A fast measurement device for distributed polarization crosstalk based on optical frequency domain frequency shift interferometry

Abstract

The present invention belongs to the field of optical fiber measurement technology, and specifically relates to an optical fiber distributed polarization crosstalk fast measurement device based on optical frequency domain interference. The device comprises 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), and is characterized in that: a high-coherence tunable laser is used to perform fast wavelength scanning on an optical signal, and a linear frequency modulated continuous light (501) is output, which passes through a polarizer (202), a device to be measured (206) and an analyzer (210) in sequence, and a beat frequency signal generated by the interference of a coupling light (704) and a transmission light (701) is obtained in the optical frequency domain, and then an optical path difference and a normalized interference signal amplitude are calculated, thereby respectively determining the polarization crosstalk position and intensity. The present invention, in combination with a tunable laser, gets rid of the dependence on an optical path scanning delay device in polarization crosstalk measurement, greatly improves the speed and dynamic range of polarization crosstalk measurement, and has a compact structure, is stable and reliable, and is easy to implement.

Description

Technical Field

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

[0002] 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. By measuring the polarization crosstalk distribution along the transmission direction, it can obtain information such as stress and temperature of the external environment. While testing and evaluating the performance of polarization optical devices, it also provides necessary guidance for the production and detection of polarization-maintaining optical fibers, the winding process of optical fiber rings, and the precise fusion alignment between polarization devices. With its advantages of high measurement sensitivity, high spatial resolution, and large dynamic range, 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.

[0003] In recent years, researchers have conducted a lot of research on improving the test performance of OCDP devices, focusing on the improvement and correction of parameters such as test range, measurement symmetry, measurement accuracy, positioning accuracy, dynamic range, measurement speed, sensitivity, as well as the suppression of noise, polarization fading, and dispersion. At the same time, they are committed to improving the test method steps and peak analysis methods.

[0004] In terms of noise suppression, in 2014, the applicant made the polarization state of the input light meet the minimum reflection condition under the Brewster angle (An optical coherent polarization measurement device capable of suppressing interference noise, 201410120901.4) to eliminate the residual reflected light signal in the device, thereby suppressing the optical interference noise; in the same year, the applicant used a combination of a high-polarization light source and a high-extinction ratio polarization device (A device and detection method for suppressing polarization crosstalk measurement noise using a light source, 201410120580.8) to suppress the measurement noise in OCDP. In terms of test range, in 2007, Oleg M. Efimov et al. from the United States disclosed a digitally controlled optical fiber delay line (Digitally controlled optical fiber delay line, US Patent, 7283708B2), which uses an optical fiber array of multiple optical fiber modules to achieve optical path extension; in 2013, the applicant disclosed an optical path extension structure composed of multiple independent optical path extension units connected in series (A large scanning range optical coherence domain polarization measurement device, 201310739313.4), which can achieve infinite extension of the scanning optical path, and the paired optical devices in each extension unit can also improve the measurement accuracy of polarization crosstalk.

[0005] However, in the existing and continuously improved OCDP scheme, there are still some performance parameter compromises and constraints. For example, sacrificing the time and speed of optical path scanning, allowing the mechanical programmable stage to perform slow scanning to ensure the high accuracy of polarization crosstalk measurement; using a mechanical programmable stage to control the optical path scanning, but the use of moving parts also reduces the reliability of the system; developing an extended optical path scanning delay to expand the test range will bring additional errors and increase the complexity of the system. In addition, since the wide-spectrum light source used in the OCDP optical coherence domain polarization measurement technology has low energy, the signal-to-noise ratio is insufficient, resulting in the dynamic range can never be greatly improved.

[0006] Based on the existing technologies and low signal-to-noise ratio problems in the above coherent domain measurements, scholars have tried to combine polarization with 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.

