BOTDA optical fiber sensing device and method based on time overlapping polarization diversity

Abstract

The invention discloses a BOTDA (Brillouin Optical Time Domain Analysis) optical fiber sensing device and method based on time overlapping polarization diversity. The device comprises a laser, a first optical fiber coupler, a first polarization controller, an intensity modulator, a microwave source, an optical isolator, a sensing optical fiber, a second optical 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 beam combiner and a first erbium-doped optical fiber amplifier. The device comprises a first optical circulator, a second erbium-doped fiber amplifier, a fiber bragg grating, a second optical circulator, a photoelectric detector, a low-pass filter and a data acquisition and analysis system. The time overlapping polarization diversity method is adopted, polarization noise caused by scrambling and polarization fading caused by the polarization traction effect are effectively avoided, and the measurement precision of the sensor is greatly improved.

Description

Technical Field

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

[0002] In Brillouin optical time-domain analysis (BOTDA) technology, ordinary single-mode optical fiber is usually used as the sensing medium to control costs. However, during the manufacturing process, single-mode optical fiber may produce birefringence due to uneven drawing or the influence of external mechanical forces such as bending and vibration, which leads to random changes in the polarization state of the input optical fiber, making it difficult for the polarization state of the pump light and the detection light to remain consistent, thus causing the polarization fading problem of Brillouin gain.

[0003] Random polarization scrambling is a common technology to solve this problem. This technology uses a polarization scrambler to randomly scramble the pump light, thereby effectively suppressing polarization fading. However, due to the randomness of the scrambling, strong measurement instability, namely polarization noise, will occur during the measurement process. This polarization noise will significantly affect the signal-to-noise ratio of the sensing system and reduce the performance of the system. The size of the polarization noise is linearly related to the size of the Brillouin gain. As the Brillouin gain increases, the polarization noise becomes stronger. In pulse-coded BOTDA fiber sensors, the pump coding sequence can generate a strong cumulative Brillouin gain, which in turn introduces a strong polarization noise, which is the dominant noise in pulse-coded BOTDA fiber sensors.

[0004] Another way to deal with polarization fading is to use polarization switching. This method inputs two pump pulses with orthogonal polarization states into the sensing fiber separately, 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 technology will introduce polarization pulling effects, which will lead to polarization fading. In addition, polarization switching technology will double the measurement time.

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

[0006] In order to solve the polarization noise and polarization fading problems in pulse-coded BOTDA optical fiber sensors, the present invention proposes a BOTDA optical fiber sensing device and method based on time-overlapping polarization diversity, which can avoid the polarization noise problem caused by polarization disturbance on the one hand, and on the other hand, different from the traditional polarization switching scheme, avoids the polarization fading caused by the polarization pulling effect, thereby greatly improving the measurement accuracy of the sensor.

[0007] The present application discloses a BOTDA optical fiber sensing device based on time overlap polarization diversity, comprising a laser, wherein the output end of the laser is connected to a first optical fiber coupler; An output end of the first optical fiber coupler is connected to a first polarization controller, an intensity modulator, an optical isolator and a sensing optical fiber in sequence, and an input end of the intensity modulator is connected to a microwave source; The other output end of the first optical fiber coupler is connected to the second optical fiber coupler, one output end of the second optical fiber coupler is connected to the first acousto-optic modulator and the second polarization controller in sequence, the other output end of the second optical fiber coupler is connected to the second acousto-optic modulator and the third polarization controller in sequence, the first acousto-optic modulator input end and the second acousto-optic modulator input end are both connected to the dual-channel arbitrary waveform generator, the second polarization controller output end and the third polarization controller output end are both connected to the polarization combiner, the output end of the polarization combiner is connected to the first erbium-doped fiber amplifier and the first optical circulator in sequence, and the reflection end of the first optical circulator is connected to the output end of the sensing optical fiber; The output end of the first optical circulator is connected to the second erbium-doped fiber amplifier, the second erbium-doped fiber amplifier is connected to the second optical circulator, the reflection end of the second optical circulator is connected to the fiber Bragg grating, and the output end of the second optical circulator is sequentially connected to a photodetector, a low-pass filter, and a data acquisition and analysis system.

[0008] Preferably, the laser is a narrow linewidth laser.

[0009] Preferably, the splitting ratios of the first optical fiber coupler and the second optical fiber coupler are both 50:50.

