A variety of applications for optical disc storage technology which share a common thread of technology are identified. Each of the applications is characterized in terms of relative importance of the optimum costs, features, performance, and data types. The similarities and differences between the various applications are highlighted.
This paper describes an experimental computer controlled digital optical disc memory system that was designed and built to track the technology, assess issues, and provide answers to a host of application oriented requirements. The system uses gas lasers for writing and reading. Unique features of the system are simultaneous eight channel parallel readout, write channel addressing and a readout channel incorporating a single custom photodetector array for extracting data as well as focus and tracking error signals. The problems of media sensitivity and archivability are discussed and sensitivity measurement techniques are described with typical examples shown.
Xerox Electro-Optical Systems is developing an information storage and retrieval system for the Library of Congress to store a data base consisting of seven million library cards. The library card image will be digitized, stored, and retrieved by a computer system and printed out on a Xerox 9700 high speed laser printer.
An Optical Disc Memory System has been developed. This system is designed for document storage and retrieval by analysing the memory system parameters, including data format and signal processing, linear actuator and access sequence, statistical error characteristics in the storage medium, and so on. The storage capacity is more than 10,000 letter-size documents on one side of a 300 mmØ disc. Every document can be randomly accessed in less than 0.7 second. System erorr rate is on the 10 -8 level.
RCA has been working on digital data storage applications of optical discs since 1976. The basic optical technology utilizes ablative recording on laser sensitive materials, providing immediate playback of the stored data. Recently, complete systems have been built which fully establish the feasibility of digital optical disc storage for high data rate (beyond 200 Mb/s), high storage density (5x1010 bits/disc) and low bit error rate (<10-8). This paper briefly describes the hardware which has been developed. The main theme of the paper is the reporting and analysis of the test results that are being derived from this hardware.
A compact optical still video recorder using Te0x(x÷1) thin film disc and a laser diode as the light source has been developed for video signal recording and playback in a real time mode. The recorder is capable of storing up to 15,000 still pictures on one side of a pregrooved disc, 200 mm in diameter with an average random access time of 0.5 seconds for any desired frame. The reflectivity of TeOx thin film recording layer which is deposited on a pregrooved plastic substrate by evaporating Te and TeO2 compound varies with absorption of irradiated thermal energy of focused laser beam. Carrier to noise ratio greater than 55 dB were achieved at incident power level of 7-9 mW, which was focused by an objective lens having numerical aperture (N.A.) of 0.5. Signal to noise ratio of more than 43 dB and resolution of more than 260 TV lines of playback video signal were obtained.
A novel optical head for digital audio disks has been successfully developed. The head consists of a GaAlAs laser diode, a quadrant PIN photodiode and a single-beam optical system for focusing and radial tracking. All of the data (RF), focusing, and radial tracking signals are derived from a single quadrant PIN photodiode. Focusing and radial tracking servocontrol of the head are characterized by two-dimensional actuation of an objective lens. The detection method of the error signals and their dependence upon both the movement of the objective lens and the tilt of the disk are described.
Relationships of laser diodes' (LD's) noise and reproduced video picture quality are experimentally studied. Single longitudinal mode LDs usually gave acceptable quality, but when these LD's oscillating mode transit to another state through quasi-multi mode, bright spots randomly appeared on the picture. When using multi longitudinal mode LDs, the picture quality were slightly poor and unacceptable, but did not show drastic degradations at any temperature. It was found that good quality of the picture required the LD's RIN to be less than 10 -14 level.
The principal thrust of the optical data storage industry to date has been the 10 billion bit optical disc system. Mass memory has been the primary objective. Another objective that is beginning to demand recognition is compact memory of 1 million to 40 million bits--on a wallet-size, laser recordable card. Drexler Technology has addressed this opportunity and has succeeded in demonstrating laser writing and readback using a 16 mm by 85 mm recording stripe mounted on a card. The write/read apparatus was developed by SRI International. With this unit, 5 micron holes have been recorded using a 10 milliwatt, 830 nanometer semiconductor-diode laser. Data is entered on an Apple II keyboard using the ASCII code. The recorded reflective surface is scanned with the same laser at lower power to generate a reflected bit stream which is converted into alphanumerics and which appear on the monitor. We are pleased to report that the combination of the DREXONTM laser recordable card ("Laser Card"), the semiconductor-diode laser, arrays of large recorded holes, and human interactive data rates are all mutually compatible and point the way forward to economically feasible, compact, data-storage systems.