[0007] In 2013, Ou Zhonghua et al. from the University of Electronic Science and Technology of China injected a linear frequency-sweep narrow-linewidth light wave into an interference system through a polarizer (A high-resolution sensing system for measuring the beat length and strain of polarization-maintaining optical fiber based on an OFDR system, 201310013952.2), and then combined it with optical heterodyne detection to achieve the purpose of accurately controlling the polarization state and obtaining a stable polarization mode, and improving the accuracy of strain measurement. In 2017, Liu Tiegen et al. from Tianjin University disclosed a polarization-sensitive DAS device (Distributed fiber acoustic sensing device and method based on dual-polarization double-sideband modulation, 201810253111.1), which combines the advantages of time domain reflection and frequency domain reflection to achieve distributed acoustic sensing with a large dynamic range and high spatial resolution. In 2020, Kerger and others from Luna Company in the United States proposed an autocorrelation and cross-correlation demodulation method for measuring polarization from the optical frequency domain (Calculation of distributed birefringence and polarization mode dispersion from optical frequency domain reflectometry, US2020028204). They used P-OFDR to obtain the back-Rayleigh scattering signal and then demodulated it to obtain the waveguide birefringence information, which greatly improved the signal-to-noise ratio and sensitivity of the system.

[0008] 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.

[0009] Since the optical frequency of a tunable laser does not change linearly over time, different optical frequency intervals will be generated between adjacent sampling points, causing the peak of the signal 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).

[0010] The present invention is based on the improvement of the prior art, and discloses a distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interference, performs wavelength scanning in the optical frequency domain, realizes transmission-type polarization crosstalk parameter measurement, and uses the above-mentioned cubic spline interpolation method to correct the nonlinear frequency sweep of the light source, thereby greatly improving the polarization crosstalk measurement rate and achieving a larger signal-to-noise ratio and dynamic range. In addition, the present invention uses a main interferometer to frequency shift the interference beat signal, thereby eliminating the 1 / f low-frequency flicker noise and the signal aliasing problem caused by the nonlinearity of the light source frequency sweep, and can also obtain the polarization crosstalk symmetry information on both sides of the main interference peak, with the advantages of small size, stable and reliable system, etc. Summary of the invention

[0011] The object of the present invention is to provide a distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interferometry, which can substantially improve the polarization crosstalk test speed and dynamic range, while overcoming the problems of low-frequency flicker noise and signal aliasing.

[0012] The purpose of the present invention is achieved by the following measures:

[0013] A distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interferometry comprises a tunable laser source module 1, a device to be measured module 2, a main interferometer module 3, an auxiliary interferometer module 4 and a signal acquisition and analysis module 5, and is characterized in that:

[0014] The tunable laser source module 1 outputs a linear frequency modulated continuous light 602, which is injected into the first coupler 601 from the first coupler input end 601a and is divided into two beams of light;

[0015] The light beam outputted from the first output terminal 601c of the first coupler is injected into the DUT module 2;

[0016] The light beam output from the DUT module 2 is injected into the main interferometer module 3, and injected into the second coupler 302 from the second coupler input end 301 thereof to be divided into two beams of light, one of which is injected into the reference arm 303 as the reference light, and the other is injected into the delay arm 304 as the test light, and passes through the main delay fiber 305 therein, and the light beams output from the reference arm 303 and the delay arm 304 are combined in the third coupler 306 and then divided into two beams, and then differentially detected by the first balanced detector 308;

[0017] The light beam outputted from the second output end 601b of the first coupler is injected into the auxiliary interferometer module 4;

[0018] The main interferometer signal 307 output by the main interferometer module 3 and the auxiliary interferometer signal 407 output by the auxiliary interferometer module 4 are jointly injected into the acquisition unit 501 in the signal acquisition and analysis module 5 to perform analog-to-digital conversion of the signal. Then, the stored digital main interferometer signal 501a and the digital auxiliary interferometer signal 501b are input into the correction unit 502 to correct the nonlinear frequency sweep of the laser. Then, the corrected main interferometer signal 502a is input into the spectrum analysis unit 503 to perform Fourier transform to obtain the frequency domain signal 503a. Then, it is input into the position calculation unit 504, in which the coupling light 704 of a certain coupling point is interfered with the transmission light 701. The generated interference beat signal is mapped to a time delay τ′ through a frequency scanning rate γ, and according to the relationship between the time delay τ′ and the optical path difference ΔL, ΔL=cτ′ / Δn, the polarization crosstalk position 708 of the coupling point is obtained; in addition, the frequency domain signal 503a obtained in the spectrum analysis unit 503 is input into the intensity calculation unit 505, and the amplitude of all interference peaks is normalized with the amplitude of the main interference peak 801 as a reference. The normalized interference peak amplitude is A=A1-A2, wherein A1 is the amplitude of the interference peak before normalization, and A2 is the amplitude of the main interference peak 801, thereby obtaining the polarization crosstalk intensity 709 of the connection point, and the distributed polarization crosstalk measurement is completed and ends 506;

