A continuous wave, single frequency intracavity optical parametric oscillator based upon the twisted-mode technique

Abstract

This document summarises a study into the elimination of spatial hole burning in the pump field of a Nd:YAG and PPLN based Intracavity Optical Parametric Oscillator (ICOPO), using the so-called ‘twisted-mode technique’. This approach relies on the manipulation of circulating field polarisation to achieve a spatially uniform electric field intensity throughout the laser gain material, thus removing the root of multi-longitudinal-mode oscillation in homogeneously broadened systems. During the course of this investigation the ‘twisted-mode’ technique was first applied to a basic 1μm Nd:YAG laser, where it proved very successful in facilitating single frequency operation. The laser cavity was then extended to accommodate the ICOPO, which delivered 93.9mW of mid-IR output for 3W of diode pump power. The implementation of the ‘twisted-mode’ technique was not found to have any significant impact on power performance of the device and once again offered substantial improvements in pump field spectral quality. Although the insertion of an etalon was necessary to achieve good frequency control, the associated drop in power was minimal compared to typical losses observed in etalon-narrowed standing-wave configurations. This can be attributed to both the low finesse that was required as well as the absence of gain unsaturated regions within the active medium, as would be observed in a spatially hole burnt system. Whilst further work will have to be carried out to optimise both the frequency behaviour and power performance of the device, the ‘twisted-mode’ technique shows clear potential as an approach to the development of a reliable, narrow line-width mid-IR spectroscopic source based on Intracavity OPO technology.