An important new application for MEMS devices is in fiber optic networks. At the micron level, MEMS-based switches route light from one fiber to another. Such an approach enables a truly photonic (completely light-based) network of voice and data traffic, since switching no longer requires conversion of light signals into digital electronic signals and then back to optical.
MEMS: not just for switching
This is important because switching using optical-electrical-optical (OEO) conversion can often cause substantial bottlenecks, preventing the realization of truly broadband networks. But MEMS and micromachined devices can be used as more than switches in the optical network. Additional applications include active sources, tunable filters, variable optical attenuators, and gain equalization and dispersion compensation devices.
The result is an end-to-end photonic network which is more reliable and cost-effective, and which has minimal performance drop-off. However the development of an all-optical network has been complex and challenging due to the integration of optics, mechanics and electronics.
Micromirrors: a key to photonic communications
While many of the key applications for MEMS devices require them to perform like valves or pumps, the functionality developed for optical networks typically revolves around MEMS-based micromirrors. Micromirrors are the fundamental micro-mechanical component for optical crossconnect switches that switch light frequencies from one set of fibers to another. This process includes an input/output port, an actuator, and a mirrored surface. When voltage is applied to the actuator, it causes the mirror to move and direct the light to a specific output port. The mirror then remains static until the light path needs to be redirected.
MEMS switches include both mechanical and microfluidic. Mechanical switches, which currently seem to offer the most reliable and flexible approach, are based on an array of micromachined mirrors that range in quantity from hundreds to thousands on a single chip.
These mirror-based switches are also classified as either two-dimensional, where they move up and down or left and right; or three-dimensional, where they can swivel in a broad range of movement.
Ensuring reliability
MEMS-based switches must be extremely reliable to meet the standards and requirements of optical telecommunications networks – they must remain in precise position over millions of operations, and they must be designed to meet stringent environmental specifications involving temperature and vibration. However, there is a high degree of confidence that mechanical MEMS devices can meet these requirements, as similar devices based on the same manufacturing processes have proven to be exceedingly robust in the automotive, military and aerospace industries.
A typical example of an optical network application is add-drop multiplexing, especially in metropolitan area networks. Though non-MEMS-based devices may range from 32 or 64 ports to 1000 ports, a number of companies are looking at lower port, scalable solutions which offer immediate manufacturability with high yields, robustness and cost-effective batch process technology. Both the metropolitan and access areas of the network have high volume requirements and may be the best opportunity to capitalize on the optical network while proving the value of MEMS.
