I. Field of the Invention
This invention relates to optical switches and, more specifically, to switches in which an optical signal is switched under the control of actuating radiation at optical wavelengths.  For example, a signal having a wavelength in the range 500 nm to 2000 nm is switched by actuating radiation having a wavelength in the range 500 nm to 2000 nm. 
II.  Related Art and other Considerations
In telecommunications it is usually essential to distinguish between an optical "signal" (ie the telecommunications traffic which is usually at 1.3 .mu.m or 1.5 .mu.m) and control radiation.  This requirement can be achieved by using different wavelengths so that the signals can be separated, eg by suitable filters. 
In other applications, eg image processing and optical composition, it may not be possible to distinguish two categories of signal and only one wavelength will be used.  It is often convenient to regard the switch as a threshold detector and an EXCLUSIVE-OR-gate could be constructed in this way. 
The optical switches usually take the form of waveguiding structures having one or more path regions.  Integrated devices comprising several switches at close proximity are also possible.  Separate path regions may be needed for signals and actuating radiation.  The switching is achieved by changing the refractive index of critical regions located in either a path region and/or cladding regions, eg a critical region may be located between two path regions.  To effect the switching, each critical region is composed of operative material which is sensitive to the actuating radiation.  The architectures of a wide variety of optical switches have already been established but the chemical nature of the active ingredient is clearly important; for example, the active ingredient should display sufficiently large changes of refractive index for practical intensities of actuating radiation and the changes should occur quickly when the actuating radiation is switched on or off.  The selection of active ingredient is a problem.