STORAGE DEVICES--THEIR POTENTIAL AND RELIABILITY PART I LASER INFORMATION RECORDING SYSTEMS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP73-00402R000100140007-2
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 53
• Document Creation Date: December 19, 2016
• Document Release Date: July 19, 2006
• Sequence Number: 7
• Publication Date: August 5, 1969
• Content Type: PAPER

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WORLD CONFERENCE
ON RECORDS
AND GENEALOGICAL SEMINAR
Salt Lake City, Utah, U.S.A.
5-8 August 1969
STORAGE DEVICES--THEIR POTENTIAL
AND RELIABILITY
LASER INFORMATION RECORDING SYSTEMS
A Survey
By
Dr. Joseph W. Shepard
3 M Company
St. Paul, Minn.
"Record Protection in
an Uncertain World"
R[RyAOIAr RCETY OF TH.E CHURCH OFF CS CHIDP73-00402ROINC 14000J\ 4 R
COPYRIGHT? 1969 A C L 50 I F.T C H JE: HRiST .OF LATTER-DAY SAINTS, INC.,
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STORAGE DEVICES -- THEIR POTENTIAL
AND RELIABILITY
LASER INFORMATION RECORDING SYSTEMS
A Survey
By
Dr. Joseph W. Shepard
3 M Company
St. Paul, Minn.
Since its advent, the laser and its unique properties have
been the subject of intense scientific and engineering activity.
Each advance has generated expanded interest and activity in a
diverse series of applications. A major application of interest,
that of recording information, is in advanced system exploration
stages.
What properties of the laser give it a pre-eminent position
in advanced information recording systems R & D?
What types of systems are being developed?
What are the current limitations in recording system appli-
cations?
What rate of progress is being made to overcome these?
What advances could be expected to develop alternate systems
approaches?
Five important characteristics of the laser as a radiant
energy source for information recording systems design are:
(1) High power density output
(2) High degree of collimation (low divergence)
(3) Monochromatic output
(4) Plane polarized output
(5) Coherency
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The first three characteristics permit achieving excellent
resolution, the second, third, and fourth permit use of a variety
of devices for modulation and deflection, while the fifth has
opened new approaches to high density storage via holography.
Taken in total, these characteristics promise high information
packing density and high speed recording in systems applications.
Potential densities of 12p/character (lp spot size) and rates of
100,000 characters/sec. are feasible compared to 100p/character
(5-10u spot size) and 60,000 characters/sec. electron beam record-
ing rates.
Prior to the advent of the laser, the only high output, radiant
energy sources were isotropic radiators, i.e. tungsten wires,
carbon arcs, etc. The output energy of an isotropic source is
omnidirectional and is distributed according to a physical law
over a wide range of wavelengths while detectors .(recording media)
normally respond to relatively narrow bandwidths. In a practical
sense, this means that these sources had to be operated at very high
temperatures to obtain even marginal power outputs in a narrow
spectral range corresponding to the detector absorption. For ex-
ample, to achieve the energy density per unit of time in a one
angstrom bandwidth attainable from a one milliwatt laser focused
to a two millimeter diameter spot would require an incandescent
source to be operated at 107 degrees Kelvin. Such temperatures
are found only in stellar interiors. Operation of isotropic
sources at very high temperatures results in short source life.
In addition, since radiation from these sources is highly divergent
- emitted in all directions - complex lens and reflector optical
systems are required to focus the light, resulting in a consider-
able power loss.
The laser generates light throughout a large volume in a
unique way that allows the emitted light to be focused with a
simple lens system to a small spot - as small as tenths of a
micron (1 micron =0.04 mil). A laser beam can be focused
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readily to achieve a power density of 1015 w/cm2 in a 10V spot
for a pulse duration of 10 nanoseconds.1 For comparison, the
radiated power at the sun's surface is about 105 s/cm2. Laser
outputs in the kilowatt range are easily attainable for times
longer than a millisecond. For our purposes, we will be con-
cerned with CW (continuous wave) lasers. These give outputs
in the milliwatt to 1 watt range over a single or very narrow
wavelength range. Consequently, for a matched detector (record-
ing medium), one has power densities in a narrow spectral range
at least thousands of times greater than from previous radiant
energy sources. There are many unusual imaging materials which
can give extremely sharp images, but which are low in sensi-
tivity limiting their usefulness. The great intensity of the
laser means that many of these materials can be explored since
extremely short exposures may be combined with high reduction
ratios and rapid access permitting storing and retrieving large
amounts of information from a small area.
As mentioned above, the monochromaticity of laser light is
exceptional - the light has nearly a single wavelength or fre-
quency. Ordinary light has colors or wavelengths ranging through
the spectrum from the blue to the red regions. A major problem
in lens design is selecting glasses of different refracting
powers and shaping the lens surfaces to focus all the different
wavelengths of the light. With laser light, these considerations
no longer apply and the optical system is simplified. The emitted
laser light beam has a single direction and does not diverge like
ordinary light so field lenses are not needed to confine the beam.
