the traditional optical domain mixing technology BOTDR system, an optical FBG filter is usually required to filter out the first-order lower sideband as the local oscillator light to mix with the Stokes light in the backward Brillouin spontaneous scattering, and the anti-Stokes light component is not utilized. Summary of the Invention

[0005] To overcome the deficiencies of the above-mentioned prior art, the present invention provides a BOTDR system and a demodulation method that fuse Stokes - anti-Stokes light. By building a BOTDR system that fuses Stokes - anti-Stokes light. In the BOTDR system, a microwave module (MWG) drives an electro-optic modulator (EOM) to use carrier-suppressed double-sideband modulation as the local oscillator light. In a single-mode coupler, the first upper sideband of the local oscillator light is mixed with the anti-Stokes light, and the first lower sideband is mixed with the Stokes light to generate a beat frequency signal with doubled signal intensity. The FPGA digital signal processing method is used to collect, filter, and extract the envelope of the beat frequency signal, avoiding the use of hardware such as optical filters, electrical band-pass filters, and envelope detectors, and reducing the complexity and hardware cost of the system.

[0006] To achieve the above object, one or more embodiments of the present invention provide the following technical solutions:

[0007] The first aspect of the present invention provides a BOTDR system that fuses Stokes - anti-Stokes light;

[0008] A BOTDR system that fuses Stokes - anti-Stokes light includes:

[0009] A narrow-linewidth laser for generating a continuous optical signal;

[0010] A coupler, whose input end is connected to the narrow-linewidth laser, the first output end is connected to the pulsed light branch, and the second output end is connected to the local oscillator light branch; the pulsed light branch includes a semiconductor optical amplifier, a random polarization scrambler, a first erbium-doped fiber amplifier, and a circulator connected in sequence, for generating and injecting pulsed light into the sensing fiber; the local oscillator light branch includes an electro-optic modulator, and the electro-optic modulator regulates the sideband frequency through a microwave module to generate a local oscillator light with carrier-suppressed double-sideband modulation;

[0011] A second erbium-doped fiber amplifier for amplifying the Stokes light and anti-Stokes light in the backward Brillouin scattering signal from the sensing fiber;

[0012] A single-mode coupler for mixing the first upper sideband of the local oscillator light with the anti-Stokes light and the first lower sideband of the local oscillator light with the Stokes light;

[0013] A balanced photodetector for converting the mixed optical signal into an electrical beat frequency signal;

[0014] An acquisition card for collecting, filtering, and envelope demodulating the electrical beat frequency signal;

[0015] The narrow-linewidth laser, semiconductor optical amplifier, first erbium-doped fiber amplifier, acquisition card, and microwave module are all connected to the computer motherboard.

[0016] As a further technical solution, the first port of the circulator is connected to the output end of the first erbium-doped fiber amplifier; the second port is connected to the sensing optical fiber; the third port is connected to the input end of the second pulsed erbium-doped fiber amplifier.

[0017] As a further technical solution, the coupler is a 10:90 coupler; the single-mode coupler is a 50:50 single-mode coupler.

[0018] As a further technical solution, the acquisition card adopts an FPGA architecture, including:

[0019] A signal acquisition unit for acquiring the electrical beat signal at a specific sampling rate;

[0020] A band-pass filtering unit for filtering out the noise in the electrical beat signal;

[0021] A digital full-wave rectification unit for extracting the signal amplitude through four-channel parallel absolute value calculation;

[0022] A low-pass filtering unit for extracting the envelope signal by using a finite impulse response filter.

[0023] The second aspect of the present invention provides a demodulation method for a BOTDR system integrating Stokes-anti-Stokes light.

