BOTDA Fiber Optic Sensing Device and Method Based on Time Overlap Polarization Diversity

Abstract

This invention discloses a BOTDA fiber optic sensing device and method based on time-overlapping polarization diversity. The device includes a laser, a first fiber coupler, a first polarization controller, an intensity modulator, a microwave source, an optical isolator, a sensing fiber, a second fiber coupler, a first acousto-optic modulator, a second polarization controller, a dual-channel arbitrary waveform generator, a second acousto-optic modulator, a third polarization controller, a polarization combiner, a first erbium-doped fiber amplifier, a first optical circulator, a second erbium-doped fiber amplifier, a fiber Bragg grating, a second optical circulator, a photodetector, a low-pass filter, and a data acquisition and analysis system. This invention employs a time-overlapping polarization diversity method, effectively avoiding polarization noise caused by polarization interference and polarization fading caused by polarization pulling effects, significantly improving the measurement accuracy of the sensor.

Description

Technical Field

[0001] This invention relates to the field of distributed fiber optic sensing technology, and in particular to a BOTDA fiber optic sensing device and method based on time overlap polarization diversity. Background Technology

[0002] In Brillouin optical time-domain analysis (BOTDA) technology, ordinary single-mode fiber is usually chosen as the sensing medium to control costs. However, single-mode fiber may develop birefringence due to uneven drawing or external mechanical forces such as bending and vibration during manufacturing. This can lead to random changes in the polarization state of the input fiber, making it difficult to maintain consistent polarization states between the pump and probe light, thus causing polarization fading in the Brillouin gain.

[0003] Random polarization scrambling is a common technique for addressing this problem. This technique uses a polarizer to randomly scramble the pump light, effectively suppressing polarization fading. However, due to the randomness of the polarization scrambling, strong measurement instability, or polarization noise, occurs during the measurement process. This polarization noise significantly affects the signal-to-noise ratio of the sensing system, degrading its performance. The magnitude of polarization noise is linearly related to the Brillouin gain; as the Brillouin gain increases, the polarization noise becomes stronger. In pulse-coded BOTDA fiber optic sensors, the pump-coded sequence can generate a very strong cumulative Brillouin gain, thus introducing strong polarization noise, which is the dominant noise in pulse-coded BOTDA fiber optic sensors.

[0004] Another approach to address polarization fading is to employ polarization switching. This method inputs two pump pulses with orthogonal polarization states into the sensing fiber, and then superimposes the Brillouin gain signals obtained under the two pump lights. For single-pulse BOTDA fiber sensors, this method can effectively suppress polarization fading. However, for pulse-coded BOTDA fiber sensors, polarization switching introduces a polarization pulling effect, leading to polarization fading. Furthermore, polarization switching doubles the measurement time.

[0005] In summary, although random polarization scrambling and polarization switching techniques can solve the polarization fading problem to some extent, they each have some limitations and cannot effectively solve the polarization noise problem in pulse-coded BOTDA fiber optic sensors. Summary of the Invention

[0006] To address the polarization noise and polarization fading issues in pulse-code BOTDA fiber optic sensors, this invention proposes a BOTDA fiber optic sensing device and method based on time-overlapping polarization diversity. On the one hand, it avoids polarization noise caused by polarization interference; on the other hand, unlike traditional polarization switching schemes, it avoids polarization fading caused by polarization traction effects, significantly improving the sensor's measurement accuracy.

[0007] This application discloses a BOTDA fiber optic sensing device based on time overlap polarization diversity, including a laser, the output end of which is connected to a first fiber optic coupler.

[0008] One output of the first fiber coupler is connected in sequence to a first polarization controller, an intensity modulator, an optical isolator, and a sensing fiber, and the input of the intensity modulator is connected to a microwave source.

