High-precision distributed birefringence testing method based on spectrum correlation

Abstract

The invention discloses a high-precision distributed birefringence testing method based on spectrum correlation, and belongs to the technical field of optical device measurement. The method is characterized in that on the basis of a polarization-maintaining fiber stress birefringence generation mechanism and a spectrum-related strain demodulation principle, internal strain between polarization orthogonal axes of the polarization-maintaining fiber is demodulated, so that a distributed birefringence value is obtained through conversion. The method comprises the following steps of: firstly, performing phase noise compensation on interference signals of orthogonal polarization axes P and S of a polarization maintaining optical fiber to be measured by using an auxiliary interference signal obtained by measurement, and then converting the interference signals of the P and S axes to a frequency domain through Fourier transform; secondly, the corresponding positions of the P axis and the S axis are intercepted through a sliding window with the length being delta z; performing inverse Fourier transform on spatial domain signals corresponding to the P-axis spatial position and the S-axis spatial position so as to transform the spatial domain signals into a spectral domain; and finally, performing mutual correlation on the corresponding signals of the P axis and the S axis. The internal stress between the P axis and the S axis of the polarization maintaining optical fiber to be measured can be calculated, and the birefringence characteristic of the polarization maintaining optical fiber to be measured can be represented by using the linear relation between the internal stress and birefringence. The method has advantages in core technical indexes such as birefringence measurement spatial resolution and measurement precision, and can be widely applied to birefringence quantitative test and evaluation of various types of polarization maintaining optical fibers and polarization maintaining optical fiber devices.

Description

Technical Field

[0001] The invention belongs to the field of distributed optical fiber measurement and relates to a high-precision distributed birefringence testing method based on spectrum correlation. 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 problems of insufficient spatial resolution and precision in the current polarization-maintaining fiber birefringence measurement, the present invention discloses a high-precision distributed birefringence test method based on spectral correlation. Based on the generation mechanism of stress birefringence of polarization-maintaining fiber and the principle of strain demodulation related to the spectrum, the internal strain between the orthogonal axes of polarization of the polarization-maintaining fiber is demodulated to obtain the distributed birefringence value. The birefringence test based on the spectral correlation method only requires a wavelength scan of several seconds to achieve the measurement of distributed birefringence, and has advantages in demodulation accuracy and spatial resolution, and the demodulation process is simple. Summary of the invention

[0010] The purpose of the present invention is to provide a high-precision distributed birefringence testing method based on spectral correlation, which solves the difficulty of achieving high-precision and high-resolution measurement of polarization-maintaining optical fiber birefringence in the background art.

[0011] 1. A high-precision distributed birefringence test method based on spectral correlation, characterized in that: based on the generation mechanism of stress birefringence of polarization-maintaining optical fiber and the strain demodulation principle related to the spectrum, the Rayleigh scattering signals of the orthogonal polarization axes of the optical fiber are respectively inquired, and then the demodulation of the spectral offset is realized by cross-correlating the Rayleigh scattering spectra of the orthogonal polarization axes, so as to calculate the distributed birefringence. The specific process is as follows:

[0012] Step 1 (S10): Polarization-maintaining fiber test. Connect one end of the polarization-maintaining fiber to be tested to the test device, perform a test, and obtain the beat frequency signals output by the auxiliary interference module (20). , and measuring the interference signals of the polarization orthogonal P-axis and S-axis output by the interferometer module (30) and ;

[0013] Step 2 (S20): Nonlinear phase noise correction. Use the beat frequency signal output by the auxiliary interference module (20) The interference signals of the polarization orthogonal P-axis and S-axis outputted by the measuring interferometer module (30) are and The nonlinear phase noise in the and ;

[0014] Step 3 (S30): Time-frequency transformation. The polarization orthogonal P-axis and S-axis signals after correction are transformed into and Perform Fourier transform to transform it from time domain to frequency domain and , obtain the back-Rayleigh scattering distribution of the polarization orthogonal P and S axes as a function of length;

[0015] Step 4 (S40): Spatial decoupling. Distributed backscattered Rayleigh signals of polarization orthogonal P and S axes are decoupled through a window with a length of Δz. and The data at the same location is intercepted;

