Mid-infrared Laser Emitted From PPLN-OPO Realized By Efficient Cavity Design

Optical parametric oscillator(OPO)is an important method to obtain mid-infrared laser source for its many advantages,such as flexible wavelength-tuning,compact structure and high conversion efficiency.The quasi-phase matching(QPM)OPO by using periodically poled nonlinear crystals is a significant way to realize the tunable mid-infrared output.Periodically poled lithium niobate(PPLN)is the most developed and widely applied quasi-phase matching crystal for its excellent characteristics in nonlinear optical frequency conversion,such as high effective nonlinear coefficient and wide transmittance range.This dissertation mainly researches the continuous-wave laser performance emitted from the QPM MgO:PPLN-OPO,and studies the tuning characteristics at different temperature and period of MgO:PPLN crystal.In addition,we designed two cavity configuration of linear cavity and V-type cavity by simulating the mode-matching between fundamental and OPO cavities.We optimized this two configurations,respectively,and obtained efficient and wide tuning mid-infrared output.The main contents are listed as follows.1.A compact linear cavity was designed for decreasing cavity loss and increasing conversion efficiency.we obtained a total output power of3.27 W under a LD incident pump power of 12.4 W,with a total optical-to-optical conversion efficiency of 26.4 %.For pursuing higher output power and conversion efficiency,the cavity modes on the MgO:PPLN crystal were theoretically calculated and compared.The result shows that the fundamental laser beam can be compressed and the mode matching between fundamental and OPO cavities at MgO:PPLN crystal can be improved when all the cavity mirrors is concave.This cavity design is beneficial to improve the OPO conversion efficiency.In this cavity the signal and idler tuning range of 1.43-1.79 ?m and2.62-4.16 ?m were obtained by changing the grating periods andtemperature of MgO:PPLN crystal.Under an incident pump power of20.1 W,a signal output power of about 4.24 W and an idler output power up to 2.11 W were obtained,leading to the conversion efficiency of31.6 %.To the best of our knowledge,this is the highest total output power with the similar cavity configuration.2.A V-type OPO cavity was designed for improving the mode-matching flexibility in cavity and increasing conversion efficiency.We first calculated the cavities modes and designed OPO system based on the calculation.The mode-matching we obtained in V-type cavity is better than that in linear cavity.The V-type configuration has a longer cavity length than the linear one.So the diffraction loss of idler light is inevitable as it reflects back from the end mirror HM and passes through the MgO:PPLN crystal once again.Therefore,we replaced the end mirror HM by another mirror HM'(curvature radius of 150 mm)with HT coated at idler wavelength and HR coated at resonant signal wavelength.With such a cavity,the signal and partial idler light outputted from mirror OC,while additional reverse idler light could directly output from end mirror HM'.The diffraction loss and reverse OPO process could be curbed by reducing the intracavity idler power in this double outputs design.Under an incident pump power of 12.4 W,a maximum signal output power of2.89 W and total idler output power of 1.44 W were obtained with a total conversion efficiency up to 34.9 %.