High-efficiency, Q-switched, mode-locked KTP OPO

Abstract

We have demonstrated an efficient, synchronously pumped, doubly resonant, KTP optical parametric oscillator (OPO). The efficiencies achieved are in excess of 60% for conversion to 2.128 μm by using a conventional mode-locked and Q-switched Nd:YAG 1.064-μm driver source. In some applications this device may be more attractive than directly using a 2.1-μm laser source. The quantum efficiency of the OPO is unity because it operates at the degenerate point (both the signal and idler waves have essentially the same wavelength at 2.128 μm), and both the signal and idler waves can be extracted to provide equally useful output power. Consequently, the overall conversion efficiency is equal to the photon conversion efficiency. Since the OPO is degenerate, it is also doubly resonant, but it does not suffer from the instability problems commonly associated with such devices. This is in part due to the large nonlinear drive, high output coupling (>95%), and simultaneous existence of many longitudinal modes. Normally, a degenerate OPO has a large frequency bandwidth. However, in KTP the phase matching must be type II, and this provides a narrow bandwidth at degeneracy of approximately 5 cm−1. This fact is extremely important for driving subsequent nonlinear stages. By using the 2-μm output power as a drive source, we have achieved frequency downconversion into the mid-IR in synchronously pumped AgGaSe2 OPO’s and we achieved 40% conversion of 2.128 μm to 3.8 plus 4.8 μm.