WO2008025999A2

WO2008025999A2 - Improvements in or relating to optical devices and systems suitable for producing synthesized holograms

Info

Publication number: WO2008025999A2
Application number: PCT/GB2007/003314
Authority: WO
Inventors: Robert Adam Munday; Olivier Pitavy
Original assignee: Spatial Imaging Limited
Priority date: 2006-09-01
Filing date: 2007-09-03
Publication date: 2008-03-06
Also published as: WO2008025999A3; GB0617209D0

Abstract

The invention relates to systems suitable for producing a digital hologram. Various techniques have been proposed. However, a disadvantage of digitisation is that, due to the power of lasers required to produce holograms, there was a risk of damage to coatings of various optical components. The present invention overcomes this problem by way of an optical device including: lenses (656, 660) placed in contact with a beam splitter/combiner (650) so as to produce first and second beams from an incident beam from a coherent source. The beam splitter/combiner is also placed in contact with first and second prisms (652, 654), whereby, in use, the device produces first and second beams which are combined to form two substantially collimated output beams, capable of producing an interference pattern at an output plane. The device may be incorporated into a digital holographic system and is ideally adapted to be retrofitted, so that, under control of suitable software it can be used to produce high quality digital holograms.

Description

IMPROVEMENTS IN OR RELATING TO OPTICAL DEVICES AND SYSTEMS

Field of the Invention

This invention relates to improvements in or relating to optical devices and systems.

More particularly, but not exclusively, the invention relates to an improvement in a device that is suitable for producing a holographic optical element, holographic micro- lens array, dot matrix hologram or a digital hologram. The invention also relates to an improvement in a related system.

Background

A holographic optical element, holographic micro-lens array or dot matrix hologram is hereinafter referred to as a digital hologram. Digital holograms have been fabricated since the early 1980's. Typically digital holograms comprise a plurality of holographic micro-lenses or pixels recorded in an array. Each micro-lens is a small, single, diffractive or refractive, holographic optical element in its own right.

Digital holograms are recorded in the form of a two dimensional array which can have any pattern or layout. Each holographic micro-lens in the array redirects light that is transmitted through it, or reflected off it, to a particular position or viewing zone in space. The precise angle at which light is deflected or reflected, from each holographic micro-lens, and hence the position of its viewing location in space, is predetermined when the digital hologram is made.

A digital hologram therefore is a complex holographic micro-lens array that can redirect incident light from a plurality of its constituent elements, to a multitude of positions or viewing locations in space. Various techniques for producing digital holograms have been proposed. Advantages of using digital techniques, over conventional analogue methods, are: firstly that production of the hologram is likely to be more controllable; and secondly production using digital techniques benefits from features and advantages often associated with digital systems.

One such feature is the ability to use information directly derived from, or stored on, a computer. However, a disadvantage of digitisation is that, because the printing of holographic micro-lenses has to be carried out sequentially, the time to record digital holograms was in some instances very much greater than the exposure time required to produce conventional analogue holograms, all of which was recorded at once.

Prior Art

A problem with holographic equipment and systems used to produce digital holograms, by way of interference of two or more light beams, has been the generation of, and subsequent control of, two or more independent beams of coherent light that are necessary to produce a holographic micro-lens. This problem becomes even more acute as pixel size decreases (a necessary requirement in order to produce higher resolution digital holograms), and micro-lens writing speeds increase (a necessary requirement in order to produce larger holograms).

Creating and controlling two or more coherent beams of light is not in itself a problem and, for example, can be readily achieved using existing optical equipment, such as aperture masks, beam splitters and diffraction gratings. However, the problem of producing two or more beams, from the same source, and controlling them in a manner to produce holographic micro-lenses, at the dimensions and write speeds, necessary to achieve high resolution large area images, is a pronounced one. One of the most successful systems currently available for producing digital holograms is sold by Spatial Imaging Limited in the UK. The system is sold under the LlGHTGATE (Trade Mark) and utilises a pair of rotating aperture masks to produce and manipulate two or more coherent laser beams, which are then focussed by means of lenses to produce the necessary interference patterns in order to record a holographic micro-lens. The aperture masks are rotated independently of one another using high speed, brushless rotary servo motors.

