CN113804301A - Distributed polarization crosstalk rapid measuring device based on optical frequency domain frequency shift interference - Google Patents
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Abstract
The invention belongs to the technical field of optical fiber measurement, and particularly relates to an optical fiber distributed polarization crosstalk rapid measurement device based on optical frequency domain interference, which comprises a tunable laser source module (1), a device module to be measured (2), an auxiliary interferometer module (3) and a signal acquisition and analysis module (4), and is characterized in that: the method comprises the steps of utilizing a high-coherence tunable laser to carry out rapid wavelength scanning on an optical signal, outputting linear frequency modulation continuous light (501), sequentially passing through a polarizer (202), a device to be tested (206) and an analyzer (210), obtaining beat frequency signals generated by interference of coupled light (704) and transmitted light (701) on an optical frequency domain, and then calculating to obtain an optical path difference and a normalized interference signal amplitude value, thereby respectively determining the polarization crosstalk position and intensity. The invention combines the tunable laser to get rid of the dependence on an optical path scanning retarder in the polarization crosstalk measurement, greatly improves the speed and the dynamic range of the polarization crosstalk measurement, and has compact structure, stability, reliability and easy realization.
Description
Technical Field
The invention belongs to the technical field of optical fiber measurement, and particularly relates to a distributed polarization crosstalk rapid measurement device based on optical frequency domain frequency shift interference.
Background
An optical coherence domain polarization measurement technique (OCDP) based on the white light interference principle is an important technique for detecting the characteristics of distributed optical polarization crosstalk, mainly aims at high-precision analysis of the position and intensity of the polarization crosstalk, can obtain information such as stress, temperature and the like of an external environment by measuring the distribution of the polarization crosstalk along the transmission direction, and provides necessary guidance for production detection of polarization-maintaining optical fibers, winding processes of optical fiber rings, accurate welding of the polarization devices and the like while testing and evaluating the performance of the polarization optical devices. The OCDP technology is widely applied to the fields of polarization-maintaining optical fiber high-precision axis alignment, device extinction ratio testing, fiber optic gyroscope ring testing and the like by virtue of the advantages of high measurement sensitivity, high spatial resolution, large dynamic range and the like.
In recent years, researchers have conducted a lot of research on the improvement of the testing performance of the OCDP device, and mainly pay attention to the improvement and correction of parameters such as the testing range, the measuring symmetry, the measuring precision, the positioning precision, the dynamic range, the measuring speed, the sensitivity, and the like, and the suppression of noise, polarization fading, and dispersion, and meanwhile, the researchers are dedicated to the improvement research of the testing method steps and the peak analysis method.
In terms of noise suppression, in 2014, the applicant satisfied the minimum reflection condition at the brewster angle (an optical coherent polarization measurement device capable of suppressing interference noise, 201410120901.4) for the polarization state of the input light to eliminate the residual reflected light signal in the device, thereby suppressing the optical interference noise; the applicant of the same year utilizes a combination of a high-polarization degree light source and a high-extinction ratio polarizer (a device and a detection method for suppressing the measurement noise of polarization crosstalk by using the light source, 201410120580.8) to suppress the measurement noise in OCDP. In terms of test range, U.S. Patent No. 2007, to Oleg m.efimov et al, discloses a Digitally controlled optical fiber delay line (US Patent, 7283708B2) that uses a fiber array of multiple fiber modules to achieve optical path expansion; the applicant disclosed in 2013 an optical path extension structure (201310739313.4) formed by connecting multiple independent optical path extension units in series, which can realize infinite extension of scanning optical path and improve the measurement accuracy of polarization crosstalk by paired optical devices in each extension unit.
However, in the existing continuously improved optical coherence domain polarization measurement technology OCDP scheme, there still exist some performance parameter compromises and pinning problems. If the time and speed of optical path scanning are sacrificed, the mechanical program control displacement table is enabled to carry out slow movement scanning to ensure the high precision of polarization crosstalk measurement; if the mechanical program-controlled displacement table is used for controlling optical path scanning, the reliability of the system is reduced due to the use of moving parts; the development of extended path scan retarders to extend the test range introduces additional errors and increases the complexity of the system. In addition, because the energy of a wide-spectrum light source used in the optical coherence domain polarization measurement technology OCDP is low, the signal-to-noise ratio is insufficient, and the dynamic range cannot be greatly improved all the time.
Based on the existing techniques and the existing problem of low signal-to-noise ratio in the coherent domain measurement, researchers have tried to combine polarization and frequency domain, and proposed a polarization-sensitive optical frequency domain reflection technique (P-OFDR), and perform measurement of polarization parameters such as distributed birefringence and polarization mode dispersion in the optical frequency domain. Meanwhile, by virtue of the advantages of high sensitivity, long testing distance and the like, the technology also measures backward Rayleigh scattering signals and is expanded to the fields of distributed vibration sensing, distributed sound wave sensing and the like.
