| High precision frequency standards and their transmission sharing have become crucial tools in various research fields,such as general relativisty effect verification,accurate punctuality and timing,navigation and positioning,precision testing of basic physical constants,and 5G communication and the Internet of Everything and engineering applications.The long-term stability of modern atomic clocks is close to 10-17.The stability and uncertainty of the state-of-art optical frequency standards have reach 10-17 and 10-18 respectively.The stability of traditional satellite-based frequency transmission technology can only reach 10-15 at 1 day,which can not meet the transmission requirements of the advanced clocks.The use of optical fiber for timefrequency transmission,with higher transmission accuracy,can meet the requirements of the new generation of time-frequency standards.However,the time-frequency signal is limited to the optical fiber,and many applications require more flexible free-space links in some special places where fiber cannot be laid.Many experiments have proved that the laser-based atmospheric tropospheric time-frequency transfer technology can meet the transfer requirements of the most accurate frequency standards available.Based on the above research background,this dissertation carried out the research of free space optical frequency transmission technology and free space optical frequency transmission technology for possible major application needs in the future.The main research contents are as follows:1.Designed a free-space optical carrier RF frequency transmission scheme based on passive phase compensation technology.Without the use of active phase detection and dynamic phase tracking,the DFB semiconductor laser is used to transmit a round-trip 1GHz radio frequency signal and mix with 3GHz of the local site to obtain a precompensated signal to achieve self-correction of phase noise which induced by atmospheric link.In addition,the first use of EDFA under the APC mode suppression optical power fluctuations caused by turbulence.This noise compensation scheme can achieve infinite dynamic range through simple phase conjugate mixing and can quickly eliminate phase noise.In addition,the system is simplified and the round-trip three wavelengths suppress the effect of Rayleigh backscatter.Based on this scheme,the 1GHz RF signal transmission experiment of 124 m indoor free space link was realized.After the delay fluctuation compensation,the RMS value of the delay jitter of the frequency signal transmitted within 5000 s is 186 fs,and fractional frequency stability of the remote site signal reaches 6.8×10-15@1 s and 3.2×10-17@1000 s.Particularly,short-term stability increased by 2 orders of magnitude2.Based on three classical turbulence models: Kolmogorov,von Karman and Greenwood-Tarazano models,a turbulence model suitable for atmospheric optical frequency transfer is established.And the effects of turbulence intensity,wind speed and transmission distance on the frequency transmission stability are studied.Theoretical analysis and simulation provide sufficient theoretical analysis and support for the overall design of the transfer system.3.Implemented the Doppler noise suppression technology in the free-space optical frequency transmission experiment,and obtained preliminary experimental verification in the laboratory short-distance free-space optical frequency transmission.The transmission stability of the remote output signal can reach 1.3×10-18 under the integration time of 10000 s.Based on the experimental results,the basic principles and technical limitations of the Doppler noise suppression technology were analyzed,which laid the foundation for further improvement of system performance in subsequent research work basis. |
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