Research On Free Space Synchronization Technology Based On Continuous Laser

High-precision time-frequency synchronization technology is widely used in communications,radar,navigation systems,astronomical observation and basic scientific research.With the development of science and technology,especially the arrival of 5G era,the require of time and frequency synchronization accuracy is getting higher and higher.Time and frequency transfer methods include optical fiber transmission,satellite transmission and free space laser transmission.This paper introduces the development status of the above three methods,and then designs the laser-based free space time transfer synchronization.It is not like the optic-al fiber transmission needs special optical channel,only need to be in the visual range,do not need to build a special physical channel.The microwave frequency channel based on satellite time-frequency transmission is getting more and more tight,and the frequency interference problem often occurs.Laser-based time-frequency transmission speed is fast and anti-interference ability is strong,there is no interference between frequencies.Therefore,laser-based free space time synchronization research is of great significance.In this paper,a round trip time transfer synchronization scheme based on a small CW laser is designed,and the time interval measurement and phase compensation of the laser transmission signal are completed.Finally,an optical experimental platform is built.This paper adopts the method of combining coarse and fine time measurement to measure the time interval.Through combining with the internal resources of the FPGA,the tap delay method is used to measure the fine time,and the direct counting method is used to measure the coarse time.This method not only ensures the accuracy,but also enlarges the measurement range.In this paper,we use the carry chain(CARRY4)cascade of Kintex-7 internal resources to construct delay lines.Due to the phenomenon of bubbling caused by carry forward and the need to convert the thermometer codes locked in the cascaded carry chain into binary numbers,this design adopts the method of division and sum to decode.In order to improve the accuracy of measurement,it is necessary to reduce the wiring delay between carry units.We use the device position constraint method to arrange the carry elements in order and channel in parallel.After verification by board level test,the results show that the time interval error of the measurement is within 190 ps.There is phase difference in time signal after free space transmission,so phase compensation is needed.Combined with the internal SelectIO resources of the FPGA,we use the method of combining coarse delay and fine delay to compensate the phase difference signals.The signal is sampled by high-speed IO,and the signal is converted by using the deserializer and the serializer.After the board level test,the test results showed that the delay fluctuation is 625 ps,and the precision delay is 78 ps.Finally,based on the above time interval measurement and phase compensation method,an optical experiment platform is built.In this paper,a free space time transfer link based on 40 m laser transmission is designed for experiment.The test results show that the delay fluctuation range of the designed synchronization system after phase compensation is 3ns,and the standard deviation is 440 ps.The overall measurement results show that the time stability is better than 200 ps,and when the time is greater than 500 s,the time stability is around 100 ps.The experimental results show that the free space time stability of laser is better than that of satellite.