Deformable Mirror - Deformable Mirror Concepts

Deformable Mirror Concepts

Segmented concept mirrors are formed by independent flat mirror segments. Each segment can move a small distance back and forth to approximate the average value of the wavefront over the patch area. Normally these mirrors have little or zero cross-talk between actuators. Stepwise approximation works poorly for smooth continuous wavefronts. Sharp edges of the segments and gaps between the segments contribute to light scattering, limiting the applications to those not sensitive to scattered light. Considerable improvement of the performance of the segmented mirror can be achieved by introduction of three degrees of freedom per segment: piston, tip and tilt. These mirrors require three times more actuators than piston segmented mirrors and they suffer from diffraction on the segment edges. This concept was used for fabrication of large segmented primary mirrors for the Keck telescopes.

Continuous faceplate concept mirrors with discrete actuators are formed by the front surface of a thin deformable membrane. The shape of the plate is controlled by a number of discrete actuators that are fixed to its back side. The shape of the mirror depends on the combination of forces applied to the faceplate, boundary conditions (the way the plate is fixed to the mirror) and the geometry and the material of the plate. These mirrors are often the most desirable implementation, as they allow smooth wavefront control with very large - up to several thousands - degrees of freedom.

MEMS concept mirrors are fabricated using bulk and surface micromachining technologies. MEMS mirrors have a great potential to be cheap. They can break the high price threshold of conventional adaptive optics. MEMS mirrors typically have high response rates, high precision and have no hysteresis, unlike other types of deformable mirrors.

Membrane concept mirrors are formed by a thin conductive and reflective membrane stretched over a solid flat frame. The membrane can be deformed electrostatically by applying control voltages to electrostatic electrode actuators that can be positioned under or over the membrane. If there are any electrodes positioned over the membrane, they are transparent. It is possible to operate the mirror with only one group of electrodes positioned under the mirror. In this case a bias voltage is applied to all electrodes, to make the membrane initially spherical. The membrane can move back and forth with respect to the reference sphere.

Bimorph concept mirrors are formed by two or more layers of different materials. One or more of (active) layers are fabricated from a piezoelectric or electrostrictive material. Electrode structure is patterned on the active layer to facilitate local response. The mirror is deformed when a voltage is applied to one or more of its electrodes, causing them to extend laterally, which results in local mirror curvature. Bimorph mirrors are rarely made with more than 100 electrodes.

Ferrofluid concept mirrors are liquid deformable mirrors made with a suspension of small (about 10 nm in diameter) ferromagnetic nanoparticles dispersed in a liquid carrier. In the presence of an external magnetic field, the ferromagnetic particles align with the field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Using proper magnetic field geometries, any desired shape can be produced at the surface of the ferrofluid. This new concept offers a potential alternative for low-cost, high stroke and large number of actuators deformable mirrors.