A method for random coding and raman pumping control of brillouin optical time domain reflectometer

Abstract

The present application relates to the technical field of distributed optical fiber sensing, and discloses a random coding and Raman pumping control method of Brillouin optical time domain reflectometer, comprising: obtaining the relative time delay of probe light and Raman pump light in the sensing optical fiber, the random coding code element time length, the probe pulse power, the Raman pump power and the candidate random coding sequence, determining the Raman gain dynamic action window according to the relative time delay and the random coding code element time length; determining the intra-code Raman gain fluctuation by combining the pre-sequence pulse distribution in the window, the probe pulse power and the Raman pump power, screening the candidate random coding sequence to form a measurement random coding sequence group; generating a random coding probe pulse by using the measurement random coding sequence group and performing Brillouin scattering signal decoding to obtain the Brillouin frequency shift and the temperature distribution along the sensing optical fiber. The present application reduces the intra-code gain inconsistency caused by the Raman pump dynamic action, and improves the stability and reliability of long-distance temperature measurement.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, and more specifically, to a random coding and Raman pump control method for a Brillouin optical time domain reflectometer. Background Technology

[0002] Brillouin optical time-domain reflectometry (OTDR) is a fiber optic sensing technology that enables distributed parameter measurement based on the Brillouin scattering effect in optical fibers. It utilizes the sensing fiber as both the transmission medium and the sensing carrier, and by detecting frequency shifts in the backscattered Brillouin signal, it achieves continuous measurement of physical quantities such as temperature and strain along the fiber. Due to its advantages of long measurement distance, high spatial resolution, strong resistance to electromagnetic interference, and suitability for monitoring complex environments, this technology is widely used in the condition monitoring of long-distance power transmission lines, bridges and tunnels, oil and gas pipelines, and large-scale engineering structures. With the continuous expansion of distributed fiber optic sensing applications, higher demands are being placed on the measurement distance, signal quality, and measurement stability of Brillouin OTDs under complex operating conditions.

[0003] To improve long-distance measurement capabilities and weak signal detection, existing Brillouin optical time-domain reflectometers typically employ methods such as optical amplification, pump enhancement, coded detection, and signal demodulation to increase the intensity of the backscattered Brillouin signal. For example, prior art CN105067143A discloses a zero-difference Brillouin optical time-domain reflectometer based on Raman amplification, which injects Raman pump light and probe pulse light into the sensing fiber, and performs distributed Raman amplification on the forward-propagating probe pulse light and the backward-propagating Brillouin scattered light to improve the signal-to-noise ratio and sensing distance of the backscattered Brillouin signal. However, in actual long-distance measurements, due to the increased length of the sensing fiber, power variations, propagation delays, and changes in the interaction state between different optical pulses during transmission affect the stability of the acquired backscattered Brillouin signal, causing amplitude variations in the scattered signal obtained in different measurement periods, which in turn affects the consistency of the Brillouin frequency shift analysis results and the accuracy of temperature measurement.

[0004] Therefore, it is necessary to design a random coding and Raman pump control method for a Brillouin optical time-domain reflectometer to solve the problems existing in the current technology. Summary of the Invention

[0005] In view of this, the present invention proposes a random coding and Raman pump control method for a Brillouin optical time-domain reflectometer, aiming to solve the problem that the intracode gain inconsistency caused by the pump dynamics of the random coding probe pulse under long-distance Raman enhancement conditions affects the stability of Brillouin frequency shift resolution.

[0006] This invention proposes a random coding and Raman pump control method for a Brillouin optical time-domain reflectometer, comprising: The relative time delay between the probe light and the Raman pump light in the sensing fiber, the duration of the random code symbol, the probe pulse power, the Raman pump power, and the candidate random code sequence are obtained. The dynamic application window of the Raman gain is determined based on the relative time delay and the duration of the random encoded symbol. Based on the preceding pulse distribution, probe pulse power, and Raman pump power within the Raman gain dynamic action window, the intracode Raman gain fluctuation of each candidate random coding sequence is determined; Candidate random coding sequences whose in-code Raman gain fluctuation does not exceed the allowable gain fluctuation are selected to form a measurement random coding sequence group; A random coded probe pulse is generated according to the measurement random coded sequence group, and the random coded probe pulse and Raman pump light are injected into the sensing fiber in the same direction; the backscattered Brillouin signal is collected, and the backscattered Brillouin signal is decoded using the measurement random coded sequence group to obtain the Brillouin frequency shift, and the temperature distribution along the sensing fiber is obtained based on the Brillouin frequency shift.

