Page 219 - Introduction to Information Optics

204 4. Switching with Optics

nllnll . Optical * Output 1

switch n n .
r
Input Output 2
(a)

Control

On , n n fc w Output 1
nnnn „ Optical n n .
switch
* Output 2
Input
(b)
Fig. 4.2. All-optical switches, (a) Self-switching, (b) Controlled switching.

In the last two decades, nonlinear optical effects have been used to switch and
route optical signals or perform logic operations on them in a number of ways.
Switches with various materials and configurations have been proposed and
demonstrated. This section discusses the principles of several all-optical switch-
ing devices, their material requirements, and their applications.

4.2.1. OPTICAL NONLINEARITY

The key element in all-optical switches is a medium with significant
nonlinear optical effect [4]. The main materials property is an intensity-
dependent refractive index n(I), where / is the total intensity of the optical field
in the medium. The nonlinear optical effect used in all-optical switching is the
third-order effect, or the Kerr effect, where

n = n n 7I. (4.5)

Here n Q is the linear refractive index and n 2 is the nonlinear refractive
coefficient. For many optical materials, n 2 is very small. For example, silica
2
20
glass has n 2 = 3 x 10~ m /W. Therefore, we cannot directly observe any
change in refractive index at low light intensity.
Nonlinear optical effect can be observed much more easily when we study
the phase shift induced by the nonlinear refractive index. The phase shift of a