GB2236418A

GB2236418A - Holographic logic device

Info

Publication number: GB2236418A
Application number: GB8922087A
Authority: GB
Inventors: William Morton
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-09-30
Filing date: 1989-09-30
Publication date: 1991-04-03
Also published as: GB8922087D0

Abstract

Two or more holograms are superimposed in such a way that when illuminated by beams of coherent light at specific angles to the medium bearing the holograms, the resulting output beam obeys the Boolean logic functions OR, and/or AND. The output beam could either be plain or could contain an information bearing image. The logic performing device can be made up of two holograms, wherein it will operate in binary mode, or by up to ten holograms, wherein it will operate in decimal mode. The holograms can either be produced by photographic means or by mechanical inscription or deposition on to a suitable medium. The medium can either be transparent or reflective.

Description

HOLOGRBPHIC LOGIC ELEMENT This invention relates to a device for perfoming logical functions using light beams and is based upon the principles of holography.

Conventional conputer systens are mlde up of a series of functional units performing such functions as addition, subtraction and comparison.

Bach of these units is in turn trade up from standard building blocks called logical elements. These logic elements perform very basic functions such as: The Boolean OR function where either one of two input signals generates an output. That is, either input signal A or input signal B gives rise to an output signal C.

The Boolean AND function where both signal A and signal B are needed to give an output signal C The Boolean AND/OR function which needs either signal A or Signal B or both to give an output signal C.

Conventionally logic elements are electronic conponents produced from semi-conductor or semi-insulator material. Although very considerable progress has been made in the development of the technology, these electronic devices are reaching their fundamental limits on size and speed of operation.

Attention is therefore being turned towards optical computers which use light beams instead of electrical currents for their operation. For such devices the fundamental limitations on their speed of operation are at least one order of nagnitude less onerous, According to the present invention there is provided a device for performing the Boolean AND and OR functions on two beams of light using the principles of holography.

According to a further embodiment of this invention there is provided a device for performing the Boolean AND and OR functions on multiple beams of light using the principles of holography.

A specific embodiment of the invention will now be described using as an example a two beam logical element.

The accompanying diagran refer, in which Figure 1 shows the principles of the production of a hologram Figure 2 shows the principles of the generation of an graze from a hologram.

Figure 3 shows the arrangement for producing a two beam logical element.

Figure 4 shows the operational arrangement for a two bean element.

The principles of holography and the taking of holograms are well established. Referring to Figure 1, a hologram is produced by recording on to a photographic plate 10 the interference pattern produced when two mutually coherent beams of.light 11 and 12 are overlaid at a slight angle to each other.

If light beams 11 and 12 are both plain, the interference pattern generated will be a series of lines perpendicular to the plane of the two beans. Such a pattern is known as an interference grating. The spacing between the lines of the interference grating is dependent only on the wavelength of the light used and the angle between the two beams.

If now a semi-transparent object 13, for example a photographic slide, were to be placed in the path of one of the beams, known as the object bean and labelled 12 in the Figure 1, the light falling on the photographic plate from this beam would no longer be plain but would be made up of a conplex pattern of subbeams.

Individual sub-beams would still interfer with the other beam, known as the reference beam 11, but the resulting interference pattern would no longer be regular and symetrical as each individual element of the object beam 12 would met the reference beam 11 at a slightly different angle. The resulting interference pattern would therefore be made up of a montage of individual interference gratings at different angles and with different line separations.

If now the photographic plate were developed and illuminated by a single plain beam of light. 21 at the same angle as the original reference bean 11 in Fig. 1, each individual grating would diffract that portion of the reference beam falling on it through an angle which depended on the spacing and direction of it'6 lines.

Assuming that the wavelength of the illuminating light is the sane as that used to produce the hologram, each grating will diffract a light beam in the same direction as the sub-beam that originally produced it. The nett effect is the reproduction.of a conposite beam of light that is made up of a series of sub-beams identical to the original object beam 12 used in the creation of the hologram. A person viewing this beam from the right, and looking through the plate as shown in Figure 2, would see an image 23 which would be an exact reproduction of the original transparency 13.

