Auxiliary interferometer delay optical fiber length optimal matching method

Abstract

The invention provides an auxiliary interferometer delay optical fiber length optimal matching method, and belongs to the field of optical fiber device testing. According to the method, the optimal matching condition between the length of an optical fiber to be measured and the length of a delay optical fiber of an auxiliary interferometer is deduced by establishing the quantitative relation between the arm length difference of a main interferometer and an auxiliary interferometer and resampling residual phase noise in optical fiber strain measurement through the optical frequency domain reflectometry (OFDR). For the condition that high-precision strain measurement is required in the full-distance scale, it can be ensured that each section of optical fiber reaches the optimal strain test precision in the mode of segmented matching and splicing after demodulation. The method has the advantages of small spatial resolution and high strain precision, and can be widely applied to high-precision testing of optical fiber devices.

Description

Technical Field

[0001] The invention relates to an optimal matching method for delaying optical fiber length of an auxiliary interferometer used in an optical frequency domain distributed strain sensing system, and belongs to the field of optical fiber measurement. Background Art

[0002] Optical frequency domain reflectometry (OFDR) technology was first proposed by W. Eickhoff et al. in 1981. It uses continuously swept light emitted by a narrow linewidth laser as the detection light. The spatial resolution of the overall system is directly determined by the tuning range of the detection light. This technology mainly uses Rayleigh scattered light in optical fiber devices for coherent detection. The frequency of coherent light is proportional to the distance of the Rayleigh scattering point in the optical fiber. By analyzing the spectrum of the coherent light, the distributed sensing information in the optical fiber can be obtained. Compared with other distributed optical fiber sensing technologies, the OFDR system has great advantages in spatial resolution and dynamic range.

[0003] In the field of optical fiber sensing strain measurement, OFDR sensing systems have been applied to structural health monitoring of bridges, houses, 3D shape sensing and aircraft structure monitoring due to their advantages of high spatial resolution, large number of limits, no measurement dead zone and single-ended detection. However, in some special monitoring occasions, such as aircraft wing shape monitoring and satellite sailboard structure health monitoring, which have extremely high requirements for strain accuracy, they usually require a test length of hundreds of meters. Level test sensitivity, thousands The measurement range is in the order of 10000, while maintaining the demodulation spatial resolution in the millimeter range.

[0004] Due to the influence of the nonlinearity of the tunable light source frequency sweep, the auxiliary interferometer has become an essential component of the current optical frequency domain reflection system for high-precision testing, and there is a matching correspondence between the auxiliary interferometer delay fiber length and the optimal test distance of the optical fiber to be tested.

[0005] In 2004, the American company LUNA proposed the use of an auxiliary interferometer to correct the errors generated by a laser with non-ideal tuning characteristics in the optical frequency domain reflection system (Apparatus and method for correcting errors generated by a laser with non-ideal tuning characteristics, US6900897B2), which greatly improved the test accuracy of the OFDR system. However, it did not find the matching correspondence between the arm length difference of the auxiliary interferometer and the arm length difference of the main interferometer.

[0006] Wang Feng and others from Nanjing University published a method for compensating the nonlinear tuning effect of lasers for optical frequency domain reflectometry (CN111397644B). An acousto-optic frequency shifter (AOM) was connected to one of the interferometer arms of the OFDR auxiliary interferometer to shift the frequency of the light wave and increase the beat signal frequency of the auxiliary interferometer. The zero crossing point of the signal was used as a reference point to correct the measurement signal of the main interferometer. However, the acousto-optic frequency shifting scheme used in the method has noise interference from the AOM itself, and does not consider the matching problem between the auxiliary interferometer delay and the main interferometer delay, which results in the final strain demodulation result failing to reach the ideal optimal value.

[0007] Zhang Lixun and others from the University of Electronic Science and Technology of China published a system and method for reducing the nonlinear phase effect of OFDR light sources (CN112461276B), which uses the test fiber as the reference fiber of the auxiliary interferometer to achieve long-distance and high spatial resolution signal detection. Since there is an optimal value matching relationship between the delay of the auxiliary interferometer and the delay of the main interferometer, directly using the device under test as the delay fiber of the main interferometer cannot achieve the best strain test accuracy.

