Non-critical phase matching KTiOAsO₄ optical parametric oscillator with a RISTRA cavity at 300  Hz

In this paper, we report a rotated image singly-resonant twisted rectangular (RISTRA) resonator implemented in a non-critical phase-matching (NCPM) KTiOAsO₄ (KTA) optical parametric oscillator (OPO). Unlike previous studies mainly focused on critically phase-matched configurations with significant spatial walk-off, this work demonstrates that the RISTRA cavity can still enhance beam quality when walk-off is negligible. The resonator introduces a 90° image rotation per round trip, which suppresses the excitation of higher order transverse modes and thus improves mode discrimination. The RISTRA cavity produces 50.3 mJ of signal and 15.5 mJ of idler energy, comparable to the planar ring cavity output of 54.0 mJ (signal) and 18.2 mJ (idler). However, the beam quality is improved: the signal M² is improved from 4.7×4.4 in the planar ring cavity to 2.9×2.8 in the RISTRA cavity, and the idler M² improves from 4.7×6.4 to 2.5×2.5. These results confirm that RISTRA remains effective for beam-quality enhancement in NCPM OPOs.