GB2198548A

GB2198548A - Hologram manufacture

Info

Publication number: GB2198548A
Application number: GB08629389A
Authority: GB
Inventors: Jon Andreasssen
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-12-09
Filing date: 1986-12-09
Publication date: 1988-06-15
Also published as: GB8629389D0

Abstract

For the manufacture of a hologram, particularly one comprising a space-variant optical interconnect, a reference beam (18) and an object beam (21) are formed from a laser beam output from a source (15), the object beam portion of the laser beam being directed through an optical fibre (20) monomode at the laser beam wavelength and the output end of which acts as a point source. The object beam and reference beam intersect and are incident on a holographic recording medium (1). To ensure each area of the medium is exposed only once when providing an array of hologram elements a masking arrangement (13, 14) is used. The position of the output end of the fibre and the relative position of the aperture (14) of the mask (13) and the medium (11) is changed between exposures. The movement of the fibre end and the masking arrangement may be computer controlled thus facilitating hologram manufacture. <IMAGE>

Description

HOLOGRAM MANUFACTURE This invention relates to the manufacture of holograms, in particular holograms for space-variant optical interconnects and spot array generators.

According to one aspect of the present invention there is provided a method of manufacturing a hologram includinq the steps of forminq a reference beam and an object beam from a

beam, the formation of the object beam comprising launching a portion of the.

beam into one end of an optical fibre which is monomode at the laser beam wavelength, the object beam being output from the other end of the optical fibre which acts as a point source therefor; causing the reference beam and the object beam to intersect and be incident on a holographic recording medium for a predetermined exposure time and processing the thus exposed holographic recording medium to form said hologram.

According to another aspect of the present invention there is provided apparatus for the manufacture of a hologram comprising a

beam source, means for forming a reference beam and an object beam from the output of the

-beam source, the means for forming the object beam including an optical fibre into one end of which a portion of the output of the

beam source is launched in use of the apparatus, the optical fibre being monomode at the

beam wavelength, the object beam being output from the other end of the fibre in use of the apparatus, which other end of the fibre acts as a point source for the object beam; means for holding a holographic recording medium; means for directing the reference beam and the object beam towards the holographic recording medium in use of the apparatus for intersection thereof and incidence on the holographic recording medium; and means for exposing the holographic recording medium to the reference and object beams for predetermined exposure time.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates, schematically, one arrangement of apparatus for the manufacture of a transmission hologram; Figs. 2a and 2b illustrate two possible hologram element arrays; Figs. 3a and 3b illustrates a possible recording sequence of hologram elements in an array in a schematic and shorthand version respectively; Figs. 3c and 3d illustrate the corresponding reconstruction stage in a schematic and shorthand version respectively; Figs. 4a and 4b illustrate another recording sequence and the corresponding reconstruction stage in shorthand form; Figs. 5a and 5b illustrate a further recording sequence and the corresponding reconstruction stage in shorthand form;; Figs. 6a and 6b illustrate yet another recording sequence and the corresponding reconstruction stage in shorthand form; Fig. 7 illustrates schematically the manufacture of a reflection hologram; Fig. 8 illustrates schematically the reconstruction stage for a reflection hologram, and Fig. 9 illustrates schematically the recording of a transmission hologram when the object beam is collimated.

Space variant optical interconnects may be employed, for example, in optical computers or for optical connections to VLSI devices, in which case metal interconnection wires are replaced by optical connections.

A space-variant optical interconnect is a means whereby a number of input optical beams can be coupled to an array of outputs, which beam is coupled to which output being determined by the interconnect. Two light beams can be swapped over, for example, without interference and by optical means.

For a spot array generator or space variant interconnect, for example, a respective hologram element for each spot, or each optical path to be interconnected, is required on a common substrate. Such holoqrams

require an object

beamtemanating from a respective point source, for each spot and a reference

beam of the same wavelength. The object and reference

beams are generally formed by splitting a laser beam into two portions and directing the two portions along different optical paths, one including an aperture in the case of the object beam, before they are both caused to be incident on a holographic recording emulsion to produce an optical interference pattern therein which when processed (developed) and illuminated by a reconstruction beam produces the required spot image.

