Continuous-wave, singly-resonant optical parametric oscillators pumped internal to Nd:YVO₄ lasers
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The advent of new quasi phase matched materials and high spectral/spatial quality pump sources has led to a renaissance in the development of continuous-wave optical parametric oscillators for the coherent generation of broadly tunable light in the mid infrared spectral region. This thesis describes a novel technique which overcomes the threshold constraints of the singly resonant oscillator (SRO) and stability constraints of the doubly resonant oscillator (DRO) traditionally associated with these devices by placing a singly resonant optical parametric oscillator in the high circulating field found within the cavity of a laser: the intracavity optical parametric oscillator. An SRO based upon the nonlinear material periodically poled LiNb0₃ (PPLN) operating internal to an all solid state, 1W diode pumped Nd:YV0₄ mini-laser is demonstrated and characterised. This system exhibits SRO threshold at only 330mW of external diode pump power, and produced a total of 70mW of extractable idler at 1W diode pump power. Through multi-parameter tuning of the poled nonlinear material we demonstrate broad tuning of the non-resonant idler through the spectrally important range 3.1 - 4μm. Novel cavity design desensitises the system to the effects of thermal lensing in the nonlinear medium, resulting in stable spatial and mean power outputs. The short term pump field stability is characterised by intensity modulation brought about by the onset of relaxation oscillations ; a consequence of placing the SRO within the cavity of the pump laser. A comparative study of SRO's based upon PPLN and the new material periodically poled RbTi0As0₄ (PPRTA) pumped internal to a high power fibre coupled diode pumped Nd:YV0₄ laser cavity is undertaken and presented. We see that although the nonlinearity and interaction length of the PPRTA is smaller than that of PPLN, the system based upon PPRTA outperforms or is at least comparable with that based upon PPLN in every respect with the exception of idler tuning range. We attribute this to the reduced sensitivity of this material to the effect of thermal lensing. Up to 440mW of extracted idler was produced by each system. The reduction of interferometric feedback of the pump field by the signal cavity mirror was found to eliminate the onset of relaxation oscillations in the case of PPRTA but not PPLN, due to thermal air currents dominating the triggering process in iii this system. Recently published Sellmeier equations and temperature derivatives for PPRTA are compared with the experimentally observed temperature tuning behaviour. The inclusion of an optical parametric oscillator within the cavity of the pump laser impacts significantly upon the transient dynamics of the pump laser in which it resides. We show experimental evidence of this effect and outline a strategy to minimise the effects of relaxation oscillations in the context of a simple numerical model which shall be derived. Possible future avenues of research are discussed in the context of the results and conclusions obtained over the course of this research program.
Thesis, PhD Doctor of Philosophy
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