Brillouin distributed optical fiber multi-parameter joint test method

Abstract

The invention provides a Brillouin distributed optical fiber multi-parameter joint test method, which comprises the following steps of: placing an optical fiber in a plurality of water bath kettles in sections to form a temperature gradient, and acquiring Brillouin gain spectrums at different temperatures through an optical fiber demodulator; carrying out Lorentz fitting on the Brillouin frequency shift to obtain the Brillouin frequency shift; dividing a non-temperature-sensing section, a transition section and a temperature-sensing section according to the cosine similarity of the Brillouin gain spectrum; parameters such as spatial resolution, spatial positioning error, temperature sensitivity coefficient, temperature measurement repeatability and spatial temperature measurement uncertainty are calculated according to the length of each section of optical fiber and the Brillouin frequency shift value, and multi-parameter joint test is realized. Multiple Brillouin distributed optical fiber sensing parameters can be calibrated through one set of device, the integration degree of the parameter calibration process is fully improved, the parameter calibration efficiency is improved, compared with a current standard method, the influence of manual judgment is reduced, dependence on prior parameters is reduced, and the method is suitable for large-scale popularization and application. And the operability, the reliability and the universality of the calibration process of the optical fiber sensing equipment are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a Brillouin distributed fiber multi-parameter joint test method. BACKGROUND

[0002] Distributed fiber sensing technology has been widely used in bridge health monitoring, ocean environment detection and power equipment diagnosis due to its advantages such as anti-electromagnetic interference, corrosion resistance and long-distance continuous monitoring. However, the performance requirements of distributed fiber sensing systems vary in different fields, so parameter calibration (such as spatial resolution, temperature sensitivity coefficient, etc.) must be performed before the system is put into use.

[0003] However, the parameter calibration method in the existing specification (GB / T 43256-2023 Distributed Fiber Strain Test System Parameter Test Method) has significant limitations: multi-parameter calibration relies on multiple sets of devices, which requires different devices to measure spatial resolution, temperature sensitivity coefficient and other parameters, resulting in complex operation process and high equipment cost; the data analysis process is not standard and objective, for example, the calculation of spatial resolution is often based on the "wave packet take-off point" judgment, but there is no standardized identification method for this point, which is easy to introduce human error; at the same time, the calibration process needs to know the spatial resolution index value provided by the manufacturer as a precondition, which affects the objectivity of the calibration results to some extent.

[0004] In summary, there is an urgent need for an integrated calibration method for the core parameters of distributed fiber sensing systems to reduce human judgment bias and dependence on prior parameters, thereby improving the operability, reliability and universality of the calibration process. SUMMARY

[0005] The present application provides a Brillouin distributed fiber multi-parameter joint test method, which can improve the integration of the distributed fiber sensing system parameter calibration method, reduce human judgment bias and dependence on prior parameters, thereby improving the operability, reliability and universality of the calibration process.

[0006] The present application adopts the following technical solutions.

[0007] A Brillouin distributed fiber multi-parameter joint test method, comprising the following steps:

[0008] First, place N water baths on the test bench.

[0009] Second step, take a fiber, fiber will be connected to the BOTDA demodulator pump light emission port one end as the fiber start, corresponding to the fiber mileage origin; Will be connected to the BOTDA demodulator probe light emission port of a section as the fiber end, corresponding to the maximum value of the fiber mileage, on the fiber continuously take N fiber section, using a ruler to measure the length of the fiber section. Each fiber section is placed in an independent water bath, according to the order of the fiber section start mileage from small to large, the length of the fiber in each water bath is L i (i = 1, 2, …, N), the length of the fiber outside each fiber section is L' i (i = 1, 2, …, N), determine the median value of the fiber mileage of each fiber section in the water bath C i (i = 1, 2, …, N);

[0010] Third step, connect the fiber end to the fiber demodulator, set the sampling frequency ΔP, sweep range, sweep step of the fiber demodulator;

[0011] Fourth step, start the test, set the temperature of each water bath in the direction of the fiber length in an increasing or decreasing manner, the temperature of different water baths is T i (i = 1, 2, …, N), record the room temperature T0 at the same time;

[0012] Each fiber section is divided into non-temperature sensitive section, front transition section, temperature sensitive section and rear transition section according to the temperature influence range;

[0013] The temperature sensitive section is the area of the fiber section located in the water bath liquid surface, the non-temperature sensitive section is the area of the fiber section located outside the water bath, the front transition section is the area between the temperature sensitive section and the previous non-temperature sensitive section, and the rear transition section is the area between the temperature sensitive section of the water bath and the next non-temperature sensitive section;

[0014] Fifth step, after the temperature of the water bath is stable, start multiple measurements to obtain the Brillouin gain coefficient at different measurement points and different sweep frequencies, and form M groups of Brillouin gain spectrum, M ≥ 10;

[0015] Sixth step, Lorenz fitting is performed on each group of Brillouin gain spectrum, and the fiber Brillouin frequency shift of all measurement points in the jth measurement is extracted, that is, the frequency corresponding to the peak point of the Brillouin gain coefficient, to form the jth temperature measurement trace V j (j = 1, 2, …, M). Seventh step, determine the critical measurement points of the non-temperature sensitive section, front transition section, temperature sensitive section and rear transition section of the fiber in the jth temperature measurement trace of the ith water bath;

[0016] Calculate the length of the non-temperature sensitive section, front transition section, temperature sensitive section and rear transition section of each fiber section, respectively D ij (i = 1, …, N; j = 1, …, M), E ij(i = 1, …, N; j = 1, …, M), F ij (i = 1, …, N; j = 1, …, M), G ij (i = 1, …, N; j = 1, …, M).

[0017] The eighth step is to record the Brillouin frequency of the kth measuring point on the non-temperature sensing section, the front transition section, the temperature sensing section, and the rear transition section of the ith water bath on the jth temperature measuring trace, which is denoted as VD ijk , VE ijk , VF ijk , VG ijk (i = 1, …, N; j = 1, …, M).

