Coupled tandem OPO operates at a slope efficiency of 35%
A group of researchers at DSO National Laboratories and the Data Storage Institute of Singapore demonstrated a coupled, tandem optical parametric oscillator (OPO) with high overall efficiency and energy in the mid-infrared region. A zinc germanium phosphide (ZGP) OPO is located inside the resonator of a potassium titanyl phosphate (KTP) OPO operating near degeneracy. The ZGP device is pumped with the high intracavity fluence inside the resonator of the LTP device. Also the nondepleted 2-µm pump beam resonating inside the first cavity is recycled by multiple passes through the ZGP OPO--the equivalent of pumping a much-longer ZGP crystal with walk-off compensation. The effect of these enhancements was to produce an overall slope efficiency of 35%, reportedly the highest obtained for two-stage OPOs.
With 1.06-µm pump energy from a multimode Nd:YAG laser, overall conversion efficiency was 5.2%, with the tandem OPO operating at about 1.4 times higher than the oscillation threshold--researchers expect efficiency as high 25% at about 4 times higher than the threshold. With a single set of mirrors, the tandem-OPO tuning range was 2.7-8 µm. No optical damage occurred throughout the experiment, and the group expects no problem pumping at a higher pulse-repetition rate because of the high thermal conductivity of ZGP and low optical loss of KTP.