In Fig. 1 there is shown a holographic emulsion 11 on a glass backing plate 12 disposed behind a mask 13 having a square aperture 14. The light beam originating from a laser 15, for example a He-Ne laser with an output of approximately lmW at a wavlength of 0.633pm, is split into two portions by beam splitter 16. One part 17 of- the beam is expanded and then collimated, as shown although it can alternatively be a converging beam, and then comprises the reference beam 18. The other part 19 of the beam is launched into one end of an optical fibre 20, which is monomode at the wavelength being employed, thus the fibre is matched to the laser. The beam output from the other end of fibre 20 comprises the object beam 21 which is shown as a diverging beam but can alternatively be collimated.The reference and object beams 18 and 21 are at an angle of approximately 300 to one another. The output end of the fibre 20 is considered to be a point source of light which for a fibre monomode at O.633pm typically has a diameter of approximately 4um. The fibre output, therfore, has a "clean" Gaussian intensity profile which forms an ideal object beam.

To produce a hologram capable of operating as a space-variant interconnect or a spot-array generator, the holographic emulsion medium 11, for example a high resolution silver halide emulsion, is multiply exposed by operation of a shutter 22 and as described hereinafter, and then processed (developed and fixed) in the appropriate manner. The processed medium is then subjected to additional processing. This may be in order to improve efficiency by converting an amplitude hologram to a phase hologram, for example by photographic bleaching; to alter the light wavelength response of the hologram; or to provide a more durable hologram, for example by baking in an oven.

During playback the hologram must be correctly positioned with respect to the reconstruction beam or beams. The developed and processed hologram may be reconstructed with a beam that is the exact conjugate (opposite in every respect) to the original reference beam, or to the object beam. Alternatively, the developed and processed hologram may be rotated through 1800 about a vertical axis so as to reverse exactly the orientation of the plate with respect to that during the exposure and reconstruct with the original reference beam or beams.

The present invention is basically concerned with the multiple exposure of the holographic medium for the production of a hologram capable of operating as a space-variant interconnect or a spot-array generator.

The multiple exposure technique is such that any one area of the holographic emulsion is only exposed once, for example by moving the apertured mask 13 between each exposure. For this purpose the mask may be movable in the directions of movement indicated at A in a plane parallel to the plane of the backing plate 12. If an area of the emulsion is exposed more than once the diffraction efficiency of the respective hologram will be reduced. For each exposure the reference beam position is the same, a point source at a different position is required for the appropriate object beam. This is achieved by repositioning the output end of the fibre 20 appropriately. For which purposes the output end of the fibre may be movable between exposures such as in the directions indicated at B. The in-dicated plane of movement of the fibre end is not necessarily parallel to the indicated plane of movement of the mask. As a result of such single exposure, space-variant interconnects and spot-array generators can be produced efficiently, the production being constrained only by the other producing steps, that is choosing the most suitable materials, processing chemicals and processing conditions.

For spot-array generating holograms it is advantageous to have the exposed areas of the holographic emulsion as "close packed" as possible without these areas overlapping. This can be achieved by carefully moving the mask between exposures; by replacing the mask with another mask between exposures; or by moving the emulsion behind a stationary mask between exposures. If the hologram elements are closely packed then they can be considered as one hologram which affects all the light of the reconstruction beam incident on the area bounded by the individual hologram elements, which one hologram comprises an overall mosaic thereof. The close-packing of "square" hologram elements 24 as indicated in Fig. 2a and achieved by use of a mask is thus preferable to that of "circular" hologram elements 25 as indicated in Fig.

2b and in effect prevents the efficiency of the overall hologram being less than that of its individual hologram elements.