[0007] Furthermore, determining the dynamic action window of the Raman gain includes: Obtain the group delay difference per unit length and the target measurement distance, and obtain the relative delay based on the group delay difference per unit length and the target measurement distance; Taking the current pulse symbol as the endpoint, the range of symbols whose cumulative duration before the current pulse symbol does not exceed the relative delay is determined as the Raman gain dynamic action window.

[0008] Furthermore, determining the Raman gain dynamic action window also includes: setting multiple measurement positions within the target measurement distance, obtaining the relative time delay based on the propagation distance of each measurement position and the unit length group time delay difference, and determining the Raman gain dynamic action window and intracode Raman gain fluctuation for each measurement position respectively.

[0009] Furthermore, when forming a group of measurement random coding sequences, the process includes: obtaining the maximum intracode Raman gain fluctuation of each candidate random coding sequence; retaining the candidate random coding sequence when the maximum intracode Raman gain fluctuation does not exceed the allowable gain fluctuation amount, and discarding the candidate random coding sequence when it exceeds the allowable gain fluctuation amount.

[0010] Furthermore, obtaining the preceding pulse distribution includes: Using each pulse symbol as the current pulse symbol, identify the preceding pulse symbol within the Raman gain dynamic action window; The time interval between adjacent pulse symbols is obtained, and the preceding pulse distribution is formed according to the arrangement order and time interval of the preceding pulse symbols.

[0011] Furthermore, determining the intra-code Raman gain fluctuation includes: The Raman pump power attenuation is determined based on the preceding pulse distribution, the probe pulse power, and the Raman pump power, and the Raman pump power recovery is determined based on the time interval between adjacent pulse symbols. The effective Raman pump power is defined as the power after depletion and recovery of the Raman pump power. The Raman gain of each pulse symbol is determined based on the effective Raman pump power, and the difference between the maximum Raman gain and the minimum Raman gain is taken as the intra-code Raman gain fluctuation.

[0012] Furthermore, determining the effective Raman pump power includes: Process the preceding pulse symbols one by one according to the temporal order of the preceding pulse distribution; The Raman pump power after processing the previous pulse symbol is used as the starting Raman pump power of the next pulse symbol. The Raman pump power recovery amount is included in the time interval between adjacent pulse symbols, and the Raman pump power depletion amount is included in the next pulse symbol, until the effective Raman pump power of the current pulse symbol is obtained.

[0013] Furthermore, when screening the candidate random coding sequences, the following steps are taken: each candidate random coding sequence has the same number of random coding symbols, and the autocorrelation sidelobes do not exceed the same autocorrelation sidelobe limit; the sequence is retained if the in-code Raman gain fluctuation does not exceed the allowable gain fluctuation amount, and is discarded if it exceeds the allowable gain fluctuation amount.

[0014] Furthermore, the process of filtering the candidate random encoded sequences also includes: When the number of retained candidate random coding sequences is less than the preset number of measurement sequences, supplementary candidate random coding sequences that meet the random coding symbol quantity requirements are reacquired. Retain supplementary candidate random coding sequences whose autocorrelation sidelobes do not exceed the autocorrelation sidelobe limit and whose in-code Raman gain fluctuations do not exceed the allowable gain fluctuations.

[0015] Furthermore, the group of measurement random coded sequences comprises: Adjust the Raman pump power, and redetermine the intracode Raman gain fluctuation of each random code sequence in the measured random code sequence group based on the adjusted Raman pump power; The random coded sequences whose Raman gain fluctuations do not exceed the allowable gain fluctuations and whose autocorrelation sidelobes do not exceed the autocorrelation sidelobe limits are retained and reassembled into a measurement random coded sequence group.

[0016] Compared with the prior art, the beneficial effects of this invention are as follows: By introducing the propagation timing relationship between the probe light and the Raman pump light in the sensing fiber into the random coding sequence screening process, the Raman gain dynamic action window is determined based on the relative time delay and the duration of the random coding symbol. Combined with the preceding pulse distribution, probe pulse power, and Raman pump power within the Raman gain dynamic action window, the intra-code Raman gain fluctuations formed after different pulse symbols within the candidate random coding sequence are subjected to Raman pumping are determined. Thus, before the random coding probe pulse is actually injected into the sensing fiber, candidate random coding sequences with intra-code Raman gain fluctuations exceeding the allowable gain fluctuation amount are screened out, ensuring that the random coding sequence group composed of the measurement random coding sequence group... The machine-coded sequence can better adapt to the current Raman pumping conditions. Compared with simply increasing the Raman pump power to enhance the far-end scattering signal or selecting the coding sequence based solely on the correlation characteristics of the random coding itself, this scheme can reduce the inter-pulse gain inconsistency caused by the arrangement of preceding pulses and dynamic changes in pumping while taking into account the correlation decoding performance of random coding and Raman amplification. It also reduces abnormal fluctuations caused by the transmission of intra-code amplitude imbalance to the correlation decoding results, making the decoded backscattered Brillouin signal maintain a more stable amplitude relationship. This improves the consistency of Brillouin frequency shift analysis results under long-distance Raman enhancement conditions and enhances the stability and reliability of temperature distribution measurement along the sensing fiber. Attached Figure Description