This paper describes a computer simulation model of an optical disc memory. Read/Write system modeling of an optical memory storage device must include all parameters that affect pit formation (the write process) as well as the quality of the playback signal (read). The model includes parameters for the read and write electronics, optics, and the media. It will be shown that the model can incorporate specifics such as acousto-optic modulator rise times, lense aberrations, and pit size variation. Results of a sensitivity study for one variable will be presented.
The optical memory servo control system performs five major functions. They are micro-processor control, disk spindle speed control, focusing, seeking and radial track-following. Preliminary laboratory results show that an average seek time of 100 msec and one track seek of 300 μsec were achieved over a total of 40,000 tracks. Radial tracking with an error less than 0.1 μm was obtained through a tracking servo. These radial tracking and random access capabilities were achieved by combining the use of a specially designed galvanometer and a translator carriage. Normal operating focus error of less than 0.1 μm was also obtained. Finally, the overall servo control system is greatly enchanced through a microprocessor controller with an adaptive control algorithm. This provides high flexibility to the servo system design in order to meet the stringent performance requirements of an optical memory system. A mathematical model of the complete servo system was developed. The analytical information was used to design and fabricate a laboratory model optical memory device.
An optical focus servo system has been designed for use with an Optical Disc Mass Data Storage system. The servo has been designed for optimum accuracy of focus control with minimal circuit components and including several features convenient to this application. The servo system exhibits correctibility of 54dB at the fundamental rotation frequency of 30 rev/sec and a system bandwidth of areater than 2 KHz. The discussion centers on the requirements of this system and emphasizes the developmental analyses, approaches and results of research in this area. The optical, mechanical and electrical considerations of the system are discussed in some detail concluding with the final design and projected views of more advanced approaches and applications.
Image recording with laser and optical disk is described. In order to use semiconductor laser sources with relatively low output power, optics with high collection efficiency was used to couple the elliptical output beam and transformed it to match the circular reading beam. Optical disk with multilayer structure provided very high recording sensitivity to further reduce the laser output power requirement.
This paper describes the design and characteristics of a three dimensional actuator for an optical readout head. In order to obtain enough servo gain, it is necessary to reduce the size and weight of the moving parts. Instead of the conventional galvano mirror, this actuator drives the objective lens, which is suspended in the collimated laser beam, towards three directions, vertical, radial and tangential, without tilting. The major advantages of this unique three dimensional actuator lie in reducing a cost of the objective lens and the number of the adjusting points.
A composite video signal consists of the luminance signal, which carries information about brightness, and the chrominance signal, which carries information about colour, hue and saturation in the object. In this paper a new method for chrominance signal generation based on binary data is presented. The problem considered here originates from the video disc system; the solution can be used for any TV system, including the broadcasting system. The chrominance signal generated according to the proposed method can be combined with the luminance signal in order to form a composite video signal, no matter whether the luminance signal carries information about brightness or is generated based on binary data.
Modern magnetic disk systems are provided with error detecting and correcting (EDAC) systems that are capable of correcting up to 12-bit single burst errors. Development of Optical Disks with densities of 25,000 bits per inch and 10,000 tracks per inch compared to 4,000 bits per inch and 200 tracks per inch for magnetic disks, requires more powerful EDAC system to maintain and possibly improve reliability and performance. This paper presents a comprehensive scheme for error recovery that can be used for Optical Disks. The scheme features a double code with interleaving and capable of correcting any single burst error of length 100 bits or less. The first code is a cyclic code which is used for detection only. The second code is a Fire code and is used for correction. Codes design, interleaving depth selection, encoding, decoding, error correcting techniques as well as implementation are presented. System simulation and the results of the simulation are discussed.
Projections of short wavelength semiconductor laser performance are based primarily on historical charts covering progress in eight different aspects of laser technology. Most of these improvements are common to lasers of all wavelengths, but some may have special relevance to the short wavelength limits. Examples are metal-organic chemical vapor deposition and molecular beam epitaxy methods of crystal growth. However, to date, the short wavelength limits of reliable lasers have been set within the "traditional" technology of liquid phase epitaxy in the GaAlAs materials system. Fairly reliable projections are made in this system to 1985 and less reliable projections beyond 1985 in other III V materials systems. Some discussion of the possibility of lasing at short wavelengths in other material systems is included.
