Page 641 - Introduction to Information Optics

11.2. Information Display Using Acousto-Optic Spatial Light Modulators 625
1.2.2. INTENSITY MODULATION OF LASER

2
Because it is common to refer to light by its intensity (i.e., 7 n = |EJ , where
/„ represents light intensity for the «th-order plane wave), we write the zeroth-
order light and the first-order light intensities, according to Eqs. (ll.lla,b), as

2
2
/ 0 = \E 0\ = 7 inccos (ac/2),
and

2
where / ine = |E inc| represents the intensity of the incident light. The value of 4;
is set to unity to represent the intensities at the exit of the Bragg cell. Figure
11.6 shows a plot of the solutions given by Eq. (11.12) and illustrates complete
energy transfer between the two coupled modes (located at a = n) for ideal
Bragg diffraction. Note that by changing the amplitude of the sound; i.e.,
through a, we can achieve intensity modulation of the diffracted beams.
In fact, one of the most popular applications for an acousto-optic Bragg cell
is its ability to modulate laser light. Figure 11.6 shows the zeroih- and
first-order diffraction curves plotted as a function of a, where P represents the
bias point necessary for linear operation. Figure 11.7 illustrates the relationship
between the modulating signal, m(t) and the intensity modulated output signal,
I(t). As shown in the figure, m(f) is biased along the linear portion of the first-
order diffraction curve.

•> a

Fig. 11.6. Complete energy transfer at a = n.