Dynamic Optics

Deformable Mirrors

The most widely used wavefront modulators are deformable mirrors that can be controlled in real time to modify wavefront shape. Control signals are either computed from wavefront errors detected by a wavefront sensor, or from data previously acquired through system modelling and/or calibration.

Some deformable mirror designs use a continuous reflective surface, while others are based on a reflective surface that is segmented or pixellated. A continuous surface provides a smooth means for correcting wavefront errors and is particularly suitable for correcting the lower-order aberrations and distortions arising in an optical system.

Low-cost continuous surface micro-machined membrane deformable mirrors have recently been brought to market. These are micro-electro-mechanical systems (MEMS) devices fabricated from silicon. They can achieve greater than fifth order correction (including +3 Diopters focal power), have resonant frequencies above 1kHz, and can have diameters up to about 50mm. They have been demonstrated to achieve good reliability, repeatability, and robustness, e.g. 1 x 10^10 full deflections at 1kHz without hysteresis, multiple shock/drop tests from circa 2m, humidity of 85% at 35 deg. C for two months - all without observable mechanical problems.

The concept behind this type of mirror is to suspend a thin reflective-coated silicon nitride membrane over an array of electrodes. A voltage potential applied between the membrane and the electrodes gives rise to an electrostatic force that deforms the flexible membrane.