Photo-Controlled Deformable Mirror (PCDM)
We have developed a novel type of deformable membrane mirror that is made by associating a metalized membrane with a monolithic photoconductive substrate. The assembly constitutes a continuous photo-controlled deformable mirror, PCDM, which removes the needs for segmentation in discrete actuators or pixels. The PCDM is driven by suitable light intensity distributions onto its photoconductive side, opposite to the reflection side. This approach, eliminating the spatial segmentation of the driving elements, largely simplifies the driving electronics; in addition, it allows realizing more flexible configurations of the effective electrodes driving the reflective surface.
A prototype PCDM was made by using a BSO crystal as a photosensitive element. The BSO is cut in the form of a thin disk, 1 mm thickness and 35 mm diameter, coated on one side with an Indium-Tin-Oxide (ITO) transparent electrode; the facing membrane is a nitrocellulose layer, 19 mm diameter and 5 µm thickness, metalized by an Ag coating and mounted on a rigid aluminum ring. Mylar spacers are introduced between the BSO and the ring supporting the membrane, in order to provide a gap of a few tens of microns. When an ac voltage is applied to the PCDM, the electrostatic pressure across the gap attracts the membrane towards the BSO substrate. Since the impedance of the BSO decreases with the light intensity, when the BSO is uniformly illuminated a large deformation in the form of a paraboloid is induced on the membrane. Once the membrane has reached an equilibrium position, further deformations can be superimposed by local point-like illuminations of the BSO.