Researches On The Key Techniques For Far-infrared DFG Comb Based On Femtosecond Pulsed Fiber Laser

Mid-and far-infrared optical frequency combs(OFCs)are very important for applications in the fields of optical frequency metrology,precision laser spectrum and coherent detection.So far,a variety of schemes for obtaining a mid-and far-infrared OFCs have been proposed.Among them,difference frequency generation(DFG)technique has attracted the most attention because the carrier envelope offset(CEO)frequency of the generated DFG comb can be automatically canceled by the DFG process only if the two-color fundamental pulses are derived from a same mode-locked laser.With the development of fiber lasers,mode-locked fiber laser has been the first choice for DFG OFCs,because of its advantages of compact structure,low cost and easy operability property.This dissertation focuses on the key principles and techniques of difference frequency generation(DFG)far-infrared OFCs based on mode-locked fiber laser,the main works and results are as follows:Firstly,based on the basic theory of difference frequency generation,the interaction behaviors of pulse propagating in nonlinear medium,through the theoretical analysis of DFG phase matching and acceptance bandwidth,DFG phase matching characteristics based on typical nonlinear crystal are studied;from the principle of optical rectification of femtosecond pulse,the key techniques of obtaining two-color pulses generation required by DFG are studied,including the generation of mode-locked femtosecond pulse,femtosecond pulse fiber amplification and high nonlinear fiber supercontinuum generation techniques;Secondly,a design scheme of DFG infrared OFC based on femtosecond mode-locked fiber laser is proposed.The fundamental two-color pulses satisfying the requirements of DFG infrared OFC are obtained by chirped pulse amplification links and the SC generation in the negative dispersion highly nonlinear fiber;the precise temporal synchronization between the two-color femtosecond pulses is realized by adding single mode fiber with low dispersion value in the fiber links and tunable optical delay line in the spatial optical path.Based on this,the achievable infrared OFC with the maximum bandwidth of 1.3?m is generated via DFG in GaSe crystal,and the tunable spectra ranges from 6 to 10 ?m;Thirdly,the effects of fundamental two-color pulse chirp and pulse duration and polarization on the bandwidth of DFG OFC are studied,a design scheme of all-polarization-maintaining(all-PM)fiber-type broadband tunable DFG OFC is proposed.By using the scheme of all-PM fiber nonlinear pulse amplification,the two-color fundamental pulses with weak chirp and narrow pulse duration are obtained,the maximum bandwidth of DFG infrared OFC reaches 2 ?m in the GaSe crystal,which is close to the theoretical value of the phase-matching acceptance bandwidth of the GaSe crystal,this is the achievable widest bandwidth fiber-type DFG infrared OFC so far;adjusting the chirp of pump pulse and pulse duration of signal pulse by changing the pump power of the fiber amplifiers,respectively,the behaviors of chirp and pulse duration on the bandwidth of DFG OFC are explored.The research results show that suppressing the amount of the chirp of pump pulse is the key to obtain broadband DFG infrared OFC.Besides,the broadband tunable DFG infrared OFC can be obtained by changing the phase-matching angle and delay line only.The above research results have important reference value for the design and development of compact and stable broadband DFG infrared OFC to meet the application requirements of optical frequency metrology,precision spectrum analysis and atmospheric environment monitoring.