A fast measurement device for fiber-optic distributed polarization crosstalk based on optical frequency domain interferometry

Abstract

The present invention belongs to the field of optical fiber measurement technology, and specifically relates to a distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interference, the device includes a tunable laser source module, a device to be measured module, a main interferometer module, an auxiliary interferometer module and a signal acquisition and analysis module, and is characterized in that: a tunable laser is used to perform fast wavelength scanning, a main interferometer is used to perform frequency shift on an interference beat frequency signal, and the position and intensity of polarization crosstalk are determined by obtaining the optical path difference between coupled light and transmitted light and the normalized interference signal amplitude, while also eliminating the signal aliasing problem caused by 1 / f flicker noise at low frequencies and light source frequency scanning nonlinearity. The present invention, combined with a tunable laser, gets rid of the dependence on an optical path scanning delay device in polarization crosstalk measurement, realizes an all-fiber structure without moving parts, enhances the reliability and stability of the test system, and greatly improves the speed and signal-to-noise ratio of polarization crosstalk measurement in principle.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber measurement, and specifically relates to an optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interference. Background Art

[0002] Polarization-maintaining fiber, also known as high birefringence fiber, increases the linear birefringence of the fiber by artificially adding stress to avoid the polarization influence caused by random fiber perturbations introduced by the external environment, thus maintaining and controlling the polarization state. Figure 2 The cross-sectional schematic diagram of the Panda-type polarization-maintaining fiber is described, which includes two orthogonal characteristic axes with different refractive indices, two stress rods and a core, in which the core is deliberately enlarged to clearly represent the light beam in it. During the manufacturing process, two stress rods are embedded on both sides of the core of the Panda-type polarization-maintaining fiber. Since the stress rods and the core have different thermal expansion coefficients, thermal stress is generated in the direction of the center line of the stress rods, and the refractive index in this direction increases, forming a slow axis. The characteristic axis perpendicular to the slow axis has a small refractive index, that is, the speed of light transmission in it is fast, so it forms a fast axis. The design of this polarization-maintaining fiber can produce sufficiently large linear birefringence, so that the optical path lengths of two orthogonal linear polarized lights transmitted in it are different in the same time, thereby forming an optical path difference, avoiding interaction and interference between the two beams of light, so that they can propagate in different polarization modes.

[0003] When the light wave passes through the coupling point (such as the fusion point, defect point, perturbation point), the linear polarized light working in the main axis will be coupled to its orthogonal axis. This phenomenon is called polarization crosstalk. By measuring the polarization crosstalk distribution along the transmission direction, the polarization performance of the device under test can be directly reflected, and the stress, temperature and other information of the external environment can be obtained. In addition, the polarization crosstalk test can not only be used to evaluate the performance of polarization-maintaining optical fiber and polarization devices and the influence of the external environment, but also provide necessary guidance for the production inspection of polarization-maintaining optical fiber, the winding process of optical fiber ring, and the precise fusion alignment between polarization devices.

[0004] Optical coherence domain polarization measurement (OCDP) based on the principle of white light interferometry is an important technology for distributed optical polarization crosstalk characteristic detection. It is mainly dedicated to the high-precision analysis of polarization crosstalk position and intensity, and then to test and evaluate the performance of polarization optical devices. OCDP technology has been widely used in the fields of high-precision alignment of polarization-maintaining optical fibers, device extinction ratio testing, and fiber gyroscope ring testing due to its advantages of high measurement sensitivity, high spatial resolution, and large dynamic range.

[0005] In terms of test speed and accuracy, in order to improve measurement efficiency and alleviate the problem of test speed and measurement accuracy being mutually restrained in optical coherence domain polarization measurement technology OCDP, in 2017, the applicant disclosed a device for scanning optical polarization devices using a common interference optical path (a device for simultaneously measuring the forward and reverse directions of a fiber optic gyroscope ring using a common optical path, 201710050099.X), which reduced the test time of the polarization crosstalk measurement device and thus improved the measurement efficiency. In terms of test range, the applicant disclosed in 2013 a structure of a differentially symmetrical optical path scanning delay device (an optical coherence domain polarization measurement device, 201310739314.9) and a device for calibrating optical delay lines (a large-range optical delay device, 201310739371.7), the former realizing automatic compensation of light intensity fluctuations while doubling the total scanning optical path, and the latter realizing continuous extension and scanning of the optical path. In terms of dynamic range, in 2016 the applicant disclosed a method for improving the dynamic range (A large dynamic range calibration method for optical coherence domain polarimeter, 201611073448.1). By changing the welding axis angle of the polarization-maintaining optical fiber, a dynamic range of more than 100 dB and high calibration accuracy were achieved.

