The Technology Of Fiber Delay Line For Time And Frequency Transfer

Fiber Optic Time and Frequency Transfer has attracted numerous attentions in decades.Due to the low power loss,low noise and and wide-bandwidth,techniques for transmitting the frequency reference signals of high stability by fibers have prominent advantages.However,the fibers are susceptible to the environment and lead to the fluctuation of time delay,which would restrict the stable remote transmission of the frequency signal.Therefore,a specific compensated device is needed to offset the fluctuation in real time.In this paper,we introduce the compensated device—fiber delay line used in Fiber Optic Time and Frequency Transfer.According to the growth modes of delay,it can be divided into two types: digital incremental and analog continuous.In the digital incremental method,fiber delay lines mainly set in advance a plurality of channels with different time delays,and then use optical switches to select different channels to achieve different time delays.It is easy to realize the large-scale integration for this approach.However,the time delay is discrete and can only be adjusted in increments.So analog continuous delay line is another alternative,in which optical paths can be altered by tuning the spatial length of light travel or by changing the refractive index of the transmission medium.Compared with the former one,this method can allow the continuous adjustment.Nevertheless,the major difficulty is the implementation of precise delay.In order to satisfy the requirement of Fiber Optic Time and Frequency Transfer,we need fiber delay lines with a large dynamic range and an adjustable delay.Thus,we proposed and designed Temperature-controlled Fiber Delay Line applied in Fiber Optic Time and Frequency Transfer whose temperature range is 9°C to 60°C and dynamic range of the optical delay is 4050 ps.According to the various parameters required for the temperature-controlled delay line,we conducted thermal simulation,determined the power of the semiconductor cooling sheet,produced a temperature-controlled device and completed experimental test of the temperature-controlled delay line.The related phase detection and control module are also designed and manufactured so that it can be served in the Fiber Optic Time and Frequency Transfer.Our work mainly includes the selection,improvement and correction of the phase detection scheme,analog-to-digital conversion of the voltage sampling,processing of internal data and feedback adjustment procedures,schematics of controlling the circuit board and fabrication of printed circuit boards.In addition,we carried out verification tests of the entire phase detection and controlling module to ensure the reliability of the module's function and the result showed that the accuracy is 0.6°.Regarding the display of data,the application of HMI screen shows a high mobility by avoiding the common computer screen.Finally,a comparative experiment was implemented to compensate for the time delay fluctuation of the Fiber Optic Time and Frequency Transfer within ±17ps,the distance of transmission is 20 km,and the test time is about 22 hours.It is better than the ±50ps stability of Tsinghua University's project.As a consequence,it successfully verified that our proposed temperature-controlled delay line and its corresponding control module can be obviously improve the performance of the Fiber Optic Time and Frequency Transfer.