[0008] Wuhan Haoheng Technology Co., Ltd. has disclosed an OFDR segmented acquisition device and method (CN111578971B) that uses an auxiliary interferometer to generate a beat signal as the external clock of the data acquisition card to achieve equal frequency interval sampling of the beat signal of the main interferometer. At the same time, the optical fiber to be tested is divided into N segments for segmented testing. Although dividing the optical fiber to be tested into shorter sections for measurement can effectively suppress the accumulation of residual phase noise, this method does not consider the matching relationship between the arm length difference of the auxiliary interferometer and the arm length difference of the main interferometer, resulting in the final test result failing to meet expectations.

[0009] Japan's Anritsu Corp publicly used an auxiliary interferometer to calibrate the nonlinear effects of the OFDR system light source frequency sweep, and found that its correction effect depends on the reference arm length difference between the main interferometer and the auxiliary interferometer, namely the auxiliary interferometer reference arm delay fiber length (Optical frequency domain reflection measurement device and optical frequency domain reflection measurement method, JP2017181115A). However, it did not accurately calculate the optimal matching relationship between the auxiliary interferometer arm length difference and the main interferometer arm length difference, and was unable to implement the adaptive dynamic matching technology of the auxiliary interferometer delay fiber.

[0010] The present invention provides an optimal matching method for the length of an auxiliary interferometer optical fiber delay ring for an optical frequency domain reflection system. The method determines the optimal matching relationship between the maximum measurement distance and the delay optical fiber length by deriving an analytical expression of the residual phase noise variance. For test conditions with relatively low strain accuracy requirements, the optimal delay optical fiber length can be dynamically matched based on the maximum distance expected to be measured. For test conditions with relatively high strain accuracy requirements, the device to be tested can be segmented and spliced ​​and reassembled after the optimal delay optical fiber matching of the independent auxiliary interferometer. In the entire 101-meter range, a strain test accuracy better than 1.5με can be obtained using a spatial resolution of 2 mm. The method can be widely used in the field of optical fiber sensing strain measurement. Summary of the invention

[0011] 1.1. An optimal matching method for the delayed fiber length of an auxiliary interferometer, wherein the matching calculation method is characterized in that: by deriving an analytical expression of the residual phase noise variance, the optimal matching relationship between the maximum measurement distance of the optical frequency domain reflection system and the delayed fiber length of the auxiliary interferometer is determined, and by controlling the delayed fiber length of the auxiliary interferometer, the resampled residual phase noise is made close to the shot noise limit to achieve the optimal strain test condition. Specifically, the method comprises the following steps:

[0012] Step 1 501: To achieve the best strain test result, the device to be tested will be measured twice. The first rough measurement will get the approximate length of the device to be tested, and then the second high-precision measurement will be performed. First, the length of the optical fiber device to be tested is preliminarily estimated to get the predicted length of the optical fiber to be tested;

[0013] Step 2 502: According to the preliminary estimated length of the optical fiber to be measured, a section of standard single-mode optical fiber close to the predicted length is selected as a delay optical fiber of one arm of the auxiliary interferometer in the high-precision optical frequency domain reflection system;

[0014] Step 3 503: After calibrating the delay of the auxiliary interferometer, a preliminary test is performed on the device under test. After obtaining the scattering spectrum information of the device under test, the precise length of the device under test can be calculated according to the calibrated delay;

[0015] Step 4 504: Determine whether to perform segmented matching measurement based on the precise length of the device under test obtained in step 3 503 and the minimum residual phase noise condition required by the test. After the device under test is tested using the OFDR system, the residual phase noise after equal frequency interval resampling correction of the obtained signal can be expressed by the formula:

[0016]

[0017] In the formula is the delay of the main interferometer, is the delay of the auxiliary interferometer, It follows the Wiener process, and its increment The mean is 0 and the variance is Gaussian normal distribution, is the line width of the tunable laser. In order to minimize the residual phase noise term after resampling, the main interferometer delay is further analyzed. Delay with auxiliary interferometer The relationship between The variance expression is:

[0018]

[0019] During the OFDR system test, after the optical fiber device to be tested is selected, the delay of the main interferometer It is determined by the difference between the optical path of the optical fiber to be measured and the optical path of the reference arm of the main interferometer and remains constant; when When the resampled residual phase noise increases with the main interferometer delay The increase of shows a concave function trend, and the extreme value of this growth trend is related to the auxiliary interferometer delay Formula (2) shows that when the main interferometer delay is completely equal to the auxiliary interferometer delay, that is, When the residual phase noise variance is the smallest, the optimal strain demodulation test effect is achieved. However, in order to achieve high-precision testing at all positions within the overall measurement range, after determining the longest test distance, the maximum value of the residual phase noise within the maximum test distance should be Assume that the equivalent fiber length of the device under test is , that is, the test length range is ,in The corresponding main interferometer delay is .make , exist The extreme points in the range are , the corresponding maximum value is .when hour If the residual phase noise of the optical fiber to be tested is to be minimized over the entire distance within the measurement range, the following conditions must be met:

