Page 253 - Introduction to Information Optics

238 4. Switching with Optics

viscosity polymers. After curing, the mold is removed, leaving behind a
microreplica of the original patterns. The main drawback of this technique is
the need for a synchrotron for collimated X-ray.

4,4.2. ELECTROSTATIC ACTUATORS

Microactuators are essential parts of MEMS optical switches. A wide
variety of actuation mechanisms have been researched in the MEMS field.
These include electrostatic, electromagnetic, piezoelectric, and thermomechani-
cal. While there is continuing research work going on in this field, it appears
that for optical switching, electrostatic actuation is an effective technique. At
microsize, it is easy to produce a high electric field for actuation. For example,
6
an electric field of 3 x 10 V/m can be generated by applying 3 V voltage across
a 1-micrometer gap. Using surface micromachining, the interface circuitry
in the silicon wafer can used to produce various patterns of control signals
for desired actuation of many elements in arrays. Other advantages of electro-
static actuation include simplicity in design, fast response, and low power
consumption.
A simple electrostatic actuator is a parallel-plate capacitor (Fig. 4.28).
Assuming the area of the plates is much greater than the separation x, its
capacitance is given by

- (4.46)
x

where e is the dielectric permitivity of the medium between the plates, and A

Area A
1

Fig. 4.28. A parallel-plate capacitor.