[0009] The other output of the first fiber coupler is connected to a second fiber coupler. One output of the second fiber coupler is sequentially connected to a first acousto-optic modulator and a second polarization controller. The other output of the second fiber coupler is sequentially connected to a second acousto-optic modulator and a third polarization controller. The inputs of the first and second acousto-optic modulators are both connected to a dual-channel arbitrary waveform generator. The outputs of the second and third polarization controllers are both connected to a polarization combiner. The output of the polarization combiner is sequentially connected to a first erbium-doped fiber amplifier and a first optical circulator. The reflecting end of the first optical circulator is connected to the output of the sensing fiber.

[0010] The output of the first optical circulator is connected to a second erbium-doped fiber amplifier, which is connected to a second optical circulator. The reflector of the second optical circulator is connected to a fiber Bragg grating, and the output of the second optical circulator is sequentially connected to a photodetector, a low-pass filter, and a data acquisition and analysis system.

[0011] Preferably, the laser is a narrow linewidth laser.

[0012] Preferably, the splitting ratio of the first fiber coupler and the second fiber coupler is 50:50.

[0013] Preferably, the input terminal of the first acousto-optic modulator is connected to the first channel signal output terminal of the dual-channel arbitrary waveform generator, and the input terminal of the second acousto-optic modulator is connected to the second channel signal output terminal of the dual-channel arbitrary waveform generator.

[0014] This application also discloses a BOTDA fiber optic sensing method based on time-overlapping polarization diversity, which is implemented using a BOTDA fiber optic sensing device and includes the following steps:

[0015] S1. Start the BOTDA fiber optic sensing device, and the laser emits a laser beam;

[0016] S2, Dual-channel arbitrary waveform generator outputs pulse code sequence;

[0017] S3. A microwave source drives an intensity modulator to perform frequency scanning, and the scanning range is adjustable.

[0018] S4. The photodetector converts the detection light of different scanning frequencies into electrical signals. The data acquisition and analysis system processes the electrical signals and demodulates the temperature and stress distribution along the sensing fiber.

[0019] Preferably, when the data acquisition and analysis system processes the electrical signal, it first decodes the cumulative Brillouin gain curves at different scanning frequency points to obtain single-pulse Brillouin gain curves, merges a series of single-pulse gain curves to obtain the Brillouin gain spectrum at each position, and performs Lorentz fitting at different positions of the sensing fiber to demodulate the Brillouin frequency shift information, reflecting the temperature stress change.

[0020] Preferably, the data acquisition and analysis system processes electrical signals using an offline processing method involving Matlab and CPU.

[0021] Preferably, the data acquisition and analysis system uses an online processing method accelerated by a field-programmable gate array (FPGA) to process electrical signals.

[0022] Preferably, the continuous light emitted by the laser is split into two beams by a first fiber coupler;

[0023] After passing through the first polarization controller, the upper-path light enters a bias voltage of The intensity modulator performs carrier-suppressed double-sideband modulation. The frequency of the microwave source driving the intensity modulator is changed, so that the frequencies of the two optical sidebands change accordingly. The double-sideband continuous light output by the intensity modulator is injected into the sensing fiber as the probe light through the optical isolator.

[0024] The downstream light is split into two beams by the second fiber coupler. These two beams are modulated into pulses by two identical code sequences generated by the dual-channel arbitrary waveform generator and the first and second acousto-optic modulators, respectively. The time delay between the two channels of the dual-channel arbitrary waveform generator is adjusted to ensure that the two optical pulses are strictly overlapped in time. Subsequently, the polarization states of the two optical pulses are adjusted by the second and third polarization controllers to align with the fast and slow axes of the polarization combiner, respectively. The combined optical pulse sequence is amplified to peak power by the first erbium-doped fiber amplifier and then used as pump light to be input into the sensing fiber through the first optical circulator.

[0025] The pump light and probe light traveling in opposite directions undergo stimulated Brillouin interaction in the sensing fiber. The probe light after the interaction enters the second erbium-doped fiber amplifier through the output of the first optical circulator. The low-frequency sideband of the probe light is amplified by the second erbium-doped fiber amplifier, and then passes through the second optical circulator and fiber Bragg grating bandpass filter. Subsequently, the probe light is converted into an electrical signal after passing through a photodetector. The electrical signal is filtered and denoised by a low-pass filter and then transmitted to the data acquisition and analysis system via a high-frequency coaxial cable. The acquired electrical signal is processed to realize distributed temperature stress measurement.

