BOTDA optical fiber sensing device based on periodic perturbation noise suppression

Abstract

The utility model relates to Brillouin distributed optical fiber sensing technology field, specifically disclose the BOTDA optical fiber sensing device based on period disturbance, including, laser, first optical fiber coupler, intensity modulator, optical isolator, sensing optical fiber, microwave source, first optical circulator, first doped erbium optical fiber amplifier, second doped erbium optical fiber amplifier, second optical circulator, fiber bragg grating, photodetector, low pass filter, data acquisition and analysis system, period disturbance module. The utility model discloses through period gradual disturbance mechanism, effectively solved the polarization noise problem in pulse coding BOTDA system due to the traditional random disturbance technique, while ensuring the elimination of polarization fading, significantly improved the system signal to noise ratio, the stability and demodulation accuracy of measurement result, provides an effective and reliable solution for high performance long distance distributed optical fiber sensing.

Description

Technical Field

[0001] This utility model relates to the field of Brillouin distributed optical fiber sensing technology, and more specifically to a BOTDA optical fiber sensing device and method based on periodic polarization noise suppression. Background Technology Optical Time-Domain Analysis (BOTDA) is a key technology that utilizes the stimulated Brillouin scattering (SBS) effect in optical fibers to achieve distributed measurement. It can continuously and in real-time acquire temperature or stress distribution information along optical fibers that are tens or even hundreds of kilometers long. Due to the advantages of optical fibers, such as resistance to electromagnetic interference, corrosion resistance, light weight, and ease of installation, BOTDA has been widely used in various fields including energy, transportation, civil engineering, geological disaster early warning, marine engineering, and national defense.

[0002] However, the implementation of BOTDA technology is limited by various factors such as the polarization characteristics of optical fibers, signal strength, nonlinear effects, and noise accumulation. Among these, polarization fading and polarization noise caused by polarization effects are one of the main bottlenecks limiting BOTDA performance. Ideally, single-mode optical fibers should maintain constant polarization. However, due to inhomogeneities in the fiber drawing process, geometric deformation, external mechanical strain, temperature disturbances, and environmental vibrations, optical fibers exhibit weak birefringence, causing the polarization state of light to change randomly during transmission. This random change directly affects the polarization matching degree when the pump and probe lights interact within the fiber. If the polarization states of the pump and probe lights are perfectly orthogonal, the Brillouin gain will be zero; if they are perfectly parallel, the Brillouin gain will reach its maximum. In reality, due to the continuous change in polarization state, the Brillouin gain will fluctuate between "zero gain" and "maximum gain," which is polarization fading. Polarization fading exhibits randomness in spatial distribution, causing severe distortion of the Brillouin gain signal at different locations, affecting measurement accuracy and stability.

[0003] To suppress polarization fading, traditional methods primarily employ random polarization scrambling (PS) techniques. This involves randomly altering the polarization state of the pump or probe light and averaging the results multiple times to obtain a stable half-maximum gain. However, while eliminating polarization fading, random polarization scrambling introduces significant polarization noise. Polarization noise arises because the polarization state of the pump or probe light is drastically and randomly changed over time by the scrambler, leading to inconsistencies in the sampled signal during the accumulation and averaging process, thus creating random interference. Especially in pulse-coded BOTDA systems, the Brillouin gain increases significantly due to the cumulative effect of coding gain, and polarization noise also intensifies, becoming a major factor limiting system performance. Another commonly used method is polarization diversity (PD) technology, which suppresses fading by alternately transmitting orthogonally polarized pump pulses and merging their gain signals. However, this method easily induces polarization pulling effects in coded BOTDA, resulting in incomplete elimination of polarization fading, and doubles the measurement time, increasing system complexity.

