A delay fiber length encoding device for an auxiliary interferometer

Abstract

This invention provides an adaptive matching optical path encoding device for delayed fiber in an auxiliary interferometer for optical frequency domain reflectance technology, belonging to the field of laser measurement instruments. It comprises three main components: a programmable optical switch, an optical fiber delay ring, and a host computer. Its key feature is that it matches the optimal length of the delayed fiber in the auxiliary interferometer to different fiber lengths. The host computer then uses the programmable optical switch to control the adaptive matching result and freely encodes and combines multiple delayed fiber rings to achieve optimal strain demodulation accuracy for different fiber lengths. This device adaptively and dynamically matches the length of the delayed fiber in the auxiliary interferometer for different fiber lengths, thereby achieving optimal strain measurement accuracy. Furthermore, it simplifies the optical path and reduces the impact of frequent disassembly and reassembly on instrument stability and performance accuracy.

Description

Technical Field

[0001] This invention belongs to the field of optical testing technology, specifically relating to an auxiliary interferometer delay fiber length encoding device for optical frequency domain reflection technology. Background Technology

[0002] Strain parameters are crucial for the development and manufacturing of optical devices, optical systems, and photonic integrated circuits. Testing strain parameters is essential during the fabrication, production, and application of fiber optic devices. Fiber optic inductive strain testing technology based on optical frequency domain reflectometry (OFDR) is widely used in fields such as large-scale building structural health monitoring, three-dimensional shape sensing, and fiber optic device testing.

[0003] Due to the influence of the nonlinearity of the frequency sweep of the tunable light source, the auxiliary interferometer has become an essential component for high-precision testing of current optical frequency domain reflection systems, and there is a matching relationship between the delay fiber length of the auxiliary interferometer and the optimal test distance of the fiber under test.

[0004] In 2004, LUNA Corporation of the United States proposed using an auxiliary interferometer to correct errors generated by a laser with non-ideal tuning characteristics in an 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 testing accuracy of the OFDR system. However, they did not find a matching correspondence between the arm length difference of the auxiliary interferometer and the arm length difference of the main interferometer.

[0005] Wang Feng et al. from Nanjing University published a compensation method (CN111397644B) for the nonlinear tuning effect of lasers in optical frequency domain reflectometers. This method involves connecting an acousto-optic frequency shifter (AOM) to one interferometer arm of an OFDR auxiliary interferometer to shift the frequency of the light wave, increasing the beat frequency signal of the auxiliary interferometer. The zero-crossing point of this signal is then used as a reference point to correct the measurement signal of the main interferometer. However, the acousto-optic frequency shifting scheme suffers from noise interference from the AOM itself and does not consider the matching problem between the time delay of the auxiliary interferometer and the time delay of the main interferometer, resulting in the final strain demodulation result failing to reach the ideal optimal value.

[0006] Zhang Lixun et al. from the University of Electronic Science and Technology of China published a system and method for reducing the influence of nonlinear phase of OFDR light sources (CN112461276B). This method uses a test fiber as the reference fiber for an auxiliary interferometer to achieve long-distance and high spatial resolution signal detection. Since there is an optimal matching relationship between the time delay of the auxiliary interferometer and the time delay of the main interferometer, directly using the device under test (DUT) as the delay fiber of the main interferometer cannot achieve the best strain testing accuracy.

[0007] Wuhan Haoheng Technology Co., Ltd. disclosed a device and method for long-distance measurement using OFDR segmented acquisition (CN111578971B). This method employs an auxiliary interferometer, whose generated beat frequency signal serves as the external clock for the data acquisition card, enabling equal-frequency interval sampling of the main interferometer's beat frequency signal. Simultaneously, the optical fiber under test is divided into N segments for segmented testing. While measuring the fiber under test in shorter segments effectively suppresses 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 results failing to meet expectations.

[0008] Anritsu Corp. of Japan disclosed that it used an auxiliary interferometer to calibrate the influence of frequency sweep nonlinearity of OFDR system light source and found that its correction effect depends on the difference in reference arm length between the main interferometer and the auxiliary interferometer, i.e., the length of the delay fiber of the auxiliary interferometer reference arm (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 difference in arm length between the auxiliary interferometer and the main interferometer, and could not realize the adaptive dynamic matching technology of the delay fiber of the auxiliary interferometer.

