Counterdirectional mode coupling in ring resonators with QPM nonlinear crystals and effects on the characteristics of cw optical parametric oscillation

The effects of coupling between clockwise and counterclockwise modes in ring resonators due to back reflections from QPM gratings in nonlinear crystals are investigated. We demonstrate, using simulations, that the non-phase-matched back reflection due to typical imperfections of the QPM grating has a relevant magnitude and can give rise to the coupling. We developed an analytical model and evaluated parameters of a singly resonant OPO in the presence of this coupling, which indicates that the threshold pump power is doubled in the limit of strong coupling. We also evaluated the effect of the coupling with respect to resonant power enhancement for cavity-enhanced nonlinear frequency mixing applications, finding that the enhancement is reduced by up to a factor four. Experimental observations based on PPLN and orientation-patterned GaAs crystals and model predictions are in good agreement. We propose and demonstrate a method for active stabilization of ring resonators with mode coupling that relies on the dissimilarity of the coupled forward and backward modes.