Abstract: A stacked OPO is disclosed wherein two or more optically nonlinear media, such as crystals, are coaxilly disposed in a single resonator. Incident radiation is coupled into the resonator, and causes parametric oscillations of the two crystals. The two crystals are independently tuned, such as by angular orientation, to produce distinct components of secondary radiation. A first one of the crystals is disposed nearer to the source of incident radiation, and a second one of the crystals is disposed nearer to the output coupler of the resonator. This causes the first crystal to experience a greater effective gain. Furthermore, the secondary radiation from the first crystal will tend to dominate and "seed" the secondary radiation from the second crystal, when their bandwidths are narrowly separated.