[0006] However, in the existing continuously improved optical coherence domain polarization measurement technology OCDP scheme, there is still a trade-off between test speed and accuracy. In order to ensure the high accuracy of polarization crosstalk measurement, the speed of optical path scanning can only be sacrificed, and the mechanical programmable displacement stage can be used for slow moving scanning. Therefore, the problem of mutual restriction between optical path scanning accuracy and speed in optical coherence domain polarization measurement technology has not been fundamentally solved. At the same time, since the optical path scanning is controlled by a mechanical programmable displacement stage, the moving parts are susceptible to external environmental interference such as stress, vibration, temperature, etc., which in turn introduces small but non-negligible deformation and errors, and also increases the instability of the system. The method of developing an optical path scanning delay device to expand the test range will also increase the complexity of the system and bring additional errors. Although the measurement range can be doubled to a certain extent, the scanning range is still limited by the length of the optical fiber delay line. A large scanning range will inevitably bring about a large volume structure and instability of the device. In addition, due to the low energy of the wide-spectrum light source used in the optical coherence domain polarization measurement technology OCDP and the limited signal-to-noise ratio, the dynamic range has never been greatly improved.

[0007] Based on the above problems of slow speed and low signal-to-noise ratio in the measurement of polarization parameters in the coherent domain, scholars have tried to find solutions in the frequency domain and proposed polarization-sensitive optical frequency domain reflectometry (P-OFDR) to measure polarization parameters such as distributed birefringence and polarization mode dispersion in the optical frequency domain. At the same time, with its advantages of high sensitivity and long test distance, this technology also measures backward Rayleigh scattering signals and is extended to distributed vibration sensing and distributed acoustic wave sensing.

[0008] In 2011, Liu Tiegen and others from Tianjin University combined optical frequency domain reflection technology with polarization control and extraction technology (Polarization-sensitive distributed optical frequency domain reflection disturbance sensing device and demodulation method, 201110237417.6), and used the polarization solution method based on Jones and Mueller matrices to obtain polarization information in the sensing fiber. In 2014, this research group also proposed a white light frequency domain interferometry measurement system (A polarization-maintaining fiber birefringence dispersion measurement method, 201410261385.7), which eliminated the mechanical scanning device, collected frequency domain interference signals through a spectrometer, and used a cubic phase function for data processing. Compared with traditional coherent domain interferometry, it not only shortened the measurement time but also improved the measurement accuracy. In 2017, OLSZAK, Artur and others in the United States disclosed a spectrally controlled white light interferometry method (True heterodyne spectrally controlled interferometry, US2017063615), which combines common optical path interference with white light interference, uses an acousto-optic modulator to generate a frequency shift in one arm of the delay line device, and then obtains a beat frequency that generates a continuous phase shift in a time-varying manner in the spectrum domain. In 2020, Kerger and others from Luna Company in the United States proposed a method for autocorrelation and cross-correlation demodulation of polarization from the optical frequency domain (Calculation of distributed birefringence and polarization mode dispersion from optical frequency domain reflectometry, US2020028204), using P-OFDR to obtain the backward Rayleigh scattered signal and then demodulate it to obtain the waveguide birefringence information, which greatly improves the signal-to-noise ratio and sensitivity of the system.

[0009] Compared with the optical coherence domain polarization measurement technology OCDP, the polarization-sensitive optical frequency domain reflectometry (P-OFDR) technology makes full use of the characteristics of tunable lasers to greatly improve the measurement speed and signal-to-noise ratio. However, it cannot measure the key polarization crosstalk parameters and has serious light source nonlinear frequency scanning problems.

[0010] Since the optical frequency of a tunable laser does not change ideally linearly over time, different optical frequency intervals will be generated between adjacent sampling points, causing the signal peak after Fourier transform to decrease and broaden, thereby degrading the spatial resolution and measurement accuracy. In 2014, Xiaoyi Bao from the University of Ottawa used an optimized frequency sweep nonlinear compensation algorithm to increase the sensing length while maintaining high measurement resolution and accuracy, and achieved a spatial resolution of 0.3 mm on a single-mode fiber with a length greater than 300 m (X. Bao, et al, Long-Range High Spatial Resolution Distributed Temperature and Strain Sensing Based on Optical Frequency-Domain Reflectometry. IEEE Photonics J. 2014, 6, 6801408.). In 2019, Zhang Xuping from Nanjing University used the zero-crossing interval of the auxiliary interferometer to perform real-time correction on the main interferometer signal and resampled it using cubic spline interpolation, while eliminating the length requirement for the auxiliary interferometer delay fiber (A method for correcting nonlinear frequency sweeping of tunable lasers in optical frequency domain reflectometry, 201910084695.9).

[0011] The present invention is based on the improvement of the prior art and provides a fast measurement device for distributed polarization crosstalk in optical fiber based on optical frequency domain interference. It realizes transmission-type polarization crosstalk parameter measurement in the optical frequency domain, gives full play to the advantages of frequency domain measurement, and uses cubic spline interpolation to correct the nonlinear frequency scanning of the light source. The present invention uses the method of spectral domain scanning to eliminate the mechanical scanning structure in the time domain, realizes an all-fiber optical path without moving parts, and the optical path scanning range is only related to the detection bandwidth and sampling rate, so the range of distributed polarization crosstalk test is expanded. In addition, due to the high signal-to-noise ratio brought by the large energy of the tunable laser source, the device can achieve a larger dynamic range while greatly improving the polarization crosstalk measurement rate. It also has the advantages of small size, stable and reliable system, and easy implementation. Summary of the invention

[0012] The purpose of the present invention is to provide a fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry, which can improve the speed of polarization crosstalk test in principle, improve the signal-to-noise ratio and dynamic range essentially, and reduce the volume of the polarization crosstalk test system in structure.