Although the aforementioned LIGHTGATE system has been highly successful, there are limitations that arose mainly due to the speed of operation of the rotary servo motors.

Reference is now made to the invention described in our co-pending, published International Patent Application Number W0-A1 -2006/003457, which describes an optical device and system for fabricating a digital hologram. So as to assist in the understanding of the present invention the complete contents of the aforementioned published International Patent Application are incorporated herein by way of reference.

The system described in our co-pending International Patent Application Number WO- A1 -2006/003457 overcomes many of the problems associated with, for example, rotating aperture masks, beam splitters and diffraction gratings, by providing an arrangement that is capable of producing a pair of twin beams which are in registration one with another and which is also capable of positioning those beams at very high speed.

The system, described in International Patent Application Number W0-A1- 2006/003457, was therefore extremely successful and provided a substantial improvement over existing systems by use of an arrangement of optical components that enabled digital holograms to be produced at much greater speeds than was previously possible. However, a limitation of the system described in International Patent Application Number W0-A1 -2006/003457, is that there is a risk of damage to optical coatings when using high power lasers, as beams are focused on the surface of optical devices. Secondly, the optical path lengths of each of the twin beams were different, making the use of low or no coherence length lasers difficult. Thirdly, there was light loss in the system due to the number of optical surfaces; and fourthly the system proved relatively awkward to align due to the nature and number of the optical components involved.

The present invention arose in order to produce an optical device and optical system which overcomes the aforementioned limitations by reducing the risk of damage to optical coatings; ensuring that the two beam paths are optically identical; reducing light lost by the system; and providing easier alignment without detracting from the ability of the optical system to produce large scale bright, high quality digital holograms at high speed.

Summary of the Invention

According to a first aspect of the present invention there is provided an optical device including: a beam deflector, which, in use, is adapted to deflect a substantially collimated beam of coherent light to produce an incident beam, comprises: a beam splitter/combiner arranged to produce first and second beams from the incident beam; and first and second prisms arranged to reflect said first and second beams towards the beam splitter/combiner, such that said first beam is inverted with respect to said second beam, whereby, in use, the first and second beams are combined by the beam splitter/combiner to form two substantially collimated output beams, capable of producing an interference pattern at an output plane.

Preferably the combination of beam splitter/combiner and prism pair forms an interferometer. Most preferably the first and second prisms are arranged such that their longitudinal axes are orthogonal with respect one to another. The result is that the first prism inverts an image and the second prism leaves the image unaltered.

Preferably lenses are located outside the interferometer, a first to focus the single input beam and a second to re-collimate and converge the two output beams. One way of achieving this is to arrange the two lenses such that the prisms are preferably at the focal lengths of the lenses and the lenses themselves are preferably a distance apart which is equal to their combined focal length.

If the two lenses have the same focal length then the input and output collimated beams have the same diameter, otherwise the output beam diameter will be bigger or smaller.

Preferably the first and second prisms are in intimate contact with the beam splitter. However, the first and second prisms may be spaced apart from the beam splitter, or there may be an index matching material placed between the prisms and the beam splitter.

Focusing means may be provided so as to vary the diameter of the incident beam and/or output beams.

Similarly coHimating means may be provided so as to vary the collimation of the incident beam and/or output beam.

An advantage of using prisms in the system, providing such prisms have a suitable refractive indices, is that total internal reflection of the incident laser beams, in the prisms, can be achieved. Light therefore reflects from inner surface of the material with the result that no reflective coatings are needed and hence higher power lasers can be safely used. Another advantage of the invention is that it can be used with very limited coherence length lasers because both optical path lengths can be identical.

Another advantage is that there are relatively few transmissive or reflective optical components and therefore more light is transmitted through the system.

Ideally prisms and beam splitters are placed in physical contact one with another and an advantage of this configuration is that because there are relatively few components the system is relatively easy to align. Furthermore the system can be miniaturised into a single optical component and thus form a very compact and stable 'holographic printing head'.

A mask may be suitably positioned at the output plane and dimensioned and arranged so as to define an aperture for forming a pixel from two beams, which beams in operation, are in registration and superimposed, one with another, so as to create an interference pattern.