[0026] The beneficial effects of this invention are:

[0027] (1) This invention does not require polarization interference. The two pump pulse sequences with perpendicular polarization states are combined and enter the sensing fiber. At the same time, the interaction with the probe light can maximize the Brillouin gain, effectively suppress polarization fading and polarization noise, reduce the number of data accumulation and averaging times, reduce the acquisition time, and improve the sensing speed.

[0028] (2) Although the time overlap of the two coding sequences in this invention produces co-frequency interference, their period is different from that of the coding sequence. During the averaging of the original data, the amplitude fluctuation caused by the co-frequency interference will not affect the demodulation of the Brillouin gain, thereby reducing both the data measurement time and the amount of original data, as well as the complexity of the system. Attached Figure Description

[0029] Figure 1 This is a BOTDA fiber optic sensing device based on time-overlapping polarization diversity, as described in an embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram illustrating the effect of cyclic coding based on time-overlapping polarization diversity in an embodiment of the present invention.

[0031] Figure 3 This is a Brillouin gain spectrum-distance 3D plot using polarization scrambling and time overlap polarization diversity in an embodiment of the present invention;

[0032] Figure 4 This is a schematic diagram of the Brillouin frequency shift of the 35m fiber at the end of the optical fiber in an embodiment of the present invention;

[0033] Figure 5 This is a schematic diagram showing the distribution of the standard deviation of the Brillouin frequency shift along the optical fiber using polarization diversity with scrambling and time overlap, according to an embodiment of the present invention.

[0034] The attached figures are labeled as follows:

[0035] 1-Laser, 2-First fiber coupler, 3-First polarization controller, 4-Intensity modulator, 5-Microwave source, 6-Optical isolator, 7-Sensing fiber, 8-Second fiber coupler, 9-First acousto-optic modulator, 10-Second polarization controller, 11-Dual-channel arbitrary waveform generator, 12-Second acousto-optic modulator, 13-Third polarization controller, 14-Polarization combiner, 15-First erbium-doped fiber amplifier, 16-First optical circulator, 17-Second erbium-doped fiber amplifier, 18-Fiber Bragg grating, 19-Second optical circulator, 20-Photodetector, 21-Low-pass filter, 22-Data acquisition and analysis system. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0037] This application discloses a BOTDA fiber optic sensing device based on time-overlapping polarization diversity, such as... Figure 1 As shown, it includes a laser 1, a first fiber coupler 2, a first polarization controller 3, an intensity modulator 4, a microwave source 5, an optical isolator 6, a sensing fiber 7, a second fiber coupler 8, a first acousto-optic modulator 9, a second polarization controller 10, a dual-channel arbitrary waveform generator 11, a second acousto-optic modulator 12, a third polarization controller 13, a polarization combiner 14, a first erbium-doped fiber amplifier 15, a first optical circulator 16, a second erbium-doped fiber amplifier 17, a fiber Bragg grating 18, a second optical circulator 19, a photodetector 20, a low-pass filter 21, and a data acquisition and analysis system 22.

[0038] The output end of laser 1 is connected to the input end of the first fiber coupler 2. The first output end of the first fiber coupler 2 is connected to the input end of the first polarization controller 3 through a single-mode fiber jumper. The output end of the first polarization controller 3 is connected to the input end of the intensity modulator 4 through a single-mode fiber jumper. The signal output end of microwave source 5 is connected to the signal input end of intensity modulator 4. The output end of intensity modulator 4 is connected to the input end of optical isolator 6 through a single-mode fiber jumper. The output end of optical isolator 6 is connected to the input end of sensing fiber 7 through a single-mode fiber jumper.

