Controlling a deformable mirror with light

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

Adaptive optics are likely to get a boost through new technology using light instead of electricity to control a deformable mirror (DM) that comes with a continuous membrane rather than segmented pixels. The technology, developed at Institut Nonlineaire de Nice, of the University of Nice-Sophia Antipolis, could be used in adaptive optical systems that measure the distortion of an incident wavefront, then correct the distortion by shaping of the DM reflected beam, among other applications.

Various schemes have been proposed or implemented for optoelectronic deformable mirrors, based either on independently actuated rigid sections made of piezoelectric material or on flexible large-area reflective membranes. The rigid sections or the elemental areas of the membrane act as independent reflective elements and are driven by respective independent actuators to get a local mirror deformation desired for correction of the locally incident wavefront.

Important static DM parameters include the number of actuators as well as their pitch, stroke, coupling and influence function. On the dynamic side, the speed of the mirror must be faster than changes of the distortions, and hysteresis and creep have to be avoided. To date, adaptive optical elements have been successfully used in specific applications such as astronomy. Further adaptation in other fields of optoelectronics – beam shaping, for example – is often limited by the complexity of the electronic circuitry driving each pixel, the discretization of the deformation, and the limited spatial resolution of the reflected images.

The optically controlled deformable mirror consists of a reflective membrane associated with a photoconductive Bi₁₂SiO₂₀ (BSO) substrate. The membrane deformation is controlled by local illumination on the BSO side of the mirror. Courtesy of the University of Nice-Sophia Antipolis.

In collaboration with researchers at Jphopto of Paris and the University of Padua in Italy, the team at Nice has addressed these issues, realizing a photo-controlled deformable mirror that is fabricated by combining a conductively coated membrane made of nitrocellulose (5-µm thickness) with a single, nonpixelated photoconductive Bi₁₂SiO₂₀ (BSO) substrate. A single AC voltage is applied across both, and the impedance of the photoconductive substrate decreases when the incident illumination increases. When the voltage increases, the capacitive effect attracts the membrane toward the photoconductive substrate, leading to a deformation of the membrane at that location in the form of a paraboloid. Once the membrane has reached an equilibrium position, further deformations can be superimposed by local point illuminations.

It might seem that this is just shifting one problem to another, but the researchers say that with today’s video projector technology, it is easy to send any illumination pattern to the mirror. Another advantage of the approach is that it is scalable from the currently used 35-mm diameter to virtually any size. The frequency behavior of the device shows that operation up to several hundred hertz is possible. Applications are foreseen in the fields of astronomy and medical diagnostics.

The next step is to produce the device with other photoconductive substrates, making it suitable for other wavelengths. This would make the deformable mirror more attractive for applications in medical and biological imaging. The same authors, in collaboration with scientists at Moncton University in New Brunswick, Canada, have demonstrated a device that achieves mid- to near-IR image conversion by thermally induced optical switching in vanadium dioxide.