[0004] Therefore, existing technologies are insufficient to effectively address both polarization fading and polarization noise issues simultaneously in pulse-code BOTDA systems. A new technical solution is urgently needed to achieve high-precision, low-noise BOTDA distributed fiber sensing. Utility Model Content

[0005] The purpose of this invention is to propose a BOTDA fiber optic sensing device based on periodic polarization noise suppression. This scheme periodically, rather than randomly, changes the polarization state of the pump light, ensuring a sufficient number of polarization states to eliminate polarization fading while significantly reducing random fluctuations in the polarization state between the pump and probe light. This effectively suppresses polarization noise, improves the system's signal-to-noise ratio and measurement accuracy, and achieves high-performance distributed temperature and stress sensing, thus solving the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention adopts the following solution, including: A laser, the output of which is connected to the input of a first fiber optic coupler; The first output terminal of the first fiber coupler is sequentially connected to the intensity modulator, the optical isolator, the sensing fiber, and the reflection terminal of the first optical circulator; the radio frequency signal input terminal of the intensity modulator is connected to the output terminal of the microwave source. The second output of the first fiber coupler is connected to a periodic polarization scrambling module; the output of the periodic polarization scrambling module is sequentially connected to the input of a first erbium-doped fiber amplifier, a first optical circulator, a second erbium-doped fiber amplifier, and the input of a second optical circulator; the reflector of the second optical circulator is connected to a fiber Bragg grating; the output of the second optical circulator is sequentially connected to a photodetector, a low-pass filter, and a data acquisition and analysis system. The periodic scrambling module is either a phase modulation module or a heterogeneous acousto-optic modulation module.

[0007] Preferably, the phase modulation module includes a first polarization controller and a dual-channel arbitrary waveform generator; the input end of the first polarization controller is connected to the output end of the first fiber coupler, and the output end is connected in sequence to the phase modulator and the semiconductor optical amplifier; The input terminals of the phase modulator and the semiconductor optical amplifier are both connected to the output terminal of the dual-channel arbitrary waveform generator.

[0008] Preferably, the phase modulator is a birefringent lithium niobate phase modulator.

[0009] Preferably, the dual-channel arbitrary waveform generator outputs a sawtooth wave electrical signal through its first output channel and an encoded sequence through its second output channel.

[0010] This application also discloses a BOTDA fiber optic sensing method based on periodic polarization noise suppression, the method being implemented using a BOTDA fiber optic sensing device, comprising: S1. Start the laser and split the light into upper and lower paths through the first fiber coupler; S2. The upper path light passes through an intensity modulator driven by a microwave source to generate a double-sideband probe light; S3. The down-path light uses a periodic polarization disturbance method to cause a periodic change in the phase difference between the two orthogonal polarization components of the light wave; the periodic progressive polarization disturbance method can be, but is not limited to, phase modulation or heterogeneous acousto-optic modulation. The polarization state of a light wave can be determined by the components of its electric field vector in the orthogonal directions and the phase difference between them. It is confirmed that the periodic asymptotic polarization technique can reduce the phase difference. The waveform exhibits a periodic linear variation with a sawtooth pattern over time, with a period of Ts. This invention ensures that within each period... The polarization is gradually varied within the radian range of 0 to 2π, thereby accessing a sufficient number of polarization states to eliminate polarization fading, while greatly reducing random fluctuations in polarization states and fundamentally suppressing polarization noise.

[0011] S4. The combined optical signal is converted into an electrical signal by a photodetector, and the distributed temperature and stress information along the sensing fiber is demodulated by the data acquisition and analysis system.

[0012] Preferably, the phase modulation method in step S3 includes: S301. The downlink light is adjusted by a polarization controller so that its polarization direction forms a 45° angle with one of the principal axes of the phase modulator, thereby generating two orthogonal polarization components. S302. A phase modulator is driven by a periodic signal, causing two orthogonal polarization components to generate a periodic relative phase difference, thereby synthesizing a light wave whose polarization state evolves periodically with the phase difference. When the phase modulator is driven by a periodic sawtooth wave signal, due to the birefringence effect, the two orthogonal polarization components will generate a periodic relative phase difference. After being synthesized at the output, their polarization state evolves periodically with the phase difference, thus forming a periodic polarization disturbance that is consistent with the period of the driving signal, i.e., the periodic sawtooth wave signal.

