热透镜效应对半整块腔型中二次谐波过程的影响

Influence of thermal lens effect on second harmonic process in semi-monolithic cavity scheme

二次谐波过程是制备高功率、宽波长范围激光的有效途径.在二次谐波过程中,晶体的热透镜效应是限制转换效率进一步提高的重要因素,热透镜效应对二次谐波转换效率的影响随着基频光功率的增大而加剧.本文理论分析了不同半整块腔型中热透镜效应对转换效率的影响关系;实验上搭建了两种腔型进行高效外腔倍频制备532 nm激光,测量其倍频转换效率随基频光功率的变化关系.对于平凹型半整块腔,在输入光功率为800 mW时,产生747 mW的532 nm激光输出,得到最佳的转换效率为93.4%±3%;对于双凹型半整块腔,在输入光功率为600 mW时,产生529 mW的532 nm激光输出,得到的最佳转换效率为88.2%±3%.研究表明,热透镜效应对双凹型半整块腔的转换效率影响相对较大,且随着腔内损耗的增加而加剧;相比于双凹型半整块腔,平凹型结构可以实现更高效的倍频转换.本文的理论及实验结果可在量子信息科学、光学频率计量以及生物医学等领域的研究中发挥重要作用.

Second harmonic generation(SHG)is an effective way to generate short wavelength laser with high power.The SHG is accompanied with the absorptions of fundamental waves and harmonic waves,which converts a fraction of the two waves deposit energy into heat,causing a temperature gradient along the radial direction of the periodically poled potassium titanyl phosphate(PPKTP)crystal.The inhomogeneous temperature distribution causes thermal lensing in the crystal.The thermal lensing effect will deform the spatial mode of the SHG cavity and result in the mode-mismatching of the fundamental wave to the SHG cavity,and therefore the conversion efficiency of SHG process is reduced.Moreover,with the increase of injected fundamental wave power,the influence caused by thermal lens becomes more and more serious.In order to obtain a high-efficiency frequency conversion,it is necessary to take the measure to minimize the effect caused by thermal lensing.In this paper,we report on a high efficiency generation of green laser at 532 nm by external cavity SHG process with a semi-monolithic standing cavity.The influences of thermal lens effect on the optimal conversion efficiency in different semi-monolithic cavities are theoretically analyzed.The variations of conversion efficiency with the pump power in“plane-concave”semi-monolithic cavity based on parallel crystal and also in“concaveconcave”semi-monolithic cavity based on concave crystal are quantitatively analyzed.In experiments,two types of cavity structures are built to measure the variation of frequency doubling conversion efficiency with pump power.For the“plane-concave”semi-monolithic cavity,the maximum green laser power of 747 mW is obtained and the corresponding conversion efficiency reaches 93.4%±3%,with 800 mW infrared laser injected.For the“concave-concave”semi-monolithic cavity,the maximum green laser power of 529 mW is obtained and the corresponding conversion efficiency is 88.2%±3%,with 600 mW infrared laser injected.The results show that the thermal lens affects the optimal conversion efficiency more seriously in“concave-concave”semimonolithic cavity than in“plane-concave”semi-monolithic cavity.Furthermore,the influence of thermal lens effect turns higher and higher with the increase of the loss in the cavity.It is obvious that the“plane-concave”semi-monolithic cavity is more suitable for the SHG process and has many potential applications in quantum optics and cold atom physics and provides a guidance for future research on high-efficiency SHG process.