Page 381 - Electrical Properties of Materials

Optical switching 363

without change of a rather brutal nature. Within picoseconds of their arrival
they are unceremoniously converted into electronic form, interrogated as to
their final destination, reconverted into optical form, and finally bundled into
the appropriate optical output fibre. Obviously, this conversion–reconversion
business is a nuisance. Actually, it is more than a nuisance. Electrons gener-
ate heat and as the density of elements increases, the point might have already
been reached where there is no easy way to keep the temperature rise to an ac-
ceptable limit. It is as if the carriages in the tube that take you from Paddington
to Liverpool Street Station are not only uncomfortable but unbearably hot as
well.
So let us see how such an interchange will be done in the future. The mov-
able mirror, whose construction was briefly discussed in Section 9.26, can of
course be constructed in two-dimensional arrays. A fine example of a 6 × 6ar-
ray, in which the angular position of each element can be controlled, is shown
in Fig. 13.24(a). Two such arrays may then be used in an optical cross-connect
switch, as shown in Figure 13.24(b). The inputs and outputs are two square
bundles of optical fibres equipped with collimators to produce small diameter
free-space beams. Each input beam is arranged to strike one of the MEMS mir-
rors in the first array. This mirror may turn to point at any mirror in the second
array, which may then turn to route the beam to its corresponding output fibre.
Switches of this type may be extremely large, with up to 1000 inputs and 1000
outputs.
What about speed? Is mechanical movement not bound to be slow? Yes,
it is, but it does not matter. It is a massively parallel operation. We can switch
simultaneously hundreds of beams. It is optics. The beams can cross each other

(a) Mirror element Local electronics

Sensing and control lines

(b) Fibre j
Mirror j
Mirror array 2
Fig. 13.24
Input fibre array Lens array Output fibre array
(a) A 6 × 6 mirror array of dual-axis
MEMS torsion mirror. (b) Optical
Mirror array 1 cross-connect constructed from two
Mirror i mirror arrays. After R.R.A. Syms,
Fibre i Lens array J. Lightwave Technol. 20, 1084, 2002.