[0013] The purpose of the present invention is achieved by the following measures:

[0014] A fast measurement device for distributed polarization crosstalk in optical fiber based on optical frequency domain interference, comprising a tunable laser source module 1, a device to be measured module 2, an auxiliary interferometer module 3 and a signal acquisition and analysis module 4, characterized in that:

[0015] The tunable laser source module 1 outputs a linear frequency modulated continuous light 501, which is injected into the first coupler 502 from the first coupler input end 502a and is divided into two beams of light;

[0016] The light beam outputted from the first output end 502c of the first coupler is injected into the DUT module 2, and passes through the polarizer input end pigtail 201, the polarizer 202, the polarizer output end pigtail 203, the first connection point 204, the DUT input end pigtail 206a, the second connection point 205, the DUT 206, the third connection point 207, the DUT output end pigtail 206b, the fourth connection point 208, the analyzer input end pigtail 209, the analyzer 210, and the analyzer output end pigtail 211 in sequence, and then is photoelectrically converted by the first detector 212;

[0017] The light beam outputted from the second output end 502b of the first coupler is injected into the auxiliary interferometer module 3;

[0018] The main interference signal 214 output by the device under test module 2 and the auxiliary interference signal 308 output by the auxiliary interferometer module 3 are input to the acquisition unit 401 in the signal acquisition and analysis module 4 to perform analog-to-digital conversion of the signals. Then, the stored digital main interference signal 401a and the digital auxiliary interference signal 401b are input to the correction unit 402 to correct the nonlinear frequency sweep of the laser. Then, the corrected main interference signal 402a is input to the spectrum analysis unit 403 to perform Fourier transform to obtain the frequency domain signal 403a, which is input to the position calculation unit 402. In the unit 404, the interference beat signal generated by the interference of the coupling light 704 at a certain coupling point and the transmission light 701 is mapped to the time delay τ′ through the frequency scanning rate γ, and the polarization crosstalk position 708 of the coupling point is obtained according to the relationship between the time delay τ′ and the optical path difference ΔL ΔL=cτ′; in addition, the frequency domain signal 403a obtained in the spectrum analysis unit 403 is input into the intensity calculation unit 405, and the amplitudes of all interference peaks are normalized with the interference peak amplitude at zero frequency as a reference, and the normalized interference peak amplitude is A=A 1 -A 2 , where A 1 is the amplitude of the interference peak before normalization, A 2 is the interference peak amplitude at zero frequency, thereby obtaining the polarization crosstalk intensity of the coupling point 709, and ending 406 after completing the distributed polarization crosstalk measurement;

[0019] The starting frequency of the tunable laser source module 1 is ν 1 , the stop frequency is ν 2 , with ν 2 -ν 1 =γτ, where γ is the frequency scanning rate and τ is the frequency scanning time, then the spatial resolution is δ≤c / (ν 2-ν 1 )Δn=c / γτΔn, where Δn is the linear birefringence coefficient of the polarization-maintaining fiber;

[0020] The optical path difference of the device under test module 2 is X 1 ;

[0021] The polarization angle of the polarizer 202 is 0°, the pigtail 201 at the input end of the polarizer is a single-mode optical fiber, and the pigtail 203 at the output end of the polarizer is a polarization-maintaining optical fiber;

[0022] The polarization angle of the polarizer 210 is 45°, the pigtail 209 at the input end of the polarizer is a polarization-maintaining fiber, and the pigtail 211 at the output end of the polarizer is a single-mode fiber;

[0023] When the measured parameter of the device under test 206 includes a high extinction ratio, the welding angle of the first connection point 204 is θ 1 =45°±25°, when the measured parameter of the device under test 206 is the polarization crosstalk of the general coupling point, the welding angle of the first connection point 204 is θ 1 =0°±2° or θ 1 =90°±2°;

[0024] The welding angle of the fourth connection point 208 is θ 2 =0°±2° or θ 2 =90°±2°;

[0025] The light beam injected into the second coupler 301 from the second coupler input end 301a is divided into two beams, one of which is injected into the reference arm 302 from the second coupler first output end 301b as reference light, and the other is injected into the delay arm 303 from the second coupler second output end 301c as test light, and passes through the delay optical fiber 304 therein. The light beams output from the reference arm 302 and the delay arm 303 are combined in the third coupler 305 and then divided into two beams, which are differentially detected by the second detector 306a and the third detector 306b respectively.

[0026] The optical path difference of auxiliary interferometer module 3 is X 2 , requiring X 2 ≥2X 1 ;

[0027] In the acquisition unit 401, the sampling time is t and the sampling rate is f s , the number of sampling points is M, and t ≥ τ is required, that is, f s ≥M / τ;

[0028] The processing method of the correction unit 402 is to use the cubic spline interpolation method to perform nonlinear frequency sweep correction of the laser on the digital main interference signal 401a;

[0029] In the intensity calculation unit 405, the real extinction ratio interference peak amplitude A of the polarizer 202 is used. 3 As a reference, the true amplitude of all interference peaks is obtained as A′=A+(A 3 -A 4 ), where A 4 is the normalized extinction ratio interference peak amplitude of the polarizer 202.

