Research On Calibration Technology Of Ultra-low Polarization Crosstalk Testing And Its Application

In the optical fiber sensing and testing system,the polarization maintaining fiber(PMF)optical circuit is mostly used as the sensing component in order to ensure a stable polarization state of optical signal transmission,in particular,polarization optical devices such as PMF,polarization maintaining coupler and integrated Y-waveguide are widely used.High-accuracy sensing system requires such devices to have ultra-low polarization crosstalk and ultra-high extinction ratio.Therefore,it is necessary to comprehensively test and evaluate the polarization characteristics of these devices,and then continuously improve the fabrication process,so as to provide high-performance core optical devices for the research and development of highaccuracy optical fiber sensing system.Optical coherence domain polarization(OCDP)measurement technology based on white light interference principle is widely used in distributed polarization crosstalk measurement,which has excellent performance such as high sensitivity,large dynamic range and long measurement distance,and can almost meet the testing requirements of various existing optical devices.However,due to the lack of performance of similar testing instruments and the lack of polarization crosstalk measurement standard system,the polarization crosstalk test cannot be calibrated effectively and the test accuracy cannot be ensured.Therefore,it is urgent to carry out the research on calibration method of ultra-low and wide-range polarization crosstalk.The difficulties of calibration are mainly reflected in two aspects:firstly,the mechanism of polarization crosstalk calibration is not clear,and it is impossible to realize the controllable and accurate generation of ultra-low polarization crosstalk;secondly,there are many factors that affect the calibration process,which will lead to bad calibration accuracy.In addition,when testing polarization optical devices and optical circuit components,the influence originated from the optical circuit under test can introduce measurement error.It is necessary to comprehensively analyze the error sources and effectively suppress them,so as to further improve the test accuracy of polarization crosstalk.This thesis is carried out according to the following idea:the research on ultra-low polarization crosstalk calibration method,the development of calibration apparatus and calibration optical circuit,the performance calibration of test instrument,the test application of polarization optical devices and components.The main research contents are as follows:(1)The high-accuracy calibration method based on single-point alignment rotation is proposed by analogy with the process of polarization crosstalk generated by PMF,and the method of ultra-low crosstalk equivalent generation by two-point polarization crosstalk superposition is proposed to expand the calibration range.Several calibration errors caused by the non-ideal device parameters and the misalignment of fiber connection point in calibration optical circuit are analyzed in detail,and the corresponding error correction formulas are given to ensure the calibration accuracy of polarization crosstalk.(2)An ultra-low and wide-range polarization crosstalk calibration apparatus is developed,which overcomes the controllable generation of second-level rotation angle,the high-precision multi-dimensional adjustment of optical fiber spatial position and the low-stress clamping technology of fiber sample.The comprehensive integration of calibration apparatus is also completed.On this basis,the calibration optical circuit based on single-point fiber rotation and two-point polarization crosstalk superposition are developed respectively.(3)The developed calibration apparatus and calibration optical circuit are used to calibrate the relevant performance of polarization crosstalk test instrument.The results show that the test range is-90～-3 dB and the full range test resolution is better than 0.2 dB.The test accuracy can reach±0.5 dB in the range of-90～-60 dB,and 10.3 dB in the range of-60～-3 dB.(4)The calibrated polarization crosstalk test instrument is used to measure the extinction ratio of Y-waveguide chip,and the measurement error from the optical circuit under test is quantitatively calculated and experimentally verified.In particular,the orthogonal alignment method is proposed to reduce the measurement error induced by optical polarizer to 0.02 dB.In addition,the influence of dispersion introduced by different lengths of PMF is studied,and the dispersion compensation processing is used to achieve effective suppression and thus restore the real measurement information.(5)The polarization characteristics test method of fiber optic gyroscope sensing optical component is proposed.The semi-closed and full-closed optical circuit composed of Ywaveguide and PMF coil are tested once respectively.By using second-order polarization crosstalk effect,the polarization characteristic information such as the polarization crosstalk of all connection points,the extinction ratio of Y-waveguide chip and the crosstalk distribution of PMF coil can be measured.It effectively solves the problem that multi-point information overlaps and cannot be separated in optical circuit test.