Research On Femtosecond Optical Frequency Comb Light Source Based On Digital Phase Locked Loop Technology

Optical frequency combs perfectly combine ultra-short pulses and ultra-stable frequencies,which have been widely used in fields such as optical atomic clocks,optical frequency measurement,molecular absorption spectroscopy,and microwave photons.Therefore,this paper focuses on the theoretical simulation and experimental research of mode-locked fiber lasers.Further,in order to obtain a stable optical frequency comb,a frequency locking study of the laser based on digital phase-locked loop technology is carried out.The main research work of this paper includes:1、Based on the nonlinear amplification loop mirror,the research on mode-locked fiber lasers has been carried out.Firstly,the evolution process of mode-locking in the time domain and frequency domain is analyzed.Then,a compact,self-starting,and stable mode-locked fiber laser is realized experimentally,and its repetition frequency,spectral width,and pulse width are 201.14 MHz,20.97 nm,and 510 fs,respectively.Further,under long-term monitoring,the laser can always maintain the mode-locked state.2、A scheme of using fiber cutting,fiber tapering and phase-locked loop to achieve precise adjustment of repetition frequency of the laser is proposed.Through fiber cutting and fiber tapering,the repetition frequency can be quickly adjusted in the order of MHz and hundreds of k Hz.After that,through temperature tuning and digital phase-locked loop,the precise adjustment and stabilization of the repetition frequency can be achieved,and mode-locked laser output with a repetition frequency of 125 MHz is obtained.3、Based on the phase-locked loop technology and Field Programmable Gate Array,a compact digital phase-locked loop is self-made.The circuit on the hardware is modified to realize the clock synchronization of the Rubidium atomic clock,Field Programmable Gate Array,and radio frequency source.After the repetition frequency is locked,the frequency fluctuation is<1 m Hz,and the Allan variance can reach 2.06×10^-12 in the average time of 1 s.4、In order to detect and lock the carrier envelope offset frequency,the generation of the supercontinuum spectrum and self-reference f-2f technology have been studied theoretically and experimentally.Two schemes are used to numerically simulate the generation of the supercontinuum,and the supercontinuum with a spectral range of1μm～2.5μm is obtained.In experiments,based on the all-fiber scheme,an octave spectrum with a spectral range of 1μm～2.2μm is generated.Then,the carrier envelope offset frequency is successfully detected when using the self-reference f-2f technology of the spatial structure and tuned when the pump source power is adjusted.And a frequency locking technology scheme based on digital phase-locked loop is proposed.