MEMS-based micro mirror laser projector can be integrated into cell phones

May 14, 2013 // By Julien Happich
For years, Fraunhofer IPMS has been working on scanning mirrors for light deflection in one and two dimensions. Alongside resonant scanners with constant sinusoidal movement, a quasi-static variation has also been available for a while, which serves to make linear movement or static orientation in a specified position possible.

Both types make compact laser projection systems possible in practice.

These could be used as pico-projector components to be integrated into smart phones, supporting projections at the size of a TV screen, even at a short projection distance. Fraunhofer IPMS now concentrates on the "laser beam steering" principle for image generation. Here, a brightness controlled laser beam (e.g. three, in red, green and blue, for full color display) is directed by a single miniaturized scanning mirror over the projection surface fast enough to generate an image which appears cohesive to the human eye.

The scanning mirrors typically have a diameter of about one millimeter and are manufactured from mono-crystalline silicon wafers using established processes in cost-effective, large batches in the MEMS clean room at Fraunhofer IPMS. Fraunhofer IPMS utilizes an electrostatic actuator with comb shaped electrodes for the necessary movement of the scanning mirrors. It was easy to make one and two dimensional scanners with conventional planar structures (i.e. mirror, outer frame and actuator combs lay on one plane). However, in this case, the resonating frequency is set, and changing the horizontal frequency, for example, is hardly possible. For another thing, the high scan frequencies bound to the double resonant principle are contrary to the trend toward increasing pixel counts in the projected image – right up to full HD.

These disadvantages are overcome with the new quasi-static scanner concept from Fraunhofer IPMS, called LinScan. It requires one subsequent micro-assembly step, along with a small modification to the mechanical design to permanently tilt the actuator combs toward each other. The Institute was able to show that the combination of a resonant actuator on the quick horizontal axis and LinScan on the slower vertical axis is possible for a two-dimensional scanner like those necessary for pico-projectors.

The laser beam can now make targeted jumps from line to line according to an externally configured frequency. This could yield pico-projectors with a SVGA