An Optical Parametric Amplifier (OPA) is based on parametric nonlinear interactions. This effect, which is due to the crystal material, produces multiple mixing wavelengths, but can also be used for parametric amplification.
In the OPA amplifier, the signal beam is passed through a crystal together with a laser beam from a shorter wavelength pump laser. The photons from the pump laser are converted into so-called Idler photons and into signal photons, which have a lower energy. The Idler photons correspond to the energy difference between the energy of the laser and signal photons. In the OPA effect, the pump energy of the laser is completely converted into signal energy and Idler energy.
Parametric optical amplifiers can also be used for four-wave mixing by exploiting the nonlinearities of the crystal or optical guide.