| Frequency synchronization technology plays a pivotal role in modern military and social life,and its application scenarios become more complex and the requirements for synchronization accuracy are increasing.Compared with the use of satellites to achieve frequency transmission,the use of lasers for frequency transmission in free space has the advantages of wider frequency band,faster speed,less interference and higher accuracy.At the same time,this transmission method does not require a special optical fiber channel like optical fiber frequency transmission,that is,the construction of a free space frequency transmission system is faster and more convenient.Based on the above advantages,the use of laser frequency transfer technology in free space has shown an irreplaceable and important role.The current work on free-space frequency transfer is mainly to improve the stability of frequency transfer through phase compensation methods.In this thesis,a optical phase compensation method is designed on the basis of the related phase compensation methods,and a complete free-space frequency transfer system is built to verify the method.The main research work and results of this thesis are as follows:Firstly,the mechanism that the laser signal is disturbed by the environment during frequency transfer in free space is studied,and the influence of atmospheric turbulence and atmospheric attenuation on the light intensity,spot and phase of the laser signal is analyzed.According to the theory,the corresponding model is obtained,and the simulation analysis is carried out from the influence of atmospheric attenuation on optical power and the phase noise caused by atmospheric turbulence.From the relevant experience and simulation results,the reasons for the influence and the corresponding improvement measures are obtained,especially the phase noise introduced by the atmospheric turbulence effect causes the frequency stability to be severely deteriorated.Therefore,it is necessary to use the phase compensation method to improve the frequency stability.Secondly,an opto-mechanical-combined optical phase compensation device is designed,and other hardware circuits and optical platforms required in the free-space frequency transfer system are designed and implemented.After the hardware system is built,the phase compensation closed-loop control algorithm is implemented in the FPGA to control the mobile platform and the piezoelectric ceramic to change the optical path,and complete the"coarse"compensation and"fine"compensation to realize the optical phase compensation method.The validity of this method is verified.Thirdly,in the experiment,a continuous laser with a wavelength of 1550 nm was used to realize the frequency transmission of a 1 GHz radio frequency signal in a 60 m free space,and the time fluctuation and Allan variance of the transmitted signal were analyzed.The experimental results show that the RMS value of the time fluctuation of the transmitted signal is about 143 fs,and the frequency stability at 1 s is 1.0×10-13,1000s by using the phase compensation method combined with"coarse"compensation and"fine"compensation.The time-frequency stabilization is 2.4×10-16. |
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