The output end of the monomode optical fibre acts as all of the required point sources for the object beams. This end can be held by an arrangement 23 with adjustable x, y and z translational stages which enables the position of the object beam to be moved very easily in between exposures, thus resulting in a very versatile recording process. The extent of movement of the output end of the fibre (object beam) between exposures describes the dimensions of the spot array. The fibre end is carefully prepared by cutting to provide a flat end face perpendicular to the fibre length, indexed matched at its ends after the fibre coatings are removed and held in an appropriate holder of said arrangement of translational stages. Typically, exposure times may be of the order of one second and the hologram elements of the order of lmm square.During reconstruction the hologram elements act as holographic lenses and each produces a respective spot of light at its

The > spot size at the focus of a holographic lens cannot be less than the resolution limit as governed by diffraction effects. The minimum feature which can be formed has a diameter d = 2 x 1.22 x) x f/D where ) is the wavelength of light employed, f is the focal length of the holographic lens and D is the diameter of the hologram.

The relative positions of the mask and the output end of the fibre for the individual exposures describe-s the interconnection properties of the hologram which are fixed at the production stage. These interconnection properties are of particular importance when producing space-variant interconnects.

Examples of interconnection geometries will now be given. Fig. 3a illustrates a hologram mosaic 30 compris-ing four separate hologram elements 31 each of which is produced as a result of exposure of the holographic emulsion through an apertured mask (omitted for clarity) by a reference beam (not shown) and an object beam output from an optical fibre 32 when the end thereof is in one of the four positions 1, 2, 3 and 4.

The hologram elements 31 were exposed in the order 1, 2, 3 and 4 with the fibre end in the respective 1, 2, 3 and 4 position. That is firstly with the mask exposing only area 1 of the emulsion, and the fibre end at position 1; secondly with the mask exposing only area 2 of the emulsion and the fibre end at position 2, and so on.

This is indicated schematically in a shorthand form in Fig. 3b which indicates, for the recording stage, at the left hand part thereof the order of exposure of the hologram elements and at the right hand part thereof the corresponding positions of the fibre end. Fig. 3c indicates the reconstruction stage, the hologram having been rotated through 1800 about a vertical axis (assuming a horizontal reference beam) for reconstruction by a reconstruction beam which may comprise the reference beam if the latter was collimated. As a result of directing the reconstruction beam 33 onto the overall hologram an array of four spots 1', 2', 3' and 4' is produced.Light incident on the hologram which was exposed first, that is number 1, is deflected and focussed down to the position of spot 1' behind the hologram, similarly light incident on the hologram which was exposed next, that is number 2, is deflected and focussed down to the position of spot 2', and so on. Thus a 2 x 2 array of spots has been generated, each spot being related to a respective position of the fibre end acting as a point source of light. Fig. 3d illustrates the reconstruction stage in the shorthand form employed in Fig. 3b.

Figs. 4a and 4b show in the same shorthand form the effect of recording the hologram elements in a different order. In this case the mask is first disposed such that with the fibre end in position 1 the top right hand element 41 is exposed first, in contrast to the top left hand element in Figs. 3a and 3b, next the mask is disposed such that with the fibre end in position 2 the bottom right hand element is exposed second, in contrast to the top right hand element in Figs. 3a and 3b, and so on. As will be appreciated, the fibre end positions are the same for both Figs. 4a and 3a, however the relative mask positions have been changed in order to expose the elements in a different order.When the hologram with the elements produced in the order indicated in Fig 4a is illuminated by a reconstruction beam, light incident on the hologram element exposed first, that is number 1 will be deflected and focussed down to the position of spot 1', light incident on the hologram element number 2 will be deflected and focussed down to the position of spot 2' and so on thus producing an array of four spots as before except that the spots have been rotated through 900 (compare the positions of the numerals 1, 1'; 2, 2' in the left and right hand parts of Fig. 4b).

