A high-precision nondestructive calibration device and method for fiber-optic sensing ring length

Abstract

This invention provides a high-precision non-destructive calibration device and method for the length of an optical fiber sensitive ring, including the implemented device structure and calibration method flow. Its key features are: two weak-reflection gratings are interconnected with the left and right pigtails of the optical fiber sensitive ring; an optical frequency domain reflectometer is used to test the scattering spectrum of the optical fiber sensitive ring; and high-precision non-destructive calibration of the length of the optical fiber sensitive ring is achieved by identifying the reflection peak positions of the weak-reflection gratings in the scattering spectrum, with testing and calibration performed simultaneously. The optical fiber sensitive ring calibration device and method established in this patent achieve a length calibration accuracy at the micrometer level, meeting the requirements of high-precision calibration. Furthermore, the calibration device is reusable and highly reliable, and can be widely used for high-precision length calibration of optical devices such as polarization-maintaining fibers, optical fiber sensitive rings, and optical fiber hydrophones.

Description

Technical fields:

[0001] This invention relates to the fields of optical measurement technology and fiber optic gyroscopes, specifically to a high-precision non-destructive calibration device and method for the length of a fiber optic sensing loop. Background technology:

[0002] Fiber optic gyroscopes, invented in the 1970s, are non-mechanical angular velocity measuring instruments based on the Sagnac effect, and they play an important role in aerospace, military, and sensing fields. As the core component of a fiber optic gyroscope system, the geometric and optical symmetries of the fiber optic sensing loop have a decisive influence on the thermally induced drift performance of the gyroscope. Therefore, the measurement and analysis of the physical properties of the fiber itself are essential.

[0003] The accurate measurement of fiber refractive index is closely related to the accuracy of distributed fiber optic sensing. In 2020, Yuan Libo et al. from Guilin University of Electronic Technology (CN 110967048 A) invented a parallel integrated Mach Zehnder interferometer with orthogonal tilted three-core fiber gratings. By detecting the output intensity after the interference of three beams, the effective refractive index of the transmission modes within N fiber cores can be calculated. In 2022, Liang Huijuan et al. from the 38th Research Institute of China Electronics Technology Group Corporation (CN 114665957A) invented a fiber refractive index measurement system and method. A vector network analyzer provides a sweep signal including different frequencies to an optoelectronic modulator. The signal is modulated into an optical signal by the optoelectronic modulator and transmitted through the fiber under test. After demodulation by a detector, the signal is returned to the vector network analyzer to analyze the phase of the different frequency signals. Further measurement is performed by changing the length of the fiber under test, thus achieving high-precision calculation of the refractive index of the fiber under test.

[0004] On the other hand, under the influence of temperature fields, thermal strain exacerbates the deterioration of optical symmetry, leading to the Mohr and Shupe effects, which significantly aggravate thermal drift in fiber optic gyroscope systems. Therefore, accurate testing and analysis of the thermal strain of the fiber optic sensing ring is particularly important. In 2022, Liu Qingwen et al. from Shanghai Jiao Tong University (CN 117451203 A) invented a large-range quasi-distributed fiber optic sensing method and system. Based on the beat frequency signal, the frequency value of the difference frequency term is obtained to calculate the optical path between two points. The temperature change and strain of the fiber are obtained by detecting the change in optical path. In the same year, Wen Kunhua et al. from Guangdong University of Technology (CN 115452213 A) invented a distributed high-precision strain measurement method for fiber optic sensing rings under temperature change conditions. Combining DTS and BOTDA systems, temperature compensation for distributed stress testing of the fiber optic sensing ring is achieved through calculation, the temperature stability of its internal stress distribution is determined, and thus the high-precision internal stress distribution of the fiber optic sensing ring is calculated. In 2023, Rao Yunjiang et al. from Zhejiang Laboratory (CN 116576897 A) invented a multi-parameter fiber optic distributed sensing system and method. This system obtains the temperature or strain changes sensed by each fiber Bragg grating by measuring the wavelength changes corresponding to wavelength defects in the reflection spectrum of each fiber Bragg grating. In 2024, Dong Yongkang et al. from Harbin Institute of Technology (CN118623781 A) invented a temperature-strain decoupling method and device for OPGW optical cables based on a zero-strain reference point and an ANN algorithm. This method uses a zero-strain reference point-based temperature-strain decoupling method to demodulate multiple Brillouin gain spectra, obtaining the corresponding temperature value for each fiber and solving the problem of accurate data demodulation. Zhang Zhuo from Harbin Engineering University (Research on Physical Field Analysis and Coupling Characteristics of Marine Fiber Optic Gyroscopes) analyzed the error mechanism of the physical field of fiber optic gyroscopes under the coupling effect of temperature field and stress field, proposed the concepts of fiber optic ring temperature matrix, stress matrix, and weight matrix, and derived the error model in the traditional integral accumulation form into the form of multiple matrix inner products, providing guidance for the design of fiber optic ring parameters and the selection of fiber and adhesive materials, thereby improving the accuracy of gyroscopes.