[0016] Step 5 (S50): strain decoupling. The intercepted polarization orthogonal axis local Rayleigh scattering information is converted into the spectral domain through inverse Fourier transform, and the offset of the polarization orthogonal P-axis and S-axis scattering spectra is further detected using a cross-correlation algorithm. At this time, the position of the cross-correlation peak is recorded as the wavelength offset. , which is converted into the internal strain of the polarization-maintaining fiber through the following formula:

[0017] (2)

[0018] In the formula, is the central wavelength of the scan, is the strain coefficient, which is determined by the propagation constant, Poisson's ratio, elastic-optical coefficient, etc.;

[0019] Step 6 (S60): Based on the birefringence and the strain in the polarization-maintaining optical fiber The linear relationship is obtained to obtain the birefringence value of the polarization-maintaining optical fiber; and steps 4 (S40) to 6 (S60) are repeated to slide the window and demodulate the birefringence distribution of each point along the length of the optical fiber one by one, thereby realizing high-precision testing of distributed birefringence.

[0020] The linear relationship between the birefringence and the internal strain between the polarization-maintaining optical fiber's orthogonal P and S axes can be described as:

[0021] Taking the Panda-type polarization-maintaining fiber as an example, the specific calculation is based on the following formula:

[0022] (3)

[0023] Where c is the photoelastic coefficient; E is Young's modulus; R is the radius of the stress region; d is the distance from the stress region 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 region and the cladding material; ΔT is the difference between the optical fiber stretching temperature and the space temperature;

[0024] In the calculation formula of polarization-maintaining fiber birefringence, except Δα and ΔT, other parameters are structural parameters of the fiber, so only Δα and ΔT need to be measured to obtain the birefringence of the corresponding polarization-maintaining fiber. The strain on the orthogonal polarization axes P and S of the polarization-maintaining fiber can also be expressed by the product of the coefficient difference Δα and the temperature difference ΔT. The specific formula is as follows:

[0025] (4)

[0026] Substituting formula (4) into formula (3), we get the expression of birefringence and internal stress of Panda-type polarization-maintaining fiber:

[0027] (5)

[0028] Compared with the existing testing methods, the advantages of the present invention are:

[0029] The present invention has a higher spatial resolution of birefringence testing; compared with the aforementioned BDG, CP-OTDR and other methods, the spatial resolution of this method depends on the wavelength scanning range of the light source, and in theory can achieve birefringence testing with a maximum spatial resolution of millimeters.

[0030] The present invention has higher birefringence test accuracy; compared with methods such as BDG based on Brillouin gain intensity detection, this method detects phase based on the spectral correlation principle and has higher accuracy. At the same time, compared with the Rayleigh scattering intensity correlation detection method, it has a better cross-correlation signal-to-noise ratio and thus has higher accuracy.

[0031] The present invention realizes measurement based on Rayleigh scattering, has good stability, and does not require fine parameter adjustment of the BDG method; it only requires one second-level wavelength scan to realize the measurement of distributed birefringence, has high test efficiency, and can be applied to the test of dynamic temperature characteristics of optical fiber sensitive rings, transverse stress sensing based on polarization-maintaining optical fiber, and other scenarios; the demodulation process based on spectral correlation is simple and the solution rate is fast. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] 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.

[0033] Figure 1 It is a flow chart of a high-precision measurement method of birefringence of polarization-maintaining optical fiber based on swept frequency interferometry;

[0034] Figure 2 It is a detailed flow chart of a high-precision measurement method for birefringence of polarization-maintaining optical fiber based on swept frequency interferometry;

[0035] Figure 3 This is a diagram of a high-precision distributed birefringence measurement device;

[0036] Figure 4 It is a high-precision distributed measurement result of the birefringence of a polarization-maintaining fiber ring. DETAILED DESCRIPTION

[0037] 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 should not be limited thereby. Specific embodiment 1

[0039] like Figure 1 The figure shows a high-precision measurement device for polarization-maintaining optical fiber birefringence based on swept frequency interferometry used in this embodiment. The selection and parameters of the main optoelectronic devices in the device are as follows:

[0040] The light source 10 is a narrow linewidth tunable laser source, the linewidth of the light source is 70kHz, the wavelength tuning range is set to 1545nm~1555nm, the sweep rate is 10nm / s, and the sweep time is 1s;

[0041] The maximum detection bandwidth of the first balanced photodetector 205, the second balanced photodetector 307 and the third balanced photodetector 308 is 80 MHz, and the peak-to-peak value of the electrical noise in the time domain V P-P <30 mV with a conversion gain of 4×104 V / W.