The linear sweep frequency narrow linewidth light wave is injected into an interference system (a high-resolution sensing system for measuring beat length and strain of a polarization maintaining optical fiber based on an OFDR system, 201310013952.2) through a polarizer by Europe, China and the like of the university of electronic science and technology in 2013, and then the purposes of accurately controlling the polarization state, obtaining a stable polarization mode and improving the strain measurement precision are achieved by combining optical heterodyne detection. 2017, Tianjin university Liu Tie root et al discloses a polarization-sensitive DAS device (a distributed optical 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 realize distributed acoustic sensing with a large dynamic range and high spatial resolution. In 2020, kener et al of Luna corporation, usa proposed an autocorrelation and cross-correlation demodulation method (US 2020028204) for measuring polarization from optical frequency domain, and the signal-to-noise ratio and sensitivity of the system were greatly improved by obtaining backward rayleigh scattering signals by P-OFDR and then demodulating them.
Compared with an optical coherent domain polarization measurement technology OCDP, a polarization-sensitive optical frequency domain reflection technology (P-OFDR) makes full use of the characteristics of a tunable laser to greatly improve the measurement speed and the signal-to-noise ratio, but cannot measure critical polarization crosstalk parameters, and has a serious problem of nonlinear frequency sweeping of a light source.
Because the optical frequency of the tunable laser does not perform ideal linear change along with time, different optical frequency intervals are generated between adjacent sampling points, so that the signal peak after Fourier transform is reduced and broadened, and further the spatial resolution and the measurement precision are degraded. Aoyao, Ottawa university in 2014, uses an optimized swept-Frequency nonlinear compensation algorithm to increase Sensing length while maintaining High measurement Resolution and accuracy, and achieves a Spatial Resolution of 0.3mm on a single-mode fiber with a length greater than 300m (X.Bao, et a1, Long-Range High Spatial Resolution Distributed measurement and string Sensing Based on Optical Frequency-Domain reflection. IEEE Photonics J.2014, 6, 6801408.). The major interferometer signal was real-time corrected by the zhuyu interval of the auxiliary interferometer in 2019 by zhuyu nah, nanjing university, and resampled by cubic spline interpolation, while eliminating the length requirement for the delay fiber of the auxiliary interferometer (a method for correcting the nonlinear sweep frequency of a tunable laser in an optical frequency domain reflectometer, 201910084695.9).
The invention discloses a distributed polarization crosstalk rapid measuring device based on optical frequency domain frequency shift interference, which is improved based on the prior art, and is characterized in that wavelength scanning is carried out on an optical frequency domain to realize transmission type polarization crosstalk parameter measurement, the nonlinear sweep frequency of a light source is corrected by the cubic spline interpolation method, and the polarization crosstalk measurement speed is greatly improved while the signal-to-noise ratio and the dynamic range are larger. In addition, the invention uses the main interferometer to carry out frequency shift on the interference beat frequency signal, thereby eliminating the problem of signal aliasing caused by 1/f low-frequency flicker noise and light source frequency sweep nonlinearity, and also acquiring the polarization crosstalk symmetry information at two sides of the main interference peak.
Disclosure of Invention
The invention aims to provide a distributed polarization crosstalk rapid measuring device based on optical frequency domain frequency shift interference, which essentially improves the polarization crosstalk testing speed and dynamic range, and simultaneously overcomes the problems of low-frequency flicker noise and signal aliasing.