[0024] A demodulation method for a BOTDR system integrating Stokes-anti-Stokes light, including:

[0025] Obtaining a continuous optical signal generated by a laser, and dividing the continuous optical signal into a pulsed optical branch and a local oscillator optical branch through a coupler;

[0026] The optical signal respectively generates pulsed light through the pulsed optical branch and injects it into the sensing optical fiber; through the local oscillator optical branch, carrier-suppressed double-sideband modulation is performed to generate the first-order upper sideband and the first-order lower sideband of the local oscillator light;

[0027] Obtaining the backward Brillouin scattering signal from the sensing optical fiber, and extracting the Stokes light and the anti-Stokes light of the backward Brillouin scattering signal;

[0028] Mixing the first-order upper sideband with the anti-Stokes light in the Brillouin scattering signal, and simultaneously mixing the first-order lower sideband with the Stokes light to generate a beat signal;

[0029] Converting the beat signal into an electrical beat signal through a balanced photodetector; demodulating the electrical beat signal by using an FPGA architecture.

[0030] As a further technical solution, the first-order upper and lower sidebands of the local oscillator light are respectively:

[0031]

[0032] In the formula, and respectively represent the electric field strengths of the first-order lower sideband and the first-order upper sideband of the local oscillator light; and respectively represent the signal electric field strength amplitudes of the first-order lower sideband and the first-order upper sideband; ω p represents the optical frequency of the incident light; ω r represents the frequency difference between the incident light and the first-order upper and lower sidebands, which is determined by the swept frequency output by the microwave module; j represents the imaginary part; and respectively represent the initial phases of the first-order lower sideband and the first-order upper sideband. In intensity modulation, their phases are the same.

[0033] As a further technical solution, the optical field strengths of the Stokes light and the anti-Stokes light in the backward Brillouin scattering signal in the optical fiber are:

[0034] E S (t,z) = E s (t,z) exp[j((ω p - ω B )t + φ s (t,z))]

[0035] E AS (t,z) = E as (t,z) exp[j((ω p + ω B )t + φ as (t,z))]

[0036] In the formula, E S (t,z) and E AS (t,z) respectively represent the electric field strengths of the Stokes light and the anti-Stokes light; E s (t,z) and E as (t,z) represent the electric field strength amplitudes of the Stokes light and the anti-Stokes light at the position z in the optical fiber; φ s (t,z) and φ as (t,z) respectively represent the cumulative phases of the Stokes light and the anti-Stokes light with respect to time at the position z in the optical fiber, which are random. ω p represents the frequency of the incident light, and ω B represents the Brillouin frequency shift of the optical fiber.

[0037] As a further technical solution, the output signal of the balanced photodetector includes the beat signal of the Stokes light and the first-order lower sideband, and the beat signal of the anti-Stokes light and the first-order upper sideband. The current representation form of the electrical beat signal is:

[0038]

[0039] In the formula, I eff (t) represents the effective current of the electrical beat signal; Δωt represents the beat signal frequency between the Brillouin frequency shift and the swept frequency, and its frequency range is from dozens of MHz to hundreds of MHz.

[0040] As a further technical solution, the demodulation of the electrical beat signal by using the FPGA architecture includes:

[0041] Preprocess the electrical beat signal to eliminate the mixed DC bias component in the signal through parameters;

[0042] Perform four-channel parallel absolute value calculation on the preprocessed signal to achieve full-wave rectification of the signal;

[0043] Use a low-pass filter to filter out the residual high-frequency harmonic components in the rectified signal and extract the envelope signal.

[0044] As a further technical solution, digital full-wave rectification can be expressed as:

[0045]

[0046] Among them, I rect (t) is the full-wave rectified signal; K is the proportionality coefficient, representing the detector sensitivity, gain factor, and system loss.

[0047] The above one or more technical solutions have the following beneficial effects:

[0048] (1) Through the double-sideband modulation technology, the first-order upper sideband of the local oscillator light is simultaneously mixed with the anti-Stokes light, and the first-order lower sideband is mixed with the Stokes light. The two generated beat signals are coherently superimposed, doubling the signal intensity and effectively enhancing the detection ability of the Brillouin scattering signal. The improvement of the signal intensity and signal-to-noise ratio enables the system to detect weaker backward Brillouin scattering signals, thereby extending the effective measurement distance of distributed fiber sensing and being applicable to ultra-long-distance temperature and strain monitoring.