[0010] Preferably, the input end of the first acousto-optic modulator is connected to the first channel signal output end of the dual-channel arbitrary waveform generator, and the input end of the second acousto-optic modulator is connected to the second channel signal output end of the dual-channel arbitrary waveform generator.

[0011] The present application also discloses a BOTDA optical fiber sensing method based on time overlap polarization diversity, which is implemented based on a BOTDA optical fiber sensing device and includes the following steps: S1, start the BOTDA optical fiber sensor device, and the laser emits laser; S2, dual-channel arbitrary waveform generator outputs pulse code sequence; S3, the microwave source drives the intensity modulator to perform frequency scanning, and the scanning range is adjustable; S4. The detection light of different scanning frequencies is converted into electrical signals through photoelectric detectors. The electrical signals are processed by the data acquisition and analysis system to demodulate the temperature and stress distribution along the sensing optical fiber.

[0012] Preferably, when the data acquisition and analysis system processes the electrical signal, it first decodes the cumulative Brillouin gain curve at different scanning frequency points to obtain a single-pulse Brillouin gain curve, combines 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 optical fiber, thereby demodulating the Brillouin frequency shift information to reflect the temperature stress change.

[0013] Preferably, the data acquisition and analysis system processes the electrical signal using an off-line processing method of Matlab and CPU.

[0014] Preferably, the data acquisition and analysis system processes the electrical signals using an online processing method accelerated by a field programmable gate array.

[0015] Preferably, the continuous light emitted by the laser is divided into two paths of light through a first optical fiber coupler; The uplink light passes through the first polarization controller and then enters the bias voltage The intensity modulator performs carrier suppressed double sideband modulation, and 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 optical fiber through the optical isolator as the detection light; The downlink light is divided into two light paths through the second optical fiber coupler. The two light paths are modulated into pulse light by the first acousto-optic modulator and the second acousto-optic modulator respectively by the coding sequences with the same codewords generated by the dual-channel arbitrary waveform generator. The time delay between the two channels of the dual-channel arbitrary waveform generator is adjusted to ensure that the two light pulses are strictly overlapped in time. Subsequently, the polarization states of the two light pulses are adjusted by the second polarization controller and the third polarization controller to align the fast axis and the slow axis of the polarization combiner respectively. The combined light pulse sequence is amplified to the peak power by the first erbium-doped fiber amplifier, and then input into the sensing fiber as pump light through the first optical circulator. The pump light and the detection light transmitted in opposite directions undergo stimulated Brillouin action in the sensing optical fiber. After the action, the detection light enters the second erbium-doped fiber amplifier through the output end of the first optical circulator. The low-frequency sideband of the detection light is amplified by the second erbium-doped fiber amplifier, and then passes through the second optical circulator and the fiber Bragg grating bandpass filter. Subsequently, the detection light is converted into an electrical signal after passing through the photodetector. After the electrical signal is filtered and denoised by a low-pass filter, it is transmitted to the data acquisition and analysis system through a high-frequency coaxial cable. The collected electrical signal is processed to realize distributed temperature stress measurement.

[0016] Beneficial effects of the present invention: (1) The present invention does not require polarization scrambling. Two pump pulse sequences with perpendicular polarization states are combined and enter the sensing optical fiber. At the same time, the interaction with the detection 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.

[0017] (2) Although the time overlap of the two coding sequences of the present invention produces co-frequency interference, its period is different from that of the coding sequence. During the averaging process of the original data, the amplitude fluctuation caused by the co-frequency interference will not affect the demodulation of the Brillouin gain, thereby reducing the data measurement time and the amount of original data, and also reducing the complexity of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A BOTDA optical fiber sensing device based on time overlap polarization diversity according to an embodiment of the present invention; Figure 2 It is a schematic diagram of the effect of cyclic coding based on time overlap polarization diversity according to an embodiment of the present invention; Figure 3 A three-dimensional diagram of Brillouin gain spectrum-distance using polarization diversity with polarization scrambling and time overlap according to an embodiment of the present invention; Figure 4 A schematic diagram of the Brillouin frequency shift of a 35m optical fiber at the end of an optical fiber according to an embodiment of the present invention; Figure 5 Schematic diagram of the distribution of the standard deviation of the Brillouin frequency shift along the optical fiber using polarization diversity with polarization scrambling and time overlap according to an embodiment of the present invention.

[0019] The reference numerals are as follows: 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 DESCRIPTION

[0020] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples.

[0021] The present application embodiment discloses a BOTDA optical fiber sensing device based on time overlap polarization diversity, such as Figure 1As 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.