[0009] This invention provides an adaptive matching optical path encoding device for the delayed fiber of an auxiliary interferometer. The design concept is as follows: based on the optimal strain demodulation accuracy matching relationship between the arm length difference between the auxiliary interferometer and the main interferometer in an optical frequency domain reflectometer, the host computer calculates the optimal matching length of the delayed fiber of the auxiliary interferometer for the current fiber under test. The original delayed fiber of the auxiliary interferometer is replaced with an adaptive optical path encoding device controlled by the host computer. According to the strain accuracy testing requirements of different fiber lengths under test, after the computer calculates the optimal optical path difference of the auxiliary interferometer, the host computer controls an optical switch to adaptively and dynamically match the optimal length of the delayed fiber of the auxiliary interferometer, thereby bringing the strain demodulation accuracy of the device under test close to the theoretical limit. This invention, based on the optimal matching method of the delayed fiber length of the auxiliary interferometer reference arm, achieves adaptive dynamic matching of the arm length difference of the auxiliary interferometer by controlling the optical switch to select the optical path through a host computer encoding method, thus significantly improving the strain testing accuracy of the fiber under test. It has advantages such as high measurement accuracy, good stability, and simple optical path structure, reducing system cost, improving testing efficiency, and saving testing costs. It can be widely used for quantitative performance testing of optical devices with high requirements for strain testing accuracy. Summary of the Invention

[0010] An auxiliary interferometer delay fiber length encoding device includes a light source module 1, an auxiliary interferometer and adaptive matching optical path encoding module 2, a main interferometer module 3, and a host computer signal processing and control module 4, characterized in that:

[0011] The tunable laser source 101 generates a linear sweeping continuous light whose frequency changes with time at a rate γ. After passing through the first coupler 102, the continuous sweeping light signal is split into two paths. One path enters the main interferometer module 3 for acquiring information about the device under test, and the other path enters the auxiliary interferometer and adaptive optical path matching encoding module 2 for acquiring the nonlinear information of the tunable laser source 101 sweeping frequency, thereby improving the demodulation accuracy of the device test strain.

[0012] The auxiliary interferometer and adaptive optical path matching encoding module according to the present invention are characterized in that:

[0013] The optical path encoding module mainly consists of n optical switches and n-1 sets of delay fibers. Depending on the different paths controlled by the host computer, this encoding module can implement arbitrary encoding methods, thus allowing for arbitrary configuration of the delay fibers. At the start of the test, based on the length of the fiber under test, the host computer calculates the optimal optical path difference between the two arms of the auxiliary interferometer corresponding to that fiber, and then automatically adjusts the output optical path of the optical path encoding module to achieve adaptive matching, thereby optimizing the test accuracy to the greatest extent.

[0014] Assuming the optical switch type used in the device is an m-channel optical switch, then the delay fiber length of the delay fiber length encoding module is encoded in m-ary mode. If calculations show that the maximum required length of the auxiliary interferometer delay fiber for this test is L.

[0015] The formula for calculating the length of the delay fiber in the m-path of optical switch 202 (No. 1) is as follows:

[0016]

[0017] The delay fiber lengths between each path of optical switch 202 are divided into m intervals with an interval of x1. The lengths of the m segments of delay fiber in optical fiber group 207 are set as: 0, x1, 2x1, ..., (m-1)x1. Optical switch 202 initially divides the delay fiber of the auxiliary interferometer into m-1 intervals, and optical switch 203 further refines the delay fiber length intervals. The formula for calculating the length of the m paths of optical fiber in optical switch 203 is as follows:

[0018]

[0019] That is, the length of the delay fiber between each path of optical switch 203 is divided into m intervals with an interval of x2. The lengths of the m segments of delay fiber in delay fiber group 208 are set as: 0, x2, 2x2, ..., (m-1)x2. The configuration of intermediate optical switch 205 and intermediate delay fiber group 209 is the same as that of optical switch 1 202 and optical switch 203. The path and number of optical switches can be selected according to the estimated longest auxiliary interferometer delay fiber length required during actual testing.

