Research On Broadband Femtosecond Mid-Infrared Lasers Based On Difference Frequency Generators

Broadband mid-infrared lasers hold great promise for many applications,including dual-comb spectroscopy and optical coherent tomography.Difference frequency generators(DFGs)are powerful tools for generating broadband mid-infrared light sources.Existing femtosecond DFG systems can only generate output with narrow bandwidths.This thesis focuses on how to expand the parametric gain bandwidths of nonlinear crystals.We investigate how to extend the parametric gain bandwidths of nonlinear crystals and the bandwidths of input fundamental waves,and how to obtain broadband mid-infrared light via femtosecond DFGs.The research results are summarized as follows.(1)A scheme to enhance the output bandwidths of femtosecond DFGs based on cascaded crystals was proposed and demonstrated.Numerical simulations show that the phase-matching bandwidth of the cascaded crystals was approximately equivalent to the sum of that of each individual crystal.In the experiment,the cascaded crystals,consisting of two1-mm-long periodically poled lithium niobite(PPLN)crystals with poling periods of 31 μm and 29 μm respectively,were used as the gain medium of a femtosecond DFG.The DFG produced a broadband idler wave having instantaneous-bandwidth over 2.7～4.05 μm,which was approximately equivalent to the sum of the bandwidths of the generated idler waves based on each individual crystal.The research demonstrates that crystal cascading is an effective technical route to expand the output bandwidths of the femtosecond DFGs.(2)A scheme to enhance the output bandwidths of femtosecond DFGs based on aperiodically poled crystals was studied.Based on a concept similar to cascaded crystals,aperiodically poled crystals could also be used to extend the phase-matching bandwidths of nonlinear crystals.A 2-mm-long aperiodically poled lithium niobite(APPLN)was designed with a poling period that varied linearly along the crystal.A femtosecond DFG based on this crystal was established,and a broadband idler wave having instantaneous-bandwidth over1.9～4.6 μm was obtained.Meanwhile,by numerical simulations,the effects of system parameters,including crystal length,pump intensity,and pump incident direction on the output characteristics,were studied.Aperiodically poled crystals have broad gain bandwidths,and can be used to avoid multi-faceted reflection losses of ordinary cascaded crystals.(3)A scheme to obtain broadband mid-infrared lasers through using a broadband pump wave and a crystal with a broad pump-acceptance bandwidth was studied.In a fiber-laser-based femtosecond DFG,for some special wavelengths,the bandwidth of the available signal wave was narrow,while the bandwidth of the pump wave could be greatly broadened.Theoretical analysis shown that a broadband idler wave could be generated by the combination of a broadband pump wave and a crystal with broad pump acceptance bandwidth.The phase-matching bandwidth of the crystal was determined by the crystal dispersions.It was found that the crystal could have a broad pump acceptance bandwidth under a certain combination of pump and signal wavelengths.For the PPLN crystal,when the wavelengths of the pump and signal waves were 1050 nm and 1525 nm respectively,a phase-matching condition could be achieved within a broad bandwidth of the pump wave.In the experiment,a femtosecond DFG,based on a 1-mm-long PPLN with the poling period of29.5 μm,generated a mid-infrared wave having an instantaneous-bandwidth covering2.72～4.15 μm.This research shows that femtosecond DFGs can generate broadband mid-infrared lasers based on broadband pump wave and a single conventional nonlinear crystal.In conclusion,this thesis studied three technical schemes to generate broadband mid-infrared lasers via femtosecond DFGs.The focus of the three schemes was how to obtain broad phase-matching bandwidths.Crystal cascading was a general scheme to expand parameter gain bandwidth of the crystal.Aperiodically poled crystal could be regarded as a special cascaded crystal and could be used to avoid multi-faceted reflection losses of ordinary cascaded crystals.However,at present,only a few kinds of crystals could be aperiodically poled.For the scheme using a crystal with a broad pump-acceptance bandwidth in a DFG,it was necessary to select the wavelengths of the pump and signal waves appropriately according to the dispersion characteristics of the crystal.