[0020]

[0021] Simplifying formula (3) yields:

[0022]

[0023] Solving equation (4) yields the optimal auxiliary interferometer delay:

[0024]

[0025] From formula (5), we can see that when the delay of the auxiliary interferometer is Under the condition of a main interferometer delay of times, the entire section of the fiber to be tested can achieve the minimum resampling residual phase noise result; that is, when the test result achieves the best strain accuracy, the relationship between the auxiliary interferometer delay fiber length and the fiber length to be tested is:

[0026]

[0027] Step 5 505: If the equivalent fiber length of the device under test is short or the distributed strain accuracy requirement of the device under test is not high, after obtaining the precise length of the fiber under test through steps 1 501 to 3 503, the auxiliary interferometer delay fiber length can be calculated according to formula (6);

[0028] Step six 506: after replacing the delay fiber of the reference arm of the auxiliary interferometer of the OFDR system according to the delay fiber length of the auxiliary interferometer calculated in step five 505, a secondary high-precision distributed strain test is performed, and the test is completed after the final distributed accurate strain result is obtained;

[0029] Step 7 507: If the strain test accuracy requirement is high, the optical fiber to be tested can be divided into multiple independent units to further improve the strain sensing accuracy within the measurement distance range. Each section of the device to be tested is individually matched with the optimal auxiliary interferometer delay optical fiber length to obtain extremely low residual phase noise in each unit. Assuming that the device to be tested is divided into n independent length units, the residual phase noise at the boundaries of adjacent units must be equal, so the division formula can be obtained:

[0030]

[0031] In the formula Divide the length of the optical fiber to be tested into intervals, Divide the delay fiber length of the auxiliary interferometer into intervals. This method can achieve the best test accuracy for longer devices under test in the entire range, thereby suppressing the problem of accuracy degradation on both sides of the optimal point;

[0032] Step eight 508: after dividing the device under test into n independent units according to the segmentation basis of the device under test in step seven 507, repeat step five 505 according to the divided intervals to respectively match the corresponding auxiliary interferometer delay fiber lengths, and replace the matched auxiliary interferometer delay fibers in sequence according to the segmentation order before conducting the test;

[0033] Step 9 509: For the device to be tested that does not require segmented testing, replace the auxiliary interferometer delay fiber to perform secondary high-precision testing and then directly perform strain demodulation of the entire fiber. If the full-range strain accuracy requirement of the device to be tested is high or the device to be tested is long, the fiber to be tested is segmented and delayed fiber matching is performed, and each section of the fiber is individually tested twice with high precision, and the demodulation results after the segmented matching test are spliced ​​in sequence, so that the full-range distributed high-precision strain test results of the entire fiber to be tested can be obtained.

[0034] Compared with the prior art, the advantages of the present invention are:

[0035] This method can not only improve the full-scale distance strain sensing accuracy of OFDR, but also guide the optimal segmented demodulation of long-distance optical fiber testing, and match the optimal auxiliary interferometer delay fiber length by minimizing the residual phase noise, thereby approaching the theoretical accuracy limit of strain testing.

[0036] This method minimizes the residual phase noise at the hardware level by directing the auxiliary interferometer delay fiber, so no additional data processing is required;

[0037] This method determines the optimal matching relationship between the maximum measurement distance of the OFDR system and the length of the delay fiber, meeting the various requirements of the optical frequency domain reflectometry system for measurement distance, strain accuracy and demodulation time. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The invention discloses a flow chart of an optimal matching method for delaying an optical fiber ring length of an auxiliary interferometer.

[0039] Figure 2 This is a schematic diagram of the overall structure of the high-precision OFDR test device used in this method.

[0040] Figure 3 The best strain demodulation results near the matching point.

[0041] Figure 4 Segment matching of strain demodulation results within the shortest distance.

