MEMS-based optics advances include large aperture scanner for 3D distance measurement

July 13, 2009--Fraunhofer IPMS (Dresden, Germany) is planning photonics demonstrations for this month's Micromachine/MEMS 2009 event (Tokyo, Japan July 29-31), including a spatial light modulator that enables the chip to directly generate analog intensities, adaptive optics, and a large aperture scanner module for 3D distance measurement. All the demonstrations are based on microelectromechanical systems (MEMS) technology, which enables some advanced functionality.

MEMS spatial light modulator
MEMS spatial light modulators (SLMs) are used to arbitrarily modulate the entire cross section of an incident beam of light simultaneously. Usually, MEMS thin film technology is used to fabricate an array of micromirrors on top of a CMOS backplane, and each mirror can be addressed and moved individually to control either the intensity or phase of one pixel.

Unlike the digitally addressed SLMs in projectors that use time multiplexing to generate grey levels, Fraunhofer IPMS's SLMs are addressed with analog voltages that enable the chip to directly generate analog intensities. Micronic Laser Systems AB's (Taeby, Sweden) mask writers exploit this feature to shift the generated patterns with accuracies far smaller than the system's pixel resolution. The mirrors of the Fraunhofer IPMS device are specified for monochromatic light between 248 and 520 nm, enabling not only the use of the DUV excimer wavelengths, but also the less aggressive (and less costly) near UV light sources.

The mirrors are promising for pattern generation for high definition interconnect (HDI) in laser direct imaging (LDI) of printed circuit boards. They could also apply to other areas that require a combination of both highest resolution and throughput. The SLM is capable of patterning 1 million pixels at a frame rate of 2 kHz (that is, 2 Gpixel/s). Fraunhofer IPMS will enable visualization of the SLM's operation. A green LED generates a light beam, and lenses and mirrors guide the light onto the SLM, where it is modulated. The reflected beam is then filtered and projected into a camera. The live image of this camera shows the pattern programmed in the SLM.

MEMS-based adaptive optics
The key component in adaptive optics (AO) is formed by the actual wavefront controlling device, and for that purpose, Fraunhofer explains, microelectromechanical systems (MEMS)-based micro mirror arrays possess several attractive features. For instance, their integrated fabrication capability lets them support large numbers of pixels, providing an exceptionally high spatial resolution for improved image reproduction--especially of higher order phase aberrations. They benefit from a step function display capability, fast mechanical response times, low power consumption, broad spectral bandwidth from infrared (IR) to deep ultraviolet (DUV) and polarization insensitivity. And compared to macro-scale systems, micro mirrors also offer the potential of a substantial cost decrease as well as device miniaturization--facilitating new possibilities for a broader commercial exploitation.

The Fraunhofer IPMS's MEMS Phase Former Kit sports a high-resolution array of 240 x 200 piston-type MEMS micro-mirror elements with 40 µm pixel size, providing 400 nm stroke at 8-bit resolution suitable for a 2π phase modulation in the visible. Driver software for Windows XP supports both a graphical user interface as well as an open ActiveX programming interface for open-and closed-loop operation. High-speed data communication is accomplished by an IEEE1394a FireWire interface, together with an electronic driving board, allowing for frame rates up to 500 Hz.
The system targets optical system developers and manufacturers in machine vision (in-situ process control through turbulent media), optical microscopy, ophthalmology, astronomy, laser pulse shaping, and diffractive optics (especially optical tweezers).

Large aperture MEMS scanner module for 3D distance measurement
Fraunhofer IPMS will present Lamda, a prototype of a large aperture 1D MEMS scanner module designed for laser radar systems. It is based on a novel, scalable MEMS array of identical silicon mirror elements with a comparatively large total scanning aperture of 2.51 x 9.51 mm and a large optical scan range of plus or minus 30 degrees. The module comprises two separate scanning channels: (a) a single scanning mirror of the collimated transmitted beam oscillates parallel to (b) a scanning mirror array of the receiver optics. Light paths of emitting and receiving optics are separated to reduce crosstalk in the final laser radar system. The receiver optics uses an array of 2 x 7 identical mirror elements resulting in a total aperture of 334 mm squared, and a filling factor of 80%.

The concept of using an array of synchronized identical MEMS mirror elements for LIDAR systems permits large reception apertures while preserving the reliability, high scanning speed, compact size and small system weight that can be expected from MEMS. Compared to systems with conventional scanner components, module enables 3D LIDAR systems to become significantly smaller and more robust. Higher scan rates can be realized without additional efforts (e.g. air bearings). Hence, the Lamda module is promising for many applications, including security, machine vision and even for portable outdoor use.

For more details on the MEMS-based systems see the descriptions on Fraunhofer IPMS's site.

Posted by Barbara G. Goode, [email protected], for Laser Focus World.