A
moisture sensitive optically nonlinear
crystal is enclosed in a hermetically sealed elongated vacuum
cell. The vacuum
cell has an input window at one end and an output window at an opposite end providing optical access to the
crystal by a
laser beam. The windows are attached to the
cell by cold-formed,
indium-
metal ram-seals. In an example of the cell in which the
crystal is arranged to generate UV
radiation from the
laser beam, the output window is located at a sufficient distance from the crystal that the flux of UV
radiation incident on the output window is below the damage threshold of the window for the UV
radiation.