[0039] The second output of the first fiber optic coupler 2 is connected to the input of the second fiber optic coupler 8 via a single-mode fiber optic patch cord. The first output of the second fiber optic coupler 8 is connected to the input of the first acousto-optic modulator 9 via a single-mode fiber optic patch cord. The output of the first acousto-optic modulator 9 is connected to the input of the second polarization controller 10 via a single-mode fiber optic patch cord. The second output of the second fiber optic coupler 8 is connected to the input of the second acousto-optic modulator 12 via a single-mode fiber optic patch cord. The output of the second acousto-optic modulator 12 is connected to the input of the third polarization controller 13 via a single-mode fiber optic patch cord. The input of the first acousto-optic modulator 9 is connected to the first channel signal output of the dual-channel arbitrary waveform generator 11. The input of the second acousto-optic modulator 12 is connected to the second channel signal output of the dual-channel arbitrary waveform generator 11. The output of the second polarization controller 10 is connected to the first input of the polarization combiner 14 via a single-mode fiber optic patch cord. The output of the third polarization controller 13 is connected to the second input of the polarization combiner 14 via a single-mode fiber optic patch cord. The output of the polarization combiner 14 is connected to the input of the first erbium-doped fiber amplifier 15 via a single-mode fiber jumper. The output of the first erbium-doped fiber amplifier 15 is connected to the input of the first optical circulator 16 via a single-mode fiber jumper. The reflector of the first optical circulator 16 is connected to the output of the sensing fiber 7 via a single-mode fiber jumper.

[0040] The output of the first optical circulator 16 is connected to the input of the second erbium-doped fiber amplifier 17 via a single-mode fiber optic patch cord. The output of the second erbium-doped fiber amplifier 17 is connected to the input of the second optical circulator 19 via a single-mode fiber optic patch cord. The output of the fiber Bragg grating 18 is connected to the reflector of the second optical circulator 19 via a single-mode fiber optic patch cord. The output of the second optical circulator 19 is connected to the input of the photodetector 20 via a single-mode fiber optic patch cord. The signal output of the photodetector 20 is connected to the input of the low-pass filter 21. The output of the low-pass filter 21 is connected to the input of the data acquisition and analysis system 22.

[0041] Laser 1 is a narrow linewidth laser, and the splitting ratio of the first fiber coupler 2 and the second fiber coupler 8 is 50:50.

[0042] The continuous light emitted by the 1KHz narrow linewidth laser 1 with an output wavelength of 1550.08nm is split into two beams by the first fiber coupler 2 with a 50:50 ratio.

[0043] After passing through the first polarization controller 3, the upper-path light enters the bias voltage. The intensity modulator 4 performs carrier-suppressed double-sideband modulation. The frequency of the microwave source 5 driving the intensity modulator 4 is changed, so that the frequencies of the two optical sidebands change accordingly. The double-sideband continuous light output by the intensity modulator 4 is injected into the sensing fiber 7 as the probe light through the optical isolator 6.

[0044] The downstream light is split into two beams by a 50:50 second fiber coupler 8. These two beams are modulated into pulses by two identical codeword sequences generated by a dual-channel arbitrary waveform generator 11, which are then modulated by a first acousto-optic modulator 9 and a second acousto-optic modulator 12. The time delay between the two channels of the dual-channel arbitrary waveform generator 11 is adjusted to ensure that the two optical pulses strictly overlap in time. Subsequently, the polarization states of the two optical pulses are adjusted by a second polarization controller 10 and a third polarization controller 13 to align with the fast and slow axes of the polarization combiner 14, respectively. The combined optical pulse sequence is amplified to a peak power of 18 dBm by a first erbium-doped fiber amplifier 15, and then used as pump light to be input into the sensing fiber 7 via a first optical circulator 16.

