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

Abstract

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 “concave-concave” 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” semi-monolithic 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.