[0017] The accompanying drawings are used to illustrate the technical solutions of the present invention and do not constitute a limitation on the scope of protection of the present invention. In the drawings: Figure 1 A flowchart of a random coding and Raman pump control method for a Brillouin optical time-domain reflectometer provided in an embodiment of the present invention; Figure 2 A flowchart for determining intracode Raman gain fluctuations and screening random encoded sequences provided in this embodiment of the invention; Figure 3 This is a schematic diagram of random coded detection and Brillouin signal processing provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the dynamic interaction window of pulse timing and Raman gain provided in an embodiment of the present invention. Detailed Implementation

[0018] The specific embodiments of the present invention will now be described with reference to the accompanying drawings. These embodiments are used to illustrate the technical solutions of the present invention and do not limit the scope of protection of the present invention. Unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0019] In some embodiments of this application, see Figure 1-4 As shown, this application proposes a random coding and Raman pump control method for a Brillouin optical time-domain reflectometer, comprising: S100: Acquire the relative time delay of the probe light and the Raman pump light in the sensing fiber, the duration of the random code symbol, the probe pulse power, the Raman pump power, and the candidate random code sequence.

[0020] S200: Determine the dynamic application window of Raman gain based on relative time delay and random coded symbol duration.

[0021] S300: Determine the intracode Raman gain fluctuation of each candidate random coding sequence based on the preceding pulse distribution, probe pulse power, and Raman pump power within the Raman gain dynamic action window.

[0022] S400: Candidate random coding sequences whose Raman gain fluctuation within the screening code does not exceed the allowable gain fluctuation amount are used to form a measurement random coding sequence group.

[0023] S500: Generates a random coded probe pulse according to the measurement random coded sequence group, and injects the random coded probe pulse and Raman pump light in the same direction into the sensing fiber. Acquires the backscattered Brillouin signal, decodes the backscattered Brillouin signal using the measurement random coded sequence group to obtain the Brillouin frequency shift, and obtains the temperature distribution along the sensing fiber based on the Brillouin frequency shift.

[0024] Specifically, the relative time delay of the probe light and the Raman pump light in the sensing fiber is determined. The relative time delay represents the time offset caused by the different group propagation characteristics of the probe light and the Raman pump light when they propagate to the same fiber position. The relative time delay can be obtained directly through calibration experiments, or it can be obtained by first acquiring the group delay difference per unit length and then accumulating it based on the propagation distance from the location to be evaluated to the incident end of the sensing fiber. The calibration of the group delay difference per unit length uses the same type of sensing fiber as the actual measurement. The propagation times of the probe light and the Raman pump light after traveling through a known length of fiber are obtained, and the ratio of the absolute value of the difference between their propagation times to the length of the calibration fiber is used as the group delay difference per unit length. Using this method, it is not necessary to add optical power detection points along the sensing fiber during the actual temperature measurement process.

[0025] The duration of a random coded symbol is the basic time length between adjacent coded positions. A pulse symbol is a coded position in the candidate random coded sequence where a probe pulse is generated. Although coded positions that do not generate a probe pulse do not result in Raman pump power attenuation, their duration is included in the time interval between adjacent pulse symbols. In this embodiment, the time interval between adjacent pulse symbols is the duration between the end of the probe pulse in the previous pulse symbol and the start of the probe pulse in the next pulse symbol.

[0026] When there is a random coded symbol that does not generate a probe pulse between two pulse symbols, the duration of the random coded symbol that does not generate a probe pulse is cumulatively included in the time interval. The probe pulse power is the peak power of the random coded probe pulse when it actually enters the sensing fiber, and the Raman pump power is the actual power of the Raman pump light when it enters the sensing fiber. The candidate random coded sequence is generated by a random number generator. The random number generator first continuously generates an original random coded sequence with a length greater than the number of random coded symbols required for a single measurement. Then, it continuously truncates the original random coded sequence from its starting position according to the preset number of random coded symbols, and each truncated segment forms a candidate random coded sequence. For example, when the preset number of random coded symbols is 512, the original random coded sequence is continuously divided into multiple candidate random coded sequences of 512 random symbols each. The number and arrangement of pulse symbols in each candidate random coded sequence are naturally determined by the original random coded result, and the position or number of pulse symbols is not manually adjusted.