[0018] The ninth step is to calculate the spatial resolution and the spatial positioning error, wherein the spatial resolution is the average of the length of the front transition section and the length of the rear transition section, that is:

[0019]

[0020] Preferably, the determination process of the length of the non-temperature sensing section, the transition section, and the temperature sensing section of the optical fiber to be measured in the seventh step comprises:

[0021] S071: taking the Brillouin frequency shift V j of the optical fiber measured in the jth measurement.

[0022] S072: calculating the start mileage ST-E ij , the end mileage ED-E ij of the front transition section, and the start mileage ST-G ij , the end mileage ED-G ij of the rear transition section corresponding to the ith temperature sensing section (corresponding to the ith water bath, i = 1, 2, …, N); at the same time, ST-E ij is also the end mileage of the ith non-temperature sensing section; ED-E ij and ST-G ij are also the start mileage and the end mileage of the ith temperature sensing section; ED-G ij is also the start mileage of the ith+1 non-temperature sensing section.

[0023] S073: according to the start mileage and the end mileage, calculating the average μ(F ij ) and the standard deviation σ(F ij ) of the Brillouin frequency shift of the ith temperature sensing section optical fiber;

[0024] S074: according to the start mileage and the end mileage, calculating the average μ(D ij ) and the standard deviation σ(D ij ) of the Brillouin frequency shift of the ith non-temperature sensing section optical fiber;

[0025] S075: Repeat S071 to S074 to calculate the starting and ending mileages of all N temperature-sensing and non-temperature-sensing segments in all M temperature measurement traces, as well as the mean and variance of the fiber optic Brillouin frequency shift.

[0026] Preferably, the method for calculating the mileage from the front transition section to the rear transition section of each optical fiber in step S072 is characterized by the following steps in the calculation process:

[0027] S0721: Take the fiber optic Brillouin frequency shift V of the j-th measurement. j In the non-temperature-sensitive section far from the transition section, select a non-temperature-sensitive fiber with a length of not less than 10m, extract the Brillouin gain spectrum at all sampling mileages, and calculate the...

[0028] The average Brillouin gain at the frequency sweep points is used to obtain the average spectrum.

[0029] S0722: For all sampling mileages described in step S0721, calculate the cosine phase of their Brillouin gain spectrum and average spectrum.

[0030] Similarity CMD j And calculate its mean μ(CMD) j ) and standard deviation σ (CMD) j );

[0031] S0723: For each water bath corresponding to the fiber optic temperature sensing segment, from C i The search begins in two directions: short mileage and long mileage. The last result in the short mileage direction satisfies CMD. j <μ(CMD) j )-3·σ(CMD j The mileage corresponding to the position of ) is the starting mileage of the previous transition section, ST-E. ij The last one in the high-mileage direction to satisfy CMD j <μ(CMD) j )-3·σ(CMD j The mileage corresponding to the position of ) is the end mileage of the subsequent transition section ED-G. ij ;

[0032] S0724: For each water bath corresponding to the fiber optic temperature sensing segment, with C i Centered on, select a length of 0.5L i The temperature-sensing fiber was used to extract the Brillouin gain spectrum at all sampling mileages, and the average Brillouin gain at all frequency sweep points was calculated.

[0033] The gain value is used to obtain the average spectrum.

[0034] S0725: For each water bath corresponding to the fiber optic temperature sensing segment, with C iFor the center, select the length of L i +0.5L i +0.5L i+1 fiber, calculate the cosine similarity CMF ij of the Brillouin gain spectrum at all measurement points and the average spectrum, and count

[0035] the mean μ(CMF ij ) and the standard deviation σ(CMF ij );

[0036] S0726: For each water bath (temperature sensing section), from C i start searching in both small and large mileage directions, the first position that meets CMF ij < μ(CMF ij )-3·σ(CMF ij ) in the small mileage direction corresponds to the end of the front transition section mileage ED-E ij ; the first position that meets CMF ij < μ(CMF ij )-3·σ(CMF ij ) in the large mileage direction corresponds to the start of the rear transition section mileage ST-G ij .

[0037] Preferably, the Lorentz fitting process described in the sixth step of the method comprises:

[0038] selecting a base function; determining initial values; iterating; outputting results;

[0039] S061, calculate the initial values of the parameters of the Brillouin function, and let the Brillouin frequency be x and the Brillouin gain value be y. The basic form of the Brillouin function is:

[0040]

[0041] where P1, P2, P3, C are undetermined coefficients. Let the measured Brillouin gain value sequence be {y n}, and the corresponding Brillouin frequency sequence be {x n}. The initial value of each undetermined coefficient is calculated as follows:

[0042]

[0043] P1=max({Y n})·P3;

[0044] C=min({Y n});

[0045] S062 uses the Levenberg-Marquardt method to perform nonlinear least squares fitting until the sum of squared residuals is less than the specified tolerance, or the number of iterations reaches the limit.

[0046] S063, return the fitted curve, fitting parameters, residuals, residual norm, and Jacobian matrix to complete the fitting. Preferably, the calculation of spatial positioning error includes the following specific calculation process:

[0047] S9201: Calculate the relative error between the i-th segment of the external optical fiber of the water bath and the measured value on each j-th temperature measurement track:

[0048]

[0049] S9202: Calculate the mean relative error of the i-th segment of the optical fiber outside the water bath:

[0050]

[0051] S9203: Fitting spatial positioning error S(L):

[0052] S(L)=F[L' i ,S(L' i )).

[0053] Preferably, it also includes calculating the temperature sensitivity coefficient, and the specific temperature sensitivity coefficient calculation process includes:

[0054] S9301: For all M temperature measurement traces, calculate the mean μ(F) of the fiber Brillouin frequency shift in the i-th temperature-sensing segment and the adjacent non-temperature-sensing segment. ij ) and μ(D ij ):

[0055]

[0056]

[0057] S9302: Calculate the temperature sensitivity coefficient C(T) of the i-th temperature sensing segment. i ), T i The temperature of the i-th sensing segment;

[0058] A method for joint testing of multiple parameters in distributed optical fibers;

[0059] S9303: Fitting temperature sensitivity coefficient C:

[0060] C = H[T i ,C(T i )).

[0061] Preferably, the temperature measurement repeatability is calculated, and the temperature measurement repeatability calculation process comprises the following steps:

[0062] S9401: for all M temperature measurement traces, the frequency shift standard deviation of the kth measurement point of the ith temperature sensing fiber is calculated:

[0063]

[0064] S9402: the temperature measurement repeatability at temperature T i is calculated.

