Apparatus for effecting harmonic conversion of a laser beam of predetermined frequency, to provide plural harmonic components of the laser beam at frequencies different from the predetermined frequency, includes a housing (40) defining a hermetically sealed chamber able to be maintained at a pressure below atmospheric pressure. Also provided are port means for evacuating the chamber, and means (36, 37) defining an optical path for the laser beam and the components thereof through the housing and the chamber. A plurality of individual holders (70, 72, 74) are arranged for retaining respective frequency conversion crystals at spaced locations in the optical path. The crystals (20, 22, 24) can be aligned individually and heated within the chamber.