[0005] In fiber optic loop testing technology, B-OTDA and optical frequency domain reflection (OFDR) are two commonly used methods. B-OTDA provides absolute measurements, but its resolution needs improvement. Conversely, OFDR has better resolution, but it is only a relative measurement relative to a reference state. Since internal stress has a significant impact on winding quality, existing research has used B-OTDA to monitor the symmetrical stress of fiber optic sensitive loops. Han Zhengying of the 41st Research Institute of China Electronics Technology Group Corporation (application of BOTDA fiber optic sensing technology in fiber optic loop quality assessment) used B-OTDA technology to measure the strain distribution of fiber optic sensitive loops under different temperature conditions. By screening winding bodies with more symmetrical strain changes at various temperatures, the performance of the gyroscope optical path was improved. Yang Jigang et al. of the Beijing Institute of Automation Control Equipment (research on methods to improve the quality of fiber optic winding bodies) addressed the problem of fiber tension asymmetry during inertial navigation winding by feeding back the strain data measured by B-OTDA to the tension control system in real time, thereby improving the stability and symmetry of tension control and significantly enhancing the symmetry of the sensitive loop. Meanwhile, OFDR technology based on Rayleigh scattering has developed rapidly in recent years. The OFDR system using the polarization-maintaining scheme can simultaneously test temperature and thermal strain. The University of Ottawa in Canada published a paper (Long-range high spatial resolution distributed temperature and strain sensing based on optical frequency-domain reflectionometry) which proposed a corresponding testing and demodulation scheme. Utilizing the characteristics that the temperature sensing coefficient is negative and the strain sensing coefficient is positive, a temperature resolution of ±0.8℃, a strain resolution of ±7.0με, and a spatial resolution of 1.3cm were achieved at a sensing distance of 170m

[74] . Yu Zhangjun et al. (Distributed Polarization Measurement for Fiber Sensing Coils: A Review) proposed a high-performance method for obtaining a dynamic range >90dB, a measurement length >10km, a full-range spatial resolution <10cm, and time-domain, frequency-domain, and time-frequency-domain diagnostics in the distributed polarization measurement of fiber sensing coils based on OFDR. W Hong et al. (Accurate measurement and enhancement of fiber coil symmetry) proposed an equivalent midpoint compensation technique based on OFDR, which can accurately test the symmetry of the fiber loop and ensure accurate adjustment of the lengths at both ends of the fiber loop, providing important insights for our research.Yu Ting et al. (CN 106706001A) improved the measurement accuracy of fiber optic gyroscopes by etching fiber gratings near the center point of the polarization-maintaining fiber optic ring and using OFDR technology to accurately locate the optical path center point of the polarization-maintaining fiber optic ring by precisely detecting the position of the fiber gratings.

[0006] However, based on extensive research, the parameters affecting the thermally induced drift of fiber optic loops and their relationships under environmental conditions remain unclear. Furthermore, the lack of suitable long-distance, high-precision, and high spatial resolution methods for temperature and thermal strain testing severely impacts measurement accuracy. Therefore, establishing new high-precision fiber optic sensing loop length calibration devices and methods is of paramount importance for parameter detection of fiber optic gyroscopes and distributed fiber optic sensing applications. Summary of the Invention:

[0007] The purpose of this invention is to provide a high-precision, stable, and reliable OFDR sensitive ring length calibration device. A further purpose of this invention is to provide a calibration method for the sensitive ring length calibration device based on OFDR beat frequency interferometry.

[0008] This invention discloses a high-precision non-destructive calibration device for the length of an optical fiber sensitive loop, characterized by comprising a TLS light source module 1, an auxiliary interferometer module 2, a main interferometer module 3, a calibration module 4, and a data processing module 5. The TLS light source module 1 injects a light beam into a first coupler 204, with a portion of the light injected into the auxiliary interferometer module 2 and the remaining portion injected into the main interferometer module 3. The calibration module 4 is connected to the main interferometer module 3, and the auxiliary and main interferometric signals are fed into a data acquisition card 502 to finally obtain the scattering characteristics of the device to be calibrated.