[0065]

[0066] Preferably, the spatial temperature measurement uncertainty is calculated, and the spatial temperature measurement uncertainty calculation process comprises the following steps:

[0067] S9501: the frequency shift standard deviation of the ith temperature sensing fiber on the jth temperature trace is calculated:

[0068]

[0069] S9502: the spatial temperature measurement uncertainty at temperature T i is calculated.

[0070]

[0071] Preferably, the mileage median estimate C i of each fiber segment is calculated. The mileage of the fiber closest to the water bath on both sides is recorded using the mileage meter mark on the surface of the fiber, and the average value is taken as the mileage median estimate C i of the fiber in the water bath.

[0072] Preferably, each of the fiber segments placed in the water bath is greater than 5 times the spatial resolution of the fiber, the temperature of the water bath is ≤90℃, and the water bath has a top cover provided with a fiber penetration hole and heat insulation cotton.

[0073] Compared with the prior art, the present application has the following beneficial effects:

[0074] The present application can calibrate multiple Brillouin distributed fiber sensing parameters through a set of devices, fully improve the integration of the parameter calibration process, speed up the parameter calibration efficiency, and support the testing and production of fiber sensing equipment. The spatial resolution calibration method adopted by the present application does not depend on the experience of calibration personnel and the factory parameters of fiber sensing equipment, and adopts objective, well-reproducible calculation, and only related to experimental data. Compared with the current standard method, the influence of manual judgment is reduced, the operability, reliability and universality of the fiber sensing equipment calibration process are fully improved. BRIEF DESCRIPTION OF DRAWINGS

[0075] Figure 1 Flow chart of the Brillouin distributed optical fiber multi-parameter joint test method of the present application.

[0076] Figure 2 Schematic diagram of the Brillouin distributed optical fiber multi-parameter joint test device provided by the embodiment of the present application.

[0077] Figure 3 Schematic diagram of the position relationship of the non-temperature sensing section, the front transition section, the temperature sensing section and the rear transition section provided by the embodiment of the present application.

[0078] Figure 4 Schematic diagram of the temperature measurement trace provided by the embodiment of the present application.

[0079] Figure 5 Schematic diagram of the start point mileage ST-E of the front transition section provided by the embodiment of the present application. ij Schematic diagram of the end point mileage ED-G of the rear transition section provided by the embodiment of the present application. ij Schematic diagram of the calculation process.

[0080] Figure 6 Schematic diagram of the end point mileage ED-E of the front transition section provided by the embodiment of the present application. ij Schematic diagram of the end point mileage ED-E of the front transition section provided by the embodiment of the present application. ij Schematic diagram of the calculation process. DETAILED DESCRIPTION

[0081] A Brillouin distributed optical fiber multi-parameter joint test method, comprising the following steps:

[0082] Firstly, N water baths are placed on a test bench.

[0083] Secondly, a piece of optical fiber is taken, one end of which to be connected to the pump light emission port of a BOTDA demodulator is recorded as the start point of the optical fiber, corresponding to the origin of the optical fiber mileage; a section to be connected to the probe light emission port of the BOTDA demodulator is recorded as the end point of the optical fiber, corresponding to the maximum value of the optical fiber mileage, N optical fiber sections are continuously taken on the optical fiber, and the length of each optical fiber section is measured by a ruler. The rear part (i.e. the large mileage direction) of each optical fiber section is placed in an independent water bath, and the length of the optical fiber in each water bath is counted as L i (i = 1, 2, …, N) in order of the start point mileage of the optical fiber section from small to large, and the length of the optical fiber outside each water bath is counted as L' i (i = 1, 2, …, N), the median value C i (i = 1, 2, …, N) of the mileage of the optical fiber in each water bath is determined.

[0084] In the application, the size of the mileage direction and the wiring mode of the BOTDA are related. The BOTDA demodulator has two sockets, one end of the optical fiber is connected to each socket, one socket emits pump light, and the other socket emits probe light. The pump light socket corresponds to the large mileage of the optical fiber, and the probe light socket corresponds to the small mileage of the optical fiber. The water bath / optical fiber segment number is from small to large, and the corresponding mileage is from small to large.

[0085] In the third step, the optical fiber end is connected to the optical fiber demodulator, and the sampling frequency ΔP, the sweep range, and the sweep step of the optical fiber demodulator are set.

[0086] In the fourth step, the test is started, and each water bath temperature is set in an incremental or decremental manner along the length direction of the optical fiber, and different water bath temperatures are T i (i = 1, 2, …, N), and the room temperature T0 is recorded at the same time.

[0087] Each optical fiber segment is sequentially divided into a non-temperature-sensitive segment, a front transition segment, a temperature-sensitive segment, and a rear transition segment according to the temperature influence range. The boundary point of the optical fiber segment is located at the start point of the non-temperature-sensitive segment.

[0088] The temperature-sensitive segment is the region of the optical fiber segment located in the water bath liquid surface, the non-temperature-sensitive segment is the region of the optical fiber segment located outside the water bath, the front transition segment is the region between the temperature-sensitive segment and the previous non-temperature-sensitive segment, and the rear transition segment is the region between the temperature-sensitive segment of the water bath and the next non-temperature-sensitive segment.

[0089] In the fifth step, after the water bath temperature is stable, multiple measurements are started to obtain the Brillouin gain coefficients at different measurement points and different sweep frequencies, and M groups of Brillouin gain spectra are formed, wherein M ≥ 10.

[0090] In the application, the distance between different measurement points is determined according to the sampling frequency ΔP, which is the spatial sampling frequency and the unit is m. This parameter is determined, and the number of measurement points is also determined.

[0091] In the sixth step, Lorentz fitting is performed on each group of Brillouin gain spectra to extract the optical fiber Brillouin frequency shift of all measurement points in the jth measurement, that is, the frequency corresponding to the peak point of the Brillouin gain coefficient, to form the jth temperature measurement trace V j (j = 1, 2, …, M).

