Ultra-low Noise Microwave Source Based On Femtosecond Optical Frequency Comb

This thesis focuses on the ultra-low noise microwave source produced by femtosecond optical frequency comb.With the development of science and technology,high-precision microwave signal have played important roles in various fields of fundamental siences,industry,national defense and medicine fields.Traditional microwave sources or standards normally rely on electronical method of microwave generation,which can not break through some electrical bottlenecks.Compared with microwave signal,light waves work at higher frequencies,so they can achieve higher frequency accuracy and lower phase noises.Optical frequency comb is such a link,which closely links the microwave frequency with the optical frequency,so that the microwave signal with lower phase noise and higher frequency accuracy can be generated by optical means.Based on the nonlinear mode-locked femtosecond ytterbium doped fiber laser frequency comb,through the precise control of the comb repetition frequency,the low-noise microwave signal based on the pulse repetition frequency of femtosecond optical frequency comb is obtained,and its noise characteristics are analyzed.We used Yb doped fiber laser based on nonlinear polarization rotation effect to realize laser mode-locked,and then precisely control the repetition frequency of the laser output to obtain a stabilized comb system.To achieve proper gain,dispersion and repetition frequency,the lengths of fibers in the cavity are calculated carefully,and the corresponding optical components are added to complete the construction of mode-locked laser.Once the laser was oscillating,the mode-locking status of laser was realized by rotating the wave plates and adjusting the distance between the grating pair.The laser can realize self-starting mode-locking and has good stability.The optical spectrum of the laser pulses and the redio frequency spectrum of the pulse train were recorded.The output repetition frequency of femtosecond fiber laser was controlled and locked by using a wide-band low-noise lock-in control loop,and the noise characteristics of various devices in the lock-in loop are measured and analyzed.Considering the influence of temperature,humidity and other external disturbances on the laser,the optimal experimental environment conditions are found.In the experiment,the fundamental frequency?143MHz?mode-locked pulse was obtained,and the locked result of repetition frequency was given.The lowest Allan deviation was 3.98×10^-13@1s.When the pump power increased,we found the phenomenon of harmonic mode-locking in the process of adjusting the distance between the wave plate and the grating,and the highest eighth harmonic mode-locked pulse was obtained in the experiment.In the same way,the repetitive frequency locking system is used to precisely control the harmonic mode-locked pulse,and the Allan deviation of the multiple frequency locking results of each harmonic mode-locked pulse is obtained,which is worse than the basic frequency locking results.The locked repetition frequency of various mode-locked pulses is analyzed by using the phase noise meter,and then the frequency divider is added to enhance the frequency control of the low frequency part,but no better frequency stability result is obtained.