[0019] 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;

[0020] The device under test module 2 is connected in sequence by a pigtail 201 at the input end of a polarizer, a polarizer 202, a pigtail 203 at the output end of a polarizer, a first connection point 204, a pigtail 206a at the input end of a device under test, a second connection point 205, a device under test 206, a third connection point 207, a pigtail 206b at the output end of the device under test, a fourth connection point 208, a pigtail 209 at the input end of an analyzer, an analyzer 210, and a pigtail 211 at the output end of an analyzer;

[0021] The optical path difference of the device under test module 2 is X1;

[0022] 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 20 at the output end of the polarizer is a polarization-maintaining optical fiber;

[0023] 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;

[0024] 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 a polarization crosstalk of a general coupling point, the fusion angle of the first connection point 204 is θ1=0°±2° or θ1=90°±2°;

[0025] The welding angle of the fourth connection point 208 is θ2=0°±2° or θ2=90°±2°;

[0026] The optical path difference of the main interferometer module 3 is X2, and it is required that X2>2X1;

[0027] The light beam injected into the fourth coupler 401 is divided into two beams, one beam is injected into the reference arm 402 as the reference light, and the other beam is injected into the delay arm 403 as the test light, and passes through the auxiliary delay optical fiber 404 therein. The light beams output from the reference arm 402 and the delay arm 403 are combined in the fifth coupler 405 and then divided into two beams, and then differentially detected by the second balanced detector 406;

[0028] The optical path difference of the auxiliary interferometer module 4 is X3, and X3 ≥ 2X2 is required;

[0029] In the acquisition unit 501, the sampling time is t and the sampling rate is f s , the number of sampling points M, requiring t ≥ τ, that is, f s ≥M / τ;

[0030] The processing method of the correction unit 502 is to use the cubic spline interpolation method to perform nonlinear frequency sweep correction of the laser on the main interferometer signal 214 .

[0031] The principle and process of performing distributed polarization crosstalk measurement on the device under test 206 are as follows:

[0032] The polarization-maintaining DUT has two orthogonal characteristic axes. There is a refractive index difference between the two orthogonal linear polarized lights, resulting in different transmission rates of light waves in the fast axis 710 and the slow axis 711. The axis with a large refractive index is the slow axis 711, and the axis with a small refractive index is the fast axis 710. The polarizer 202 converts the linear frequency modulated continuous light 602 emitted by the narrow linewidth tunable laser module 1 into linear polarized light and injects it into the working main axis of the DUT 206 (such as the fast axis 710). According to the attached Figure 2 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 fusion angle of the first connection point 204 is θ1=90°, and the fusion angle of the fourth connection point 208 is θ2=0°. The linearly 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 710 coupled to the slow axis 711), forming coupled light 704. This phenomenon is called polarization crosstalk. The linearly polarized light that has been working on the main axis is the transmission light 701, and time delay, optical path difference and phase difference will occur between the transmission light 701 and the coupled light 704. The fourth connection point 208 with a polarization crosstalk intensity of -40dB corresponds to a coupled light 703, and the first connection point 204 with a polarization crosstalk intensity of -30dB corresponds to b coupled light 702. The optical path difference between the a-coupled light 703 propagating on the slow axis 711 and the transmission light 701 is recorded as L1=cτ1 / Δn, and the optical path difference between the b-coupled light 702 propagating on the slow axis 711 and the transmission light 701 is recorded as L2=cτ2 / Δn, where c is the speed of light, and τ1 and τ2 are the delays of the a-coupled light 703 and the b-coupled light 702, respectively. Therefore, the polarization crosstalk position 708 of the corresponding connection point can be obtained by only obtaining τ1 and τ2.

