Frequency quadrupling is a process of nonlinear frequency conversion where the resulting optical frequency is four times that of the input laser beam, which means that the wavelength is reduced by a factor of 4. This can be accomplished with two sequential frequency doublers (Figure 1). Another possibility would be to use a single frequency doubler and two sum frequency generation stages for mixing with residual pump light, but that approach is not common.
A commonly used frequency quadrupling configuration begins with a continuous-wave or pulsed Nd:YAG laser at 1064 nm for generating 532-nm light in a first frequency doubler stage (based e.g. on LBO = lithium triborate) and then 266 nm in a second stage (based e.g. on CLBO = cesium lithium borate). Such ultraviolet light is useful e.g. for pumping a dye laser or an optical parametric oscillator, for Raman spectroscopy in flames, or for material processing, e.g. the writing of fiber Bragg gratings.
Limited Lifetime due to Crystal Degradation
As explained in the article on frequency tripling, nonlinear crystals can be degraded by the intense ultraviolet light during operation. For frequency quadrupling, the correspondingly shorter UV wavelength can even increase such problems and lead to short lifetimes of crystals and other optics. Otherwise, similar aspects apply as discussed in the article on frequency tripling.
The RP Photonics Buyer's Guide contains 13 suppliers for nonlinear crystals and related equipment for frequency quadrupling. Among them:
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