[0027] See Figure 4 As shown, when determining the dynamic application window for Raman gain, the start time of the current pulse symbol is taken as the end point of the window. Complete symbols are accumulated sequentially from the end point of the window according to the duration of the randomly encoded symbols. Accumulation stops when adding a previous complete symbol would cause the accumulated duration to exceed the relative delay. The range of complete symbols that have already been accumulated and whose accumulated duration does not exceed the relative delay is defined as the dynamic application window for Raman gain. Therefore, when the relative delay is not an integer multiple of the duration of the randomly encoded symbols, symbols exceeding the accumulation range are not discounted.

[0028] For each candidate random coded sequence, each pulse symbol is sequentially taken as the current pulse symbol, and the preceding pulse symbols within the Raman gain dynamic action window are identified. The positions of the preceding pulse symbols are recorded in chronological order, and the time intervals between adjacent pulse symbols are obtained. The preceding pulse distribution is formed by the arrangement order and time intervals of the preceding pulse symbols. The preceding pulse distribution thus simultaneously reflects the number of pulse symbols within the window and the recovery interval between pulse symbols, and is used not only to determine the number of consecutive pulses.

[0029] The Raman pump power attenuation and Raman pump power recovery are calibrated before the evaluation of candidate random coding sequences. In this embodiment, a calibration fiber of the same type as the sensing fiber is used. A single probe pulse and two probe pulses with different time intervals are emitted under different Raman pump powers and probe pulse powers. The Raman pump wavelength is separated at the output of the calibration fiber, and the change of Raman pump power over time is collected. The difference between the Raman pump power before the probe pulse and the lowest Raman pump power after the probe pulse is applied is taken as the Raman pump power attenuation. The power increment recovered from the lowest Raman pump power after the probe pulse to the arrival of the next probe pulse is taken as the Raman pump power recovery. This establishes the calibration relationship between the initial Raman pump power, probe pulse power, pulse symbol time interval, and the Raman pump power attenuation and Raman power recovery. When the actual parameters are between adjacent calibration parameters, linear interpolation between adjacent calibration points is used to obtain the Raman pump power attenuation and Raman power recovery.

[0030] The Raman pump reference power at each measurement location, without the preceding pulse, is obtained based on the input Raman pump power, the attenuation per unit length of the sensing fiber at the Raman pump wavelength, and the propagation distance at the measurement location. When calculating the effective Raman pump power, the Raman pump power recovery is considered in the direction of increasing power, and the recovered Raman pump power does not exceed the Raman pump reference power. The Raman pump power reduction is considered in the direction of decreasing power, and the reduced Raman pump power is not lower than 0.

[0031] See Figure 2 As shown, when calculating the effective Raman pump power of the current pulse symbol, the preceding pulse symbols are processed one by one according to the time sequence of the preceding pulse distribution. First, the pump power when the Raman pump light propagates to the corresponding measurement position and has not yet been affected by the preceding pulse within the window is taken as the initial Raman pump power. After processing one preceding pulse symbol, the Raman pump power attenuation is obtained based on the probe pulse power and the Raman pump power before processing. Before entering the next pulse symbol, the Raman pump power recovery is obtained based on the time interval between two pulse symbols, and the recovered Raman pump power is taken as the initial Raman pump power of the next pulse symbol. The recovered Raman pump power does not exceed the Raman pump reference power at the same propagation position before being affected by the preceding pulse attenuation. This process continues in the above order until the effective Raman pump power at the arrival of the current pulse symbol is obtained.

[0032] Furthermore, Raman gain is expressed in dB. Under the conditions of the same type of calibrated fiber, the same propagation distance, and the same probe pulse power, the probe pulse power is obtained with the Raman pump light on and off, respectively, and the ratio of the probe pulse power under the two conditions is converted into Raman gain. By changing the Raman pump power, a calibration relationship between effective Raman pump power and Raman gain is established. The same Raman gain representation method is used in the evaluation of candidate random coding sequences. Based on the pre-calibrated relationship between effective Raman pump power and probe pulse Raman gain, the Raman gain of each pulse symbol in the same candidate random coding sequence is determined. The difference between the maximum and minimum Raman gain within the same candidate random coding sequence is taken as the intra-code Raman gain fluctuation. Thus, for different candidate random coding sequences with the same number of random coding symbols, the number and arrangement of pulse symbols are naturally formed by the random coding result, and different preceding pulse distributions can form different intra-code Raman gain fluctuations.

