WO1998029767A1

WO1998029767A1 - Method and apparatus for creating holographic patterns

Info

Publication number: WO1998029767A1
Application number: PCT/GB1997/003563
Authority: WO
Inventors: Robert Adam Munday; Walter Kilian Clarke
Original assignee: Munday Robert A; Clarke Walter K
Priority date: 1997-01-04
Filing date: 1997-12-31
Publication date: 1998-07-09
Also published as: AU5565598A

Abstract

Apparatus and method for producing a plurality of holographic diffraction gratings wherein two or more interfering beams (24) are generated by means of an apertured mask (20). The mask is formed with one or more aperture (22) and is located in the path of the light source (12) and acts to baffle or block the light such that the light passes through only in the region of the apertures thereby creating the number and angle of beams required.

Description

METHOD AND APPARATUS FOR CREATING HOLOGRAPHIC PATTERNS

The present invention relates to holograms, and more specifically to apparatus and methods for creating holographic patterns.

Holographic images are well-known and may be used to create the impression of a three dimensional object from the illumination of a two-dimensional hologram. Such holographic images have been utilised in various practical applications, such as on credit cards, tickets and security passes for authentication purposes, and have also been utilised for packaging purposes. They are also becoming increasingly popular for purely artistic and decorative use. However, the cost of initial production and subsequent reproduction of holograms and the difficulty of viewing holograms in diffuse lighting has limited their application.

It has been proposed in US 5,291,317 that holographic images can be created from a plurality of individual spots, each spot forming a holographic diffraction grating of a pre- determined grating spacing and angular orientation for that spot, the so-called "dot matrix" holograms. The apparatus required to produce such holograms is complex and expensive, this being reflected in the cost of producing the hologram. A further disadvantage of this prior system when used in combination with a continuous wave laser is that the speed at which the hologram can be created is limited by the need to physically rotate the diffraction grating or beam splitter to a given position, to pause the diffraction grating or beam splitter momentarily and to open and close a shutter to make a holographic exposure. This results in the production of holograms being slow and therefore expensive. This prior system is also limited in its ability to create a small spot size, due to its relatively complex optics . Accordingly, it is an object of the present invention to provide a method and apparatus for the production of holograms which provide improvements in relation to one or more matters discussed above, or generally.

According to the present invention there is provided apparatus for producing a plurality of holographic diffraction gratings, a corresponding method of producing a plurality of holographic diffraction gratings and a hologram produced by said apparatus and method.

In an embodiment of the invention there is provided apparatus for producing a plurality of holographic diffraction grating patterns for the production of dot matrix-type holograms, said apparatus comprising a source of light such as a continuous wave laser, pulsed laser, light emitting diode (LED) or any other more conventional source, including monochromatic lamps, means for diverging the light source (such as a fibre optic, conventional lens or lenses or a holographic optical element (HOE) ) may be provided, unless the light source used naturally emits a divergent beam of light (such as a conventional bulb or LED, means for generating two or more interfering beams, means for focusing said beams, such as conventional lens or lenses or HOE, and a photosensitive region onto which said beams are directed to a spot such that a holographic diffraction grating is created within said spot.

The means for generating two or more interfering beams comprises a mask. The mask may comprise a thin plate, constructed from aluminium or any other suitable material.

The required number of beams are produced by forming a corresponding number of apertures or holes in the mask.

These holes can be displayed anywhere on the mask, within the area of the focusing lens.

The plate is located in the path of the light beam and when the light beam encounters the plate, the plate acts to baffle or block the light such that light passes through only in the region of the holes thus creating the number and angle of beams required. The beams created are focused by said optic to a common focal point, or spot, onto a photosensitive region such as a photoresist plate. As the beams created by selective baffling or blocking of the light beam are all of equal focal length and they can be focused more easily and accurately than would be the case if the beams had been formed by the traditional splitting of the light source which can create beams of different focal lengths.

Furthermore, as the beams created by selective baffling or blocking do not contact any other lens, mirror or the like until the beams are focused, this serves to maintain the focal length. This provides a smaller spot which results in better resolution and colour definition in the final hologram pattern.

In a further aspect of the invention, the plate may be adapted to be rotated rapidly. As the mask spins, the light source is caused to pulse at the appropriate moment for a given diffraction angle whereby greatly increasing the number of exposures or spots which can be created per second.

