Analysis of cavity-length detuning in diffraction-grating narrowed picosecond optical parametric oscillators

We report on the detailed investigation of picosecond synchronously pumped optical parametric oscillators (OPOs) with a Littrow-mounted diffraction grating as the tuning element, operating close to and far from degeneracy. In addition to the expected line-narrowing effect, we observe a fivefold increase of the resonator-length detuning tolerance when compared to conventional mirror-based OPOs. When the OPO resonator length is altered, we also measure reproducible variations of the emitted wavelength associated with lateral beam displacements that were not reported before. We show that all of these effects can be explained by a geometric adaptation of the resonant beam path within the cavity to maintain a constant cavity round-trip time.