Optical disk technology today finds widespread application in VLP, CD and DS equipment. The required specifications of lenses for such systems are stated. Types of lenses which meet these requirements are discussed. Their image quality is given by RMS of wave aberration, by point spread function especially by encircled energy or by MTF. Due to the automatic servo focussing technique a certain amount of field curvature can be tolerated. Methods of measuring the image quality of such lenses are shown e.g. examination of point spread function, interferometric measurement of wave aberration and MTF measurement. The results are compared with theoretical values.
The introductory remarks show that the properties of light sources and discs require the use of objectives similar to those used in microscopes. Conventional lasers and semiconductor lasers have a Gaussian intensity distribution. Bearing this in mind the ideal intensity distribution is calculated and the results are shown as "light mountains". Finally, the influence of aberrations and tolerances is discussed.
An investigation was made of the possibilities of optical recording on bubble-forming metal/thermodegradeable-plastic bilayers incorporated in front-surface trilayer devices. Two distinct types of trilayer device were chosen for study. It was found that a device consisting of a 14 nm-thick metal layer, a 200 nm-thick thermodegradeable-plastic dielectric layer and a 100 nm-thick aluminum layer had promising properties. That is, a power of only 6.5 mW focussed into a 0.8 μm spot at 442 nm (the helium-cadmium laser line) was required to write a 5 MHz-pattern with 50% duty cycle. This pattern is observed with high contrast at 633 nm (the helium-near laser line), where the initial reflectivity is 0.9, and it provides signal to noise in a 30 kHz bandwidth of 45 dB. The trilayer device is also calculated to have the same writing sensitivity at 830 nm (the gallium arsenide laser line).
Bubble-forming media are an attractive method of achieving high density archival information storage via optical direct-read-after-write (DRAW) techniques. Several parameters are important in the characterization of DRAW media, including sensitivity, recording latitude, defect density, and environmental stability. The purpose of this paper is to report these and other media characteristics for bubble-forming optical recording media.
Hydrogenated semiconductor films have been studied for their application as possible optical recording media. We characterize three different recording processes, which are based on hydrogen evolution in amorphous (a) hydrogenated semiconductor materials without melting. Bulges (or bubbles) of ≤μm diameter are created in a-Si:H. The written marks in a-Ge:H resemble a circular sponge-like microswelling. At higher intensity, ablation without melting is occurring in a-Si:H, where holes with sharp edges are produced. These recording materials could be used for low cost archival disks, which would be also adaptable to replication.
We are developing new media at Burroughs. The preliminary test results are very encouraging, and our media appear to have better sensitivity and lifetime than the existing Te media. The media are compatible with an overcoat.
A new recording medium for optical disks was investigated. The new medium, called Te-C film, is characterized by a high sensitivity and stability, compared to Te films. This paper presents the optical recording characteristics for Te-C films on PMMA substrates and its stability under stressed conditions. Then, the high sensitivity and stability origins are discussed.
Thin films of SbOx, GeOx, TeOx, MoOx, InOx and PbOx ( x is smaller than the stoichiometric value for each component ) were found to have a feasibility to be applied to an optical recording disc. Upon heating, these thin films are critically changed in their optical constants at 150°, 280°, 120°, 150°, 120° and 100°C, respectively. The contrast ratios before to after heat treatment of the GeOx, TeOx and MoOx films of 1000 - 2000 Å thick are very high, they are 50, 20 and 15, respectively. Among them, the TeOx (x ≈1.1) film was excellent in both the sensitivity and the contrast ratio, and an optical recording disc was prepared by depositing the TeOx on a PMMA disc, substrate. Video-signals can be recorded and played back using a laser diode (λ =8300 Å, N.A.= 0.5) at the laser power of 6 - 8 mW incident. The carrier to noise ratio at 5 MHz was in excess of 55 dB.
