Bulk micromachined 2D lens scanners for transparent optical fiber switches

Abstract

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 &micron 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...

A two-dimensional f-.THETA. micro optical lens scanner with electrostatic comb-drive XY-stage

“…We also used the same figure to read the scan angle θ calculated by using θ = x/f , where f is the focal length, 1 mm. A 10-µm-displacement (which was needed for optical switching to the nearest neighborhood fiber coupled over a distance of 20 cm) was obtained within 40 V in both the X and the Y directions; the voltage was almost 1/10 of that of the previously reported device of the same device footprint [4]. The mechanical resonant frequency at the fundamental mode in the X-direction (1.40 kHz) was higher than that in the Y-direction (0.55 kHz), which was thought to be acceptable for OXC applications.…”

“…In our previous report [4], we used a silicon suspension in the shape of an "H" with a lens at the center part, in which the suspension was directly attracted by the nearby electrostatic electrode. However, the displacement was limited to almost 1/3 of the initial electrostatic gap, for the mechanism was governed by the pull-in limit of the parallel-plate actuator.…”

“…The inner stage has a thin silicon lens of a 3.4-micron-sag and a 260-microndiameter (focal length of 1.0 mm) with four pairs of folded-beam suspensions of 2 microns wide, 500 microns long and 30 microns high. The silicon lens profile was transferred by the isotropic reactive ion etching (RIE) from a thermally reflowed photoresist pattern [4,9]. The microactuator patterns were made in the SOI layer by the high aspect-ratio DRIE, after which the handle wafer was etched from the backside to make the internal stages.…”

“…To propose a cost-effective and agile optical alignment system, we have developed a position adjustable silicon micro lens collimator integrated with a silicon-micromachined XY-stage [3,4]. The lens can be driven along the XY plane in the lateral directions only (see Figure 1 (a)), such that the optical axis can be finely adjusted to the incoming light.…”

Design and fabrication of MEMS optical switches

“…These include realizations based on optical MEMS [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32], thermal optical [33][34][35], semiconductor optic amplifiers (SOA) [36], liquid crystals [37], electro-holography [38], electro-optical LiNb03 [39], electronically waveguide Bragg grating [9], acousto-optical [40][41][42][43] and opto-optical switches [44][45][46][47][48][49], respectively.…”