According to the invention there is also provided a method for producing holographic diffraction gratings, said method comprising the steps of providing a source of light such as a continuous wave laser, pulsed laser, light emitting diode (LED), or any other more conventional source, including monochromatic lamps, the step of providing means for diverging said light source, such as fibre optic, conventional lens or lenses or a holographic optical element (HOE) may be provided, unless the light source used naturally emits a divergent beam of light (such as a conventional bulb or LED) , the step of providing means for generating two or more interfering beams, the step of focusing said beams, by means of a conventional lens or lenses or an HOE, providing a photosensitive region and the step of directing said beams to a spot on said photosensitive region such that a holographic diffraction grating is created within said spot.

The method further comprises the step of utilising a mask adapted to generate two or more interfering beams by means of selective baffling or blocking light generated from the light source.

In a further aspect of the invention, the method comprises the step of causing the mask to rotate, and the step of directing pulsed light towards the mask.

According to a further aspect of the invention there is provided a hologram produced by the apparatus and/or methods as hereinbefore described.

According to a further aspect of the invention there is provided apparatus for producing a plurality of holographic diffraction gratings for the production of dot matrix-type holograms, as hereinbefore described wherein the means for generating two or more interfering beams comprises a liquid crystal display (LCD) mask, or any other solid state spatial light modulators (SLM) . The required number of beams are produced by displaying a corresponding number of transparent areas or "holes" on the LDC mask. These "holes" can be positioned anywhere on the LCD mask, within the area of the focusing optic. The relative location of the holes determines the angle of diffraction produced.

When the light beam hits the LCD mask, the region of the mask around the holes serves to baffle or block the light such that it passes through only in the region of the "holes" thus creating the number and angle of beams required. The beams created are focused by said focusing optic to a common focal point, or spot, onto a photosensitive region such as a photoresist plate.

The "holes" in the LCD mask can be created and removed quickly and easily by selective application of an electronic field as is well-known. The^' LCD mask does not need to be moved so as to provide the required number, position, shape and angle of beams needed to form the spots required for the holographic image, but rather the "holes" formed within the mask can be selectively formed anywhere within the LCD and "opened" and "closed" electronically as required. This provides a number of advantages over traditional systems which use stepper motors and the like to move HOE or splitter to produce the same effect, namely that the vibration caused by such motors which causes the light source to be moved out of alignment is eliminated.

The use of an LCD mask to create the required interfering beams provides a means whereby two or more such beams can be created without the need for a rotating HOE, diffraction grating, beam splitter or the like and is not limited by the need to have a separate shutter. Accordingly, the speed at which spots can be created is greatly increased. For example, LCD masks can produce in the region of 4000 exposures or spots per second compared with the mechanical systems which use diffraction gratings and beam splitters which produce in the region of 4 to 10 exposures or spots per second. A hologram with 3 million exposures would take approximately 8 days on a conventional system. The method and apparatus of the present invention aims reduce this a matter of minutes.

A further advantage provided by the invention is that a separate shutter is not required to shutter the beam as in traditional systems, wherein the beam is shuttered whilst the HOE and resist plate, carrying the photosensitive material move to the next position. The LCD mask also acts as a static shutter thereby reducing the complexity and cost of the apparatus.

Another advantage provided by the invention is that by utilising an LCD mask the intensity of the interfering beams may be modulated by modulating the transmittance of the "holes" in the mask. In this way subtle colour variations of the diffraction spots can be achieved. It would much more difficult to achieve this using traditional systems.

In an embodiment, a static baffle may be used to mask off the areas of the LCD not being used. This simply blocks the light travelling Through the "dark" or "closed" areas of the LCD, and enables the use or low contrast LCD screens. This is a different configuration from using a rotating mask in combination with the LCD.

A still further advantage of the invention is that by reducing the complexity of the apparatus, the interfering beams created by the apparatus remain virtually identical in length and it is therefore possible to utilise a very short coherent length light source, even a conventional UV lamp could be used to expose photoresist.

According to this aspect of the invention there is also provided a method for producing holographic diffraction gratings, as described previously, characterised by the step of providing utilising an liquid crystal display (LCD) mask adapted to generate two or more interfering beams. The required number of beams are produced by the step of displaying a corresponding number of transparent areas or "holes" on the LDC mask. These "holes" can be displayed anywhere on the LCD mask, within the area of the focusing lens. The "holes" in the LCD mask can be created and removed quickly and easily by the step of selective application of an electronic field as is well-known, and can be "opened" and "closed" electronically as required.