[0033] The allowable gain fluctuation is calibrated based on the target temperature measurement accuracy before formal measurement. A calibration fiber segment with a constant temperature and axial strain is selected, and the reference Brillouin frequency shift at each sampling position of the calibration fiber segment is obtained through multiple averaging measurements. Under the condition that the number of random coding symbols, probe pulse power, and Raman pump power remain constant, measurements are performed using candidate random coding sequences obtained according to the same random coding generation and segmentation rules. For each candidate random coding sequence, the root mean square error of the measured Brillouin frequency shift at each sampling position of the calibration fiber segment relative to the reference Brillouin frequency shift is calculated. Based on the pre-calibrated Brillouin frequency shift temperature coefficient, the preset temperature measurement allowable error is converted into the Brillouin frequency shift allowable error. Among the candidate random coding sequences whose Brillouin frequency shift root mean square error does not exceed the Brillouin frequency shift allowable error, the largest intra-code Raman gain fluctuation is determined as the allowable gain fluctuation. When the same temperature measurement accuracy requirement is used for the same measurement, the allowable gain fluctuation remains unchanged.

[0034] The autocorrelation sidelobe limit is used to constrain the correlation decoding performance of the random coded sequence itself. Normalized autocorrelation is calculated for candidate random coded sequences with the same number of random coded symbols, the autocorrelation peak is normalized to 1, and the maximum absolute value of the autocorrelation at the non-zero delay position is used as the autocorrelation sidelobe evaluation metric. Under the reference measurement condition of turning off the Raman pump light, correlation decoding is performed on the candidate random coded sequences, and the root mean square error of the Brillouin frequency shift is calculated. Among the candidate random coded sequences whose root mean square error of the Brillouin frequency shift does not exceed the allowable error of the Brillouin frequency shift, the largest autocorrelation sidelobe evaluation metric is used as the autocorrelation sidelobe limit. During the formal screening, candidate random coded sequences whose autocorrelation sidelobes do not exceed the autocorrelation sidelobe limit and whose intra-code Raman gain fluctuation does not exceed the allowable gain fluctuation are retained.

[0035] See Figure 3 As shown, after screening, the retained candidate random coding sequences are grouped into a measurement random coding sequence group. Random coding probe pulses are generated sequentially according to the measurement random coding sequence group. These pulses, along with the Raman pump light, are injected into the sensing fiber in the same propagation direction, and the received signal is then scattered towards the Brillouin. For each random coding sequence in the measurement random coding sequence group, correlation decoding is performed using the random coding sequence used during transmission. The decoding results obtained from the measurement random coding sequence group are then combined to obtain the Brillouin gain spectrum and Brillouin frequency shift along the sensing fiber. During temperature measurement, the axial strain of the sensing fiber remains constant during the measurement period or has been pre-calibrated.

[0036] Using the Brillouin frequency shift at each sampling location under a known reference temperature as the location reference value, the temperature change is obtained based on the change in the current Brillouin frequency shift relative to the location reference value and a pre-calibrated Brillouin frequency shift temperature coefficient. The temperature change at each sampling location is added to the reference temperature at that location to obtain the current temperature at each sampling location. The temperature distribution along the sensing fiber is then formed according to the order of the sampling locations on the sensing fiber. For initial strains present at different locations but remaining constant during the measurement period, the fixed frequency shift caused by the initial strain is already included in the Brillouin frequency shift location reference value at each location and is not considered in the temperature change calculation.

[0037] This embodiment, based on the above embodiments, further considers the issue of the relative delay between the probe light and the Raman pump light changing with propagation distance. Multiple measurement positions are set within the target measurement distance, and the relative delay is obtained using the propagation distance and the group delay difference per unit length for each measurement position. A dynamic Raman gain window is established for each measurement position. Measurement positions are sequentially set along the target measurement distance from the incident end of the self-sensing fiber, ensuring that the relative delay increment between two adjacent measurement positions due to the propagation distance difference is no greater than the duration of one random code symbol. The end of the target measurement distance is set as the last measurement position. If the relative delay increment between two initially set adjacent measurement positions exceeds the duration of one random code symbol, a new measurement position is added between the two measurement positions until all adjacent measurement positions meet the above condition.

