Difference frequency generation combined with intracavity absorption

A previous study in which the optical parametric oscillator (OPO) has been shown to provide a light source suitable for intracavity absorption spectroscopy, is extended to difference frequency generation. The following experimental situation is discussed: in a nonlinear material an incident monochromatic pump wave, together with an incident many-mode idler wave, creates a many-mode wave at the difference frequency (signal wave) which is made to resonate in a cavity. The latter contains an absorbing medium whose absorption line is smaller than the signal bandwidth. For this device, the enhancement factor ξ is calculated. Large values of ξ can be expected only for pump powers which exceed the threshold value for optical parametric oscillation. Otherwise, the enhancement will be due only to the resonator effect.