1C.G. Young et al, "Optical Avalanche Laser", Journal of Applied
Physics, 4319-24, 37 (1966)
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One of the more important characteristics of laser light is
that it can be obtained plane polarized. Electromagnetic radiation,
which includes normal light, vibrates perpendicular to the direction
of light beam propagation, but in all possible directions. Plane
polarized light vibrates in only one direction. In the laser,
emissions are stimulated by means of reflection at the end mirror
in the laser assembly and only light polarized in one plane is
produced. This property is very important in recording system
design, i.e. modulation, and for some systems, also deflection.
The coherency characteristic of laser light means that un-
like normal light, laser radiation comes out in a single train
of waves rather than a superposition of many waves incoherently
spaced at random. These waves can be controlled and shaped
accurately to give images sharper than those obtained with ordi-
nary light. This characteristic permits holographic recording.
Laser action can be achieved in a variety of materials in
different states of matter.
(1) Solid
(2) Gas
(3) Liquid
(4) Junction (semiconductor)
In solid and liquid lasing materials, certain ions located
as "impurities" in the material are the active radiators. In
the original ruby laser, a sprinkling of chromium ions (Cr+++)
distributed in an aluminum oxide crystal lattice operate as the
emitters. In other solid and certain liquid lasers, various
rare-earth ions are incorporated for the same role. In gas-
discharge lasers, the active role may be played by certain atoms,
ions, or by simple inorganic molecules, such as carbon dioxide.
In semiconductor junction lasers, the crystal lattice of the host
material itself has a primary role along with that of the
"impurities" in the light emitting process.
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U LL 1.
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CONTINUOUS WAVE
LASERS
PUMP
INPUT
POWER
OUTPUT
POWER
YAG
RoD
Sodium in Mercury
"?'apor discharge
1 KW
20-30W
lamp
YAG
1.7 KW
20W
RoD
Tungsten Filament
1.7 KW
27W
3 cm long
1.7 KW
.04W
0.5 cm diameter
1.5 KW
1/8W
Carbon Dioxide-
nitrogen-
Direct discharge
through tube,
10 KW 1KW
helium gas
D.C. or A. C.
Direct discharge 10-20 W 1.5-1MW 0.63211
D.C.
I Carbon Dioxide-
Ln
nitrogen-
helium gas
tube length: 1.8m (-6 ft.) Direct discharge
diameters: 2.5-)0cm (-'1-4 in.) through tube, 2 KW 150W 10.611
(water cooling required A.C. or D.C.
for continuous operation)
Helium-neon gas
12" x 8" x 5"
Tube lifetime:
over 5,000 hrs.
Krypton-Argon
Krypton
Argon
Direct discharge 1 KW 300MW
60 cycle A.C. 50OMW
Direct discharge 1-2KW 1.5-0.3W
OUTPUT
WAVELENGTH
1.0411
1.3411
1.0611
.6711
.5311
10.611
0.514511
O.488?
0.476211, 0.520811
0.5682U 0.6471p
Direct discharge 1-2KW 1.0-0.1W 0.4579p, 00476511,
0.488011, 0.496511
0.501711, 0.514511
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MIRROR
/ r
YAG CRYSTAL
WATER COOLED
MIRROR
99.8% REFLECTIVE AT 1.06
TRANSMISSIVE AT 5300 A?
LASER BEAM
5300 Ao
PUMPING
LAMPS
(Tungsten Iodine)
FREQUENCY
DOUBLER
(Barium sodium niobiate)
CW YAG LASER
1/8W OUTPUT AT 5300 A?
FOR 1500 W PUMPING POWER
FIGURE 1
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MIRROR
HELIUM-NEON
GAS MIXTURE
D C POWER
HELIUM-NEON LASER
FIGURE 2
MIRROR
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Typical solid materials besides ruby which may be used are
glasses containing neodymium ions (Nd+++), "YAG", or containing
a variety of rare-earth ions, "alphabet YAG", or a calcium phos-
phate containing neodymium, "FAP". Although ruby, the first
laser, required a very high power, pulsed, flash lamp to achieve
laser oscillations, later developed materials YAG and FAP, can
oscillate continuously using a tungsten filament source as the
"pump". Table 1 lists typical continuous wave lasers which have
been used as sources in laser information recording systems.
The lasers with outputs in the visible spectrum (0.5 - 0.6u)
have been used to record on wavelength sensitive media, i.e.
silver halide emulsions, Dry Silver films, while the lasers
with outputs in the infrared (1.0p - 10u) have been used to record
via thermal effects, i.e., heating or evaporating a metal film,
burning a hole in a film. Figure 1 is a schematic of a YAG
laser, while Figure 2 is a schematic of a helium neon gas laser.