[0009] The calibration method for the high-precision non-destructive calibration device for the length of the fiber optic sensing loop is as follows:

[0010] (1) Two weak reflection gratings are connected to the left and right pigtails of the fiber optic sensitive ring; the right fiber 407 of the fiber optic ring is connected to the first calibration grating 402, the left fiber 408 of the fiber optic ring is connected to the second calibration grating 404, and the end of the left pigtail 409 of the fiber optic ring is cut flat and placed in the alcohol solution 405.

[0011] (2) Measure the length of the reflection peak at the end of the fiber loop S1, and measure the position of the first calibration grating S2 and the position of the second calibration grating S3;

[0012] (3) Connect the optical path of the optical frequency domain reflectometer and start the TLS light source 101;

[0013] (4) The scattering spectrum of the fiber optic sensitive ring was tested using an optical frequency domain reflectometer;

[0014] (5) Identify the position of the reflection peak of the weak reflection grating in the scattering spectrum; find the end reflection peak according to the intensities S1, S2 and S3, and calibrate the grating pigtail lengths S2 and S3;

[0015] (6) Define U1, U2, and U3 as calibration peak 1, calibration peak 2, and calibration peak 3, respectively; observe whether the intensity of the reflection peak at the end of the fiber ring reaches U1~U2, and whether the reflection intensity of the fiber grating reaches U3.

[0016] (7) If the intensity of the reflection peak at the end of the fiber ring reaches U1 to U2 and the reflection intensity of the fiber grating reaches U3, then record the reflection intensity of the end peak and the fiber grating.

[0017] (8) If the intensity of the reflection peak at the end of the fiber ring does not reach U1~U2, or the intensity of the fiber grating does not reach U3, change the alcohol concentration and replace the corresponding fiber grating until the intensity of the reflection peak at the end of the fiber ring reaches U1~U2 and the intensity of the fiber grating reaches U3. Record the intensity of the end peak and the reflection intensity of the fiber grating to complete the calibration.

[0018] 1. The auxiliary interferometer module 2 includes: a 45° polarizer 202, a first coupler 204, a first circulator 207, a second coupler 209, a first Faraday rotator mirror 212, a second Faraday rotator mirror 213, and a first balanced photodetector 214. Light undergoes beat frequency interference at the second coupler 209.

[0019] (1) The 45° polarizer 202 is connected to the TLS light source 101 via the first flange 201.

[0020] (2) The left arm 218 of the first coupler is connected to the 45° polarizer 202 via the second flange 203.

[0021] (3) The first circulator A port 221 is connected to the lower right arm 220 of the first coupler through the fourth flange 206.

[0022] (4) The upper right arm 224 of the second coupler is connected to the port B 222 of the first circulator via the fifth flange 208.

[0023] (5) The lower right arm 215 of the second coupler and the port C 223 of the first circulator are respectively connected to the first balanced photodetector 214.

[0024] (6) The first Faraday rotating mirror 212 is connected to the sixth flange 210 and the upper left arm 225 of the second coupler.

[0025] (7) The second Faraday rotating mirror 213 is connected to the seventh flange 211 and the lower left arm 226 of the second coupler.

[0026] (8) The first optical fiber 216, the second optical fiber 217, the left arm 218 of the first coupler, the upper right arm 219 of the first coupler, and the lower right arm 220 of the first coupler are all polarization-maintaining optical fibers;

[0027] (9) The first circulator A port 221, the first circulator B port 222, the first circulator C port 223, the upper right arm 224, the lower right arm 215, the upper left arm 225, and the lower left arm 226 of the second coupler are all single-mode optical fibers.

[0028] 2. The main interferometer module 3 includes: a third coupler 302, a fourth coupler 306, a second circulator 309, a first polarization-maintaining beam splitter 318, a second polarization-maintaining beam splitter 326, a second balanced photodetector 320, and a third balanced photodetector 322. Light undergoes beat frequency interference at the fourth coupler 306.

[0029] (1) The right arm 301 of the third coupler is connected to the upper right arm 219 of the first coupler via the third flange 205.

[0030] (2) The upper right arm 305 of the fourth coupler is connected to the upper left arm 303 of the third coupler through the eighth flange 304.

[0031] (3) The second circulator A port 310 is connected to the lower left arm 307 of the third coupler through the ninth flange 308.

[0032] (4) The second circulator C port 312 is connected to the lower right arm 314 of the fourth coupler through the tenth flange 313.

[0033] (5) The right arm 317 of the first polarization-maintaining beam splitter is connected to the upper left arm 315 of the fourth coupler via the eleventh flange 316.

[0034] (6) The right arm 325 of the second polarization-maintaining beam splitter is connected to the lower left arm 323 of the fourth coupler via the twelfth flange 324.

[0035] (7) The second balanced photodetector 320 is connected to the upper left arm 319 of the first polarization-maintaining beam splitter and the upper left arm 327 of the second polarization-maintaining beam splitter, respectively.