[0033] However, the splitting ratio setting of the optical device and the circuit acquisition setting will introduce a DC component at the low frequency, generating additional power. At the same time, there is severe 1 / f flicker noise and frequency scanning nonlinearity at the low frequency, which will cause signal aliasing problems. Therefore, the optical path difference of the main interferometer module 3 should be used to move the interference peak containing polarization crosstalk information to the non-low frequency to obtain complete and correct information.

[0034] The scanning test process of the device of the present invention is shown in the attached figure. Figure 2As shown, the slope γ in the figure 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 frequency-shifted a coupled light interference signal 705 in the time domain and the frequency-shifted b coupled light interference signal 706 in the time domain. The specific process is as follows: In the optical frequency domain, the beat frequency between the transmission light 701 and the coupled light 704 is mapped to a delay through the frequency scanning rate γ by using the linear frequency modulated continuous light 602, that is, the beat frequencies Δf1 and Δf2 corresponding to the fourth connection point 208 and the first connection point 204 are used, and the delays τ1 and τ2 are obtained from the mapping relationship. Then, the delay τ′ is introduced by using the optical path difference of the main interferometer module 3, so that the delays of the transmission light 701, the a coupled light 703 and the b coupled light 702 are increased by τ′ at the same time, thereby simultaneously increasing the beat frequency Δf1′ between the transmission light 701 and the frequency-shifted a coupled light 712, and the beat frequency Δf2′ between the transmission light 701 and the frequency-shifted b coupled light 713. By measuring Δf1' and Δf2', τ1+τ' and τ2+τ' are obtained from the mapping relationship, and then the optical path difference L3=c(τ1+τ') / Δn and L2=c(τ2+τ') / Δn generated by the corresponding connection point are demodulated according to the relationship between the time delay and the optical path difference, 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 amplitude of the main interference peak 801 as a reference, and the normalized interference peak amplitude is A'=A1-A2, wherein A1 is the amplitude of the interference peak before normalization, and A2 is the amplitude of the main interference peak 801, thereby obtaining the polarization crosstalk intensity 709 corresponding to the coupling point.

[0035] For the Y-waveguide DUT 206Y, the method for determining the polarization crosstalk position 708 is as follows:

[0036] 1) The main delay fiber 305 is a single-mode fiber with a geometric length of Δl, and its corresponding optical path is calculated as X2=Δl×n, where n is the refractive index of the single-mode fiber;

[0037] 2) The pigtail 203 at the output end of the polarizer is a panda-type polarization-maintaining fiber with a geometric length of l1, and its corresponding optical path is calculated as S1=l1×Δn; the pigtail 209 at the input end of the analyzer is a panda-type polarization-maintaining fiber with a geometric length of l2, and its corresponding optical path is calculated as S2=l2×Δn, where Δn is the linear birefringence coefficient of the polarization-maintaining fiber;

[0038] 3) The geometric length of the pigtail 206a at the input end of the device under test is l3, the geometric length of the pigtail 206b at the output end of the device under test is l4, and the chip length of the Y-waveguide device under test 206Y is l5. The corresponding optical path is calculated and recorded as S=S3+S4+S5=l3×Δn+l4×Δn+l5×Δn Y , where Δn is the linear birefringence coefficient of the polarization-maintaining fiber, Δn Yis the linear birefringence of the Y-waveguide chip;

[0039] 4) If the working mode of the Y waveguide chip is fast axis 710 working, the position corresponding to the characteristic peak of the extinction ratio interference is |X2+S1+S2+S3+S4+S5|; if the working mode of the Y waveguide chip is slow axis 711 working, the position corresponding to the characteristic peak of the extinction ratio interference is |X2+S1+S2+S3+S4-S5|;

[0040] 5) 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 |X2+S1|, |X2+S3+S1|, |X2+S4+S2|, and |X2+S2|, respectively. If l1<l2, the position distribution of the interference peaks is |X2+S1|<|X2+S3+S1|<|X2+S2|<|X2+S4+S2|; if l1>l2, the position distribution of the interference peaks is |X2+S2|<|X2+S4+S2|<|X2+S1|<|X2+S3+S1|;

[0041] 6) 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 |X2+S2|207, and the second connection point 205 and the fourth connection point 208 respectively, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are |X2+S1+S2+S4|, |X2+S1+S2|, |X2+S1+S2+S3+S4|, |X2+S1+S2+S3|.