[0013] S303, the light wave is input to the semiconductor optical amplifier, and the second channel output terminal of the dual-channel arbitrary waveform generator outputs an encoded sequence to drive the semiconductor optical amplifier, thereby modulating the light wave into synchronous encoded pulse light; Preferably, the acousto-optic modulation method in step S3 uses a hetero-frequency acousto-optic modulation module. The hetero-frequency acousto-optic modulation module includes a second fiber coupler, the input end of which is connected to the second output end of a first fiber coupler; the first output end of the second fiber coupler is sequentially connected to a first acousto-optic modulator and a second polarization controller; the second output end of the second fiber coupler is sequentially connected to a second acousto-optic modulator and a third polarization controller; the input ends of the first and second acousto-optic modulators are both connected to the output end of a dual-channel arbitrary waveform generator; and the output ends of the second and third polarization controllers are both connected to the input end of a polarization combiner. The modulation method is as follows: S301, The second fiber optic coupler splits the downstream light into two paths, which are respectively input to the first acousto-optic modulator and the second acousto-optic modulator; S302, the first acousto-optic modulator and the second acousto-optic modulator have a fixed frequency difference, so that the light waves generate a relative phase difference before beam combining; S303. The two optical signals are adjusted by two polarization controllers to keep them orthogonal to each other, and then combined by a polarization combiner.

[0014] Compared with the prior art, the embodiments of this utility model have at least the following advantages or beneficial effects: First, the periodic polarization disturbance technique of this invention can effectively eliminate polarization noise. On this basis, the signal-to-noise ratio of the system of this invention is significantly improved. The average signal-to-noise ratio of the Brillouin gain on the entire sensing fiber is increased by about 4dB. This improvement makes the dominant noise source of the system change from gain-related polarization noise to beat frequency noise of probe light and spontaneous Brillouin scattering.

[0015] Secondly, the improved signal-to-noise ratio directly leads to a significant improvement in measurement stability, effectively suppressing amplitude fluctuations in the measurement signal and providing high-quality raw data for subsequent accurate demodulation of the Brillouin shift. This improved measurement stability ultimately translates into a substantial increase in system measurement accuracy. The measurement uncertainty at the end of the sensing fiber in this invention is significantly reduced from approximately 1.5MHz in the random polarization scheme to approximately 0.6MHz, a reduction of approximately 2.5 times. Furthermore, this invention also demonstrates superior performance in spatial resolution verification.

[0016] In summary, this invention effectively solves the polarization noise problem caused by the traditional random polarization scrambling technique in pulse code BOTDA systems through a periodic progressive polarization scrambling mechanism. While ensuring the elimination of polarization fading, it significantly improves the system's signal-to-noise ratio, the stability of measurement results, and the demodulation accuracy, providing an effective and reliable solution for high-performance long-distance distributed optical fiber sensing. Attached Figure Description

