Optical communication networks require switches for fiber links. Free-space optical switches have the potential of high coupling efficiencies and are investigated for fabrication by silicon surface-micromachining. Due to batch processing, this technology can provide large volume production of high-performance devices at low cost. My goal is to design and build a (2x2)-optical switch for multimode fibers that allows one to switch two input fibers to two output fibers in parallel. This is accomplished by using a polysilicon plate coated with gold, which serves as a mirror and reflects the light to the fibers. The mirror is hinged to a slider and stands vertically on the chip. Comb-driven vibromotors actuate the slider and move the mirror in and out of the optical path.
Characterizing existing mirror structures for their possible application in a switch showed that they could not immediately be used and that new devices were necessary. The transfer from a 4-layer process to a commercially more attractive process with three layers of polysilicon necessitated the design of a novel hinge. The new approach is based on cantilevers and requires only two structural layers of polysilicon for slider, mirror, hinge, and locking mechanism. The later one has also been redesigned and led to considerably improved behavior during assembly. The novel hinge design and improved locking mechanism make it possible to build a mirror with high accuracy and without wobble.
Initial experiments show a slider velocity of >1 dsec which would make possible switching speeds of less than 100 m. With off-chip actuation of the slider we obtain coupling with -1.9 to -2.0 dB loss. The fibers will eventually be integrated on the chip by the means of v-grooves. For the initial project described here, they were aligned off-chip with xyz-stages.
This research shows the functionality of an actuated, precisely locked mirror and the capability to use the mirror for fiber-coupling at relatively low loss. Thereby, the feasibility of a surface-micromachined optical switch is proven.
March 31, 1996
Friedberger, A (1996). Development of a Free-Space Optical Switch for Fiber Communications Based on Actuated, Silicon Surface-Micromachined Mirrors. United States: University of California, Berkeley.