High-precision distributed birefringence measuring device based on frequency sweep interference

Abstract

The invention discloses a high-precision distributed birefringence measurement device based on frequency sweep interference. The high-precision distributed birefringence measurement device comprises a tunable laser source, an auxiliary interferometer module, a measurement interferometer module, a polarization maintaining optical fiber module to be measured and a data acquisition and processing module. The method is characterized in that independent inquiry of a polarization orthogonal P axis and a polarization orthogonal S axis of a polarization-maintaining optical fiber to be measured is realized by designing a complete polarization multiplexing measurement interferometer, and scattering signals of the polarization orthogonal axis are clearly separated by adopting a polarization beam splitter, so that linear birefringence is inverted by representing internal stress between the P axis and the S axis. Based on a stress birefringence generation mechanism and a sweep frequency interference measurement principle, compared with an existing distributed birefringence measurement method, the device can avoid mutual interference of polarization orthogonal axes, and lays a foundation for long-distance and high-precision distributed birefringence measurement; the method has advantages in core technical indexes such as birefringence measurement spatial resolution, measurement precision and the like, and takes second-level measurement speed into account. The device can be widely applied to quantitative testing and evaluation of important indexes such as polarization maintaining performance and birefringence uniformity of various types of polarization maintaining optical fibers and polarization maintaining optical fiber devices.

Description

Technical Field

[0001] The invention belongs to the technical field of optical device measurement, and in particular relates to a high-precision distributed birefringence measurement device based on swept frequency interferometry. Background Art

[0002] Polarization Maintaining Fiber (PMF) is a specially designed single-mode optical fiber that achieves stable optical signal transmission by controlling and maintaining the polarization state of the transmitted light, ensuring the quality and transmission efficiency of the optical signal, and enabling the optical communication system to better cope with complex transmission environments. Therefore, it is widely used in optical communications, optical sensors, optical devices and other fields.

[0003] Birefringence is one of the key parameters of polarization-maintaining optical fiber. In optical fiber communication, the birefringence characteristics in the optical fiber will cause the polarization state of the optical signal to change, thus affecting the transmission and demodulation of the optical signal. A high-precision polarization-maintaining optical fiber birefringence measurement device can better study the birefringence phenomenon, control the optical signal transmission characteristics in the optical fiber, and thus improve the performance and stability of the optical fiber communication system.

[0004] Birefringence measurement technology can be divided into two types: lumped and distributed birefringence measurement technology. The existing lumped birefringence measurement technologies mainly include dynamic pressurization method, magneto-optical demodulation method, white light interferometry method, broadband light source method, Sagnac interferometry method, etc. For example, Zhao Yao and others from the 41st Institute of China Electronics Technology Group used interference technology to measure the geometric length between the two stress-applied points of the polarization-maintaining optical fiber to be tested and the optical length between the corresponding coupling points, which greatly improved the measurement accuracy of the beat length of the polarization-maintaining optical fiber (A polarization-maintaining optical fiber beat length measurement device and measurement method, 202010760887.X). Although the above method can achieve high-precision birefringence measurement, it can only test the average birefringence or beat length of 1~2 meters of optical fiber, and cannot achieve distributed measurement, which is far from meeting the high-precision and distributed birefringence measurement requirements of polarization-maintaining devices such as polarization-maintaining optical fiber and optical fiber sensitive ring.

[0005] The current distributed birefringence measurement technology can be divided into distributed measurement technology based on Brillouin scattering or Rayleigh scattering. In 2012, Dong Yongkang and others from Harbin Institute of Technology used Brillouin Dynamic Gratings (BDG) measurement technology to achieve birefringence distribution measurement of polarization-maintaining optical fibers with a length of more than 500m, and overcame the problem of long measurement time (CN201210059817.7). In 2020, the same method was used to achieve birefringence measurement of polarization-maintaining optical fibers with a length of 3030m at a spatial resolution of 40cm (Optics Express. 27(2):951-964), and the measurement time was about tens of minutes.

