optical parametric oscillator

Light source similar to a laser, but based on 'non-linear optical gain' from parametric amplification rather than on stimulated emission.

Notes:

The device is a powerful solid-state source of broadly tunable coherent radiation. It consists of a crystal, usually BBO (BaB₂O₄), located inside an optical resonator and pumped by a very intense laser beam (typically provided by a pulsed neodymium laser or a diode laser). The pump beam (wavelength $λ p$ frequency $ν p$ ) is partially converted into two coherent beams, the signal and the idler, with wavelengths ( $λ s$ , $λ I$ ) and frequencies ( $ν s$ , $ν I$ ) such that $ν s + ν i = ν p$ . By simultaneous rotation of the crystal and adjustment of the optical resonator, the wavelength of the signal beam is continuously tunable, theoretically from $λ p$ to $2 × λ p$ and practically over a slightly more reduced range.
For example, for $λ p = 355 nm$ (3^rd harmonic of a Nd:YAGlaser), $ν s$ can be tuned from $400 nm$ (with $λ i ≈ 3.15 μm$ ) up to $600 nm$ (with $λ i ≈ 870 nm$ ).
This 'splitting of one photon into two photons' is the reverse of the 'sum frequency mixing' used, for instance, to generate the 3^rd harmonic of a laser emission by mixing in a convenient crystal the fundamental and the frequency doubled beams (a way to get the 3^rd harmonic much more efficiently than by pure frequency tripling as described under harmonic frequency generation).

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 378

Acronym: OPO