Abstract:We report the design, fabrication and electromechanical performance of a newly developed micro electromechanical XYscanner for micro-optical spatial light modulation using the silicon micromachining technology. A two-dimensional stage is integrated with a silicon micro lens to scan a transmitting infrared light of 1.55-micronwavelength by the mechanism of the f -θ lens scanner. Mechanical displacement of up to ±10 microns (optical angle of ±0.57 degrees) was obtained with a drive voltage of 30 V; optical simulation has suggested that the scanner can be used to construct a free-space optical crossconnect of 9-input by 9-output port counts.
Phone: +81-3-5452-6276 / Fax: +81-3-5452-6250 /Contact H. Toshiyoshi at huok2iis.u-tokvo.ac.iu '' also with VLSI Design and Education Center: University of TokyoAs an alternative approach of constructing 3D MEMS optical crossconnectors (OXC), we propose bulk micromachined electrostatic 2D lens-scanner arrays instead of tilting mirrors. Having carefully investigated the optical and mechanical designs of MEMS OXC, we have concluded that collimator lenses have the most significant influence upon the optomechanical switching performance. In particular, fiber array's core-position dispersion with respect to the collimator lenses gives random offset in the direction of the collimated light beams (as shown in Fig. I); this leads to large optical insertion loss and limited number of optical channels that could be accommodated. When scanning mirrors are used as a beam steering device, the mirror size has to be designed large enough not to suffer from beam-clipping loss; this may also lead to degraded mechanical switching response.On the other hand, the fiber-core deviation would not be a serious problem if collimator lenses are individually position-controlled with sub-micron accuracy. At the same time, such scanning lenses work as a beam steering device when each piece is two-dimensionally actuated with large enough mechanical stroke. Figure 2 illustrates the schematic view of our OXC using a microlens scanner array with a fiber anay. The MEMS scanning lens chip is placed at its focal length from the fiber facet in order to steer the collimated beams. Fundamental concept has been reported elsewhere [I].Different from our previous device made of photoresist microlens on a polysilicon surface micromachined XY-stage, we have developed bulk-micromachined 2D scanners with silicon lenses as shown in Fig. 3. The center disk (diameter 300 microns) supported at the middle of " H shaped suspensions (width 6 microns) is a silicon microlens. Stationary electrodes are arranged around the suspensions for making X-and Y-motion of the lens by electrostatic forces. In our prototype, a 4x4 array of such microlens scanners has been designed with a 1-mm pitch. Figure 4 shows simplified fabrication process using a 50-micron-SO1 (silicon-on-insulator) wafer. Microlens patterns formed by the photoresist reflow technique are transferred to silicon by isotropic RIE (reactive ion etching). Spherical shape of silicon lens and high-aspect ratio structures of electrostatic actuator are made by engineering the mask-combination prepared at the beginning of the process: aluminum for field-protection during isotropic RIE of microlens, and LOCOS oxide for Deep RIE of actuator parts. Figure 5 shows a typical optica1,microscope image of silicon microlens array (test run, diameter 260 microns, sag 9 microns, focal length estimated to be 400 microns). Our first mechanical test has shown µn displacement in the X-direction with a dc 250 V (Fig. 6), which could potentially steer a beam by 2 mm at a 10-cm distance. Typical mechanical resonance was found aro...
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