According to a still further embodiment of the invention there is provided apparatus for producing a plurality of holographic diffraction gratings for the production of dot matrix-type holograms, as hereinbefore described wherein the means for generating two or more interfering beams comprises a plate mask as previously described, is used in combination with an LCD mask of the type also previously described. The LCD mask is placed adjacent the plate mask in the light path and is used to effectively close or partially close the aperture (s) of the plate mask. In use, the LCD mask is used to create a grey scale and/or positional modulation of a hole or holes in the plate mask. By utilising the plate mask together with the physical mask and using the two in combination this provides the means whereby the colour of the spots or pixels created by focusing the beams on the resist plate can be changed, and the position of the interference beams in real time. A further advantage is that the presence of the plate mask can be used to overcome any contrast limitations of the LCD mask by acting to baffle or block the light.

An additional advantage of this embodiment is that the speed at which an area of different pixels is exposed can be increased by incorporating multiple optical heads in the recording system. Light could be most easily directed to such multiple optical heads using an optical fibre delivery system although conventional optics and/or multiple light sources could also be employed. This embodiment is most applicable to the LCD only configuration, and the preferred method of delivering light to the multiple heads would be to use multiple light sources, such as LEDs , or a single collimated beam of light covering all the heads at the same time. This particular embodiment of the invention may be employed to create "holographic" compact discs for the storage of information such as audio, visual, computer data and the like. In this way, each spot or pixel can represent multiple states by reflecting light at any of a number of angles. This will greatly increase the information storage capacity of such a medium which traditionally uses a two state "on/off" system.

According to a further embodiment of the invention there is a holographic pattern formed by any of the apparatus or methods herein before described.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying illustrative drawings in which:

Figure 1 shows schematically apparatus for producing holographic diffraction gratings in accordance with an embodiment of the invention;

Figure la shows schematically apparatus for producing holographic diffraction gratings in accordance with an embodiment of the invention;

Figures 2, 2a and 2b show diagrammatically an example of aperture patterns from the apparatus of Figures 1 and la;

Figure 3 shows schematically apparatus for producing holographic diffraction gratings in accordance with a further embodiment of the invention;

Figures 4, 4a and 4b show diagrammatically an example of aperture patterns from the apparatus of Figure 3 ; Figure 5 shows schematically apparatus for producing holographic diffraction gratings in accordance with a further embodiment of the invention; and

Figures 6, 6a and 6b show diagrammatically an example of aperture patterns from the apparatus of Figure 5.

As shown in Figure 1, apparatus for producing holographic diffraction gratings 10 comprises a light source 12 to provide a light beam 14. The light beam 14 is directed to and passes through an optical head comprising a light diverging optic, such as a lens, fibre optic or the like 16, although this is not necessary if the light source used emits a divergent beam of light (such as a conventional bulb or LED) .

The objective or optic 16 may be adapted to function as a printer-like head which could be scanned over a large area to make large patterns quickly. The light beam 14 is then directed to an optional collimating optic 18, if required.

Located behind said optic 18 is a plate 20 formed with apertures or holes 22. When the light beam 14 encounters the plate 20, the plate effectively baffles or blocks the light beam 14 allowing light to pas^'s through the plate only in the regions of the holes 22. This has the effect of producing the required number and type of interfering beams 24 required to produce a particular diffraction grating. A focusing optic 26 acts to focus the interfering beams 24, created by the plate 20, to a focal point. The focused beams

28 pass through an optional aperture if required. This aperture serves to shield the areas of unexposed sensitive material from any stray light produced by the system.

The beams 28 are directed to a spot on a resist plate 30 such that a holographic diffraction grating is created in that spot. The resist plate is mounted on an XY stage (not shown) the movement of which is controlled by a suitable computer control system (not shown) . The apparatus may be mounted on a vibration isolating optical table.

An alternative manner in which the light beam 14 can be selectively baffled or blocked to provide the required interference beams is shown in Figure la. Located behind said optic 18 is a liquid crystal display (LCD) mask 32. The LCD mask 20 is provided with a number of transparent regions or "holes" 22. The location, size and shape of the holes 22 may be changed as required by selective application of an electronic field to the liquid crystal display 32.

Another alternative configuration would be to use a reflecting LCD spatial light modulator, which effectively provides a solid state modulating mirror. A single beam would be directed onto the LCD chip and the number and angle of beams required would be reflected off.

When the light beam 14 encounters the LCD mask 32, the mask 20 effectively acts to selectively block the light beam 14, allowing light to pass through only in the regions of the holes 22. This has the effect of producing the required number and type of interfering beams 24 required to produce a particular diffraction grating.

