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Guiding Light

Cancer diagnosis is serious stuff and enormously important. But other uses for MEMS are lighter at heart. At the University of Michigan’s MEMS research center, Clark T. Nguyen envisions a Dick Tracy future based on MEMS resonators and frequency filters for cell phones and other communications gadgets. These are the components that ensure transmission takes place at specific frequencies and allow receivers to tune into particular frequencies plucked from the cacophony filling the airwaves. Unlike the pinkie-nail-sized quartz crystal resonators used in cell phones, whose oscillations are in the form of mechanical waves traveling back and forth along the solid crystal’s atomic framework, MEMS resonators have moving parts more akin to pendulums. Yet thousands could fit into the same space taken up by one of their crystal rivals.

As researchers like Nguyen miniaturize radio frequency components for cell phones, pagers, Global Positioning System receivers and other technologies, devices can become more selective in the frequencies they receive or transmit. Such selectivity means they require less power. “We are looking at a process that can take the whole cell phone and fit it onto a wristwatch, or a ring on your finger,” Nguyen says.

Others predict they are on the verge of using MEMS to transform the communication infrastructure based on optical fibers and other light-based technologies. Considering that the light-carrying core of optical fibers is about 9 micrometers in diameter, it’s no surprise that systems engineers at places like Lucent Technologies would look to diminutive devices to help them steer light through fiber networks. “We believe MEMS is going to really revolutionize how photonic switching gets done,” says David Bishop, head of microstructure physics research at Lucent’s Bell Laboratories in Murray Hill, N.J. Some of the first of these microswitches for light could show up in the communication network in the next year or so, says Bishop.

“For the moment, optical switches are heavy and expensive pieces of equipment,” explains Bishop. “Some involve just taking the end of a fiber, connecting it to a motor and you move it [from one fiber to another] that way.” Instead of moving the fibers, MEMS promises more elegant, cheap and reliable ways of optical switching using tiny mirrors and lenses-in other words, by guiding the light. “The nice thing about photons is they are small and don’t weigh much, so you can use a micromachine to move them around,” says Bishop.

In one MEMS design, Bishop and colleagues have fabricated a “thermally deformable micromirror” that can change its focal distance. Smaller than a poppy seed, it looks like a radar dish with eight individually tiltable triangular wedges (each made of gold-coated silicon). With this design, light coming from eight fibers can be precisely recombined (multiplexed) into downstream fibers positioned at several of the adjustable mirrors’ focal points.

Devices like these could help reduce the vulnerability of photonic networks to failure. If an optical fiber gets severed in the middle of the night, explains Bishop, you would like to be able to reconfigure the path of light in that network without having to send a truck out to fix the fiber. As Bishop sees it, pressing a button that sends a little jolt of light or electricity to the invisible wires of a poppy-seed-sized mirror might be all it will take to reroute the light and keep the lines open.

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