[0030] The principle and process of performing distributed polarization crosstalk measurement on the device under test 206 are as follows:

[0031] The polarizer 202 converts the linear frequency modulated continuous light 501 output by the narrow linewidth tunable laser source module 1 into linearly polarized light, and injects it into the working axis (such as the fast axis 802) of the device under test 206. Figure 3 As shown in the optical path tracking principle diagram, the two ends of the device under test 206 are connected to the 0° polarizer 202 and the 45° analyzer 210 respectively, generating two connection points with different polarization crosstalk intensities. The welding angle of the first connection point 204 is θ 1 =90°, the welding angle of the fourth connection point 208 is θ 2 =0°. The linear polarized light working in the main axis will be coupled to its orthogonal axis when passing through these two connection points (such as the fast axis 802 is coupled to the slow axis 801), forming coupled light 704, and the linear polarized light that has always worked on the main axis is the transmission light 701, so there will be a time delay, optical path difference and phase difference between the transmission light 701 and the coupling light 704. The fourth connection point 208 with a polarization crosstalk intensity of -40dB corresponds to a coupling light 703, and the first connection point 204 with a polarization crosstalk intensity of -30dB corresponds to b coupling light 702. The optical path difference between the a coupling light 703 and the transmission light 701 generated by the transmission of the slow axis 801 is recorded as L 1 =cτ 1 / Δn, the optical path difference between the coupling light 702 and the transmission light 701 generated by the transmission on the slow axis 801 is recorded as L 2 =cτ 2 / Δn, where c is the speed of light, τ 1 and τ 2 a is the time delay of coupled light 703 and b is the time delay of coupled light 702. Therefore, we only need to obtain τ 1 and τ 2 The polarization crosstalk position 708 of the corresponding connection point can be obtained.

[0032] In the optical coherence domain, a mechanical optical path scanning delay device is used to compensate for the optical path difference. The length of the optical path scanning corresponds to the optical path difference generated at different coupling points. Only when the optical path difference is compensated to zero can the coupled light at a certain coupling point interfere with the transmitted light, thereby forming a corresponding distributed white light interference signal in the time domain. The scanning test process of the device of the present invention is shown in the attached figure. Figure 3As shown in the figure, the slope γ represents the frequency scanning rate, Δτ represents the test time, and the time domain interference signal 707 is represented by sinusoidal waves of different frequencies, including the time domain a coupled light beat frequency signal 706 and the time domain b coupled light interference signal 705. In the optical frequency domain, the linear frequency modulated continuous light 501 is used to map the beat frequency between the transmission light 701 and the coupling light 704 to a time delay through the frequency scanning rate γ, that is, the beat frequency Δf corresponding to the first connection point 204 and the fourth connection point 208 is obtained by measurement and Fourier transform. 1 and Δf 2 , and then the delay τ is obtained from the mapping relationship 1 and τ 2 Then, according to the relationship between time delay and optical path difference, the optical path difference L generated by the corresponding connection point is demodulated. 1 =cτ 1 / Δn and L 2 =cτ 2 / Δn, thereby obtaining the polarization crosstalk position 708. At the same time, the amplitude of the interference peak corresponding to the first connection point 204 and the fourth connection point 208 is normalized with the extinction ratio interference peak of the polarizer 202 as a reference, and the normalized interference peak amplitude is A'=A 1 -A 3 , where A 1 is the amplitude of the interference peak before normalization, A 2 is the amplitude of the extinction ratio interference peak of the polarizer 202, thereby obtaining the polarization crosstalk intensity 709 corresponding to the connection point.

[0033] For the Y-waveguide DUT 206Y, the method for determining the polarization crosstalk position 708 is as follows:

[0034] 1) The pigtail 203 at the output end of the polarizer has a geometric length of l 1 The corresponding optical path length of the polarization-maintaining fiber is calculated and recorded as S 1 = l 1 ×Δn; the pigtail 209 at the input end of the analyzer has a geometric length of l 2 The corresponding optical path length of the polarization-maintaining fiber is calculated and recorded as S 2 = l 2 ×Δn; where Δn is the linear birefringence coefficient of the polarization-maintaining optical fiber.

[0035] 2) The geometric length of the pigtail 206a at the input end of the device under test is l 3 , the geometric length of the pigtail 206b at the output end of the device under test is l 4 , the chip length of the Y-waveguide DUT 206Y is l 5 , calculate the corresponding optical path, recorded as S = S 3 +S 4 +S 5 = l3 ×Δn+l 4 ×Δn+l 5 ×Δn Y , where Δn is the linear birefringence coefficient of the polarization-maintaining fiber, Δn Y is the linear birefringence of the Y-waveguide chip;

[0036] 3) If the working mode of the Y waveguide chip is fast axis 802 operation, the position corresponding to the characteristic peak of the extinction ratio interference is |S 1 +S 2 +S 3 +S 4 +S 5 |; If the working mode of the Y waveguide chip is slow axis 801, the position corresponding to the extinction ratio interference characteristic peak is |S 1 +S 2 +S 3 +S 4 -S 5 |;