[0092] In the seventh step, the critical measurement points of the non-temperature-sensitive segment, the front transition segment, the temperature-sensitive segment, and the rear transition segment of the optical fiber in the jth temperature measurement trace of the ith water bath are determined.

[0093] The lengths of the non-temperature-sensitive segment, the front transition segment, the temperature-sensitive segment, and the rear transition segment of each optical fiber segment are calculated and are respectively denoted as D ij (i = 1, …, N; j = 1, …, M), E ij (i = 1, …, N; j = 1, …, M), and F ij(i = 1, …, N; j = 1, …, M), G ij (i = 1, …, N;

[0094] j = 1, …, M).

[0095] The eighth step is to record the Brillouin frequency of the kth measuring point on the non-temperature sensing section, the front transition section, the temperature sensing section, and the rear transition section of the ith water bath on the jth temperature measuring trace, which is denoted as VD ijk , VE ijk , VF ijk , VG ijk (i = 1, …, N; j = 1, …, M).

[0096] The ninth step is to calculate the spatial resolution and the spatial positioning error, wherein the spatial resolution is the average of the length of the front transition section and the length of the rear transition section, that is:

[0097]

[0098] In an embodiment of the present application, the determination process of the length of the non-temperature sensing section, the transition section, and the temperature sensing section of the optical fiber to be measured in the seventh step and the average and standard deviation of the Brillouin frequency shift of the non-temperature sensing section and the temperature sensing section include:

[0099] S071: take the Brillouin frequency shift V j of the optical fiber measured in the jth time.

[0100] S072: calculate the start mileage ST-E ij , the end mileage ED-E ij of the front transition section corresponding to the ith temperature sensing section (corresponding to the ith water bath, i = 1, 2, …, N), and the start mileage ST-G ij , the end mileage ED-G ij of the rear transition section; at the same time, ST-E ij is also the end mileage of the ith non-temperature sensing section; ED-E ij and ST-G ij are also the start mileage and the end mileage of the ith temperature sensing section; ED-G ij is also the start mileage of the ith+1 non-temperature sensing section.

[0101] S073: according to the start mileage and the end mileage, calculate the average μ(F ij ) and the standard deviation σ(F ij ) of the Brillouin frequency shift of the ith temperature sensing section optical fiber;

[0102] S074: according to the start mileage and the end mileage, calculate the average μ(D ij ) and the standard deviation σ(D ij ) of the Brillouin frequency shift of the ith non-temperature sensing section optical fiber.

[0103] S075: Repeat S071 to S074, and calculate the mean and variance of the start and end mileages and the fiber Brillouin frequency shift of all N temperature sensing sections and non-temperature sensing sections in all M temperature measurement traces.

[0104] In an embodiment of the present application, the calculation method of the front transition section to the rear transition section mileage of each fiber in step S072, the calculation process includes the following steps:

[0105] S0721: Take the fiber Brillouin frequency shift V j in the jth measurement, and extract the Brillouin gain spectrum at all sampling mileages in the non-temperature sensing section of the fiber with a length of not less than 10 m, and calculate the average Brillouin gain value at all frequency sweeping points to obtain the average spectrum;

[0106] S0722: For all sampling mileages described in step S0721, calculate the cosine similarity CMD j between the Brillouin gain spectrum and the average spectrum, and calculate the mean μ(CMD j ) and the standard deviation σ(CMD j );

[0107] S0723: For the fiber temperature sensing section corresponding to each water bath, search from C i to the small mileage and the large mileage, and the mileage corresponding to the last position that satisfies CMD j < μ(CMD j )-3·σ(CMD j ) in the small mileage direction is the start mileage ST-E ij of the front transition section; the mileage corresponding to the last position that satisfies CMD j < μ(CMD j )-3·σ(CMD j ) in the large mileage direction is the end mileage ED-G ij of the rear transition section;

[0108] S0724: For the fiber temperature sensing section corresponding to each water bath, take the temperature sensing section fiber with a length of 0.5L i centered at C i , extract the Brillouin gain spectrum at all sampling mileages, and calculate the average Brillouin gain value at all frequency sweeping points to obtain the average spectrum;

[0109] S0725: For the fiber temperature sensing section corresponding to each water bath, take the temperature sensing section fiber with a length of L i centered at C i , extract the Brillouin gain spectrum at all sampling mileages, and calculate the average Brillouin gain value at all frequency sweeping points to obtain the average spectrum; i i+1 ​cosine similarity CMF of the Brillouin gain spectrum at all measurement points to the average spectrum ij , and statistics of the mean μ(CMF ij ) and the standard deviation σ(CMF ij );

[0110] S0726: For each water bath (temperature sensing section), search from C i to the small and large mileage directions, and the mileage corresponding to the first position satisfying CMF ij < μ(CMF ij )-3·σ(CMF ij ) in the small mileage direction is the end point mileage ED-E ij of the front transition section; the mileage corresponding to the first position satisfying CMF ij < μ(CMF ij )-3·σ(CMF ij ) in the large mileage direction is the start point mileage ST-G ij .

[0111] In an embodiment of the present application, the Lorentz fitting process in the sixth step includes:

[0112] selecting a base function, determining an initial value, iterating, and outputting a result;

[0113] S061, calculate the initial value of each parameter of the Brillouin function, and denote the Brillouin frequency as x and the Brillouin gain value as y. The basic form of the Brillouin function is:

[0114]

[0115] where P1, P2, P3, and C are to-be-determined coefficients. Denote the measured Brillouin gain value sequence as {y n}, and the corresponding Brillouin frequency sequence as {x n}. The initial value of each to-be-determined coefficient is calculated as follows:

[0116]

[0117] P1=max({Y n})·P3;

[0118] C=min({Y n});

[0119] S062, perform nonlinear least squares fitting using the Levenberg-Marquardt method until the sum of squares of residuals is less than a specified tolerance or the number of iterations reaches a limit;

[0120] S063, return the fitted curve, fitting parameters, residuals, residual norm and Jacobian matrix, complete the fitting.

[0121] Preferably, the spatial positioning error is calculated, and the specific calculation process of the spatial positioning error comprises:

[0122] S9201: Calculate the relative error of the i-th section of the optical fiber outside the water bath outside the water bath and the measured value on each j temperature measurement trace:

[0123]

[0124] S9202: Calculate the relative error mean of the i-th section of the optical fiber outside the water bath:

[0125]

[0126] S9203: Fit the spatial positioning error S(L):

[0127] S(L) = F[L' i , S(L' i )].