[0049] (2) The envelope extraction link of the beat signal in the present invention is realized by using the FPGA digital signal processing method, avoiding the additional shot noise and thermal noise caused by the introduction of the envelope detector, and avoiding the use of hardware such as optical filters, electrical band-pass filters, and envelope detectors. This not only simplifies the system structure but also reduces the hardware cost and maintenance difficulty.

[0050] The advantages of the additional aspects of the present invention will be partially given in the following description, partially will become obvious from the following description, or will be understood through the practice of the present invention. Description of the Drawings

[0051] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments and descriptions thereof of the invention are used to explain the invention and do not constitute an improper limitation to the invention.

[0052] Figure 1 It is a structural diagram of a BOTDR system with double-sideband modulation for the first embodiment.

[0053] Figure 2 It is a structural diagram of the method for the second embodiment.

[0054] Figure 3 It is a signal flow diagram of the local oscillator light and the signal light mixing, and the band-pass filter and the ideal Brillouin gain spectrum convolution in the second embodiment.

[0055] Figure 4 It is a structural diagram of the digital demodulation FPGA of the beat signal in the second embodiment.

[0056] Figure 5 It is a schematic diagram of the comparison result of the signal-to-noise ratio between double-sideband modulation and single-sideband modulation in the second embodiment. Detailed implementation manners

[0057] It should be noted that the following detailed descriptions are all exemplary and are intended to provide a further description of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this invention belongs.

[0058] It should be noted that the terms used herein are only for describing the specific implementation manners and are not intended to limit the exemplary implementation manners according to this invention.

[0059] Without conflict, the embodiments in this invention and the features in the embodiments can be combined with each other.

[0060] The present invention generates first-order upper and lower sidebands by modulating the local oscillator light, mixes them with the anti-Stokes light and the Stokes light in the Brillouin scattering signal respectively, and the two generated beat signals are superimposed and enhanced. Subsequently, the enhanced beat signal is converted into an electrical signal by a balanced photodetector, and then the FPGA is used to collect, filter the beat signal and extract its envelope signal, thereby reducing the complexity and hardware cost of the system and avoiding the shot noise and thermal noise introduced by the band-pass filter and the envelope detector.

[0061] Embodiment 1

[0062] This embodiment discloses a BOTDR system integrating Stokes-anti-Stokes light;

[0063] As Figure 1As shown, a BOTDR system that combines Stokes and anti-Stokes light includes:

[0064] A narrow linewidth laser, which is used to generate a continuous optical signal with a wavelength of 1550 nm and performs beam splitting through a coupler. In this embodiment, the coupler (PMC) is a 10:90 coupler. Its input end is connected to the narrow linewidth laser, and the first output end is connected to the pulsed light branch. The pulsed light branch includes a semiconductor optical amplifier (SOA), a random polarization scrambler (RPS), a first erbium-doped fiber amplifier (EDFA1), and a circulator connected in sequence. In the pulsed light branch, the coupler sequentially modulates the optical signal with a proportion of 10% through the semiconductor optical amplifier to form a pulsed light sequence. Subsequently, the pulsed light is introduced into the random polarization scrambler to eliminate the polarization state correlation of the optical signal. The output end of the first erbium-doped fiber amplifier is connected to the first port of the circulator. After being amplified by the first erbium-doped fiber amplifier, the pulsed light is input into the circulator through the first port of the circulator and injected into the sensing fiber through the second port of the circulator. Among them, the sensing fibers are also connected through flanges.

[0065] The second output end of the coupler is connected to the local oscillator light branch. The coupler inputs the optical signal with a proportion of 90% into the electro-optic modulator (EOM) to achieve carrier-suppressed double-sideband modulation, and its ±1st order sideband frequencies are precisely regulated by the electrical signal output by the microwave module (MWG). Specifically, the microwave module controls the frequency difference between the first-order upper and lower sidebands and the carrier of the local oscillator light modulated by the electro-optic modulator to obtain the first-order upper sideband and the first-order lower sideband of the local oscillator light.

[0066] Among them, the narrow linewidth laser, the semiconductor optical amplifier, the first erbium-doped fiber amplifier, the acquisition card, and the microwave module are all connected to the computer motherboard, and the operation of the above structures is controlled through the computer motherboard.

