High Quality Content by WIKIPEDIA articles! An optical parametric oscillator (OPO) is a parametric oscillator which oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency into two output waves of lower frequency () by means of second order nonlinear optical interaction. The sum of the output waves frequencies is equal to the input wave frequency: . For historic reasons, the two output waves are called "signal" and "idler", where the wave with higher frequency is called signal. A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, .