FEBRUARY 23--Adaptive optics (AO) is mainly used for the compensation of spatially and timely varying wavefront disturbances within an optical system for an enhancement in optical imaging through inhomogeneous or turbulent media. Originally evolved from astronomy to compensate for atmospheric turbulences, AO techniques also can be used for aberration correction of the human eye in ophthalmology and for imaging through biological tissue in optical microscopy or for any kind of object recognition in machine vision. Furthermore, there are applications in laser-beam shaping, as well as in ultrafast laser-pulse modulation.
The key component is formed by the actual wavefront-controlling device. For that purpose MEMS (microelectromechanical system) micromirror arrays possess several attractive features. Due to their integrated fabrication capability they can support large pixel numbers providing high spatial resolution for improved reproduction, especially of higher-order phase aberrations. They also benefit from a step function display capability, fast mechanical response times, low power consumption, broad spectral bandwidth from IR down to DUV, and polarization insensitivity.
Compared to previous macroscale systems, micromirrors also offer potentially substantial cost decrease, as well as a significant device miniaturization facilitating completely new opportunities for broader commercial exploitation. The Fraunhofer IPMS (www.ipms.fraunhofer.de) has developed a complete MEMS phase former kit. The key component is a high-resolution MEMS micromirror array of 240 x 200 piston-type mirror elements with 40-μm pixel size providing 400-nm stroke at 8-bit resolution. Full user programmability and control is established by driver software for Windows XP PCs supporting both a graphical user interface and an open ActiveX programming interface for open-loop and closed-loop operation.
High-speed data communication is accomplished by an IEEE 1394a FireWire interface together with an electronic driving board allowing for maximum frame rates of up to 500 Hz. To visualize the potential for optical imaging enhancement, a complete AO demonstrator system has been implemented. It comprises a projection system, where objects of different complexity are imaged through adaptive optics onto a CCD camera. Phase errors are introduced by rotating phase plates. Using a Shack-Hartmann sensor and the Fraunhofer IPMS MEMS micromirror for wavefront sensing and correction respectively, the obtainable imaging improvement can be assessed by means of the recorded CCD picture, which is also projected onto a large screen.
The device will be demonstrated at OPTATEC 2006 (20--23 June 2006, Frankfurt/Germany) at the joint booth of the Fraunhofer Gesellschaft.