[0006] The birefringence / beat length measurement technology based on Rayleigh scattering mainly includes: In 2019, Rao Yunjiang and others from the University of Electronic Science and Technology of China proposed a distributed birefringence measurement method based on CP-ΦOTDR (CN 201910430425.9). This method introduces chirped pulses for measurement. Only a single pulse and an incident light of any polarization state are required to be input into the sensing optical fiber to be measured to achieve distributed birefringence measurement.

[0007] Of the two methods mentioned above, the birefringence distribution measurement technology based on Brillouin scattering usually has a complex device and requires careful design of the pump pulse and the detection pulse to satisfy the conditions for sufficient Brillouin scattering in order to obtain a strong Brillouin signal. On the one hand, the spatial resolution is intrinsically limited by the phonon lifetime, and only sub-meter measurements can be achieved; on the other hand, limited by the non-local effect, the birefringence measurement length and measurement accuracy are restricted. The longer the test distance, the more serious the accuracy degradation, and even distortion at the far end. The distributed birefringence measurement technology based on CP-ΦOTDR also has the problem of insufficient spatial resolution. Limited by the time domain positioning method, it can only achieve sub-kilometer measurement lengths and meter-level spatial resolutions, which is far from meeting the current testing requirements for polarization-maintaining optical fibers and devices.

[0008] Relatively speaking, the distributed birefringence measurement technology based on Rayleigh scattering detection in the optical frequency domain has a higher spatial resolution. In 2015, Zhou et al. (Optics Communications, 2015, (345): 62-66) used the Rayleigh scattering intensity correlation to obtain the birefringence delay, thereby characterizing the distributed birefringence. Although its spatial resolution reached 7.8 cm, the measurement length was only 1.2 m and the measurement accuracy was only 2×10 -5 The main problem is that the information of the orthogonal axes of polarization is aliased, resulting in a low signal-to-noise ratio in the process of identifying the propagation constant difference, and the spatial resolution, accuracy and length of the birefringence measurement are limited.

[0009] In view of the problem of insufficient spatial resolution and precision in the current polarization-maintaining fiber birefringence measurement, the present invention discloses a high-precision measurement device for polarization-maintaining fiber birefringence based on swept-frequency interferometry. The design concept of the device is: utilizing the high resolution and high precision advantages of swept-frequency interferometry measurement, proposing the use of swept-frequency interferometry to achieve the spatial position positioning of the optical fiber, and realizing the high-precision measurement of distributed birefringence by characterizing the linear birefringence through the stress birefringence effect. The device adopts a fully polarization-multiplexed measurement interferometer 30 to simultaneously and independently query and resolve the polarization orthogonal axes of the polarization-maintaining optical fiber 50 to be measured, thereby avoiding the degradation of the birefringence measurement accuracy and spatial resolution caused by the mutual entanglement of the polarization orthogonal axis signals. The present invention is based on the principle of swept-frequency interferometry and has the advantages of high measurement spatial resolution, high measurement accuracy, and fast test speed; the multiplexed optical path structure is simple, and the stability and consistency of the polarization orthogonal axis measurement are good. It can be widely used in the quantitative testing and evaluation of important indicators such as polarization maintenance performance and birefringence uniformity of various types of polarization-maintaining optical fibers and polarization-maintaining optical fiber devices. Summary of the invention

[0010] The purpose of the present invention is to provide a high-precision distributed birefringence measurement device based on swept frequency interferometry, which solves the problem that it is difficult to achieve high-precision measurement of the birefringence of polarization-maintaining optical fiber in the background technology.

