Page 644 - Introduction to Information Optics

628 11. Information Display with Optics

by ignoring the factor 1.8 for simplicity. T = D/v s is the transit time for the
sound wave to cross the light beam and the modulation bandwidth for the
acousto-optic modulator is, therefore, the inverse of the sound propagation
time through the light beam. In practice, the bandwidth can be increased by
focusing the light inside the sound column. By special beam-steering tech-
niques, it is possible to further increase the allowable bandwidth. The interested
reader is referred to the existing literature [11].

11.2.3. DEFLECTION OF LASER

In contrast to intensity modulation, where the amplitude of the modulating
signal is varied, the frequency of the modulating signal is changed for
applications in laser deflection. We, therefore, have the so-called acousto-optic
deflector (AOD), as shown in Fig. 11.9, where the acousto-optic device
operates in the Bragg regime.
In Fig. 11.9, the angle between the first-order beam and the zeroth-order
beam is defined as the deflection angle 4> d. We can express the change in the
deflection angle A0 d upon a change AQ of the sound frequency as

(11.16)
2n v.

Using a He-Ne laser (/, ~ 0.6 /mi), a change of sound frequency of 20 MHz

Q+AQ

Fig. 11.9. Acousto-optic deflector.