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4
Opportunities in Storage and Display
INTRODUCTION
Optical storage and display subsystems are used for input and output of
data in information processing systems. Optical printers, which use optical
beams to write on drums that transfer the image to the paper, are also part of
the input/output subsystem but are not treated in this report.
There is already a large U.S. industrial and military market for these optical
subsystems, but they also have a strategic importance for future information-
handling systems. As information-processing systems grow in complexity to
handle an ever-broader range of applications as well as more sophisticated
applications, the demand to store massive amounts of data and to display and
print large amounts of data becomes critical. The optical technologies used in
these subsystems promise the capability to store and display more information
than their mechanical, electronic, and magnetic counterparts. Therefore, having
a strong technology base in these optical and optoelectronic materials and de-
vices is potentially critical to having a leading-edge information-processing
system.
The word data is to be interpreted in this section in its broadest sense, that
is, information relating to entertainment, culture, education, and business as
well as the more common scientific connotation of the word. Thus these
information systems affect virtually all human activity.
38
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OPPORTUNITIES IN STORAGE AND DISPLAY
OPIICAL STORAGE
Importance of Optical Storage
39
Measured in the hundreds of billions of dollars, information storage is an
industry that isvitalto this country's military, cultural, and economic well-being.
One type of storage, archival storage, is dominated by printing on paper
(95 percent of the words written are on paper). The other type of storage is on-
line, rapid-access storage, which is currently dominated by magnetic storage
media such as tapes and disks. On-line storage itself is more than a $50 billion
per year industry and is growing at a rate of about 15 percent per year. The
long-term trend, as cost permits, is to store all information electronically on-
line because of the inherent ease in accessing, manipulating, and disseminating
the information. This chapter will thus emphasize on-line storage applications.
Optical storage media store information more densely than their magnetic
counterparts and thus potentially at lower cost. Therefore optical storage tech-
nology could displace large segments of the magnetic storage market. Optical
storage also has the potential of storing information much more densely than
paper and thus could also displace this market segment, although in the near
term optical storage technology is too expensive for many archival applications.
In computer data storage, for example, an optical disk can store 500 times
more data than a floppy disk of comparable size. For high-speed data access,
an optical disk can store 50 times more data than the competing magnetic rigid
disk. With "jukebox" configurations where the disks are interchanged on a disk
reader, the storage can be greatly expanded at the expense of access time. A
more detailed description of this application has been given by Chi (1981~.i
In the area of archival storage, one optical disk is capable of storing the
entire Encyclopaedia Britannica of 43 million words and 24 thousand pictures.
Ten optical storage boxes could store all of the information currently contained
in the Library of Congress. This type of system has been described in detail by
Bartolini (1982~.2 In the entertainment/arts field, the compact disc--an optical
disk--is already replacing the phonograph record. In this case, the larger storage
capacity coupled with digital encoding has been utilized to improve audio
fidelity.
The exceptionally large storage capabilities of optical media make possible
a third, unique application based on combining digital and video information
on the same disk. As an example, consider a future "interactive encyclopedia"
where a stored subject consists of words, pictures, and video movies to fully
describe the subject, complete with user-activated cross-references or elabora-
tion capability. This application today is a $100 million per year industry that
is projected to be a $1 billion industry by the 1990s. Besides having an economic
impact, this technology could lead to new concepts in learning, literature, or the
arts, that go beyond what is available ~ a written textbook. For example, the
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40
PHO TONICS
"interactive novel," which allows its reader to select from many potential plots
and thus to take part in the story, is made more feasible with a large, quickly
branchable, storage medium such as optical storage.
Since the storage of data is so critically important in todays world and since
future generations of data storage are likely to include optical storage, the
United States must gain and maintain a leadership position in optical storage
technology. This critical need is amplified even further when one considers that
optical storage can displace many types of storage technology in various fields,
from computers to literature to audio/video. As with many of the optical
technologies, the dominant development activity appears to be in Japan.
Storage Systems
Storage systems can be divided into three types. The fast is the multiple
read and write system, currently a $70 billion per year industry that is dominated
by magnetic media. Optical storage systems of this type are under develo`pment
at many major computer and communications companies worldwide. The
second type is archival storage where media are written once by the user but can
be read many times. Todays market is dominated by magnetic tapes, micro-
f0, and printed paper. The audio and video disk technology has already been
reengineered in Japan and introduced into this storage market. The third type
of system is the read-only system, where replicas of a master disk are distributed
to users. In this case, only preexisting data are available to the user, who cannot
generate data on this system. This market is immense and difficult to quantify
as it includes photographic film (including movies), phonograph records, and
printed paper. As mentioned above, optical storage has penetrated the audio
application segment of this market and is starting to penetrate the printed paper
segment. For the read/write applications, high storage density and data access
speed comparable to those of the magnetic hard disk are key technical require-
ments, whereas in archival storage applications, the cost and permanence of the
stored information are critical.
