Page 380 - Electrical Properties of Materials

362 Optoelectronics

(a) 50

I t

0
0 I i 90
(b) 50

I t3
I t2
I t
Fig. 13.23
(a) The I t versus I i relationship as I t4
determined by the construction in
Fig. 13.22. (b) The I t versus I i I t1
characteristic as it would be 0
measured. 0 I il I i I ih 90

the upper curve. In the reverse direction, when I i decreases, I t will suddenly
jump from I t3 to the lower curve at I t4 and will then follow the lower curve.
What is bistability good for? Quite obviously, just as in the case of the sim-
ilarly looking ferrite hysteresis loop, we can make memory elements out of
them. By adding for example a switching beam to a holding beam the device
can switch from a low output state to a high output state.
Summarizing, our nonlinear cavity has yielded components both for lo-
gical arithmetic and storage. The hope is that one day they will be parts of
all-optical computers. Their main advantage in the applications discussed in
the present section is speed. The physical mechanism causing the nonlinearity
is fast. Switching speeds of the order of 1 ps have been measured.

13.10 Optical switching

MEMS were mentioned in Section 9.26. They represent a new way of doing
things. Parts of the structures produced that way can actually move, so it is
possible, for example, to produce movable mirrors which can redirect a beam
of light. But that is exactly the thing we need for optical switching. We need
it badly. The present practice is rather cumbersome. It may be likened to the
plight of the traveller who wants to travel from Oxford to Cambridge in the
comfort of a railway carriage. He can certainly take a train from Oxford to
Paddington Station, London, but there he is forced to disembark. He must then
travel by tube to Liverpool Street Station from where he is allowed the luxury
of boarding another train. The journey by tube is a nuisance. Signals travelling
in optical fibres face the same problems. They can rarely reach their destination