[0042] Compared with the prior art, the advantages of the present invention are:

[0043] 1. The present invention is a device for fast measurement of low-noise distributed polarization crosstalk in the optical frequency domain, which uses a main interferometer to frequency shift the interference beat signal, thereby eliminating the influence of 1 / f flicker noise at low frequencies and the influence of DC components introduced by photoelectric device acquisition, while avoiding the signal aliasing problem caused by the nonlinearity of the light source frequency sweep at low frequencies. It can not only obtain the polarization crosstalk symmetry information on both sides of the main interference peak, but also improve the system signal-to-noise ratio.

[0044] 2. The present invention realizes transmission-type polarization crosstalk parameter measurement in the optical frequency domain, and uses a highly coherent 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 high energy of the light source, the device can greatly improve the test rate and achieve a larger dynamic range.

[0045] 3. 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 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

[0046] Figure 1 This is a schematic diagram of a device for rapid measurement of distributed polarization crosstalk based on optical frequency domain frequency-shift interferometry.

[0047] Figure 2 This is a schematic diagram of the principle of fast measurement of distributed polarization crosstalk based on optical frequency domain frequency shift interferometry.

[0048] Figure 3 It is a schematic diagram of a device for fast measurement of distributed polarization crosstalk of Y waveguide based on optical frequency domain frequency shift interferometry.

[0049] Figure 4 It is a schematic diagram of a device for fast measurement of distributed polarization crosstalk on an optical fiber ring based on optical frequency domain frequency shift interferometry.

[0050] Figure 5 It is the test result of the distributed polarization crosstalk in the audio domain of the optical fiber ring. DETAILED DESCRIPTION

[0051] In order to more clearly illustrate the distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interferometry proposed by the present invention, the present invention is described in more detail below in combination with the embodiments and drawings, but this should not limit the protection scope of the present invention.

[0052] Specific implementation method 1, a device for fast measurement of Y waveguide polarization crosstalk in the optical frequency domain, as shown in the attached Figure 3 As shown:

[0053] It consists of four parts: tunable laser source module 1, device under test module 2, main interferometer module 3, auxiliary interferometer module 4 and signal acquisition and analysis module 5, among which:

[0054] 1) The tunable laser source module 1 outputs a linear frequency modulated continuous light 602, which is injected into the first coupler 601 from the first coupler input end 601a and is divided into two beams of light;

[0055] 2) The light beam outputted from the first output terminal 601c of the first coupler is injected into the device under test module 2;

[0056] 3) The light beam output from the device under test module 2 is injected into the main interferometer module 3, and injected into the second coupler 302 from the second coupler input end 301 thereof to be divided into two beams of light, one of which is injected into the reference arm 303 as the reference light, and the other is injected into the delay arm 304 as the test light, and passes through the main delay fiber 305 therein, and the light beams output from the reference arm 303 and the delay arm 304 are combined in the third coupler 306 and then divided into two beams, and then differentially detected by the first balanced detector 308;

[0057] The light beam outputted from the second output end 601b of the first coupler is injected into the auxiliary interferometer module 4;

[0058] The main interferometer signal 307 output by the main interferometer module 3 and the auxiliary interferometer signal 407 output by the auxiliary interferometer module 4 are input into the signal acquisition and analysis module 5 for acquisition, processing and demodulation. The signal acquisition and analysis module 5 is composed of a data acquisition card 507 and a computer processing unit 508; the data acquisition card 507 is controlled by the labview software and is used to synchronously acquire and store the analog voltage signal after the photoelectric conversion and convert it into a digital signal; the computer processing unit 508 is used to control the generation and reception of the signal, and to process and demodulate the digital signal.

