GB2384865A

GB2384865A - Depolariser with random orientated regions of polarisation state modulating material

Info

Publication number: GB2384865A
Application number: GB0202455A
Authority: GB
Inventors: Smith Mark Anthony Gleeson; Maurice Stanley
Original assignee: Holographic Imaging LLC
Current assignee: Holographic Imaging LLC
Priority date: 2002-02-02
Filing date: 2002-02-02
Publication date: 2003-08-06
Also published as: GB0202455D0

Abstract

A depolariser (2) has a plurality of randomly orientated crystals (4) which may be a birefringent material held in a carrier matrix (6). The crystals (4), preferably calcite crystals, are of sufficient size to induce a half wave retardation of incident light. The polarisation state of any given light ray transmitted through the depolariser (2) is dependent upon the orientation of the crystal(s) (2) through which it passes.

Description

DEPOLARISER The present invention relates to a device for depolarising light originating from a polarised or partially polarised light source.

In many optical systems, it is desirable to deliver unpolarised light having a high intensity and which is easily modulated. Light from incandescent light sources (eg. Hg lamp) although perfectly unpolarised, is neither intense nor easily modulated. On the other hand, laser light is intense and easily modulated, but generally polarised. There is therefore a need for a simple method of depolarising polarised light emitted from a laser.

US 4507785 discloses an unpolarised electro-optically Q-switched laser which incorporates a calcite prism and a Pockels cell between a pair of mirrors. The prism is selected to give a low-angle walk-off arranged that when a quarter wave length voltage is applied to the Pockels cell, the first pass will be cancelled by an equal and opposite walk-off during the return path. Such an arrangement is complex and of limited application.

Depolarisers comprising a crystalline quartz wedge with a compensating fused silica wedge to correct the angular deviation are known. The optic axis of the quartz wedge lies in the plane of the wedge and at 450 to the input polarisation. A significant disadvantage of such a depolariser is that a spatially regular variation of polarisation is imposed across the quartz wedge. The depolarisation is not random.

It is an object of the present invention to provide a simple depolariser which may be used to depolarise light from any polarised (or partially polarised) source.

According to a first aspect of the present invention, there is provided a depolariser comprising a plurality of randomly orientated regions of a polarisation state modulating material in a carrier matrix, whereby the polarisation state of a given light ray after transmission through the depolariser is dependent upon the path of that light ray through the depolariser.

Since each light ray has a different path through the depolariser, the relative polarisation states of any two light rays after transmission are independent of the relative input polarisation states of those two light rays.

The regions may be constituted by discrete particles of the polarisation state modulating material. However, said regions may be constituted by discrete volumes of liquid or semi-solid polarisation state modulating material.

Examples of materials which modulate the polarisation state of light include birefringent materials, birefringent liquid crystal materials in their mesophase (s), liquid crystal-based materials (eg. NCAP or PDLC sheet which comprises droplets of liquid crystal dispersed in a polymer matrix formed into a thin film, the scattering state of the film being electrically tuneable), optically active materials such as organic crystal materials and

aligned polymer particles (e. g. PVA, polyamide, polythene), or any material which exhibits anisotropy with respect to polarisation of light.

Preferably, each region is of a sufficient size to minimise light scatter. In general, scattering is reduced as the size of the regions increases relative to the wavelength of the light to be depolarised. However, there may be embodiments in which the scattering of light is required or beneficial, in which case some of said regions are preferably of sufficiently small size to promote light scatter.

Preferably, at least a fraction of the regions are of a sufficient size to be capable of modulating any given input polarisation state to any other possible polarisation state. It will be understood that the actual degree of modulation will be dependent on the orientation of the region relative to the incident light. Most preferably said fraction is at least half of the regions. It will be understood that such an arrangement increases the probability of the transmitted light being completely unpolarised (i. e. consisting of light having an equal amount of all polarisation states). The size of region required to achieve such modulation will be dependent upon the nature of the material used.

Preferably, a majority (and more preferably substantially all) of the regions are below a maximum predetermined size. If the regions are too large, localised polarisation effects will be observed. Clearly, the maximum predetermined size will be dependent on the material used.

If particles of birefringent material are used, these may be crystalline or non-crystalline. Crystalline materials may be uniaxial (i. e. hexagonal tetragonal or trigonal crystal structures). Specific examples of uniaxial crystals are tourmaline, calcite, quartz, sodium nitrate and rutile titanium oxide. Alternatively, the crystals may be biaxial (i. e. orthorhombic, monoclinic and triclinic). An example of a biaxial crystal is mica (KHASO).

If liquid crystal materials are used they must be in their mesophase at operational temperatures (i. e. normally room temperature). One example of a suitable liquid crystal material is 4-cyano-4'-n-pentyl-p-terphenyl.