[0011] The present invention provides a high-precision distributed birefringence measurement device based on swept frequency interference, comprising a tunable laser source 10, an auxiliary interferometer module 20, a measuring interferometer module 30, a data acquisition and processing module 40, and a polarization-maintaining optical fiber 50 to be measured, wherein the frequency modulated continuous light wave generated by the tunable laser source 10 is divided into two beams by a first polarization-maintaining coupler 101 and respectively injected into the auxiliary interferometer module 20 and the measuring interferometer module 30; a light beam with a small amount of energy is output from a first output end 101a of the first polarization-maintaining coupler and injected into the auxiliary interferometer module 20 to form an auxiliary interference signal; a light beam with most of the energy is output from a second output end 101b of the first polarization-maintaining coupler and injected into the measuring interferometer module 30; the measuring interferometer module 30 is connected to the polarization-maintaining optical fiber 50 to be measured. The interference electrical signals output from the auxiliary interferometer module 20 and the measuring interferometer module 30 are transmitted to the data acquisition and processing module 40, and the distributed birefringence measurement result of the polarization-maintaining optical fiber 50 to be measured is obtained after data processing and demodulation.

[0012] The auxiliary interferometer module 20 is characterized in that the auxiliary interferometer module 20 is an unbalanced Michelson or Mach-Zehnder interferometer, and the interferometer delay is not less than:

[0013] (2)

[0014] Wherein, n and L are the refractive index and length of the polarization-maintaining optical fiber 50 to be measured, and c is the speed of light in vacuum.

[0015] The measuring interferometer module 30 is characterized in that: the frequency modulated continuous light wave is output from the second output end 101b of the first polarization-maintaining coupler and then enters the measuring interferometer module 30; first, the polarization state of the injected light is adjusted to 45° through the 45° polarizer 301, and then it is injected into the second polarization-maintaining coupler 302 and is divided into two beams; one of the light signals is output from the first output end 302a of the second polarization-maintaining coupler and then passes through the third polarization-maintaining coupler 304; the other light is output from the second output end 302b of the second polarization-maintaining coupler and then enters the first port 303a of the polarization-maintaining circulator; the light is injected into the polarization-maintaining optical fiber 50 to be measured from the second port 303b of the polarization-maintaining circulator; then, part of the reflected signal light is injected into the measuring interferometer module 30 again from the third port 303c of the polarization-maintaining circulator, and then is input into the third polarization-maintaining coupler 304; the two light signals are injected into the third polarization-maintaining coupler 3 04, and its interference signal is divided into two beams; the two optical signals are received by the first polarization beam splitter 305 and the second polarization beam splitter 306 respectively, and the P-axis and S-axis optical signals of the polarization-maintaining optical fiber to be measured in the interference signal are separated; the optical signal output from the first output end 305a of the first polarization beam splitter and the optical signal output from the second output end 306b of the second polarization beam splitter enter the first photoelectric balanced detector 307 for differential detection, and the optical signal output from the first output end 306a of the second polarization beam splitter and the optical signal output from the second output end 305b of the first polarization beam splitter enter the second photoelectric balanced detector 308 for differential detection; finally, the optical signal from the output end 307c of the first photoelectric balanced detector and the optical signal from the output end 308c of the second photoelectric balanced detector enter the data acquisition and processing module 40 for data acquisition and demodulation.

[0016] The data acquisition and processing module 40 is characterized in that: the third acquisition unit 403 in the data acquisition and processing module 40 acquires the auxiliary interference signal generated by the auxiliary interference module 20, and transmits it to the PC end 404 after completion; the first acquisition unit 401 and the second acquisition unit 402 respectively acquire the interference signals of the P axis and the S axis of the measured polarization-maintaining optical fiber output by the measurement interferometer module 30; then the two groups of interference signals are also transmitted to the PC end 404, and after data processing and demodulation, high-precision measurement of the birefringence of the measured polarization-maintaining optical fiber 50 can be achieved.

[0017] The 45° polarizer can adjust the polarization state of the injected optical signal to 45°, so that the optical signal is uniformly injected into the P axis and S axis of the polarization-maintaining optical fiber.