[0037] 4) After the light beams are coupled once through the first connection point 204, the second connection point 205, the third connection point 207, and the fourth connection point 208, the positions of the corresponding four first-order polarization crosstalk interference characteristic peaks are S 1 |、|S 3 +S 1 |、|S 4 +S 2 |、|S 2 |, if l 1 <l 2 , then the position distribution of the interference peak is |S 1 |<|S 3 +S 1 |<|S 2 |<|S 4 +S 2 |; if l 1 >l 2 , then the position distribution of the interference peak is |S 2 |<|S 4 +S 2 |<|S 1 |<|S 3 +S 1 |;

[0038] 5) When the light beam is coupled twice through the first connection point 204 and the third connection point 207, the first connection point 204 and the fourth connection point 208, the second connection point 205 and the third connection point 207, and the second connection point 205 and the fourth connection point 208, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are respectively |S 1 +S 2 +S4 |、|S 1 +S 2 |、|S 1 +S 2 +S 3 +S 4 |、|S 1 +S 2 +S 3 |.

[0039] Compared with the prior art, the advantages of the present invention are:

[0040] 1. The present invention is a fiber-optic distributed polarization crosstalk fast measurement device based on optical frequency domain interference, which realizes transmission-type polarization crosstalk parameter measurement in the optical frequency domain, giving full play to the advantages of frequency domain measurement, and using a high-coherence tunable laser source to quickly scan the wavelength of the optical signal, thereby obtaining the intensity and position information of the polarization crosstalk after frequency domain demodulation. Due to the high signal-to-noise ratio brought by the large energy of the light source, the device can greatly improve the test rate and achieve a larger dynamic range.

[0041] 2. The present invention can eliminate the mechanical scanning delay structure in the traditional coherent domain polarization test. The optical path scanning range is no longer limited by the length of the optical fiber delay line, but only related to the detection bandwidth and sampling rate. Therefore, the range of the distributed polarization crosstalk test has been greatly expanded. Since the device realizes an all-fiber optical path without moving parts, it has the advantages of compact structure, stability and reliability, and easy implementation, which is conducive to the promotion and popularization of new technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The present invention is a schematic diagram of a device for rapid measurement of optical fiber distributed polarization crosstalk based on optical frequency domain interferometry.

[0043] Figure 2 This is a schematic diagram of the cross-sectional structure of a panda-type polarization-maintaining optical fiber.

[0044] Figure 3 This is a schematic diagram of the principle of fast measurement of fiber distributed polarization crosstalk based on optical frequency domain interferometry.

[0045] Figure 4 It is a schematic diagram of a device for fast measurement of distributed polarization crosstalk on a Y waveguide.

[0046] Figure 5 It is a schematic diagram of a device for fast measurement of distributed polarization crosstalk on an optical fiber ring.

[0047] Figure 6 It is the test result of the distributed polarization crosstalk in the audio domain of the optical fiber ring.

[0048] Figure 7 It is the time domain test result of distributed polarization crosstalk of the fiber ring. DETAILED DESCRIPTION

[0049] In order to more clearly illustrate the optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interference proposed by the present invention, the present invention is described in more detail below in combination with embodiments and drawings, but this should not limit the protection scope of the present invention.

[0050] Specific implementation 1, a Y-waveguide distributed polarization crosstalk fast measurement device, as shown in the attached Figure 4 shown.

[0051] It consists of four parts: tunable laser source module 1, device under test module 2, auxiliary interferometer module 3 and signal acquisition and analysis module 4, among which:

[0052] 1) The tunable laser source module 1 outputs a linear frequency modulated continuous light 501, which is injected into the first coupler 502 from the first coupler input end 502a and split into two beams of light;

[0053] 2) The light beam outputted from the first output end 502c of the first coupler is injected into the DUT module 2, and after passing through the polarizer 202, the pigtail 203 at the output end of the polarizer, the first connection point 204, the pigtail 206a at the input end of the DUT, the second connection point 205, the Y-waveguide DUT 206Y, the third connection point 207, the pigtail 206b at the output end of the DUT, the fourth connection point 208, the pigtail 209 at the input end of the analyzer, and the analyzer 210, the first detector 212 performs photoelectric conversion;

[0054] 3) The light beam outputted from the second output end 502b of the first coupler is injected into the auxiliary interferometer module 3, and is split into two beams by the second coupler 301, and respectively injected into the reference arm 302 and the delay arm 303, the delay arm 303 includes a delay optical fiber 304, and the light beams outputted from the two arms of the interferometer are combined in the third coupler 305 and then split into two beams, and then differentially detected by the second detector 306a and the third detector 306b respectively;

[0055] 4) The main interference signal 214 output by the device under test module 2 and the auxiliary interference signal 308 output by the auxiliary interferometer module 3 are input into the signal acquisition and analysis module 4 for acquisition, processing and demodulation. The signal acquisition and analysis module 4 is composed of a data acquisition card 407 and a computer processing unit 408; the data acquisition card 407 is controlled by labview software and is used to synchronously acquire and store the analog voltage signal after photoelectric conversion and convert it into a digital signal; the computer processing unit 408 is used to control the generation and reception of the signal, and to process and demodulate the digital signal.

