Tiny Mirrors Could Reflect Optical Future

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The Holy Grail of pure optical switching may lie in a technology called Micro Electro-Mechanical Systems, or MEMS, which a growing number of optical-networking companies have started to adopt.

The technology is centered on an optical-network switch application that uses tiny mirrors to route beams of light from fiber to fiber in a network cross-connect or node. Theoretically, the all-optical switch eliminates the need to convert optical signals to electric ones, then back to optical. That process occurs in traditional switches.

Some industry observers still see MEMS technology as "far out there." But a growing contingent of technology companies is chasing down potential applications for the technology that could apply to cable systems.

Both Nortel Networks and Lucent Technologies have introduced MEMS-based optical switches. Nortel's Xros division has created an optical cross-connect consisting of two facing 6-inch-by-6-inch arrays of 1,152 mirrors each.

Incoming beams of light are directed to their destinations by these movable mirrors. An incoming fiber-optic channel can be switched as a whole to any output port.

Lucent's WaveStar Lamda-Router, designed by Bell Labs, initially routes up to 256 individual signals and scales to switch 1,024 signals. In April, Lucent announced that Global Crossing Ltd. would employ the all-optical switch.

ADC HOPS ON

Also jumping on the MEMS bandwagon was ADC Telecommunications Inc., which in October announced a strategic co-development and production agreement with MEMSCAP, a provider of MEMS-based communications components. Armed with MEMS technology, ADC plans to develop low-port-count optical switches, variable optical attenuators and tunable optical filters.

The low-count optical switch will incorporate a 2-by-2 or 1-by-2 input/output configuration. It will serve phone companies' immediate need to more efficiently activate redundant synchronous optical network (SONET) fiber rings, when necessary, explained Marty Nyman, director of ADC's advanced photonics integration center.

The MEMS-based switches could also be used to reduce specific multiplexed wavelengths at the access portion of large fiber networks, allowing voice or data traffic to move on or off fiber at the wavelength level.

ADC may develop other variants, such as 1-by-4 or 1-by-8 input/output configurations for use in switches or optical cross-connects.

Though optical-switching MEMS applications have garnered a good deal of the attention, the technology is also being developed for other optical network elements and functions, including variable attenuators, tunable lasers, tunable filters and dispersion compensation, according to a report by Deutsche Banc Alex Brown.

This trend is reflected in ADC's development of MEMS-based variable optical attenuators and tunable optical filters.

Nyman insisted that variable optical attenuators are necessary in fiber networks that use dense-wave-division multiplexing. Different wavelengths, he said, respond differently to amplification, creating the need to reduce or increase the amplification level of wavelengths as they enter optical receivers.

If a signal enters the receiver with an amplification that's too high, it could overdrive the receiver.

"MEMS is a very compact way to activate a lightpath," said Nyman. It provides "a reliable and fairly stable activation medium."

A MEMS attenuator device includes a shutter placed into the path of a signal as it moves from fiber to fiber. By varying the amount of light that seeps in, the MEMS actuator can be controlled to reduce or increase the amount of energy that passes through.

The development of MEMS-based tunable optical filters is driven by the need to screen out specific wavelengths of light at nodes and cross-connects, said Nyman. A tunable filter selectively determines which wavelengths come out of that filter. As networks evolve and turn more toward DWDM on fibers carrying high channel counts, the need for filtering out specific wavelengths becomes greater.

"What the network operator is eventually trying to do is offer services at the wavelength level," said Nyman.


DEMAND BANDWIDTH

Large-bandwidth customers, such as Amazon.com, may want to reserve large chunks of bandwidth for special events when they expect server demand to be very high, such as a greatly anticipated book release.

Using the type of pure optical switching that MEMS facilitates, dedicated wavelengths can be quickly provisioned and reserved with the help of "smart" software. After the demand passes, they can be reallocated.

Some entrepreneurs have talked about selling bandwidth as a commodity, perhaps through a futures market in which bidders would vie for specific wavelengths. "But you need a smart network to do that," said Nyman.

At this stage in MEMS' optical development, specific applications of the technology have not been developed for cable TV hybrid fiber-coax networks. Will that change?

"I have to believe as the cable market starts to deploy more wavelengths, the answer is yes," said Nyman.

A leading MSO executive, speaking on background, said he is not evaluating MEMS technology at this point, but noted that "it's really kind of fun technology."

A Time Warner Cable spokesman said his company is not involved with MEMS technology. Sister telco Time Warner Telecom said the company is considering MEMS optical technology and expects to evaluate it in first-quarter 2001.

But the benefits of pure optical switching are not lost on the industry's engineers.

"There's a lot of demand for being able to rapidly provision optical services," said

Excite@Home Corp. chief technology officer Milo Medin. With today's gear, that provisioning is usually accomplished manually, with a technician making fiber connections on a patch panel.

Though he's not a big believer in the bandwidth-on-demand model within the context of Internet-protocol networks, Medin said: "Wavelength sales are going to be a big factor in 2001. It's not about bandwidth-on-demand. It's about rapid provisioning of services."

Keeping a signal in the optical domain-and bypassing its electrical conversion at a switch-could save a significant amount of money, Medin added.

Harmonic Inc. director of market development Chris Bonang said MEMS is on the company's radar screen and seems most applicable for tunable filters and tunable lasers. Still, such products using MEMS "are pretty far out there at this point," he said.

Though MEMS will surely play a big role in the development of pure optical switching, "MEMS alone is not the only answer to make optical switching functional," said Naimish Patel, director of networks for Sycamore Networks Inc.

Instead, Patel said the software that resides on top of MEMS is key to making the remote provisioning of circuits and other switching functions a reality.

Sycamore, which is partnering with MEMS component makers OMM Inc. and Corning to develop three-dimensional MEMS technology, has not yet announced products that incorporate MEMS, but "we're definitely interested in MEMS technology," Patel said.

Pure optical switches have been the most economical to deploy in the core of networks that carry terabits of data from one point to another. But Patel said that all-optical switching would eventually hit the network's edge and be present in the metropolitan network environment.

As Internet traffic continues to skyrocket, the quest for an all-optical switching fabric will mean that such companies as Sycamore will continue to invest in MEMS technology.

Just when-and if-MEMS makes its way to cable engineers' agendas remains to be seen.

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