[0042] The sampling rate of acquisition card 401 is set to 180MHz / s. After compensation, its effective number of bits is 8.6 bits, the spurious-free dynamic range is 67.4 dB, and the no-load electrical noise is V P-P =2.8 mV.

[0043] The first coupler 101, the fourth coupler 302 and the fifth coupler 304 are all polarization-maintaining couplers, and the remaining couplers are all single-mode couplers. The splitting ratio of the first polarization-maintaining coupler 101 is 98:2, the splitting ratio of the fourth polarization-maintaining coupler 301 is 99:1, and the splitting ratios of the other couplers are all 50:50.

[0044] The auxiliary interferometer module 20 adopts a Mach-Zehnder interferometer, in which the extended optical fiber is set to have a length of 130 m and the refractive index of the used single-mode optical fiber is 1.456.

[0045] The polarization-maintaining fiber to be tested is a panda-type polarization-maintaining fiber, whose cladding radius r 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; and the photoelastic coefficient c is 4.95×10 -12 ; Poisson's ratio It is 0.186.

[0046] The specific implementation of the method for high-precision measurement of birefringence of polarization-maintaining optical fiber based on swept frequency interferometry of the present invention is as follows: a high-precision measurement device for high-precision measurement of birefringence of polarization-maintaining optical fiber based on swept frequency interferometry is as follows: Figure 3As shown, the continuous frequency sweeping light emitted by the light source 10 is divided into two beams by the first polarization-maintaining coupler 101, and 2% of the light signal is injected into the auxiliary interferometer module 20 to generate an auxiliary interference signal for eliminating the phase noise of the beat signal output by the measuring interferometer module 30; in addition, 98% of the light signal is injected into the measuring interferometer module 30, and the polarization state of the injected light is first adjusted to 45° by the 45° polarizer 301. Then, the polarized light is divided into two beams by the fourth polarization-maintaining coupler 302, and 1% of the light signal is injected into the reference arm; 99% of the light signal is injected into the measuring arm where the polarization-maintaining circulator 303 is located, and then enters the polarization-maintaining optical fiber 50 to be measured through the polarization-maintaining circulator 303, and the reflected signal light is merged again after passing through the polarization-maintaining circulator 302; after the main interference signal passes through the polarization beam splitters 305 and 306 respectively, the beat signals of the orthogonal polarization axes P and S of the polarization-maintaining optical fiber to be measured are separated, and enter the second photoelectric balanced detector 307 and the third photoelectric balanced detector 308 respectively to convert the light signal into an electrical signal. The auxiliary interference signal and the main interference signal generated by the main interference module 30 are respectively collected by the acquisition card 401 in the data acquisition and processing module 40, and the data is transmitted to the PC terminal 402 for processing using the method proposed by the present invention.

[0047] A high-precision measurement method for polarization-maintaining optical fiber birefringence based on swept frequency interferometry, the specific operation steps are as follows:

[0048] The first step is to measure and collect the auxiliary interference signal generated by the auxiliary interferometer module (20) and the interference signals of the polarization-maintaining optical fiber P axis to be measured generated by the measuring interferometer module (30) Interference signal with S axis ;

[0049] In the second step, a resampling method is used to measure the interference signals of the P-axis and S-axis of the polarization-maintaining optical fiber to be measured output by the interferometer module (30). and The phase noise in the filter is corrected; the beat frequency signal output by the auxiliary interference module (20) is used Determine the resampling points with equal frequency intervals, and then use the interpolation algorithm to interpolate the two sets of interference signals on the P axis and S axis. and Processing is performed to obtain the signal value at the resampling point, and finally a resampled interference signal with equal frequency intervals is obtained. and ; At this time, the beat frequency signal of the P axis It can be expressed as:

