Optical Switch Offers 1.4-dB Loss

Gary Boas

Optical telecom networks have relied on electronic switching, but all-optical switching presents an attractive alternative because it is less expensive, takes up less space and consumes less power. Microelectromechanical systems (MEMS) have been suggested as a means to achieve large-scale photonic cross connects, and researchers at Calient Networks Inc. in Goleta, Calif., have reported a 288 x 288-channel switch using this approach.

A 288 x 288-channel optical cross connect based on micromirrors features a median loss of 1.4 dB.

The 3-D MEMS switch's design is relatively simple. An optical signal enters via an input fiber and is directed to one of the MEMS mirrors through a lens array. The mirror reflects the signal to another mirror, which reflects it again to the output lens array and the proper output fiber. Because MEMS mirrors rotate on two axes, they can move the signal in three dimensions.

Insertion loss was a focus in the design of the switches. Photonic switches do not enable regeneration in each fiber span, so maintaining a low, stable and uniform insertion loss is vital, especially for networks in which the signal encounters multiple switching nodes and fiber spans. The researchers incorporated a theoretical loss model into the design, and the completed cross connect displays a median measured loss of 1.4 dB, easily within the target of <3 dB.

They also designed the system for use within a broad range of signal wavelengths -- 1260 to 1625 nm -- carefully positioning the fibers to avoid bending loss at longer wavelengths. As a result, the wavelength-dependent loss of a typical light path was just 0.4 dB.

Chandrasekhar Pusarla, a Calient researcher on the project, believes that all-optical cross connects will likely replace the electronic switches currently used in many optical networks, especially if the telecom market fails to see a reversal of fortune in the near future. Besides their smaller footprints and power economy, he said, the switches enable network providers to eliminate lasers, receivers, transponders, multiplexers and redundant electronic switch ports at transit nodes, resulting in lower capital and operating costs. Moreover, because they are bit-rate, protocol- and wavelength-independent, such switches can support any combination of transport formats and 1-, 10- and 40-Gb/s signals.

Calient introduced a 256 x 256 matrix switch in October 2001 and a 128 x 128 switch in June.