[0038] For the same candidate random coded sequence, the intracode Raman gain fluctuation at each measurement location is calculated, and the maximum intracode Raman gain fluctuation is selected from the calculation results at multiple measurement locations. If the maximum intracode Raman gain fluctuation does not exceed the allowable gain fluctuation, the random coded sequence is considered to meet the Raman gain consistency requirement within the target measurement distance range. If the maximum intracode Raman gain fluctuation exceeds the allowable gain fluctuation, the random coded sequence is discarded. Therefore, the selection result is not determined by the average performance of a single local measurement location, but by unfavorable propagation locations within the target measurement distance range, avoiding random coded sequences meeting requirements at the near end but exhibiting significant intracode gain imbalance at the far end.

[0039] When the number of candidate random coding sequences retained after screening is less than the preset number of measurement sequences, the random number generator regenerates the original random coding sequence and continues to segment it continuously with the same number of random coding symbols. Each newly obtained segment is used as a supplementary candidate random coding sequence. The autocorrelation sidelobes and intracode Raman gain fluctuations of the supplementary candidate random coding sequences are determined. Supplementary candidate random coding sequences that meet the screening criteria are added to the measurement random coding sequence group. During the generation and segmentation of supplementary candidate random coding sequences, the pulse symbol positions are not adjusted based on existing random coding sequences, nor are they manually removed based on whether they are duplicates of existing random coding sequences. If the number of measurement random coding sequences is still less than the preset number of measurement sequences, supplementary candidate random coding sequences are generated and screened in the same way until the number of measurement random coding sequences reaches the preset number of measurement sequences.

[0040] The preset number of measurement sequences is determined before the formal measurement. With the number of random coded symbols and the probe pulse power remaining constant, the number of random coded sequences participating in decoding is increased in ascending order. Signal acquisition and random coding / decoding are performed for each sequence quantity separately. The root mean square error of the Brillouin frequency shift of the calibration fiber segment is calculated, and the total acquisition time is recorded. Among the sequences whose total acquisition time does not exceed the preset allowable acquisition time and whose root mean square error of the Brillouin frequency shift does not exceed the allowable error of the Brillouin frequency shift, the smallest number of random coded sequences is selected as the preset number of measurement sequences. The preset allowable acquisition time is predetermined based on the single measurement cycle allowed by the target measurement task, and the preset number of measurement sequences remains constant during the same measurement process.

[0041] This embodiment also processes the random coding adaptation after Raman pump power changes. The Raman pump power is divided into power levels according to the power steps that the Raman pump source can stably and repeatedly control. After the target measurement distance and probe pulse power are determined, the Raman pump power is selected step by step from low to high. After adjusting the Raman pump power, the intracode Raman gain fluctuation of each random coding sequence in the measurement random coding sequence group is re-determined based on the adjusted Raman pump power. Random coding sequences whose intracode Raman gain fluctuation does not exceed the allowable gain fluctuation and whose autocorrelation sidelobes do not exceed the autocorrelation sidelobe limit are retained. If the number of retained sequences is insufficient, supplementary screening is performed according to the aforementioned method for supplementing candidate random coding sequences.

[0042] Subsequently, the backscattered Brillouin signal at the target measurement distance is acquired, and the root mean square error of the Brillouin frequency shift is calculated. When a certain Raman pump power level simultaneously satisfies the conditions of the root mean square error of the Brillouin frequency shift not exceeding the allowable error and the number of randomly encoded measurement sequences not less than the preset number of measurement sequences for the first time, the Raman pump power is used as the current measurement Raman pump power. If an effective Brillouin frequency shift cannot be obtained, it is determined that the current Raman pump power level does not meet the Brillouin frequency shift error condition, and the Raman pump power is further increased. If reaching the maximum allowable power of the Raman pump source still cannot simultaneously meet the above conditions, it is determined that the current target measurement distance and probe pulse power combination does not meet the measurement requirements, and the target measurement distance or probe pulse power is adjusted before re-performing the screening. Under the same temperature measurement accuracy requirements, the allowable gain fluctuation and autocorrelation sidelobe limits remain unchanged.

[0043] In a set of benchmark verifications, random coding parameters verified in previous BOTDR experiments were used: a random coding sequence length of 512 bits, a single coding pulse width of 20 ns, a preset measurement sequence number of 30 sets, and an input fiber detection pulse peak power of approximately 10 mW. The random coding symbol duration was determined based on the symbol period set in the random coding generator. The random coding symbol duration and the single coding detection pulse width were set separately; only when the two values ​​were the same was the single coding detection pulse width used as the random coding symbol duration. Previous random coding BOTDR experiments achieved an effective sensing distance of 64.76 km without using the Raman pump control method of this embodiment. The experimental results served as the comparison benchmark for the Raman pump control verification in this embodiment.