[0018] The calculation formula of birefringence of Panda-type polarization-maintaining fiber is expressed as follows:

[0019] (3)

[0020] Where c is the photoelastic coefficient; E is Young's modulus; is the strain coefficient; R is the radius of the stress area; d is the distance from the stress area to the center of the optical fiber core; r is the radius of the optical fiber cladding; Δα is the difference in linear expansion coefficient between the stress area and the cladding material; ΔT is the difference between the optical fiber stretching temperature and the space temperature.

[0021] The resampling method is used to correct the phase noise of the P-axis and S-axis signals of the polarization-maintaining fiber through auxiliary interference signals; then the RBS spectra of the two axes are correlated to solve the internal stress between the axes; finally, the linear relationship between the relative stress and birefringence between the two orthogonal polarization axes of the polarization-maintaining fiber is used to characterize the birefringence characteristics of the optical fiber to be tested.

[0022] The linear relationship between birefringence and internal strain between the orthogonal polarization axes of polarization-maintaining optical fibers is calculated according to the following formula:

[0023] (4)

[0024] Therefore, the spatial resolution of the distributed birefringence measurement achieved by this method depends on the scanning range of the frequency modulated continuous light wave, which can usually reach the centimeter level; the measurement accuracy depends on the wavelength recognition accuracy of the light source and demodulation process, which can reach 10 -7 Magnitude.

[0025] Compared with the prior art, the advantages of the present invention are:

[0026] The present invention has significant advantages in the performance indicators of distributed birefringence measurement. The device is based on the principle of swept frequency interferometry and has the advantages of high measurement spatial resolution and high measurement accuracy compared with existing time domain methods such as BOTDA and CP-OTDR.

[0027] The present invention designs a fully polarization-multiplexed measurement interferometer that can simultaneously and independently query polarization orthogonal axes, avoiding performance degradation caused by the mutual entanglement of polarization orthogonal axis signals, improving the stability and consistency of polarization orthogonal axis measurement, and simplifying the optical path structure through multiplexing.

[0028] The invention is based on the stress birefringence principle and only requires a wavelength scan of several seconds to obtain birefringence distribution information, with a fast test speed; it is suitable for birefringence distribution measurement of various polarization-maintaining optical fibers and polarization-maintaining devices and has excellent applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To more clearly illustrate the device design of the embodiments of the present invention, the following briefly introduces the drawings of the embodiments of the present invention, wherein the drawings are only used to illustrate some embodiments of the present invention, but not to limit all embodiments of the present invention thereto.

[0030] Figure 1 It is a high-precision distributed birefringence measurement device based on swept frequency interferometry;

[0031] Figure 2 This is a specific embodiment diagram of high-precision polarization-maintaining optical fiber distributed birefringence measurement;

[0032] Figure 3 It is the time domain signal diagram of the P-axis and S-axis of the polarization-maintaining optical fiber to be tested. DETAILED DESCRIPTION

[0033] In order to more clearly explain the application objectives, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with embodiments and drawings, but the protection scope of the present invention shall not be limited thereby. Specific embodiment 1

[0035] like Figure 1 The figure shows a high-precision measurement device for birefringence of a polarization-maintaining optical fiber 50 to be measured based on swept frequency interferometry. The selection and parameters of the main optoelectronic components of the device are as follows:

[0036] The line width of the tunable light source 10 is less than 60 kHz, the wavelength tuning range is set to be 1545 nm to 1555 nm, the sweep rate is 10 nm / s, and the sweep time is 1 s.

[0037] The maximum detection bandwidth of the first balanced photodetector 307 and the second balanced photodetector 308 are both 80 MHz, the saturated differential detection power is 55 μW, and the common mode rejection ratio is 25 dB.

[0038] The sampling rates of the first acquisition unit 401 , the second acquisition unit 402 and the third acquisition unit 403 are all set to 180 MHz / s.

[0039] The first coupler 101, the second coupler 302 and the third coupler 304 are all polarization-maintaining couplers. The splitting ratio of the first polarization-maintaining coupler 101 is 98:2, the splitting ratio of the second polarization-maintaining coupler 302 is 99:1, and the splitting ratio of the third polarization-maintaining coupler is 50:50.

