Crystals of lithium niobate were passively exposed to the space environment of the Long Duration Exposure Facility (LDEF). Three of the four crystals contained volume holograms. Although the crystals suffered the surface damage characteristic of that suffered by other components on the Georgia Tech tray, the crystals remained suitable for the formation of volume holograms

Callen, W. Russell

Gaylord, Thomas K.

English

NASA Technical Reports Server

N93-29697
HOLOGRAPHIC DATA STORAGE CRYSTALS FOR THE LDEF*
W. Russell Callen
School of Electrical Engineering
Georgia Institute of Technology
Atlanta, Georgia 30332-0250
Phone: 404/894-2912, FAX: 404/853-9171
Thomas K. Gaylord
School of Electrical Engineering
Georgia Institute of Technology
Atlanta, Georgia 30332-0250
Phone: 404/894-2931, FAX: 404/853-9171
SUMMARY
Crystals of lithium niobate were passively exposed to the space environment of
LDEF. Three of the four crystals contained volume holograms. Although the crystals
suffered the surface damage characteristic of that suffered by other components on the
Georgia Tech tray, the crystals remained suitable for the formation of volume
holograms.
INTRODUCTION
Lithium niobate is a significant electro-optic material, with potential
applications in ultra high capacity data storage and processing systems. Lithium
niobate is the material of choice for many integrated optical devices and holographic
mass memory systems. The objective of the experiment is to test the spaceworthiness
of electro-optic crystals for use in ultrahigh capacity space data storage and
retrieval systems.
VOLUME HOLOGRAPHIC STORAGE
Volume holographic storage offers a unique capability for ultrahigh capacity data
storage and processing systems. In addition to the potential storage of up to 10"
bits in a single crystal, holographic storage is insensitive to point damage of the me-
dium. Holographic storage is particularly suitable for the processing of data in page-
oriented form and can be used either in a read-write-erase mode or in an archival
*This work was performed under NASA Contract No. NAS1-15370.
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storage mode. By using electro-optic beam deflectors, mechanical motion can be
eliminated. The principal advantages of volume holographic storage are listed in
Table I.
To record the volume holograms in the lithium niobate, two plane waves, produced
by the same laser, are interfered within the crystal, as shown in Fig. 1. The
interference maxima and minima produce a corresponding refractive index variation in
the photorefractive crystal, which produces the volume hologram. By passing one of
these beams, the "object" beam through a data page mask prior to incidence on the
crystal, digital data in page oriented format may be stored as a volume hologram. The
data page may be displayed by subsequent illumination with the reference beam alone.
This writing and reading process is shown in Fig. 2. By rotation of the crystal with
respect to the laser beam, multiple pages of data can be Stored. Figure 3 illustrates
the data storage in a page-oriented optical phase holographic memory.
OPTICAL SYSTEM
Much of our effort centered on developing a precise system for writing and
evaluating the holograms. Systems were developed for hologram formation with both
helium-neon (_ = 632.8 nm) and argon (_ = 514.5 nm) lasers. The apparatus for the
holography study with helium-neon lasers is shown in Fig. 4. This apparatus achieves _
page rotation by rotating the reference beam angle with respect to a Stationary object
beam and stationary recording crystals. The reference beam angle is controlled by a
stepper motor that drives a rotating mirror. The angular rotation resulting from a :
single step is 19.6 mirroradians. [I]
LDEF EXPERIMENT
For the LDEF experiment, holograms were recorded in 10 mm x 10 mm x 2 mm samples
of iron-doped lithium niobate, with the optic axis lying in the plane of the surface;
The crystals were specified to be iron doped to .005 mole percent iron in the melt.__
The samples were flown as part of the overall Georgia Tech experiment, LDEF experiment
S0050, "Investigation of the Effects Of Long-Duration Exposure on Active Optical
Components," with principal investigator M.D. Blue. Our experiment consisted of the
materials shown in Table 2.
The diffraction_efficiency (diffracted power divided by incident power) as a
function of read beam angle for a typical plane wave hologram is shown in Fig. 5.
This diffraction efficiency could be used to monitor the degradation of the hologram
with _me. ' ,__ _
RESULTS
To date, no holograms have been observed remaining in any of the samples because
of the long exposure time involved. Although the crystals were recovered intact, they
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suffered the same surface damage characteristic of that of other optical components on
the Georgia Tech tray. A very significant result is that the crystals still retained
their photosensitivity. A recently recorded hologram from one of the LDEF crystals is
shown in Fig. 6.
ACKNOWLEDGMENTS
The authors express their appreciation to NASA for sponsoring this research, and
to Mr. Mark Lehi Jones for the photographs shown in Fig. 6.
REFERENCE
I , J.E. Weaver, "Angular Addressing Properties of Volume Fourier Transform Holograms
in Iron-doped Lithium Niobate," Ph.D. Thesis, Georgia Institute of Technology,
1979.
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(b)
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(c)
Volume hologram recording (a) interfering beams (b) crystal recording (c)
recorded holograms.
ING
Figure 2. Writing and reading a data page hologram.
ANGLE ACCESS
ON[ TOTAL _G[ SINICE0 0.2*APART FROM
GLE LOCATIONS
104 bits x 102 x 10 2 x 103 = 10II bits
Figure 3. Data storage in a page-oriented optical holographic memory.
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Optical SyStem Diagram
Hologram recording and reading apparatus (helium-neon laser)
(reference ]).
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10" 3 THEORY _ EXPERIMENT
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DEVIATION FROM BRAGG ANGLE
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Figure 5. Diffraction efficiency versus angle (reference I).
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Figure 6.
(a )____ _ ._
._ ,__ :_
(b)
Hologram produced with LDEF crystal (a) object beam data page (b) hologram
of object beam.
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ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
Table ]. Characteristics of volume holographic storage,
VOLUME HOLOGRAPHIC STORAGE
HIGH INFORMATION CAPACITY
REDUNDANT
READ-WRITE-ERASE OR ARCHIVAL
NON MECHANICAL (ELECTRONIC & OPTICAL)
INHERENT TWO-DIMENSIONAL STORAGE
PROCESSING CAPABILITY
Table 2. LDEF samples.
FI"VE SAMPLES FLOWN
1. LITHIUM NIOBATE, HEAT TREATED
MAXIMUM SENSITIVITY, BLANK
FOR
2. LITHIUM NIOBATE, PLANE WAVE HOLOGRAM,
HELIUM-NEON LASER
3. LITHIUM NIOBATE,
ARGON LASER
PLANE WAVE HOLOGRAM,
4. LITHIUM NIOBATE, SPOKE PATTERN HOLOGRAM,
ARGON LASER
5. GLASS CONTROL SAMPLE
•
Two CONTROL CRYSTALS
LITHIUM NIOBATE, PLANE WAVE HOLOGRAM,
HELIUM NEON LASER
2. LITHIUM NIOBATE,
ARGON LASER
PLANE WAVE HOLOGRAH,
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Holographic data storage crystals for the LDEF

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