[0042] Figure 5 Long-distance segmented matching optimal auxiliary interferometer delayed fiber splicing strain demodulation results. DETAILED DESCRIPTION

[0043] In order to clearly illustrate the optimal matching method of the delay fiber ring length of an auxiliary interferometer of the present invention, the present invention is further described in conjunction with the embodiments and drawings, but this should not limit the protection scope of the present invention. Specific implementation method one:

[0045] like Figure 2The optical frequency domain distributed strain sensing system used in this embodiment is shown. The selection and parameters of the main optoelectronic devices of the system are as follows:

[0046] The light source 101 is a narrow linewidth tunable laser source, and the wavelength scanning range is set to 154nm (1483nm-1637nm), the scanning speed is 200nm / s, and the scanning time is 0.77s;

[0047] The maximum detection bandwidth of the first balanced photodetector 208, the second balanced detector 305 and the third balanced detector 306 are all 200 MHz;

[0048] The sampling rate of acquisition card 401 is set to 90MHZ / s;

[0049] The auxiliary interferometer delay fiber 207 is a standard single-mode fiber, and its refractive index n=1.456. The auxiliary interferometer delay fiber is calculated and replaced according to the optimal matching method of the present invention to ensure the optimal strain test effect;

[0050] The splitting ratio of the first coupler 102 and the third coupler 301 is 1:99, and the splitting ratio of the second coupler 202 and the fourth coupler 304 is 50:50;

[0051] The sensing optical fiber 307 used here has a length of 101 m. Two measurements are quickly performed without interference from the external environment, and then the strain demodulation of the optical fiber to be measured is performed with a spatial resolution of 2 mm.

[0052] Since the time interval between the two measurements is very short and no strain is applied to the optical fiber during this process, the strain fluctuation relative to the zero strain condition can be approximately equivalent to the strain demodulation accuracy of the system, thereby characterizing the magnitude of the residual phase noise.

[0053] The test fiber length in the experiment is 101m, and calculation shows that the optimal delay fiber length of the auxiliary interferometer is 83.7m.

[0054] The test results obtained using this calculation method show that when the delay fiber of the auxiliary interferometer is configured at the optimal length, the worst strain accuracy within the entire 101m range of the tested fiber is 3.1 , the strain demodulation results are as follows Figure 3 shown.

[0055] In order to further improve the strain accuracy within the entire measurement distance range, the optical fiber to be measured is divided into multiple independent sections, and the optimal auxiliary interferometer delay fiber length corresponding to each independent unit is calculated separately, so that the residual phase noise of each section is lower than the required index requirement.

[0056] However, when the delay fiber length is less than or equal to 24 m, the residual phase noise within the optimal measurement distance range is close to the critical value, and the value of the participating phase noise is close to the optical shot noise.

[0057] When the light particle noise dominates all the noise, the system noise reaches the theoretical minimum. Under the experimental conditions, when the light particle noise dominates, the strain accuracy can reach the theoretical optimal value. Figure 4 The results of using a 24 m delay fiber to measure a 29 m fiber under test are shown. The worst strain accuracy is 1.2 (Close to 1 Theoretical best accuracy limit).

[0058] In order to make the variance of the residual phase noise as close as possible to the photoelastic noise, the delay fiber lengths of the auxiliary interferometer are set to 24m, 32m, 39m, 46.5m, 56.75m, 62m, 69m, 76.5m, 83.5m, 91.5m and 98.5m respectively. The final segmented splicing test results are shown in Figure 5 shown.