The aim of the invention is achieved by the following measures:
the utility model provides a distributed polarization crosstalk quick measuring device based on optical frequency domain frequency-shift interference, includes tunable laser source module 1, device module 2, main interferometer module 3, supplementary interferometer module 4 and signal acquisition analysis module 5 that awaits measuring, characterized by:
the tunable laser source module 1 outputs chirped continuous light 602, which is injected into the first coupler 601 from the first coupler input end 601a and is divided into two beams;
injecting the light beam output by the first output end 601c of the first coupler into the device to be tested module 2;
the light beam output from the device module 2 to be tested is injected into the main interferometer module 3, the light beam is injected into the second coupler 302 from the second coupler input end 301 and is divided into two beams, one beam is used as reference light and is injected into the reference arm 303, the other beam is used as test light and is injected into the delay arm 304, the two beams pass through the main delay optical fiber 305, the light beams output by the reference arm 303 and the delay arm 304 are combined in the third coupler 306 and then are divided into two beams, and then the two beams are subjected to differential detection by the first balanced detector 308;
the light beam output by the second output end 601b of the first coupler is injected into the auxiliary interferometer module 4;
the start frequency of the tunable laser source module 1 is v1The termination frequency is v2Has v at2-v1Where τ is the frequency sweep time, the spatial resolution is δ ≦ c/(v)2-v1) Δ n ═ c/γ τ Δ n, where Δ n is the linear birefringence of the polarization maintaining fiber;
the device module 2 to be tested is sequentially connected with a polarizer input end tail fiber 201, a polarizer 202, a polarizer output end tail fiber 203, a first connecting point 204, a device to be tested input end tail fiber 206a, a second connecting point 205, a device to be tested 206, a third connecting point 207, a device to be tested output end tail fiber 206b, a fourth connecting point 208, a polarization analyzer input end tail fiber 209, a polarization analyzer 210 and a polarization analyzer output end tail fiber 211;
the optical path difference of the device module 2 to be tested is X1；
The polarizing angle of the polarizer 202 is 0 degree, the polarizer input end pigtail 201 is a single mode fiber, and the polarizer output end pigtail 20 is a polarization maintaining fiber;
the polarization analyzing angle of the polarization analyzer 210 is 45 degrees, the tail fiber 209 at the input end of the polarization analyzer is a polarization maintaining fiber, and the tail fiber 211 at the output end of the polarization analyzer is a single mode fiber;
when the parameter to be measured of the device under test 206 includes a high extinction ratio, the welding angle of the first connection point 204 is θ1When the parameter to be measured of the device under test 206 is the polarization crosstalk of the common coupling point, the welding angle of the first connection point 204 is θ ± 25 °10 ° ± 2 ° or θ1＝90°±2°；
The welding angle of the fourth connection point 208 is θ20 ° ± 2 ° or θ2＝90°±2°；
The optical path difference of the main interferometer module 3 is X2Requires X2＞2X1；
The light beam injected into the fourth coupler 401 is divided into two beams, one beam is injected into the reference arm 402 as reference light, the other beam is injected into the delay arm 403 as test light, and passes through the auxiliary delay fiber 404, and the light beams output by the reference arm 402 and the delay arm 403 are divided into two beams after being combined in the fifth coupler 405, and then are differentially detected by the second balanced detector 406;
the optical path difference of the auxiliary interferometer module 4 is X3Requires X3≥2X2；
In the acquisition unit 501, the sampling time is t, and the sampling rate is fsThe number of sampling points is M, t is required to be more than or equal to tau, namely fs≥M/τ；
The processing method of the correction unit 502 is to perform nonlinear sweep frequency correction of the laser on the main interferometer signal 214 by cubic spline interpolation.
The principle and process of performing distributed polarization crosstalk measurement on the device under test 206 is as follows:
the polarization-maintaining device to be tested has two orthogonal characteristic axes, and refractive index difference exists between two linearly polarized light beams orthogonal in the directions, so that the transmission rates of light waves in the fast axis 710 and the slow axis 711 are different, the axis with the large refractive index is the slow axis 711, and the axis with the small refractive index is the fast axis 710. Polarizer 202 modulates the linearity emitted by narrow-linewidth tunable laser module 1The frequency continuous light 602 becomes linearly polarized and is injected into the principal axis of operation (e.g., fast axis 710) of the dut 206. According to the optical path tracking schematic diagram shown in fig. 2, two ends of the device under test 206 are respectively connected to the 0 ° polarizer 202 and the 45 ° analyzer 210 to generate two connection points with different polarization crosstalk intensities, and the welding angle of the first connection point 204 is θ1The welding angle of the fourth connection point 208 is θ at 90 °20 deg.. Linearly polarized light operating in the principal axis couples to its orthogonal axis (e.g., fast axis 7110 couples to slow axis 711) through these two junctions, forming coupled light 704, a phenomenon known as polarization crosstalk. However, the linearly polarized light that has been working on the principal axis is the transmission light 701, and a time delay, an optical path difference, and a phase difference are generated between the transmission light 701 and the coupling light 704. The fourth connection point 208 with-40 dB of polarization crosstalk strength corresponds to the a-coupled light 703, and the first connection point 204 with-30 dB of polarization crosstalk strength corresponds to the b-coupled light 702. The optical path difference between the transmitted light 701 and the light generated by the propagation of the a-coupled light 703 on the slow axis 711 is denoted as L1＝cτ1The optical path difference between the transmitted light 701 and the light generated by the/Δ n, b-coupled light 702 propagating on the slow axis 711 is denoted as L2＝cτ2Δ n, where Δ n is the refractive index difference between the fast axis 710 and the slow axis 711, c is the speed of light, τ1And τ2The time delays of the a-coupled light 703 and the b-coupled light 702, respectively. Thus only τ needs to be acquired1And τ2The polarization crosstalk location 708 corresponding to the connection point is obtained.
