optical parametric oscillator

An optical parametric oscillator (OPO) is a parametric oscillator that 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 output wave with higher frequency is called signal. A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, .