Research On Polarization Control In Fiber-Optic Frequency Transfer System

As the instability of the optical frequency standard,represented internationally by the strontium atomic light clock,has achieved the E-19 level,the standard provides high-precision measurement tools for numerous research areas such as navigation and positioning,definition of fundamental physical quantities,geodesy.All these applications require the transmission of high-precision optical frequency signals over long distances,so high-precision transmission technology for optical frequency signals is essential.In recent years,optical fiber links have been regarded as the ideal medium for optical frequency signal transmission due to their low loss and high reliability,and their transmission accuracy is about four to five orders of magnitude higher than that of traditional satellite-based frequency transmission methods.Hence,optical fiberbased optical frequency transmission technology provides a powerful technical support for remote transmission and comparison applications of optical frequency standards.However,as long distance common single-mode fibers are used in the transmission process,the stress on the fiber can cause random changes in the polarization state of the transmitted light,and external temperature changes can also cause a slow drift on the polarization state of the transmitted light.Therefore,in the optical frequency transfer system for out-loop beat frequency measurement,if the polarization state of the transmitted light is different from that of the local reference light,then it will affect the beat frequency power and further deteriorate the transfer signal-to-noise ratio,so the polarization control technology is used to control the polarization state of the transmitted light in the optical frequency transfer engineering has important research significance and practical value.In this paper,the research on polarization control of long-haul optical fibers is carried out in the context of optical frequency transfer to achieve polarization control in optical frequency transfer,and the main work is as follows.First,according to the Stokes vector expression and transmission matrix of the optical polarization state and delay amount adjustable polarization controller,the mathematical model of polarization control in the case of different wavelet-cascaded is derived,and the model of several typical wavelet-cascaded is simulated and verified by Matlab software,the simulation results show that at least three wavelet cascades are needed,i.e.0°-45° The simulation results show that at least three waveplates cascaded,i.e.0°-45°,are needed to achieve non-blind control from any input polarization state to any output polarization state.The limitations of a three-wave cascade polarization control system are also raised,as the polarization control system will not work properly when one of the waveplates reaches its maximum adjustment range.Therefore,through simulation,a four-wave cascade polarization controller,i.e.0°-45°-0°-45° polarization controller,is finally chosen for the next experimental study.Second,a polarization control system for optical frequency transfer is designed and built,including hardware and software.The hardware includes the power detection module,the main control circuit module and the polarization controller module.The software is based on a gradient descent algorithm and a forced reset method to design the polarization control flow and write the polarization control program.An interferometer test platform was built to test the reliability and stability of the polarization control system.The experimental tests were carried out on a short fiber,a50 km laboratory spooled fiber and a 260 km field fiber.The results show that the system is able to increase the feedback radio-frequency power to the maximum by adjusting the polarization state of the transmitted light in about 24 ms,and it can operate stably for a long time in the field fiber test,which meets the engineering requirements of optical frequency transfer.Third,the polarization control system was installed before the optical frequency transfer receiver and the optical frequency transfer engineering tests were carried out on 450 km of laboratory spooled fiber and 260 km of field fiber respectively.The test results show that without polarization control,changes in polarization can lead to deterioration of the out-of-loop beat-note power,thus interrupting the optical frequency transmission and preventing continuous operation of the optical frequency transfer;after the polarization control system has controlled the polarization state of the transmitted light,the optical frequency transfer system achieves long-term stable operation: in the 450 km laboratory spooled fiber and 260 km field fiber tests In the 450 km laboratory spooled fiber and 260 km field fiber tests,the data utilization rate of optical frequency transfer was increased from 70% and 94% to nearly 100%,and the stability index of integration time 10000 s was obtained,which was 7.2E-20/10000 s and 1.4E-19/10000 s respectively,meeting the requirements of transmission accuracy for off-site high-precision optical clock comparison.