[0056] The selection of the main optoelectronic components of the invention and their performance parameters are as follows:

[0057] 1) The tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning, with a wavelength tuning range of 1490nm to 1630nm, a high coherence mode, a wavelength scanning rate of 80nm / s, and a wavelength scanning time of 1.75s, such as a TSL-770 laser from Santec;

[0058] 2) The photosensitive materials of the first detector 212, the second detector 306a and the third detector 306b are all InGaAs, the common mode rejection ratio is 25dB, the light detection range is 900-1700nm, the maximum detection bandwidth is 80MHz, the saturated differential detection power is 55uW, the transimpedance gain is 50000V / A, and the minimum noise equivalent power is The peak responsivity is 1A / W, such as using the NewFocus 1817 balanced detector;

[0059] 3) The 16-bit sampling rate of the data acquisition card 407 is 11.25MHz, the number of sampling points is 21M, and the sampling time is about 1.87s. Combined with the laser for external triggering, such as the acquisition card model M4i:4471-x8 of Spectrum Instrumentation, the sampling time is greater than the wavelength scanning time, and the parameter design is reasonable;

[0060] 4) The splitting ratio of the first coupler 502 is 2:98, the splitting ratio of the second coupler 301 is 50:50, and the splitting ratio of the third coupler 305 is 50:50. The extinction ratio is greater than 20 dB, the insertion loss is less than 0.5 dB, and the operating wavelength covers the 1550 nm band;

[0061] 5) The working wavelength of the polarizer 202 covers the 1550nm band, the polarization angle is 0°, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the pigtail 201 at the input end of the polarizer is a single-mode optical fiber, and the pigtail 203 at the output end of the polarizer is a panda-type polarization-maintaining optical fiber with a diameter of 125um, and the length of the polarization-maintaining pigtail is l 1 =10m, calculate the corresponding optical path S 1 = l 1 ×Δn≈0.5×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0062] 6) The working wavelength of the polarizer 210 covers the 1550nm band, the polarization angle is 45°, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the pigtail 211 at the output end of the polarizer is a single-mode fiber, and the pigtail 209 at the input end of the polarizer is a panda-type polarization-maintaining fiber with a diameter of 80um, and the length of the polarization-maintaining pigtail is l 2 =20m, calculate the corresponding optical path S2 = l 2 ×Δn≈1×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 .

[0063] 7) The welding angle of the first connection point 204 is θ 1 =45°, the welding angle of the fourth connection point 208 is θ 2 =0°;

[0064] 8) The Y-waveguide DUT 206Y uses lithium niobate crystal as the chip substrate, has an extinction ratio of 60 dB, works in the fast axis 802, and the geometric length of the pigtail 206a at the input end of the DUT is l 3 =1m, the diameter is 125um, and the geometric length of the pigtail 206b at the output end of the device under test is l 4 =2m, diameter is 80um, length of Y waveguide chip is l 5 =3.5cm, calculate the corresponding optical path S=S 3 +S 4 +S 5 = l 3 ×Δn+l 4 ×Δn+l 5 ×Δn Y ≈4770um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 , Linear birefringence Δn of Y waveguide chip Y Take 9.34×10 -2 ;

[0065] 9) The auxiliary interferometer module 3 adopts a Mach-Zehnder fiber interferometer structure in which the single-arm delay fiber 304 is Δl=1m. The optical path difference X corresponding to the auxiliary interferometer module 3 in this embodiment is calculated. 2 =Δl×n=1.468m, where the refractive index n of the single-mode optical fiber is 1.468; calculate the optical path difference X corresponding to the device under test module 2 in this embodiment 1 =S 1 +S 2 +S≈0.02m, satisfying X 2 ≥2X 1 , the parameters are designed reasonably;

[0066] 10) The position corresponding to the characteristic peak of extinction ratio interference is |S 1 +S 2 +S 3 +S 4 +S 5 |≈19770um;

[0067] 11) After the light beams are coupled once through the first connection point 204, the second connection point 205, the third connection point 207, and the fourth connection point 208, the positions of the corresponding four first-order polarization crosstalk interference characteristic peaks are |S 1 |=0.5×10 4 um,|S 3 +S 1 |=0.55×10 4 um,|S 4 +S 2 |=1.1×10 4 um,|S 2 |=1×10 4 um, because l 1 <l 2 , so the position distribution of the interference peak is |S 1 |<|S 3 +S 1 |<|S 2 |<|S 4 +S 2 |;

[0068] 12) When the light beam is coupled twice through the first connection point 204 and the third connection point 207, the first connection point 204 and the fourth connection point 208, the second connection point 205 and the third connection point 207, and the second connection point 205 and the fourth connection point 208, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are respectively |S 1 +S 2 +S 4 |=1.6×10 4 um,|S 1 +S 2 |=1.5×10 4 um,|S 1 +S 2 +S 3 +S 4 |=1.65×10 4 um,|S 1 +S 2 +S 3 |=1.55×10 4 um.

[0069] Specific embodiment 2, a distributed polarization crosstalk fast measurement device for an optical fiber ring, as shown in the attached Figure 5 shown.