However, the setting of the splitting ratio of the optical device and the circuit acquisition setting can introduce direct current components at low frequency to generate extra power, and meanwhile, serious 1/f flicker noise and sweep frequency nonlinearity exist at the low frequency to bring about the problem of signal aliasing, so that the interference peak containing polarization crosstalk information is shifted to a non-low frequency position by using the optical path difference of the main interferometer module 3 to obtain complete and correct information.
The scanning test process of the device of the present invention is shown in fig. 2, in which the slope γ represents the frequency scanning rate, Δ τ represents the test time, and the sinusoidal waveforms with different frequencies are used to respectively represent the time-domain interference beat signal 707, including the frequency-shifted a-coupled optical beat signal 706 in the time domain and the frequency-shifted b-coupled optical beat signal in the time domainThe frequency signal 705. The specific process is as follows: the beat frequency between the transmitted light 701 and the coupled light 704 is mapped to the time delay by the frequency sweep rate γ using the chirped continuous light 602 in the optical frequency domain, i.e. the corresponding beat frequency Δ f of the first connection point 204 and the fourth connection point 208 is used1And Δ f2The time delay tau is obtained from the mapping relation1And τ2. And then, the optical path difference of the main interferometer module 3 is used for introducing a time delay tau ', so that the time delays of the transmission light 701, the a-coupled light 703 and the b-coupled light 702 are simultaneously increased by tau', and further, the beat frequency delta f between the transmission light 701 and the frequency shift a-coupled light 712 is simultaneously increased1Beat frequency Δ f between transmission light 701 and frequency-shifted b-coupled light 7132'. By measuring Δ f1' and Δ f2', and then τ is derived from the mapping1+ τ' and τ2+ tau', then demodulating the optical path difference L generated by the corresponding connection point according to the relation between the time delay and the optical path difference3＝c(τ1+ τ')/Δ n and L2＝c(τ2+ τ')/Δ n, resulting in a polarization crosstalk location 708. Meanwhile, the amplitudes of the interference peaks corresponding to the first connection point 204 and the fourth connection point 208 are normalized by taking the amplitude of the main interference peak 801 as a reference, and the normalized amplitude of the interference peak is a ═ a1-A2Wherein A is1To normalize the amplitude of the front interference peak, A2The amplitude of the main interference peak 801, and thus the polarization crosstalk strength 709 corresponding to the coupling point is obtained.
For the Y-waveguide DUT 206Y, the method of determining the polarization crosstalk position 708 is as follows:
1) the main delay fiber 305 is a single mode fiber with a geometric length Δ l, and its corresponding optical path is calculated and recorded as X2Δ l × n, where n is the refractive index of the single-mode fiber;
2) the tail fiber 203 at the output end of the polarizer has a geometric length of l1The panda type polarization maintaining fiber calculates the corresponding optical path and is marked as S1＝l1X Δ n; the polarization analyzer input end pigtail 209 has a geometric length of l2The panda type polarization maintaining fiber calculates the corresponding optical path and is marked as S2＝l2X Δ n, where Δ n is the linear birefringence of the polarization maintaining fiber;
3) the geometrical length of the tail fiber 206a at the input end of the device to be tested is l3The geometrical length of the tail fiber 206b at the output end of the device under test is l4The chip length of the Y waveguide device under test 206Y is l5Calculating the corresponding optical path, and recording as S ═ S3+S4+S5＝l3×Δn+l4×Δn+l5×ΔnYWhere Δ n is the linear birefringence of the polarization maintaining fiber, Δ nYIs the linear birefringence of the Y waveguide chip;
4) if the operation mode of the Y waveguide chip is fast axis 710 operation, the position corresponding to the extinction ratio interference characteristic peak is | X2+S1+S2+S3+S4+S5L, |; if the operation mode of the Y waveguide chip is the slow axis 711 operation, the position corresponding to the characteristic peak of extinction ratio interference is | X2+S1+S2+S3+S4-S5|；
5) When the light beam is 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|、|X2+S2If l1＜l2The position distribution of the interference peak is | X2+S1|＜|X2+S3+S1|＜|X2+S2|＜|X2+S4+S2L, |; if l1＞l1The position distribution of the interference peak is | X2+S2|＜|X2+S4+S2|＜|X2+S1|＜|X2+S3+S1|；
6) When the light beam passes 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 | X, respectively2+S2After |207, the second connection point 205 and the fourth connection point 208 are coupled twice, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are respectively | X2+S1+S2+S4|、|X2+S1+S2|、|X2+S1+S2+S3+S4|、|X2+S1+S2+S3|。
Compared with the prior art, the invention has the advantages that:
1. the invention relates to a device for quickly measuring low-noise distributed polarization crosstalk in an optical frequency domain, which uses a main interferometer to carry out frequency shift on an interference beat signal, further eliminates the influence of 1/f flicker noise at a low frequency and the influence of direct current components introduced by collection of a photoelectric device, simultaneously avoids the problem of signal aliasing caused by nonlinear sweep frequency of a light source at the low frequency, can obtain the symmetry information of the polarization crosstalk at two sides of a main interference peak, and can also improve the signal-to-noise ratio of a system.