[0045] The pump light and probe light traveling in opposite directions undergo stimulated Brillouin interaction in the sensing fiber 7. The resulting probe light enters the second erbium-doped fiber amplifier 17 through the output of the first optical circulator 16. The low-frequency sideband of the probe light is amplified by the second erbium-doped fiber amplifier 17, then bandpass filtered by the second optical circulator 19 and fiber Bragg grating 18. Subsequently, the probe light is converted into an electrical signal by the photodetector 20. The electrical signal is filtered and denoised by the low-pass filter 21, and then transmitted to the data acquisition and analysis system 22 via a high-frequency coaxial cable. The sampling rate is set to 100 MSa / s. The acquired electrical signal is processed to obtain the Brillouin gain spectrum of each sensing point in the fiber, and the Brillouin frequency shift information is further demodulated to realize distributed temperature stress measurement.

[0046] like Figure 2 This diagram illustrates the effect of the cyclic coding based on time-overlapping polarization diversity in this application. Taking cyclic coding as an example, each cyclic coding sequence consists of N-bit coding pulses. The duration of each bit determines the spatial resolution of the system. The coding pulse interval is determined by both the sensing distance and the number of coding bits. It is necessary to ensure that the duration of the cyclic coding sequence is slightly longer than the time it takes for the light wave to travel one round trip in the entire sensing fiber. In this application, two optical pump pulse sequences are generated with identical codewords. The polarization state of one pump pulse sequence is adjusted to the X-direction by the second polarization controller 10; the polarization state of the other pump pulse sequence is adjusted to the Y-direction (perpendicular to the X-direction) by the third polarization controller 13. The polarization states of the two pump pulse sequences are orthogonal and completely overlap in time. The interaction between the pump light with the X-direction polarization state and the probe light with the Y-direction polarization state maximizes the Brillouin gain at each sensing location, effectively suppressing polarization pulling. Furthermore, since polarization interference is not used, the generation of polarization noise can be effectively avoided.

[0047] Another embodiment of this application discloses a BOTDA fiber optic sensing method based on time-overlapping polarization diversity, which is implemented using a BOTDA fiber optic sensing device and includes the following steps:

[0048] S1. Start the BOTDA fiber optic sensing device, and laser 1 emits a laser beam;

[0049] S2, Dual-channel arbitrary waveform generator 11 outputs pulse code sequence;

[0050] S3, microwave source 5 drives intensity modulator 4 to perform frequency scanning, and the scanning range is adjustable;

[0051] S4. The photodetector 20 converts the detection light of different scanning frequencies into electrical signals. The data acquisition and analysis system 22 processes the electrical signals and demodulates the temperature and stress distribution along the sensing fiber 7.

[0052] Specifically, when processing electrical signals, the data acquisition and analysis system 22 first decodes the cumulative Brillouin gain curves at different scanning frequency points to obtain single-pulse Brillouin gain curves. A series of single-pulse gain curves are then merged to obtain the Brillouin gain spectrum at each location. Lorentz fitting is performed at different locations on the sensing fiber to demodulate the Brillouin frequency shift information, reflecting changes in temperature stress. In this embodiment, the data acquisition and analysis system 22 can process electrical signals either offline using Matlab and a CPU, or online using a field-programmable gate array (FPGA) for acceleration.

[0053] like Figure 3 The figures show the Brillouin gain spectrum-distance 3D plots measured at the fiber optic end using both the conventional polarization scrambling scheme and the scheme proposed in this application. It can be seen that the Brillouin gain spectrum measured using the scheme proposed in this application is smoother and has less fluctuation compared to the conventional polarization scrambling scheme. Higher demodulation accuracy is obtained when Lorentz fitting is performed on the measured Brillouin gain spectrum to solve for the Brillouin frequency shift. A significant Brillouin gain spectrum shift can be observed at approximately 49.515 km from the fiber optic end, which is caused by the change in the fiber's Brillouin frequency shift (the fiber optic end is spliced ​​with a 6-meter fiber with a Brillouin frequency shift of 10.80 GHz).

[0054] like Figure 4 The figure shows the Brillouin frequency shift of the 35m fiber at the end of the fiber obtained by demodulation using the sampling scheme of this application. The inset in the middle shows the Brillouin frequency shift of the entire sensing fiber. For the measurement of a 6m fiber with a Brillouin frequency shift of 10.80GHz, the length of the fiber from 10% to 90% is approximately 2m, proving that the spatial resolution of the system is 2m, which is consistent with a pump pulse width of 20ns.