The interfering beams 24 created are focused as described above. The type of aperture or hole patterns which may be formed in a plate 20 or a LCD mask 32 to create the required spot or pixel or diftractive effect are shown in Figures 2, 2a and 2b. The aperture pattern of Figure 2 will produce a "single colour" diftractive spot, the pattern shown in Figure 2a will produce "multi-coloured" and "multi-angled" diffractive spots and the pattern shown in Figure 2b will produce an alternative diffractive effect as a result of the differently shaped "holes".

A further embodiment of the invention is shown in Figure 3. In this embodiment, the light beam 14 is generated from a pulsed laser. The plate 20 is mounted so as to be rotatable. In use, the mask 20 is caused to spin continuously and quickly, and the laser is caused to pulse at the appropriate moment for a given diffraction angle.

In this embodiment, very short exposures would need to be made using a pulsed light source. Alternative light sources, such as continuous wave (CW) lasers could be used but the rotation of the mask would need to be slowed or paused momentarily whilst the exposure was made.

In this embodiment the number and type of interfering beams 24 required are produced by the pulsed light hitting the rotating plate 20 at the appropriate moment, that is when the light beam 14 is aligned with the required hole 22 for a particular diffraction grating. A focusing optic 26 acts to focus the interference beams 24, created by the mask 20, to a focal point. The focused beams 28 are directed to a spot on a photosensitive region 30 such that a holographic diffraction grating is created in that spot.

The type of aperture or hole patterns which may be formed in the mask 20 to create the required spot or pixel or diffractive effect are shown in Figures 4, 4a and 4b. The aperture pattern of Figure 4 will produce a "single colour" diffractive spot, the pattern shown in Figure 4a will produce "multi-coloured" and "multi-angled" diffractive spots and the pattern shown in Figure 4b will produce an alternative diffractive effect as a result of the differently shaped "holes".

As shown in Figure 5, interfering beams 24 can be formed by utilising the plate mask 20 and the LCD mask 32 in combination. The plate mask 20 is located behind the optic 18 and the LCD mask 32 is located behind the plate mask although the positions of the plate and the LCD are interchangeable. The position of the apertures 22 formed in the LDC mask 32 are used to modulate and determine the position and number of holes 22 through which light will pass. The LCD mask can be used to effectively close an aperture as shown at 38 or partially close an aperture 22 in the plate mask 20 thus enabling the position and number of the beams 24 created to be continuously variable.

Figures 6, 6a and 6b show this combined effect. Figure 6 shows an apertured plate mask acting as a primary light baffle although the positions of the plate and the LCD are interchangeable. Figure 6a shows the LCD mask with zones or apertures having varying degrees of contrast and acts to determine the position and number of "holes" through which light can pass. Figure 6b shows how combining the two can produce varying types of interfering beams from passing through "holes" of different shapes and degrees of modulation as shown at 40, 42, 44 and 46.

An advantage of the invention is because the interfering beams are not formed by any kind of conventional beam splitting, the interfering beams are effectively the same beam, the focal length of the interfering beams created can be kept short and of the same length. This enables the beams to be focused more accurately which facilitates the production of very small spots resulting in very high resolution patterns being created. Spot size is limited primarily (but not only) by the expanding and focusing optics of which there need be very few in the present invention.

The present invention utilises very high quality optics and by focusing the beam(s) through two optics this makes it possible to produce very small focused spots with very high resolution. Such high resolution patterns produced by very small spots are important in various security, authentication, machine-readable and covert hologram applications.

A further advantage of the invention is that because the apparatus generates the required interfering beams without the need to utilise beam splitters, HOEs, mirrors and numerous focusing optics, the optical head of the present invention is much simpler and lends itself to miniaturisation, indeed a plurality of optical heads could be utilised within the apparatus to produce the required holographic pattern.

Claims

1. Apparatus for producing holographic diffraction gratings comprising said apparatus comprising a source of light such as a continuous wave laser, pulsed laser, LED or other more conventional source, including monochromatic lamps, means for diverging the light source (such as a fibre optic, conventional lens or lenses or a holographic optical element (HOE) ) unless the light source naturally emits a divergent beam of light, means for generating two or more interfering beams, means for focusing said beams, such as conventional lens or lenses or HOE, and a photosensitive region onto which said beams are directed to a spot such that a holographic diffraction grating is created within said spot characterised in that the means for generating two or more interfering beams comprises an apertured mask.

2. Apparatus for generating interference beams for the production of holographic diffraction gratings characterised in that said apparatus comprises a mask.

3. Apparatus according to claim 1 or claim 2 characterised in that said mask is in the form of an plate having one or more apertures formed therein.

