Page 176 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 155

Trench isolation Silicon Input 2

Electrostatic Micromirror
comb actuator
Input 1 Output 1

Output 2
Unactuated
(cross state)

Slots for optic fibers

Optic fiber Silicon
oxide

Actuated
(bar state)
Input 2

Input 1 Output 1

Output 2
Figure 5.14 Illustration of a 2 × 2 binary reflective optical switch fabricated using SOI wafers and
DRIE. An electrostatic comb actuator controls the position of a micromirror. In the cross state, light
from an input fiber is deflected by 90º. In the bar state, the light from that fiber travels
unobstructed through the switch. Side schematics illustrate the signal path for each state.

middle of the intersection and reflects the light by 90º, thereby altering the path of
data communication—this is the cross state. Applying approximately 70V to the
actuator combs causes the mirror to retract, letting the light pass through unob-
structed—this is the bar state. Arraying the 2 × 2 switch element in both directions
creates a generalized M × N switch matrix.
The slots must normally accommodate optical fibers, typically 150 to 250 µm
in diameter. The depth of the slot must be such that the center of the fiber aligns
with the center of the micromirror. Because light diverges upon leaving the fiber,
the mirror must be significantly larger than the fiber core itself (the core is a central
area that carries light, typically about 10 µm in diameter for single-mode fibers
[33]). In the demonstration from the University of Neuchâtel, the mirror height is
identical to the depth of the groove, approximately 75 µm. Insertion loss, a meas-
ure of the light-coupling efficiency between input and output fibers—it is the ratio
of intensities at the output to the input—greatly depends on the alignment accuracy