Research On Active Phase Compensation Technology Based On Optical Frequency Comb In Free Space Frequency Transmission

With the increasing atomic frequency standards and the rapid development of wireless communication technology,the requirements for high-precision frequency transmission are gradually increasing.Highprecision time-frequency transmission technology is widely used in navigation,deep space exploration,precision timing,national defense and military and other advanced technology fields.Traditional time-frequency transmission schemes,including satellite two-way time-frequency transmission and global navigation satellite systems,can no longer meet the requirements of high-precision atomic frequency standard transmission.At present,the frequency transmission scheme with optical fiber link as the carrier has become more and more mature,and it can meet the requirements of high-precision frequency transmission.The transmission mode using optical fiber as the medium has the advantages of small transmission loss and high stability,but it has a strong dependence on the mature optical fiber network that has been built,and the cost of laying optical fiber cables in a large area is expensive.At the same time,it is difficult to dynamically adjust after the completion of the transmission site,and the flexibility is poor.Therefore,when there is no fiber optic network laid,we can use free space optical links as a supplement to the optical fiber network to quickly establish communication connections between various sites.Free space frequency transfer technology can effectively compensate for the shortcomings of optical fiber and microwave transmission.In order to achieve higher transmission accuracy,it is necessary to compensate for the impact of atmospheric environment during optical signal transmission.Therefore,in the context of the above research,this paper focuses on the research of frequency transmission system using the free space atmosphere as the laser transmission link.In this process,because the atmospheric link is affected by a variety of environmental factors,such as the influence of atmospheric turbulence,platform vibration,temperature changes,etc.,the transmitted frequency signal will accumulate the phase noise of the upper link,and ultimately affect the stability of frequency transmission.Therefore,it is necessary to design a phase compensation scheme to suppress the influence of the above environmental factors on the frequency transmission instability.In order to solve this problem,an active phase compensation method based on optical frequency comb is proposed,and the realized frequency transmission system can effectively suppress the phase noise caused by atmospheric turbulence,and the following work is mainly carried out:1.The theoretical analysis and experimental system construction of optical frequency comb and microwave atomic reference locking are carried out.The locking system uses FLOM-PD as the phase detector of the phase locked loop,generates an error signal based on the phase difference between the optical frequency comb generated by the mode locked laser and the microwave atomic reference,and performs feedback control on the mode locked laser to adjust its output repetition frequency.When the loop is successfully locked,the frequency stability of the optical frequency comb output by the system reaches 5.99 ×10-6 at 1 second,reaching 2.59×10-18 at 1000 seconds.2.The theoretical analysis of the active phase compensation experiment of free space fr-equency transmission and the construction of the experimental system are carried out.Based on the principle of photoelectric phase-locked loop,the active phase compensation system uses piezoelectric controller,piezoelectric ceramic,mode-locked laser and free space link as the equivalent voltage-controlled oscillator,and directly uses the output of free space link as the output of the equivalent voltagecontrolled oscillator,and precisely locks the output on the microwave reference signal through the phase-locked loop,thus realizing the active phase compensation.When the free space link noise is actively compensated,the frequency stability of the system is 5.15×10-14 at 1 second,6.29×10-18 at 1000 seconds,the frequency stability is improved by more than one order of magnitude compared with that without compensation.