[0059] The selection and parameters of the main optoelectronic devices in this scheme are as follows:

[0060] 1) The tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning, with a wavelength tuning range of 1490 to 1630 nm, a high coherence mode, a wavelength scanning rate of 80 nm / s, and a wavelength scanning time of 1.75 s, such as a TSL-770 laser from Santec;

[0061] 2) The photosensitive materials of the first balanced detector 308 and the second balanced detector 406 are both 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;

[0062] 3) The 16-bit sampling rate of the data acquisition card 507 is 11.25MHz, the number of sampling points is 25M, and the sampling time is about 2.22s. 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;

[0063] 4) The splitting ratio of the first coupler 601 is 2:98, the splitting ratio of the second coupler 302, the third coupler 306, the fourth coupler 401, and the fifth coupler 405 is 50:50, the extinction ratio is greater than 20dB, the insertion loss is less than 0.5dB, and the operating wavelength covers the 1550nm band;

[0064] 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 fiber, and the pigtail 203 at the output end of the polarizer is a panda-type polarization-maintaining fiber with a diameter of 125um. The length of the polarization-maintaining pigtail is l1=20m. The corresponding optical path S1=l1×Δn≈1×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0065] 6) The working wavelength of the polarization analyzer 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 polarization analyzer is a single-mode fiber, and the pigtail 209 at the input end of the polarization analyzer is a panda-type polarization-maintaining fiber with a diameter of 80um. The length of the polarization-maintaining pigtail is l2=10m. The corresponding optical path S2=l2×Δn≈0.5×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 .

[0066] 7) The welding angle of the first connection point 204 is θ1 = 45°, and the welding angle of the fourth connection point 208 is θ2 = 0°;

[0067] 8) The Y-waveguide DUT 206Y uses lithium niobate crystal as the chip substrate, has an extinction ratio of 60 dB, and works at the fast axis 710. The geometric length of the DUT input end pigtail 206a is l3 = 1 m, and the diameter is 125 um. The geometric length of the DUT output end pigtail 206b is l4 = 2 m, and the diameter is 80 um. The length of the Y-waveguide chip is, and the corresponding optical path S is calculated as S3 + S4 + S5 = l3 × Δn + l4 × Δn + l5 × Δn Y ≈4770um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 , linear birefringence Δn of the Y waveguide chip Y Take 9.34×10 -2 ;

[0068] 9) The main interferometer module 3 adopts a Mach-Zehnder fiber interferometer structure in which the main delay fiber 305 is Δl=3m. The optical path difference X2 corresponding to the main interferometer module 3 in this embodiment is calculated as follows: 6um, where the refractive index n of the single-mode optical fiber is 1.468; the optical path difference X1=S1+S2+S=19770um corresponding to the device under test module 2 in this embodiment is calculated, and X2>2X1 is satisfied, and the parameter design is reasonable;

[0069] 10) The auxiliary interferometer module 4 uses an auxiliary delay fiber 404 for Δl ref =7m Mach-Zehnder fiber interferometer structure, calculate the optical path difference X2=Δl corresponding to the auxiliary interferometer module 4 in this embodiment ref ×n=10.276×10 6 um, where the refractive index n of the single-mode optical fiber is 1.468; therefore, X3≥2X2 is satisfied, and the parameter design is reasonable;

[0070] 11) The position corresponding to the characteristic peak of the extinction ratio interference is |X2+S1+S2+S3+S4+S5|=4423770um;

[0071] 12) 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 respectively

[0072] |X2+S1|=4414000um, |X2+S3+S1|=4414500um,

[0073] |X2+S4+S2|=4410000um, |X2+S2|=4405000um. Since l1>l2, the position distribution of the interference peak is |X2+S2|<|X2+S4+S2|<|X2+S1|<|X2+S3+S1|;

[0074] 13) 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 respectively, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are |X2+S1+S2+S4|=4420000um, |X2+S1+S2|=4415000um, |X2+S1+S2+S3+S4|=4420500um, |X2+S1+S2+S3|=4415500um.

[0075] Specific embodiment 2, a device for fast measurement of optical fiber circular polarization crosstalk in the optical frequency domain, as shown in the attached Figure 4 shown.