Ideally the mask is perpendicular with the optical axis and defines a tessellating shaped aperture. Because the beams are confined by the mask, and due to the fact that they are in registration and superimposed one with another as they pass through the mask, the combined interfering beams have identical characteristics due to the screening effect of the mask. The result is a bright, high quality hologram comprising pixels, which substantially cover the entire holographic supporting surface.

Ideally the mask has two sets of substantially parallel sides that are most preferably all the same length and arranged at right angles, so as to define a suitably shaped aperture that produces square shaped pixels. However, it will be appreciated that triangular, hexagonal or indeed any other shaped pixel may be formed by the mask. In an alternative embodiment even a letter or sequentially varying shaped pixel may be used for the purposes of producing a secure hologram in which is encoded or embedded a unique identifier, such as a watermark label.

Such a sequentially varying or alternating mask may comprise a ferro electric device or spatial light modulator (SLM), which is addressable electronically in order to alter its shape and dimensions in real time. Therefore optical encoding of information, such as security data, into a hologram at a microscopic level, is achievable under direct control of software for example. Therefore using this technique such data is difficult to reproduce, as it is not only extremely small, but can also be encrypted.

A holographic pixel and its holographic grating can be produced by reduction of the mask image size, by way of image reduction optics, which enables the resolution of the hologram (in pixels per mm²) and therefore the usable spatial frequency range to be increased. The consequence of this is that the quality of the final hologram is improved.

The above mentioned optical device is hereinafter referred to as a scanning interferometer. Because it is envisaged to fabricate the optical device as a stand alone system, it may be retrofitted to existing optical systems. Suitable software may also be provided to enable existing systems to be modified to operate with the scanning interferometer.

An advantage of the scanning interferometer is that two coherent beams are produced, and displacement of the first beam from the principal axis is exactly replicated by displacement of the second beam, but in an opposite direction. As the size of the angle of deflection in both x-z and y-z planes, from the principal optical axis is identical in both beams, any rotation (0), of the beams about the principal optical axis, also occurs simultaneously. Thus the beams are easier to manipulate and control. Preferably means is provided to vary the diameter of the collimated beam. A reducing/enlarging lens arrangement is ideally adapted to vary the beam diameter. An advantage of this feature is that the mask aperture can be filled with a uniform intensity of light. This provides more consistent pixel quality, which diffract light uniformly and at the same intensity across their entire surface, thus a superior quality hologram is obtained.

In a preferred arrangement, a pixel size of less than 30μm x 30μm is achieved. Ideally the pixel size is less than 20μm x 20μm and most preferably it is less than 10μm x 10μm.

Advantages of the above mentioned embodiments, in digital holography, are that two exactly identical beams are produced whose characteristics can be manipulated simultaneously and at very high speed. Under control of suitable software, coherent light beams can be displaced controllably and rapidly whilst registering and superimposing them at a common output point.

According to another aspect of the invention beams are displaced to different positions, relative to the optical axis, in order to produce a desired holographic grating at a high speed and in a controllable manner. This is one factor that relates the speed of manipulation of the two beams to the speed of exposure. The amount and speed of displacement is typically measured as pixels per second or exposures per second. In this sense the beams can be displaced typically at a rate of 0.2 kHz to 2 kHz and ideally in excess of 5 kHz using certain optical beam deflectors or scanners. Digital hologram recording times are therefore greatly reduced.

It will be appreciated that the invention, under control of a suitable controller, for example a microprocessor operating according to suitable software, is capable of producing a pair of superimposed beams whose spot sizes are manageable and small. Moreover as the two beams emanate from a common source they are absolutely identical in every respect (including coherence and polarisation). Furthermore the two beams may be manipulated i.e. displaced laterally in both the x-z and y-z planes and can therefore effectively be rotated about the principal optical axis, by an angle (0) at very high speeds and with accuracy and precision.

An acousto-optic modulator (AOM) may be used as a beam deflector. An advantage of an acousto-optic beam deflector is that it can be operated at high frequencies, typically in excess of 5 kHz, and preferably in excess of 10 kHz, thereby enabling holographic production rates to be further increased.

An alternative beam deflector utilises a piezoelectric (PZ) element. A further alternative beam deflector is a galvanometric beam deflector and is typically capable of exposing 3 kHz or 3000 points per second.

