Mirrorless optical parametric oscillators

We have considered two novel configurations for optical parametric oscillators (OPO's): transversely-pumped counter-propagating and nondegenerate backward OPO's due to the distributed feedback provided through the opposite propagation directions of the signal and idler. In both configurations, by changing the incident angle of the pump beam, one can tune the output frequency in a large range. The threshold pump powers for the oscillation can be as low as similar to 10 W for the transversely-pumped counter-propagating OPO's and 44 W for the nondegenerate backward OPO's. The quasi-phase matching is achieved by spatially modulating second-order susceptibility along the growth direction based on semiconductor alternating thin layers or asymmetric quantum-well domain structures or by electric-field poling in conventional second-order nonlinear materials. The nondegenerate backward OPO's offer the most efficient conversion among all the configurations for the OPO's having the same threshold pump power. The transversely-pumped counter-propagating OPO's have the optimal pump power.