Continuous-wave, intracavity singly-resonant optical parametric oscillators based upon Nd-doped laser gain media

Abstract

This thesis describes a system which overcomes one of the limitations faced by traditional continuous-wave optical parametric oscillators (cw OPOs). In this approach the high circulating field found within a laser cavity is exploited to reach the high threshold powers required by a singly-resonant oscillator (SRO) by placing the nonlinear crystal inside the cavity, creating an intracavity singly-resonant oscillator (ICSRO). These ICSROs are based upon cw diode-pumped solid-state lasers which, in themselves, are not necessarily widely tunable. However, by placing the OPO internal to the laser cavity means that it now includes a frequency converter and the two outputs produced, called the signal and idler, can be tuned over broad ranges in the mid and far infrared. The inclusion of an OPO internal to the cavity of a laser has a significant impact upon the transient dynamics of the pumping field. Experimental evidence of these effects are presented and an approach by which they can be damped is outlined. SRO systems based upon periodically-poled LiNbO₃ (PPLN) and periodically-poled RbTiOAsO₄ (PPRTA) pumped internal to diode-pumped Nd:YVO₄ lasers have been demonstrated in both high (15 W) and low (3 W) power regimes and limits to the practical pumping power have been investigated. Extractable idler output powers of 440 mW and 66 mW have been observed for diode pump powers of ~12 W and 3 W respectively. This idler power was shown to be capable of tuning over the ranges 3158 to 4024 nm for PPLN and 3538 to 3404 nm for PPRTA. The spectral quality of the down-converted radiation generated has been studied. The signal (resonated wave) is found to be single frequency and under such conditions the idler wave acquires the linewidth/frequency spectrum of the pump laser. Most applications for such devices require a single frequency output, and techniques to achieve this are discussed along with other refinements that could improve the overall performance of these systems. This should result in devices that are capable of delivering 10s to 100s of mW of continuous wave tunable output from compact geometries. Diode-pumped solid-state laser technology developments, quasi-phase-matched materials and the implementation of the intracavity technique, along with simple, inexpensive components could result in the realisation of straightforward and cost-effective devices.