[0128] Preferably, it also includes calculating the temperature sensitivity coefficient, and the specific temperature sensitivity coefficient calculation process comprises:

[0129] S9301: For all M temperature measurement traces, calculate the mean μ(F ij ) and μ(D ij ) of the i-th temperature sensing section and the adjacent non-temperature sensing section fiber Bragg frequency shift:

[0130]

[0131]

[0132] S9302: Calculate the temperature sensitivity coefficient C(T i ) of the i-th temperature sensing section, T i is the temperature of the i-th temperature sensing section;

[0133] Distributed optical fiber multi-parameter joint test method

[0134] S9303: Fit the temperature sensitivity coefficient C:

[0135] C = H[T i , C(T i )].

[0136] Preferably, it also includes calculating the temperature measurement repeatability, and the specific calculation process of the temperature measurement repeatability comprises:

[0137] S9401: For all M temperature measurement traces, calculate the standard deviation of the frequency shift of the kth measurement point of the ith temperature sensing fiber segment:

[0138]

[0139] S9402: Calculate the temperature measurement repeatability at temperature T i :

[0140]

[0141] Preferably, the spatial temperature measurement uncertainty is also calculated, and the calculation process of the spatial temperature measurement uncertainty includes:

[0142] S9501: Calculate the standard deviation of the frequency shift of the ith temperature sensing fiber segment on the jth temperature trace:

[0143]

[0144] S9502: Calculate the spatial temperature measurement uncertainty at temperature T i :

[0145]

[0146] Preferably, the mileage median estimate C i of each fiber segment is calculated using the mileage meter mark on the surface of the fiber, recording the fiber mileage of the two sides closest to the water bath, and taking the average as the mileage median estimate C i of the fiber in the water bath.

[0147] The following is described by a specific embodiment:

[0148] A Brillouin distributed optical fiber multi-parameter joint testing method, the testing method comprising the following steps:

[0149] First, place 5 water baths on the test bench.

[0150] Second, as shown in Figure 2 , take 11 fiber segments of different lengths on an optical fiber at intervals, measure the length of the fiber segment using a ruler, and place the fiber segments in series in the water bath, and according to the order of the starting mileage of the fiber segment from small to large, the length of the fiber in the water bath is counted as L i (i = 1, 2, 3, 4, 5), and the length of the fiber outside the water bath is counted as L' i (i = 1, 2, 3, 4, 5, 6), as shown in Figure 2 . At the same time, using the mileage meter mark on the surface of the fiber, record the fiber mileage of the two sides closest to the water bath, and take the average as the mileage median estimate C i of the fiber in the water bath, and the estimation result is shown in Table 1.

[0151] Table 1 Mileage median estimate of optical fiber in water bath

[0152]

[0153] Thirdly, the optical fiber end is connected to the optical fiber demodulator, and the sampling frequency ΔP = 0.1 m, the sweep frequency range: 10650 MHz ~ 11200 MHz, and the sweep frequency step: 1 MHz of the optical fiber demodulator are set.

[0154] Fourthly, the test is started, the water bath temperature is set in an increasing or decreasing manner, and different water bath temperatures are T1 = 90℃, T2 = 80℃, T3 = 70℃, T4 = 60℃, and T5 = 50℃, as shown in the following table. Figure 2

[0155] As shown in the following table, each optical fiber section is divided into a non-temperature-sensitive section, a front transition section, a temperature-sensitive section, and a rear transition section according to the temperature influence range; the temperature-sensitive section is the area of the optical fiber section located in the water bath liquid surface, the non-temperature-sensitive section is the area of the optical fiber section located outside the water bath, the front transition section is the connecting part of the temperature-sensitive section and the previous non-temperature-sensitive section, and the rear transition section is the connecting part of the temperature-sensitive section of the water bath and the next non-temperature-sensitive section. Figure 3

[0156] Fifthly, after the water bath temperature is stable, multiple measurements are started to obtain the Brillouin gain coefficients at different measurement points and different sweep frequencies, and 10 groups of Brillouin gain spectra are formed.

[0157] Sixthly, the Brillouin gain spectra are Lorenz fitted, and the fiber Brillouin frequency shifts (i.e., the frequencies corresponding to the peak points of the Brillouin gain coefficients) of all measurement points in each measurement are extracted, which are recorded as temperature measurement traces Vj (j = 1 ~ 10). As shown in the following table, the first temperature measurement trace V1 is as shown in the following table. j Figure 4

[0158] Seventhly, the critical measurement points of the non-temperature-sensitive section, the front transition section, the temperature-sensitive section, and the rear transition section of the optical fiber on the first to tenth temperature measurement traces are determined, and the lengths of the non-temperature-sensitive section (excluding the first water bath), the front transition section, the temperature-sensitive section, and the rear transition section are calculated and recorded as D ij (i = 2 ~ 5, j = 1 ~ 10), E ij (i = 1 ~ 5, j = 1 ~ 10), F ij (i = 1 ~ 5, j = 1 ~ 10), G ij (i = 1 ~ 5, j = 1 ~ 10), and the positional relationship of the non-temperature-sensitive section, the front transition section, the temperature-sensitive section, and the rear transition section of the optical fiber is shown in the following table. Figure 3

[0159] ​​​​​The eighth step is to record the Brillouin frequency of the kth measuring point on the non-temperature sensing section, the front transition section, the temperature sensing section, and the rear transition section on the 1st-10th temperature measuring trace of the 1st-5th water bath, which is recorded as VD ijk , VE ijk , VF ijk , VG ijk (i = 1-5, j = 1-10).

[0160] The ninth step is to calculate the spatial resolution, spatial positioning error, temperature sensitivity coefficient, temperature measurement repeatability, and spatial temperature measurement uncertainty.

[0161] Further, the optical fiber segment placed in the water bath is greater than 5 times the spatial resolution of the optical fiber, the device comprises a water bath, a distributed optical fiber, an optical fiber demodulator, and a thermometer, the temperature of the water bath should be ≤90℃, the top cover of the water bath is provided with an optical fiber penetration hole and heat insulation cotton, and the optical fiber outside the water bath should not be wound or subjected to tension.