4. Apparatus according to claim 3 characterised in that the apertures formed in said plate are within the area of the focusing lens.

5. Apparatus according to any of the previous claims characterised in that said plate is located in the path of the light emanating from said light source.

6. Apparatus according to claim 5 characterised in that said plate selectively baffles or blocks light emanating from the light source such that light passes through only in the region of the apertures thereby forming one or more beams .

7. Apparatus according to any of the previous claims characterised in that said apertured plate is adapted to rotate.

8. A method of producing holographic diffraction gratings, said method comprising the steps of providing a source of light such as a continuous wave laser, pulsed laser, LED or other more conventional source, including monochromatic lamps, providing means for diverging said light source unless the light source naturally emits a divergent beam of light, such as fibre optic, conventional lens or lenses or a holographic optical element (HOE) , the step of providing means for generating two or more interfering beams, the step of focusing said beams, such as conventional lens or lenses or an HOE, providing a photosensitive region and the step of directing said beams to a spot on said photosensitive region such that a holographic diffraction grating is created within said spot characterised by the step of utilising an apertured mask adapted to generate two or more interfering beams by means of selective baffling or blocking light generated from the light source.

9. A method according to claim 8 characterised by the step of placing an apertured mask in the form of a plate in the path of the light source, causing said light source to be selectively baffled or blocked such that light passes through only in the region of the apertures thereby forming one or more beams .

10. Apparatus according to claim 1 or claim 2 characterised in that said mask comprises an electronically controlled electro-optic crystals, such as a liquid crystal display (LCD) .

11. Apparatus according to claim 10 characterised in that apertures are formed within the LDC by selective application of an electronic field to create one or more transparent areas.

12. Apparatus according to claim 11 characterised in that said LCD is located in the path of the light emanating from said light source.

13. Apparatus according to claim 12 characterised in that said LCD selectively baffles or blocks light emanating from the light source such that light passes through only in the region of the apertures thereby forming one or more beams.

14. Apparatus according to claim 13 characterised in that the transmittance of the regions of transparency of the LCD can be varied.

15. A method according to claim 8 characterised by the step of placing an apertured mask in the form of an electronically controlled electro-optic crystals, such as a liquid crystal display (LCD) in the path of the light source, causing said light source to be selectively baffled or blocked such that light passes through only in the region of the apertures thereby forming one or more beams.

16. Apparatus according to claim 1 or claim 2 characterised in that said mask comprises an apertured plate in combination with an electronically controlled electro-optic crystals, such as a liquid crystal display (LCD) in which apertures or regions of transparency can be created and removed by selective application of an electronic field.

17. Apparatus according to claim 16 characterised in that said apertured plate is located adjacent the LCD mask in the light path.

18. Apparatus according to claim 17 characterised in that said apertured plate and said LCD both act to selectively baffle or block light emanating from the light source such that light passes through only in the region of the apertures formed in both masks thereby forming one or more beams.

18. Apparatus according to claim 17 characterised in that said LCD effectively closes or partially closes the apertures formed in the plate mask.

19. A method according to claim 8 characterised by the step of placing an apertured mask in the form of a combination of an apertured plate and an electronically controlled electro- optic crystals, such as a liquid crystal display (LCD), in the path of the light source, causing said light source to be selectively baffled or blocked such that light passes through only in the region of the apertures formed in both the plate and the LDC thereby forming one or more beams.

20. A holographic pattern formed by the apparatus and methods as set out in the preceding claims.

21. Apparatus for producing holographic diffraction gratings comprising said apparatus comprising a source of light such as a continuous wave laser, pulsed laser, LED or other more conventional source, including monochromatic lamps, means for generating two or more interfering beams, means for focusing said beams, such as conventional lens or lenses or HOE, and a photosensitive region onto which said beams are directed to a spot such that a holographic diffraction grating is created within said spot characterised in that the means for generating two or more interfering beams comprises an apertured mask.

22. A method of producing holographic diffraction gratings, said method comprising the steps of providing a source of light such as a continuous wave laser, pulsed laser or other more conventional source, including monochromatic lamps, the step of providing means for generating two or more interfering beams, the step of focusing said beams, such as conventional lens or lenses or an HOE, providing a photosensitive region and the step of directing said beams to a spot on said photosensitive region such that a holographic diffraction grating is created within said spot characterised by the step of utilising an apertured mask adapted to generate two or more interfering beams by means of selective baffling or blocking light generated from the light source.

23. Apparatus for producing holographic diffraction gratings wherein one or more optical heads are used to create said grating.