Figs. 5a and Sb show in the same shorthand form the effect of recording the hologram elements in another order. The mask is disposed such that the bottom right hand element 51 is exposed first with the fibre end in position 1, then the bottom left hand element is exposed with the fibre end in position 2, and so on.When the hologram with the elements produced in the order indicated in Fig. 5a is illuminated by a reconstruction beam, light incident on the first exposed hologram, number 1, will be deflected and focussed down onto the position spot 1', light incident on the hologram element number 2 will be deflected and focussed down onto the position of spot 2' and so on, thus producing an array of four spots as before except that there has been rotation through 1800, this can alternatively be considered as diagonal interchange of the spots. (Compare the two sets of numbers in Fig. 5b).

Figs. 6a and 6b show in the same shorthand form the effect of recording the hologram elements in a further order. The mask is disposed such that the bottom right hand element 61 is exposed first with the fibre end in position 1, then the top right hand element is exposed with the fibre end in position 2, and so on. When the hologram with the elements produced in the order indicated in Fig. 6a is illuminated by the reconstruction beam, light incident on hologram element 1 is deflected and focussed down onto the position of spot 1', light incident on hologram element 2 is deflected and focussed down onto the position of spot 2', and so on. Once again producing an array of four spots but as can be seen from the two sets of numbers in Fi.g 6b in this case there has been diagonal interchange of spots 1 and 3 whilst spots 2 and 4 have held their positions.

The interconnection behaviour of the holograms can be predicted by comparing the sets of numbers in the left and right hand portions of the recording stages (Figs. 3b, 4a, 5a and 6a). In Fig. 3b the numbers are in the same relative positions and thus the generated array of spots will correspond to the fibre end positions. In Figs. 6a numbers 1 and 3 have been interchanged whilst 2 and 4 remain unchanged.

In all of the examples quoted, however, incident light (the reconstruction beam) is spatially redistributed by the hologram in a manner dependent upon the order in which the hologram elements were produced, the successive positions of the end of the fibre being the same in each case. In these space-variant interconnects the output position of a beam of light is dependent on which hologram it is incident to, that is its input position.

Arrays containing larger numbers of spots can be produced in the same manner. The number of hologram elements would generally have to be more than or equal to the number of spots generated.

The production of such multi-element holograms will be particularly facilitated by microcomputer/ computer control of the recording process. It is envisaged that such control may be employed, for example, to move the end of the fibre to the next position between exposures; to move the mask, move a new mask into place or to move the holographic material, as appropriate between exposures; to control the exposure times and to allow the apparatus time to settle after any movement before an exposure is taken. Preferably the computer would be programmed so that it is merely necessary to input the required array-type/dimensions or the required interconnection pattern, from which the correct exposure sequence would be automatically determined and carried out.

In the above only transmission holograms have been considered, however reflection holograms, for interconnects or array generators, can be constructed by a similar procedure. In this case, however, the reference beam 71 (Fig. 7) is now incident on the opposite side of the holographic material 72 to the object beam 73, and two identical masks 74 are required during an exposure, one on each side of the holographic material 72 and very close thereto. Whereas the holographic material has so far been described as an emulsion on a glass plate, it may alternatively been in film form. The fibre end 75 producing the object beam 73 and the masks 74 are moved as described above between exposures. The plate is processed and placed in its original position but rotated through 1800 about the vertical axis, assuming a horizontal reference beam.The reference beam must be orthogonal to the holographic plate in both the horizontal and vertical planes/directions. In this case the resulting spot images (76, Fig. 8) are on the same side of the processed hologram Nn as the reconstruction (reference) beam 71.

Such a reflection holographic array generator/ interconnect, also acts as a wavelength filter since the volume hologram, a complex grating/lattice structure, only reflects/diffracts light with a wavelength and orientation/angle that satisfies the Bragg condition.

In the foregoing the object beams have been produced by a point source of light (fibre end) whose position only has been altered slightly and only divergent object beams have been specifically employed although the possibility of collimated object beams was briefly referred to. With use of collimated object beams the recording procedure has to be altered slightly. It is the angle the collimated object beam is incident to the masked holographic emulsion which must be changed between exposures and to achieve this both the position and the angle of the fibre (together with that of a collimating lens) must be altered. Fig. 9 illustrates the recording arrangement for a transmission hologram with a collimated object beam.Holographic emulsion 80 on a glass plate 81 is exposed via an apertured mask 82 to a reference beam 83 and an object beam 84 produced by a fibre end 85 and collimated by a lens 86. In order to change both the position and angle, an additional degree of movement of the object beam, or rather of the fibre end and lens producing it, is required namely movement in a semi-circular arc centred on the exposed areas of the holographic emulsion. This movement must be made in conjunction with the previously described movement of the ojbect beam/fibre end and the holographic mask.

