The Study On Optical Frequency Comparison And Transfer Via Optical Fibers

The high performance of frequency transfer and comparison is the basis of the development of scientific research.In order to meet the high precision requirements of optical frequency transfer and comparison,the optical frequency transfer and comparison with fiber as the medium have become the a hotspot in the field of time and frequency measurement.High-precision optical frequency signals are widely used in many basic scientific research and engineering technology fields.This technology will provide an indispensable time-frequency benchmark for much scientific research.In recent years,the optical clocks have achieved 10-18 level at uncertainty and instability,the traditional comparison method between remote clocks is satellite two-way comparison,the frequency stability of only 10-16 / day,unable to meet the high precision comparison of optical clocks,so the optical fiber for the transfer medium of high-precision two-way optical phase comparison technology came into being.This paper extends the two-way comparison method which proposed by Calosso et al.to demonstrate a two-way optical phase comparison scheme based on local measurement,and establishes a demonstration system over a 50-km spool fiber.Two optical signals of the same source are injected into a bidirectional fiber from both ends,and one of the carriers is sent back from the remote end.In this way,the phases of the two signals can be compared without the implementation of active fiber noise control.This scheme has the distinct advantage of measuring the beat notes only at local site without supplying extra time synchronization signal to trigger the remote instruments.Using a demonstration system with a 50-km fiber spool,the second-order frequency stability is 2.2×10-16,and long-term stability is 5.5×10-21/40000 s.The scheme has less devices,low cost,and higher accuracy.Because there is no closed-loop control of the lock and it has a high system reliability and continuous operation capacity.The current research on the field of optical frequency transfer is focused on ensuring the accuracy of optical transfer under the premise of further improve the transfer distance.At present,the researches at home and abroad mainly focus on spool fiber or short-range field fiber,yet in order to achieve the goal of high-precision optical transfer throughout the country,it is urgent to verify the frequency transfer performance of ultra-long distance fiber link.This paper introduces the principle of optical frequency transfer,and tests the fiber link from Xi'an to Xianyang and an 871.6-km loop in Jiangsu Province.It can be seen that the Xi'an-Xianyang 98-km fiber-optic link is similar to the noise of the fiber-optic link in sections of Jiangsu Province,and there are many non-periodic disturbances.It is found that the transfer precision of Nanjing-Taizhou line is the highest which is based on the phase noise theory.Its frequency stability of 1000-km is 10-19/10000 s.It can be expected to achieve a longer distance transfer through multi-level system cascade,to achieve a nationwide high-precision optical transfer network construction and to provide strong technical support to the future of basic research,high-tech applications and other fields.