Page 634 - Introduction to Information Optics

618 11. Information Display with Optics

modern holographic techniques called optical scanning holography and syn-
thetic aperture holography. In Sec. 11.4, we discuss information display using
electro-optic modulators. We provide background for the electro-optic effect
and discuss two types of electro-optic SLMs: electrically addressed SLMs and
optically addressed SLMs. The use of these types of SLMs for 3-D display will
also be discussed. Finally, in Sec. 11.5, we make some concluding remarks.

11.2. INFORMATION DISPLAY USING ACOUSTO-OPTIC
SPATIAL LIGHT MODULATORS

Acousto-optics deals with the interaction between light and sound. It can
result in light beam deflection, amplitude modulation, phase modulation,
frequency shifting, and spectrum analysis [1]. Devices and systems based on
acousto-optic interaction have played and continue to play a major role in
various types of optical information processing [2, 3]. In later sections, we
cover acousto-optic interactions using a plane-wave scattering model and
discuss how intensity and frequency modulation of a laser beam can be
accomplished by acousto-optic interaction. Finally, we present laser television
displays using these modulation effects.

11.2.1. THE ACOUSTO-OPTIC EFFECT

An acousto-optic modulator (AOM) or Bragg cell is a spatial light modu-
lator that consists of an acoustic medium, such as glass, to which a piezoelectric
transducer is bonded. When an electrical signal is applied to the transducer, a
sound wave propagates through the acoustic medium, causing perturbations in
the index of refraction proportional to the electrical excitation, which in turn
modulates the laser beam traversing the acoustic medium.
There are a variety of ways to explain acousto-optic interaction [1-11]. An
instinctive approach considers the interaction of sound and light as a collision
of photons and phonons [7]. Basically, the conservation of energy and
momentum laws are applied to the process of collision. If we denote the wave
vectors of the incident plane wave of light, scattered or diffracted plane wave
of light, and sound plane wave by k 0, /c + 1, and K, respectively, the conserva-
tion of momentum may be written as

tik + l=tik 0 + hR, (11.1)

where h = h/2n and h denotes Planck's constant. From Fig. ll.l(a) and the
division of Eq. (11.1) by h, it is apparent that the condition for wave matching