OPTICAL PARAMETRIC OSCILLATORS FOR MID INFRARED RADIATION

Abstract

In the aim of generating laser source whose wavelength is tunable in the mid IR (3-5 µm) range, we have demonstrated the feasibility of using both single-stage and double- stage OPO for such laser wavelength conversion. In the single-stage OPO approach, we report a mid infra-red AgGaS2 optical parametric oscillator (OPO) pumped by nanosecond 1.064 µm Nd:YAG laser. Its wavelength tuning range is from 2.8 to more than 4.2 µm. Using singly resonating (SRO) configuration, the maximum absolute conversion efficiency obtained is about 10% (slope efficiency ~ 16%) which is equivalent to a pump depletion of 30-40%. In the double-stage OPO approach, we have demonstrated two different configurations: a) ZnGeP2 OPO pumped in series by a degenerate KTP OPO and b) a novel configuration where the ZnGeP2 OPO is placed within the cavity of the degenerate KTP OPO. For both configurations, the starting pump laser is a nanosecond 1.064 µm Nd:YAG laser. For the first configuration, we report a high efficiency ZnGeP2 OPO pumped by another KTP OPO in a multimode pumped tandem OPO configuration. The maximum optical-to-optical and slope efficiencies were 32% and 42.5% respectively. This configuration also provided tunable multi-band radiation in the 2.03 - 2.32 µm range and 2.9-6.2 µm range simultaneously. However, its overall cascaded efficiency (from 1.06µm to mid IR) of ~ 3.1 % is not as good as that of the second coupled tandem OPO configuration where the second ZnGeP2 OPO is placed within the resonator of the first KTP OPO. With a multimode Nd:YAG laser, this novel configuration gives an overall optical-to-optical efficiency (from 1.06µm to mid IR) of 5.2 % when operating only ~1.4 times above oscillation threshold. The measured overall slope efficiency is attractively high at 35%. With a single set of mirrors, we obtained a broad wavelength tuning range from 2. 7 to 8 µm. Therefore, we believe this configuration is promising in the tandem OPO route to mid IR. Beside the experimental results, we have also implemented our own theoretical mode to predict a) the oscillation threshold of a pulsed doubly resonating OPO operating near degeneracy and b) conversion efficiency of pulsed nanosecond OPO.