[0022] The output end of the laser 1 is connected to the input end of the first optical fiber coupler 2, the first output end of the first optical fiber coupler 2 is connected to the input end of the first polarization controller 3 through a single-mode optical 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 optical fiber jumper, the signal output end of the microwave source 5 is connected to the signal input end of the intensity modulator 4, the output end of the intensity modulator 4 is connected to the input end of the optical isolator 6 through a single-mode optical fiber jumper, and the output end of the optical isolator 6 is connected to the input end of the sensing optical fiber 7 through a single-mode optical fiber jumper.

[0023] The second output end of the first fiber coupler 2 is connected to the input end of the second fiber coupler 8 through a single-mode fiber jumper, the first output end of the second fiber coupler 8 is connected to the input end of the first acousto-optic modulator 9 through a single-mode fiber jumper, and the output end of the first acousto-optic modulator 9 is connected to the input end of the second polarization controller 10 through a single-mode fiber jumper. The second output end of the second fiber coupler 8 is connected to the input end of the second acousto-optic modulator 12 through a single-mode fiber jumper, the output end of the second acousto-optic modulator 12 is connected to the input end of the third polarization controller 13 through a single-mode fiber jumper, the input end of the first acousto-optic modulator 9 is connected to the first channel signal output end of the dual-channel arbitrary waveform generator 11, and the input end of the second acousto-optic modulator 12 is connected to the second channel signal output end of the dual-channel arbitrary waveform generator 11. The output end of the second polarization controller 10 is connected to the first input end of the polarization combiner 14 through a single-mode fiber jumper, and the output end of the third polarization controller 13 is connected to the second input end of the polarization combiner 14 through a single-mode fiber jumper. The output end of the polarization combiner 14 is connected to the input end of the first erbium-doped fiber amplifier 15 through a single-mode fiber jumper, the output end of the first erbium-doped fiber amplifier 15 is connected to the input end of the first optical circulator 16 through a single-mode fiber jumper, and the reflection end of the first optical circulator 16 is connected to the output end of the sensing fiber 7 through a single-mode fiber jumper.

[0024] The output end of the first optical circulator 16 is connected to the input end of the second erbium-doped fiber amplifier 17 through a single-mode fiber jumper, the output end of the second erbium-doped fiber amplifier 17 is connected to the input end of the second optical circulator 19 through a single-mode fiber jumper, the output end of the fiber Bragg grating 18 is connected to the reflection end of the second optical circulator 19 through a single-mode fiber jumper, the output end of the second optical circulator 19 is connected to the input end of the photodetector 20 through a single-mode fiber jumper, the signal output end of the photodetector 20 is connected to the input end of the low-pass filter 21, and the output end of the low-pass filter 21 is connected to the input end of the data acquisition and analysis system 22.

[0025] The laser 1 is a narrow line width laser, and the splitting ratios of the first optical fiber coupler 2 and the second optical fiber coupler 8 are both 50:50.

[0026] The continuous light emitted by the 1KHz narrow linewidth laser 1 with an output wavelength of 1550.08nm is split into two paths of light through the 50:50 first optical fiber coupler 2.

[0027] After the uplink light passes through the first polarization controller 3, the bias voltage is The intensity modulator 4 performs carrier suppressed double sideband modulation, and 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 optical fiber 7 through the optical isolator 6 as the detection light.

[0028] The downlink light is divided into two paths through the 50:50 second fiber coupler 8. The two paths are modulated into pulse light by the first acousto-optic modulator 9 and the second acousto-optic modulator 12 respectively by the coding sequence with the same codewords generated by the dual-channel arbitrary waveform generator 11. The time delay between the two channels of the dual-channel arbitrary waveform generator 11 is adjusted to ensure that the two paths of light pulses are strictly overlapped in time. Subsequently, the polarization states of the two paths of light pulses are adjusted by the second polarization controller 10 and the third polarization controller 13 to align the fast axis and the slow axis of the polarization combiner 14 respectively. The combined light pulse sequence is amplified to a peak power of 18 dBm by the first erbium-doped fiber amplifier 15, and then input into the sensing fiber 7 as pump light through the first optical circulator 16.

