Buried Long Period Grating For Laser Applications

One type of aperture-sharing device, the buried long period grating (BLPG), is described in this paper. The BLPG functions as a buried segmented mirror whose primary function is to spatially redirect, by reflections, an antiparallel laser beam and its corresponding low power broad band LWIR (long wavelength infrared) return beam. The aperture sharing unit consists of a pair of BLPGs, the second BLPG being used to restore spatial coherence across the LWIR wavefront. Other system functions of the BLPG such as autoalignment and beam sampling are discussed. The optical performance of the device is discussed in terms of energy losses due to material dispersion diffraction loss, degradation of resolution resulting from diffraction, segment fabrication tolerances, and thermally induced structural deformations due to laser beam heating. Both transient and steady-state thermal and structural analysis were performed on the device. One result from the analysis was the value of the burying dielectric thickness above the segment tips that minimized the stress within the device under laser beam irradiation. Fabrication consisted of separately tooling segmented surfaces in the cooled substrate and in the burying dielectric (CVD ZnSe and ZnS) followed by application of a metallic coating. These two segmented surfaces were joined with an appropriate bonding agent. The critical fabrication step is to use the bonding materials that have the following properties: (1) approaches full cure during fabrication, (2) minimum of outgassing with temperature and with time, (3) stable with aging (minimize surface distortion), (4) high thermal conductivity, and (5) flexible bond line to absorb thermal expansion mismatch between the dissimilar substrate materials. The exposed dielectric surface is polished flat, vacuum baked, and dichoric coated to reflect a laser beam and transmit a LWIR beam. Some top surface distortion is introduced during vacuum baking and during dichroic coating. Present work is towards reducing these fabrication temperature deformations by accurately mixing the bonding components to give different compositions. Preliminary work indicates good device performance in a laser environment.