[0036] (8) The third balanced photodetector 322 is connected to the lower left arm 321 of the first polarization-maintaining beam splitter and the lower left arm 328 of the second polarization-maintaining beam splitter, respectively.

[0037] (9) The right arm 301 of the third coupler, the upper left arm 303 of the third coupler, the lower left arm 307 of the third coupler, the upper right arm 305 of the fourth coupler, the A port 310 of the second circulator, the B port 311 of the second circulator, the C port 312 of the second circulator, the lower right arm 314 of the fourth coupler, the upper left arm 315 of the fourth coupler, the lower left arm 323 of the fourth coupler, the right arm 317 of the first polarization-maintaining beam splitter, the right arm 325 of the second polarization-maintaining beam splitter, the upper left arm 319 of the first polarization-maintaining beam splitter, the upper left arm 327 of the second polarization-maintaining beam splitter, the lower left arm 321 of the first polarization-maintaining beam splitter, and the lower left arm 328 of the second polarization-maintaining beam splitter are all polarization-maintaining fibers.

[0038] 3. The module to be calibrated 4 includes: fiber optic ring 403, first calibration grating 402, second calibration grating 404, and alcohol solution 405.

[0039] (1) The alcohol solution 405 has a reflectance of 0.2%.

[0040] (2) The fiber 407 on the right side of the fiber optic ring is fused to the first calibration grating 402.

[0041] (3) The fiber 408 on the left side of the fiber optic ring is fused with the second calibration grating 404.

[0042] (4) The right-side tail fiber 406 is connected to the second circulator B port 311 through the thirteenth flange 401.

[0043] (5) Cut the end of the left tail fiber 409 flat and put it into alcohol solution 405.

[0044] (6) The fiber ring and its left and right sides, the calibration grating and its pigtail are all polarization-maintaining fibers.

[0045] 4. The data processing module 5 includes: a data processor 501 and a data acquisition card 502.

[0046] (1) The first balanced photodetector 214 is connected to the data acquisition card 502 through the first electrical wire 503.

[0047] (2) The second balanced photodetector 320 is connected to the data acquisition card 502 through the second electrical wire 504.

[0048] (3) The third balanced photodetector 322 is connected to the data acquisition card 502 through the third electrical wire 505.

[0049] (4) The data acquisition card 502 is connected to the data processor 501 via the fourth wire 506.

[0050] The purpose of this invention is to provide a device and method for accurately calibrating the length of an optical fiber sensing ring by precisely detecting the positions of the fiber gratings on both sides of the ring. By constructing an optical frequency domain testing system, this invention solves the problem of inaccurate positioning of scattering information within the optical fiber ring in gyroscope technology. This invention is applicable to multi-parameter testing and performance evaluation of optical fiber rings, and to calibrating the length of optical fibers under rapid environmental changes in distributed optical fiber sensing systems.

[0051] The schematic diagram of the OFDR-based calibration device proposed in this invention is attached. Figure 1 As shown, its characteristics are:

[0052] 1. It consists of a TLS light source 101, a 45° polarizer 202, a first coupler 204, a first circulator 207, a second coupler 209, a first Faraday rotator 212, a second Faraday rotator 213, a first balanced photodetector 214, a third coupler 302, a second circulator 309, a fourth coupler 306, a first polarization-maintaining beam splitter 318, a second polarization-maintaining beam splitter 326, a second balanced photodetector 320, a third balanced photodetector 322, and a calibration device 801.

[0053] 2. The TLS light source 101 provides input light through a 45° polarizer 202 and a first coupler 204. 1% of the output light enters the first circulator 207 through the first connecting fiber 802, then enters the second coupler 209 through the B port of the first circulator and the second connecting fiber 803. The remaining 99% of the output light enters the third coupler 302 through the third connecting fiber 804.

[0054] 3. Of the light entering the second coupler 209, 50% is reflected by the first Faraday rotator mirror 212 after passing through the fourth connecting fiber 805 to form the first reflected light 806, and the other 50% is reflected by the second Faraday rotator mirror 213 after passing through the delay fiber 807 to form the second reflected light 808. The first reflected light 806 and the second reflected light 808 undergo beat frequency interference at the second coupler 209. A portion of the beat frequency interference signal is sequentially input into the first balanced photodetector 214 through the second connecting fiber 803, the B port of the first circulator, the C port of the first circulator, and the fifth connecting fiber 809. The other portion of the beat frequency interference signal is also input into the first balanced photodetector 214 through the sixth connecting fiber 810. The two portions of the beat frequency interference signal are differentially detected.

