Reduced Print Thru in HEL Deformable Mirrors

This project will investigate a passive solution to the deformable mirror print through problem. No additional sensor or control subsystems will be required. The high reflector surface will be positioned on the backside of the thin deformable transparent glass sheet. First order theory (uniform temperature through the thickness of the thin glass sheet) indicates that for most glasses there is a thickness that makes this backside reflector athermal with respect to actuator thermal expansion. The thermal expansion and dn/dT of the thin glass sheet cancels the change in optical path caused by the thermal expansion of the actuators and their heating of the mirror substrate. The thermal optic distortion of mirrors heated by simulated actuators will be measured interferometrically and compared to theory. Besides the required change in optical path with temperature, the glass must have physical and chemical properties suitable for fabrication into a thin polished sheet that will be HR coated on one side and AR coated on the other. This project will investigate the most practical actuators, transparent materials and fabrication techniques and apply finite element analysis to guide design modifications to minimize any effects of thermal gradients produced by the actuators and/or laser beam.