[0050]

[0051] in, It is to measure the amplitude of the interferometer beat frequency light signal; is the reflection coefficient; is the initial frequency of the light source; is the tunable rate of the light source; is the time delay between the polarization-maintaining fiber to be measured and the reference arm of the measuring interferometer;

[0052] The third step is to resample the interference signals of the P and S axes after phase noise compensation in the second step. and Perform fast Fourier transform to transform the time domain signal into the frequency domain signal and , thereby obtaining the Rayleigh backscattering spectra of the P and S axes of the tested polarization-maintaining optical fiber as the length of the tested polarization-maintaining optical fiber changes respectively;

[0053] The fourth step is to measure the frequency domain signals of the P-axis and S-axis of the polarization-maintaining optical fiber to be tested through a window with a length of Δz. and The corresponding positions are intercepted respectively;

[0054] The fifth step is to convert the intercepted frequency domain signals corresponding to the two axes into the spectral domain through inverse Fourier transform, and determine the relative wavelength offset of the fast and slow axes of the polarization-maintaining optical fiber to be tested in the spectral domain through cross-correlation demodulation. ;

[0055] In the sixth step, the linear relationship between the birefringence of the polarization-maintaining optical fiber to be measured and the internal strain between its orthogonal polarization axes F and S can be used to achieve high-precision distributed measurement of the birefringence of the Panda-type polarization-maintaining optical fiber.

[0056]

[0057] The above embodiments describe in detail some implementation methods of a method for high-precision measurement of birefringence of polarization-maintaining optical fiber 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 be regarded as the protection scope of the present invention.

Claims

1. A high-precision distributed birefringence testing method based on spectral correlation, characterized by: Based on the generation mechanism of stress birefringence of polarization-maintaining optical fiber and the principle of strain demodulation related to the spectrum, the Rayleigh scattering signals of the orthogonal axes of polarization of the optical fiber are queried respectively, and then the demodulation of the spectrum offset is realized by cross-correlating the Rayleigh scattering spectra of the orthogonal axes of polarization, so as to calculate the distributed birefringence. The specific process is as follows: Step 1 (S10): Polarization-maintaining fiber test. Connect one end of the polarization-maintaining fiber to be tested to the test device, perform a test, and obtain the beat frequency signals output by the auxiliary interference module (20). , and measuring the interference signals of the polarization orthogonal P-axis and S-axis output by the interferometer module (30) and ; Step 2 (S20): Nonlinear phase noise correction. Use the beat frequency signal output by the auxiliary interference module (20) The interference signals of the polarization orthogonal P-axis and S-axis outputted by the measuring interferometer module (30) are and The nonlinear phase noise in the and ; Step 3 (S30): Time-frequency transformation. The polarization orthogonal P-axis and S-axis signals after correction are transformed into and Perform Fourier transform to transform it from time domain to frequency domain and , obtain the back-Rayleigh scattering distribution of the polarization orthogonal P and S axes as a function of length; Step 4 (S40): Spatial decoupling. Distributed backscattered Rayleigh signals of polarization orthogonal P and S axes are decoupled through a window with a length of Δz. and The data at the same location is intercepted; Step 5 (S50): strain decoupling. The intercepted polarization orthogonal axis local Rayleigh scattering information is converted into the spectral domain through inverse Fourier transform, and the offset of the polarization orthogonal P-axis and S-axis scattering spectra is further detected using a cross-correlation algorithm. At this time, the position of the cross-correlation peak is recorded as the wavelength offset. , which is converted into the internal strain of the polarization-maintaining fiber through the following formula: （1） In the formula, is the central wavelength of the scan, is the strain coefficient, which is determined by the propagation constant, Poisson's ratio, elastic-optical coefficient, etc.; Step 6 (S60): Based on the birefringence and the strain in the polarization-maintaining optical fiber The linear relationship is obtained to obtain the birefringence value of the polarization-maintaining optical fiber; and steps 4 (S40) to 6 (S60) are repeated to slide the window and demodulate the birefringence distribution of each point along the length of the optical fiber one by one, thereby realizing high-precision testing of distributed birefringence.

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