Abstract: Apparatus and method for high-power multi-function millimeter-wavelength (THz-frequency) signal generation using OPO and DFG in a single cavity. In some embodiments, the OPO-DFG cavity includes an optical parametric oscillator (OPO) non-linear material that receives pump light IP having pump-light frequency and generates two different lower intermediate frequencies of light—an OPO-signal beam IS and a spatially/temporally overlapping OPO-idler beam II. A difference-frequency generator non-linear material then receives the two intermediate-frequency beams II and IS, and the DFG then generates a THz-frequency output signal that has a frequency equal to the difference between the two intermediate frequencies. In some embodiments, a single-piece crystal of non-linear material is used for both OPO and DFG functions.