[0055] 4. Of the light entering the third coupler 302, 1% passes through the seventh connecting fiber 811 to enter the fourth coupler 306, and the remaining 99% passes through the eighth connecting fiber 812 and the second circulator 309, sequentially entering the calibration device 801 from the second circulator's A port and B port via the ninth connecting fiber 813. The third reflected signal 814, formed after reflection, passes through the ninth connecting fiber 813, the second circulator's B port, the second circulator's C port, and the tenth connecting fiber 815, and is also input into the fourth coupler 306, generating a beat frequency interference signal.

[0056] 5. 50% of the beat frequency interference signal generated at the fourth coupler 306 enters the first polarization-maintaining beam splitter 318 through the eleventh connecting fiber 816, and the other 50% enters the second polarization-maintaining beam splitter 326 through the twelfth connecting fiber 817.

[0057] 6. The light entering the first polarization-maintaining beam splitter 318 is partly input to the second balanced photodetector 320 through the thirteenth connecting fiber 818, and partly input to the third balanced photodetector 322 through the fourteenth connecting fiber 819.

[0058] 7. Part of the light entering the second polarization-maintaining beam splitter 326 is input to the second balanced photodetector 320 through the fifteenth connecting fiber 820, and the other part is input to the third balanced photodetector 322 through the sixteenth connecting fiber 821.

[0059] The flowchart of the calibration method for a high-precision non-destructive calibration device for the length of an optical fiber sensing loop proposed in this invention is attached. Figure 2 As shown, its characteristics are:

[0060] Step 701 involves connecting two weakly reflective gratings to the left and right pigtails of the fiber optic sensitive ring. The right fiber 407 of the fiber optic ring is connected to the first calibration grating 402, and the left fiber 408 of the fiber optic ring is connected to the second calibration grating 404. The end of the left pigtail 409 of the fiber optic ring is cut flat and placed in the alcohol solution 405.

[0061] Step 702 involves measuring the length S1 of the reflection peak at the end of the fiber optic loop, and measuring the position S2 of the first calibration grating and the position S3 of the second calibration grating.

[0062] Step 703 involves connecting the optical path of the optical frequency domain reflectometer and starting the TLS light source 101.

[0063] Step 704 involves testing the scattering spectrum of the fiber optic sensitive loop using an optical frequency domain reflectometer. The scattering signal is tested at room temperature and normal conditions, and the scattering spectrum signal is obtained by performing a Fourier transform.

[0064] Step 705 involves identifying the position of the reflection peak of the weakly reflective grating in the scattering spectrum. The end reflection peaks are located according to intensities S1, S2, and S3, and the grating pigtail lengths S2 and S3 are calibrated.

[0065] Step 706 is to observe and determine whether the intensity of the reflection peak at the end of the fiber ring reaches U1 to U2, and whether the reflection intensity of the fiber grating reaches U3.

[0066] If, in step 706, the intensity of the reflection peak at the end of the fiber ring reaches U1 to U2, and the reflection intensity of the fiber grating reaches U3, then step 707 is executed: record the reflection intensity of the end peak and the fiber grating.

[0067] If, in step 706, the reflection peak intensity at the end of the fiber optic ring does not reach U1-U2, or the reflection intensity of the fiber grating does not reach U3, then proceed to step 708: change the alcohol concentration and replace the corresponding fiber grating, return to step 701, etc., until the reflection peak intensity at the end of the fiber optic ring reaches U1-U2 and the reflection intensity of the fiber grating reaches U3, and then proceed to step 707: record the reflection intensity of the end peak and the fiber grating. Calibration is complete.

[0068] The present invention proposes a high-precision non-destructive calibration device and method for the length of an optical fiber sensitive ring. Its basic principle is the working principle of an optical frequency domain reflectometer. The length of the optical fiber sensitive ring is accurately calibrated by using an OFDR system formed by an interferometer to accurately position the fiber gratings at both ends of the optical fiber ring.

[0069] Device calibration principle:

[0070] Let the optical fields of the reference signal and the measurement signal be...

[0071]

[0072] The third coupler 302 has a beam splitting ratio of 1:99, therefore E2 = 99E1, and the interference intensity of the two signals is...

[0073]

[0074] When this interference signal is split by the fourth coupler (306) with a splitting ratio of 50:50, the output beams of the two arms will have a phase difference of π / 2. Therefore, the interference signal is split into two beams of equal intensity and detected by balanced photodetectors. The photocurrent generated by the two balanced detectors can be expressed as follows:

[0075]

[0076] in Let Δl represent the transmitted light intensity in the reference arm and the measuring arm, respectively, and Δl be the optical path delay of the measuring arm compared to the reference arm. From the above equation, it can be seen that the photocurrent generated by the balanced photodetector contains both DC and AC components; the DC component can be eliminated through differential calculation. By solving the Rayleigh scattering signal spectrum and comparing it with the calibrated positions of the two optical fibers, the fiber loop length can be deduced from the spectrum.