Claims

1. An optimal matching method for delay fiber length of an auxiliary interferometer, wherein the matching calculation method is characterized by: By deriving the analytical expression of the residual phase noise variance, the optimal matching relationship between the maximum measurement distance of the optical frequency domain reflection system and the delay fiber length of the auxiliary interferometer is determined. By controlling the delay fiber length of the auxiliary interferometer, the resampled residual phase noise is close to the shot noise limit to achieve the optimal strain test conditions. The specific process is that the method includes the following steps: Step 1 (501): To achieve the best strain test result, the device to be tested is measured twice. The first rough measurement is used to obtain the approximate length of the device to be tested, followed by a second high-precision measurement. First, the length of the optical fiber device to be tested is preliminarily estimated to obtain the predicted length of the optical fiber to be tested; Step 2 (502): based on the preliminary estimated length of the optical fiber to be measured, select a section of standard single-mode optical fiber close to the predicted length as a delay optical fiber for one arm of the auxiliary interferometer in the high-precision optical frequency domain reflection system; Step 3 (503): After calibrating the delay of the auxiliary interferometer, a preliminary test is performed on the device under test. After obtaining the scattering spectrum information of the device under test, the precise length of the device under test can be calculated based on the calibrated delay; Step 4 (504): Determine whether segmented matching high-precision measurement is required based on the precise length of the device under test obtained in step 3 (503) and the minimum residual phase noise condition required by the test. After the device under test is tested using the OFDR system, the residual phase noise of the obtained signal after equal frequency interval resampling correction can be expressed by the formula: In the formula is the delay of the main interferometer, is the delay of the auxiliary interferometer, It follows the Wiener process, and its increment The mean is 0 and the variance is Gaussian normal distribution, is the line width of the tunable laser. In order to minimize the residual phase noise term after resampling, the main interferometer delay is further analyzed. Delay with auxiliary interferometer The relationship between The variance expression is: During the OFDR system test, after the optical fiber device to be tested is selected, the delay of the main interferometer It is determined by the difference between the optical path of the optical fiber to be measured and the optical path of the reference arm of the main interferometer and remains constant; when When the resampled residual phase noise increases with the main interferometer delay The increase of shows a concave function trend, and the extreme value of this growth trend is related to the auxiliary interferometer delay Formula (2) shows that when the main interferometer delay is completely equal to the auxiliary interferometer delay, that is, When the residual phase noise variance is the smallest, the optimal strain demodulation test effect is achieved. However, in order to achieve high-precision testing at all positions within the overall measurement range, after determining the longest test distance, the maximum value of the residual phase noise within the maximum test distance should be Assume that the equivalent fiber length of the device under test is , that is, the test length range is ,in The corresponding main interferometer delay is .make , exist The extreme points in the range are , the corresponding maximum value is .when hour If the residual phase noise of the optical fiber to be tested is to be minimized over the entire distance within the measurement range, the following conditions must be met: Simplifying formula (3) yields: Solving equation (4) yields the optimal auxiliary interferometer delay: From formula (5), we can see that when the delay of the auxiliary interferometer is Under the condition of a main interferometer delay of times, the entire section of the fiber to be tested can achieve the minimum resampling residual phase noise result; that is, when the test result achieves the best strain accuracy, the relationship between the auxiliary interferometer delay fiber length and the fiber length to be tested is: Step 5 (505): If the equivalent optical fiber length of the device under test is short or the distributed strain accuracy requirement of the device under test is not high, after obtaining the precise length of the optical fiber under test through steps 1 (501) to 3 (503), the auxiliary interferometer delay optical fiber length can be calculated according to formula (6); Step six (506): after replacing the auxiliary interferometer reference arm delay fiber of the OFDR system according to the auxiliary interferometer delay fiber length calculated in step five (505), a secondary high-precision distributed strain test is performed, and the test is completed after the final distributed precise strain result is obtained; Step 7 (507): If the strain test accuracy requirement is high, the optical fiber to be tested can be divided into multiple independent units to further improve the strain sensing accuracy within the measurement distance range. Each section of the device to be tested is individually matched with the optimal auxiliary interferometer delay optical fiber length to obtain extremely low residual phase noise in each unit. Assuming that the device to be tested is divided into n independent length units, the residual phase noise at the boundaries of adjacent units must be equal, so the division formula can be obtained: In the formula Divide the length of the optical fiber to be tested into intervals, Divide the delay fiber length of the auxiliary interferometer into intervals. This method can achieve the best test accuracy for longer devices under test in the entire range, thereby suppressing the problem of accuracy degradation on both sides of the optimal point; Step eight (508): after dividing the device under test into n independent units according to the segmentation basis of the device under test in step seven (507), repeat step five (505) according to the divided intervals to respectively match the corresponding auxiliary interferometer delay fiber lengths, and replace the matched auxiliary interferometer delay fibers in sequence according to the segmentation order before testing; Step 9 (509): For the device to be tested that does not require segmented testing, the auxiliary interferometer delay fiber is replaced to perform a secondary high-precision test, and then the strain demodulation of the entire fiber section can be directly performed. If the full-range strain accuracy requirement of the device to be tested is high or the device to be tested is long, the fiber to be tested is segmented and delayed fiber matching is performed, and each section of the fiber is individually tested twice with high precision, and the demodulation results after the segmented matching test are spliced ​​in sequence, so that the full-range distributed high-precision strain test results of the entire fiber to be tested can be obtained.

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