[0070] The selection of the main optoelectronic components of the invention and their performance parameters are as follows:

[0071] 1) The tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning, with a wavelength tuning range of 1510nm to 1620nm. The high coherence mode is turned on, the light source output power is 6.4mW, the wavelength scanning rate is 80nm / s, and the wavelength scanning time is 1.375s, such as the TSL-550 laser of Santec Company;

[0072] 2) The photosensitive materials of the first detector 212, the second detector 306a and the third detector 306b are all InGaAs, the common mode rejection ratio is 25dB, the light detection range is 900-1700nm, the maximum detection bandwidth is 80MHz, the saturated differential detection power is 55μW, the transimpedance gain is 50000V / A, and the minimum noise equivalent power is The peak responsivity is 1A / W, such as using the NewFocus 1817 balanced detector;

[0073] 3) The 16-bit sampling rate of the data acquisition card 407 is 11.25MHz, the number of sampling points is 16M, the sampling time is about 1.42s, and the external triggering is combined with the laser. The voltage setting range is 2V. If the acquisition card model M4i:4471-x8 of Spectrum Instrumentation is used, the sampling time is greater than the frequency scanning time, and the parameter design is reasonable;

[0074] 4) The splitting ratio of the first coupler 502 is 2:98, the splitting ratio of the second coupler 301 is 50:50, and the splitting ratio of the third coupler 305 is 50:50. The extinction ratio is greater than 20 dB, the insertion loss is less than 0.5 dB, and the operating wavelength covers the 1550 nm band;

[0075] 5) The working wavelength of the polarizer 202 covers the 1550nm band, the polarization angle is 0°, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the pigtail 201 at the input end of the polarizer is a single-mode optical fiber, and the pigtail 203 at the output end of the polarizer is a panda-type polarization-maintaining optical fiber with a diameter of 80um, and the length of the polarization-maintaining pigtail is l 6 =20m, calculate the corresponding optical path S 6 = l 6 ×Δn≈1×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0076] 6) The working wavelength of the polarizer 210 covers the 1550nm band, the polarization angle is 45°, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the pigtail 211 at the output end of the polarizer is a single-mode fiber, and the pigtail 209 at the input end of the polarizer is a panda-type polarization-maintaining fiber with a diameter of 80um, and the length of the polarization-maintaining pigtail is l 7=20m, calculate the corresponding optical path S 7 = l 7 ×Δn≈1×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0077] 7) The welding angle of the first connection point 204 is θ 1 =0°, the welding angle of the fourth connection point 208 is θ 2 =0°;

[0078] 8) The fiber ring 206R is composed of a Panda-type polarization-maintaining fiber with a diameter of 80 μm. The length measured by the optical time domain reflectometer OTDR is l 8 =1854m, calculate the corresponding optical path S 8 = l 8 ×Δn ring ≈1.08m, where the linear birefringence of the fiber ring Δn ring 5.81×10 -4 ;

[0079] 9) The auxiliary interferometer module 3 uses a single-arm delay fiber 304 for Δl 2 =3m Mach-Zehnder fiber interferometer structure, calculate the optical path difference X corresponding to the auxiliary interferometer module 3 in this embodiment 2 =Δl 2 ×n≈4.4m, where the refractive index n of the single-mode optical fiber is 1.468; calculate the optical path difference X corresponding to the device under test module 2 in this embodiment 1 =S 6 +S 7 +S 8 ≈1.10m, satisfying X 2 ≥2X 1 , the parameters are designed reasonably;

[0080] The frequency domain test results are as follows Figure 6 The ordinate of the interference peak 1 601 represents the normalized polarization crosstalk intensity corresponding to the analyzer 210, and the abscissa represents the normalized optical path position x corresponding to the analyzer 210. 1 = 0m; the ordinate of the interference peak 2 602 represents the polarization crosstalk intensity C corresponding to the fourth connection point 208 2 =-30.2dB, the horizontal axis represents the optical path position x corresponding to the fourth connection point 208 2 =8.5mm, so the geometric length l of the polarizer 210 7real =(x 2 -x 1 ) / Δn=17.04m, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4The vertical coordinate of the interference peak 3 603 represents the first connection C 1 =0dB The polarization crosstalk intensity C corresponding to the junction 204 3 =-27.6dB, the horizontal axis represents the optical path position x corresponding to the first connection point 204 3 =1085.6mm;

[0081] The vertical coordinate of the interference peak 4 604 represents the polarization crosstalk intensity C corresponding to the polarizer 202. 4 =-25.09dB, the horizontal axis represents the optical path position x corresponding to the polarizer 202 4 =1094.15mm, so the geometric length of the polarizer is l 6real =(x 4 -x 3 ) / Δn=17.14m, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 ; From the test results, the geometric length of the fiber ring 206R is l 8real =(x 3 -x 2 ) / Δn ring ≈1854m, which is consistent with the length measured by the optical time domain reflectometer OTDR.

[0082] The optical fiber ring 206R has a relatively obvious first characteristic peak 605 and a second characteristic peak 606. A series of characteristic peaks between the interference peak 2 602 and the interference peak 3 603 include the distributed information of the layer and turn changes of the optical fiber ring 206R. According to the background noise 607 of the test result, the dynamic range can be estimated to be about 105dB.

[0083] The time domain test results are as follows Figure 7 As shown, the horizontal axis is the sampling time in seconds, and the vertical axis is the signal amplitude in V. The time required for one test is about 1.375 seconds.