[0055] like Figure 5The figure shows the distribution of the standard deviation of the Brillouin frequency shift along the optical fiber when using the conventional polarization scrambling scheme and the scheme of this invention. The standard deviation value can reflect the uncertainty of the Brillouin frequency shift measurement. As can be seen from the figure, due to the influence of polarization noise, the STD value of the random polarization scrambling scheme is always greater than that of the scheme of this application, and the difference between the two increases with the increase of the sensing distance.

[0056] For BOTDA fiber optic sensors employing random polarization scrambling, a polarizer is placed in the pump light path to suppress polarization fading, perturbing the polarization state of the pump light. Due to the randomness of the polarizer, measurement instability arises, requiring extensive accumulation and averaging of the acquired Brillouin gain signal to reduce the impact of polarization noise, resulting in significant time consumption. Compared to BOTDA fiber optic sensors using random polarization scrambling, the proposed solution eliminates the need for polarization scrambling. Two pump pulse sequences with perpendicular polarization states are combined and fed into the sensing fiber, interacting with the probe light to maximize the Brillouin gain and effectively suppress polarization fading and polarization noise. This significantly reduces the number of data accumulation and averaging operations, lowers the acquisition time, and increases the sensing speed.

[0057] For BOTDA fiber optic sensors employing traditional polarization diversity techniques, to avoid co-frequency interference, two pump sequences with perpendicular polarization states must be separated either by time or by frequency. This increases both data acquisition time and the amount of raw data, as well as system complexity. In this application, although the temporal overlap of the two coded sequences produces co-frequency interference, their periods differ from the coded sequence periods. During the averaging of the raw data, amplitude fluctuations caused by co-frequency interference do not affect the demodulation of the Brillouin gain. Therefore, this application reduces both data measurement time and the amount of raw data, as well as system complexity.

[0058] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A BOTDA fiber optic sensing device based on time-overlapping polarization diversity, characterized in that, Includes a laser (1), the output end of which is connected to a first fiber optic coupler (2); One output of the first fiber coupler (2) is connected in sequence to the first polarization controller (3), intensity modulator (4), optical isolator (6) and sensing fiber (7), and the input of the intensity modulator (4) is connected to the microwave source (5). The other output of the first fiber coupler (2) is connected to the second fiber coupler (8). One output of the second fiber coupler (8) is connected to the first acousto-optic modulator (9) and the second polarization controller (10) in sequence. The other output of the second fiber coupler (8) is connected to the second acousto-optic modulator (12) and the third polarization controller (13) in sequence. The input of the first acousto-optic modulator (9) and the input of the second acousto-optic modulator (12) are both connected to the dual-channel arbitrary waveform generator (11). The output of the second polarization controller (10) and the output of the third polarization controller (13) are both connected to the polarization combiner (14). The output of the polarization combiner (14) is connected to the first erbium-doped fiber amplifier (15) and the first optical circulator (16) in sequence. The reflecting end of the first optical circulator (16) is connected to the output of the sensing fiber (7). The output end of the first optical circulator (16) is connected to the second erbium-doped fiber amplifier (17), the second erbium-doped fiber amplifier (17) is connected to the second optical circulator (19), the reflective end of the second optical circulator (19) is connected to the fiber Bragg grating (18), and the output end of the second optical circulator (19) is connected in sequence to the photodetector (20), the low-pass filter (21), and the data acquisition and analysis system (22).

2. The BOTDA fiber optic sensing device based on time overlap polarization diversity according to claim 1, characterized in that, The laser (1) is a narrow linewidth laser.

3. The BOTDA fiber optic sensing device based on time overlap polarization diversity according to claim 2, characterized in that, The splitting ratio of the first fiber coupler (2) and the second fiber coupler (8) is 50:

50.