[0067] When the pulsed light generated by the pulsed light branch is input into the sensing fiber, the stimulated in-fiber spontaneous backward Brillouin scattering signal is directionally output through the third port of the circulator, and the Stokes light and anti-Stokes light in the backward Brillouin scattering signal are amplified by the second erbium-doped fiber amplifier (EDFA2); the input end of the second erbium-doped fiber amplifier is connected to the third port of the circulator, and the output end is connected to the first input end of the single-mode coupler. The single-mode coupler is a 50:50 coupler. Its second input end is connected to the output end of the electro-optic modulator, and is used to mix the first-order upper sideband of the local oscillator light with the anti-Stokes light and the first-order lower sideband of the local oscillator light with the Stokes light, and input the mixed optical signal into the balanced detector (BPD) through the output end of the single-mode coupler. In this embodiment, the balanced detector is a balanced photodetector, which is used to convert the mixed optical signal into an electrical beat signal.

[0068] The beat signal after optoelectronic conversion is collected, filtered, and demodulated by a data acquisition card (DAQ) based on the FPGA architecture. Among them, the data acquisition card based on the FPGA architecture includes: a signal acquisition unit, a band-pass filtering unit, a digital full-wave rectification unit, and a low-pass filtering unit.

[0069] Among them, the signal acquisition unit collects the electrical beat signal at a sampling rate of 1 GS / s, and filters out the noise in the electrical beat signal through the band-pass filtering unit; the digital full-wave rectification unit extracts the signal amplitude through four-channel parallel absolute value calculation; the low-pass filtering unit extracts the envelope signal using a finite impulse response filter. Specifically, the low-pass filtering unit consists of a finite impulse response filter (FIR) defined by an IP core. The passband cut-off frequency of the filter is 150 MHz (amplitude fluctuation <0.1 dB), and the stopband cut-off frequency is 180 MHz (attenuation >75 dB). The basic clock frequency of the digital full-wave rectification unit and the low-pass filtering unit is 250 MHz. In the timing architecture design, the 1 GS / s input data stream is decoupled into four sub-data streams in the 250 MHz clock domain through a four-phase parallel processing architecture. Affected by the characteristics of the filter and the data rate, this module uses 172 DSP resources to implement an 86th-order FIR filter.

[0070] Embodiment 2

[0071] This embodiment discloses a demodulation method for a BOTDR system that combines Stokes-anti-Stokes light;

[0072] As Figure 2 shown, a demodulation method for a BOTDR system that combines Stokes-anti-Stokes light includes:

[0073] Step S1, obtaining a continuous optical signal generated by a laser.

[0074] A continuous optical signal with a wavelength of 1550 nm is generated by a narrow-linewidth laser and split through a coupler. The continuous optical signal is divided into a pulsed light branch and a local oscillator light branch.

[0075] Step S2, the optical signal generates pulsed light through the pulsed light branch and injects it into the sensing fiber; the local oscillator light is subjected to carrier-suppressed double-sideband modulation through the local oscillator light branch to generate the first-order upper sideband and the first-order lower sideband of the local oscillator light.

[0076] In step S2, the microwave module controls the electro-optic modulator to modulate the frequency difference between the first-order upper and lower sidebands of the local oscillator light and the carrier. For example, the swept frequency range of the microwave module is 10.15 GHz - 10.75 GHz, and the frequency difference between the first-order upper and lower sidebands of the local oscillator light and the carrier can also be swept within the range of 10.15 GHz - 10.75 GHz. The first-order upper and lower sidebands of the local oscillator light can be expressed as:

[0077]

[0078] In the formula, and respectively represent the electric field strengths of the first-order lower sideband and the first-order upper sideband of the local oscillator light; and respectively represent the signal electric field strength amplitudes of the first-order lower sideband and the first-order upper sideband; ω p represents the optical frequency of the incident light; ω r represents the frequency difference between the incident light and the first-order upper and lower sidebands, which is determined by the swept frequency output by the microwave module; j represents the imaginary part; and respectively represent the initial phases of the first-order lower sideband and the first-order upper sideband. In intensity modulation, their phases are the same.