Technology Elements
An optical storage system consists of a number of key technology elements.
These are the storage media, the reading and writing head, and the electronic
interfaces and system design and software.S,6 As shown schematically in Figure
4.1, semiconductor lasers are used as a source of light that is focused onto a very
small spot on the optical storage media. The small spot causes a material
change when the information is to be written. The spot is read by a change in
the light reflected off the small spot (of the order of 1 micron in size). The spot
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OPPORTUNITIES IN STORAGE AND DISPLAY
/
-
-
-
-
-
-
Disk rotation
-
.~
-
-
- 3,
——Laser beam
41
.aser movement
FIGURE 4.1 Diagram of optical storage system. The same laser may be used for both reading
and writing.
sequence is converted to valid data or information sequences by the storage
system protocols and sent to the data processing system or to a specialized
"librarian" computer.
Media
There are a number of different materials for recording these dots of infor-
mation. Table 4.1 lists some of these materials, the storage processes, and the
relative advantages of each. The main approaches used today are ablative for
archival storage and phase change for read/write writing, coupled with
magneto-optical readout. Key issues in the media today are the read and write
optical power required, media lifetime, and media cost. Current methods for
writing information on optical disks use the laser as a heat source for removing
small spots of material (the ablative process) or for causing a phase change in
a small spot of the material. The photonic nature of the laser beam is utilized
only in that the light wavelength sets the limits on the size of the written spot.
Because of the heating requirement, high-power lasers (of the order of 50 mW)
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42
TABLE 4.1 Various Optical Storage Media and Characteristics
PHO TONICS
Type Storage Advantages Disadvantagcs
Read-only
Injection molding Pit formation Stability
Low replication cost
High mastering cost
Write-once
Ablative Pit formation Sensitivity Optical write power
(Te alloy) Most developed Potential for bit error
Data permanence
Ablative Bubble formation High SNR Stability
(dye polymer) Low manufacturing cost Potential for bit error
Phase change Optical reflectivity Lower write power Lifetime
change in contact Low SNR
overcoat
Reversible
Magneto-optic Magnetic domain Reversibility Low SNR
switching Most advanced technique Mcdia cxpcnsc
Kerr effect readback Mcdia passivation
Phase change Optical reflectivity High SNR (potential) Lin~itcd reversibility
change Simpler optical system Higher write power
Single-pass overwrite
Dye polymer Materials flow Cost Limited reversibility
Stability Lifetime
Read/writc speed
are critical for the writing process. The reading of optically stored information
is based on a change in reflectivity or a change in polarization of the reflected
light; this process requires less laser power, only enough to obtain an adequately
noise-free signal.
Read/Write Head
The key element of the read/write head is the semiconductor Ejection
laser. Critical issues are the optical output power, device lifetime at high power,
shorter light wavelength for smaller spots and high recording density, modula-
tion speed, and optical beam quality. Expensive, bulky gas lasers are available
for writing but are limited primarily to the equipment that makes "masters."
Adequate injection lasers for small spot writing (about 50 mW of single-mode
optical power with adequate product life) are only begunning to be developed.
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OPPORTUNITIES IN STORAGE AND DISPLAY
43
Read-only injection lasers are available, with Japanese industry the primary
supplier. The reflected light is read by a photodetector, usually segmented into
four quadrants, for use in tracking a spot's position and improving the signal
strength with respect to system noise. The beam is focused by a series of optical
lenses and beam splitters and a miniature beam-positioning system based on
"voice coil" actuators. The key attributes are efficiency in delivering power to
a small spot and low mass so that the head can be rapidly repositioned.
Storage System Architecture
The architecture of an optical storage system refers to the way the data are
formatted onto the disk, the way the data are read from the disk, and the way
the computer system interacts with the optical storage system. For example, in
read-only memory, contiguous data sequences are rearranged into noncon-
tiguous dot patterns on the disk and coded with error-correcting codes in order
to reduce the number of erroneously read bits by more than 7 orders of
magnitude. These formats are often critical to the practicality of a technology
for an application. For example, a factor retarding the use of an ideal write-
once storage with infinite storage capability and zero cost is the lack of good
ways to store various copies of a particular piece of information so that the
proper (i.e., the most recent) copy is retrieved efficiently. (It appears that the
human brain is ahead of computer science in this respect.) The panel will not
cover these architectural aspects of optical storage systems in detail, except to
point out where they seem to be controlling the development of the hardware.