The laser has been used in two modes to achieve information
recording:
(1) Holography
(2) Direct writing
HOLOGRAPHIC - DIGITAL MEMORY SYSTEMS
In holography, a laser beam is split into an object beam
which shines on the object, and a reference beam which shines
directly on a recording film. Light reflected from an object
when combined with light from a reference source forms an inter-
ference pattern on a film, Figure 3. The resulting interference
pattern bears no resemblance to the object. When the film is
developed and the beam of a laser is passed through it, the
object is reconstructed in three dimensions at a focal point.
Ordinary photography using lenses records only the intensity of
the light (the variations in brightness) reflected from an object.
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MIRROR
BEAM DIVERGING
SPLITTER LENS
DIVERGING
LENS
HOLOGRAPHIC RECORDING
REFERENCE
MIRROR
FIGURE 3
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Holography, which does not employ a lens, records both wave
patterns and intensity patterns. When light strikes a point on
an object, it is reflected back from the point in ever increasing
concentric rings -- like the rings of waves caused by a pebble
dropped into a pool. A complex object has innumerable reflection
points so that a large number of sets of wave rings are reflected.
Taken all together, these reflections make up an extremely com-
plex pattern that is distinctive to the object being illuminated.
While holographic principles have been known for two decades,
the laser provides the intense source of coherent light needed
to stimulate renewed interest in applications.
A hologram for information storage purposes is a record of
a three-dimensional, stationary, interference pattern in a photo-
sensitive material. A typical, thin, silver halide emulsion
stores only a "fringe system" that is essentially a cross-section
of the interference pattern. If the holograph is recorded in a
material much thicker than the average fringe spacing of the
pattern, it can be considered a "volume" hologram. The volume
hologram has redundancy in that it is equivalent to a large
number of thin holograms of the same interference pattern.
There is no fundamental difference between the recording of a
thin (surface) hologram and a volume hologram. Differences do
appear when one wishes the series of thin holograms in a volume
hologram to represent a corresponding series of different objects.
In doing this, one is seeking to increase the capacity to store
information. A volume hologram is "readable" only from a very
narrow angular interval. Outside this range, the image intensity
is very low. A series of superimposed holograms can be recorded
and read-out by operating at selected rotational increments
larger than the "read-out" angle interval for a volume hologram.
Typically, the discrimination interval in an alkali halide
crystal can be 0.1? permitting successive holograms to be re-
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corded and read out at 0.2? or larger increments.
A variety of techniques are being explored seeking to exploit
holography as a basis for high density, digital data memory
systems.
(1) Multiple exposure holograms in which different
objects are exposed separately with the same
reference beam at different angles.1
(2) Multiple exposure holograms in which the
reference beam and the element are changed.2
The reference beam is part of the code by
which objects are retrieved.
(3) Multiple exposure holograms in which the
reference beam is changed and coded by
changing the position of a ground glass
plate between exposures.3
The interference patterns are diffused over the entirety
of the recording medium. The thicker the recording medium, the
greater the number of images that can be stored in it. For
maximum usefulness, the number of superimposed images should
be large. However, each succeeding image will reduce the resol-
ution of those preceeding. In the limit, the number of holograms
that can be recorded on a single plate is limited by the required
resolution and the finite information limit of the recording
medium. It is interesting to note that the latter is also a
major limiting factor in integral imaging.
1G. W. Strike, F. H. Westerveldt, R. G. Zech, Proc. I.E.E.E.
55, 109 (1967)
2M. Marchant, D. Knight, Optica Acta, 14, 199 (1967)
3R. J. Collier, K. S. Pennington, Appl. Opt. 6, 1091 (1967)
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Volume holography places severe requirements on the record-
ing material.
(1) Extremely high resolution
(2) High dimensipnal stability
(3) Low grain noise
(4) Good optical quality
In addition to special silver halide photographic emulsions,
a number of photochromic1 and alkali halide crystals have been
used in exploring this technique. The imaging mechanism in the
latter materials is based on various color center transitions.
Some of their properties of major interest are:
(1) Very high resolution
(2) Reversible transitions permit erasing and revision
(3) The transitions are nearly instantaneous and re-
quire no development
Use of a dichromated gelatin holographic plate, which has
the advantage of reducing the laser beam power required for
retrieval by a factor of 20, has been reported by Bell Laboratories
personnel.2 This means that on "read-out" as much as 96% of the
reconstructing beam power passes through the hologram to the
detector array. Previous recording materials permitted less than
6% of the reconstructing beam power to illuminate the detector
array. In the recording technique, a dichromated gelatin film
is exposed using an argon laser beam and developed by gently
agitating it in water. Following this, it is rinsed in isopropanol
or some other aqeous solvent and air dried. It is then coated
with a lacquer and air dried. Present techniques permit achiev-
ing 2,000 lines per mm. However, the material appears capable
of achieving 4,000 lines per mm.
