Progress in the development phases of a new optical material that exhibits very low thermal lensing and robustness against thermal shock will be reported. Material, thermal, and optical properties of the current material formulation, called OFG-04, have been determined on small- and mid-scale samples. Manufacture scale-up to full-scale has been completed and flight windows prepared. Follow-on efforts are now beginning for an optimized formulation that will exhibit even lower bulk absorptance and OPD change with temperature rise.
We begin with a brief review of prior work relating to optical windows for use with high power laser beams. A typical window must provide pressure separation between system segments, ultra-low loss, and small wavefront distortion of the many outgoing laser beams and signal returns despite heating by the high energy laser beam. Historically, two approaches have been examined to improve such windows.
Using conventional materials like fused silica and sapphire for critical window components in a high-power laser system can lead to intolerable thermal distortions and optical path difference effects. A new oxyfluoride glass is being developed which has the unique property of possessing a negative thermo-optic coefficient (dn/dT) in the near- and mid-wave infrared. Specifically, the refractive index (n) of oxyfluoride glass decreases as the temperature increases. The distortions caused by thermal expansion of the glass during laser irradiation are partly offset by the negative dn/dT. This paper specifically addresses optical properties and surface finishing of oxyfluoride glass compared to fused silica. Normarski micrographs and surface profiles were measured to inspect the surface quality since smooth surfaces are essential for suppressing surface scattering and absorption. The refractive index and thermo-optic coefficient were measured using null polarimetry near the Brewster angle. Low dn/dT is required for laser windows. Transmittance spectra were measured to deduce the extinction coefficient by comparing with the transmittance calculated from the refractive index and to screen for unwanted absorption from contaminants including hydrocarbon oils, polishing residue, and water or -OH groups. Total integrated scattering was measured for both surface and bulk scattering. All measurements were done on 1.0- and 1.5-inch-diameter witness samples.
An account of the recently completed, 1-1/2 year ABL risk reduction deformable mirror (DM) program will be given. In this effort, candidate sub-scale deformable mirrors (SSDM) supplied by tow vendors were tested for manufacture and performance quality. The down-selected mirror was then provided with a newly-developed, high performance, multiple wavelength, low stress, very low absorptance faceplate optical coating. The SSDM was then operated for cycle number and stroke, and under high laser flux, that exceeds the operational requirements of the ABL DMs. The successful results for these tests, carried out with the contributions of a multifaceted integrated product team, will be reported.
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