2. The invention realizes the transmission-type polarization crosstalk parameter measurement in the optical frequency domain, and utilizes the high-coherence tunable laser source to carry out rapid wavelength scanning on 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 testing speed and simultaneously realize a larger dynamic range.
3. The invention can avoid the structure of a mechanical scanning delayer in the traditional coherent domain polarization test, the optical path scanning range is not limited by the length of the optical fiber delay line any more, and only relates to the detection bandwidth and the sampling rate, so the range of the distributed polarization crosstalk test is greatly expanded. The device realizes the all-fiber light path without moving parts, so the device has the advantages of compact structure, stability, reliability, easy realization and the like, and is beneficial to the popularization of new technology.
Drawings
FIG. 1 is a schematic diagram of an apparatus for distributed polarization crosstalk fast measurement based on optical frequency domain frequency-shift interference.
FIG. 2 is a schematic diagram of the principle of distributed polarization crosstalk fast measurement based on optical frequency domain frequency-shift interference.
FIG. 3 is a schematic diagram of an apparatus for fast measurement of distributed polarization crosstalk based on optical frequency domain frequency-shift interference on a Y waveguide.
FIG. 4 is a schematic diagram of an apparatus for performing distributed polarization crosstalk fast measurement based on optical frequency domain frequency-shift interference on an optical fiber loop.
Fig. 5 is a frequency domain test result of distributed polarization crosstalk for a fiber optic ring.
Detailed Description
In order to more clearly illustrate the distributed polarization crosstalk fast measuring apparatus based on optical frequency domain frequency shift interference of the present invention, the following describes the present invention in more detail with reference to the following embodiments and the accompanying drawings, but should not limit the scope of the present invention.
the interferometer comprises a tunable laser source module 1, a device module to be measured 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 chirped continuous light 602, which is injected into the first coupler 601 from the first coupler input end 601a and is divided into two beams;
2) injecting the light beam output by the first output end 601c of the first coupler into the device to be tested module 2;
3) the light beam output from the device module 2 to be tested is injected into the main interferometer module 3, the light beam is injected into the second coupler 302 from the second coupler input end 301 and is divided into two beams, one beam is used as reference light and is injected into the reference arm 303, the other beam is used as test light and is injected into the delay arm 304, the two beams pass through the main delay optical fiber 305, the light beams output by the reference arm 303 and the delay arm 304 are combined in the third coupler 306 and then are divided into two beams, and then the two beams are subjected to differential detection by the first balanced detector 308;
the light beam output by the second output end 601b of the first coupler is injected into the auxiliary interferometer module 4;
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 consists of a data acquisition card 507 and a computer processing unit 508; the data acquisition card 507 is controlled by labview software and is used for synchronously acquiring and storing the analog voltage signals subjected to photoelectric conversion and converting the analog voltage signals into digital signals; the computer processing unit 508 is used to control the generation and reception of signals, as well as to process and demodulate digital signals.
The main photoelectric devices of the scheme are selected and the parameters are as follows:
1) the tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning, the wavelength tuning range is 1490-1630 nm, the high-coherence mode is started, the wavelength scanning rate is 80nm/s, the wavelength scanning time is 1.75s, and for example, a TSL-770 type laser of Santec company is adopted;
2) the photosensitive materials of the first balanced detector 308 and the second balanced detector 406 are all InGaAs, the common mode rejection ratio is 25dB, the optical detection range is 900-1700 nm, the maximum detection bandwidth is 80MHz, the saturation 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 a 1817 type balance detector of New Focus company;
3) the 16-bit sampling rate of the data acquisition card 507 is 11.25MHz, the number of sampling points is 25M, the sampling time is about 2.22s, and external triggering is performed by combining a laser, for example, the model number of the Spectrum Instrumentation company is M4 i: 4471-x8, the sampling time is longer than the wavelength scanning time, and the parameter design is reasonable;