A. Reich, G. H. Dorion, Optical & Electro-Optical Information
Processing, Chap.31, p.567, MIT Press, Cambridge, Mass, (1965)
2Bell Laboratories Record 46, 276 (1968)
12
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A disadvantage of all the above materials is their relative
insensitivity compared to silver halide emulsions.
Incident power density on the recording medium is relatively
low in volume holography. With a helium-neon laser, exposure times
for photographic films have been in the range of 10 to 15 minutes.
The advent of the 1 watt argon gas laser has decreased this re-
quirement by more than an order of magnitude to the 10 second
range. Over the near term, this exposure time limits this technique
chiefly to systems where rapid read-out is a requirement, but the
data input can be relatively slow.
optical switched holographic optical memories has been examined.1
Fundamental physical limits in basic electro-optical and acousto-
optical processes limit the maximum value of the capacity-speed
product. Speed is the rate at which the laser beam can be
switched from one address to another randomly selected one,
while capacity is the number of distinct beam positions the
deflector device can produce.
CSP = Nat/2 va
CSP = The capacity speed product
Na = The total number of addresses in a memory
va = The rate (addresses/sec) of random addressing
The highest capacity-speed product for electro-optical de-
flectors achievable with presently known materials is in the
range of 109sec.-1. For current acousto-optical deflectors
(400 MHz maximum available acoustic frequency), the highest
capacity speed product is 108sec.-1. The maximum number of
available addresses limited by practical deflector element
size and optics is about 106addresses.
1F.M.Smits, L.E.Gallaher, Bell System Tech.J. 46, 1267-78,
(1967)
The design considerations for electro-optical and acousto-
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If one seeks higher information density by storing bits
in the form of a holographic arrays at each address, the sensi-
tivity of the detectors employed and the light power reaching
each bit become limiting factors. Using reliable lasers oper-
ating in the visible region with from 1 watt to 100 milliwatts
output, one could expect to attain total memory capacities of
108 bits divided into address positions of 10" bits each with
an address access time of less than lop sec.
Holographic digital memory systems.have a number of ad-
vantages:
(1) Reduced sensitivity to dust since the infor-
mation about each bit is spread over the entire
image area. The presence of dust will reduce image
intensity, but not cause complete information loss.
(2) Uniformity of illumination is also not very
critical.
(3) Exact positioning of the individual hologram
is not very critical. Slight displacement of the
beam with relation to the image will still permit
reconstruction of the image.
(4) Complex arrays of focusing lenses,are not
required, eliminating optical losses.
On the other hand, reconstruction is very sensitive to the
angular position at which the beam addresses the hologram.
Translational sensitivity is traded for angular sensitivity,
since it is easier to design a mechanical system in which close
angular tolerances are maintained in the presence of mechanical
vibration and thermal changes than one in which absolute distances
(spacings) must remain constant.
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On the basis of calculations, the theoretical storage capacity
of a 1 cm3 alkali halide crystal has been determined to be 3 x 1011
bits l. Interestingly, if one were to sequentially record with
a modulated 1 micron diameter laser beam laying down a two-
dimensional array of bits in the same volume of medium (a series
of 1 mil thick, 1 cm2 slices) the theoretical maximum storage
capacity is about 4 x 1010 bits. On this basis, depending on
the ability to coat films of the medium less than a mil thick,
the theoretical recording densities of the two techniques would
converge in the limit. The relative simplicity, efficiency,
access time, and economics of either direct writing or the
holographic technique in approaching the theoretical infor-
mation limit of a medium will be a decisive factor in practi-
cal system design.
A photo-image data storage unit has been reported which is
capable of storing 100, 35mm holographic images on a one square
inch potassium bromide crystal.2 A helium-neon laser is used
in the system. Work is under way to achieve a 1000 image storage
capacity. The crystal element has a storage capability of 106
bits per cm2.
Experiments have been reported in which 1000 holograms
have been superimposed in the same area of a 9" square photo-
graphic plate.3 Theoretical calculations indicated as many
as 105 holograms could be obtained while retaining a practical
signal to noise ratio (S/N) for read-out (10 db.) when an array
of separated point sources (digital information) is the subject.
The limitation appears to be not one of optical signal to noise
ratio but one of available intensity and electrical S/N in the
detector. Bell Laboratories has reported the ability to store
temporarily up to 1000 holographic images on a crystalline cube
1P. J. Van Heerden, Appl. Optics, 2, 764 (1963)
2Laser Focus, p. 12, March 14, 1966
3J. T. LaMacchia, D. L. White, Appl. Optics, 7, p.91-4,(1968)
15
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of lithium niobate.