[0077] As can be seen, the Rayleigh scattering spectrum results based on OFDR contain the position information of the calibration gratings on both sides, which can be used to accurately infer the length of the fiber optic ring and complete the calibration of the fiber optic ring length.

[0078] Compared with the prior art, the advantages of the present invention are as follows:

[0079] 1. This invention utilizes a weak grating fusion splicing method to perform non-destructive calibration of the length of the fiber optic sensing ring without damaging its optical and mechanical properties.

[0080] 2. This invention utilizes the reflection peak of the scattering spectrum of a weak grating in an optical frequency domain reflectometer to achieve accurate fiber ring length calibration, meeting the requirements of high-precision calibration. The length calibration accuracy can reach the micrometer level.

[0081] 3. This device is resistant to interference and has good repeatability, ensuring the accuracy and reliability of calibration results. It can be widely used for high-precision length calibration of optical devices such as polarization-maintaining optical fibers, optical fiber sensitive rings, and optical fiber hydrophones. Attached image description:

[0082] Figure 1 This is a schematic diagram of the calibration principle of Optical Frequency Domain Reflectometry (OFDR).

[0083] Figure 2 This is a flowchart of the fiber optic sensitive ring calibration method;

[0084] Figure 3 This is a diagram of the fiber optic ring structure to be calibrated;

[0085] Figure 4 This is a diagram of a high-precision non-destructive calibration device for the length of a fiber optic sensing ring.

[0086] Figure 5 This is the scattering spectrum of the fiber optic sensitive ring.

[0087] Figure 6 This is the second calibration grating and end reflection peak diagram of the fiber optic sensitive ring.

[0088] Figure 7 This is the reflection peak diagram of the first calibration grating of the fiber optic sensitive ring. Detailed implementation method:

[0089] To clearly illustrate the high-precision non-destructive calibration device and method for the length of an optical fiber sensing ring according to the present invention, the present invention will be further described in conjunction with the accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0090] Figure 3 This is the structure of the fiber optic ring to be calibrated according to the present invention. Its characteristics are:

[0091] The fiber optic ring structure to be calibrated consists of a fiber optic ring 403, a first calibration grating 402, a second calibration grating 404, and an alcohol solution 405.

[0092] The reflectivity of the grating and the alcohol solution should be as low as possible, for example, below 2‰.

[0093] When the calibration device 4 is connected to the optical path of the OFDR test system, the swept light first passes through the second calibration grating 404. The reflected light, carrying the position information of the second calibration grating, returns to the optical path of the OFDR test system via the right pigtail 406. The transmitted light undergoes a slight reflection and transmission at the first calibration grating 404 after passing through the fiber ring 403 to be calibrated. This reflected light, carrying the position information of the first calibration grating, returns to the optical path of the OFDR test system. This transmitted light also undergoes a slight reflection and transmission at the second calibration grating 404 after passing through the fiber ring 403, and is reflected by the alcohol solution 405 via the left pigtail. The reflected light sequentially passes through the second calibration grating 404, the fiber ring 403 to be calibrated, and the first calibration grating 402, and returns to the optical path of the OFDR test system via the right pigtail 406. This process obtains the position information of the calibration gratings on both sides of the fiber ring, thereby accurately obtaining the length information of the fiber ring.

[0094] Figure 4 This diagram illustrates a high-precision, non-destructive calibration device for the length of a fiber optic sensing loop, comprising a TLS light source module 1, an auxiliary interferometer module 2, a main interferometer module 3, a calibration module 4, and a data processing module 5. The calibration module 4 has a structure similar to... Figure 3 The structure of the fiber optic ring to be calibrated is the same.

[0095] Combination Figure 4 One embodiment of the present invention is as follows: the frequency sweep range of the TLS light source 101 is set to 10nm~160nm and the frequency sweep speed is 10nm / s. The emitted frequency sweep light enters the auxiliary interferometer module 2 by 1% and the main interferometer module 3 by 99% respectively through the first coupler 204.

[0096] 1% of the light entering the main interferometer module 3 passes through the upper left arm 303 of the third coupler and the upper right arm 305 of the fourth coupler before entering the fourth coupler 306, while the remaining 99% passes through the lower left arm 307 of the third coupler and the second circulator 309.

