Electronic Design

Thermal Actuator Ruggedizes Fast Optical MEMS Router

The potential of optical microelectromechanical systems (MEMS) in the context of optical switching was recently underscored by the release of Lucent's WaveStar LambdaRouter. According to the company, this device is capable of directing 10 times today's Internet traffic in just one second. Such potential also has been the driving force behind efforts from companies like Cronos Integrated Microsystems Inc., Research Triangle Park, N.C. This particular firm used a thermal actuator to realize an extremely rugged optical-MEMS device that overcomes the questionable reliability issues surrounding other MEMS implementations.

Lucent's router uses a series of microscopic mirrors to instantly direct and route optical signals from fiber to fiber at 16 times the speed of electrical switches. It leverages off research from Bell Laboratories, Murray Hill, N.J. (see "Optical MEMS Answer High-Speed Networking Requirements," Electronic Design, April 5, 1999, p. 85. Or point your browser to www.elecdesign.com, keyword search "Optical MEMS.")

The LambdaRouter is but one approach, however. Cronos' sliding mirror device, while not as scalable, does have the potential for a much greater degree of robustness. Its theory of operation is fairly straightforward, too. The actuating mechanism is thermal—the same thermal actuator that drives the company's MEMS relay. When heated, the beam expands and moves in the plane of the chip. In the figure, it would move from the top left to the bottom right.

Jesko von Windheim, vice president of marketing at Cronos, suggests imagining a credit card. "Hold it between your fingers and squeeze it. The card will curve or bend in a similar fashion \[to the beam\]. Now, we attach a mirror to the rib that's attached to the beam, and put a mirror on the beam so that the mirror gets moved back and forth." When driven all the way through, hooks on either side of the beam form a mechanical latch to hold the beam in place. The latches can be subsequently opened to allow the beam to spring back.

The relatively simple device targets optical switching and attenuation applications, but it has limited scalability potential. According to von Windheim, a 4-by-4 array would be its limit. Beyond that, each mirror would take up too much space. "Theoretically you could do it, but there are more economical alternatives," von Windheim noted. As the result of proprietary processing techniques, the "perfect" mirror used in the device is key to its operation.

The sliding-mirror device features an operating voltage and power consumption of 5 V and 500 mW (per mirror) respectively, and a switching time of under 50 ms. As for latching, with power removed, the mirrors remain in their most recent state. Mirror dimensions are in the realm of 200 by 200 µm, with 1-mm spacing (see the figure). Cronos is soliciting interest from select customers. General sampling is planned for early in the first quarter of this year.

For more information, go to www.memsrus.com. Or contact Jesko von Windheim at (919) 248-4132.

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