[0044] When implementing this method, the probe wavelength is approximately 1550 nm, and the Raman pump wavelength is approximately 1450 nm. After further optical amplification of the probe pulse, the Raman pump power reduction calibration is re-performed using the actual calibrated peak power of the fiber-entry probe pulse, instead of continuing to use the 10 mW reference value. For long-distance verification, 100 km can be set as the target measurement distance, and the random coding sequence screening and Raman pump power selection are completed according to the aforementioned multi-measurement location rules. 100 km represents the target measurement distance set in this embodiment; the final effective sensing distance is determined by whether the actual Brillouin frequency shift root mean square error meets the Brillouin frequency shift allowable error.

[0045] In practice, the group delay difference per unit length between the Raman pump light near 1450nm and the probe light near 1550nm is first calibrated on a single-mode fiber of the same type and known length. Then, the relative delay is obtained according to the propagation distance at different measurement positions. The Raman gain dynamic action window for each measurement position is defined according to the random coding symbol duration set by the random coding generator. The effective Raman pump power and Raman gain are calculated for each pulse symbol in the candidate random coding sequence. Among candidate random coding sequences with the same number of random coding symbols, different candidate random coding sequences with randomly arranged pulse symbols are selected. The maximum intra-symbol Raman gain fluctuation of each candidate random coding sequence is calculated and compared. The number and arrangement of pulse symbols in different candidate random coding sequences are naturally formed by the random coding results and are not manually adjusted.

[0046] The Raman gain dynamic action window referred to in this application is not a pre-fixed range of symbols, but rather a complete range of symbols determined based on the relative time delay between the probe light and the Raman pump light at the target propagation position and the duration of the randomly encoded symbols. When the relative time delay changes, the number of complete symbols included in the Raman gain dynamic action window changes accordingly; symbols exceeding the relative time delay are not included in the Raman gain dynamic action window. The preceding pulse distribution is not simply a statistical count of the number of pulses within the window, but also includes the sequential arrangement of preceding pulse symbols and the time interval between adjacent pulse symbols. The Raman pump power attenuation reflects the reduction in the current pump state caused by the passage of a pulse symbol, while the Raman pump power recovery reflects the degree of recovery of the pump state from the end of the previous pulse to the arrival of the next pulse. By continuously transmitting the attenuated pump state and the recovery state within the interval according to the pulse time sequence, the effective Raman pump power actually faced by each current pulse symbol can be obtained, and the Raman gain distribution in the same randomly encoded sequence can then be obtained accordingly. This process avoids summarizing the entire random coding sequence using only the average duty cycle or the number of consecutive pulses, allowing the interaction between the local timing of the random coding and the dynamics of the Raman pump to be incorporated into the actual screening process of the random coding sequence.

[0047] This embodiment evaluates candidate random coding sequences generated naturally by a random number generator and obtained by continuous segmentation according to a fixed number of random coding symbols, while keeping the number of random coding symbols and the probe pulse power constant. By identifying differences in Raman pump attenuation and recovery states caused by different preceding pulse distributions, random coding sequences prone to intracode gain imbalance under specific Raman pump conditions and propagation distances are screened out. The number and arrangement of pulse symbols in the candidate random coding sequences are naturally formed by the random coding results; the pulse symbol positions or numbers are not manually adjusted during the screening process. Simultaneously, the maximum intracode Raman gain fluctuation among multiple measurement positions is used for screening, preventing random coding sequences that meet the near-end criteria but are mismatched at the far end from entering the final measurement. After adjusting the Raman pump power, the same criteria are re-executed, thus simultaneously achieving the goal of increasing Raman pump power to enhance the far-end signal and maintaining the stability of random coding correlation decoding. The pump dynamic relationship is calibrated and calculated before measurement, eliminating the need to add segmented optical power detection points along long-distance sensing fibers during actual sensing, which helps maintain the engineering advantages of single-end measurement using a Brillouin optical time-domain reflectometer.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for random coding and Raman pump control of a Brillouin optical time-domain reflectometer, characterized in that, include: The relative time delay between the probe light and the Raman pump light in the sensing fiber, the duration of the random code symbol, the probe pulse power, the Raman pump power, and the candidate random code sequence are obtained. The dynamic application window of the Raman gain is determined based on the relative time delay and the duration of the random encoded symbol. Based on the preceding pulse distribution, probe pulse power, and Raman pump power within the Raman gain dynamic action window, the intracode Raman gain fluctuation of each candidate random coding sequence is determined; Candidate random coding sequences whose in-code Raman gain fluctuation does not exceed the allowable gain fluctuation are selected to form a measurement random coding sequence group; A random coded probe pulse is generated according to the measurement random coded sequence group, and the random coded probe pulse and Raman pump light are injected into the sensing fiber in the same direction; the backscattered Brillouin signal is collected, and the backscattered Brillouin signal is decoded using the measurement random coded sequence group to obtain the Brillouin frequency shift, and the temperature distribution along the sensing fiber is obtained based on the Brillouin frequency shift.