[0162] Further, the start and end mileages of the front and rear transition sections of the temperature sensing section are calculated as follows, taking the temperature measuring trace V1 shown in Figure 4 as an example.

[0163] (1) In the non-temperature sensing section far away from the transition section, an optical fiber with a length of not less than 10m is selected, and the Brillouin gain spectrum at all positions is extracted; for example, the interval of 2035m-2045m in Figure 4 , and the average spectrum of the Brillouin gain spectrum is calculated by the average value of the Brillouin gain value at each sampling mileage.

[0164] (2) The cosine similarity CMD1 of the Brillouin gain spectrum at all measuring points in the interval of 2035m-2045m and the average spectrum is calculated, and the average value μ(CMD1) and the standard deviation σ(CMD1) are counted.

[0165] (3) The cosine similarity CMD 1k of the Brillouin gain spectrum at all measuring points and the average spectrum in the first step is calculated.

[0166] (4) For each water bath (temperature sensing section), the CMD1 value of each measuring point is calculated from C i , and the start mileage of the front transition section ST-E i1 (i = 1-5) is searched in the small mileage direction, and the end mileage of the rear transition section ED-G i1 (i = 1-5) is searched in the large mileage direction.(i = 1 to 5). For temperature measurement trajectory V1, the measurement results of the starting mileage of the front transition section and the ending mileage of the rear transition section for all 5 water baths are as follows: Figure 5 As shown.

[0167] (5) For each water bath (temperature sensing section), with C i Centered on (i = 1 to 5), a length of 0.5L is selected. i For the temperature-sensing fiber (i = 1 to 5), extract the Brillouin gain spectrum at all measuring points. Taking the first water bath (temperature-sensing section) as an example, the median estimated mileage of the temperature-sensing fiber is C1 = 2053m, and the fiber length inside the water bath is L1 = 2m. Extract the Brillouin gain spectrum at the measuring points in the interval from 2052.5m to 2053.5m, and calculate the average spectrum.

[0168] (6) For each water bath (temperature sensing section), with C i Centered on a length of L i +0.5L i +0.5L i+1 For an optical fiber (i = 1 to 5), calculate the cosine similarity (CMF) between the Brillouin gain spectrum and the average spectrum at all measurement points. i1 And calculate its mean μ(CMF) i1 ) and standard deviation σ (CMF) i1 Taking the first water bath (temperature sensing section) as an example, the median estimated fiber optic mileage in the temperature sensing section is C1 = 2053m. The fiber optic length inside the water bath is L1 = 2m, and the fiber optic lengths outside the water bath are L1 = 5m and L2 = 5m. The Brillouin gain spectrum of the measuring points in the interval from 2049.5m to 2056.5m is extracted, and the cosine similarity (CMF) between the Brillouin gain spectrum and the average spectrum at all measuring points is calculated. 11 And calculate its mean μ(CMF) 11 ) and standard deviation σ (CMF) 11 ).

[0169] (7) For each water bath (sensing section), from C i The search begins in two directions: short mileage and long mileage. The first search in the short mileage direction satisfies the CMF (Continuous Merge Format). ij <μ(CMF) ij )-3·σ(CMF ij The mileage corresponding to the position of ) is the end mileage of the first transition section ED-E. ij The first one to meet CMF in the high-mileage direction ij <μ(CMF) ij )-3·σ(CMF ij The mileage corresponding to the position of ) is the starting mileage of the subsequent transition section, ST-G. ij For the first water bath, the measurement results of the end mileage of the first transition section and the beginning mileage of the second transition section are as follows:Figure 6 As shown in Table 1, for the first temperature measurement trace and the first water bath, the start and end points and lengths of each fiber optic segment are measured.

[0170] Table 1 shows the start and end points and lengths of the first temperature measurement trace and the first optical fiber in the water bath.

[0171] Optical fiber segment Starting Milepost (m) Ending Milepost (m) Length (m) Front Transition Section ST-E 11 = 2051.6 ED-E 11 = 2052.4 E 11 = 0.8 Temperature-Sensing Section ED-E 11 = 2052.4 ST-G 11 = 2053.7 F 11 = 1.3 Back Transition Section ST-G 11 = 2053.7 ED-G 11 = 2058.5 G 11 = 0.8

[0172] Furthermore, based on the starting and ending mileages, the mean value μ(F) of the Brillouin frequency shift of the fiber in the i-th temperature-sensing segment is calculated. ij ) and standard deviation σ(F ij Based on the starting and ending mileages, calculate the mean μ(D) of the Brillouin frequency shift in the i-th non-temperature-sensitive fiber segment. ij ) and standard deviation σ(D ij Repeat the above steps to calculate the starting and ending mileages of all N temperature-sensing and non-temperature-sensing segments in all M temperature measurement traces, as well as the mean and variance of the fiber optic Brillouin frequency shift.

[0173] Furthermore, the Lorentz fitting process is as follows:

[0174] (1) Calculate the initial values ​​of the parameters of the Brillouin function. Let the Brillouin frequency be x and the Brillouin gain be y. Then the basic form of the Brillouin function is:

[0175]

[0176] Where P1, P2, P3, and C are undetermined coefficients. The measured Brillouin gain sequence is denoted as {y}. n}, the corresponding Brillouin frequency sequence is {x n The initial values ​​of each undetermined coefficient are calculated as follows:

[0177]

[0178] P1 = max({Y n})·P3

[0179] C = min({Y n})

[0180] (2) Use the Levenberg-Marquardt method to perform nonlinear least squares fitting until the sum of squared residuals is less than the specified tolerance or the number of iterations reaches the limit.

[0181] (3) Return the fitted curve, fitting parameters, residuals, residual norms and Jacobian matrix to complete the fitting.

[0182] Further, the spatial resolution is the average of the front transition section and the rear transition section, that is:

[0183]

[0184] For the test device in Figure 2 , after 10 rounds of testing, the front transition section length E and the rear transition section length G of the optical fiber in each water bath are shown in Table 2. Substituting the formula can obtain that the spatial resolution of the measured instrument is X = 0.82 m.