Based on the dichromated gelatin holographic film reported
above, L. K. Anderson1 has described a holographic optical
memory system for bulk data storage with a predicted capacity of
100 million bits of data with a random access time as short as
one microsecond. For comparison, a typical twister memory using
16 permanent magnet twister memory modules has a capacity of
5.8 million bits and an average access time of about 4 to 5
microseconds. While this does not represent the largest possible
twister memory - factors of large capacity, short access time,
and cost warrant a searching exploration of other techniques. A
32 by 32 matrix of holograms with about 1.2 mm for each hologram
(page) is employed as the storage plane. Electro-optic, X-Y
deplection of the read-out beam projects reconstructed data on an
8 x 8 matrix of phototransistors. A hologram about 1 mm in
diameter can readily store 10,000 bits of information. The
current experimental system has a capacity of one thousand,
4,000 bit pages (4 million bits). The deflector used can
address the 32 page by 32 page memory plane in 6 microseconds.
Recently, engineers from I.B.M.'s System Development Div-
ision reported on a laser memory system capable of retrieving
blocks (arrays) of information from a 9" square holographic
plate (photographic) in 10 microseconds.2 This is a thousand
times faster than current magnetic disc and drum storage units.
The holographic display is read-out by an array of photodetectors.
The system is reported to have a potential of storing a 100
million bits on the 9" square plate (2 x 106 bits/cm2) which
is a hundred times greater capacity than current magnetic
devices.
1L.K. Anderson, Bell Laboratories Record, 46, 318-25 (1968)
2N.E.R.E.M., Fall, 1968; Product. Engrg. 39, 38 (Dec.,1968)
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For comparison with the holographic approach to digital
memory systems, consider two direct writing systems under de-
velopment. Minneapolis Honeywell, I.B.M., and Precision
Instrument Company are working with laser digital storage
systems using thin film recording media in which the bits are
put down sequentially by a modulated laser beam rather than as
an array in a hologram, and read-out sequentially. In these
systems, recording can be accomplished at very high rates.
Honeywell's system1 employs an evaporated thin film of a
compound of manganese and bismuth. The magnetic and optical
properties of the compound polarize light passing through the
film rotating it clockwise or counter-clockwise depending on
the direction of thin film magnetization. Honeywell is reported
to be using a 1W argon laser beam. Transmission of a low power
laser beam does not disturb the magnetization pattern represent-
ing the stored digital information and rotation of the laser
beam polarization plane can be readily detected. Data can be
written or erased by pulsing the beam to high intensity and
simultaneously applying a magnetic field. At 680 degrees
Fahrenheit (produced by the high intensity, focused, laser beam),
the managanese-bismuth compound loses its magnetization and as
it cools will align itself with an applied magnetic field
changing the magnetization pattern. The system has a capability
of storing 106 bits/cm2 with a read rate of 100 million bits
per second. This is roughly 100 times faster than present
magnetic devices.
D. Chen, J. R. Ready, R. L. Aagard, E. Bernol G., Laser
Focus, p.18-22, (March 1968); Instrumentation Technology,
15, 14 (1968)
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I.B.M.'s system1 employs a europium oxide film at low
temperature (20?K). The advantages of using a europium oxide
memory material are large magneto-optic effects are produced
permitting fast read-out rates, thermowriting at low temper-
atures requires much less energy, fast switching speeds can be
used for the read-write lasers because of the low energy re-
quirement, and small area magnetization reversals can be
sustained. Using a 10 MW laser,bits less than 3p in diameter
were written in 10 nanoseconds. Experimental work indicated
that. operational temperatures as high as 77?K may be feasible
in the future.
Work on a transparent ferrimagnetic garnet material
suitable for magneto-optical memory has been reported by
Bell Laboratories pers-onnel.2 A memory consisting of a close-
packed mosaic of small thin Gd IG crystals operating at 14?K
with a maximum density of 2.5 x 105 bits per square inch is
proposed as practical.
Precision Instrument's system employs a one-watt argon
laser beam which is frequency modulated by input recording
signals to "burn" holes in a thin metallic film,3 (metallic
aluminum has been suggested) deposited on a polyester base.
