A novel use of liquid-crystal television (LCTV) is described. It is shown that, if the phase nonuniformity of the LCTV is corrected by a liquid gate, then a simple computer-generated hologram can be written and coherently reconstructed

Diep, Joseph

Liu, Hua-Kuang

Mok, Fai

Psaltis, Demetri

English

Caltech Authors - Main

748 OPTICS LETTERS / Vol. 11, No. 11 / November 1986
Real-time computer-generated hologram by means of
liquid-crystal television spatial light modulator
Fai Mok
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125
Joseph Diep
Department of Optical Engineering, University of La Verne, La Verne, California 91750
Hua-Kuang Liu
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109
Demetri Psaltis
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125
Received March 17, 1986; accepted August 8, 1986
A novel use of liquid-crystal television (LCTV) is described. It is shown that, if the phase nonuniformity of the
LCTV is corrected by a liquid gate, then a simple computer-generated hologram can be written and coherently
reconstructed.
It is well known that spatial light modulators (SLM's)
are essential for real-time optical information process-
ing. For that reason, much research has been devoted
to them, and many types of SLM have been devel-
oped.1
Most of the existing SLM's are either forbiddingly
expensive or still in the research and development
stages. Recently an extremely inexpensive liquid-
crystal television (LCTV) was investigated in relation
to its use in optical data processing.2-10 One advan-
tage of the LCTV is that it can be addressed either
electronically or optically. However, because of the
thickness variations over the aperture of the screen of
the devices, its usefulness for coherent optical process-
ing is limited.
We have examined the thickness nonuniformities of
the LCTV screen in a Mach-Zehnder interferometer.
We have successfully corrected the nonuniformities by
submerging the screen in a liquid gate and have dem-
onstrated the feasibility of using this device to write
and reconstruct a computer-generated hologram.
The results are briefly reported below.
The operating principle of the LCTV has been de-
scribed. 2' 3 The device has a 5.4 cm by 4.4 cm screen
that consists of a two-dimensional mosaic of individ-
ually addressable liquid-crystal cells that contain a 90-
deg twisted nematic liquid crystal sandwiched be-
tween two glass plates. These plates have, respective-
ly, transparent horizontal-line and vertical-line elec-
trodes deposited on their inner surfaces facing the
liquid crystal. Two linear polarizer sheets (polariz-
ers) are glued to the outer surfaces of the glass plates.
The polarization axes of these polarizers are parallel.
When no electric field is applied, the plane of polariza-
tion for linearly polarized light is rotated through 90
deg by the twisted liquid-crystal molecules, and no
light is transmitted through the second polarizer.
However, with an electric field applied, the twist and
tilt of the liquid-crystal molecules are altered, result-
ing in transmission of a controllable and variable frac-
tion of light. The input signal to the LCTV can be
from a TV receiver, a computer, or a TV camera.
A Mach-Zehnder interferometer was set up to ana-
lyze the optical quality of the LCTV screen. A 6.4-
cm2 (1-in.2 ) region of the LCTV screen was illuminat-
ed by a collimated He-Ne laser beam. The results are
shown in Fig. 1(a). It can be seen that more than five
fringes are produced, indicating thickness nonunifor-
mities of the LCTV screen amounting to about 0.046
gm per pixel. It has been found that the phase non-
uniformity is due mainly to the poor quality of the
polarizer sheets. Because of the nonuniformity,
Fraunhofer diffraction patterns show fuzziness in the
various orders. 3
We investigated two ways of removing the phase
distortions. The first was to correct the phase varia-
tion by a phase-conjugation method. We used a 10-
mW He-Ne laser to produce a holographic filter of the
LCTV screen and addressed it with a phase-conjugat-
ed beam projected through a holographic plate. Un-
fortunately, the beam reconstructed through the
LCTV was too weak to be observed visually because of
the low diffraction efficiency of the hologram and the
low power of the laser. The effectiveness of the phase-
conjugation technique was, therefore, not conclusively
shown. However, Casasent and Xia have proved that
0146-9592/86/110748-03$2.00/0 © 1986, Optical Society of America
November 1986 / Vol. 11, No. 11 / OPTICS LETTERS 749
this is a viable way of correcting the phase-nonunifor-
mity problem.7
We then used a liquid-gate correction technique.
The LCTV screen was submerged in a liquid gate
(NRC Model 550) filled with an index-matching non-
conductive mineral oil with a refractive index of about
1.45. In this case, the optical path length through the
liquid gate was made nearly constant. Because it is
hard to match simultaneously the different indices of
the LCTV screen (glass plates plus polarizers), a small
amount of phase variation remained. Nevertheless,
we have found that most of the thickness variation of
the TV screen was corrected. Figure 1(b) shows an
interferogram of the liquid gate without the LCTV
screen. Note that much less than one fringe appears
across the illuminated region when the liquid gate
(filled with mineral oil) is placed in the path of the
Mach-Zehnder interferometer.
Figure 1(c) is an interferogram of the LCTV screen
submerged in the same liquid gate.
(a)
(b)
(b)
(c)
Fig. 1. Phase-nonuniformity correction of the LCTV in a
Mach-Zehnder interferometer (MZI) as shown by (a) an
interferogram with LCTV in a MZI, (b) an interferogram
with liquid-filled liquid gate in a MZI, and (c) an interfero-
