YAG CRYSTALS.

This contract validated the conclusion tentatively presented in the previous report that growth station design and control had evolved to an optimum level for the current scale of operations so that laser quality NdYAG crystals large enough to permit fabrication of 14 inch diameter by 2 inch long rods core free can now be produced routinely. This capability was demonstrated by delivery to Ft. Monmouth of eight such rods during the month of July. In addition, three 3mm X 50mm NdYAG rods and one undoped YAG crystal large enough to permit fabrication of 14 inch X 2 inch rods were delivered in fulfillment of the final stage requirements specified under the present contract. Evaluation of the rods via passive i.e., non-lasing optical tests indicated a state-of-the-art quality profile that was generally quite encouraging High angle scattering losses fell in a range of 0.1 to 0.2 per cent per centimeter of length. Optical homogeneity, as gauged by Twyman-Green interferometric examination, was high from zero to two fringes on the 3mm X 50mm rods and two to three fringes on the 14 inch X 2 inch rods. Appearance between crossed polars of the best rods revealed only a negligible diffuse residual strain pattern a few specimens showed more pronounced localized disturbances. Far-field photographs produced patterns that were generally consistent with the other measurements. Author