4. The BOTDA fiber optic sensing device based on time overlap polarization diversity according to claim 3, characterized in that, The input terminal of the first acousto-optic modulator (9) is connected to the first channel signal output terminal of the dual-channel arbitrary waveform generator (11), and the input terminal of the second acousto-optic modulator (12) is connected to the second channel signal output terminal of the dual-channel arbitrary waveform generator (11).

5. A BOTDA fiber optic sensing method based on time-overlapping polarization diversity, characterized in that, Based on the BOTDA fiber optic sensing device according to any one of claims 1-4, the process includes the following steps: S1. Start the BOTDA fiber optic sensing device and the laser (1) emits laser light; S2, Dual-channel arbitrary waveform generator (11) outputs pulse code sequence; S3, Microwave source (5) drives intensity modulator (4) to perform frequency scanning, and the scanning range is adjustable; S4. The photodetector (20) converts the detection light of different scanning frequencies into electrical signals. The data acquisition and analysis system (22) processes the electrical signals and demodulates the temperature and stress distribution along the sensing fiber (7).

6. The BOTDA fiber optic sensing method based on time overlap polarization diversity according to claim 5, characterized in that, When the data acquisition and analysis system (22) processes the electrical signal, it first decodes the cumulative Brillouin gain curve at different scanning frequency points to obtain the single-pulse Brillouin gain curve, merges a series of single-pulse gain curves to obtain the Brillouin gain spectrum at each position, and performs Lorentz fitting at different positions of the sensing fiber to demodulate the Brillouin frequency shift information, reflecting the temperature stress change.

7. The BOTDA fiber optic sensing method based on time-overlapping polarization diversity according to claim 6, characterized in that, The data acquisition and analysis system (22) processes electrical signals using an offline processing method with Matlab and CPU.

8. The BOTDA fiber optic sensing method based on time overlap polarization diversity according to claim 7, characterized in that, The data acquisition and analysis system (22) uses an online processing method accelerated by field-programmable gate array to process electrical signals.

9. The BOTDA fiber optic sensing method based on time overlap polarization diversity according to claim 8, characterized in that, The continuous light emitted by the laser (1) is split into upper and lower beams by the first fiber coupler (2). After passing through the first polarization controller (3), the upper-path light enters the bias voltage range of... The intensity modulator (4) performs carrier-suppressed double-sideband modulation, and changes the frequency of the microwave source (5) driving the intensity modulator (4) so ​​that the frequencies of the two optical sidebands change accordingly. The double-sideband continuous light output by the intensity modulator (4) is injected into the sensing fiber (7) through the optical isolator (6) as the probe light. The lower light is split into two light paths by the second fiber coupler (8). The two light paths are generated by the dual-channel arbitrary waveform generator (11) with two identical codewords and are modulated into pulse light by the first acousto-optic modulator (9) and the second acousto-optic modulator (12). The time delay between the two channels of the dual-channel arbitrary waveform generator (11) is adjusted to ensure that the two light pulses are strictly overlapped in time. Subsequently, the polarization state of the two light pulses is adjusted by the second polarization controller (10) and the third polarization controller (13) to align with the fast axis and slow axis of the polarization combiner (14) respectively. The combined light pulse sequence is amplified to the peak power by the first erbium-doped fiber amplifier (15) and then used as pump light to be input into the sensing fiber (7) through the first optical circulator (16). The pump light and probe light traveling in opposite directions undergo stimulated Brillouin interaction in the sensing fiber (7). The probe light after the interaction enters the second erbium-doped fiber amplifier (17) through the output end of the first optical circulator (16). The low-frequency sideband of the probe light is amplified by the second erbium-doped fiber amplifier (17), and then passes through the second optical circulator (19) and the fiber Bragg grating (18) for bandpass filtering. Subsequently, the probe light is converted into an electrical signal after passing through the photodetector (20). The electrical signal is filtered and denoised by the low-pass filter (21) and then transmitted to the data acquisition and analysis system (22) through the high-frequency coaxial cable. The acquired electrical signal is processed to realize distributed temperature stress measurement.

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