[0185]

[0186] Further, the specific calculation process of the spatial positioning error includes:

[0187] (1) Calculate the relative error of the i-th section of the optical fiber outside the water bath on each j-th temperature measurement trace and the measurement value:

[0188]

[0189] (2) Calculate the average relative error of the i-th section of the optical fiber outside the water bath:

[0190]

[0191] (3) Fit the spatial positioning error:

[0192]

[0193] Further, the specific calculation process of the temperature sensitivity coefficient includes:

[0194] (1) For all M temperature measurement traces, calculate the average μ(F ij ) and μ(D ij ) of the i-th temperature sensing section (temperature T i ) and the adjacent non-temperature sensing section optical fiber Brillouin frequency shift: i

[0195]

[0196]

[0197] (2) Calculate the temperature sensitivity coefficient of the i-th temperature sensing section:

[0198]

[0199] (3) Fit the temperature sensitivity coefficient:

[0200] C = H[T i , C(T i )]

[0201] Further, the temperature measurement repeatability specific calculation process comprises:

[0202] (1) calculating the frequency shift standard deviation of the i-th temperature sensing section optical fiber at the k-th measuring point:

[0203]

[0204] (2) calculating the temperature measurement repeatability at the temperature Ti:

[0205]

[0206] Further, the space temperature measurement uncertainty specific calculation process comprises:

[0207] (1) calculating the frequency shift standard deviation of the i-th temperature sensing section optical fiber on the j-th temperature trace:

[0208]

[0209] (2) calculating the space temperature measurement uncertainty at the temperature Ti:

[0210]

[0211] The above description is only the preferred specific implementation of the present application, but the protection scope of the present application is not limited to this, any skilled person in the art should consider that the equivalent replacement or change of the technical solution and the inventive concept of the present application within the technical range disclosed by the present application should be within the protection scope of the present application.

Claims

1. A method for joint testing of multiple parameters in Brillouin distributed optical fibers, characterized in that, The steps include the following: The first step is to place N water baths on the test bench; The second step involves taking an optical fiber and designating the end that will be connected to the pump light transmitter of the BOTDA demodulator as the fiber start point, corresponding to the origin of the fiber mileage. The section that will be connected to the probe light transmitter of the BOTDA demodulator is designated as the fiber end point, corresponding to the maximum fiber mileage. N consecutive fiber segments are measured along the fiber, and their lengths are measured using a ruler. The rear of each fiber segment is placed in a separate water bath. The fiber lengths in each water bath are recorded as L, arranged in ascending order of the fiber start point mileage. i (i = 1, 2, ..., N), the length of each fiber segment outside the water bath is denoted as L'. i (i = 1, 2, ..., N), determine the median estimated mileage C of each fiber segment within the water bath. i (i = 1, 2, ..., N); The third step is to connect the fiber optic end to the fiber optic demodulator and set the sampling frequency ΔP, sweep range, and sweep step size of the fiber optic demodulator. The fourth step is to begin the experiment. The temperature of each water bath is set incrementally, either increasing or decreasing, along the length of the optical fiber. The temperatures of the different water baths are T0. i (i = 1, 2, ..., N), and simultaneously record the room temperature T0; Each fiber segment is divided into non-temperature-sensitive segment, front transition segment, temperature-sensitive segment, and rear transition segment according to the temperature influence range. The temperature-sensing section is the area where the optical fiber segment is located inside the liquid surface of the water bath, the non-temperature-sensing section is the area where the optical fiber segment is located outside the water bath, the front transition section is the area between the temperature-sensing section and the previous non-temperature-sensing section, and the rear transition section is the area between the temperature-sensing section of the water bath and the next non-temperature-sensing section. Fifth step: After the water bath temperature stabilizes, start multiple measurements to obtain the Brillouin gain coefficients at different measurement points and different sweep frequencies, and form M groups of Brillouin gain spectra, where M≥10. Step 6: Perform Lorentz fitting on each group of Brillouin gain spectra, extract the fiber Brillouin frequency shift at all measurement points in the j-th measurement, i.e., the frequency corresponding to the peak point of the Brillouin gain coefficient, and form the j-th temperature measurement trace V. j (j = 1, 2, ..., M); Step 7: Determine the critical measurement points of the i-th water bath on the j-th temperature measurement trace for the non-temperature sensing section, the front transition section, the temperature sensing section, and the rear transition section of the optical fiber. Calculate the lengths of the non-temperature-sensing section, the front transition section, the temperature-sensing section, and the rear transition section of each fiber segment, and denote them as D. ij (i=1,…,N; j=1,…,M), E ij (i=1,…,N; j=1,…,M), F ij (i=1,…,N; j=1,…,M), G ij (i=1,…,N; j=1,…,M); Step 8: Record the Brillouin frequency of the i-th water bath at the k-th measuring point on the j-th temperature measurement line, across the non-sensing section, the pre-transition section, the sensing section, and the post-transition section. Denote this frequency as VD. ijk VE ijk VF ijk VG ijk (i=1,…,N; j=1,…,M); Step 9: Calculate the spatial resolution and spatial positioning error. The spatial resolution is the average of the lengths of the front and rear transition segments, i.e.:

2. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, The process of determining the lengths of the non-temperature-sensitive section, transition section, and temperature-sensitive section of the optical fiber under test, as well as the mean and standard deviation of the Brillouin frequency shift in the non-temperature-sensitive section and the temperature-sensitive section, in step seven includes: S071: Take the fiber optic Brillouin frequency shift V from the j-th measurement. j ; S072: Calculate the starting mileage ST-E of the preceding transition segment corresponding to the i-th temperature sensing segment (corresponding to the i-th water bath, i = 1, 2, ..., N). ij Destination Mileage ED-E ij and the starting mileage of the subsequent transition section ST-G ij ED-G (Destination Mileage) ij Meanwhile, ST-E ij It is also the endpoint mileage of the i-th non-temperature-sensing segment; ED-E ij With ST-G ij It also marks the starting and ending points of the i-th temperature-sensing segment; ED-G ij It is also the starting mileage of the (i+1)th non-temperature-sensing segment; S073: Calculate the mean μ(F) of the Brillouin frequency shift of the fiber optic cable in the i-th temperature-sensing segment based on the starting and ending mileages. ij ) and standard deviation σ(F ij ); S074: Calculate the mean μ(D) of the Brillouin frequency shift of the i-th non-temperature-sensitive fiber segment based on the starting and ending mileages. ij ) and standard deviation σ(D ij ); S075: Repeat S071 to S074 to calculate the starting and ending mileages of all N temperature-sensing and non-temperature-sensing segments in all M temperature measurement traces, as well as the mean and variance of the fiber optic Brillouin frequency shift.

