Mirrorless optical parametric oscillators (MOPO) represent a special class of parametric devices based on three-wave nonlinear interaction in which the generated photons counter-propagate. Owing to the phase-matching condition of the counter-propagating waves, MOPOs can sustain oscillation without mirrors and present unique and useful tuning and spectral properties. In this paper, we will review our recent advances in structuring technology to achieve quasi-phase matching periodicities as short as 500 nm in Rb-doped KTiOPO4, which are necessary to compensate for the large phase mismatch. We will also review the performance of MOPOs both in the ps- and ns- pumping regime. In the latter, our crystals reach single-pass conversion efficiencies exceeding 50%, with mJ-level output energies.
High-energy mid-infrared nanosecond sources are required in a number of applications including biomedicine, remote sensing, and standoff countermeasures, to name just a few. Sources which serve these applications include mid-infrared fiber and solid-state lasers, quantum cascade lasers, as well as optical parametric oscillators (OPO).
One of the most practical means of generating tunable mid-infrared output is by using cascaded parametric downconversion from 1 μm, where efficient and reliable high-energy nanosecond lasers are well established. The overall efficiency of the cascade relies heavily on the efficiency of the first down-conversion stage where it is beneficial to employ quasi-phase matched crystals such as periodically-poled Rb:KTiOPO4 (PPRKTP). Ultimately, the pulse energy at 2 μm and the optimum design of the first cascade will depend on the maximum intensity which could be safely applied to these crystals and therefore these schemes mandate investigation of nanosecond laser-induced damage threshold in KTiOPO4 (KTP) and Rb:KTiOPO4 (RKTP) at 1.064 μm and 2 μm. In the context of high-energy systems, where the beams are at most loosely focused, the limiting energy fluence will be determined by the laser induced damage threshold (LIDT) of the bare surface. Therefore the LIDT of the bare surface is the lowest LIDT which has to be taken into account in design of robust 2 μm parametric systems. We report surface LIDT measurements in KTP and RKTP with nanosecond pulses at 1.064 μm and 2.1 μm. We find that the reported LIDT for the bulk is far higher than that of the surface and therefore is unsuitable as a guide for the 2 μm parametric system designs. LIDT values for KTP and RKTP with nanosecond pulses at 2 μm have not been reported so far to the best of our knowledge.