DETAILED DESCRIPTION The basic principles of the invention will be described in accordance with the embodiment illustrated in FIGS.
1-3.
FIG.
1 illustrates schematically the basic components of the switch.
A semiconductor substrate, 10, in this example silicon, includes in one major surface at two adjacent edges, a first and second set of grooves.
Grooves in the first set are shown as 11-14 and those of the second set as 15-18.
It will be appreciated that not all the grooves are illustrated for purposes of clarity in the Figure.
Each groove is etched, in accordance with known techniques, so that an optical fiber may be mounted and aligned therein.
The first set of grooves, 11-14, receives optical fibers, 20-23, respectively, and the second set of grooves, 15-18, receives fibers, 24-27, respectively.
In this particular example, the grooves are formed to align graded-index-of-refraction lenses, such as 28, attached to the ends of the fibers in order to collimate the light to and from the fibers.
Alternatively, the grooves may be formed to align only fibers without lenses.
Fibers 20-23, in this example each carry an input signal, and fibers 24-27 are adapted to receive the input signals under certain conditions to provide the switched output.
At the edge of the substrate opposite to the input fibers, 20-23, is an array of photodetectors illustrated schematically as region 30.
The array comprises a linear array of standard photodetectors, four of which are illustrated schematically as circles, 31-34.
Each photodetector is aligned with one of the input fibers to receive a signal therefrom.
The photodetectors could be integrated into the semiconductor by known techniques or could be discrete elements aligned in grooves in the semiconductor.
Each photodetector is electrically coupled to some addressing logic circuitry, illustrated as 35, which electrically addresses the elements of the switch.
(For the sake of clarity, this latter connection is shown schematically by four wires to the substrate
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Fault Detection 