[0079] Step S3: Obtain the backward Brillouin scattering signal from the sensing optical fiber, and extract the Stokes light and anti-Stokes light of the backward Brillouin scattering signal.

[0080] After the backward Brillouin scattering signal in the sensing optical fiber passes through the third port of the circulator, it is amplified by the second erbium-doped fiber amplifier, and then mixed with the local oscillator light in the single-mode coupler. The optical field intensities of the Stokes light and the anti-Stokes light in the backward Brillouin scattering signal in the optical fiber are expressed as:

[0081] E S (t,z) = E s (t,z)exp[j((ω p - ω B )t + φ s (t,z))]

[0082] E AS (t,z) = E as (t,z)exp[j((ω p + ω B )t + φ as (t,z))]

[0083] In the formula, E S (t,z) and E AS (t,z) respectively represent the electric field strengths of the Stokes light and the anti-Stokes light; E s (t,z) and E as (t,z) represent the electric field strength amplitudes of the Stokes light and the anti-Stokes light at the position z in the optical fiber;; φ s (t,z) and φ as (t,z) respectively represent the cumulative phases of the Stokes light and the anti-Stokes light with respect to time at the position z in the optical fiber, which are random. ω prepresents the frequency of the incident light, ω B represents the Brillouin frequency shift of the optical fiber.

[0084] Step S4: Mix the first-order upper sideband with the anti-Stokes light in the Brillouin scattering signal, and at the same time mix the first-order lower sideband with the Stokes light to generate a beat signal.

[0085] The output signal of the balanced photodetector includes the beat signal between the Stokes light and the first-order lower sideband, and the beat signal between the anti-Stokes light and the first-order upper sideband. The current representation of the electrical beat signal is:

[0086]

[0087] where I eff (t) represents the effective current of the electrical beat signal; Δωt represents the frequency of the beat signal between the Brillouin frequency shift and the swept frequency, and its frequency range is from dozens of MHz to hundreds of MHz.

[0088] Step S5: Convert the beat signal into an electrical beat signal through a balanced photodetector; demodulate the electrical beat signal using an FPGA architecture.

[0089] The Brillouin time-domain electrical beat signal detected and output by the balanced photodetector is filtered by a band-pass filter. In the frequency domain, the spectrum of the beat signal, that is, the convolution of the Brillouin gain spectrum and the frequency response of the band-pass filter, is a gain spectrum approximated by a Lorentz function, which can be expressed as:

[0090]

[0091] where h[n] represents the amplitude-frequency response of the band-pass filter; g[n + mv ref ,z] represents the Brillouin scattering signal at the fiber axial position z. The process of mixing the local oscillator light and the signal light, and the frequency-domain convolution process of the band-pass filter and the ideal Brillouin scattering signal are as Figure 3 shown.

[0092] Demodulate the beat signal using digital full-wave rectification and low-pass filtering methods to extract the envelope signal.

[0093] Among them, digital full-wave rectification can be expressed as:

[0094]

[0095] where I rect (t) is the full-wave rectified signal; K is the proportionality coefficient, representing the detector sensitivity, gain factor, and system loss. The envelope signal obtained by filtering out the high-frequency components in the full-wave rectified signal I rect (t) through low-pass filtering is the amplitude signal of the Brillouin scattering signal:

[0096]

[0097] Among them, I envelope (t) is the envelope signal; LPF is the low-pass filter; the local oscillator light changes the sweep frequency to obtain Brillouin scattering time-domain signals with different amplitudes.

[0098] Deploy a beat signal demodulation module in the FPGA of the acquisition card. The digital demodulation process of the optical path beat signal based on the FPGA is as Figure 4 shown. In the preprocessing stage of the beat signal, the mixed DC bias component in the signal is first eliminated, Figure 4 and the parameters in it are used to remove the DC component. The parameter represents the difference between the DC bias amount and 0, and is a constant; the collected signal is subtracted from the parameter in the FPGA, making the intensity of the beat signal symmetric about 0V, thus completing the elimination of the DC bias. The full-wave rectification stage is realized by a four-channel parallel absolute value calculation unit, and this process realizes zero-delay amplitude detection through hardware combinational logic; the low-pass filtering stage filters out the residual high-frequency harmonic components in the rectified signal to complete the extraction of the envelope signal.