[0017] Figure 1 This is an overall structural diagram of the present invention; Figure 2 This is a structural diagram of the phase modulation module of this utility model; Figure 3 This is a structural diagram of the heterogeneous frequency acoustic-optical modulation module of this utility model; Figure 4 This is a schematic diagram of the phase modulation method of this utility model; Figure 5 This is a schematic diagram of the heterogeneous frequency acoustic-optical modulation method of this utility model; Figure 6 This is a schematic diagram of the optical polarization state principle of this utility model; Figure 7 This is a schematic diagram of the random scrambling principle of this utility model; Figure 8 This is a schematic diagram of the principle of periodic gradual perturbation of the present invention; Figure 9 This is a distribution diagram of Brillouin gain along the entire sensing fiber under the random and periodic polarization scrambling schemes of this utility model. Figure 10 This is a diagram showing the distribution of Brillouin gain along the fiber end under the random and periodic polarization scrambling schemes of this invention. Figure 11 The graph shows the signal-to-noise ratio of the Brillouin gain for decoding a single pulse under the random and periodic scrambling schemes of this invention as a function of distance. Figure 12 The images show the three-dimensional Brillouin gain measured at the end of the optical fiber under the random and periodic polarization interference schemes of this invention, where (a) is the three-dimensional image under the random polarization interference scheme and (b) is the three-dimensional image under the periodic polarization interference scheme. Figure 13 The distribution of Brillouin frequency shift and its standard deviation along the optical fiber obtained by the random and periodic polarization scrambling schemes of this invention is shown in (a), where (a) is the distribution of Brillouin frequency shift along the optical fiber obtained by the random and periodic polarization scrambling schemes, and (b) is the distribution of the standard deviation of Brillouin frequency shift along the optical fiber obtained by the random and periodic polarization scrambling schemes. Figure 14 The figures show a comparison of the Brillouin frequency shift measurement results near the hotspot using the random and periodic perturbation schemes of this invention, where (a) is a comparison of the results under the random perturbation scheme and (b) is a comparison of the results under the periodic perturbation scheme. As shown in the figure: 1-Laser, 2-First fiber coupler, 3-Intensity modulator, 4-Microwave source, 5-Optical isolator, 6-Sensing fiber, 7-First polarization controller, 8-Phase modulator, 9-Dual-channel arbitrary waveform generator, 10-Semiconductor optical amplifier, 11-First erbium-doped fiber amplifier, 12-First optical circulator, 13-Second erbium-doped fiber amplifier, 14-Fiber Bragg grating, 15-Second optical circulator, 16-Photodetector, 17-Low-pass filter, 18-Data acquisition and analysis system, 19-Second fiber coupler, 20-First acousto-optic modulator, 21-Second acousto-optic modulator, 22-Second polarization controller, 23-Third polarization controller, 24-Polarization combiner. Detailed Implementation

[0018] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present utility model. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present utility model without creative effort are within the protection scope of the present utility model.

[0019] Example 1: Reference Figure 1-3 As shown, this utility model provides a BOTDA fiber optic sensing device based on periodic polarization noise suppression: Laser 1, the output end of which is connected to the input end of the first fiber coupler 2; The first output end of the first fiber coupler 2 is sequentially connected to the intensity modulator 3, the optical isolator 5, the sensing fiber 6, and the reflection end of the first optical circulator 12; the radio frequency signal input end of the intensity modulator 3 is connected to the output end of the microwave source 4. The second output terminal of the first fiber coupler 2 is connected to the periodic polarization scrambling module; the output terminal of the periodic polarization scrambling module is sequentially connected to the input terminals of the first erbium-doped fiber amplifier 11, the first optical circulator 12, the second erbium-doped fiber amplifier 13, and the second optical circulator 15; the reflective terminal of the second optical circulator 15 is connected to the fiber Bragg grating 14; the output terminal of the second optical circulator 15 is sequentially connected to the photodetector 16, the low-pass filter 17, and the data acquisition and analysis system 18. The periodic scrambling module is either a phase modulation module or a heterogeneous acousto-optic modulation module.

[0020] according to Figure 2 As shown, the phase modulation module includes a first polarization controller 7 and a dual-channel arbitrary waveform generator 9; the input end of the first polarization controller 7 is connected to the output end of the first fiber coupler 2, and the output end is connected in sequence to the phase modulator 8 and the semiconductor optical amplifier 10; The input terminals of the phase modulator 8 and the semiconductor optical amplifier 10 are both connected to the output terminal of the dual-channel arbitrary waveform generator 9.

[0021] Specifically, the phase modulator 8 is a birefringent lithium niobate phase modulator.