The beam is swept over the medium by a combination of tape
movement and reflection from a rotating mirror (helical scan)
or alternately by movement of the tape alone. 1.511 "holes" are
made on 1011 centers. A 111 laser beam of lower intensity (3%
of recording intensity) is used for read-out. The laser is
kept in the same position and the film is driven at the same
1G. V. Fan, J. H. Greiner, J.Applied Physics, 39, 1216-18 (1968)
2J. T. Chang, J. F. Dillon, Jr., V. F. Gianola, J.Applied Physics,
36, 1110-11 (1965)
3Product Engineering, 39, 21-3 (May, 1968); Industrial Electronics
6, 153 (April, 1968)
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MANCHESTER CODING
YES - BLACK FOLLOWED BY WHITE
NO - WHITE FOLLOWED BY BLACK
MIRROR
EMULSION ---?-
KOP SLIT CALCITE
CRYSTAL LENS ! MIRROR LENS
MOTOR
PHOTOMULTIPLIER
TUBE
INCOMING
TO BE RECORDED DISPLAY
DATA OSCILLOSCOPE
f ?pH &TcO oS f E 200MEi # ES-XS 00100140007-2
FIGURE A
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speed as during recording. A flying spot scanner guided along
the track by remote controls directed by a galvanometer keeps
the laser spot in the center of the track. A photodetector
beneath the film serves as the read-out detector for the laser
spot. The medium has a recording density of 2 x 106 bits/cm2
and the system is capable of a read-out rate of 2 x 106 bits/
sec. The manufacturer claims the recording tape has about
1000 times the capacity of present magnetic tapes. One limitation
is that the recording process is not reversible. The record is
permanent. In some applications, this could be an advantage.
The degree of advantage could depend on relative cost per unit
of storage among other factors.
Another direct writing, digital information storage system
is Photostore, a data processor developed and used by the United
States Air Force for automatic translation.1 A laser beam
focused by a lens is used to record photographically on a disc
of film ten inches in diameter. Ten million bits of information
are stored in one square inch (about 106 bits/cm2). The recorded
information comprises a very large dictionary and tables of
grammar. Manchester coding is used, i.e. binary digits are
recorded by two marks - a black followed by a white represents
one binary digit, the reverse, a white followed by a black
represents the other, Figure 4. The photosensitive film is
attached to a metal disc mounted on a spindle with a large
moment of inertia and is rotated at high speed, 1800 rpm, to
give a million bit per second recording rate. The image being
focused on the film is a rectangle which sweeps out a square
recording. Simultaneously, a track border is recorded by using
the same lens to form an image of a mask illuminated by an
auxilliary source. Between the laser and the lens, nine crystals
1G. W. King, Discovery, 27, 19-22 (1966)
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of calcite shift the image over one mark width when the
polarization of the beam is switched to that of the extra-
ordinary ray by a KDP electro-optic crystal device. The
unswitched polarization is that of the ordinary ray. The
modulating voltage applied to the KDP crystal is gated
electronically by the data to be recorded. Most high
resolution materials are sufficiently transparent to transmit
part of the beam to a photomultiplier tube. The current
signal from the photomultiplier goes to a comparator to be
matched with the original electrical signal which created the
modulation. The comparator confirms the correct signal was
sent and that an image was formed.
For a commercial system, recording rates of a million
bits a second are desirable. To achieve sufficiently sharp
images (minimum "smear"), the exposure time must be extremely
short -- fractions of a microsecond. To ensure the marks
have acceptably sharp edges, the modulation must be applied
and become effective during a fraction of the exposure time --
about ten nanoseconds (10-0 sec.). During this period, ten
thousand volts must be applied across the faces of the KDP
modulating crystal. This is a severe problem and new materials
having lower power requirements are becoming available and
should permit practical applications operation.
Currently, direct writing systems operate in essentially a
facsimile mode. A modulated laser beam is scanned in sequential
lines across the recording medium. The input signal to the
modulator may be from an optical scanner of some type or from a
specially formated, computer generated, magnetic tape.
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Direct writing systems involve the following elements:
(1) A continuous wave laser source
(2) A modulator to impress information on the
source beam
(3) A deflector to scan the modulated signal
over the recording medium
(4) A recording medium in a transport device
The basic means of modulating laser radiation is by exploit-
ing the polarized nature of the beam.
When application of an electric field to an optical medium
results in a perturbation of its refractive properties, the
phenomenon is called the electro-optic effect. In some solids
and in liquids, when the changes in refractive index are pro-
portional to the square of the applied field, it is called the
Kerr effect. In crystalline solids lacking a center of symmetry,
changes in refractive index display a linear relationship to the
applied voltage. This is called the Pockels effect. Rotation
of the plane of polarization of a light wave as it travels through
a substance in a direction parallel to an applied magnetic field
is called the Faraday effect. A variety of gases, liquids, and
solids show this effect.
Light polarized in a certain direction can have its plane of
polarization rotated on emergence from devices employing these
effects, Figure 5. By selection of proper cell thickness and
applying sufficient voltage, the direction of polarization can be
made to rotate 90?. A polarizer placed in the path of the emerg-
ing beam with its polarization axis oriented 90? with respect to
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ELECTRO-OPTIC MODULATION
APPLIED
VOLTAGE
LASER
BEAM
POLARIZER
LINEAR
POLARIZATION
POLARIZER
LINEAR
POLARIZATION
ELLIPTICAL POLARIZATION
ELECTRO-OPTIC
MATERIAL
ELECTRO-OPTIC MODULATION - LINEARLY POLARIZED LIGHT ON PASSING
THROUGH THE ELECTRO-OPTIC MATERIAL IS MADE MORE OR LESS ELLIPTICAL-
LY POLARIZED DEPENDING ON THE MAGNITUDE OF THE APPLIED VOLTAGE
I HUS LIMITING THE EXTENT TO WHICH THE BEAM WILL PASS THROUGH THE
SECOND POLARIZED ORIENTED AT 900 TO THE INITIAL POLARIZER.
