An amorphous layer is used as a protective coating for hygroscopic nonlinear optical crystals. The amorphous layer consists of one or more alkali metal borates and/or alkali earth metal borates. The amorphous layer slows or prevents water and/or oxygen from diffusing into the hygroscopic nonlinear optical crystal, thus simplifying handling, storage and operating environmental requirements. One or multiple additional coating layers may be placed on top of the amorphous layer, with the additional coating layers including conventional optical materials. The thicknesses of the amorphous layer and/or additional layers may be chosen to reduce reflectance of the optical component at one or more specific wavelengths. The coated nonlinear optical crystal is used in an illumination source utilized in a semiconductor inspection system, a metrology system, or a lithography system.