Optical data storage systems employing large varieties of different recording materials have been investigated in great detail over the last few years. This paper will review most of the known classes of optical recording media and identify materials with potential interest for specific applications.
High-resolution electrophotographic recording media have a number of characteristics which make them attractive for holographic data storage and laser recording. Exposure sensitivity, resolution and storage capacity are in some cases comparable to Lippmann photographic emulsions. We have evaluated the recording and storage performance of several electrophotographic recording media: Scott Graphics TEP film (absorption medium) and Kalle AG and Scott Graphics photoplastic recording films (phase media). The Scott Graphics photoplastic recording films were created by adding a 1 μm thick layer of thermoplastic to Scott Graphics P5-003 and Laser Recording TEP films. We measured various sensitometric and data storage parameters, particularly diffraction efficiency and signal-to-noise ratio, and operational properties such as performance consistency, image permanence and recyclability. Experimental data showed a broad range of exposure sensitivity, resolution and storage capacity and revealed a number of problem areas. Nonetheless, these recording materials appear promising for some holographic and laser recording applications.
An experimental and analytical study of hole formation in metal films is reported. The threshold for hole formation was found to be melting. Three time constants relating the film and substrate parameters are defined which relate sensitivity to exposure pulse duration. The use of metal alloys is also discussed.
IR laser heating for thermal development and erasure of data is presented with results showing holographic diffraction efficiency, signal to noise ratio, thermoplastic surface condition, recording area size and IR energy requirements.
General characteristics of photopolymer imaging systems are reviewed with comments on applicability for laser recording. Two imaging systems are emphasized in detail; namely, a Dylux® proofing paper material which produces a visible dye on exposure to UV and an experimental holographic photopolymer. A review of the characteristics of these materials is presented. Formulation variations for the color forming Dylux® are noted which have potential for improved laser recording response. After reviewing exposure procedures and diffraction characteristics of the experimental holographic photopolymer, applications and comments on potential new developments are given. Finally, a brief description of a new 'KTP' crystal-potassium titanyl phosphate - is described which is ideal for frequency doubling neodymium lasers.
Laser recording and readout measurements using the photodichroic crystal KF:LiF indicate that the material has potential as an input transducer and an adaptive spatial filter in an optical spectrum analyzer, These crystals are among the most photosensitive recording materials without gain, can be produced in large sizes at relatively low cost, and can be operated close to room temperature using thermoelectric coolers. Experiments using a high precision laser scanner providing 96.6 MHz subnanosecond pulses at 514 nm show that adequate recording can be accomplished with exposures less than 10 mJ/cm2. An MTF of 40% at 72 ℓ/mm was measured with uniform readout illumination of the recorded signal, but this is reduced if the signal is recovered by rescanning due to the convolution of the scanning beam profile with the recorded spot profile. The readout is destructive at the recording wavelength but can be readout with a much reduced decay rate by using 488 nm. The linear dynamic range in the Fourier plane was measured to be 40 dB.
The overall performance of laser imagery transmission systems is often determined by the recording material used to pro-duce hard copy. Baseline recording material specifications frequently include an exposure sensitivity of about 200 ergs/cm2, spectral sensitivity matched to a high reliability laser, and resolution greater than 250 lpi. An inline dry-processing capability is generally a key requirement. Dry silver film and paper, such as the panchromatic 3M Type 7771 paper and Type 7869 film and the orthochromatic Kodak Ektamate 150 paper, currently best satisfy these needs. As part of an ongoing effort, we have experimentally characterized the 3M and Kodak dry silver materials. We have measured sensitometric and information recording properties as a function of development conditions and investigated operational characteristics such as shelf life, image permanence and environmental sensitivity. Experimental data in the form D-log E, CTF and AIM curves will be pre-sented. In addition, problem areas common to dry silver materials will be discussed. A general conclusion of our studies is that dry silver films and papers are satisfactory for the majority of laser imagery transmission applications, but that they require several improvements.
High density storage of data in photosensitive electrooptic crystals has been achieved. One of the crystals with the highest writing sensitivity is LiNb03 doped with iron. A key factor in its high sensitivity involves the nature of the charge transport in the crystal. A series of measurements has been carried out to study the charge transport by photocurrent measurements and electrooptic measurements. The two techniques are shown to give consistent results. When described in terms of an equivalent internal electric field a dependence on light intensity is required.
