Mid-infrared Optical Parametric Oscillator Technology

3～5μm laser had lots of applications in military.There were many methods to generate the laser.To get the high-repetition-rate,high-average-power 3～5μm laser,every method's advantages and disadvantages were analysed and it was sure that ZGP optical parametric oscillator (OPO for short) was one of the best ways.The pump laser of the 3-5 μm ZGP optical parametric oscillator was near 2μm.There were two main generation methods:one was the YAG,YLF solid lasers doped Tm or Ho;the other was Nd:YAG pumped KTP OPO.The former were all quasi-three level systems.It was difficult to ganerate high power laser in room teperature.So the way using KTP OPO to generate 2μm laser was ascertained.The theoretical models of sigle resonant OPO(SRO for short) and doubly resonant OPO(DRO for short) were established.In the base of the nonlinear coupling wave equations,the thresholds and the conversion efficiency curves of the two models were got.During calculating the conversion efficercy,the uniform-plane-wave was first taken into account and then the Guass beams further.The results showed that:DRO's threshold was low but its conversion efficiency high;SRO's threshold was high but its efficiency low.The acceptable angles both of non-critical phase matching and critical phase matching,the dispersion angles both of Type I phase matching and Type II phase matching ,and the OPO's output linewidth were also calculated.Begun with the KTP crystal refractive index ellipse equation and Sellmeier equation,the tuning characteristic of the crystal about the angle 0 was analyzed when the 1.064μm continuous Nd:YAG laser pumped the Type II phase matching KTP crystal .The angle tuning curse was also calculated. As a result,when the angle θ was 52.6°,the signal and the idle wave were degenerated at 2.128μm but with perpendicular polarizations.To restrain the dispersion effect, the intracavity optical parametric oscillator(short for OPO) and the short crystals connecting external OPO were combined.A experimental set of intracavity doubly resonant optical parametric oscillator with two short KTP crystals connecting was designed.There were two main tuning means of ZGP OPO:angle tuning and teperature tuning.Because of the difficult teperature control in the teperature tuning,the angle tuning was chosen. In the base of ZGP crystal refractive index ellipse equation and Sellmeier equation, the ZGP OPO's angle tuning curves of Type I phase matching and Type II phase matching was calculated.After analysis and comparison,the Type I phase matching ZGP OPO was chosen.The OPO's configuration,the crystal length,the cavity length and the coatings of mirrors were also designed.According to the theoretical calculation,the maximum 310mW 3～5μm laser at 4.0kHz repetition rate was got in experiment,while the power of p-polarization 2μm was 2.6W.The optical-to-optical efficiency was 12%.The beam quality,the spot distritution of 2μm was also measured.The output power of 2μm at different frequencies in the rage of 3.0kHz to 8.0kHz was measured,and also the 3～5μm.The angle tuning curve of ZGP OPO was gotten.It was quite in accrod with the theoretical...