[0084] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A fast optical fiber distributed polarization crosstalk measurement device based on optical frequency domain interferometry, comprising a tunable laser source module (1), a device to be measured module (2), an auxiliary interferometer module (3) and a signal acquisition and analysis module (4), wherein: 1) The tunable laser source module (1) outputs linear frequency modulated continuous light (501), which is injected into the first coupler (502) from the first coupler input end (502a) and divided into two beams of light; 2) the light beam outputted from the first output end (502c) of the first coupler is injected into the device under test module (2), and passes through the polarizer input end pigtail (201), the polarizer (202), the polarizer output end pigtail (203), the first connection point (204), the device under test input end pigtail (206a), the second connection point (205), the device under test (206), the third connection point (207), the device under test output end pigtail (206b), the fourth connection point (208), the analyzer input end pigtail (209), the analyzer (210), and the analyzer output end pigtail (211) in sequence, and then undergoes photoelectric conversion by the first detector (212); 3) The light beam output from the second output end (502b) of the first coupler is injected into the auxiliary interferometer module (3) from the input end (301a) of the second coupler; the light beam injected into the second coupler (301) from the input end (301a) of the second coupler is divided into two beams, one beam as reference light is injected into the reference arm (302) from the first output end (301b) of the second coupler, and the other beam as test light is injected into the delay arm (303) from the second output end (301c) of the second coupler and passes through the delay optical fiber (304); the light beams output from the reference arm (302) and the delay arm (303) are combined in the third coupler (305) and then divided into two beams, which are differentially detected by the second detector (306a) and the third detector (306b) respectively; 4) The main interference signal (214) output by the device under test module (2) and the auxiliary interference signal (308) output by the auxiliary interferometer module (3) are input into the acquisition unit (401) in the signal acquisition and analysis module (4) to perform analog-to-digital conversion of the signals, and then the stored digital main interference signal (401a) and the digital auxiliary interference signal (401b) are input into the correction unit (402) to correct the nonlinear frequency sweep of the laser, and then the corrected main interference signal (402a) is input into the spectrum analysis unit (403) to perform Fourier transform to obtain the frequency domain signal (403a); the frequency domain signal (403a) is input into the position calculation unit (404) in which the interference generated by the coupling light (704) at a certain coupling point and the transmission light (701) is converted into the frequency domain signal (403a); The beat frequency signal is mapped to a time delay τ' through a frequency scanning rate γ, and the polarization crosstalk position (708) of the coupling point is obtained according to the relationship between the time delay τ' and the optical path difference ΔL, ΔL=cτ' / Δn, where c is the speed of light in a vacuum, and Δn is the linear birefringence coefficient of the polarization-maintaining optical fiber; in addition, the frequency domain signal (403a) obtained in the spectrum analysis unit (403) is input into the intensity calculation unit (405), and the amplitudes of all interference peaks are normalized with the interference peak amplitude at zero frequency as a reference. The normalized interference peak amplitude is A=A1-A2, where A1 is the amplitude of the interference peak before normalization, and A2 is the interference peak amplitude at zero frequency, thereby obtaining the polarization crosstalk intensity (709) of the coupling point, and the distributed polarization crosstalk measurement is completed and ends (406).

2. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: The starting frequency of the tunable laser source module (1) is ν1, the ending frequency is ν2, ν2-ν1=γτ, where τ is the frequency scanning time, and the spatial resolution is δ≤c / (ν2-ν1)Δn=c / γτΔn, where Δn is the linear birefringence coefficient of the polarization-maintaining optical fiber.

3. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: The optical path difference of the device under test module (2) is X1; the optical path difference of the auxiliary interferometer module (3) is X2, and it is required that X2≥2X1.

4. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: The polarization angle of the polarizer (202) is 0°, the pigtail (201) at the input end of the polarizer is a single-mode optical fiber, and the pigtail (203) at the output end of the polarizer is a polarization-maintaining optical fiber; The polarization angle of the polarizer (210) is 45 degrees, the pigtail (209) at the input end of the polarizer is a polarization-maintaining optical fiber, and the pigtail (211) at the output end of the polarizer is a single-mode optical fiber.

5. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: When the measured parameter of the device under test (206) includes a high extinction ratio, the fusion angle of the first connection point (204) is θ1=45°±25°; when the measured parameter of the device under test (206) is polarization crosstalk of a general coupling point, the fusion angle of the first connection point (204) is θ1=0°±2° or θ1=90°±2°; The welding angle of the fourth connection point (208) is θ2=0°±2° or θ2=90°±2°.

6. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: In the acquisition unit (401), the sampling time is t and the sampling rate is f s , the number of sampling points is M, τ is the interference delay, and t≥τ is required.

7. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: The processing method of the correction unit (402) is to use a cubic spline interpolation method to perform nonlinear frequency sweep correction of the laser on the digital main interference signal (401a).

8. The optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interferometry according to claim 1 is characterized by: In the intensity calculation unit (405), the amplitudes of all interference peaks are normalized with reference to the amplitude of the extinction ratio interference peak of the polarizer (202), and the normalized interference peak amplitude is A'=A-A3, wherein A3 is the amplitude of the extinction ratio interference peak of the polarizer (202).

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