Special fiber bragg grating suitable for vehicle-mounted environment and preparation method thereof

Abstract

The invention belongs to the technical field of fiber bragg grating preparation, and provides a special fiber bragg grating suitable for a vehicle-mounted environment and a preparation method thereof.The preparation method comprises the steps that uniformity analysis is conducted on a grating period written by femtosecond laser, whether the grating period distortion phenomenon occurs or not is judged, and if yes, the fiber bragg grating is obtained; if yes, constructing and comparing a write-in data set about the laser scanning speed and the repetition frequency on the normal grating period and the distorted grating period, judging whether the source cause of the grating period distortion phenomenon is disjunction of the laser scanning speed and the repetition frequency, if yes, performing fluctuation analysis on the laser scanning speed and the repetition frequency, and if not, performing fluctuation analysis on the laser scanning speed and the repetition frequency; according to the method, the preferential adjustment object and the corresponding adjustment limit value or adjustment combination are determined, and then targeted adjustment is implemented, so that the problem of grating period distortion can be effectively solved, the uniformity of the grating period is realized, and the quality and stability of femtosecond laser writing into the grating are guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber grating preparation, in particular to a special optical fiber grating suitable for a vehicle-mounted environment and a preparation method thereof. Background Art

[0002] In vehicle-mounted scenarios, vehicles are exposed to complex and changeable environmental factors such as vibration, temperature fluctuations, and shock. Fiber Bragg gratings (FBGs), due to their high sensitivity to physical parameters, can be used to build vehicle-mounted status monitoring systems. However, there are still many deficiencies in the preparation of fiber Bragg gratings. Therefore, it is of great significance to study a special fiber Bragg grating suitable for vehicle-mounted environments and its preparation method. In the technical field of femtosecond laser grating writing, the uniformity of the grating period is a key indicator to ensure the performance of the grating. However, in the current femtosecond laser writing process, the grating period is often distorted, and its period uniformity is difficult to be effectively guaranteed, and there are often deviations to varying degrees. The occurrence of this phenomenon is closely related to the lack of coordinated control of key parameters such as laser scanning speed and repetition frequency. Due to the lack of in-depth understanding of the correlation and fluctuation characteristics of these parameters during the writing process, and the lack of precise adjustment basis and methods, it is impossible to timely discover and solve the problem of parameter disconnection or improper matching, which in turn makes the grating quality stability poor, making it difficult to meet the strict requirements of high-precision application scenarios for grating period uniformity and consistency, seriously restricting the further development and application of femtosecond laser grating writing technology.

[0003] To this end, the present invention provides a special fiber Bragg grating suitable for a vehicle-mounted environment and a preparation method thereof. Summary of the Invention

[0004] In order to make up for the deficiencies of the prior art, at least one technical problem raised in the background technology is solved.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a method for preparing a special fiber Bragg grating suitable for a vehicle-mounted environment, comprising the following steps: Analyze the uniformity of the grating period written by the femtosecond laser to determine whether there is grating period distortion; If it occurs, a normal write database and a distorted write database are constructed based on the laser scanning speed and repetition frequency of the normal grating period and the distorted grating period. Through comparative analysis, it is determined whether the root cause of the grating period distortion phenomenon is the disconnection between the laser scanning speed and the repetition frequency. If so, then analyze the fluctuations of the laser scanning speed and repetition frequency. If the fluctuation of the laser repetition frequency is higher than or equal to the fluctuation of the laser scanning speed, then determine that the laser repetition frequency is the priority adjustment object. Otherwise, analyze the temperature peak value under the combination of laser repetition frequency and laser scanning speed to determine whether the priority adjustment object is the laser repetition frequency. If the priority adjustment object is the laser repetition frequency, determine whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak value. If so, determine the laser repetition frequency adjustment limit value. Otherwise, determine the adjustment combination database of the laser repetition frequency and the laser scanning speed. The laser repetition frequency is adjusted according to the laser repetition frequency adjustment limit value or the laser repetition frequency and the laser scanning speed are adjusted according to the adjustment combination database.

[0006] Furthermore, the process of uniformity analysis of the grating period is as follows: Obtain the laser scanning speed and laser repetition frequency of the femtosecond laser when writing the grating stripes, and perform proportional calculation to obtain the grating period; All written grating periods are obtained and summarized to obtain a grating period sequence. The coefficient of variation of the grating period sequence is calculated. If the coefficient of variation is greater than or equal to the coefficient of variation threshold, it indicates that the grating period uniformity is low and grating period distortion occurs.