Two new developments in the thermoplastic-photoconductor medium technology for optical recording are reported. The first concerns a method for utilization of the medium for bit-serial recording which expands the range of potential applications of the medium. The second concerns a unique medium configuration which leads to improved performance in terms of higher diffraction efficiency, long latency between exposure and development and most importantly, a significant reduction of frost noise.
Performance of transparent electrophotographic films is discussed in terms of physical and sensitometric properties as well as RMS Granularity, Detective Quantum Efficiency, AIM and MTF curves. Data is presented on most of these properties and it is shown that such are utilized in viable commercial information recording systems.
The proprietary product of Coulter Information Systems, designated KC-Film, will be described in terms of physical makeup, operating characteristics and areas of application. KC-Film is a crystalline, transparent photoconductor made up of semiconductors so tiny that the layer contains 100 million to the square millimeter. The surface voltages utilized with KC-Film are very much lower than normally found in electrophotographic recording work. This results in the ability to produce extended grey scale continuous tone imagery without edge effects. KC-Film has sensitivity comparable to high resolution silver halide films and has spectral sensitivity throughout the visible region. The application possibilities demonstrated to this time include high resolution continuous tone imagery, offset, printing, high speed contact printing, microfilming, plain paper copying, and multi-megacycle laser recording. Presently available coating capability allows production of films up to 32 inches wide by 4000 feet long.
We discuss the development of an optical disc recorder with an on-line storage capacity of 1010 bits. Recording is done by laser machining of micro-sized pits in a thin tellurium film allowing direct-read-after-writing (DRAW) of the information. The recording format maps each bit of information to a recorded pit. The tellurium is deposited on a clear plastic disc configured to be self-protecting and easily handled. A major program objective was to demonstrate the feasibility of an inexpensive recorder and disc.
A summary is given of recent work on laser-addressed smectic liquid crystal projection displays. Details of the cell construction are given along with some considerations on optimizing contrast ratio. Two display demonstrations are described. The first employs a GaAs laser, has 500x500 picture element capacity, and operates at keyboard-entry speeds. The second demonstration uses an argon laser, has 1024x1024 picture elements, two color capability, and can be written in several seconds.
Thin film evaporation and sputtering methods in conjunction with photolithographic fabrication tech-niques provide a simple means for producing useful electro-optical devices. Multilayer Cd1_xZnxS (CZS) compound photoconductive film deposited with rf sputtering exhibits a spectral response which can be varied between 360 nm to 530 nm. The CZS films exhibit a rise time of l μ sec, have good optical transmission characteristics, exhibit a photoconductive gain of approximately 10, respond to 400 mw laser pulses in a 1 mm spot and provide film breakdown strength of 30 x 104 Volt/cm. Multilayer CZS films exhibit less photoconductive gain and higher breakdown strength than single layer films. Sn doped indium oxide (ITO) films are produced with magnetron rf sputtering. The conductivity and transmissivity of the ITO films are critically influenced by the amount of free oxygen present during the sputtering deposition. An O2 partial pressure of 4 x 10-5 results in greater than 80% transmissivity at 600 nm with a bulk conductivity of 100 mhos/cm. The film transmissivity can be substantially increased in the visible spectrum by post heat treatment in an air oven at 300°C for 20 minutes. A scanning cursor device for graphical data extraction was fabricated using various thin film techniques.
A new photorefractive effect, resulting from the interaction of high-intensity, short-duration laser pulses with propagating acoustic waves, has been found in LiNbO3. The acoustic wave pattern is stored as changes in the index of refraction (δn). The δn is proportional to the rf amplitude, and increases sublinearly with the number of laser pulses and as the 1.3 power of the incident laser energy density. The decay time varies from a few hours, when only green illumination (530 nm) is used, to several weeks when combined green and infrared illumination (1060 nm) are used. The On can be erased by exposure to ultraviolet radiation or by annealing at 250°C. This acousto-photorefractive effect has been utilized to construct an acousto-optic memory correlator wherein both the stored On and the On produced by a "live" propagating acoustic wave simultaneously modulate a low-power cw laser beam. The resultant detected signal is proportional to the correlation integral. The memory correlator operated at a center frequency of 10 MHz with a 1-MHz bandwidth. A large variety of complex signals, such as chirps and Barker codes, was stored and subsequently correlated. The stored signal strength is about 30 dB below that of the original live signal. Successful correlation with a live signal was achieved several weeks after storage.
Some aspects of copying equipment technology are described as bearing on the use of powder or liquid toner systems. Problems inherent to both systems are described. Literature references are made to be used as a basic introduction to current approaches in the use of powder or liquid toners. These approaches suggest future trends.