With divergent object beams transmission-mode holographic space-variant interconnects and array generators with efficiencies up to 35% can be achieved with silver halide emulsions. It is considered that if dichromated gelatin were used as the hologram medium then efficiencies approaching 100% could be achieved. In general such holographic interconnects can be recorded at wavelengths where the following are available, namely: relatively efficient photosensitive materials; optical fibres monomode at these wavelengths; and laser light sources of adequate power. However, the finished products can be used at wavelengths different to their recording wavelength, without significant aberrations, by using special material processing techniques or by using corrective holograms to counter aberrations.

It should be noted that for the construction of all of the holographic elements referred to above, the two optical beams which interfere must have the same polarisation. The polarisation of the object beam may be altered by passage through the optical fibre, and thus a method or means of altering the polarisation of the object beam may be required in order to compensate for any polarisation change due to the fibre. One method of achieving this involves placing a quarter wave plate in front of the fibre after the beamsplitter, which plate converts linearly polarised light into circularly polarised light or elliptically polarised light depending on orientation.Pressure is applied to the fibre at the output end, by for example tightening the grip of ferrules thereon, and this stress induces a birefringence, or extra birefringence, into the fibre which serves to alter the state of polarisation of light passing through that region of the fibre and therefore changes the state of polarisation at the fibre output.

Hence the s-tate of polarsiation at the fibre output can be determined by adjusting the angle of the optical axis of the quarter wave plate with respect to the incoming beam and adjusting the pressure placed on the fibre at a particular point.

The present invention thus provides a method of manufacturing volume holograms, particularly volume holographic arrays for use as spot array generators or space-variant optical interconnect devices, in which the use of special optical fibres, those monomode at the recording laser beam wavelength, facilitates manufacture as well as providing a versatile method of manufacture.

The use of a mask or masks interposed between holographic medium and the reference and object beams overcomes the fan-out low diffraction efficiency problem of holographic array generation by ensuring that each section of the emulsion (holographic medium) is only exposed once. The appropriate positioning of the special fibre used as the object beam source and of the mask may be carried out automatically under computer control, thereby facilitating hologram production, particularly volume holograms for space-variant optical interconnect devices.

Such volume holograms, which are all-optically generated, can selectively focus down and deflect an array of input beams to form an output array as chosen during manufacture. Whereas computers can be employed to actually generate the holograms required for interconnects, for example, they do this relatively slowly and can only achieve relatively thin holograms with only 40% efficiency at best. Producing the holograms optically as proposed above but using computer control of the positioning etc facilitates the hologram manufacture and with the appropriate materials can achieve closer to 1008 efficiency.

Claims

CLAIMS:-

1. A method of manufacturing a hologram including the steps of forming a reference beam and an object beam from a

beam, the formation of the object beam comprising launching a portion of the

beam into one end of an optical fibre which is monomode at the

beam wavelength, the object beam being output from the other end of the optical fibre which acts as a point source therefore; causing the reference beam and the object beam to intersect and be incident on a holographic recording medium for a predetermined exposure time and processing the thus exposed holographic recording medium to form said hologram.

2. A method as claimed in claim 1 wherein the hologram comprises an array of hologram elements each formed by exposing a respective portion of the holographic recording medium to the reference beam and the object beam for a respective exposure time.

3. A method as claimed in claim 2 wherein the respective portions of the holographic recording medium are exposed through masking means whereby each portion is only exposed once.