[0007] Furthermore, the normal grating period and the distorted grating period are obtained as follows: Compare the grating period to the ideal period range; If the grating period is within the ideal period range, the grating period is marked as a normal grating period; If the grating period is not within the ideal period range, the grating period is marked as a candidate distorted grating period; Based on the candidate distorted grating periods, calculating the contribution of each candidate distorted grating period, wherein the contribution is the squared deviation value of each candidate distorted grating period; If the contribution is greater than or equal to the preset contribution, the candidate distorted grating period is marked as a distorted grating period.

[0008] Furthermore, the construction process of the normal write database and the distorted write database is as follows: Integrate the laser scanning speed and laser repetition frequency during normal grating period writing into a normal writing data group, and summarize them to obtain a normal writing database; Integrate the laser scanning speed and laser repetition frequency during periodic writing of the distorted grating into a distorted writing data group, and summarize them to obtain a distorted writing database; The comparison and analysis process is as follows: Compare the normal write database and the distorted write database to obtain the write deviation value and the cycle deviation value; Sum the cycle deviation value and the write deviation value to obtain the write disjoint value; If the write disjoint value is greater than or equal to the write disjoint threshold, it indicates that the root cause of the grating period distortion phenomenon is the disjointness between the laser scanning speed and the repetition frequency.

[0009] Furthermore, the period deviation value is obtained as follows: In the normal writing database, the ratio of the laser scanning speed and the laser repetition frequency in each normal writing data group is calculated and summed to obtain the average value of the normal grating period; in the distorted writing database, the ratio of the laser scanning speed and the laser repetition frequency in each distorted writing data group is calculated and summed to obtain the average value of the distorted grating period; The absolute difference between the normal grating period average and the distorted grating period average is calculated to obtain the period deviation value; The write deviation value is the absolute deviation of the data correlation coefficients corresponding to the normal write database and the distorted write database respectively.

[0010] Furthermore, the fluctuation analysis process of the laser scanning speed and the laser repetition frequency is as follows: Summarize the laser scanning speed when writing each grating period, and calculate the coefficient of variation to obtain the speed variation coefficient; Summarize the laser repetition frequency when writing each grating period, and calculate the coefficient of variation to obtain the frequency variation coefficient; If the frequency variation coefficient is greater than or equal to the speed variation coefficient, the laser repetition frequency is determined to be the priority adjustment object.

[0011] Furthermore, the process of performing fluctuation analysis of the laser scanning speed and the laser repetition frequency also includes: If the frequency coefficient of variation is less than the speed coefficient of variation, then reanalysis is performed, specifically: The normal grating period is averaged to obtain the normal grating period average, the laser repetition frequency when all grating periods are written is averaged to obtain the laser repetition frequency average, and the product of the normal grating period average and the laser repetition frequency average is calculated to obtain the laser scanning speed average; The average laser repetition frequency and the average laser scanning speed are used as the simulation parameter group. The fiber period is simulated and written according to the simulation parameter group using COMSOL software. After the simulation is completed, the temperature peak is extracted according to the temperature field contour map. If the temperature peak is greater than or equal to the preset temperature threshold of the optical fiber material, the laser repetition frequency is adjusted as a priority; If the temperature peak is less than a preset temperature threshold of the optical fiber material, calculating a temperature deviation between the temperature peak and the preset temperature threshold of the optical fiber material; If the temperature deviation is less than the preset temperature deviation, the laser repetition frequency is adjusted as a priority.

[0012] Furthermore, whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak is determined by fitting the temperature peaks at different laser repetition frequencies and judging the goodness of fit; If it exists, obtain a fitting model after fitting the temperature peak values ​​under different laser repetition frequencies, input the preset temperature threshold into the fitting model, and output the laser repetition frequency adjustment limit value; If it does not exist, according to the simulated writing process of the optical fiber cycle, a simulation parameter group with a temperature peak value less than a preset temperature threshold is obtained, and all simulation parameter groups are summarized to obtain an adjustment combination database.

