Deep ultraviolet radiation of solid-state near-infrared (~ 1 μm) lasers is often generated as the second harmonic of the frequency doubled beam, i.e. the wavelength of ~260 nm, and as the sum harmonic frequency, the wavelength of ~200 nm. For these processes usually BBO and CLBO crystals are used. The big advantage in the fourth harmonic generation of the BBO crystal is its high nonlinear coefficient (1.69 pm/V), however, its walk-off angle (84.5 mrad) is quite big and limits the exploitable length of the crystal. The nonlinear coefficient of the CLBO crystal is half of that for BBO (0.83 pm/V), but the walk-off is small (28.8 mrad). Unpleasant with the CLBO is its hygroscopicity, which has to be compensated by keeping the crystal at higher temperature, 150 °C recommended . Nevertheless, the elevated temperature is beneficial for lowering the nonlinear absorption which proceeds as two photon absorption . Regarding the nonlinear absorption, both crystals do not differ much. Our study concerns the CLBO crystals as convertors into the deep ultraviolet (DUV) range.
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