Competitive Environment
Initial research in optical storage systems was carried on by a number of
U.S. and European labs: Philips on laser tracking concepts and error correcting
coding, INCA on high-powered lasers and archival systems, IBM on magneto-
optic read/write media, and IBM/MCI and AT&T Bell Labs on ablative media.
Products were developed but never introduced in the commercial marketplace
(e.g., Storage Technology Corporation's gigabit file system), largely because
U.S. industry could not identify near-term large markets. Today the primary
research and development and product work is done in Japan at the large
electronic companies. Current products are compact audio and video disks and
archival systems for personal computer and workstation applications.
Read/write systems are near the product introduction stage. In this as in other
areas reviewed in this report, the lack of leadership in optoelectronic and
photonic manufacturing technologies in the United States represents a serious
threat to a significant, emerging area of economic competition.
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44
PHO TONICS
Key Enabling Technology Needs
In a number of areas in optical storage, technological improvements are
critical to product competitiveness. Technology elements that are in a position
to be incorporated into products in the next 5 years are called enabling technol-
ogy. Elements that need a longer, sustained research effort are considered in
the next subsection.
In the area of read/write heads, there is clearly a need for improved
high-power laser sources. Maintaining good beam characteristics and
demonstrating long life at high power are key issues. High power may come
from more efficient single devices or from arrays of interacting lasers. There
is good reason to expect that these enabling improvements are possible. New
advances in the quality of compound semiconductor growth, such as the use of
molecular-beam epitaxy (MBE) or metal-organic chemical vapor deposition
(MOCVD), are allowing newlaser structures to be explored. Lasers with quan-
tum well confinement, new alloy compositions, and nonabsorbing mirrors are
being researched; the result could be lower laser threshold currents, improved
efficiency, shorter wavelength, and the ability to operate a device at higher
power without damage. Planar-processed laser structures might lead to better
ways of coupling the optical beam from the laser or improving the beam
properties. An alternative approach to high optical power is to pump small
Nd-YAG rod lasers with diode lasers. This approach is not as attractive from
a wavelength, packaging density/mass, or efficiency viewpoint.
Manufacturable packaging technology for lasers and the optical head
is an important component of assuring a good optical source. Migration from
the assembly of miniature but discrete optical elements to planar-processed,
self-aligned elements is critical to offering a competitive technology.
· Low-mass read/write heads are important to fast data access. The in-
tegration of signal source and detection and perhaps some of the processing
functions into an OEIC chip could provide a low-mass head with enhanced
capability and functions for optical storage systems. Advances in laser sources
as well as in optoelectronics require a high degree of sophistication and
materials control in the processing of compound semiconductors. These areas
will require sustained research and development activity, even though some
forms of integration can be developed today.
Another aspect of faster data access lies in multiple-track reading and
multiple "platter" systems. The integration mentioned above helps packaging
density in these types of systems but needs to be extended to cover arrays of
function, for example, arrays of read/write lasers that are simultaneously and
independently accessing tracks on the disk.
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OPPORTUNITIES IN STORAGE AND DISPLAY
45
The key problem in read/write applications is the availability of a
reversible, high-contrast material. Improvement of existing materials must be
vigorously pursued. The photo-induced change in optical reflective properties
must occur rapidly and be stable for long periods of time without degradation.
Finally, it must be possible to repeatably read and write the memory without
degradation. The challenge here is similar to that for the optical logic
applications: a large optical change triggered by a small optical impulse (a
large optical nonlinear effect) is desired to clearly delineate two optical states.
The problem is compounded by the need for large areas of this nonlinear
material (compared to a logic chip), but the regions do not have to be interac-
tive or unbred. Currently available systems are based on funs that can be cycled
between stable and metastable phases with accompanying reflectivity or
Magnetic Kerr effect changes. The quality of such funs is limiting current
applications, and significant advances in the storage media would have a major
impact on read/write technology.
Key Research Areas
· A longer-term need is the advance of laser technology, particularly
small semiconductor injection diode lasers. Here a key development is the
migration of lasers to shorter wavelength, either by new laser materials or by
frequency doubling of existing lasers. Basic materials research is needed in
nonlinear optical materials for frequency doubling, with emphasis on com-
patibility with low-mass read-write heads.
