Crystal housing apparatus for adjustably positioning cooperating non-linear optical crystals in a laser beam generating system, includes a housing (50) with entry (60) and exit (62) windows for respective laser beams, respective holder means (72, 74, 76) to retain respective crystals within the housing in optical alignment with the windows, and means (86) mounting the respective holder means for individual fine rotational adjustment of the crystals about respective axes. Individual temperature control elements (120) are mounted in association with the respective holder means for controlling the temperature of the respective crystals and means (110, 114) is provided for effecting the fine rotational adjustment of the crystals.