Term: optical parametric oscillator (OPO)
https://doi.org/10.1351/goldbook.OT07438

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

Notes:
0) The device is a powerful solid-state source of broadly tunable @C01130@. It consists of a crystal, usually BBO (BaB2O4), located inside an optical resonator and pumped by a very intense @L03459@ beam (typically provided by a pulsed @N04103@ or a @D01747@). The pump beam (@W06659@ λp frequency ν p) is partially converted into two coherent beams, the signal and the idler, with wavelengths (λs, λI) 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 @W06659@ of the signal beam is continuously tunable, theoretically from λp to 2 x λp and practically over a slightly more reduced range.
1) For example, for λp = 355 nm (3rd harmonic of a Nd:YAG @L03459@), ν s can be tuned from 400 nm (with λi ≈ 3.15 µm) up to 600 nm (with λi ≈ 870 nm).
2) This 'splitting of one photon into two photons' is the reverse of the 'sum frequency mixing' used, for instance, to generate the 3rd harmonic of a @L03459@ emission by mixing in a convenient crystal the fundamental and the frequency doubled beams (a way to get the 3rd harmonic much more efficiently than by pure frequency tripling as described under @H02744@).

Related Terms:
1) coherent radiation (http://doi.org/10.1351/goldbook.C01130).
2) laser (http://doi.org/10.1351/goldbook.L03459).
3) neodymium laser (http://doi.org/10.1351/goldbook.N04103).
4) diode laser (http://doi.org/10.1351/goldbook.D01747).
5) wavelength (http://doi.org/10.1351/goldbook.W06659).
6) harmonic frequency generation (http://doi.org/10.1351/goldbook.H02744).
7) light source (http://doi.org/10.1351/goldbook.L03526).
8) parametric amplification (http://doi.org/10.1351/goldbook.PT07440).
9) stimulated emission (http://doi.org/10.1351/goldbook.S06015).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 378 (https://doi.org/10.1351/pac200779030293)

Citation: 'optical parametric oscillator (OPO)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.OT07438

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