w_p - w_s = w_i (or 1/l_p -1/l_s = 1/l_i in wavelength)

It splits one high-energy photon into two low-energy photons.

For example:

Optical Parametric Generation (OPG) is an inverse process of Sum Frequency Generation. It splits one high-frequency photon (pumping wavelength, l_p) into two low-frequency photons (signal, l_s, and idler wavelength, l_i). If two mirrors are added to form a cavity as shown in following Figure, an Optical Parametric Oscillator (OPO) is established. For a fixed pump wavelength, tilting a crystal can generate an infinite number of signal and idler wavelengths. Therefore, OPO is an excellent source for generating wide tunable range coherent radiation. BBO, KTP, LBO and LiNbO₃ are good crystals for OPO and Optical Parametric Amplifier (OPA) applications.

Optical Parametric Oscillator