21.1 INTRODUCTION

All-optical switching is a process by which light, usually in the form of digital communications signals, is routed from one transmission channel to another, or modulated,without intermediate conversion to another format. Previous options in this process have included the following sequence of events: detection of an optical signal, an electronic routing decision, electronic triggering of a laser in the appropriate output channel, and generation of a new optical pulse. In the all-optical switching format, the signal remains in the optics domain and its properties are altered so that it is routed to a specific output.

The term all-optical switchinghas now taken on two separate meanings. In the first, the switching operation is controlled electrically for example, via the electro-optic effect and the induced phase changes are used interferometrically in a 2 2 switch. Switching between more than two channels is implemented by ganging together multiple 2 2 switches.

The second use for this term is when the operation is controlled optically. That is, an optical beam (rather than an electrical signal) changes the optical properties of the medium and leads to interferometric control of the output. This approach typically utilizes an intensity ( I)-controlled change in the refractive index ? n( I). In the simplest case, ? n = n ₂ I, where n ₂ is the Kerr coefficient.1

Both...