[0013] Furthermore, the adjustment process is: If there is a linear mapping relationship between the laser repetition frequency and the temperature peak value and the relationship is positively proportional, then when the grating period is written, if the grating period is not equal to the normal grating period, the laser repetition frequency is adjusted, and the laser repetition frequency cannot exceed the laser repetition frequency adjustment limit value. Conversely, if there is an inverse proportional relationship, the laser repetition frequency cannot be lower than the laser repetition frequency adjustment limit value. If there is no linear mapping relationship between the laser repetition frequency and the temperature peak, the laser repetition frequency and the laser scanning speed are adjusted according to the simulation parameter group included in the adjustment combination database.

[0014] A special optical fiber Bragg grating suitable for a vehicle-mounted environment is prepared by a special optical fiber Bragg grating preparation method suitable for a vehicle-mounted environment.

[0015] The beneficial effects of the present invention are as follows: a uniformity analysis is performed on the grating period written by the femtosecond laser to determine whether grating period distortion occurs. If so, a writing data group is constructed for the normal grating period and the distorted grating period regarding the laser scanning speed and the repetition frequency, and a normal writing database and a distorted writing database are obtained respectively. By comparing and analyzing the normal writing database and the distorted writing database, it is determined whether the root cause of the grating period distortion phenomenon is the disconnection between the laser scanning speed and the repetition frequency. If so, a fluctuation analysis of the laser scanning speed and the repetition frequency is performed. If the fluctuation of the laser repetition frequency is higher than or equal to the fluctuation of the laser scanning speed, the laser repetition frequency is determined to be the priority adjustment object. Otherwise, the temperature peak value under the combination of the laser repetition frequency and the laser scanning speed is analyzed to determine whether the priority adjustment object is the laser repetition frequency. If The priority adjustment object is the laser repetition frequency. It is determined whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak. If so, the laser repetition frequency adjustment limit value is determined. Otherwise, the adjustment combination database of the laser repetition frequency and the laser scanning speed is determined. The laser repetition frequency is adjusted according to the laser repetition frequency adjustment limit value or the laser repetition frequency and the laser scanning speed are adjusted according to the adjustment combination database. The present invention analyzes the uniformity of the grating period written by the femtosecond laser, combines the writing data comparison, fluctuation analysis and temperature peak correlation analysis of the laser scanning speed and repetition frequency, determines the priority adjustment object and the corresponding adjustment limit value or adjustment combination, and then implements targeted adjustment. It can effectively solve the problem of grating period distortion, achieve the uniformity of the grating period, and ensure the quality and stability of the grating written by the femtosecond laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described below with reference to the accompanying drawings.

[0017] Figure 1 This is a flowchart of the steps of a method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to an embodiment of the present invention; Figure 2 It is a logic judgment diagram of a method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to an embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the technical means, creative features, objectives and effects achieved by the present invention easier to understand, the present invention is further described below in conjunction with specific implementation methods.