A second image 24 is generated on the viewer'; side of the plate and may be visualised by placing a screen in this position.

A hologram of a solid objects can be prepared in the same way by illuminating it with an object beam 12 and allowing the light reflected from it's surface to interfer with the reference beam 11.

The present invention makes use of one other property of holography. By carefully adjusting the exposure tines it is possible to record more than one hologram onto the same area of photographic emulsion. If each hologram is recorded with the reference beam flaking a different angle to the photographic plate, individual images can be reproduced by illuminating the developed plate at that same angle.

According to this invention in it's simplest manifestation, two holograms are recorded on the same photographic plate, as shown in Figurre 3.

In both cases the object beam c;r: is plain, that is does not have a transparent object placed in it, and strikes the photographic plate 30at the same angle 33. This angle is usually but not necessarily 90 degrees, that is the object beam is perpendicular to the plate.

To produce the first hologram a plain reference beam 34 is arranged to strike the photographic plate 30 at an angle 35. The photographic plate is then exposed to the resulting interference pattern for a pre-determined time.

Text, a second hologram is produced by maintaining the object beam in it's original orientation and arranging for a new plain reference beam 36 to strike the photographic plate 30 at an angle 37. The plate 30 is then further exposed to the resulting interference pattern for a pre-determined period.

The photographic plate 30 is then developed to produce the resulting hologram.

Refering to Figure 4, if now the developed hologram 40 is illuminated by a beam of coherent light 44 at an angle 45 to the plate which is the same as angle 35 in Figure 3, that is by a beam which is identical to the original reference beam 34, the result is a bean of light 42 emerging from the right of the plate which is identical to the original object beam 32.

If now-the developed hologram 40 is illuminated by a beam of coherent light 46 at an angle 47 to the plate which is the same as angle 37 in Figure 3, that is by a beam which is identical to the original reference beam 36, the result is a beam of light 42 emerging from the right of the plate which is identical to the original object beam 32.

The illuminating beams 44 and 46 are usually but not necessarily of the same wavelength as the beams used to produce the hologram.

By this means the requirements of a simple primary logical element performing the Boolean AND/OR function can be produced. That is, an output beam 42 results if the hologram is illuminated by input beam 44 OR by input bean 46, or if the hologram is illuminated by beans 44 AND 46 together.

The logical function OR, that is the above without the acconpanying AND function can be achieved by inserting what is known as a "half-wave plate" into either illuminating beams 44 or 46.

A half-wave plate is a piece of transparent material whose thickness is carefully controlled to be equal to an odd multiple of half the wavelength of the light in that material. The effect is to "slow" the light so that it is half a wavelength out of phase with a comparable bean that did not pass through the plate.

If these two beams then interact, the phase difference results in destructive interference taking place between them. If the beams are of equal intensity, total extinction occurs.

Applying a half-wave plate to one of the illuminating beans, output beam 42 is still produced when the hologram is illuminated by either beams 44 or 46 alone, but when illuminated by both, the half-wave plate causes one bean to be out of phase with the other and they therefore destructively interfer with each other and so do not generate the output beam 42. That is, the Boolean function OR/NAKD is produced where BAND is the shorthand notation for Not AND.

Similarly, if the original hologram where produced with both reference beans 34 and 36 simultaneously, the resulting developed hologram would not generate an output beam 42 when illuminated by either beam 44 or beam 46 in isolation. However, when illuminated by both beans 44 and 46 simultaneously, output beam 42 would be generated. That is, the Boolean function AKD/NOR is produced where NOR is the shorthand notation for Not OR.

The above example describes a logic element that performs the fundamental logical functions on two input signals. Such a two input system would be said to operate on binary notation. Binary notation is that currently used in conventional computer systems as current electronic components are most effective when operation in a two state mode, that is either on or off.