4) the splitting ratio of the first coupler 601 is 2: 98, the splitting ratios of the second coupler 302, the third coupler 306, the fourth coupler 401 and the fifth coupler 405 are 50: 50, the extinction ratios are all larger than 20dB, the insertion loss is all smaller than 0.5dB, and the working wavelength covers a 1550nm waveband;
5) the working wavelength of the polarizer 202 covers 1550nm waveband, the polarizing angle is 0 degree, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the tail fiber 201 at the input end of the polarizer is a single-mode fiber, the tail fiber 203 at the output end of the polarizer is a panda-type polarization maintaining fiber with the diameter of 125um, and the length of the polarization maintaining tail fiber is l1The corresponding optical path length S is calculated at 20m1＝l1×Δn≈1×104um, in which the linear birefringence Δ n of the polarization-maintaining fiber is 5 × 10-4；
6) The working wavelength of the analyzer 210 covers 1550nm waveband, the analyzing angle is 45 degrees, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the tail fiber 211 of the output end of the analyzer is a single-mode fiber, the tail fiber 209 of the input end of the analyzer is a panda type polarization maintaining fiber with the diameter of 80um, and the length of the polarization maintaining tail fiber is l2Calculate the corresponding optical path length S at 10m2＝l2×Δn≈0.5×104um, in which the linear birefringence Δ n of the polarization-maintaining fiber is 5 × 10-4。
7) The welding angle of the first connecting point 204 is theta1The welding angle of the fourth connection point 208 is θ at 45 °2＝0°；
8) The Y waveguide device to be tested 206Y takes lithium niobate crystal as a chip substrate, the extinction ratio is 60dB, the Y waveguide device to be tested works in the fast axis 710, and the geometric length of the tail fiber 206a at the input end of the device to be tested is l31m, the diameter is 125um, and the geometrical length of the tail fiber 206b at the output end of the device under test is l42m, diameter 80um, length of Y waveguide chip, calculating corresponding optical path S ═ S3+S4+S5＝l3×Δn+l4×Δn+l5×ΔnYApproximately 4770um, where the linear birefringence Δ n of the polarization maintaining fiber takes the form of 5 × 10-4Linear birefringence of a Y-waveguide chip, Δ nYTake 9.34X 10-2；
9) The main interferometer module 3 adopts a Mach-Zehnder fiber interferometer structure in which the main delay fiber 305 is 3m Δ l, and calculates the optical path difference X corresponding to the main interferometer module 3 in this embodiment2＝Δl×n＝4.404×106um, wherein the refractive index n of the single mode fiber is 1.468; calculating the optical path difference X corresponding to the device module 2 to be tested in the embodiment1＝S1+S2+ S-19770 um, satisfying X2＞2X1The parameter design is reasonable;
10) the auxiliary interferometer module 4 uses the auxiliary delay fiber 404 as Δ lrefCalculating the optical path difference X corresponding to the auxiliary interferometer module 4 in the embodiment as a 7m Mach-Zehnder fiber interferometer structure3＝Δlref×n＝10.276×106um, wherein the refractive index n of the single mode fiber is 1.468; thus satisfying X3≥2X2The parameter design is reasonable;
11) the position corresponding to the extinction ratio interference characteristic peak is | X2+S1+S2+S3+S4+S5|＝4423770um；
12) When the light beam is 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|＝4414000um、|X2+S3+S1|＝4414500um、|X2+S4+S2|＝4410000um、|X2+S2Since l is 4405000um1＞l2So that the positions of the interference peaks are distributed as | X2+S2|＜|X2+S4+S2|＜|X2+S1|＜|X2+S3+S1|；
13) When the light beams are 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 | X |2+S1+S2+S4|＝4420000um、|X2+S1+S2|＝4415000um、|X2+S1+S2+S3+S4|＝4420500um、|X2+S1+S2+S3|＝4415500um。
The selection of the main photoelectric devices of the device and the performance parameters thereof are as follows:
1) the tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning, the wavelength tuning range is 1510nm to 1620nm, the high-coherence mode is started, the light source output power is 6.4mW, the wavelength scanning rate is 80nm/s, and the wavelength scanning time is 1.375s, for example, a TSL-550 type laser of Santec company is adopted;
2) the photosensitive materials of the first balanced detector 308 and the second balanced detector 406 are all InGaAs, the common mode rejection ratio is 25dB, the optical detection range is 900-1700 nm, the maximum detection bandwidth is 80MHz, the saturation 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 a 1817 type balance detector of New Focus company;