An image reduction system may be placed between the point at which the two output beams converge and superimpose with one another (at the output plane or the object) and the point at which the holographic image is to be recorded (the image). An advantage of this is that the recorded holographic pixel and its holographic grating can be reduced in size and therefore both the resolution of the hologram (in pixels per mm) and the usable spatial frequency range is increased. Again a direct consequence of this is that the quality of the final hologram is improved.

In all the abovementioned aspects it will be understood that the angle of deflection of the first beam (Ψ) is in an opposite sense, about the principal optical axis, to the angle of deflection of the second beam (θ).

Preferred embodiments of the invention will now be described, by way of exemplary examples only, and with reference to the Figures generally, and specifically Figures 2, 3 and 4, in which:

Brief Description of the Figures Figure 1 shows a diagrammatical overview of an embodiment of an optical system which produces focussed output beams suitable for the production of digital holograms;

Figure 2 shows an example of a collimated beam input/output system;

Figure 3 shows a diagrammatical overview of an embodiment of an optical device, for example for use in the system of Figure 2; and

Figure 4 shows an alternative embodiment of an embodiment of an optical device, for example for use in the system of Figure 2.

Detailed Description of Preferred Embodiments of the Invention

Referring to Figure 1 as background and to Figures 2, 3 and 4, in which like parts bear the same reference numerals, there is shown a device 10 incorporated in an optical system 100 for producing a hologram (not shown) at an output plane 200. The output plane 200 may be located at either of two positions 200a or 200b in dependence upon whether an image reduction device 300 is included.

The optical system 100 includes a modulated laser 60 for producing a coherent light beam 12 and a scanner 630 for deflecting the light beam 12 in accordance with control signals from a controller. A photoresist coated plate or other holographic supporting medium is located at the output plane 200a or 200b and it is here where the hologram is formed. The photoresist coated plate is typically supported so that relative displacement is achieved between the system and the photoresist coated plate (or other light sensitive recording medium), so that pixels can be defined at different locations on the photoresist coated plate.

The embodiment shown in Figure 2 includes a beam reducing/enlarging lens arrangement 20. The lens arrangement 20 is used to precondition the beam 12 prior to it being deflected by scanner 630. Preconditioning of beam 12 is carried out in order to vary the diameter of beam 12 that is incident on scanner 630.

The scanner 630 or beam deflector 630 may comprise a piezoelectric (PZ) element, an acousto-optic device (AOD), a galvanometric beam deflector or other beam deflection device.

Light beam 12 passes to scanner 630 where it is deflected along x and y axes in accordance with signals received from a controller 61. The scanner 630 alters the angle of incidence of the laser beam 12 to optical device 10 in both x and y directions. Beam 12 is split by beam splitter/combiner 650 into a first beam 12A and a second beam 12B (not labelled in 2,3,4).

Referring again briefly to Figure 1 , the particular orientation of beam splitter 50, lenses 46a, 47a, 48a and 49a and beam combiner 55 in the optical device 10; their relative position one to another and the focal lengths of the lenses 46a, 47a, 48a and 49a ensure that the angle of deflection (Θ1) of the first beam 12A is the same as, but opposite to, the angle of deflection (Θ2) of the second beam 12B. This configuration ensures that two exactly identical beams are produced and that the displacement of beam 12A is faithfully reproduced, but in an opposite sense, by beam 12B.

Referring now to Figure 2, the optical device 10, shown incorporated as part of a system 100, shows light beam 12 emanating from laser 60 passing through a lens arrangement 20 that acts as a beam reducer or enlarger. Lens arrangement 20 is optional and includes, for example, a series of co-linear focussing lenses. The optical device 10 produces a pair of optically identical beams 12A and 12B in a manner similar to the device 10 shown in Figure 1.

In the embodiment shown in Figure 2 the optical device 10 comprises: a beam splitter 650, first and second lenses 656 and 660 and first and second prism 652 and 654. Scanner 630, in use, is preferably located at lens 656. Beam splitter 650 produces first 12A and second 12B beams, from the incident beam 12, which first and second beams 12A and 12B are capable of being displaced about the principal optical axis. The first and second prisms are arranged such that their longitudinal axes are orthogonal with respect one to another. The result is that the first prism inverts an image and the second prism leaves the image unaltered. This occurs as a result of the relationship between the beamsplitter, prisms and lenses. The beams are then collimated and diverged from a common point in an output plane by lenses 660.