[0020] The nth optical switch 206, with its minimum delay fiber group 210, is the most precise adjustment switch for the delay fiber length encoding device in the auxiliary interferometer. The formula for calculating the length of the m-path delay fibers in the nth optical switch 206 is as follows:

[0021]

[0022] That is, the length of the delay fiber between each path of optical switch n 206 is x n The interval is divided into m intervals, and the lengths of the m delay fiber segments in the minimum delay fiber group 210 are respectively set as: x n 2x n ..., m·x n This ensures that when all n optical switches are selected from the maximum length of each group of delay fibers, the maximum auxiliary interferometer delay fiber length is reached during the pre-estimated calculation, thereby achieving high-precision auxiliary interferometer length encoding.

[0023] By connecting the optical path encoding module to one arm of the auxiliary interferometer module, the optical frequency domain reflectometer auxiliary interferometer can extract the nonlinear information of the light source sweep frequency, and convert the light source sweep frequency information into interference fringes of the auxiliary interferometer, which are then extracted by the first balanced detector 211 and converted into electrical signals that are input into the host computer signal processing and control module 4.

[0024] The main interferometer module 4 and the host computer signal processing and control module 5 of the present invention are characterized by:

[0025] The test optical signal input to the main interferometer is split into two paths by the second coupler 301. One path is input to the reference arm of the main interferometer, and the polarization controller 302 ensures that the intensity of the optical signal in the P and S directions in the interferometer is equal, thereby achieving optimal test accuracy. The other path enters the test arm, is input from port 1 of the circulator 303 and output from port 2, and outputs the optical information of the device under test from port 303 through reflection. After interfering with the reference signal, the two paths are collected by the second balanced detector 305 and converted into an electrical signal, thereby extracting the optical information of the device under test.

[0026] The host computer signal processing and control module 4 includes a data acquisition card 401, a computer 402, a first balanced detector 211, and a second balanced detector 305. The acquired interference fringe signals are converted into electrical signals and input into the signal acquisition card 401 for recording, and then into the computer 402 for data processing. The computer 402 is also responsible for controlling the optical switches in the adaptive matching optical path encoding device of the auxiliary interferometer, thereby realizing the switching of delay fibers of different lengths in the auxiliary interferometer.

[0027] Compared with the prior art, the advantages of the present invention are as follows:

[0028] This device can reduce residual phase noise when OFDR system performs strain measurement, thereby improving strain demodulation accuracy. The host computer performs adaptive dynamic matching according to the length of the fiber under test. The optical path coding device always keeps the current difference between the arm length of the optical frequency domain reflectometer auxiliary interferometer and the arm length of the main interferometer in the best matching state, so that the strain test accuracy approaches the theoretical limit.

[0029] This device, controlled by a host computer based on arm length difference matching calculations, performs adaptive delay fiber matching on the optical path encoding device, thereby improving system feasibility and stability. To meet the needs of different scenarios, different optical path encoding levels can be achieved by adjusting the number of optical switch paths, enhancing system adaptability. This design simplifies the overall testing process, avoiding issues such as system disassembly, thus improving operational convenience and saving time and costs. With continuous technological optimization, this device will demonstrate great potential in a wider range of applications in the future. Attached Figure Description

[0030] Figure 1 A schematic diagram of an auxiliary interferometer delay fiber adaptive matching optical path coding device used in an OFDR system;

[0031] Figure 2 This is a schematic diagram of an optical frequency domain reflection system auxiliary interferometer delay fiber adaptive matching optical path coding device based on the Mach-Zehnder (MZ) interferometer structure;

[0032] Figure 3 This is a schematic diagram of an optical frequency domain reflection system auxiliary interferometer delay fiber adaptive matching optical path coding device based on the Mach Michelson (MI) interferometer structure;

[0033] Figure 4 This is a schematic diagram of a quaternary auxiliary interferometer delay fiber adaptive matching optical path encoding device based on the Michelson interferometer structure. Detailed Implementation

