Optical MEMS for Free-Space Communication

The convergence of MEMS technology with communication and digital circuitry makes high-speed, low power, free-space communication links over distances up to several km possible. MEMS corner cube retroreflectors (CCRs) are proposed to work as a passive optical transmitter, sending collected information back to an interrogating center. MEMS scanning micromirrors are proposed to steer a modulated laser beam in order to establish a secure optical link between rapidly moving platforms.
Sub-millimeter-sized quad CCRs are fabricated by assembling two side mirrors onto an actuated bottom mirror. An angular alignment accuracy of < 0.06° is achieved through locking the two side mirrors using spring flexures and protrusion-notch structures. The quad CCR incorporates a gap-closing actuator to deflect a base mirror, allowing their reflectivity to be modulated up to 7 kb/s by a drive voltage less than 5 V. A 180-m free-space optical communication link using a CCR as the transmitter is demonstrated. CCRs have been integrated into miniature, autonomous “Smart Dust” nodes that constitute a distributed wireless sensor network. A signal-to-noise ratio analysis of CCR-based links is also presented, considering the impact of CCR dimensions, link distance, and other factors.
An SOI/SOI wafer bonding process is developed to fabricate scanning micromirrors using lateral actuation. The process is an extension of the SOItechnology and can be used to fabricate stacked high-aspect-ratio structures withwell-controlled thicknesses. The fabricated one-axis micromirror scans up to 21.8°optically under a DC actuation voltage of 75.0 V and has a resonant frequency of 3.6kHz. The fabricated two-axis micromirror scans up to 15.9° optically for the inner axis at 71.8 V and 13.2° for the outer axis at 71.2 V. The micromirror is observed to be quite durable and resistant to shocks.
Torsional beams with T-shaped cross sections are introduced to replace rectangular torsional beams in two-axis MEMS micromirrors in order to reduce the cross-coupling between the two axial rotations. The fabricated bi-directional two-axis micromirror works up to ±7° for the outer-axis and from -3° to 7° for the inner-axis under DC actuation.
Joseph M. Kahn
Alice M. Agogino
Publication date: 
November 30, 2004
Publication type: 
Ph.D. Dissertation
Zhou, L. (2004). Optical MEMS for Free-space Communication. United States: University of California, Berkeley.

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