Research On Two-Way Phase Comparison Method Based Optical Frequency Comb

With the gradual improvement of the level of science and technology and the accelerated development of the communication field,the accuracy of atomic clocks used to generate time-frequency standard has reached the order of 10-19,which has brought the demand for improving the accuracy of frequency transfer.High precision frequency transfer plays important roles in navigation,deep space exploration,atomic clock comparison and other large-scale scientific fields and people’s livelihood fields.The traditional method of frequency transfer is mainly based on satellite link,and the current achievable frequency instability by the method is 10-15/day,which cannot meet the requirements of current frequency standards and applications.Optical fiber has become the best medium for frequency transfer due to its low loss,small size,low price,strong antielectromagnetic interference and other characteristics.At present,the transfer realized by optical fiber has obtained the frequency transfer instability of 10-20/104 s.However,in the actual research process,when frequency transfer is carried out through optical fiber,due to the influence of various factors such as the external temperature change and mechanical vibration,the phase jitter in the transfer process is introduced,which reduces the stability of frequency transfer and affects the quality of received signal.Eliminating the phase noise introduced by optical fiber has become one of the key works of high-precision frequency transfer.This paper mainly focuses on applying the traditional two-way phase comparison method to the frequency transfer system based on the optical frequency comb.By changing the measurement and compensation method of the conventional one-way round-trip comparison,the link jitter is eliminated by the two-way comparison method.The optical signal only needs to be transferred in one direction along the optical fiber link,the distance of the optical signal through the link is shortened,the link delay is reduced,and the compensation response speed is improved.The main work of this paper is as follows:1.Carried out theoretical analysis and experimental construction of the mode-locked laser used in this paper,analyzed the internal structure and characteristics of different implementation methods of the modelocked laser,built a mode-locked laser based on the nonlinear polarization rotation mechanism,obtained the optical frequency comb signal with a repetition rate of 250 MHz,and used it as the laser source in the two-way phase comparison experiment.2.Carried out the theoretical analysis and experimental construction that the repetition frequency of the optical frequency comb is locked to the rubidium clock signal,analyzed the repetition frequency locked scheme of the optical frequency comb and its internal transfer function expression,and completed the experimental construction of the locked module on the theoretical basis.This module realized the output of optical pulse signal with stable repetition frequency,and the frequency offset achieved in the experiment reaches the order of mHz.After being locked,the instability of the optical frequency comb can reach 2.0×10-14/s and 2.0×10-17/100 s.3.Proposed a two-way phase comparison scheme based on optical frequency comb.Through principle analysis and formula derivation,and compared with the current one-way round-trip method,the system transfer stability evaluation is completed and the feasibility of the scheme is verified.We conducted a two-way phase comparison experiment on a 25 km optical fiber in the laboratory and tested its performance.After eliminating the phase jitter through the two-way phase comparison technology,the frequency instability of the system can reach 1.6×10-14/s and 2.1×10-16/100 s.