gram with LCTV submerged in liquid gate in a MZI.
(a)
Fig. 2. Computer-generated hologram of a circle: (a) on a
conventional TV monitor, (b) on LCTV; (c) coherent-optical
reconstruction.
With the phase distortion of the LCTV mostly cor-
rected, several coherent-optical data-processing ex-
periments can be performed, including an optical
matched-filtering experiment.3 We thought it inter-
esting to try to write a computer-generated hologram
(a)
750 OPTICS LETTERS / Vol. 11, No. 11 / November 1986
on the LCTV and reconstruct it through coherent
light. A 100 X 100 pixel binary phase kinoform"l of a
circle was first generated using an IBM PC computer.
Figure 2(a) shows the kinoform displayed on a conven-
tional TV monitor. The kinoform was produced by
binary encoding of the phase, while the amplitude was
kept constant, at each pixel of the Fourier transform of
a circle. Basically, this kinoform consisted of on-axis,
phase-modulated patterns written into the liquid-
crystal layer through the electrodes controlled by the
computer output. When the collimated laser light
was transmitted through the liquid-crystal layer, the
phase of the light was modulated. The output analyz-
er-polarizer served the function of transforming the
phase variations into amplitude variations. However,
only the vertical resolution of the kinoform matched
that of the LCTV exactly. The LCTV interpolated
the missing horizontal points to form an image as
shown in Fig. 2(b).
The LCTV, submerged in the oil-filled NRC liquid
gate, was then placed in an optical Fourier-transform
system. The resulting holographic reconstruction of
the circle is shown in Fig. 2(c). As expected, the ellip-
tical appearance of the circle is due to the nonunity
aspect ratio of the high-resolution format of the IBM
PC. A zero-order dc spot has been blocked for clearer
image presentation. The high-intensity dc spot not
shown here is believed to be caused by the noncon-
stant rotations of polarization of the pixels of the same
logic level.
To show the necessity of using the liquid gate in the
present work, we also recorded a binary phase holo-
gram of the letter 0 on a piece of film and placed it in
front of the LCTV screen with the TV electronics
turned off. The reconstruction of this composite fil-
ter was fuzzy. After the LCTV was removed, the
reconstruction became well defined. Hence we may.
conclude that the unimproved LCTV can introduce
enough phase distortion to cause failure in reconstruc-
tion.
The results described in this Letter demonstrate the
usefulness of an inexpensive LCTV as a spatial light
modulator for coherent-optical processing in the writ-
ing and reconstruction of a single computer-generated
hologram. Some similar results were achieved by
Psaltis et at., who used a magneto-optics device for
reconstruction of a hologram."2 These findings
should make it much easier and less expensive for
many optics laboratories to conduct experiments on
coherent and incoherent optical processing.
We are currently investigating the use of a large-
memory correlator13 with the LCTV as a real-time
incoherent-to-coherent transducer and with its zero
order suppressed.
The research described in this Letter was carried
out by the Jet Propulsion Laboratory, California Insti-
tute of Technology, and was sponsored by the Physics
Division of the U.S. Army Research Office and the
National Aeronautics and Space Administration.
We would like to thank J. W. Goodman of Stanford
University and A. Johnston of Jet Propulsion Labora-
tory for helpful discussion on the use of liquid in the
liquid gate.
J. Diep is an academic part-time employee at Jet
Propulsion Laboratory, California Institute of Tech-
nology.
References
1. G. R. Knight, in Optical Information Processing, S. H.
Lee, ed., Vol. 48 of Topics in Applied Physics (Springer-
Verlag, Berlin, 1981).
2. J. A. McEwan, A. D. Fisher, P. B. Rolsma, and J. N. Lee,
J. Opt. Soc. Am. A 2(13), P8 (1985).
3. H. K. Liu, J. A. Davis, R. A. Lilly, Opt. Lett. 10, 635
(1985).
4. D. A. Gregory, Appl. Opt. 25, 467 (1986).
5. M. Young, Appl. Opt. 25, 1024 (1986).
6. A. M. Tai, Appl. Opt. 25, 1380 (1986).
7. D. Casasent and S. F. Xia, Opt. Lett. 11, 398 (1986).
8. H. K. Liu, S. Y. Kung, and J. Davis, Opt. Eng. 25, 853
(1986).
9. F. T. S. Yu, J. Jutamulia, and X. L. Huang, Appl. Opt.
25, 3324 (1986).
10. J. A. Davis, R. A. Lilly, K. D. Kreng, and H. K. Liu, Proc.
Soc. Photo-Opt. Instrum. Eng. 613, 245 (1986).
11. L. B. Lesem, P. Hirsch, and J. A. Jordan, Jr., IBM J. Res.
Dev. 13, 150 (1969).
12. D. Psaltis, E. G. Paek, and S. S. Venkatesh, Opt. Eng. 23,
698 (1984).
13. D. Gregory and H. K. Liu, Appl. Opt. 23,4560 (1984).


Real-time computer-generated hologram by means of liquid-crystal television spatial light modulator

A novel use of liquid-crystal television (LCTV) is described. It is shown that, if the phase nonuniformity of the LCTV is corrected by a liquid gate, then a simple computer-generated hologram can be written and coherently reconstructed. It is well known that spatial light modulators (SLM&apos;s) are essential for real-time optical information process-ing. For that reason, much research has been devoted to them, and many types of SLM have been devel-oped.1 Most of the existing SLM&apos;s are either forbiddingly expensive or still in the research and development stages. Recently an extremely inexpensive liquid-crystal television (LCTV) was investigated in relation to its use in optical data processing.2-10 One advan-tage of the LCTV is that it can be addressed either electronically or optically. However, because of th

Real-time computer-generated hologram by means of liquid-crystal television spatial light modulator

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

CiteSeerX