A class of optical storage media is described which consists of thin films, microscopically textured by reactive ion etching. These surfaces have random arrays of decoupled columns or cones with cross-sectional dimensions less than 100 nm. This structure yields a spatially varying, graded refractive index resulting in low specular reflectance for visible wavelengths. Reflective spots ~1 μm in diameter have been produced on the surfaces by local melting with a low power laser beam. This has been demonstrated for highly stable materials such as Si and Ge with threshold energies as low as ~1 nJ. The fabrication, microstructure, writing parameters and encapsulation of the media are discussed.
Amorphous rare earth-transition metal films as media of magneto-optical memory are described. Improvements of amorphous GdCo film in stabilizing recorded bits and in making uniform film were performed. High speed recording and reading experiment were carried out on GdCo disk of 150 mm in diameter, and excellent stability of recorded bits, considerable uniformity, and high sensitivity for recording were confirmed. Thermal stability and spectra of Kerr rotation angle of GdCo film were also measured. It is shown that GdCo film is very promising material for magneto-optical memory.
The power spectral density of the read signal for a magneto-optical readout system based on the polar Kerr effect has been analyzed. A criterion for optimum performance deriving from the signal to noise ratio is then defined and it is shown that a quadrilayer disk structure which acts as an interference device for the enhancement of the Kerr effect is superior to other structures. Results of a thermal calculation for the quadrilayer are then reported. These results clearly indicate that the quadrilayer can be designed to match different levels of laser power available for writing.
A number of TbFe thin films on 2"x2" glass slides were evaluated as candidates for an erasable memory. Maximum bit density is better then 1μm center to center, limited by the resolving power of the optics. The homogeneity of the films is very good, at least on a microscopic level. The required incident power in 25 ns pulses for writing a 50% bit is in several cases below 5 mW. A variation of a factor of 2 in write power can be tolerated without an appreciable loss in either resolution or error rate. An externally applied biasing field, H, equal to one quarter of the value of coercivity, Hc, is adequate both for writing and for erasing a bit. As fields of the order of 0.6 Hc did not affect the signal, it appears that the tolerance of the system to variations in H is high. From the limited number of samples that have been measured, one can only make educated guesses about optimum values of these parameters. It seems that the maximum in performance lies somewhere in the 100-1000 A and 800-3500 Oe range. However the maximum is very broad, so that probably anything in this range should be adequate.
Metallic optical recording layers such as Te are degraded on exposure to oxygen and water. Both oxygen and moisture are necessary for the oxidation of Te to proceed at a significant rate. In this report, the feasibility of protecting thin metal layers from oxidation is examined in light of the known vapor permeability characteristics of selected organic polymeric materials. Experimental results are presented for multilayer Te structures.
To establish the high density disc system, the most important point is how discs are produced stably on high rate of yield. From the point of this view we developed a new laser cutting machine of master disc and a novel duplicator of replica disc. This cutting machine which is compactly designed has a improved optical system which are stabel against temperature fluctuation and other environmental disturbance. And we can produce high quality discs at high rate of index time by our disc duplicator using liquid photo-polymer resin which is cured by ultra-violet ray.
Laser beam is focused on a rotating disk. As the reflected light intensity VDC is modulated by irregularity of an optical disk surface, noise VAC in the VDC is observed. Effect of the irregularity of the optical disk surface is given by S/ND = 20 log(VDC/2√2VAC). If the noise is not increased in recording and the reflected light is modulated 100% by the carrier signal in playback, carrier to noise ratio C/N reaches this S/ND. The carrier signal was recorded on an Al-Cr thin film (observed S/ND was 81 dB) on the glass substrate (S/ND was 87 dB), obtained C/N was 69 dB.
One of the key elements of an optical storage system is the recording medium. An important part to the successful development of media technology are the various techniques for characterization of the media. In this paper, several of the techniques are described. It is shown that useful information concerning the performance of the media can be derived without actual recording and playback. These techniques are obviously important when the recording technology is based on a non-erasable, write-once, read-only type of media.
For high density optical recording requiring a high numerical aperture objective, it has been found that substrate incident recording suffers from spherical aberration because of substrate thickness variations. To overcome this problem a novel protective cartridge-enclosed disc has been developed which provides the advantages of air incident recording without the presence of spherical aberration. This cartridge protects the disc from atmospheric dust and virtually eliminates the possibility of internal dust generation both during operation and during storage. In addition, all components, including dust seals, have an expected life of at least 10 years. Key optical and mechanical considerations are discussed as well as experimental results obtained from an operational prototype.