[0022] Specifically, the dual-channel arbitrary waveform generator 9 outputs a sawtooth wave electrical signal through its first output channel and a cyclic encoded sequence through its second output channel.

[0023] according to Figure 3 As shown, the heterogeneous frequency acoustic-optic modulation module includes a second fiber coupler 19, the input end of which is connected to the second output end of the first fiber coupler 2; the first output end of the second fiber coupler 19 is sequentially connected to the first acoustic-optic modulator 20 and the second polarization controller 22; the second output end of the second fiber coupler 19 is sequentially connected to the second acoustic-optic modulator 21 and the third polarization controller 23; the input ends of the first acoustic-optic modulator 20 and the second acoustic-optic modulator 21 are respectively connected to the first channel output end and the second channel output end of the dual-channel arbitrary waveform generator 9; the output ends of the second polarization controller 22 and the third polarization controller 23 are both connected to the input end of the polarization combiner 24; and the output end of the polarization combiner 24 is connected to the input end of the first erbium-doped fiber amplifier 11.

[0024] The frequencies of the radio frequency drive sources of the first acousto-optic modulator 20 and the second acousto-optic modulator 21 are different.

[0025] Example 2: refer to Figure 4-14 A BOTDA fiber optic sensing method based on periodic polarization noise suppression, applied to the aforementioned device, includes: S1. Start laser 1 and split the light into upper and lower paths through the first fiber coupler 2; Laser 1 is activated, outputting a narrow linewidth laser with a wavelength of 1550.08 nm and a linewidth of 1 kHz. This laser is then split into upper and lower paths by fiber coupler 2 at a 50:50 splitting ratio.

[0026] S2. The upper path light passes through the intensity modulator 3 driven by the microwave source 4 to generate double-sideband probe light; The upper-path light first enters intensity modulator 3, and the bias voltage of intensity modulator 3 is set to... It operates in carrier-suppressed double-sideband modulation mode. The microwave source 4 outputs a swept microwave signal with a frequency range of 10.76 GHz to 10.96 GHz, with a step of 4 MHz, which drives the intensity modulator 3 to generate double-sideband probe light. This probe light is then injected at a power of -6 dBm into the input end of the 49.5 km long sensing fiber optic cable 6 after passing through the optical isolator 5.

[0027] S3. The down-path light uses a periodic polarization disturbance method to cause a periodic change in the phase difference between the two orthogonal polarization components of the light wave; the periodic polarization disturbance method can be, but is not limited to, phase modulation or heterogeneous acousto-optic modulation. The basic principle of polarization state control is as follows: according to Figure 6 The polarization state of the light wave shown can be determined by the components of its electric field vector in two orthogonal directions. and and the phase difference between them Sure.

[0028] according to Figure 7 As shown, in the traditional random perturbation scheme, Random jumps within the range of 0 to 2π can cover all polarization states, but they introduce significant randomness, leading to polarization noise.

[0029] according to Figure 8 As shown, this invention employs a periodic polarization disturbance method to reduce the phase difference. It exhibits a sawtooth waveform that changes periodically and linearly over time, with a period of [period value missing]. This scheme ensures that within each cycle... By traversing the radian range from 0 to 2π, the appropriate amount of polarization states that can eliminate polarization fading is visited, while the random fluctuations of polarization states are greatly reduced, thus fundamentally suppressing polarization noise.

[0030] according to Figure 4 As shown, the phase modulation method includes: S301. The downlink light is adjusted by the polarization controller 7 so that its polarization direction is at a 45° angle with one of the principal axes of the phase modulator 8, thereby generating two orthogonal polarization components. The downlink light is first adjusted to a 45° linear polarization by the polarization controller 7, and then injected into the phase modulator 8.

[0031] Birefringent lithium niobate ( The phase modulator 8 (PM) is the core device. It utilizes the difference in response efficiency of its fast axis and slow axis to the modulation signal to realize the periodic change of the polarization state of the output light.