[0019] Example 1: Please refer to Figure 1-Figure 2 As shown, a method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to an embodiment of the present invention includes: Step 1: Analyze the uniformity of the grating period written by the femtosecond laser to determine whether there is grating period distortion; In step one, the femtosecond laser writing method is used to write the grating period. This method does not require hydrogen, is resistant to high temperatures (sapphire gratings can reach 1800°C), retains the mechanical strength of the coating, and has flexible and adjustable period. Given the harsh conditions of high temperatures (such as brake disc temperatures > 500°C), vibration, and electromagnetic interference in the vehicle environment, femtosecond laser writing is the best choice. In step 1, the process of uniformity analysis of the grating period written by the femtosecond laser is as follows: Obtain the laser scanning speed and laser repetition frequency of the femtosecond laser when writing the grating stripes, and perform proportional calculation to obtain the grating period; The grating period is calculated as follows: Λ=v / f, where Λ is the grating period, v is the laser scanning speed (instantaneous) when the grating stripes are written, and f is the laser repetition frequency (instantaneous) when the grating stripes are written. Obtain all written grating periods, summarize and integrate them to obtain a grating period sequence, and calculate the coefficient of variation of the grating period sequence, where the coefficient of variation is the ratio of the mean value of the grating period sequence to the standard deviation; Preferably, the coefficient of variation is compared to a coefficient of variation threshold; If the coefficient of variation is greater than or equal to the coefficient of variation threshold, it means that the grating period uniformity is low and grating period distortion occurs; If the coefficient of variation is less than the coefficient of variation threshold, it means that the grating period uniformity is high and there is no grating period distortion; Step 2: If it occurs, a data set of writing about the laser scanning speed and repetition frequency is constructed for the normal grating period and the distorted grating period, obtaining a normal writing database and a distorted writing database respectively. By comparing and analyzing the normal writing database and the distorted writing database, it is determined whether the root cause of the grating period distortion phenomenon is the disconnection between the laser scanning speed and the repetition frequency; In step 2, if grating distortion occurs, the normal grating period and the distorted grating period are obtained as follows: Compare the grating period to the ideal period range; If the grating period is within the ideal period range, the grating period is marked as a normal grating period; If the grating period is not within the ideal period range, the grating period is marked as a candidate distorted grating period; Among them, the ideal cycle range can be constructed in the following way: Based on the grating period sequence, the mean μ and standard deviation σ of all grating periods in the grating period sequence are calculated. The ideal period range is constructed based on the mean μ and standard deviation σ of the grating period, that is, [μ-kσ, μ+kσ], where k is a coefficient and the value of k is 2 or 3. For example, in high-precision scenarios, the value of k is 2, and it is usually 3. Based on the candidate distorted grating periods, the contribution of each candidate distorted grating period is calculated, where the contribution is the squared deviation of each candidate distorted grating period. The specific calculation is: Deviation squared = , where Λ i represents the i-th candidate distorted grating period, μ is the mean of all grating periods; If the contribution is greater than or equal to the preset contribution, the candidate distorted grating period is marked as a distorted grating period; If the contribution is less than the preset contribution, no action will be taken; It can be understood that the contribution reflects the contribution of the distorted grating period to the unevenness (discreteness) of the grating period sequence. The greater the contribution, the more distorted the grating period is and the higher the distortion. In step 2, the construction process of the normal write database and the distorted write database is as follows: Integrate the laser scanning speed and laser repetition frequency during normal grating period writing into a normal writing data group, and summarize them to obtain a normal writing database; Integrate the laser scanning speed and laser repetition frequency during periodic writing of the distorted grating into a distorted writing data group, and summarize them to obtain a distorted writing database; In step 2, the comparison and analysis process is as follows: In the normal writing database, the ratio of the laser scanning speed to the laser repetition frequency (normal grating period) in each normal writing data group is calculated and summed to obtain the normal grating period mean; In the distorted writing database, the ratio of the laser scanning speed to the laser repetition frequency (distorted grating period) in each distorted writing data group is calculated and summed to obtain the average distorted grating period; The absolute difference between the normal grating period average and the distorted grating period average is calculated to obtain the period deviation value; Calculate the data correlation coefficients of the normal write database and the distorted write database respectively, and calculate the absolute difference to obtain the write deviation value; The calculation process of the data correlation coefficient is: Based on the normal writing database, the laser scanning speeds of each writing data group in the normal writing database are integrated into a laser scanning speed sequence according to the time sequence, and the laser repetition frequency is integrated into a laser repetition frequency sequence according to the time sequence; The Pearson correlation coefficient method is used to calculate the Pearson correlation coefficient between the laser scanning speed sequence and the laser repetition frequency sequence, that is, the data correlation coefficient of the normal writing database. The method for obtaining the data correlation coefficient of the distorted writing database is the same as the method for obtaining the data correlation coefficient of the normal writing database. Sum the cycle deviation value and the write deviation value to obtain the write disjoint value; In some embodiments, the write disjoint value is compared to a write disjoint threshold; If the write disjoint value is greater than or equal to the write disjoint threshold, it indicates that the root cause of the grating period distortion phenomenon is the disjointness between the laser scanning speed and the repetition frequency; If the write disjoint value is less than the write disjoint threshold, it means that the root cause of the grating period distortion phenomenon is not the disjointness between the laser scanning speed and the repetition frequency, and other causes should