[0029] The pump light and the detection light transmitted in opposite directions undergo stimulated Brillouin action in the sensing optical fiber 7, and the detection light after the action enters the second erbium-doped fiber amplifier 17 through the output end of the first optical circulator 16. The low-frequency sideband of the detection light is amplified by the second erbium-doped fiber amplifier 17, and then band-pass filtered by the second optical circulator 19 and the fiber Bragg grating 18. Subsequently, the detection light is converted into an electrical signal after passing through the photodetector 20. After the electrical signal is filtered and de-noised by the low-pass filter 21, it is transmitted to the data acquisition and analysis system 22 via a high-frequency coaxial cable, and the sampling rate is set to 100MSa / s. The collected electrical signal is processed to obtain the Brillouin gain spectrum of each sensing point of the optical fiber, and the Brillouin frequency shift information is further demodulated to realize distributed temperature stress measurement.

[0030] like Figure 2 This is a schematic diagram of the effect of the cyclic coding based on time overlap polarization diversity of the present application. Taking cyclic coding as an example, each group of cyclic coding sequences 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 the sensing distance and the number of coding bits. It is necessary to ensure that the duration of the cyclic coding sequence is slightly greater than the time it takes for the light wave to propagate a round trip in the entire sensing optical fiber. In the application, the codewords of the two optical pump pulse sequences are the same. 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 and the detection light in the X-direction polarization state and the Y-direction polarization state can maximize the Brillouin gain of each sensing position, and can effectively suppress polarization pulling. In addition, since polarization scrambling is not used, the generation of polarization noise can be effectively avoided.

[0031] Another embodiment of the present application also discloses a BOTDA optical fiber sensing method based on time overlap polarization diversity, which is implemented based on a BOTDA optical fiber sensing device and includes the following steps: S1, start the BOTDA optical fiber sensor device, and laser 1 emits laser; S2, dual-channel arbitrary waveform generator 11 outputs pulse code sequence; S3, the microwave source 5 drives the intensity modulator 4 to perform frequency scanning, and the scanning range is adjustable; S4. The detection light of different scanning frequencies is converted into an electrical signal by the photoelectric detector 20. The electrical signal is processed by the data acquisition and analysis system 22 to demodulate the temperature and stress distribution along the sensing optical fiber 7.

[0032] Specifically, when the data acquisition and analysis system 22 processes the electrical signal, it first decodes the cumulative Brillouin gain curves at different scanning frequency points to obtain a 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 optical fiber, and then demodulates the Brillouin frequency shift information to reflect the temperature stress change. In this embodiment, the data acquisition and analysis system 22 can process the electrical signal in an offline processing method using Matlab and CPU, or in an online processing method accelerated by a field programmable gate array.

[0033] like Figure 3 The three-dimensional diagrams of the Brillouin gain spectrum-distance measured at the end of the optical fiber using the conventional scrambling scheme and the scheme of the present application are shown. It can be seen that the measured Brillouin gain spectrum of the present application scheme is smoother and has smaller fluctuations than the conventional scrambling scheme. When the measured Brillouin gain spectrum is subjected to Lorentz fitting to solve the Brillouin frequency shift, a higher demodulation accuracy can be obtained. An obvious Brillouin gain spectrum offset can be seen at about 49.515 km from the end of the optical fiber, which is caused by the change in the Brillouin frequency shift of the optical fiber (a 6-meter optical fiber with a Brillouin frequency shift of 10.80 GHz is fused to the end of the optical fiber).

[0034] like Figure 4 The figure shows the Brillouin frequency shift of the 35m fiber at the end of the fiber obtained by demodulation of the sample solution of this application, and the middle illustration shows the Brillouin frequency shift of the entire sensing fiber. For the measurement of 6 meters of fiber with a Brillouin frequency shift of 10.80GHz, the length of the Brillouin frequency shift from 10% to 90% is about 2m, proving that the spatial resolution of the system is 2m, which is consistent with the 20ns pump pulse width.

[0035] like Figure 5 The figure shows the distribution of the standard deviation of the Brillouin frequency shift along the optical fiber when the conventional scrambling scheme and the scheme of the present invention are used. The standard deviation value can reflect the uncertainty of the Brillouin frequency shift measurement. It can be seen from the figure that the random scrambling scheme has a STD value that is always greater than the scheme of the present application due to the influence of polarization noise, and the difference between the two increases with the increase of the sensing distance.

[0036] For the BOTDA fiber sensor using random scrambling technology, in order to suppress polarization fading, a scrambler is set on the pump light path to disturb the polarization state of the pump light. Due to the randomness of the scrambler, the measurement instability is caused, and a large amount of cumulative averaging processing is required for the collected Brillouin gain signal to reduce the influence of polarization noise, which results in a large consumption of acquisition time. Compared with the BOTDA fiber sensor using random scrambling technology, the solution proposed in this application does not require scrambling. The two-way polarization state perpendicular pump pulse sequence is combined and enters the sensing fiber. At the same time, the interaction with the detection light can maximize the Brillouin gain, effectively suppressing polarization fading and polarization noise. This effectively reduces the number of data cumulative averaging times, reduces the acquisition time, and improves the sensing speed.