[0099] The low-pass filter is composed of a finite impulse response filter (FIR) defined by an IP core. The cut-off frequency of the passband of the filter is 150 MHz (amplitude fluctuation <0.1 dB), and the cut-off frequency of the stopband is 180 MHz (attenuation >75 dB). The basic clock frequency of the filter module is 250 MHz. In the timing architecture design, the 1 GS / s input data stream is decoupled into four sub-data streams in the 250 MHz clock domain through a four-phase parallel processing architecture. Affected by the characteristics of the filter and the data rate, this module uses 172 DSP resources to implement an 86th-order FIR filter.

[0100] Furthermore, the two-sideband signal-to-noise ratio test is carried out by calculating the signal-to-noise ratio. The signal-to-noise ratio of the Brillouin scattering time-domain signal is defined as the ratio of the signal voltage amplitude at the end of the optical fiber to the noise amplitude, and is expressed as follows:

[0101]

[0102] Among them, SNR E (dB) is the signal-to-noise ratio of the Brillouin scattering time-domain signal; V signal is the signal voltage amplitude; V noise is the noise amplitude.

[0103] Combined with Figure 5, the Brillouin scattering time-domain signal is measured at the center frequency (the peak frequency of the Brillouin gain spectrum, 10.45 GHz). Under double-sideband modulation, the Brillouin scattering time-domain signal is 20.7 dB. Compared with the single-sideband modulation (16.6 dB) method, the signal-to-noise ratio is increased by 4.1 dB. The improvement of the signal-to-noise ratio enables the system to detect weaker backward Brillouin scattering signals, thus extending the effective measurement distance of distributed fiber optic sensing and being applicable to ultra-long-distance temperature and strain monitoring.

[0104] Although the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present invention.

Claims

1. A BOTDR system that integrates Stokes - anti - Stokes light, characterized in that, Including: A narrow linewidth laser for generating a continuous optical signal; A coupler, whose input end is connected to the narrow linewidth laser, the first output end is connected to the pulsed light branch, and the second output end is connected to the local oscillator light branch; the pulsed light branch includes a semiconductor optical amplifier, a random polarization scrambler, a first erbium-doped fiber amplifier, and a circulator connected in sequence, for generating and injecting pulsed light into the sensing fiber; the local oscillator light branch includes an electro-optic modulator, and the electro-optic modulator regulates the sideband frequency through a microwave module to generate a local oscillator light with carrier-suppressed double-sideband modulation; A second erbium-doped fiber amplifier for amplifying the backward Brillouin scattering signal from the sensing fiber and extracting the Stokes light and anti-Stokes light of the backward Brillouin scattering signal; A single-mode coupler for mixing the first-order upper sideband of the local oscillator light with the anti-Stokes light and the first-order lower sideband of the local oscillator light with the Stokes light; A balanced photodetector for converting the mixed optical signal into an electrical beat signal; An acquisition card for acquiring, filtering, and envelope demodulating the electrical beat signal; The narrow linewidth laser, semiconductor optical amplifier, first erbium-doped fiber amplifier, acquisition card, and microwave module are all connected to the computer motherboard.

2. The BOTDR system integrating Stokes and anti-Stokes lights according to claim 1, wherein The first port of the circulator is connected to the output end of the first erbium-doped fiber amplifier; the second port is connected to the sensing fiber; the third port is connected to the input end of the second pulsed erbium-doped fiber amplifier.

3. A BOTDR system integrating Stokes and anti-Stokes light, characterized in that, The coupler is a 10:90 coupler; the single-mode coupler is a 50:50 single-mode coupler.

4. A BOTDR system integrating Stokes - anti - Stokes light, as claimed in claim 1, wherein, The acquisition card adopts an FPGA architecture and includes: A signal acquisition unit for acquiring the electrical beat signal at a specific sampling rate; A band-pass filtering unit for filtering out the noise in the electrical beat signal; A digital full-wave rectification unit for extracting the signal amplitude through four-channel parallel absolute value calculation; A low-pass filtering unit for extracting the envelope signal using a finite impulse response filter.

