Page 217 - Introduction to Information Optics

202 4. Switching with Optics

Control

(a)
Control
Channel !

Channel 2
(b)

Fig. 4.1. Two configurations of optical switches, (a) On-ofF switch, (b) Routing switch.

below what is required in optical communication systems (50 Gbits/s). For
application in free-space switching fabrics, these switches are bulky and difficult
to integrate into a compact unit.
Recent technological developments have led to three very important types
of optical switching devices. The first one, ultrafast all-optical switches, is based
on nonlinear optical effects. It has many applications in digital optical signal
processing, and can have a switching speed over 100 Gbits/s. The second one
is fast electro-optic modulators using LiNbO 3 or semiconductor quantum
wells. The third is based on a microelectromechanical system (MEMS). In
contrast to conventional optomechanical systems that use bulky parts, these
switching elements are batch fabricated by micromachining techniques. They
are therefore smaller, lighter, faster, and cheaper than their bulk counterparts,
and can be monolithically integrated with other optical components. While the
speed of these switches is still slow, they can form large arrays in a compact
unit and be used in networks with massive parallel connections.

4.1. FIGURES OF MERITS FOR AN OPTICAL SWITCH

There are several basic parameters used for evaluating the performance of
an optical switch. These include on-off ratio, bandwidth or switching time,
insertion loss, power consumption, and cross talk between channels.
The on-off ratio (also called contrast ratio) is the ratio of the maximum
transmitted light intensity / max to the minimum transmitted light intensity / min ,
and is often described in decibels:

/? on - off = 101og(/ max// mj n). (4.1)

This ratio measures the quality of generated data by the switch and is related