An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency (and shorter wavelength). Two types of devices are currently common, frequency doublers often based on lithium niobate, potassium titanyl phosphate (KTP) or lithium triborate (LBO), and frequency triplers typically made of potassium dihydrogen phosphate (PTP) or KTP. Both are widely used in optical experiments that use lasers as a light source.
There are two processes that are commonly used to achieve the conversion, second harmonic generation (SHG, also called frequency doubling), or sum frequency generation which sums two non-similar frequencies. Direct third harmonic generation (THG, also called frequency tripling) also exists, but known examples of these devices are very low efficiency and are generally not used in practice. "Real world" triplers use a two-stage process, combining two photons using SHG, then adding a third using summing.
Optical frequency multipliers are very common in high-power lasers, notably those used for inertial confinement fusion (ICF) experiments. ICF attempts to use a laser to heat and compress a target containing fusion fuel, and it was found in experiments with the Shiva laser that the infrared frequencies generated by the laser lost most of its energy in the hot electrons being generated early in the heating process. In order to avoid this problem much shorter wavelengths needed to be used, and experiments on the OMEGA laser and NOVETTE laser validated the use of frequency tripling KDP crystals to convert the laser light into the ultraviolet, a process that has been used on almost every laser-driven ICF experiment since then.