[0097] The 1% of light passing through the upper left arm 303 of the third coupler and the upper right arm 305 of the fourth coupler serves as a reference signal. The 99% of light passing through the lower left arm 307 of the third coupler and the second circulator 309 then passes through the first calibration grating 402, the fiber optic loop 403, and the second calibration grating 404 before returning along the same path as a test signal carrying the position information of the two calibration gratings. The two signals undergo beat frequency interference at the fourth coupler 306 and are then received by a data acquisition card 502 with a sampling frequency of 180MHz.

[0098] Trigger the TLS light source 101 and data acquisition card 502 to calibrate the calibration gratings on both sides of the fiber optic ring to be calibrated.

[0099] Collect test data. Observe and determine whether the intensity of the reflection peak at the end of the fiber optic loop reaches U1 to U2, and whether the reflection intensity of the fiber grating reaches U3. If the conditions are met, complete the calibration of the fiber optic loop and record the temperature sensor data at the corresponding time. If the conditions are not met, change the alcohol concentration, replace the corresponding fiber grating, and restart the fiber optic loop calibration process until the conditions are met. Record the reflection intensity of the end peak and the fiber grating to complete the calibration.

[0100] According to the calibration method of the high-precision non-destructive calibration device for the length of the fiber optic sensing loop of the present invention, it is possible to obtain... Figure 5 Fiber optic sensitive ring scattering spectrum Figure 6 The second calibration grating and end reflection peak diagram of the fiber optic sensitive loop and Figure 7 This is the reflection peak diagram of the first calibration grating of the fiber optic sensitive ring.

[0101] This calibration device and method meet the requirements of high-precision calibration, and the positional accuracy can reach the millimeter level.

Claims

1. A high-precision non-destructive calibration device for the length of an optical fiber sensing loop, characterized in that: The system includes a TLS light source module (1), an auxiliary interferometer module (2), a main interferometer module (3), a calibration module (4), and a data processing module (5). The TLS light source module (1) injects the light beam into the first coupler (204), injects part of the light into the auxiliary interferometer module (2), and injects the other part of the light into the main interferometer module (3). The calibration module (4) is connected to the main interferometer module (3), and the auxiliary interference signal and the main interference signal are fed into the data acquisition card (502) to obtain the scattering characteristics of the device to be calibrated. The calibration is performed using the aforementioned high-precision non-destructive calibration device for the length of the fiber optic sensing loop. The steps are as follows: (1) Two weak reflection gratings are connected to the left and right pigtails of the fiber sensing ring; the right fiber (407) of the fiber ring is connected to the first calibration grating (402), the left fiber (408) of the fiber ring is connected to the second calibration grating (404), and the end of the left pigtail (409) of the fiber ring is cut flat and placed in the alcohol solution (405). (2) Measure the length of the reflection peak at the end of the fiber loop S1, and measure the position of the first calibration grating S2 and the position of the second calibration grating S3; (3) Connect the optical path of the optical frequency domain reflectometer and start the TLS light source (101); (4) The scattering spectrum of the fiber optic sensitive ring was tested using an optical frequency domain reflectometer; (5) Identify the position of the reflection peak of the weak reflection grating in the scattering spectrum; find the end reflection peak according to the lengths S1, S2 and S3, and calibrate the lengths S2 and S3 of the grating pigtail; (6) Define U1, U2, and U3 as calibration peak 1, calibration peak 2, and calibration peak 3, respectively; observe whether the intensity of the reflection peak at the end of the fiber ring reaches U1~U2, and whether the reflection intensity of the fiber grating reaches U3. (7) If the intensity of the reflection peak at the end of the fiber ring reaches U1 to U2 and the reflection intensity of the fiber grating reaches U3, then record the reflection intensity of the end peak and the fiber grating. (8) If the intensity of the reflection peak at the end of the fiber ring does not reach U1~U2, or the intensity of the fiber grating does not reach U3, change the alcohol concentration and replace the corresponding fiber grating until the intensity of the reflection peak at the end of the fiber ring reaches U1~U2 and the intensity of the fiber grating reaches U3. Record the intensity of the end peak and the reflection intensity of the fiber grating to complete the calibration.