[0040] The auxiliary interferometer module 20 may adopt a Mach-Zehnder interferometer, the extended optical fiber length is 130 m, and the refractive index of the used single-mode optical fiber is 1.456.

[0041] The frequency modulated continuous light wave is emitted from the tunable laser source 10, and is split into two beams by the first polarization-maintaining coupler 101 and then injected into the auxiliary interferometer module 20 and the measuring interferometer module 30 respectively;

[0042] A beam of light signal is output from the first output end 101a of the first polarization-maintaining coupler and injected into the auxiliary interferometer module 20 to form an auxiliary interference signal, and then injected into the data acquisition and processing module 40; another beam of light signal is output from the second output end 101b of the first polarization-maintaining coupler and injected into the measurement interferometer module 30;

[0043] After entering the measuring interferometer module, the injected light will first pass through the 45° polarizer 301 to adjust the polarization state of the injected light to 45°, and then pass through the second polarization-maintaining coupler 302 to be divided into two beams; one of the light signals enters the reference arm of the measuring interferometer and is directly injected into the third polarization-maintaining coupler 304; the other light passes through the polarization-maintaining circulator 303 and is injected into the polarization-maintaining optical fiber module 50 to be measured, and the reflected signal light passes through the polarization-maintaining circulator 303 again to enter the measuring interferometer module 30, and then is input into the third polarization-maintaining coupler 304; the two light signals interfere in the third polarization-maintaining coupler 304, and the interference signal is divided into two beams; the two light signals are received and transmitted by the first polarization beam splitter 305 and the second polarization beam splitter 306 respectively. The P-axis and S-axis optical signals of the polarization-maintaining optical fiber to be tested in the interference signal are separated; the optical signal output from the first output end 305a of the first polarization beam splitter and the optical signal output from the second output end 306b of the second polarization beam splitter enter the first photoelectric balanced detector 307 for differential detection, and the optical signal output from the first output end 306a of the second polarization beam splitter and the optical signal output from the second output end 305b of the first polarization beam splitter enter the second photoelectric balanced detector 308 for differential detection; finally, the optical signal from the output end 307c of the first photoelectric balanced detector and the optical signal from the output end 308c of the second photoelectric balanced detector are respectively collected by the collection unit 402 and the collection unit 403 in the data collection and processing module 40.

[0044] The beat frequency signal output by the auxiliary interference module 20 is used to determine the resampling points with equal frequency intervals, and then the two groups of interference signals of the P axis and the S axis are processed by the interpolation algorithm to obtain their signal values ​​at the resampling points, and finally the resampled interference signals with equal frequency intervals are obtained.

[0045] The polarization-maintaining fiber to be tested is a panda-type polarization-maintaining fiber, the cladding radius r of which is 62.5 μm; the stress zone radius R is 12.6 μm; the distance d from the stress zone to the core is 23.9 μm; the Young's modulus E is 65 GPa; the photoelastic coefficient c is 4.95×10 -12 ; Poisson's ratio It is 0.186.

[0046] Therefore, the calculation formula for the birefringence of the polarization-maintaining optical fiber to be measured can be calculated as follows:

[0047] (5)

[0048] The relative wavelength offset between the P-axis and S-axis of the polarization-maintaining fiber is obtained by cross-correlation demodulation. , by substituting it into the calculation formula, we can get the high-precision measurement result of the birefringence of the polarization-maintaining optical fiber to be measured.

[0049] The above embodiment describes in detail an implementation of a high-precision distributed birefringence measurement device based on swept frequency interferometry proposed by the present invention. However, for those skilled in the art, various modifications and variations can be made to the above specific embodiments without departing from the principles of the present invention, and various technical features and structures proposed by the present invention can be combined in different ways, but they do not exceed the protection scope of the present invention, and these should all be regarded as the protection scope of the present invention.