The optical device 10 may be retrofitted to an existing optical system 100. Suitable software may also be provided to enable exiting systems 100 to be modified to operate with the device.

It will be appreciated that modification of the system 100 to receive the optical device 10, may be required.

It will be further appreciated that use of the optical device 10 enables the production of high quality digital holograms to be fabricated faster than has previously been achievable. Moreover as the device 10 can be retrofitted to existing holographic fabrication systems 100, under supervision of suitable control software and hardware, it will be further appreciated that the invention provides a relatively cheap way of upgrading existing systems to provide superior quality holographic production systems.

It is understood that control hardware and software, as well as data carriers supporting said software, for use with the aforementioned device 10 and/or system 100, falls within the scope of the invention.

Figures 3 and 4 shows a diagrammatical view of an optical device 10 that is suitable for use in a system for producing digital holograms, for example as shown in Figures 1 and 2. Advantages of the embodiment shown by Figures 2, 3 and 4 are that the device is lightweight and compact; there is small light loss due to the small surface area. Furthermore alignment is not necessary as the optical components are pre- aligned at manufacture. This also improves shock resistance.

Referring to Figures 3 and 4, there are depicted to embodiments of an optical device 10 in which like parts bear the same reference numerals. Device 10 includes a beam splitter 650, lenses 656 and 660 and first and second right angle prisms 652 and 654. Beam splitter 650 also acts as a beam combiner when combining beams reflected from prisms 654 and 656. The embodiment in figure 4 is equivalent to that shown in Figure 3 except that beam splitter 650, focusing lenses 656 and 660 and first and second prisms 652 and 654 are all fused to form a single optical component that is miniaturised, pre-aligned and shock resistant.

Referring again to Figures 2, 3 and 4, scanner 630, deflects a collimated beam of coherent light. Beam splitter/combiner 650 produces first 12A and second 12B beams (not labelled), from the incident beam 12, which first and second beams 12A and 12B are capable of being displaced about the principal optical axis. This occurs as a result of the relationship between the beam splitter 650 and first and second prisms 652 and 654. Beam splitter 650 then combines first and second beams reflected and the combined beams are then collimated and converged to a common point on an output plane by lens 660.

A mask 200a is placed at an output plane of the optical system 100. A resist or other photosensitive recording medium is located at the output plane 200b. An image relay system, which preferably is used as an image reduction system 300, is also present; the purpose of the image relay or image reduction system 300 is explained below.

Lens arrangement 20 is optional and is used to condition beam 12 in order to vary the beam diameter so as to give an even intensity of light across the aperture mask 500. An x-y beam deflector 30 typically comprises a piezoelectric (PZ) element, an acousto optic device (AOD) , galvanometric beam deflector or other beam deflection device. The beam 12 is split by a beam splitter 650 into two beams 12a and 12b. Light beam 12 passes to the beam deflector 30 where it is deflected along x and y axes in both x-z (Θ1) and y-z planes (Θ2) in accordance with control signals received from a controller (not shown).

An image relay system 300, which may preferably be an image reduction system (although may be used as an image enlargement system), is positioned between aperture mask 200a and recording plane 200b, to relay and preferably reduce, an image of the aperture mask onto the recording plane 200b.

Device 10 may be referred to as a scanning interferometer and, in use, produces two collimated (or focused) output beams that converge symmetrically about the principal optical axis (PA) at the output plane. Optical device 10 may be retrofitted to an existing optical system. Suitable software may also be provided to enable exiting systems to be modified to operate with the device.

It will be appreciated that modification of the system 100 may be required to enable it to be fitted to alternative optical systems.

Use of the optical device 10 therefore enables the production of high quality digital holograms to be fabricated faster than has previously been achievable. Moreover as the device can be retrofitted to existing holographic fabrication systems, under supervision of suitable control software and hardware, it will be further appreciated that the invention provides a relatively cheap way of upgrading existing systems to provide superior quality holographic production systems.

The invention has been described by way of examples only and it will be appreciated that variation may be made to the embodiments described without departing from the scope of the invention.