be investigated; It is understandable that the consistency comparison of the ratio of laser scanning speed and laser repetition frequency (period deviation value) focuses on the macroscopic overall characteristics: through the mean deviation of the laser scanning speed and laser repetition frequency of normal and distorted periods, it is judged whether the average ratio of the two is significantly deviated, and the overall ratio offset problem can be quickly identified (such as the overall ratio of laser scanning speed and laser repetition frequency is too large or too small), but it cannot reflect the matching of laser scanning speed and laser repetition frequency in dynamic changes (for example, even if the overall mean deviation is small, there may be a local instantaneous disconnection between laser scanning speed and laser repetition frequency); the laser scanning speed and laser repetition frequency Linear correlation comparison focuses on dynamic correlation features: the linear correlation strength between the laser scanning speed and the laser repetition frequency is used to determine whether the two maintain synchronous changes during the writing process (ideally, the laser scanning speed and the laser repetition frequency fluctuate proportionally). This can capture local instantaneous disconnects (such as a sudden jump in the laser scanning speed while the laser repetition frequency is not synchronized, or vice versa). Even if the overall laser scanning speed and laser repetition frequency mean deviation is not large, dynamic matching failures can be identified through a decrease in correlation. The combination of the two can cover both scenarios of overall proportional offset and local dynamic disconnection, effectively improving analysis accuracy and accurately determining the root cause of grating period distortion. Step 3: If yes, analyze the fluctuations of the laser scanning speed and repetition frequency. If the fluctuation of the laser repetition frequency is greater than or equal to the fluctuation of the laser scanning speed, determine that the laser repetition frequency is the priority adjustment object. Otherwise, analyze the temperature peak value under the combination of laser repetition frequency and laser scanning speed to determine whether the priority adjustment object is the laser repetition frequency. In step 3, the fluctuation analysis process of the laser scanning speed and laser repetition frequency is as follows: Summarize the laser scanning speed when writing each grating period, and calculate the coefficient of variation to obtain the speed variation coefficient; Summarize the laser repetition frequency when writing each grating period, and calculate the coefficient of variation to obtain the frequency variation coefficient; If the frequency variation coefficient is greater than or equal to the speed variation coefficient, the laser repetition frequency is determined to be the priority adjustment object; If the frequency coefficient of variation is less than the speed coefficient of variation, reanalysis is performed; Among them, the reasons for choosing the ones with large coefficient of variation (large volatility) as the priority adjustment objects are: Reason 1: The essence of grating period distortion is the instability of the ratio of v to f (v / f). This instability is often dominated by drastic fluctuations in one parameter (such as instantaneous velocity jumps caused by mechanical vibration, or high-frequency fluctuations in f due to insufficient laser source stability). The parameter with large fluctuations is the main contributor to the disconnect between the two. For example, if the fluctuation amplitude of v is 5 times that of f, then the impact of v fluctuations on the stability of the v / f ratio is far greater than that of f. Prioritizing adjustment of this parameter can directly eliminate the core cause of distortion and is more targeted than blindly adjusting both parameters. Reason 2: Adjusting parameters consumes resources (such as hardware debugging, control algorithm optimization, and time costs). Parameters with high volatility have a higher perturbation weight on the v / f ratio: Assuming that adjusting a parameter with high volatility reduces its standard deviation by 50%, the overall volatility of v / f may be reduced by 40%. However, adjusting a parameter with low volatility, even if the standard deviation is reduced by the same 50%, may only reduce the overall volatility by 5%. Therefore, prioritizing adjustments to parameters with high volatility can significantly improve cycle uniformity with less resource investment, in line with the principle of efficient optimization. Reason three: The formation of a normal grating period relies on a strong linear correlation between v and f (e.g., a Pearson correlation coefficient close to 1). However, this correlation is destroyed during distortion. Parameters with large fluctuations are the main drivers of this correlation. For example, if v suddenly jumps without f following synchronously, the linear relationship between v and f will be broken. Prioritizing adjustment of this parameter to converge its fluctuations can quickly restore synchronization between the two, stabilize the v / f ratio, and ultimately return the grating period to the ideal range. Reason 4: In harsh environments like those on vehicles, where there are high temperatures, vibrations, and electromagnetic interference, parameter fluctuations may be amplified (for example, vibration can cause unstable scanning mechanism speeds, and electromagnetic interference can affect the laser repetition rate). Parameters with large fluctuations are often more sensitive to environmental interference. Prioritizing adjustments can enhance their anti-interference capabilities (for example, upgrading the anti-shake performance of the drive motor for V or optimizing the anti-electromagnetic interference design of the laser source for F). This improves the robustness of the system in complex environments from the source and prevents recurring distortion. The reanalysis process is as follows: The normal grating period is averaged to obtain the normal grating period average, the laser repetition frequency when all grating periods are written is averaged to obtain the laser repetition frequency average, and the product of the normal grating period average and the laser repetition frequency average is calculated to obtain the laser scanning speed average; The mean value of laser repetition frequency and the mean value of laser scanning speed are used as the simulation parameter group; Use COMSOL software to simulate the fiber cycle according to the simulation parameter set. After the simulation is completed, select "Grid Node Data" through the "Export" function of COMSOL software to export the temperature field data. The temperature field data includes the temperature values ​​of the grid nodes. Use MATLAB, Python (such as Matplotlib library), or Origin to draw a temperature field contour map. Extract the temperature peak value based on the temperature field contour map and compare it with the preset temperature threshold of the optical fiber material: If the temperature peak is greater than or equal to the preset temperature threshold of the optical fiber material, the laser repetition frequency is