[0076] The selection of the main optoelectronic components of the invention and their performance parameters are as follows:

[0077] 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;

[0078] 2) The photosensitive materials of the first balanced detector 308 and the second balanced detector 406 are both 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;

[0079] 3) The 16-bit sampling rate of the data acquisition card 507 is 11.25MHz, the number of sampling points is 18M, the sampling time is about 1.6s, combined with the laser for external triggering, the voltage setting range is 5V, 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 number design is reasonable;

[0080] 4) The splitting ratio of the first coupler 601 is 2:98, the splitting ratio of the second coupler 302, the third coupler 306, the fourth coupler 401, and the fifth coupler 405 is 50:50, the extinction ratio is greater than 20dB, the insertion loss is less than 0.5dB, and the operating wavelength covers the 1550nm band;

[0081] 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 fiber, and the pigtail 203 at the output end of the polarizer is a panda-type polarization-maintaining fiber with a diameter of 80um. The length of the polarization-maintaining pigtail is l6=20m. The corresponding optical path S6=l6×Δn≈1×10 4 um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0082] 6) The working wavelength of the polarization analyzer 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 polarization analyzer is a single-mode fiber, and the pigtail 209 at the input end of the polarization analyzer is a panda-type polarization-maintaining fiber with a diameter of 80um. The length of the polarization-maintaining pigtail is l7=20m. The corresponding optical path S7=l7×Δn≈1×10 4um, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4 .

[0083] 7) The welding angle of the first connection point 204 is θ1 = 45°, and the welding angle of the fourth connection point 208 is θ2 = 0°;

[0084] 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 l8=1854 m. The corresponding optical path S8=l8×Δn is calculated. ring ≈1.08m, where the linear birefringence of the fiber ring Δn ring 5.81×10 -4 ;

[0085] 9) The main interferometer module 3 adopts a Mach-Zehnder fiber interferometer structure in which the main delay fiber 305 is Δl=3.3m. The optical path difference X2=Δl×n=4844400um corresponding to the main interferometer module 3 in this embodiment is calculated, where the refractive index n of the single-mode fiber is 1.468; the optical path difference X1=S6+S7+S8≈1.10m corresponding to the device under test module 2 in this embodiment is calculated, which satisfies X2>2X1, and the parameter design is reasonable;

[0086] 10) The auxiliary interferometer module 4 uses an auxiliary delay fiber 404 for Δl ref =7m Mach-Zehnder fiber interferometer structure, calculate the optical path difference X3 corresponding to the auxiliary interferometer module 4 in this embodiment = Δl ref ×n=10.276×10 6 um, where the refractive index of the single-mode fiber n is 1.468; therefore, X3≥2X2 is satisfied, and the parameter design is reasonable;

[0087] The frequency domain test results are as follows Figure 5 The ordinate of the interference peak 801 represents the normalized polarization crosstalk intensity C1=0 dB corresponding to the polarizer 210, and the abscissa represents the optical path position x1=4796.47 mm corresponding to the polarizer 210. Therefore, the geometric length of the main delay fiber 305 is Δl real =x1 / n=3.267m, where the refractive index n of the single-mode optical fiber is 1.468; the ordinate of the second interference peak 802 represents the polarization crosstalk intensity C2=-30.7dB corresponding to the fourth connection point 208, and the abscissa represents the optical path position x2=4804.98mm corresponding to the fourth connection point 208, so the geometric length l of the analyzer 210 7real =(x2-x1) / Δn=17.02m, where the linear birefringence coefficient Δn of the polarization-maintaining fiber is 5×10 -4The ordinate of the interference peak 803 represents the polarization crosstalk intensity C3 = -27.96 dB corresponding to the first connection point 204, and the abscissa represents the optical path position x3 = 5881.15 mm corresponding to the first connection point 204; the ordinate of the interference peak 804 represents the polarization crosstalk intensity C3 = -25.36 dB corresponding to the polarizer 202, and the abscissa represents the optical path position x4 = 5889.7 mm corresponding to the polarizer 202, so the geometric length l of the polarizer 6real =(x4-x3) / Δn=17.12m, 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 =(x3-x2) / Δn ring ≈1854m, which is consistent with the length measured by the optical time domain reflectometer OTDR.