Claims

1. An optical device for use with a beam deflector, which deflects a substantially collimated beam of coherent light to produce an incident beam, comprises: a beam splitter/combiner arranged to produce first and second beams from the incident beam; and first and second prisms arranged to reflect said first and second beams towards the beam splitter/combiner, such that said first beam is inverted with respect to said second beam, whereby, in use, the first and second beams are combined by the beam splitter/combiner to form- two substantially collimated output beams, capable of producing an interference pattern at an output plane.

2. An optical device according to claim 1 wherein the combination of beam splitter/combiner and prisms form an interferometer.

3. An optical device according to claim 1 or 2 wherein the first and second prisms are arranged such that their longitudinal axes are orthogonal with respect one to another.

4. An optical device according to any preceding claim wherein lenses are located outside the interferometer, a first to focus the single input beam and a second to re- collimate and converge the two output beams.

5. An optical device according to claim 4 wherein the lenses are arranged such that the prisms are preferably at the focal lengths of the lenses and the lenses themselves are preferably a distance apart which is equal to their combined focal length.

6. An optical device according to any preceding claim, wherein the first and second prisms are in intimate contact with the beam splitter.

7. An optical device according to claim 5 wherein the first and second prisms are spaced apart from the beam splitter.

8. An optical device according to claim 6 wherein an index matching material placed between the prisms and the beam splitter.

9. An optical device according to any preceding claim wherein focusing means is provided so as to vary the diameter of the incident beam and/or output beams.

10. An optical device according to any preceding claim wherein collimating means is provided so as to vary the collimation of the incident beam and/or output beam.

11. An optical device according to any preceding claim wherein a mask is positioned at the output plane and dimensioned and arranged so as to define an aperture for forming a pixel from two beams, which beams in operation, are in registration and superimposed, one with another, so as to create an interference pattern.

12. An optical device according to claim 11 wherein the mask is arranged orthogonal with respect to the optical axis and defines a tessellating shaped aperture.

13. An optical device according to claim 12 wherein the mask defines an aperture for producing pixels from the group comprising: squares, triangles and hexagons.

14. An optical device according to any preceding claim for use in producing a secure hologram in which is encoded or embedded a unique identifier, such as a watermark label.

15. An optical device according to any preceding claim includes a sequentially varying mask comprising a ferro electric device, which is addressable electronically in order to alter its shape and/or dimensions in real time.

16. An optical device according to any of claims 1 to 14 includes a sequentially varying mask comprising a spatial light modulator (SLM), which is addressable electronically in order to alter its shape and/or dimensions in real time.

17. An optical device according to either claim 15 or 16 wherein a driver operating the sequential variation of the mask is encrypted under suitable control software.

18. A system including: a laser, a scanner and a lens train and further including the optical device according to any preceding claim.

19. A system according to claim 18 further including means to vary the diameter of the collimated beam.

20. A system according to claim 18 or 19 further including a reducing/enlarging lens adapted to vary the beam diameter.

21. A system according to any of claims 18 to 20, when dependent on claim 11 , wherein the mask is provided to vary the diameter of the collimated beam to provide a pixel size of less than 30μm x 30μm.

22. A system according to any of claims 18 to 20, when dependent on claim 11, wherein the mask is provided to vary the diameter of the collimated beam to provide a pixel size of less than 20μm x 20μm.

24. A system according to any of claims 18 to 20, when dependent on claim 11, wherein the mask is provided to vary the diameter of the collimated beam to provide a pixel size of less than 10μm x 10μm.

25. A system according to any of claims 18 to 24 wherein a control of a suitable controller, for example a microprocessor operating according to suitable software, is capable of producing a pair of superimposed beams whose spot sizes are manageable and small.

26. A system according to any of claims 18 to 25 wherein an acousto-optic modulator (AOM) may be used as a beam deflector.

27. A system according to any of claims 18 to 25 wherein a piezoelectric (PZ) element is used as a beam deflector.

28. A system according to any of claims 18 to 25 wherein a galvanometric beam deflector is used as a beam deflector.

29. A system according to any of claims 18 to 28 includes software for operating and controlling the system so as to wherein a

30. A method of displacing beams of coherent light to different positions, relative to a common optical axis, in order to produce a desired holographic grating at a high speed and in a controllable manner, so as to create characterised in that the exposure rate of pixels is in excess of 3 kHz and ideally in excess of 5 kHz.