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  • Double Duty for MEMS-Based Optical Switch

Photonics Spectra
Oct 2005
Breck Hitz

Optical switches based on microelectromechanical systems (MEMS) are at the heart of next-generation “transparent” optical telecommunications networks. Nearly all of today’s networks are “opaque” in the sense that the optical signal is periodically converted to an electrical one for amplification and switching. A transparent system is one in which the optical signal propagates from one end to the other, and the switching is done optically.

Double Duty for MEMS-Based Optical Switch
Figure 1. In a microelectromechanical systems (MEMS) optical switch, incoming light (red arrows) reflects from individually programmable MEMS mirrors so that any fiber on the left can be optically connected to any fiber on the right. Courtesy of Calient Networks Inc.

A MEMS-based switch connects the signal on an input fiber to any of several output fibers with two planar arrays of tiny, adjustable mirrors (Figure 1). Because the individual electrostatically activated mirrors have so little momentum, switching speeds are on the order of 10 ms.
The optical power in different channels tends to become unbalanced as the signals propagate over long distances, however, so engineers add variable optical attenuators to the network to restore the balance.

Recently, scientists at Calient Networks Inc., a MEMS switch manufacturer in Goleta, Calif., suggested that detuning a MEMS switch can introduce the required variable attenuation and eliminate the need for separate variable attenuators.

Double Duty for MEMS-Based Optical Switch
Figure 2. The calculated attenuation contours for mirror misalignments in the X and Y directions indicate that attenuation sufficient to compensate for any expected channel imbalance can be introduced by a MEMS-based switch. The example shown is for an optical path in the switch with a large angular displacement. ©2005 IEEE.

The researchers calculated that as much as 20 dB of loss — sufficient to compensate for any expected channel imbalance — could be added to a channel by detuning a mirror (Figure 2). Important parameters in a real optical network are the stability of the attenuation introduced and the crosstalk between channels introduced by misaligning the mirrors.

The scientists built an experimental transmission system to evaluate these parameters. They found that crosstalk could be maintained at less than −50 dB and that the mirror misalignment did not introduce significant instability to the system.

In separate bit-error-rate measurements, they determined that, rather than misaligning one mirror as shown in Figure 2, it was preferable to introduce a small misalignment to both the input and output mirrors. This approach reduced the switch’s sensitivity to vibration.

microelectromechanical systems (mems)
Refers to micron-size complex machines that have physical dimensions suitable for the fabrication of optical switches for use in state-of-the-art communications networks.
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