3. The distributed optical fiber multi-parameter joint testing method according to claim 3, characterized in that, The method for calculating the mileage from the front transition section to the rear transition section of each optical fiber in step S072 is characterized by the following steps in the calculation process: S0721: Take the fiber optic Brillouin frequency shift V of the j-th measurement. j In the non-temperature-sensitive section far from the transition section, select a non-temperature-sensitive fiber with a length of not less than 10m, extract the Brillouin gain spectrum at all sampling mileages, and calculate the average Brillouin gain value at all frequency sweep points to obtain the average spectrum. S0722: For all sampling mileages described in step S0721, calculate the cosine similarity (CMD) between their Brillouin gain spectrum and average spectrum. j And calculate its mean μ(CMD) j ) and standard deviation σ (CMD) j ); S0723: For each water bath corresponding to the fiber optic temperature sensing segment, from C i The search begins in two directions: short mileage and long mileage. The last result in the short mileage direction satisfies CMD. j <μ(CMD) j )-3·σ(CMD j The mileage corresponding to the position of ) is the starting mileage of the previous transition section, ST-E. ij The last one in the high-mileage direction to satisfy CMD j <μ(CMD) j )-3·σ(CMD j The mileage corresponding to the position of ) is the end mileage of the subsequent transition section ED-G. ij ; S0724: For each water bath corresponding to the fiber optic temperature sensing segment, with C i Centered on, select a length of 0.5L i The temperature-sensing fiber is used to extract the Brillouin gain spectrum at all sampling mileages and calculate the average Brillouin gain value at all frequency sweep points to obtain the average spectrum. S0725: For each water bath corresponding to the fiber optic temperature sensing segment, with C i Centered on a length of L i +0.5L i +0.5L i+1 For an optical fiber, calculate the cosine similarity (CMF) between the Brillouin gain spectrum and the average spectrum at all measurement points. ij And calculate its mean μ(CMF) ij ) and standard deviation σ (CMF) ij ); S0726: For each water bath (temperature sensing section), from C i The search begins in two directions: short mileage and long mileage. The first search in the short mileage direction satisfies the CMF (Continuous Merge Format). ij <μ(CMF) ij )-3·σ(CMF ij The mileage corresponding to the position of ) is the end mileage of the first transition section ED-E. ij The first one to meet CMF in the high-mileage direction ij <μ(CMF) ij )-3·σ(CMF ij The mileage corresponding to the position of ) is the starting mileage of the subsequent transition section, ST-G. ij .

4. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, The Lorentz fitting process described in step six includes: Choose basis functions; determine initial values; iterate; output results; S061, calculate the initial values ​​of each parameter of the Brillouin function. Let the Brillouin frequency be x and the Brillouin gain be y. Then the basic form of the Brillouin function is: Where P1, P2, P3, and C are undetermined coefficients; the measured Brillouin gain sequence is denoted as {y}. n }, the corresponding Brillouin frequency sequence is {x n The initial values ​​of each undetermined coefficient are calculated as follows: P1=max({Y n })·P3; C=min({Y n }); S062 uses the Levenberg-Marquardt method to perform nonlinear least squares fitting until the sum of squared residuals is less than the specified tolerance, or the number of iterations reaches the limit. S063 returns the fitted curve, fitting parameters, residuals, residual norm, and Jacobian matrix, completing the fitting process.

5. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, The calculation of spatial positioning error includes the following specific calculation process: S9201: Calculate the relative error between the i-th segment of the external optical fiber of the water bath and the measured value on each j-th temperature measurement track: S9202: Calculate the mean relative error of the i-th segment of the optical fiber outside the water bath: S9203: Fitting spatial positioning error S(L): S(L)=F[L' i ,S(L' i )]。 6. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, It also includes calculating the temperature sensitivity coefficient, and the specific calculation process for the temperature sensitivity coefficient includes: S9301: For all M temperature measurement traces, calculate the mean μ(F) of the fiber Brillouin frequency shift in the i-th temperature-sensing segment and the adjacent non-temperature-sensing segment. ij ) and μ(D ij ): S9302: Calculate the temperature sensitivity coefficient C(T) of the i-th temperature sensing segment. i ), T i The temperature of the i-th sensing segment; A method for joint testing of multiple parameters in distributed optical fibers; S9303: Fitting temperature sensitivity coefficient C: C=H[T i ,C(T i )]。 7. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, It also includes calculating the repeatability of temperature measurements, and the specific calculation process for the repeatability of temperature measurements includes: S9401: For all M temperature measurement traces, calculate the standard deviation of the frequency shift at the k-th measurement point in the i-th temperature sensing segment of the optical fiber: S9402: Calculation temperature T i Temperature measurement repeatability under these conditions; 8. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, It also includes calculating the uncertainty of space temperature measurement, and the specific calculation process of the space temperature measurement uncertainty includes: S9501: Calculate the standard deviation of the frequency shift of the i-th temperature-sensing fiber segment on the j-th temperature trace: S9502: Calculation temperature T i Uncertainty in space temperature measurement:

9. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, Median estimated mileage C for each fiber segment i Using the kilometer markers on the surface of the optical fibers, the mileage of the two optical fibers closest to the water bath was recorded, and the average value was taken as the median estimated value C of the mileage of the optical fibers inside the water bath. i .

10. The distributed optical fiber multi-parameter joint testing method according to claim 1, characterized in that, Each fiber segment placed in the water bath has a spatial resolution greater than 5 times that of the fiber optic cable. The temperature of the water bath should be ≤90℃. The water bath has a top cover with fiber optic penetration holes and heat insulation cotton.

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