Coupled tandem optical parametric oscillator (OPO):?an OPO within an OPO

We have demonstrated, for the first time to our knowledge, a coupled tandem optical parametric oscillator (OPO) configuration in which a second ${\mathrm{ZnGeP}}_2$ OPO is placed within the resonator of the first, ${\mathrm{KTiOPO}}_4$, OPO. A significant enhancement in the overall cascaded efficiency of this OPO compared with standard two-stage OPO’s was observed. With a multimode Nd:YAG laser, an overall optical-to-optical efficiency (from $1.06 \mathit{\mu }\mathrm{m}$ to the mid IR) of 5.2% was obtained from operating only $\sim 1.4$ times above oscillation threshold. The measured overall slope efficiency was attractively high at 35%. With a single set of mirrors we obtained a broad wavelength-tuning range from 2.7 to $8 \mathit{\mu }\mathrm{m}$.