Incompatibility between cavity resonances and wavevector matching: influence on threshold and beam structures of OPOs

We investigate theoretically and experimentally the interplay between cavity and double-refraction in continuous-wave optical parametric oscillators. We show that very basic geometrical effects can prevent transverse wavevector matching for the TEM₀₀ modes, and thus increase the threshold and change dramatically the beam structures when using a resonant (or double-pass) pump. We extend the work of Boyd and Kleinman to take these cavity-induced mismatches into account, and test experimentally the theoretical results using a cw type II KTP OPO. The transverse wavevector mismatch leads to the appearance of transverse modes with increasing order (up to TEM_40-0) and surprisingly induces structures that are different on signal and idler. The theory is able to predict quantitatively all selected transverse modes, and allows a simple interpretation in transverse Fourier space.