Enhanced power from a doubly resonant optical parametric oscillator by choice of resonator length

The output energy from a doubly resonant optical parametric oscillator (OPO) pumped by a multi-mode beam has been found to have a significant maximum when the optical length of the OPO matches that of the pump source, even if this length is considerably longer than the shortest possible for the OPO. We have observed this effect in a ZnGeP2-based OPO, and reproduced it in numerical simulations. A similar effect is also seen in simulations of a singly resonant OPO. It is also shown that this length effect may increase the OPOs tolerance to a wide pump bandwidth.