Continuously frequency-tunable CW optical parametric oscillators and their application to spectroscopy

Abstract

Within the past few years, renewed interest has been generated in the field of continuous wave optical parametric oscillators. This has been due in part, to the wider range of nonlinear materials now available, and also to the improvements being made in the pump sources for these devices. OPO resonator design has also been stimulated and the use of monolithic / semi-monolithic cavities which display high levels of passive stability has been realised. The work to be described herein was prompted by a desire to develop a doubly resonant optical parametric oscillator with frequency stable, high integer ratio signal-idler tunable outputs pumped in the green spectral region. Given a pump source constrained to be in the green spectral region, the nonlinear material of choice for the optical parametric oscillator (OPO) was lithium triborate (LBO). The target device for this work was a tunable, type I LBO doubly resonant oscillator (DRO) with a signal: idler frequency output ratio of 3 : 1. This configuration necessitates a high operating temperature which, it was predicted, would lead to problems with the nonlinear OPO crystal coatings. LBO, as will be discussed later, suffers from an anisotropic thermal expansion which leads it to expand on heating in two crystal directions but to contract in the third crystal direction. This has in the past led to problems with the dielectric coatings applied to the surfaces of the crystal whereby the coatings and the crystal become separated upon temperature cycling. The solution to this problem developed within this thesis, involves the use of index-matching fluid to attach cavity mirrors directly onto the faces of the nonlinear material. The cavity mirrors, constructed from a substrate which exhibits an isotropic thermal expansion, could then be dielectrically coated and temperature cycled successfully. The index-matching fluid would provide the contact necessary to maintain high finesse optical cavities. The first device to be built in this way was a type EL, LBO DRO which operated successfully with signal and idler output wavelengths of 946 and 1126 nm respectively. Other devices were also investigated during this time, including the target device, and they are discussed later within this thesis. However, the first device developed displayed frequency stability characteristics far in excess of what had been expected theoretically and a thermal feedback mechanism was proposed to explain its behaviour. This device was presented at The Conference on Lasers and Electro-Optics (C.L.E.O.) in 1995 and a discussion with a fellow delegate brought to my attention an independent proposal of a similar thermal feedback mechanism used to explain the enhanced frequency stability within other parametric devices. A period of work was then undertaken to prove the existence of this mechanism and to characterise it fully. The thermal feedback mechanism was subsequently computer modelled successfully and a solid-state laser was designed and built in order to probe this mechanism experimentally. Whilst technical problems were encountered which prevented a full characterisation of the said mechanism, evidence for it was found in the micro-operating characteristics of such a solid-state laser pumped, type II, pseudo-monolithic DRO.