Highly efficient second harmonic generation by using four pass quadrature frequency conversion

Abstract

ABSTRACT A novel frequency converter architecture of four-pass quadrature frequency conversion scheme was developed aiming to achiev high conversion efficiency. A high conversion efficiency of more than 80 % has been achieved in this scheme forfrequency doubling of 1064-nm in KTP with a low input fundamental laser intensity of 76 MW/cm2. A second-harmonicoutput of 486 mJ was obtained with 607 mJ of the input 1064-nm fundamental laser at 10 Hz.Keywords : quadrature frequency conversion, second-harmonic generation (SHG), KTP, Nd:YAG laser 1. INTRODUCTIONS Frequency conversion through nonlinear processes is widely used for extending the utility of existing laser. Inparticular, generation of pulsed green output is important for application for pumping Ti:sapphire amplifier. The mosteffective way to accomplish this has been through second-harmonic generation (SHG) of 1064-nm Nd:YAG lasers innonlinear optical crystals1'2. For SHG at 1064-nm for Nd:YAG lasers, a commonly employed laser source, conversionefficiency normally exceeds 50 %36• Efficient SHG from the fundamental laser wavelength is also essentially important forthe frequency up-conversion because the performance of higher-order harmonic generation is very much dependent on this.Additionally, in many applications and experiments, a high second-harmonic conversion efficiency is desirable for thepurpose of enhancing the signal-to-noise ratio or reducing the size of the required laser source.In order to achieve high conversion efficiency, a quadrature frequency conversion scheme has been proposed7.Figure 1 shows the quadrature doubling scheme used for SHG. In this scheme, the planes formed by the input laser beampropagation direction and optic axes of two type II crystals are arranged to be orthogonal. The input laser beam leftunconverted from the first crystal due to nonuniformities of the input beam intensity can be converted efficiently in thesecond crystal. Thus, the sensitivity to input laser beam profile, angle and thermal misadjustment is lower than that ofsingle crystal scheme. The second-harmonic output produced in the first crystal is not at the correct polarization forinteraction in the second. Thus, the second-harmonic output from the first crystal passes through the second without back-conversion.In this paper, we describe a four-pass quadrature frequency conversion scheme by using polarization rotation forpumping Ti:sapphire amplifier and report a frequency conversion efficiency of above 80 % for SHG with a low inputfundamental laser beam intensity of 76 MW/cm2. The incorporation of quadrature and multi-pass features in this schemeprovides a frequency conversion performance superior to that of comparable lasers in its class. This scheme can be easilyscaled up by increasing the size of the nonlinear optical crystal to accommodate larger input fundamental laser beam cross-section.