Deformable mirror
Deformable mirrors represent the most convenient tool for wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control system in adaptive optics.
The shape of the mirror can be controlled with a speed that is appropriate for compensation of dynamic aberrations present in the optical system. In practice the mirror shape should be changed much faster than the process that should be corrected, as the correction process, even for a static aberration, may take several iterations.
The mirror has many degrees of freedom. Typically, these degrees of freedom are associated with the mechanical actuators and it can be roughly taken that one actuator corresponds to one degree of freedom.
Deformable mirror concepts
Segmented deformable mirrors are formed by independent flat mirror segments. Each segment can move a small distance back and forward to approximate the average value of the wavefront over the patch area. Normally these mirror have little or zero cross-talk between actuators. Stepwise approximation works bad for smooth continuous wavefronts. Sharp edges of the segments and gaps between the segments contribute to the light scattering, limiting the applications to those non-sensitive to scattered light. Considerable improvement of the approximation 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.
Continuous faceplate deformable mirrors with discrete
actuators are formed by the front
surface of a thin deformable plate. 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 considered to be
the best, as they allow smooth wavefront control with very
large - up to several thousands - degrees of freedom.
Membrane deformable mirror 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 the electrostatic electrode actuators that can be positioned under the membrane and also over the membrane. If there is any electrodes positioned over the membrane, they should be transparent. It is possible to operate the mirror with only one group of electrodes positioned under the mirror. In this case a bias voltage should be applied to all electrodes, to make the membrane initially spherical. The membrane can move back and forth with respect to the reference sphere.
Bimorph deformable mirrors are formed by two or more layers of different materials. One or more layers is fabricated from piezoelectric or electrostrictive material. Electrode structure is patterned on the active layer to facilitate local response. The mirror is deformed vhen a voltage is applied to one or more electrodes, causing them to extend laterally, which results in local mirror deformation. Bimorph mirrors are rarely made with more than 100 electrodes.
MEMS deformable mirrors are fabricated using technologies or bulk or surface micromachining. MEMS mirrors have a great potential to break the high price threshold of conventional adaptive optics.