Page 642 - Introduction to Information Optics

626 11. Information Display with Optics

Intensity modulated output, l(t)

Fig. 11.7. Intensity modulation system.

It should be clear that intensity modulation can also be achieved by using
the zeroth-diffracted order. However, note that the two diffracted orders
process information in the linear regions with opposite slopes. This suggests
that any demodulated electrical signal received via the first diffracted order will
be 180° out of phase with the electrical signal received via the zeroth-diffracted
order. This has been demonstrated experimentally [15].
For intensity modulation using acousto-optics devices, it is instructive to
estimate the modulation bandwidth. The modulation bandwidth, J5, can be
defined as the difference between the highest and lowest sound frequencies at
which the normalized diffracted intensity drops by 50%. To estimate this, we
2
2
assume a is small such that (<5()/4) »(a/2) ; i.e., under the case of weak
inteaction. The normalized intensity is, therefore, given by

(11.13)
/ \2