In the area of storage media, basic materials research is fundamental
to future development. In this context, a variety of other phenomena have been
explored for optical information storage. These include photochemical hole
burning, holographic storage, and ferro-electric polarization-change storage.
Although attractive in some respects, photochemical hole burning requires
ultralow temperatures for long-term stability. Holographic storage mechanisms
maybe of considerable importance in image processing (discussed in Chapter 3)
but may not be competitive with the already high information storage density
capability of optical compact disks. Ferroelectric ceramic materials may
become important if the recent rapid progress continues in developing thin-f~
materials with good storage characteristics and low voltage requirements.
However, these materials also appear to be of greater importance in image
processing due to the inherently fast read/write speed of this medium. While
all these areas are long-shot technologies, research into new methods of optical
storage should be encouraged. All the current methods, as illustrated in
Table 4.1, utilize thermal heating by a laser beam for writing information, and
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46
PHO TONICS
this inherently requires appreciable power. Other approaches based on
photonic effects in materials should be possible, for example, an absorbed
photon causing either a photo-induced charge-transfer reaction or a photo-
induced change in molecular conformation leading to a metastable change in
optical properties. One might hope that other phenomena based on photonic
effects in materials would lead to significant improvements in read/write head
requirements or media stability.
· The ability to store exceedingly high densities of information in archival
systems is coming and may lead to conceptually new applications. Advances
may depend critically on nonhardware factors such as storage system design
(software and architecture) and the storage-computer system interface. New
architectures are needed to organize and access vast quantities of information
at a high rate of speed. As software and application development has histori-
cally occurred at a slower rate than development of hardware, it is important
to encourage early work in this area by sponsoring seed research and by
supplying early prototypes to system designers.
OPTICAL DISPLAYS
Importance of Optical Displays
Displays take electronic information and convert it to images and text for
human viewing. Since displays are viewed, they necessarilyinvolve optics. How-
ever, their operation is better described by how they are addressed or written;
in this sense they can be either electronic or photonic. The commercial market
in displays was about $7 billion in 1986 and is projected to be over $9 billion by
1990. The market is dominated by the cathode ray tube (CRT) display, which
is currently at about 75 percent of the market share. There is a growing market
in flat panel display technology, driven by the desire for compact, low-voltage
(light-weight, low-power-consumption) displays.
Today's displays are addressed primarily by electrons and hence are
electronic displays. Examples are the beam-addressed CRT and the matrix-
addressed flat panel displays (Figure 4.2~. Photonic displays are addressed by
light beams. An example is a display where a laser beam writes on a liquid
crystal cell (thermal writing process). Although U.S. and European corpora-
tions are involved in research and development and commercialization of
displays, the main effort appears to be from the major Japanese electronic
corporations. One reason for their dominance may be that the Japanese
internal market is also the largest in the world.
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OPPORTUNITIES IN STORAGE AND DISPLAY
Addressing
Hard Wired ~ I
r r I
Beam
Production Type of
of Light Beam
Nonemissive | Emissive Photon | Electron
1
Liquid Crystal Light-Emitting Diode Laser-Liquid Crystal Cathode-Ray Tube
Electrochromic Electroluminescent
Electrophoretic Plasma Panel
FIGURE 4.2 Examples of beam-addressed and matrix-addressed displays.
Display Status
Flat Panel Displays
47
The major activity in flat panel displays today is in electronically matrix-
addressed panels. A voltage is applied to each picture element either to induce
light to be emitted (emissive) or to cause a change in optical transmission or
reflection of light from an external light source (non-emissive). As these
displays are not "photonic" in the above defined sense, their characteristics and
status will be covered only briefly.
The relative advantages and disadvantages of some representative display
technologies are summarized in Table 4.2. A more detailed description of these
displays was given in a special issue of IEEE Spectrum (1985~. The liquid
crystal display, matrix addressed, is the dominant flat panel technology (at about
a 15 percent share of the total display market in 1987), primarily in applications
that cannot use CRTs. Earlier problems of slow switching speed and low con-
trast have been solved by active matrix elements involving thin-film transistors
or diodes in conjunction with the liquid crystal at each picture element. High
(30:1) contrast is now possible at video refresh rates and large viewing angles.