[0032] The output light of the linearly polarized laser source is adjusted by a polarization controller (PC) so that its polarization direction is at a 45° angle with a principal axis (fast axis or slow axis) of the phase modulator 8, thereby effectively decomposing the incident light into two orthogonal polarization components that propagate along the fast axis and the slow axis.

[0033] S302. The phase modulator 8 is driven by a periodic signal to generate a periodic relative phase difference between two orthogonal polarization components, thereby synthesizing a light wave whose polarization state evolves periodically with the phase difference. The first channel of the dual-channel arbitrary waveform generator 9 outputs a sawtooth wave voltage signal with a frequency of 1.7 kHz, which drives the phase modulator 8 to generate a periodic, continuous, and gradual change in the polarization state of the output light. Then, this continuously polarized light enters the semiconductor optical amplifier 10.

[0034] When the phase modulator is driven by a periodic sawtooth wave signal, the two orthogonal polarization components will generate a periodic relative phase difference due to birefringence. After being combined at the output, their polarization state evolves periodically with the phase difference, thus forming a periodic polarization disturbance that coincides with the period of the driving signal.

[0035] S303, the light wave input semiconductor optical amplifier 10, the second channel output terminal of the dual-channel arbitrary waveform generator 9 outputs a 419-bit cyclic encoded sequence to drive the semiconductor optical amplifier 10, thereby modulating the light wave stripe into synchronous encoded pulse light; The second channel of the dual-channel arbitrary waveform generator 9 generates a 419-bit cyclic encoded sequence (symbol pulse width 20 ns, sequence period 603.36 µs). This encoded electrical signal serves as a switching signal to drive the semiconductor optical amplifier 10, modulating continuous light carrying a periodically varying polarization state into synchronized encoded pulse light. Finally, the encoded pulse light is amplified to a peak power of 19 dBm by the first erbium-doped fiber amplifier 11 and injected into the output end of the sensing fiber 6 through the first optical circulator 12.

[0036] according to Figure 5 As shown, the acousto-optic modulation method in step S3 includes: S301, the second optical fiber coupler 19 splits the downstream light into two paths, which are respectively input to the first acousto-optic modulator 20 and the second acousto-optic modulator 21; The downstream light is split into two paths again by the second fiber coupler 19 at a splitting ratio of 50:50. The two paths then enter the first acousto-optic modulator 20 and the second acousto-optic modulator 21, respectively.

[0037] S302, the first acousto-optic modulator 20 and the second acousto-optic modulator 21 have a fixed frequency difference, so that the light waves generate a relative phase difference before beam combining; The two acousto-optic modulators have different RF drive frequencies, one with a drive frequency of... The other is ,in The frequency difference is constant. This frequency difference causes a relative phase difference between the two optical signals to accumulate linearly over time before they are combined. .

[0038] S303. The two optical signals are adjusted by the second polarization controller 22 and the third polarization controller 23 respectively to keep them orthogonal to each other, and then combined by the polarization combiner 24.

[0039] The two channels of the dual-channel arbitrary waveform generator 9 generate identical 419-bit cyclic encoded sequences with a symbol pulse width of 20 ns and a sequence period of 603.36 µs. The time delay between the two channels is finely adjusted to ensure that the two sequences are strictly overlapped in time. The two encoded electrical signals drive the first acousto-optic modulator 20 and the second acousto-optic modulator 21, respectively, to modulate the two continuous light sources into synchronized encoded pulses. The polarization states of the two pulses are precisely adjusted by the second polarization controller 22 and the third polarization controller 23, respectively, to align them with the fast and slow axes of the polarization combiner 24, i.e., to maintain mutual orthogonality. The combined pump pulse is amplified to a peak power of 18 dBm by the first erbium-doped fiber amplifier 11, and then injected into the output of the sensing fiber 6 through the first optical circulator 12.