FIGURE 5
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the plane of polarization of the light beam entering the electro-
optic device will not pass light whose plane of polarization has
not been rotated 90? by application of voltage to the device. By
alternating the applied voltage one can "pulse code" modulate the
light beam. A variety of crystals and liquids are used commercially
in devices and others are being developed. Ammonium dihydrogen
phosphate crystals and nitrobenzene (liquid) have been used
commercially. A number of alkali tantalate niobate salts, i.e.
potassium tantalate niobate (KTN), show promise of requiring
lower power. Power requirements at high frequencies are a major
problem in modulation using electro-optic devices.
Laser modulation has been achieved by placing an electro-
optic crystal, i.e. lithium niobate inside a helium-neon laser
cavity and deflecting the laser beam so as to miss the end mirror.1
The usable modulation can be increased theoretically by as much
as one hundred times provided no optical losses are introduced.
Using an aperture in the above case to reduce the beam diameter
resulted in a 6% loss but enabled an increase of about 50 times
in diffracted power. In one set of experiments at 500 MHz,
approximately one watt of RF power was needed to achieve a
maximum.
A lithium tantalate modulator (Figure 6) has been reported
to produce 80 percent modulation of the intensity of a red,
helium-neon laser beam over a bandwidth of 200mc/sec. using only
200 milliwatts of power from a transistorized amplifier.2
1A. E. Siegman, C. F. Quate, J. Bjorkholm, G. Francois,
Appl. Phys. Letters 5, 1 (1964)
2D. F. Nelson, Scientific American, 218, 17 (1968)
R. A. Laudise, Bell Laboratories Record 46, 3 (1968)
23
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APPLIED (MODULATING)
VOLTAGE
CRYSTAL
ROCHON
PRISM
REFLECTING
COATING
QUARTZ
LENS WEDGE
HEAT
SINK
THE BEAM EMERGES FROM THE CRYSTAL ELLIPTICALLY POLARIZED AND THE
COMPONENT OF POLARIZATION PERPENDICULAR TO THE INPUT COMPONENT IS
DEFLECTED BY A ROCHON PRISM TO FORM THE OUTPUT BEAM.
LITHIUM
ApprovelAorNRIAeakA211/0q/ly?ATA~7M- AARAOT4907-2
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The p-n junction modulator makes use of an electro-optic
effect in a semiconductor crystal, Figure 7. Because of the
extreme narrowness of the p-n junction region, precise focusing
of the beam is required. A constant reverse bias is used to
augment the electric field across the junction while the modu-
lating voltage is impressed on top of the constant field.
Polarizers at 90 degress to each other are used at opposite
ends of the device to achieve modulation. Power dissipation
within the diode crystal is a major limiting factor. A 1.5mm
long diode can modulate the intensity of a helium-neon laser
beam by up to 80%.1 About 150 milliwatts can be dissipated
limiting the diode operation to about a 100 MHz per second
bandwidth. Improvements in materials and development work on
mounting and device design are needed. Mounting techniques
to improve heat dissipation and design for multiple beam passes
could extend the modulation range considerably.
An efficient magneto-optic modulator requires a material
that gives the largest Faraday rotation per unit of optical
loss from absorption, Figure 8. Crystalline chromium tribromide,
the material frequently used, has to be cooled within a few
degrees of absolute zero in order to achieve the desired magnetic
properties. Recently, a region of very high transparency has
been found in magnetic yttrium-iron-garnet (YIG) and it can be
used at room temperature. A 40% amplitude modulation of a
helium-neon laser beam has been achieved with a YIG-based device
at a bandwidth of 200mc with a power expenditure of 0.1 watt.2
An alternate technique of intensity modulation involves use
of an ultrasonic diffraction cell, Figure 9. The operation of
this device is based on modification of the refractive index of
1D. F. Nelson, Scientific American 218, 17 (1968)
2lbid
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MODULATING VOLTAGE
SUPERPOSED ON REVERSE
BIAS VOLTAGE
PN JUNCTION FUNCTIONS AS AN ELECTRO.OPTIC MATERIAL ELLIP-
TICALLY POLARIZING THE BEAM TO A DEGREE DEPENDENT ON
THE MODULATING VOLTAGE.