adjusted first. The purpose is: although the fluctuation of the laser repetition frequency is small, under the current fluctuation, the temperature field caused by the laser repetition frequency does not meet the temperature requirements of the optical fiber material during optical fiber writing, so it is necessary to adjust the laser repetition frequency first. If the temperature peak is less than a preset temperature threshold of the optical fiber material, calculating a temperature deviation between the temperature peak and the preset temperature threshold of the optical fiber material; If the temperature deviation is less than the preset temperature deviation, the laser repetition frequency is adjusted first. This is because: since the temperature deviation is less than the preset temperature deviation, it means that the temperature peak is close to the preset temperature threshold of the optical fiber material. Therefore, adjusting the laser repetition frequency first is beneficial to prevent the laser repetition frequency from causing the temperature peak to exceed the preset temperature threshold of the optical fiber material, thereby ensuring the quality of the optical fiber material. If the temperature deviation is greater than or equal to the preset temperature deviation, the laser scanning speed is adjusted as a priority; Step 4: If the priority adjustment object is the laser repetition frequency, determine whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak. If so, determine the laser repetition frequency adjustment limit value. Otherwise, determine the adjustment combination database for the laser repetition frequency and the laser scanning speed. In step 4, if the priority adjustment object is the laser repetition frequency, the process of determining whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak is as follows: By simulating writing optical fiber cycles at different laser repetition frequencies, the temperature peaks at different laser repetition frequencies are obtained; The fiber period is simulated by calculating the laser scanning speed corresponding to the different laser repetition frequencies and the normal fiber period, and performing the simulated writing of the fiber period according to the different laser repetition frequencies and the corresponding laser scanning speeds. The simulation process has been described above and will not be elaborated here. The temperature peaks at different laser repetition frequencies were fitted using the least squares method, and the goodness of fit was calculated. If the goodness of fit is greater than or equal to the preset goodness of fit, it means that there is a linear mapping relationship between the laser repetition frequency and the temperature peak; If the goodness of fit is less than the preset goodness of fit, it means that there is no linear mapping relationship between the laser repetition frequency and the temperature peak; In step 4, if it exists, the laser repetition frequency adjustment limit value is determined as follows: Obtaining a fitting model after fitting the temperature peaks at different laser repetition frequencies, inputting a preset temperature threshold into the fitting model, and outputting a laser repetition frequency adjustment limit value; In step 4, if it does not exist, the process of determining the adjustment combination database of laser repetition frequency and laser scanning speed is as follows: According to the simulated writing process of the optical fiber cycle, a simulation parameter group with a temperature peak value less than a preset temperature threshold is obtained, and all simulation parameter groups are summarized to obtain an adjustment combination database; Step 5: Adjust the laser repetition frequency according to the laser repetition frequency adjustment limit value or adjust the laser repetition frequency and laser scanning speed according to the adjustment combination database; In step 5, the process of adjusting the laser repetition frequency and the laser scanning speed is as follows: If there is a linear mapping relationship between the laser repetition frequency and the temperature peak value and the relationship is positively proportional, then when the grating period is written, if the grating period is not equal to the normal grating period, the laser repetition frequency is adjusted, and the laser repetition frequency cannot exceed the laser repetition frequency adjustment limit value. Conversely, if there is an inverse proportional relationship, the laser repetition frequency cannot be lower than the laser repetition frequency adjustment limit value. If there is no linear mapping relationship between the laser repetition frequency and the temperature peak, the laser repetition frequency and the laser scanning speed are adjusted according to the simulation parameter group contained in the adjustment combination database; The technical solution of the embodiment of the present invention is as follows: uniformity analysis is performed on the grating period written by the femtosecond laser to determine whether grating period distortion occurs. If so, a writing data set regarding the laser scanning speed and repetition frequency is constructed for the normal grating period and the distorted grating period, and a normal writing database and a distorted writing database are obtained respectively. By comparing and analyzing the normal writing database and the distorted writing database, it is determined whether the root cause of the grating period distortion phenomenon is the disconnection between the laser scanning speed and the repetition frequency. If so, a fluctuation analysis of the laser scanning speed and the repetition frequency is performed. If the fluctuation of the laser repetition frequency is higher than or equal to the fluctuation of the laser scanning speed, the laser repetition frequency is determined to be the priority adjustment object. Otherwise, the temperature peak value under the combination of the laser repetition frequency and the laser scanning speed is analyzed to determine whether the priority adjustment object is the laser repetition frequency. If the priority adjustment object is the laser repetition frequency, determine whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak value. If so, determine the laser repetition frequency adjustment limit value. Otherwise, determine the adjustment combination database for the laser repetition frequency and the laser scanning speed, adjust the laser repetition frequency according to the laser repetition frequency adjustment limit value, or adjust the laser repetition frequency and the laser scanning speed according to the adjustment combination database. The present invention analyzes the uniformity of the grating period written by the femtosecond laser, combines the written data comparison, fluctuation analysis and temperature peak correlation analysis of the laser scanning speed and repetition frequency, determines the priority adjustment object and the corresponding adjustment limit value or adjustment combination, and then implements targeted adjustment, which can effectively solve the grating period distortion problem, achieve the uniformity of the grating period, and ensure the quality and stability of the femtosecond laser written grating.