Claims

1. A high-precision distributed birefringence measurement device based on swept frequency interferometry, comprising a tunable laser source (10), an auxiliary interferometer module (20), a measurement interferometer module (30), a data acquisition and processing module (40), and a polarization-maintaining optical fiber to be measured (50), wherein: The frequency modulated continuous light wave generated by the tunable laser source (10) is divided into two beams by a first polarization-maintaining coupler (101) and respectively injected into an auxiliary interferometer module (20) and a measuring interferometer module (30); a light beam with a small amount of energy is output from a first output end (101a) of the first polarization-maintaining coupler and injected into the auxiliary interferometer module (20) to form an auxiliary interference signal; a light beam with most of the energy is output from a second output end (101b) of the first polarization-maintaining coupler and injected into the measuring interferometer module (30); the measuring interferometer module (30) is connected to a polarization-maintaining optical fiber (50) to be measured. The interference electrical signals output from the auxiliary interferometer module (20) and the measuring interferometer module (30) are transmitted to a data acquisition and processing module (40), and after data processing and demodulation, a distributed birefringence measurement result of the polarization-maintaining optical fiber (50) to be measured is obtained.

2. The auxiliary interferometer module (20) according to claim 1, characterized in that: The auxiliary interferometer module (20) is an unbalanced Michelson or Mach-Zehnder interferometer, and its interferometer delay is not less than , where n and L are the refractive index and length of the polarization-maintaining optical fiber (50) to be tested, is the speed of light in a vacuum.

3. The measuring interferometer module (30) according to claim 1, characterized in that: The frequency modulated continuous light wave is outputted from the second output end (101b) of the first polarization-maintaining coupler and then enters the measuring interferometer module (30); the polarization state of the injected light is first adjusted to 45° through a 45° polarizer (301), and then injected into the second polarization-maintaining coupler (302) and split into two beams; one of the light signals is outputted from the first output end (302a) of the second polarization-maintaining coupler and then passes through a third polarization-maintaining coupler (304); the other light beam is outputted from the second output end (302b) of the second polarization-maintaining coupler and then enters the first port (303a) of the polarization-maintaining circulator; Light is injected from the second port (303b) of the polarization-maintaining circulator into the polarization-maintaining optical fiber (50) to be tested; then part of the reflected signal light is injected again from the third port (303c) of the polarization-maintaining circulator into the measuring interferometer module (30) and then input into the third polarization-maintaining coupler (304); two light signals interfere in the third polarization-maintaining coupler (304), and the interference signal is split into two light signals; the two light signals are respectively received by the first polarization beam splitter (305) and the second polarization beam splitter (306), and the P-axis and S-axis light signals of the polarization-maintaining optical fiber to be tested in the interference signal are separated; and the first output end (303c) of the first polarization beam splitter (304) is output from the first output end (303d) of the first polarization beam splitter. 5a) and the light signal output from the second output end (306b) of the second polarization beam splitter enter the first photoelectric balanced detector (307) for differential detection; the light signal output from the first output end (306a) of the second polarization beam splitter and the light signal output from the second output end (305b) of the first polarization beam splitter enter the second photoelectric balanced detector (308) for differential detection; finally, the light signal at the output end (307c) of the first photoelectric balanced detector and the light signal at the output end (308c) of the second photoelectric balanced detector enter the data acquisition and processing module (40) for data acquisition and demodulation.

4. The data acquisition and processing module (40) according to claim 1, characterized in that: The third acquisition unit (403) in the data acquisition and processing module (40) acquires the auxiliary interference signal generated by the auxiliary interference module (20), and transmits it to the PC end (404) after completion; the first acquisition unit (401) and the second acquisition unit (402) respectively acquire the interference signals of the P axis and the S axis of the polarization-maintaining optical fiber to be measured output by the measurement interferometer module (30); then the two groups of interference signals are also transmitted to the PC end (404), and after data processing and demodulation, high-precision measurement of the birefringence of the polarization-maintaining optical fiber to be measured (50) is achieved.

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