The challenge remaining is to manufacture such displays at costs competitive
with those of CRTs in order to penetrate the higher-volume market. In
addition, new materials such as ferroelectric liquid crystals, which can respond
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48
TABLE 4.2 Relative Advantages and Disadvantages of
Representative Display Technologies
Type Advantages Disadvantages
Beam-addressed
-
Cathode ray tube High resolution High voltage
Good addressability Large depth
High contrast High ambient lifetime
Flexibility Corner edge focus
Color capability circuitry
High luminous efficiency High maintenance cost
Heavy
Emissive flat panel
Light-emitting diode Extremely fast Short persistence
High resolution Poor luminous efficiency
Rugged Brightness uniformity
Reliable High peak currents
Low voltage No blue light
Expensive in large arrays
Electroluminescent display Rugged Moderate luminous
High contrast efficiency
Inherent memory Moderate luminance
Expensive drivers
Non-emissive flat panel
Liquid crystal display
Electrochromatic display
Passive display crystal
Memory possible
Very high resolution
No contrast loss in ambient
light
Passive display
High contrast
Inherent memory
Low switching voltage
External illumination
required
Temperature range
Addressing, multiplexing
Viewing angle
Contrast limitations
External illumination
required
Difficult to matrix-
address
Electrode and electrolyte
stability
Slow switching speed
PHO TONICS
more rapidly to external fields and can exhibit memory, could have a significant
impact on a variety of applications for this display technology.
Plasma panel displays and vacuum fluorescent displays, utilizing a gas dis-
charge to emit light from a picture element to the viewer, represent a slowly
growing market (from 8 percent of the market in 1987 to 9 percent in 1990),
primarily in specialized, high-information-content displays.
Electroluminescent (EL) panels are also present in the marketplace.9~0
Materials research directed toward improved EL efficiency, better reliability,
and full color would benefit this technology. A common problem in the "gas"
displays is the high cost of driver electronics.
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OPPORTUNITIES IN STORAGE AND DISPLAY
Photonic Displays
49
Laser beams have been used to write on liquid crystals or photoconductive
media to produce images, much as an electron beam writes on a phosphor in a
CRT. Large-area and high-resolution displays have been demonstrated in
specialized applications.
Interactive Displays
The interaction between the display and its viewer is distinct from how the
display is made or addressed but may be impacted by photonics. An example
is the eye-controlled display, where the orientation of the viewer's eyeball deter-
mines the display's cursor position. Although the concepts and necessary
technology are available, a successful product has not emerged. An exciting
application would be a variable resolution display, where the portion of the
display being focused on by the user is high-resolution (with greater information
content), while the peripheral region has low resolution (minimum information
density displayed).
Key Enabling Technology Needs
The key technology developments needed in displays are in the flat panel
electronic displays. The panel does not currently see any key enabling
technology elements in photonic displays that would allow them to displace
existing electronic display technologies or open new fields of application.
Key Research Areas
There should be continued research in new materials and subsystem con-
f~gurations, which might lead to a photonic display that is superior to its
electronic counterpart in the long term. Potential advantages of using photonics
could result from the fact that light beams do not interact, leading to novel mul-
ti-beam addressing schemes. In addition, new photoexcited display media may
have higher resolution or image-storage capability. The promise of the
hologram as the key element of a three-dimensional display remains a
tantalizing goal. New concepts are needed to advance photonic displays if they
are to become competitive. Some of the technologies developed for optical
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50
PHO TONICS
information processing or optical storage may be applicable to photonic
displays.
REFERENCES
Chi, C. S. 1981. High density-for disk memories. IEEE Spectrum 18:39.
Bartolini, R. A. 1982. Optical recording: High density information storage
and retrieval. Proceedings of the IEEE 70:589-597.
Bruno, R. 1987. Making compact disks interactive. IEEE Spectrum
24(November):40-45.
4. Freese, R. P. 1987. Erasable optical disks. IEEE Spectrum 23 (Feb-
ruary3:41-45.
Bartolini, R. A., A. E. Bell, R. E. Flory, M. Lurie, and F. W. Spong. 1978.
Optical disk systems emerge. IEEE Spectrum 15(August):20-28.
White, R. 1980. Disk-storage technology. Scientific American 243
(August):138-148.
7. 1987. Electronic Display World (7~:7-8. Stanford, California:Stanford
Resources, Inc.
8. IEEE. 1985. Special Issue on Display Technologies. IEEE Spectrum
22:52-67.
9. Tannas, Jr., L. E. 1986. Electroluminescence catches the public eye. IEEE
Spectrum 23~0ctober):37-42.
10. Manuel, T. 1987. The picture brightens in flat panel technology.
Electronics 60~11~:55-58.
11. Levine, J. L. 1984. Performance of an eye-tracker for office use.
Computers in Biology and Medicine 14~1~:77.
Representative terms from entire chapter:
flat panel