PN JUNCTION MODULATOR
FIGURE 7
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CONSTANT MAGNETIC
FIELD
INFRARED
LASER
BEAM
POLARIZER
MODULATING
VOLTAGE
ANALYZER
MODULATED
MAGNETIC
FIELD FROM
COIL
MAGNETO-OPTIC (FARADAY EFFECT) MODULATOR
FIGURE 8
MODULATED
OUTPUT
BEAM
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LENS
HE-NE
LASER
MODULATOR
VOLTAGE
--
ISIWIK
UNDEFLECTED
BEAM STOP
DEFLECTED
ACOUSTIC OUTPUT
CELL
IN THE ACOUSTIC CELL THE PERIOD PATTERN OF LAYERS OF ALTERNATELY
HIGHER AND LOWER REFRACTIVE INDEX IS CAPABLE OF ACTING LIKE A DIF-
FRACTION GRATING AND EFFICIENTLY DEFLECTING LIGHT THROUGH A SMALL
ANGLE.
ULTRASONIC MODULATION
FIGURE 9
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a transparent material, i.e. glass or water, by a stress wave.
An ultrasonic stress wave traveling through a transparent material
will deflect light in a manner comparable to a ruled diffraction
grating. The angle of deflection will be directly proportional
to the wavelength of light and the frequency of the ultrasonic
stress wave. A control signal is used to modulate the stress
wave. Use of a sufficiently small light beam coupled with a
small or moderate carrier amplitude will result in diffracted
light intensity variations corresponding to variations in the
control signal. An optical stop is used to remove undefracted
light. With a high index glass medium, operating frequencies
in the 40MHz range have been achieved..1 Bandwidth limitations
and low contrast generally have been problems in this technique.
DEFLECTION
Basically, three types of devices have been employed to
achieve deflection.
1. Mechanical Scanners involving use of electro-
2. variable Diffraction Devices
a. Electro-optic effects have been employed in a
number of devices, i.e. a series of prisms, to
achieve variable refraction at a dielectric
interface or by an index gradient.2
1R. Adler, I.E.E.E. Spectrum 4, 42-54 (1967)
2J. F. Lotspeich, I.E.E.E. Spectrum 5, 42-52 (1968)
25
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b. Acoustic (ultrasonic) waves traveling through a
transparent medium produce sinusoidal refractive
index variations which can be used to deflect a
laser beam.1
3. Birefringent Deflection Devices employing a combination
of a polarization modulation element combined with a
passive birefringent polarization discriminator (crystal).
The polarization state of the incident laser beam is
varied by the modulator and this variation is converted
by the discriminator into a linear or an angular dis-
placement to yield two distinct beams, Figure 10. A
linear chain of these devices will produce a regular
array of discrete beam positions.
Although an imposing array of deflection devices are avail-
able, system constraints for high speed, high information density
recording rapidly converge one to limited approaches. The follow-
ing requirements are minimal for microform recording:
(1) Resolution capability in excess of 100 lines/mm.
(2) Aperture size and focal length consistent with
diffraction limitations.
(3) Dynamic optical surface uniformity during
scanning.
(4) Ability of high speed scanning elements to
sustain severe dynamic stress.
Phase front distortion must be reduced to negligible values
for all components used in a high resolution deflection device.
Theoretically, all the techniques can provide nearly diffraction
1R. Adler, I.E.E.E. Spectrum, 4, 42-54 (1967); A. Korpel,R.Adler,
P. Desmeres, W. Watson, Proc. I.E.E.E. 54, 1429 (1966)
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LASER
BEAM
BIREFRINGENT
DEFLECTION UNIT
sum
BINARY
SELECTION
PRISM
900 ELECTRO-OPTIC 90? ELECTRO-OPTIC
SWITCH
SWITCH
BIREFRINGENT DEFLECTION DEVICE
FIGURE 10
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limited performance. Practically,_optical distortion is presently
a problem with electro-optic refractors using the new high sensi-
tivity materials such as potassium tantalate (KTN), with variable
refractors at high frequencies for which dielectric losses heat
the medium non-uniformly, and for acoustic (ultrasonic) standing
wave refractors.
Using cascaded binary birefringent deflection devices,
resolutions (discrete spot positions) greater than 106 appear
feasible with present technology.1 Scanning rate is ultimately
limited by power dissipation in the electro-optic crystals and
is in the 106 deflections/second range for presently available
materials. This type of device is best suited for random access
deflection applications (digital recording).
For direct writing, computer output, and microrecording
applications, scanning by electro-optic type deflectors is
fundamentally resolution limited by diffraction effects and
wave front distortion within the deflecting material. Temp-
erature gradients play a role in the latter. Beiser2 character-
izes the maximum resolution of a linear gradient type deflector
as follows: 2/3
R = 1.46 (W/aa)
max n 1
where Rmax = maximum number of spots per scan width W
W = scan width in same units as A
A = wavelength of light beam
a = aperture and flux shape factor (1