[0040] The polarization states of the two optical signals are adjusted by two polarization controllers to maintain mutual orthogonality, and then combined by polarization combiner 24. The phase difference between the two orthogonal polarization components of the combined light wave is... Therefore, the polarization state of the combined beam is periodic. It undergoes periodic linear changes.

[0041] In summary, the aforementioned periodic asymptotic polarization is achieved through any technique that can cause a periodic change in the phase difference between two orthogonal polarization components of a light wave. Specific techniques for achieving periodic polarization include, but are not limited to: using a phase modulator 8, which, through its birefringence effect, generates a periodic relative phase difference between the two orthogonal polarization components under the action of a periodic driving signal, such as a sawtooth wave or a sine wave; and using hetero-frequency acousto-optic modulation technology, which combines two optical signals with a fixed frequency difference, utilizing their beat frequency effect to generate a periodically changing relative phase difference.

[0042] S4. The combined optical signal is converted into an electrical signal by the photodetector 16, and the distributed temperature and stress measurements along the sensing fiber are demodulated by the data acquisition and analysis system 18.

[0043] The pump light and probe light propagate in opposite directions within the sensing fiber. The probe light, after stimulated Brillouin scattering in the sensing fiber, is output from the first optical circulator 12. Its low-frequency sideband is amplified by the second erbium-doped fiber amplifier 13, and then guided by the second optical circulator 15 to the fiber Bragg grating 14 for bandpass filtering. The filtered optical signal is converted into an electrical signal by the photodetector 16. This electrical signal is then filtered and denoised by a low-pass filter 17 with a cutoff frequency of 50 MHz before being acquired by the data acquisition and analysis system 18 at a sampling rate of 100 MSa / s.

[0044] The data acquisition and analysis system 18 performs matrix multiplication decoding on the accumulated Brillouin gain signal acquired at each frequency sweep point to reconstruct the single-pulse Brillouin gain. By traversing all frequency sweep points, a Brillouin gain spectrum (BGS) is constructed for each sensing point. Subsequently, a Lorentz curve is fitted to the BGS at each location to accurately demodulate the Brillouin frequency shift (BFS) at that point. The change in BFS is linearly related to the temperature or stress experienced by the optical fiber, thereby enabling distributed temperature and stress measurement along the sensing optical fiber.

[0045] As attached Figure 9 As shown, the distribution of the decoded single-pulse Brillouin gain along the entire 49.5 km sensing fiber, obtained using both random and periodic polarization scrambling methods in 10 consecutive measurements, indicates that both methods can effectively eliminate polarization fading. However, further observation of the magnified view at the fiber end (49.2–49.6 km) reveals further limitations.

[0046] like Figure 10 As shown, the Brillouin gain fluctuation range measured by the random polarization perturbation method is much larger than that measured by the periodic asymptotic polarization perturbation method. This phenomenon intuitively proves that the periodic asymptotic polarization perturbation method can effectively eliminate polarization noise. Based on this, the system signal-to-noise ratio is significantly improved.

[0047] like Figure 11 As shown, the relationship between the signal-to-noise ratio (SNR) of the decoded single-pulse Brillouin gain and distance is quantitatively demonstrated under two methods. Since the periodic asymptotic polarization scrambling eliminates polarization noise, the average SNR of the Brillouin gain across the entire sensing fiber is improved by approximately 4 dB. This improvement transforms the dominant noise source of the system from gain-related polarization noise to beat frequency noise from the probe light and spontaneous Brillouin scattering.

[0048] The improvement in signal-to-noise ratio directly leads to a significant improvement in measurement stability.

[0049] like Figure 12 As shown, by comparing the three-dimensional Brillouin gain spectrum obtained at the fiber optic end using the two polarization scrambling methods, it is evident that the Brillouin gain in the random polarization scrambling scheme exhibits drastic fluctuations with distance and frequency distribution. However, with the proposed method, the Brillouin gain spectrum surface becomes smoother and more stable. This improvement demonstrates that the periodic asymptotic polarization scrambling technique effectively suppresses amplitude fluctuations in the measured signal, providing high-quality raw data for subsequent accurate demodulation of the Brillouin frequency shift.

