Application Note: An Introduction To Optical Parametric Oscillators

Unlike a tunable laser, which employs various energy strategies to deliver relatively broad coverage, an optical parametric oscillator (OPO), in a process similar to harmonic generation, relies on the nonlinear response of a medium to a driving field (the pump laser beam) to convert photons of one wavelength to photons of other, longer wavelengths.

Specifically, in the parametric process, a nonlinear medium (usually a crystal) converts the high energy photon (the pump wave) into two lower energy photons (the signal and idler waves).