Method has been developed for doping lithium niobiate crystals with transition metal to increase rate at which crystal can record optical data. Discovery may facilitate development of system for analog storage of TV frames, printed pages, photographs, and other visual information

Aronson, H.

Loiacono, G. M.

English

NASA Technical Reports Server

March 1973	 B73-10095 
•	 NASA TECH BRIEF 
 Marshall Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
A New Optical Recording Medium 
The problem: 
One of the next advances in computer capabilities, a 
step which has been just out of reach, is the develop-
ment of an erasable, optical-recording material for 
computer memories. Such a development would permit 
the rapid, inexpensive storage and recall of entire pic-
tures as a single unit. It would permit the analog storage 
of TV frames, printed pages, photographs and other 
visual information that must currently be stored as a 
sequence of ones and zeros. 
Several optical storage candidates have been under 
consideration for some time. Among these is a lithium-
rare earth crystal, lithium niobiate (LiNb0 3 ). Crystals 
of LiNb0 3 have several favorable properties for use as 
an optical recording medium. They have a high infor-
mation capacity, a high readout efficiency, and are 
erasable. In addition, they require no chemical develop-
ing and can be used to store three-dimensional holo-
grams. However, LiNb0 3 has one severe drawback; it is 
not sensitive enough to record at the rate required in 
computer processing. 
The solution: 
For a time it had seemed as though work on a suit-
able optical recording medium had come to a standstill. 
Thin films, thermoplastics, and other alternatives to 
LiNb0 3 all had inherent limitations offering little hope 
for improvement. But it has recently been discovered 
that LiNb0 3 crystals can record data at a much higher 
rate when doped with transition metals. 
How it's done: 
If an LiNb0 3 crystal is doped during growth with a 
transition metal such as iron, copper, or manganese, 
the optical properties are significantly enhanced, and 
the crystal becomes a highly promising optical recording 
medium. 
The crystals are doped with from 0.01 to 0.5 weight-
percent of the transition metal. After growth, they are
Mirror	 Beam Expander 
Information 
Laser	 ,Transparency 
Beam
Focussing Splitter	
.
 
Lens
Crystal 
BLOCK DIAGRAM OF HOLOGRAPH arrangement used to 
record information in single crystal LiNb03. Readout is accom-
plished by blocking off the object beam and illuminating the 
hologram with the reference beam 
annealed, X-ray mounted, and sliced into 20 x 20 mm 
wafers one mm thick. Iron seems to be the best dopant. 
Iron-doped crystals will store a hologram image in as 
little as seven milliseconds using a commercial argon 
laser. 
The recording process in LiNb0 3 is based on the 
electro-optic properties of the crystal. During crystal 
growth, the transition metal ions, alter the optical 
absorption, electron concentrations, and trapping center 
concentrations. Electrons in the crystal are photo-
excited when exposed to light. The spatial distribution 
of these excited electrons maps the intensity pattern 
of the incident light field. The photoexcited electrons 
drift out of the high intensity areas and are subsequently 
retrapped with an accumulation of electrons in regions 
of the crystal where the light intensity is less. The final 
result is a space charge pattern that is positive in regions 
of high optical intensity and negative in, regions of low 
intensity. The space charge generates an electric field 
that locally modulates the index of refraction by the 
electro-optic effect.
(continued overleaf) 
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Aeronautics and Space Administration. Neither the United States	 information contained in ,his document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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To store a hologram image in the crystal (see figure). 
light from an argon laser is split into two beams. One of 
the beams is modulated by the information transparency 
and focused to a 2-mm spot on the crystal. The second 
beam (the reference beam) intersects the signal beam at 
the crystal. The interference pattern of the two beams 
is recorded. The reference beam, by itself, is used to 
reproduce the original signal. 
Another remarkable discovery is that the diffraction 
efficiency of the crystal can be increased by a factor of 
2 to 5 when the same wavelength is used for readout as 
for storage. This discovery and the discovery of the 
transition metal enhancement effect may open up many 
new fields in optical recording, storage, and display. 
Large quantities of digital information may be stored 
and randomly accessed for an on-line optical memory in 
a computer complex. TV frames, printed matter, and 
other optical information may be stored and processed 
optically.
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Code A&PS-TU 
Marshall Space Flight Center, Alabama 35812 
Reference: B73-10095 
Patent status: 
Inquiries concerning rights for the commercial use of 
this invention should be addressed to: 
Patent Counsel 
Marshall Space Flight Center 
Code A&PS-PAT 
Marshall Space Flight Center, Alabama 35812 
Source: H. Aronson and

G. M. Loiacono of

Isomet Corp.

under contract to

Marshall Space Flight Center

(MFS-22348)
n 
S 
B73-10095
	 Category 03, 04, 09


A new optical recording medium

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