2. The high-precision non-destructive calibration device for the length of an optical fiber sensing loop according to claim 1, characterized in that: The auxiliary interferometer module (2) includes: a 45° polarizer (202), a first coupler (204), a first circulator (207), a second coupler (209), a first Faraday rotator (212), a second Faraday rotator (213), and a first balanced photodetector (214); light undergoes beat frequency interference at the second coupler (209); (1) A 45° polarizer (202) is connected to a TLS light source (101) via a first flange (201); (2) The left arm (218) of the first coupler is connected to the 45° polarizer (202) via the second flange (203); (3) The first circulator A port (221) is connected to the lower right arm (220) of the first coupler through the fourth flange (206); (4) The upper right arm (224) of the second coupler is connected to the B port (222) of the first circulator via the fifth flange (208); (5) The lower right arm (215) of the second coupler and the C port (223) of the first circulator are respectively connected to the first balanced photodetector (214); (6) The first Faraday rotating mirror (212) is connected to the second coupler via the sixth flange (210) and the upper left arm (225); (7) The second Faraday rotating mirror (213) is connected to the lower left arm (226) of the second coupler via the seventh flange (211); (8) The first optical fiber (216), the second optical fiber (217), the left arm (218) of the first coupler, the upper right arm (219) of the first coupler, and the lower right arm (220) of the first coupler are all polarization-maintaining optical fibers; (9) The first circulator A port (221), the first circulator B port (222), the first circulator C port (223), the upper right arm (224) of the second coupler, the lower right arm (215) of the second coupler, the upper left arm (225) of the second coupler, and the lower left arm (226) of the second coupler are all single-mode optical fibers.

3. The high-precision non-destructive calibration device for the length of an optical fiber sensing loop according to claim 1, characterized in that: The main interferometer module (3) includes: a third coupler (302), a fourth coupler (306), a second circulator (309), a first polarization-maintaining beam splitter (318), a second polarization-maintaining beam splitter (326), a second balanced photodetector (320), and a third balanced photodetector (322); light undergoes beat frequency interference at the fourth coupler (306); (1) The right arm (301) of the third coupler is connected to the upper right arm (219) of the first coupler via the third flange (205); (2) The upper right arm (305) of the fourth coupler is connected to the upper left arm (303) of the third coupler via the eighth flange (304); (3) The second circulator A port (310) is connected to the lower left arm (307) of the third coupler via the ninth flange (308); (4) The second circulator C port (312) is connected to the lower right arm (314) of the fourth coupler via the tenth flange (313); (5) The right arm (317) of the first polarization-maintaining beam splitter is connected to the upper left arm (315) of the fourth coupler via the eleventh flange (316); (6) The right arm (325) of the second polarization-maintaining beam splitter is connected to the lower left arm (323) of the fourth coupler via the twelfth flange (324); (7) The second balanced photodetector (320) is connected to the upper left arm (319) of the first polarization-maintaining beam splitter and the upper left arm (327) of the second polarization-maintaining beam splitter respectively; (8) The third balanced photodetector (322) is connected to the lower left arm (321) of the first polarization-maintaining beam splitter and the lower left arm (328) of the second polarization-maintaining beam splitter respectively; (9) The right arm (301) of the third coupler, the upper left arm (303) of the third coupler, the lower left arm (307) of the third coupler, the upper right arm (305) of the fourth coupler, the A port (310) of the second circulator, the B port (311) of the second circulator, the C port (312) of the second circulator, the lower right arm (314) of the fourth coupler, the upper left arm (315) of the fourth coupler, the lower left arm (323) of the fourth coupler, the right arm (317) of the first polarization-maintaining beam splitter, the right arm (325) of the second polarization-maintaining beam splitter, the upper left arm (319) of the first polarization-maintaining beam splitter, the upper left arm (327) of the second polarization-maintaining beam splitter, the lower left arm (321) of the first polarization-maintaining beam splitter, and the lower left arm (328) of the second polarization-maintaining beam splitter are all polarization-maintaining fibers.

4. The high-precision non-destructive calibration device for the length of an optical fiber sensing loop according to claim 1, characterized in that: The module to be calibrated (4) includes: an optical fiber ring (403), a first calibration grating (402), a second calibration grating (404), and an alcohol solution (405); (1) The alcohol solution (405) has a reflectance of 0.2%. (2) The fiber (407) on the right side of the fiber ring is fused to the first calibration grating (402); (3) The fiber (408) on the left side of the fiber ring is fused with the second calibration grating (404); (4) The right tail fiber (406) is connected to the second circulator B port (311) through the thirteenth flange (401); (5) Cut the end of the left tail fiber (409) flat and put it into the alcohol solution (405); (6) The fiber ring and its left and right sides, the calibration grating and its pigtail are all polarization-maintaining fibers.

5. A high-precision non-destructive calibration device for the length of an optical fiber sensing loop according to claim 1, characterized in that... The data processing module (5) includes: a data processor (501) and a data acquisition card (502); (1) The first balanced photodetector (214) is connected to the data acquisition card (502) through the first electrical wire (503); (2) The second balanced photodetector (320) is connected to the data acquisition card (502) via the second electrical wire (504); (3) The third balanced photodetector (322) is connected to the data acquisition card (502) through the third electrical wire (505); (4) The data acquisition card (502) is connected to the data processor (501) via the fourth electrical wire (506).

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