Dispersion Compensation Method for Accuracy Improvement of Optical Frequency Domain Polarimetry
Objective Long-distance polarization-maintaining fiber is mainly used in submarine cables,optical fiber sensor networks,navigation and positioning,geophysical surveys,and other fields.One of the key concerns in the measurement field is the polarization crosstalk of the core component of the optical fiber gyroscope used for navigation and positioning.The optical frequency domain polarimetry(OFDP)is a new polarization measurement method that has advantages such as long measurement distance,high sensitivity,and small measurement times.It can be widely used to accurately characterize polarization-maintaining fibers and their devices.The main performance limitation of OFDP is interference phase error.While some parts of this error,such as the tunable light source's intrinsic phase noise,sweep frequency nonlinearity,and ambient noise,have been effectively suppressed,residual interference phase noise still exists.This residual noise can degrade the accuracy of polarization crosstalk measurements and is mainly caused by dispersion in the test optical path and birefringent dispersion in the device under test.Existing optical fiber dispersion compensation methods are mostly applied to absolute distance measurements and do not meet the requirements of distributed and transmitted optical polarimeters.We propose an OFDP optical path scheme(SR-OFDP)based on a self-reference interferometer.The accuracy of polarization crosstalk measurement is enhanced through the application of distributed iterative dispersion compensation technology.We hope that the chromatic dispersion verification method and the concept of distributed iterative dispersion compensation proposed in this study will contribute to the advancement of distributed dispersion compensation techniques in the optical frequency domain.Methods We first review the basic principles and testing scheme of OFDP.We then theoretically analyze the phase distortion caused by chromatic and birefringent dispersion and discuss the corresponding suppression schemes.Notably,due to the similarity between the phase term introduced by chromatic dispersion and the swept nonlinearity of the light source,it is possible to match the dispersion coefficient by adjusting the length of the delay ring in the interferometer.Then,we employ the SR-OFDP optical path scheme,where phase noise induced by chromatic dispersion in the optical path is eliminated through interpolation resampling.To mitigate the phase error caused by birefringent dispersion in the long-distance polarization-maintaining fiber ring,we propose a distributed iterative dispersion compensation scheme based on optimal criteria.The core idea is to use the criterion function to obtain the total second-and third-order dispersions of the measured fiber,and construct the corresponding dispersion compensation convolution kernel to convolve with the compensated signal,and finally,the phase error of wave number domain is compensated by dispersion in space domain.Results and Discussions In the OFDP accuracy optimization scheme,the experimental design and results for dispersion suppression are shown as follows.Firstly,experiments are designed to identify the source of chromatic dispersion in the optical path.The original single-mode fiber inside the auxiliary interferometer is replaced with a dispersion compensation fiber with a dispersion coefficient of-100 to-200 ps/(nm/km),simulating the case of mismatched dispersion coefficients in the interferometer.When the dispersion coefficients are approximately the same,a 500-meter polarization-maintaining fiber is tested.The amplitude accuracy of the polarization crosstalk peak at the main peak position improves from-25.5 dB to 0 dB,and the spatial resolution increases from 22.41 m to 9.6 cm(Fig.4),confirming that the dispersion in the measured optical path is caused by differences in the interferometer's dispersion coefficient.Subsequently,the SR-OFDP optical path scheme is employed to suppress this dispersion(Fig.5).Based on this optical path,distributed iterative dispersion compensation technique is used to obtain the distributed polarization crosstalk results of a 9.5 km PMF with a sensitivity of-105 dB in 2 s.After dispersion compensation,the amplitude accuracy of the end peak significantly improves by 20 dB(Fig.6).The forward and backward alignment results of measured fiber dispersion compensation indicate that the dispersion compensation algorithm has good spatial accuracy(Fig.7).In addition,after dispersion compensation,10 groups of repeated test results show that the standard deviation of each position along the fiber length is as low as 0.3 dB(Table 1),indicating that the results after the dispersion compensation algorithm exhibit good stability.Conclusions In the present study,the dispersion compensation method described is used to improve the accuracy of the polarimeter in the optical frequency domain,filling the gap in fiber dispersion compensation methods for optical frequency domain interferometry.Compared with traditional polarization measurement technologies(such as OCDP),which only satisfy a single index,the SR-OFDP technology with dispersion compensation capability offers distributed measurement,high sensitivity,and long-distance testing.The test speed can reach the order of seconds,and its excellent comprehensive performance is incomparable to other technologies.In the future,OFDP technology will play a key role in the production and fault analysis of high-precision optical fiber gyroscopes,provide more accurate testing means for optical fiber devices,components,and optical paths,and promote the further development of distributed polarization crosstalk testing technology.
distributed optical measurementoptical frequency domain polarimeterdispersion compensationpolarization maintaining fiber and deviceoptical fiber gyroscope
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