2. The method according to claim 1, characterized in that, Determining the dynamic action window for Raman gain includes: Obtain the group delay difference per unit length and the target measurement distance, and obtain the relative delay based on the group delay difference per unit length and the target measurement distance; Taking the current pulse symbol as the endpoint, the range of symbols whose cumulative duration before the current pulse symbol does not exceed the relative delay is determined as the Raman gain dynamic action window.

3. The method according to claim 2, characterized in that, Determining the Raman gain dynamic action window further includes: setting multiple measurement positions within the target measurement distance, obtaining the relative time delay based on the propagation distance of each measurement position and the unit length group time delay difference, and determining the Raman gain dynamic action window and intracode Raman gain fluctuation for each measurement position respectively.

4. The method according to claim 3, characterized in that, When forming a measurement random coding sequence group, the process includes: obtaining the maximum intracode Raman gain fluctuation of each candidate random coding sequence; retaining the candidate random coding sequence when the maximum intracode Raman gain fluctuation does not exceed the allowable gain fluctuation amount, and discarding the candidate random coding sequence when it exceeds the allowable gain fluctuation amount.

5. The method according to claim 1, characterized in that, When acquiring the preceding pulse distribution, the process includes: Using each pulse symbol as the current pulse symbol, identify the preceding pulse symbol within the Raman gain dynamic action window; The time interval between adjacent pulse symbols is obtained, and the preceding pulse distribution is formed according to the arrangement order and time interval of the preceding pulse symbols.

6. The method according to claim 1, characterized in that, Determining the intracode Raman gain fluctuation includes: The Raman pump power attenuation is determined based on the preceding pulse distribution, the probe pulse power, and the Raman pump power, and the Raman pump power recovery is determined based on the time interval between adjacent pulse symbols. The effective Raman pump power is defined as the power after depletion and recovery of the Raman pump power. The Raman gain of each pulse symbol is determined based on the effective Raman pump power, and the difference between the maximum Raman gain and the minimum Raman gain is used as the intra-code Raman gain fluctuation.

7. The method according to claim 6, characterized in that, Determining the effective Raman pump power includes: Process the preceding pulse symbols one by one according to the temporal order of the preceding pulse distribution; The Raman pump power after processing the previous pulse symbol is used as the starting Raman pump power of the next pulse symbol. The Raman pump power recovery amount is included in the time interval between adjacent pulse symbols, and the Raman pump power depletion amount is included in the next pulse symbol, until the effective Raman pump power of the current pulse symbol is obtained.

8. The method according to claim 1, characterized in that, When screening the candidate random coding sequences, the following steps are taken: each candidate random coding sequence has the same number of random coding symbols, and the autocorrelation sidelobes do not exceed the same autocorrelation sidelobe limit; the sequence is retained if the in-code Raman gain fluctuation does not exceed the allowable gain fluctuation amount, and is discarded if it exceeds the allowable gain fluctuation amount.

9. The method according to claim 8, characterized in that, The process of filtering the candidate random encoded sequences also includes: When the number of retained candidate random coding sequences is less than the preset number of measurement sequences, supplementary candidate random coding sequences that meet the random coding symbol quantity requirements are reacquired. Retain supplementary candidate random coding sequences whose autocorrelation sidelobes do not exceed the autocorrelation sidelobe limit and whose in-code Raman gain fluctuations do not exceed the allowable gain fluctuations.

10. The method according to claim 9, characterized in that, After assembling the measurement random coded sequence group, it includes: Adjust the Raman pump power, and redetermine the intracode Raman gain fluctuation of each random code sequence in the measured random code sequence group based on the adjusted Raman pump power; The random coded sequences whose Raman gain fluctuations do not exceed the allowable gain fluctuations and whose autocorrelation sidelobes do not exceed the autocorrelation sidelobe limits are retained and reassembled into a measurement random coded sequence group.

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