4. A method as claimed in claim 3 wherein the respective portions of the holographic recording medium are exposed one at a time and the position of the other end of the optical fibre is changed between exposures, each respective portion of the holographic recording medium having a different position of the other end of the optical fibre associated therewith.

5. A method as claimed in claim 4 wherein the masking means comprises an apertured mask disposed between the other end of the optical fibre and the holographic recording medium and including the step of changing the position of the aperture between exposures whereby to expose different portions of the holographic recording medium.

6. A method as claimed in claim 4 wherein the masking means comprises an apertured mask disposed between the other end of the optical fibre and the holographic recording medium and including the step of changing the mask for one with an aperture in a different place between exposures whereby to expose a different portion of the holographic recording medium, there being one said mask for each hologram element of the array.

7. A method as claimed in claim 4 wherein the masking means comprises an apertured mask disposed between the other end of the optical fibre and the holographic recording medium and including the step of changing the position of the holographic recording medium between exposures whereby to expose different portions thereof.

8. A method as claimed in any one of the preceding claims wherein the hologram is a transmission hologram and wherein for the formation thereof the reference beam and the object beam are directed towards the holographic recording medium from one side thereof.

9. A method as claimed in any one of claims 1 to 4 wherein the hologram is a reflection hologram and wherein for the formation thereof the reference beam and the object beam are directed towards the holographic recording medium from opposite sides thereof.

10. A method as claimed in claim 9 as appendant to claim 4, wherein the masking means comprises a pair of apertured masks disposed on opposite sides of the holographic recording medium with their apertures aligned, and including the step of changing the position of the apertures between exposures whereby to expose different portions of the holographic recording medium.

11. A method as claimed in claim 9 as appendant to claim 4, wherein the masking means comprises a pair of apertured masks disposed on opposite sides of the holographic recording medium with their apertures aligned, and including the step of changing the position of the holographic recording medium between exposures whereby to expose different portions thereof.

12. A method as claimed in claim 4 including the step of collimating the object beam output from the other end of the optical fibre, and wherein when the position of the other end of the fibre is changed correspondingly changing the angle at which the collimated object beam is incident on the holographic recording medium.

13. A method as claimed in claim 4 or any one of claims 5 to 12 as appendant to claim 4 wherein the change in position of the other end of the optical fibre is controlled by a computer.

14. A method as claimed in claim 13 and wherein the computer also controls the masking of the holographic recording medium whereby each portion thereof is only exposed once.

15. A method as claimed in claim 2 wherein the hologram comprises a space-variant optical interconnect device.

16. A method as claimed in claim 2 wherein the hologram comprises a spot array generator.

17. Apparatus for the manufacture of a hologram comprising a

beam source, the means for forming the object beam including an optical fibre into one end of which a portion of the output of the

beam wavelength, the object beam being output from the other end of the fibre in use of the apparatus, which other end of the fibre acts as a point source for the object beam; means for holding a holographic recording medium; means for directing the reference beam and the object beam towards the holographic recording medium in use of the apparatus for intersection thereof and incidence on the holographic recording medium; and means for exposing the holographic recording medium to the reference and object beams for a predetermined exposure time.

18. Apparatus as claimed in claim 17, wherein the hologram is comprised of an array of hologram elements each formed by exposing a respective portion of the holographic recording medium for a respective exposure time, and including masking means whereby the respective portions of the holographic recording medium are only exposed once.

19. Apparatus as claimed in claim 18 including means for changing the position of the other end of the optical fibre between exposures, each respective portion of the holographic recording medium having a different position of the other end of the optical fibre associated therewith.

20. Apparatus as claimed in claim 19 wherein the masking means includes an apertured mask and including means for causing relative movement between the mask and the holographic recording medium between exposures whereby different portions thereof can be exposed.

21. Apparatus as claimed in claim 20 wherein the position changing of the other end of the optical fibre and the relative movement between the mask and the holographic recording medium is controlled by a computer.

22. A method as manufacturing a hologram substantially as herein described with reference to the accompanying drawings.

23. Apparatus for the manufacture of a hologram substantially as herein described with reference to the accompanying drawings.