[0037] For the BOTDA fiber sensor using traditional polarization diversity technology, in order to avoid co-frequency interference, the two pump sequences with perpendicular polarization states are either separated by time or by frequency. On the one hand, this increases the data acquisition time and the amount of raw data, and on the other hand, it increases the complexity of the system. Although the time overlap of the two coding sequences in this application produces co-frequency interference, its period is different from the coding sequence period. In the process of averaging the raw data, the amplitude fluctuation caused by co-frequency interference will not affect the demodulation of the Brillouin gain. Therefore, this application can reduce both the data measurement time and the amount of raw data and the complexity of the system.

[0038] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

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

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

50.

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

5. A BOTDA optical fiber sensing method based on time overlap polarization diversity, characterized in that: The BOTDA optical fiber sensing device according to any one of claims 1 to 4 is implemented, comprising the following steps: S1, start the BOTDA optical fiber sensor device, and the laser (1) emits laser; S2, a dual-channel arbitrary waveform generator (11) outputs a pulse code sequence; S3, the microwave source (5) drives the intensity modulator (4) to perform frequency scanning, and the scanning range is adjustable; S4. The detection light of different scanning frequencies is converted into an electrical signal by a photoelectric detector (20), and the electrical signal is processed by a data acquisition and analysis system (22) to demodulate the temperature and stress distribution along the sensing optical fiber (7).

6. The BOTDA optical fiber 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 curves at different scanning frequency points to obtain a single-pulse Brillouin gain curve, combines a series of single-pulse gain curves to obtain a Brillouin gain spectrum at each position, and performs Lorentz fitting at different positions of the sensing optical fiber, thereby demodulating the Brillouin frequency shift information to reflect the temperature stress change.

7. The BOTDA optical fiber sensing method based on time overlap polarization diversity according to claim 6, characterized in that: The data acquisition and analysis system (22) processes the electrical signal using an off-line processing method of Matlab and CPU.

8. The BOTDA optical fiber sensing method based on time overlap polarization diversity according to claim 7, characterized in that: The data acquisition and analysis system (22) processes the electrical signal using an online processing method accelerated by a field programmable gate array.

9. The BOTDA optical fiber sensing method based on time overlap polarization diversity according to claim 8, characterized in that: The continuous light emitted by the laser (1) is divided into two paths of light, upper and lower, through the first optical fiber coupler (2); The uplink light passes through the first polarization controller (3) and then enters the bias voltage The intensity modulator (4) performs carrier suppressed double sideband modulation, changes the frequency of the microwave source (5) driving the intensity modulator (4), so that the frequencies of the two optical sidebands change accordingly, and the double sideband continuous light output by the intensity modulator (4) is injected into the sensing optical fiber (7) through the optical isolator (6) as detection light; The downlink light is divided into two light paths through a second optical fiber coupler (8), and the two light paths are modulated into pulse light by a first acousto-optic modulator (9) and a second acousto-optic modulator (12) respectively using a coding sequence with the same codewords generated by a dual-channel arbitrary waveform generator (11). 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 states of the two light pulses are adjusted by a second polarization controller (10) and a third polarization controller (13) to align the fast axis and the slow axis of the polarization combiner (14) respectively. The combined light pulse sequence is amplified to a peak power by a first erbium-doped fiber amplifier (15), and then input into the sensing optical fiber (7) as pump light through a first optical circulator (16); The pump light and the detection light transmitted in opposite directions undergo stimulated Brillouin action in the sensing optical fiber (7). After the action, the detection light enters the second erbium-doped fiber amplifier (17) through the output end of the first optical circulator (16). The low-frequency sideband of the detection light is amplified by the second erbium-doped fiber amplifier (17), and then band-pass filtered by the second optical circulator (19) and the fiber Bragg grating (18). Subsequently, the detection light is converted into an electrical signal after passing through a photoelectric detector (20). After the electrical signal is filtered and de-noised by a low-pass filter (21), it is transmitted to a data acquisition and analysis system (22) via a high-frequency coaxial cable. The collected electrical signal is processed to realize distributed temperature stress measurement.

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