[0034] To clearly illustrate the present invention, an auxiliary interferometer delay fiber adaptive matching optical path coding device, the present invention will be further described in conjunction with embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention. Specific implementation method one:

[0036] In the optical frequency domain reflection system based on the Mach-Zehnder (MZ) interferometer, as shown in the attached... Figure 3 As shown in the optical path structure, the optical signal entering the auxiliary interferometer passes through the delayed fiber configured by the adaptive matching optical path encoding device and is directly output by the path selected by the nth optical switch 26 as the delayed optical signal of the auxiliary interferometer reference arm. It undergoes beat frequency interference with the local oscillator optical signal in the balanced detector 35, thereby correcting the accuracy degradation problem of the OFDR system caused by the nonlinearity of the light source sweep frequency. Specific Implementation Method Two:

[0038] In an optical frequency domain reflection system using a Michelson (MI) interferometer as an auxiliary interferometer, as shown in the attached... Figure 2 As shown in the optical path structure, a Faraday rotator needs to be connected after each path of optical switch 49 at the end of the adaptive matching optical path encoder, as shown in the attached diagram. Figure 2As shown in the example, there are two paths after optical switch n. The first and second paths are each connected to a Faraday rotator mirror 44. Similarly, if there are n paths after optical switch n 49, then n Faraday rotator mirrors are needed. The reference light signal, after passing through a specific length delay fiber selected in the fiber length encoding device, is reflected by the Faraday rotator mirror, and after passing through twice the fiber delay loop length, returns to the first photodetector 43 and interferes with the local oscillator reference light. This interference is used to correct the accuracy degradation problem in the OFDR system caused by the frequency sweep nonlinearity of the light source. Specific implementation method three:

[0040] Different fiber delay loop encoding methods with different bases can be implemented depending on the number of optical switch paths selected, as shown in the attached figure. Figure 4 The example shown is an auxiliary interferometer with a Michelson interferometer structure. A four-channel optical switch can be used to configure the fiber delay loop of the auxiliary interferometer using a quaternary encoding method, thereby achieving higher precision control of the fiber length range.

[0041] Taking the auxiliary interferometer with the longest requirement of 128 meters for the device under test as an example, a total of three sets of optical switches and fiber delay rings are used, namely, the intermediate optical switch 68 and all intermediate delay fiber rings 78, 79, 80 and 81 are not configured. The first set of fiber delay rings can initially be configured with lengths ranging from 0 to 32 meters, 33 to 64 meters, 65 to 96 meters, or 97 to 128 meters. Specifically, the lengths of the first set of fiber delay rings are configured as follows: first delay ring 70 = 0 meters, second delay ring 71 = 32 meters, third delay ring 72 = 64 meters, and fourth delay ring 73 = 96 meters. The second set of fiber delay rings can be further subdivided into the required length ranges. The lengths of the second set of fiber delay rings are configured as follows: fifth delay ring 74 = 0 meters, sixth delay ring 75 = 8 meters, seventh delay ring 76 = 16 meters, and eighth delay ring 77 = 24 meters. The lengths of the third set of fiber delay rings are configured as follows: ninth delay ring 82 = 0 meters, tenth delay ring 83 = 2 meters, eleventh delay ring 84 = 4 meters, and twelfth delay ring 85 = 6 meters. This allows for dynamic encoding of the quaternary delay fiber length within a 128-meter range, enabling rapid matching of the auxiliary interferometer delay fiber length required for testing.

