Page 259 - Introduction to Information Optics

244 4. Switching with Optics

The size of the bridges is on the order of 80 /mi x 15 /mi. Because the inertia
of the microbridges is small and they only need to move a small distance,
switching time from the nondiffraction state to the diffraction state is on the
order of 20 nanoseconds, and required driving voltage is 3.5 V. With this fast
switching time and the latching property of the microbridges, this type of
switching device has many potential applications. Optical modulators, light
valves, and projection displays have been demonstrated using this technology.

4.4.3.2. Micromirror Arrays
Micromirror arrays have been used for image processing and display
applications. Several examples of micromirror-based crossbar switches have
already been demonstrated. The main benefits of these switching arrays include
large numbers of optical channels, low signal attenuation, and a very compact
volume integrated on a chip. With proper design, the integrated switch system
may also be capable of detecting and identifying the data content of incoming
optical channels and reconfiguring the switching pattern accordingly.
The backbone of crossbar switches is a two-dimensional N x N array of
micromirrors. Micromirrors are ultrasmall movable structures fabricated by
using micromachining. They are normally electrostatically actuated. The most
commonly used ones are torsion mirrors (Fig. 4.31). Each micromirror is
attached on a torsion hinge which allows rotational movement of the mirror.
The rotation is controlled individually to assume either a reflective or non-
reflective position. Therefore, the switch array can perform any arbitrary
switching between the N incoming channels and JV output channels.
An interesting and important device in micromirror arrays is the digital
mirror device (DMD) which has been developed by Texas Instruments [41].
The DMD is a micromechanical spatial light modulator array which consists
of a matrix of tiny torsion mirrors (16-/mi base) supported above silicon
addressing circuitry by small hinges attached to support posts. Each mirror can
be made to rotate 10° about its axis by applying a potential difference between
the mirror and the addressing electrode. The DMD is now commercialized for
high-luminance TV projection. Arrays of up to 2048 x 1152 with full address-
ing circuitry are available. The response time of each mirror is 10 /is, and the
addressing voltage is 5 V. In the wake of developments by Texas Instruments,
several research groups are now working on DMD -like devices for switching
purposes.
For switches in cross-connection of optical fiber arrays and optical routing,
micromirrors with large rotation angles (45-90°) are required. With larger
rotation angle, the switching time and driving voltages become larger. One
example is the 45° vertical torsion mirror for 2 x 2 switching array (Fig. 4.34)
[42]. Currently, the required driving voltage is 80 V and the switching time is
80 /mi. Better versions of optical switches arrays with higher density, lower