[0020] Example 2: A special fiber Bragg grating suitable for a vehicle-mounted environment according to the embodiment of the present invention is prepared by the method for preparing a special fiber Bragg grating suitable for a vehicle-mounted environment described in the above-mentioned Example 1.

[0021] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the foregoing embodiments. The foregoing embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a special fiber Bragg grating suitable for an in-vehicle environment, characterized by: The following steps are involved: Analyze the uniformity of the grating period written by the femtosecond laser to determine whether there is grating period distortion; If it occurs, a normal write database and a distorted write database are constructed based on the laser scanning speed and repetition frequency of the normal grating period and the distorted grating period. Through comparative analysis, it is determined whether the root cause of the grating period distortion phenomenon is the disconnection between the laser scanning speed and the repetition frequency. If so, then analyze the fluctuations of the laser scanning speed and repetition frequency. If the fluctuation of the laser repetition frequency is higher than or equal to the fluctuation of the laser scanning speed, then determine that the laser repetition frequency is the priority adjustment object. Otherwise, analyze the temperature peak value under the combination of laser repetition frequency and laser scanning speed to determine whether the priority adjustment object is the laser repetition frequency. If the priority adjustment object is the laser repetition frequency, determine whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak value. If so, determine the laser repetition frequency adjustment limit value. Otherwise, determine the adjustment combination database of the laser repetition frequency and the laser scanning speed. The laser repetition frequency is adjusted according to the laser repetition frequency adjustment limit value or the laser repetition frequency and the laser scanning speed are adjusted according to the adjustment combination database.

2. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 1, characterized in that: The process of uniformity analysis of the grating period is as follows: Obtain the laser scanning speed and laser repetition frequency of the femtosecond laser when writing the grating stripes, and perform proportional calculation to obtain the grating period; All written grating periods are obtained and summarized to obtain a grating period sequence. The coefficient of variation of the grating period sequence is calculated. If the coefficient of variation is greater than or equal to the coefficient of variation threshold, it indicates that the grating period uniformity is low and grating period distortion occurs.

3. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 1, characterized in that: The normal grating period and the distorted grating period are obtained as follows: Compare the grating period to the ideal period range; If the grating period is within the ideal period range, the grating period is marked as a normal grating period; If the grating period is not within the ideal period range, the grating period is marked as a candidate distorted grating period; Based on the candidate distorted grating periods, calculating the contribution of each candidate distorted grating period, wherein the contribution is the squared deviation value of each candidate distorted grating period; If the contribution is greater than or equal to the preset contribution, the candidate distorted grating period is marked as a distorted grating period.

4. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 3, characterized in that: The construction process of the normal write database and the distorted write database is as follows: Integrate the laser scanning speed and laser repetition frequency during normal grating period writing into a normal writing data group, and summarize them to obtain a normal writing database; Integrate the laser scanning speed and laser repetition frequency during periodic writing of the distorted grating into a distorted writing data group, and summarize them to obtain a distorted writing database; The comparison and analysis process is as follows: Compare the normal write database and the distorted write database to obtain the write deviation value and the cycle deviation value; Sum the cycle deviation value and the write deviation value to obtain the write disjoint value; If the write disjoint value is greater than or equal to the write disjoint threshold, it indicates that the root cause of the grating period distortion phenomenon is the disjointness between the laser scanning speed and the repetition frequency.

5. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 4, characterized in that: The period deviation value is obtained as follows: In the normal writing database, the ratio of the laser scanning speed and the laser repetition frequency in each normal writing data group is calculated and summed to obtain the average value of the normal grating period; in the distorted writing database, the ratio of the laser scanning speed and the laser repetition frequency in each distorted writing data group is calculated and summed to obtain the average value of the distorted grating period; The absolute difference between the normal grating period average and the distorted grating period average is calculated to obtain the period deviation value; The write deviation value is the absolute deviation of the data correlation coefficients corresponding to the normal write database and the distorted write database respectively.

6. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 5, characterized in that: The fluctuation analysis process of the laser scanning speed and the laser repetition frequency is as follows: Summarize the laser scanning speed when writing each grating period, and calculate the coefficient of variation to obtain the speed variation coefficient; Summarize the laser repetition frequency when writing each grating period, and calculate the coefficient of variation to obtain the frequency variation coefficient; If the frequency variation coefficient is greater than or equal to the speed variation coefficient, the laser repetition frequency is determined to be the priority adjustment object.

7. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 6, characterized in that: The process of performing fluctuation analysis of the laser scanning speed and the laser repetition frequency also includes: If the frequency coefficient of variation is less than the speed coefficient of variation, then reanalysis is performed, specifically: The normal grating period is averaged to obtain the normal grating period average, the laser repetition frequency when all grating periods are written is averaged to obtain the laser repetition frequency average, and the product of the normal grating period average and the laser repetition frequency average is calculated to obtain the laser scanning speed average; The average laser repetition frequency and the average laser scanning speed are used as the simulation parameter group. The fiber period is simulated and written according to the simulation parameter group using COMSOL software. After the simulation is completed, the temperature peak is extracted according to the temperature field contour map. If the temperature peak is greater than or equal to the preset temperature threshold of the optical fiber material, the laser repetition frequency is adjusted as a priority; If the temperature peak is less than a preset temperature threshold of the optical fiber material, calculating a temperature deviation between the temperature peak and the preset temperature threshold of the optical fiber material; If the temperature deviation is less than the preset temperature deviation, the laser repetition frequency is adjusted as a priority.

8. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 7, characterized in that: Whether there is a linear mapping relationship between the laser repetition frequency and the temperature peak is determined by fitting the temperature peaks at different laser repetition frequencies and judging the goodness of fit. If it exists, obtain a fitting model after fitting the temperature peak values ​​under different laser repetition frequencies, input the preset temperature threshold into the fitting model, and output the laser repetition frequency adjustment limit value; If it does not exist, according to the simulated writing process of the optical fiber cycle, a simulation parameter group with a temperature peak value less than a preset temperature threshold is obtained, and all simulation parameter groups are summarized to obtain an adjustment combination database.

9. The method for preparing a special fiber Bragg grating suitable for an in-vehicle environment according to claim 8, characterized in that: The adjustment process is as follows: If there is a linear mapping relationship between the laser repetition frequency and the temperature peak value and the relationship is positively proportional, then when the grating period is written, if the grating period is not equal to the normal grating period, the laser repetition frequency is adjusted, and the laser repetition frequency cannot exceed the laser repetition frequency adjustment limit value. Conversely, if there is an inverse proportional relationship, the laser repetition frequency cannot be lower than the laser repetition frequency adjustment limit value. If there is no linear mapping relationship between the laser repetition frequency and the temperature peak, the laser repetition frequency and the laser scanning speed are adjusted according to the simulation parameter group included in the adjustment combination database.

10. A special fiber Bragg grating suitable for vehicle-mounted environments, characterized by: The special fiber Bragg grating is prepared by a special fiber Bragg grating preparation method suitable for a vehicle-mounted environment as described in any one of claims 1 to 9.

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