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. SHG is accompanied by absorptions of fundamental waves and harmonic waves, which make a fraction of the two waves deposit energy as heat and cause 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 the conversion efficiency of SHG process is so 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 measures 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 influence of thermal lens effect on the optimal conversion efficiency in different semi-monolithic cavities was theoretically analyzed. The variation of conversion efficiency with the pump power in "plane-concave" semi-monolithic cavity based on parallel crystal and "concave-concave" semi-monolithic cavity based on concave crystal was quantitatively analyzed. In experiments, two types of cavity structures were 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 was obtained and the corresponding conversion efficiency reaches (93.43)%, with injecting of 800 mW infrared laser. For the "concave-concave" semi-monolithic cavity, the maximum green laser power of 529 mW was obtained and the corresponding conversion efficiency is (88.23)%, with injecting of 600 mW infrared laser. The results show that the thermal lens impacts on the optimal conversion efficiency more serious in "concave-concave" semi-monolithic cavity in comparing with "plane-concave" semi-monolithic cavity. What's more, the influence of thermal lens effect gets higher and higher with the increase of the loss in the cavity. It is obviously that the "plane-concave" semi-monolithic cavity is more suitable for the SHG process and has huge potential in quantum optics and cold atom physics and provides a guide for future research on high-efficiency SHG process.

A New Nonlinear Photothermal Iterative Theory for Port-Wine Stain Detection

“…Figure 2 shows the results in the low−frequency range PWS samples with thicknesses d = 0.5 mm and d = 0.8 mm. Both the fundamental and second harmonics on the posterior surface of the samples obtained by the two numerical methods decrease with increasing thickness, which is in good agreement with the results predicted in the literature [28,29]. For the fundamental and second harmonics obtained by the conventional perturbation method, there is no significant change in the effect of the sample thickness on the results, and there is no significant difference between the fundamental and second harmonics.…”

“…As observed from Figure 5a, the fundamental frequency amplitud tional to the optical energy p w , which is consistent with the results of linea observed from Figure 5b,c, the amplitudes of the higher−order harmonics are p to the square of the light energy, which is consistent with the theoretical deriv To demonstrate the validity of the new theory, two numerical methods analyze and compare the results for the variation in the amplitude with sam on the posterior surface of 2219 aluminum alloy samples. The two method iterative numerical method and the conventional perturbation method [28,2 The effectiveness of the new iterative numerical method for solving To demonstrate the validity of the new theory, two numerical methods were used to analyze and compare the results for the variation in the amplitude with sample thickness on the posterior surface of 2219 aluminum alloy samples. The two methods are the new iterative numerical method and the conventional perturbation method [28,29].…”

“…The two method iterative numerical method and the conventional perturbation method [28,2 The effectiveness of the new iterative numerical method for solving To demonstrate the validity of the new theory, two numerical methods were used to analyze and compare the results for the variation in the amplitude with sample thickness on the posterior surface of 2219 aluminum alloy samples. The two methods are the new iterative numerical method and the conventional perturbation method [28,29].…”

“…and light is absorbed by the sample and converted into heat, generating a photoacoustic signal through thermal expansion, where τ is the pulse width, t 0 is the pulse center, and w P is the luminous flux (mJ/cm 2 ). The fundamental equation of the photoacoustic imaging theory is based on the thermal diffusion equation; therefore, the thermal diffusion equation and boundary conditions in the sample are [28,29]:…”

“…Subscripts behind the comma mean the calculation of partial derivatives for the corresponding subscripts. The endothermic source in the sample is Q(z, t), which can be expressed as [28]:…”