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, the external triggering is performed by combining a laser, the voltage setting range is 5V, and if the model number of a Spectrum Instrumentation company is M4 i: 4471-x8, the sampling time is longer than the frequency scanning time, and the parameter design is reasonable;
4) the splitting ratio of the first coupler 601 is 2: 98, the splitting ratios of the second coupler 302, the third coupler 306, the fourth coupler 401 and the fifth coupler 405 are 50: 50, the extinction ratios are all larger than 20dB, the insertion loss is all smaller than 0.5dB, and the working wavelength covers a 1550nm waveband;
5) the working wavelength of the polarizer 202 covers 1550nm waveband, the polarizing angle is 0 degree, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the tail fiber 201 at the input end of the polarizer is a single-mode fiber, the tail fiber 203 at the output end of the polarizer is a panda-type polarization maintaining fiber with the diameter of 80um, and the length of the polarization maintaining tail fiber is l6The corresponding optical path length S is calculated at 20m6＝l6×Δn≈1×104um, in which the linear birefringence Δ n of the polarization-maintaining fiber is 5 × 10-4；
6) The working wavelength of the analyzer 210 covers 1550nm waveband, the analyzing angle is 45 degrees, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the tail fiber 211 of the output end of the analyzer is a single-mode fiber, the tail fiber 209 of the input end of the analyzer is a panda type polarization maintaining fiber with the diameter of 80um, and the length of the polarization maintaining tail fiber is l7The corresponding optical path length S is calculated at 20m7＝l7×Δn≈1×104um，Wherein the linear birefringence (Δ n) of the polarization maintaining fiber is 5 × 10-4。
7) The welding angle of the first connecting point 204 is theta1The welding angle of the fourth connection point 208 is θ at 45 °2＝0°；
8) The fiber ring 206R is composed of panda-type polarization maintaining fiber with diameter of 80um, and the length measured by OTDR is l8Calculating corresponding optical path length S as 1854m8＝l8×Δnring1.08m, where the linear birefringence Δ n of the fiber loopringIs 5.81X 10-4；
9) The main interferometer module 3 adopts a Mach-Zehnder fiber interferometer structure in which the main delay fiber 305 is 3.3m in Δ l, and calculates the optical path difference X corresponding to the main interferometer module 3 in this embodiment2Δ l × n is 4844400um, where the refractive index n of the single-mode fiber is 1.468; calculating the optical path difference X corresponding to the device module 2 to be tested in the embodiment1＝S6+S7+S81.10m and satisfies X2＞2X1The parameter design is reasonable;
10) the auxiliary interferometer module 4 uses the auxiliary delay fiber 404 as Δ lrefCalculating the optical path difference X corresponding to the auxiliary interferometer module 4 in the embodiment as a 7m Mach-Zehnder fiber interferometer structure3＝Δlref×n＝10.276×106um, wherein the refractive index of the single-mode fiber n is 1.468; thus satisfying X3≥2X2The parameter design is reasonable;
the frequency domain test results are shown in figure 5. The ordinate of the interference peak I801 represents the normalized polarization crosstalk intensity C corresponding to the analyzer 21010dB, the abscissa represents the optical path position x corresponding to the analyzer 21014796.47mm, the geometric length of the main delay fiber 305 is therefore Δ lreal＝x1N is approximately equal to 3.267m, wherein the refractive index n of the single-mode fiber is 1.468; the ordinate of the second interference peak 802 represents the polarization crosstalk intensity C corresponding to the fourth connection point 2082The abscissa represents the optical path position x corresponding to the fourth connection point 208 at-30.73 dB24804.98mm, the geometric length l7 of the analyzer 210 is thereforereal＝(x2-x1) The linear birefringence Δ n of the polarization maintaining fiber is 5 × 10-4(ii) a The ordinate of the interference peak three 803 represents the polarization crosstalk intensity C corresponding to the first connection point 2043The abscissa represents the optical path position x corresponding to the first connection point 204, at-27.96 dB35881.15 mm; the ordinate of the interference peak four 804 represents the polarization crosstalk intensity C corresponding to the polarizer 2024The abscissa represents the optical path position x corresponding to the polarizer 202 at-25.36 dB45889.7mm, the geometric length l of the polarizer is therefore6real＝(x4-x3) The linear birefringence Δ n of the polarization maintaining fiber is 5 × 10 ═ 17.12m-4(ii) a The optical fiber loop 206R has a corresponding geometric length l according to the test result8real＝(x3-x2)/ΔnringIs approximately equal to 1854m and is consistent with the length actually measured by the OTDR.
