Date of Award
Doctor of Engineering Science in Electrical Engineering
Raj Pratap Misra
This dissertation proposes a novel form of optical ROM for the digital computer. This memory makes use of holographic techniques because of the advantages offered by holographic memories over conventional optical ones, especially with regard to dust and scratch sensitivity.
These advantages have long been recognized by workers in the field, indeed several holographic optical memories have already been proposed.
A survey of the state of the art of digital computer memories is offered in the introductory part of the thesis. Particular emphasis is placed on optical memories. A brief review of basic holographic theory is also included for reference.
As shown by the above survey random-access holographic ROM's to this day characteristically offer access times of 1 microsecond or more. This limitation is imposed by the response time of the devices used to deflect the read-out laser beam in order to effect access to the various page addresses.
This dissertation proposes to bypass this difficulty be using the radiation from the phosphor of a CRT tube, rather than laser radiation, for holographic readout. This proposal is based on the hypothesis that a viable random access holographic computer memory can be designed so as not to require for readout the high degree of coherence characterizing laser radiation. The advantage of utilizing this specific technique is that current CRT technology, having produced phosphors with nanosecond decay times and deflection circuitry with frequency response in excess of 500 MHz, can be used to reduce the read-out time by a factor of one hundred from that noted above.
To verify this hypothesis, a memory system that employed the CRT as the source of read-out radiation is designed. A mathematical model of this system is formulated from the basic principles of physical optics, taking into account the non-point and non-monochromatic characteristics of the CRT source. The model is meant for digital computer simulation of the system's behavior in order to determine feasibility and optimize system parameters.
A conventional mathematical model, unfortunately, proves to be inadequate in practice because of the excessive computation time required. A second model is formulated based on a less conventional approach, namely on a physical and geometrical interpretation and consequent approximation of various elements of the system.
On the basis of this model a simulation of the system is performed and the parameters of a CRT ROM capable of storing 105 bits per square centimeter of film are determined.
The holograms required to implement a memory at such a density cannot be practically produced by optical means. It is shown, however, that production of such holograms can be achieved by means of a computer synthesizing technique.
The design of an appropriate synthesizing system is described. This system uses computer-generated pulse duration modulation on the z-axis input of a CRT to produce the desired hologram.
Practical feasibility of this approach is investigated and proved experimentally by producing test holograms and verifying their information storing ability.
As part of the optimization study of this system the effect of interbit diffraction interference on the bit-to-bit read-out signal non-uniformity is investigated. This non-uniformity, which is not peculiar to the CRT system but constitutes a problem shared by most optical memories, results in output data errors.
A qualitative analysis shows that a tilted rectangular aperture can be selected so as to cause less diffraction interference than is caused by a circular one.
A quantitative theoretical analysis of the phenomenon is presented. For the sake of mathematical convenience this analysis is limited to circular apertures. The relationship between diffraction interference and aperture size is shown to be non-monotonic and to possess local minima of significant depth.
Experimental verification of the above theoretical results is described. These experiments show the presence of other sources of readout non-uniformity and error. In the case of circular apertures the non-uniformity due to interbit diffraction interference is shown to be dominated by the effect of other sources of disturbance whenever the aperture diameter is larger than 50 mils.
Computer synthesized holograms, as described in this dissertation, are shown to present some advantages in this respect.
Barrett, William A., "Synthetically-generated holographic memory system utilizing partially coherent light" (1974). Dissertations. 1295.