Page 376 - Electrical Properties of Materials

358 Optoelectronics

beam, and it is affected everywhere in the same way by the modulation. Spatial
light modulators do the same thing, but different parts of the beam are differ-
ently affected. A simple definition would be that a spatial light modulator is
a device which gives a desired light distribution over a certain area. Thus, in
principle, all programmable display devices may be regarded as spatial light
modulators, including possibly a display at a railway station which announces
the departure of trains. Other examples are a cathode-ray tube in a television
set or a liquid crystal display in a calculator.
I shall discuss here the operation of only one of the modern spatial light
modulators, which may also be called an incoherent-to-coherent light con-
verter. Such a device is needed because coherent light is usually more suitable
for further processing than incoherent light.
A schematic diagram of such a converter is shown in Fig. 13.18. In the
absence of input incoherent light from the left (the writing beam) the pho-
toconductor does not conduct, and consequently there is a high voltage drop
across the photoconductor and a low voltage across the liquid crystal (in prac-
tice the liquid crystal layer is much thinner than the photoconductor). The role
of the liquid crystal is to transmit or to absorb the coherent light (reading light)
coming from the right, depending on whether there is a voltage across it or
not. Thus, the intensity modulation of the writing beam is converted into the
intensity modulation of the reading beam. The optical isolator is usually in the
form of a wide-band dielectric mirror, which separates the writing and reading
beams from each other.

Optical isolator

Photoconductor Liquid crystal

Writing Reading
beam beam

~
Glass Glass
Fig. 13.18
An incoherent-to-coherent light Transparent
converter. electrodes