Researches On Broadband Femtosecond Mid-Infrared Laser Generation Based On Optical Parametric Oscillators

Mid-infrared laser sources with broad instantaneous-bandwith are critical for many applications,including infrared microspectroscopy,environmental monitoring,medical diagnosis,and ultrashort pulse generation.The purpose of this thesis is to investigate how to expand the instantaneous-bandwidth of the output from synchronously pumped optical parametric oscillators(SPOPOs).The main research results are summarized as follows:(1)A scheme to achieve broadband mid-infrared laser emission through intra-cavity spectral combining in SPOPOs is proposed and demonstrated.By adding a spatially-dispersive element and multiple reflectors into an SPOPO cavity,multiple channels with different central-wavelengths could work simultaneously and independently in a single resonator.This scheme enables an effective extension of the mid-infrared spectral bandwidth by using a single pump source and a single nonlinear crystal.In our experiment,the intracavity spectral beam combination of two channels was achieved,and a broadband midinfrared laser was obtained,covering 2937-3375 nm,with-10 dB bandwidth of 13.2THz,nearly twice as much as the pump bandwidth(6.7THz).(2)A scheme to realize broad-band chirped-pulse oscillation by enhancing the selfphase modulation(SPM)effect and optimizing the intra-cavity dispersion in SPOPOs is proposed and demonstrated.A Zn Se plate was introduced to provide additional SPM effect,resulting in spectral broadening of the oscillating waves.In two experimental configurations based on period-poled lithium niobate(PPLN)crystals with different lengths,broadband mid-infrared laser emssion covering 2.9-3.9 ?m(-20 dB)and 3.1-4.5 ?m(-30 dB)were achieved,respectively.The effects of intra-cavity dispersion and pump power on the output spectral bandwidth,as well as the effects of intra-cavity dispersion distribution and nonlinear gain distribution on the output spectral shape are also studied through simulations and experiments.This offers a technical route to achieve output with an instantaneousbandwidth only limited by the gain bandwidth of the nonlinear crystal from an SPOPO.(3)A scheme to realize broadband mid-infrared laser emission based on SPOPOs with chirped quasi-phase matching(QPM)crystals is demonstrated.The formation mechanism of the broadband chirped pulses is clarified theoretically and verified in numerical simulations.In the experiment,utilizing an aperiodically-poled Lithium-niobate(APPLN)crystal,chirped pulse operation in a SPOPO was achieved.An idler output with an average power of64 mW,covering 2.9-5.0?m with a-30 dB bandwidth of 44.1THz was obtained.To the best of our knowledge,this represents the first demonstration of mid-infrared laser emission fully covering the 3-5 ?m infrared transmission window with a single-resonant OPO system.