The optical fiber ring 206R has a first characteristic peak 805 and a second characteristic peak 806 which are relatively obvious, and a series of characteristic peaks between the second interference peak 802 and the third interference peak 803 contain distributed information of layer change and turn change of the optical fiber ring 206R.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. The utility model provides a distributed polarization crosstalk quick measuring device based on optical frequency domain frequency-shift interference, includes tunable laser source module (1), device module (2) that awaits measuring, main interferometer module (3), supplementary interferometer module (4) and signal acquisition analysis module (5), characterized by:
1) the tunable laser source module (1) outputs chirp continuous light (602), and the chirp continuous light is injected into the first coupler (601) from the input end (601a) of the first coupler and is divided into two beams of light;
2) injecting the light beam output by the first output end (601c) of the first coupler into a device module to be tested (2);
3) a light beam output from the device module to be tested (2) is injected into the main interferometer module (3), the light beam is injected into the second coupler (302) from the input end (301) of the second coupler and is divided into two beams, one beam is used as reference light and is injected into the reference arm (303), the other beam is used as test light and is injected into the delay arm (304), the two beams pass through the main delay optical fiber (305), the light beams output by the reference arm (303) and the delay arm (304) are combined in the third coupler (306) and then are divided into two beams, and then differential detection is carried out by the first balanced detector (308);
4) a beam injection auxiliary interferometer module (4) output by the second output end (601b) of the first coupler;
5) a main interferometer signal (307) output by a main interferometer module (3) and an auxiliary interferometer signal (407) output by an auxiliary interferometer module (4) are injected into an acquisition unit (501) in a signal acquisition and analysis module (5) together to perform analog-to-digital conversion of the signals, then a stored digital main interferometer signal (501a) and a stored digital auxiliary interferometer signal (501b) are input into a correction unit (502), nonlinear sweep frequency of a laser is corrected, then the corrected main interferometer signal (502a) is input into a spectrum analysis unit (503), and Fourier transform is performed to obtain a frequency domain signal (503 a); then, the position is input into a position calculation unit (504), an interference beat signal generated by interference of the coupling light (704) and the transmission light (701) of a certain coupling point is mapped into a time delay tau ' through a frequency scanning rate gamma, and a polarization crosstalk position (708) of the coupling point is obtained according to the relation delta L between the time delay tau ' and an optical path difference delta L, namely c tau '/delta n; the frequency domain signal (503a) acquired by the spectrum analysis unit (503) is input into an intensity calculation unit (505), the amplitudes of all interference peaks are normalized by taking the amplitude of the main interference peak (801) as a reference, and the normalized amplitude of the interference peak is A-A1-A2Wherein A is1To normalize the amplitude of the front interference peak, A2The amplitude of the main interference peak (801) is obtained, so as to obtain the polarization crosstalk intensity (709) of the coupling point, and the distributed polarization crosstalk measurement is finished (506).
2. Tunable laser source module (1) according to claim 1, characterized in that:
the start frequency of the tunable laser source module (1) is v1Finally, finallyStop frequency is v2Has v at2-v1Where τ is the frequency sweep time, the spatial resolution is δ ≦ c/(v)2-v1) Δ n ═ c/γ τ Δ n, where Δ n is the linear birefringence of the polarization maintaining fiber.
3. The device under test module (2) according to claim 1, wherein:
the device module (2) to be tested is sequentially connected with a polarizer input end tail fiber (201), a polarizer (202), a polarizer output end tail fiber (203), a first connecting point (204), a device to be tested input end tail fiber (206a), a second connecting point (205), a device to be tested (206), a third connecting point (207), a device to be tested output end tail fiber (206b), a fourth connecting point (208), a polarization analyzer input end tail fiber (209), a polarization analyzer (210) and a polarization analyzer output end tail fiber (211);
the optical path difference of the device module (2) to be tested is X1。
4. A polarizer (202) and analyzer (210) according to claim 3, characterized by:
the polarizing angle of the polarizer (202) is 0 degrees, the tail fiber (201) at the input end of the polarizer is a single-mode fiber, and the tail fiber (203) at the output end of the polarizer is a polarization maintaining fiber;
the polarization analyzing angle of the polarization analyzer (210) is 45 degrees, the tail fiber (209) at the input end of the polarization analyzer is a polarization maintaining fiber, and the tail fiber (211) at the output end of the polarization analyzer is a single mode fiber.
5. The first connection point (204) and the fourth connection point (208) of claim 3, wherein:
when the parameter to be measured of the device to be measured (206) contains high extinction ratio, the welding angle of the first connecting point (204) is theta1When the parameter to be measured of the device under test (206) is the polarization crosstalk of a common coupling point, the welding angle of the first connecting point (204) is theta10 ° ± 2 ° or θ1＝90°±2°；
The welding angle of the fourth connecting point (208) is theta20 ° ± 2 ° or θ2＝90°±2°。
6. A main interferometer module (3) according to claim 1, wherein:
the optical path difference of the main interferometer module (3) is X2Requires X2＞2X1。
7. The auxiliary interferometer module (4) according to claim 1, wherein:
the light beam injected into the fourth coupler (401) is divided into two beams, one beam is injected into a reference arm (402) as reference light, the other beam is injected into a delay arm (403) as test light, the two beams pass through an auxiliary delay optical fiber (404), the light beams output by the reference arm (402) and the delay arm (403) are divided into two beams after being combined in a fifth coupler (405), and then differential detection is carried out by a second balanced detector (406);
the optical path difference of the auxiliary interferometer module (4) is X3Requires X3≥2X2。
8. The acquisition unit (501) of the signal acquisition and analysis module (5) according to claim 1, characterized in that:
in the acquisition unit (501), the sampling time is t, and the sampling rate is fsThe number of sampling points is M, t is required to be more than or equal to tau, namely fs≥M/τ。
9. The calibration unit (502) of the signal acquisition and analysis module (5) according to claim 1, characterized in that:
the processing method of the correction unit (502) is to carry out nonlinear sweep frequency correction of the laser on the main interferometer signal (214) by utilizing a cubic spline interpolation method.
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