[0050] The improvement in measurement stability ultimately translates into a substantial increase in the system's measurement accuracy.

[0051] like Figure 13As shown, the distribution of the estimated Brillouin frequency shift along the entire sensing fiber in 10 consecutive measurements is displayed under the two schemes. The Brillouin frequency shift values ​​obtained by the two methods are highly consistent at each fiber location, and there is no significant systematic deviation, indicating that the Brillouin frequency shift measurement based on the periodic asymptotic polarization method is accurate.

[0052] More importantly, from such Figure 13 (b) shows the relationship between the Brillouin frequency shift measurement uncertainty (measured by standard deviation) and distance. It can be seen that after adopting the periodic asymptotic polarization perturbation scheme, the measurement uncertainty at the end of the sensing fiber is significantly reduced from approximately 1.5 MHz in the random polarization perturbation scheme to approximately 0.6 MHz, a reduction of approximately 2.5 times. This improvement is consistent with... Figure 11 The 4dB signal-to-noise ratio improvement shown is a perfect match.

[0053] In terms of spatial resolution verification, this invention also demonstrates superior performance.

[0054] like Figure 14 As shown, in the test of heating a 5-meter-long fiber segment to 40°C at the far end of the fiber, the periodic asymptotic polarization method significantly reduced the measurement fluctuation of Brillouin frequency shift while maintaining a spatial resolution of 2 meters (corresponding to a pulse width of 20 ns). This resulted in smoother Brillouin frequency shift curves in the heated and unheated areas, further verifying the advantages of this method in improving measurement accuracy.

[0055] The above description is merely an example and illustration of the structure of this utility model. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the structure of the utility model or exceed the scope defined in the claims, they should all fall within the protection scope of this utility model.

Claims

1. A BOTDA fiber optic sensing device based on periodic polarization noise suppression, characterized in that, Includes a laser (1), the output end of which is connected to the input end of a first fiber optic coupler (2); The first output end of the first fiber coupler (2) is connected in sequence to the intensity modulator (3), the optical isolator (5), the sensing fiber (6), and the reflection end of the first optical circulator (12); the radio frequency signal input end of the intensity modulator (3) is connected to the output end of the microwave source (4). The second output end of the first fiber coupler (2) is connected to the periodic polarization scrambling module; the output end of the periodic polarization scrambling module is sequentially connected to the input end of the first erbium-doped fiber amplifier (11), the first optical circulator (12), the second erbium-doped fiber amplifier (13), and the input end of the second optical circulator (15); the reflective end of the second optical circulator (15) is connected to the fiber Bragg grating (14); the output end of the second optical circulator (15) is sequentially connected to the photodetector (16), the low-pass filter (17), and the data acquisition and analysis system (18). The periodic scrambling module is either a phase modulation module or a heterogeneous acousto-optic modulation module.

2. The BOTDA fiber optic sensing device based on periodic polarization noise suppression according to claim 1, characterized in that, The phase modulation module includes a first polarization controller (7) and a dual-channel arbitrary waveform generator (9); the input end of the first polarization controller (7) is connected to the output end of the first fiber coupler (2), and the output end is connected in sequence to the phase modulator (8) and the semiconductor optical amplifier (10). The input terminals of the phase modulator (8) and the semiconductor optical amplifier (10) are both connected to the output terminal of the dual-channel arbitrary waveform generator (9).

3. The BOTDA fiber optic sensing device based on periodic polarization noise suppression according to claim 2, characterized in that, The phase modulator (8) is a birefringent lithium niobate phase modulator.

4. The BOTDA fiber optic sensing device based on periodic polarization noise suppression according to claim 2, characterized in that, The dual-channel arbitrary waveform generator (9) outputs a periodic drive signal through its first output channel and an encoded sequence through its second output channel.

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