Claims

1. A delay fiber length encoding device for an auxiliary interferometer used in OFDR, comprising a light source module (1), an auxiliary interferometer and an adaptive matched optical path encoding module (2), a main interferometer module (3), and a host computer signal processing and control module (4), characterized in that: The tunable laser source (101) generates a linear sweeping continuous light whose frequency changes with time at a rate γ. After passing through the first coupler (102), the continuous sweeping light signal is split into two paths. One path enters the main interferometer module (3) for acquiring information about the device under test, and the other path enters the auxiliary interferometer and adaptive optical path matching encoding module (2) for acquiring the nonlinear information of the tunable laser source (101) sweeping frequency, thereby improving the device test strain demodulation accuracy. The optical path encoding module mainly consists of n optical switches and n-1 sets of delay fibers. Depending on the different optical switch paths controlled by the host computer, the encoding module can achieve an arbitrary encoding method, thereby arbitrarily configuring the delay fibers. At the start of the test, the host computer calculates the optical path difference between the two arms of the optimal auxiliary interferometer corresponding to the fiber under test based on the length of the fiber under test, and then automatically adjusts the output optical path of the optical path encoding module to achieve adaptive matching, thereby optimizing the test accuracy to the greatest extent. Assuming the optical switch type used in the device is an m-channel optical switch, the corresponding delay fiber length of the delay fiber length encoding module is encoded in m-ary mode; the maximum length of the delay fiber required for the auxiliary interferometer in this test is set to L, as calculated. The formula for calculating the length of the m-path delay fiber of optical switch No. 1 (202) is as follows: The delay fiber lengths between each path of optical switch 1 (202) are divided into m intervals with an interval of x1. The lengths of the m segments of delay fiber in optical fiber group 1 (207) are set as: 0, x1, 2x1, ..., (m-1)x1. Optical switch 1 (202) initially divides the delay fiber of the auxiliary interferometer into m-1 intervals, and optical switch 2 (203) further refines the delay fiber length intervals. The formula for calculating the length of the m paths of optical fiber in optical switch 2 (203) is as follows: That is, the length of the delay fiber between each path of optical switch 2 (203) is divided into m intervals with an interval of x2. The lengths of the m segments of delay fiber in optical fiber group 2 (208) are set as: 0, x2, 2x2, ..., (m-1)x2 respectively. The configuration of intermediate optical switch (205) and intermediate delay fiber group (209) is the same as that of optical switch 1 (202) and optical switch 2 (203). The path and number of optical switches are selected according to the estimated length of the longest auxiliary interferometer delay fiber required in the actual test process. The minimum delay fiber group (210) of the nth optical switch (206) is the most precise adjustment switch for the delay fiber length encoding device of the auxiliary interferometer; the formula for calculating the length of the m-path delay fibers of the nth optical switch (206) is as follows: That is, the length of the delay fiber between each path of optical switch n (206) is x n The interval is divided into m intervals, and the lengths of the m delay fiber segments in the minimum delay fiber group (210) are respectively set as: x n 2x n ..., m·x n This ensures that when all n optical switches are selected from the maximum length of each group of delay fibers, the maximum auxiliary interferometer delay fiber length is reached during the pre-estimated calculation, thereby achieving high-precision auxiliary interferometer length encoding. By connecting the optical path coding module to one arm of the auxiliary interferometer module, the optical frequency domain reflectometer auxiliary interferometer can extract the nonlinear information of the light source sweep frequency, and convert the light source sweep frequency information into the interference fringes of the auxiliary interferometer, which are then extracted by the first balanced detector (211) and converted into electrical signals input into the host computer signal processing and control module (4).

2. The auxiliary interferometer delay fiber length encoding device for OFDR according to claim 1, characterized in that: The test optical signal input to the main interferometer is split into two paths by the second coupler (301). One path is input to the reference arm of the main interferometer. The polarization controller (302) ensures that the intensity of the optical signal in the P and S directions in the interferometer is equal, thereby achieving the optimal test accuracy. The other path enters the test arm and is input from the first port of the circulator (303) and output from the second port. The optical information of the device under test is reflected and output from the third port of the circulator (303). After interfering with the reference signal, the two paths are collected by the second balanced detector (305) and converted into an electrical signal, thereby extracting the optical information of the device under test. The host computer signal processing and control module (4) includes a data acquisition card (401), a computer (402), a first balanced detector (211), and a second balanced detector (305). The acquired interference fringe signal is converted into an electrical signal and then input into the signal acquisition card (401) for recording and then into the computer (402) for data processing. The computer (402) is also responsible for controlling each optical switch in the adaptive matching optical path coding device of the auxiliary interferometer, thereby realizing the switching of the delay fiber of the auxiliary interferometer of different lengths.

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