In theory a workable computer system could be devised by substituting the binary holographic logic elements described above with the electronic elements in a conventional system design.

A computer system that is designed to operate in binary notation does however have one disadvantage. Human beings differ from computers in as such as they do not use binary notation but instead use decimal notation, that is one based on units of ten. This means that at the interface between humans and computers a translation from decimal to binary and vice versa has to take place. Conventionally this is done by the machine, but it does require additional special purpose circuitry to achieve.

With the holographic logic elements described above it is possible and practicable to construct a logic element that operates in decimal notation as the number of individual holograms that can be recorded on to one area of photographic emulsion is not limited to two. By selecting exposure tines such that their sum is less than or equal to that exposure tine needed to saturate the photographic emulsion it is possible to overlay up to 10 holograms.

Therefore a composite holographic logic element made up of 10 individual holograms, each produced with the reference beam at a different angle to the photographic plate, would produce a decimal notation logical elenent.

In the above example the holograms were produced in a photographic emulsion supported on a transparent plate. They could equally well be produced in a photographic emulsion deposited onto a reflective surface.

In the above examples the holograms were produced by a photographic process. A photographic emulsion was exposed to the various interference patterns and subsequently developed to produce the required holograms.

However, as each interference pattern is generated by two plain beams it consists of a simple interference grating pattern of parallel lines. The spacing between the lines of each grating and their orientation relative to the other gratings could be calculated mathematically and the composite pattern simulated, for example on a computer.

The required composite hologram could therefore also be produced by mechanically inscribing this pattern on to a plate, either transparent or reflective, by a mechanical process such as machining, etching, engraving, deposition or sinilar methodologies.

In the above example the output bean, labelled 42 in Figure 4, has been assumed to be a plain beam. In a further embodinent of the invention the device is constructed with either the reference beams, labelled 34 and 36 in Figure 3, or the object beam, labelled 32 in Figure 3, or any combination of these, distortd by the insertion of a transparent medium such as a photographic slide containing an image which contains coded information in either analogue form such as a picture or a set of characters, or in digital form.

The generation of this image is achieved by illuminating the developed hologram with an illuminating beam which is identical to the original reference beam. That is, a beam at the same angle to the hologram as the original beam, or a beam that has been distorted in some way such that it is identical in cross-section to the original beam.

An example of the latter would be the insertion into the reference beam of a liquid crystal cell with it's electrodes arranged and activated in such a way that discrete patterns of transparency and opaqueness could be set up within it by the application of an appropriate electrical signal.

Another example would be the insertion into the reference beam of a transparent piezo-electric device which can be distorted into discrete shapes by the application of appropriate electrical signals.

In this way an optical information storage device can be achieved.

Claims

1. A device for performing the Boolean AND and OR logical functions on beams of light using the principles of holography and operating in binary notation mode.

2. A device for performing the Boolean logical functions on beams of light using the principles of holography as claimed in Clain 1 wherein a half-wave plate inserted into one illuminating beam supresses the AND function and only allows the OR function to be performed.

3. A device for performing the Boolean logic functions on beams of light using the principles of holography as claimed in Claims 1 and 2 wherein operation is in decimal notation mode.

4. A device for performing the Boolean logic functions on beams of light using the principles of holography as claimed in Claims 1, 2 and 3 wherein the holographic patterns are produced by photographic means.

5. A device for performing the Boolean logic functions on beams of light using the principles of holography as claimed in Claims 1, 2 and 3 wherein the holographic patterns are produced by mechanical inscription or deposition of the mathematically simulated patterns.

6. A device for performing the Boolean logic functions on beams of light using the principles of holography as claimed in Claims 1, 2, 3, 4 and 5 wherein the holographic patterns are formed on a transparent medium.

7. A device for performing the Boolean logic functions on beams of light using the principles of holography as claimed in Claims 1, 2, 3, 4 and 5 wherein the holographic patterns are formed on a reflective medium.