5. A demodulation method for a BOTDR system that combines Stokes and anti-Stokes light, characterized in that, Including: Obtaining the continuous optical signal generated by the laser and dividing the continuous optical signal into a pulsed light branch and a local oscillator light branch through a coupler; The optical signals respectively generate pulsed light through the pulsed light branch and inject it into the sensing fiber; and generate the first-order upper sideband and first-order lower sideband of the local oscillator light through carrier-suppressed double-sideband modulation in the local oscillator light branch; Obtaining the backward Brillouin scattering signal from the sensing fiber and extracting the Stokes light and anti-Stokes light of the backward Brillouin scattering signal; Mixing the first-order upper sideband with the anti-Stokes light in the Brillouin scattering signal, and at the same time mixing the first-order lower sideband with the Stokes light to generate a beat signal; Converting the beat signal into an electrical beat signal through a balanced photodetector; and demodulating the electrical beat signal using an FPGA architecture.

6. The demodulation method of a BOTDR system integrating Stokes and anti-Stokes light according to claim 5, characterized in that, The first-order upper and lower sidebands of the local oscillator light are respectively: Wherein, and respectively represent the electric field strengths of the first-order lower sideband and the first-order upper sideband of the local oscillator light; and respectively represent the signal electric field strength amplitudes of the first-order lower sideband and the first-order upper sideband; ω p represents the optical frequency of the incident light; ω r represents the frequency difference between the incident light and the first-order upper and lower sidebands, which is determined by the swept frequency output by the microwave module; j represents the imaginary part; and respectively represent the initial phases of the first-order lower sideband and the first-order upper sideband. In intensity modulation, their phases are the same.

7. The demodulation method of a BOTDR system integrating Stokes - anti - Stokes light according to claim 5, characterized in that, The optical field intensities of the Stokes light and anti-Stokes light in the backward Brillouin scattering signal in the fiber are: E S E(t, z) = s E(t, z) exp[j((ω p - ω B )t + φ s (t, z))] E AS (t,z) = E as (t,z) exp[j((ω p + ω B ) t + φ as (t,z))] where, E S (t, z) and E AS (t, z) respectively represent the electric field strengths of the Stokes light and the anti-Stokes light; E s (t, z) and E as (t, z) represent the electric field strength amplitudes of the Stokes light and the anti-Stokes light at the position z in the optical fiber; φ s (t, z) and φ as (t, z) respectively represent the cumulative phases of the Stokes light and the anti-Stokes light with time at the position z in the optical fiber, which are random; ω p represents the optical frequency of the incident light, and ω B represents the Brillouin frequency shift of the optical fiber.

8. The demodulation method of a BOTDR system integrating Stokes - anti - Stokes light according to claim 5, characterized in that, The output signal of the balanced photodetector includes the beat signal of the Stokes light and the first-order lower sideband, and the beat signal of the anti-Stokes light and the first-order upper sideband, and the effective photocurrent signal is: Where, I eff (t) represents the effective current of the electrical beat signal; Δωt represents the beat signal frequency of the Brillouin frequency shift and the swept frequency, and its frequency range is from dozens of MHz to hundreds of MHz.

9. The demodulation method of a BOTDR system integrating Stokes - anti - Stokes light according to claim 5, characterized in that, The demodulating the electrical beat signal using the FPGA architecture includes: Preprocess the electrical beat signal to eliminate the mixed DC bias component in the signal through parameters; Perform four-channel parallel absolute value calculation on the preprocessed signal to achieve full-wave rectification of the signal; Use a low-pass filter to filter out the remaining high-frequency harmonic components in the rectified signal and extract the envelope signal.

10. The demodulation method of a BOTDR system integrating Stokes and anti-Stokes light as claimed in claim 9, characterized in that, Digital full-wave rectification is expressed as: where I rect (t) is a full-wave rectified signal; K is a proportionality coefficient representing detector sensitivity, gain factor, and system loss.

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