Research On The Key Techniques Of The Local And Remote Self-Reference Optical Clock Comparison

The high-precision comparisons between optical clocks have important applications in fundamental physical research,metrology,geodesy,et al,enabling searches for the temporal variations of fundamental constants,test of general relativity,and geopotential measurements with centimeter-level resolution.Carrying out comparisons between different types of optical clocks with high precision is important for the performance evaluation of optical clocks,and it is also the premise for realizing the next generation of second definition by optical frequency standards.At present,the frequency uncertainty and instability of various optical clocks worldwide entered 10-18 level,far exceeding the cesium fountain clocks which enable realization of the second.The accuracy of absolute frequency measurement of the optical clocks is limited by cesium fountain clocks at the order of 10-16.However,by directly measuring the frequency ratios between different kinds of optical clocks,comparison with higher precision can be achieved at the level of the optical clocks themselves.Comparison between remote optical clocks can use communication fiber links as the medium of optical frequency transmission.However,as the permitting wavelength of the communication fiber is limited by attenuation,it is necessary to firstly convert the frequency of local optical clock to communication wavelength,and then transfer it to the remote site via fiber link to realize the comparison.The main work during my Ph.D.were around the realization methods of local and remote optical clock comparison,which can be categorized as the following aspects:1.A 1560 nm ultra-stable laser system was built.By using the Pound-Drever-Hall technique of laser frequency stabilization,a frequency-stabled laser source at communication wavelength was generated for optical frequency ratio measurement and fiber frequency transmission experiments.2.A self-reference scheme for directly measuring optical frequency ratios was designed and examined.This measuring method was based on the transfer oscillator scheme of optical frequency comb.By down-converting the optical frequency to radio frequency,the high-precision measurement of optical frequency ratio was realized in a self-reference manner without the help of external radio frequency reference.Precision of 10-21 level was realized in the experiment with the self-made circuit system.3.An optical frequency transfer method was constructed based on the frequency ratio measurement scheme mentioned above,for transferring the optical clock frequency to fiber communication wavelength.Through this method,the frequency of laser at target wavelength can be directly locked to the reference optical frequency at arbitrary preset ratio.In the experiment of transferring the 729 nm ultra-stable laser to 1560 nm,an accuracy of 10-20 was achieved.The method was also applied to transfer the stability of a 1064 nm ultra-stable laser to 1070 nm laser for 27Al+ optical clock,and successfully improved the stability of the 27 Al+clock by 6.7 times.4.Optical frequency transfer in a 60 km urban fiber link was realized.The transmission was carried on the 60 km round-trip fiber linking Innovation Academy for Precision Measurement Science and Technology,and Huazhong University of Science and Technology.By actively compensating the phase noise in the fiber link,the frequency transfer instability of 2.4 × 10-17 at 1 s was achieved,meeting the needs for remote comparison of optical clocks.