[0088] The optical fiber ring 206R has a relatively obvious first characteristic peak 805 and a second characteristic peak 806. A series of characteristic peaks between the second interference peak 802 and the third interference peak 803 contain the distributed information of the layer and turn changes of the optical fiber ring 206R.

[0089] 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 distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interferometry, comprising a tunable laser source module (1), a device to be measured module (2), a main interferometer module (3), an auxiliary interferometer module (4) and a signal acquisition and analysis module (5), wherein: 1) The tunable laser source module (1) outputs linear frequency modulated continuous light (602), which is injected into the first coupler (601) from the first coupler input end (601a) and divided into two beams of light; 2) the light beam outputted from the first output end (601c) of the first coupler is injected into the device under test module (2); the device under test module (2) is connected in sequence by a pigtail fiber (201) at the input end of the polarizer, a polarizer (202), a pigtail fiber (203) at the output end of the polarizer, a first connection point (204), a pigtail fiber (206a) at the input end of the device under test, a second connection point (205), the device under test (206), a third connection point (207), a pigtail fiber (206b) at the output end of the device under test, a fourth connection point (208), a pigtail fiber (209) at the input end of the analyzer, an analyzer (210), and a pigtail fiber (211) at the output end of the analyzer; 3) The light beam output from the device under test module (2) is injected into the main interferometer module (3), and injected into the second coupler (302) from the second coupler input end (301) thereof to be divided into two light beams, one of which is injected into the reference arm (303) as the reference light, and the other is injected into the delay arm (304) as the test light, and passes through the main delay optical fiber (305), and the light beams output from the reference arm (303) and the delay arm (304) are combined in the third coupler (306) and then divided into two beams, and then differentially detected by the first balanced detector (308); 4) The light beam outputted from the second output end (601b) of the first coupler is injected into the auxiliary interferometer module (4) by the fourth coupler (401); the light beam injected into the fourth coupler (401) is divided into two beams, one beam is injected into the reference arm (402) as the reference light, and the other beam is injected into the delay arm (403) as the test light, and passes through the auxiliary delay optical fiber (404); the light beams outputted from the reference arm (402) and the delay arm (403) are combined in the fifth coupler (405) and then divided into two beams, and then differentially detected by the second balanced detector (406); 5) The main interferometer signal (307) output by the main interferometer module (3) and the auxiliary interferometer signal (407) output by the auxiliary interferometer module (4) are jointly injected into the acquisition unit (501) in the signal acquisition and analysis module (5) to perform analog-to-digital conversion of the signals, and then the stored digital main interferometer signal (501a) and the digital auxiliary interferometer signal (501b) are input into the correction unit (502) to correct the nonlinear frequency sweep of the laser, and then the corrected main interferometer signal (502a) is input into the spectrum analysis unit (503) to perform Fourier transform to obtain the frequency domain signal (503a); then the frequency domain signal (503a) is input into the position calculation unit (504), in which the coupling light (704) of a certain coupling point is interfered with the transmission light (701) to generate a signal. The interference beat signal generated is mapped into 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 (503a) obtained in the spectrum analysis unit (503) is input into the intensity calculation unit (505), and the amplitudes of all interference peaks are normalized with the amplitude of the main interference peak (801) 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 amplitude of the main interference peak (801), thereby obtaining the polarization crosstalk intensity (709) of the coupling point, and the distributed polarization crosstalk measurement is completed and ends (506).

2. The distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift 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 distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift 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.

4. The distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift 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°.

5. The distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift 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 main interferometer module (3) is X2, requiring X2>2X1; the optical path difference of the auxiliary interferometer module (4) is X3, requiring X3≥2X2.

6. The distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interferometry according to claim 1 is characterized by: In the acquisition unit (501), 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 distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shift interferometry according to claim 1 is characterized by: The processing method of the correction unit (502) is to use a cubic spline interpolation method to perform nonlinear frequency sweep correction of the laser on the main interferometer signal (214).

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