For a birefringent material, modulation of an input polarisation state is dependent upon the degree of birefringence exhibited by the material (i. e. the no and np values of the material). To modulate an input polarisation state to any other polarisation state requires at least a half wave phase shift (positive or negative) across a single particle/region. Thus, a fraction (preferably at least half) of the particles preferably have a diameter > ÀI2 (no-np).

Preferably, the depolariser additionally comprises a substrate, such as a glass plate, on a side of which the carrier matrix is supported. However, it will be understood that the carrier matrix may be in the form of a sheet, block or other shaped article which is self-supporting, thereby obviating the need for a separate substrate for support.

In one series of embodiments, particularly but not exclusively suitable in the case of particulate polarisation state modulating materials, the carrier matrix is a binding agent such as an adhesive or curable polymer which is preferably capable of securing the particles in the carrier matrix to the substrate (when present).

In a second series of embodiments, particularly suitable in the case of nonparticulate polarisation state modulating materials, particularly liquid crystal and other flowable materials, the carrier matrix is a porous material the polarisation state modulating material residing in pores within the carrier matrix.

Conveniently, the carrier matrix and/or substrate (when present) are optically isotropic, and more preferably light transparent. Most preferably, when a birefringent polarisation state modulating material is used, the refractive index of the carrier matrix (and substrate when present) is midway between the ordinary and extraordinary refractive indices of the polarisation state modulating material in order to minimise scatter.

More than one polarisation state modulating material may be used. Each polarisation state modulating material may be in the same or different carrier matrix.

Where the depolariser comprises a support substrate, one or more carrier matrices may be provided on more than one side of the substrate.

The present invention also resides in a process for manufacturing a depolariser in accordance with the first aspect.

The present invention further resides in a process for manufacturing a depolariser comprising the steps of :- (i) dispersing a polarisation state modulating material in a flowable material to form a dispersion, (ii) coating a substrate on at least one side thereof with said dispersion, and (iii) setting said flowable material whereby to form a carrier matrix for said polarisation state modulating material, wherein discrete regions of said polarisation state modulating material are formed in the carrier matrix.

Examples of the invention will now be described by way of example only, with reference to the accompanying drawings in which, Figure 1 is a schematic view of polarised light incident on a depolariser in accordance with the present invention, and Fig 2 is a schematic view of a device incorporating the depolariser of Figure 1.

Referring to Figure 1, a depolariser 2 comprises a plurality of calcite crystals 4 embedded in a film 6 of a UV-curable glue (Norland-60), coated on a glass plate 8.

To make the depolariser 2, calcite is rapidly quenched in liquid nitrogen and powdered using a mortar and pestle to the desired crystal size. A solution of powdered calcite in Norland-60 is then formed, at least sufficient Norland-60 being added such that the mixture is free flowing, with the resultant solution being dip coated onto the glass substrate 8 in a per se known manner. It will be understood that the solution can be applied in other ways such as by spraying or spreading. The film 6 is cured by exposure to UV radiation, and once hardened serves as a carrier matrix for the calcite 4.

In the above embodiment, the quantity of calcite 4 and Norland-60 used is such that substantially all of the calcite 4 is embedded in the film 6 (i. e. no calcite is exposed above the surface of the film 6). Such an arrangement avoids unwanted light scattering. However in other embodiments, larger crystals and/or a thinner film layer can be used to ensure that calcite is exposed at the surface for those applications where light scattering is required. As a further alternative, to avoid exposure of calcite at the surface, a further layer (eg. Norland-60) can be applied over the Norland- 60/calcite layer to provide a smooth non-scattering surface.

Other embodiments can be envisaged in which the glass plate 8 is coated on both sides, the material on the second side being calcite or any other polarisation state modulating material. In addition more than one layer may be formed on the same side of the glass plate. Each additional layer may comprise the same or different polarisation state modulating material.

In certain application, it may be desirable to form layers comprising mixtures of two or more polarisation state modulating materials.

In general, a light ray incident on a birefringent crystal, as will be well understood by a skilled person, is double refracted to produce two parallel and spaced apart light rays (usually referred to as the o-and e-rays). More importantly in the present application, the two emerging rays will have orthogonal polarisation states (parallel to and perpendicular to the optical axis of the crystal). These effects are well known to the skilled person, see for example'Optics", E. Hecht, 2nd Ed., Ch. 8,1987, Addison-Wesley Publishing Company.

Thus, each (polarised) ray from a laser (not shown) is resolved into two orthogonal components each having an intensity determined by the angle of incidence relative to the optical axis of the particular crystal it is incident upon. Since individual rays across the incident light beam will have a different path through the depolariser, each ray will pass through one or more crystals having randomly different optical axis orientations (illustrated by arrows in figure 1). The net result is a beam of substantially randomly polarised (i. e. unpolarised) light. In order to obtain as high a randomisation in transmitted polarisation as possible, a majority of the calcite crystals have a diameter of > 1. cm to ensure at least a half wave retardation through those crystals (no and ne for calcite are 1.658 and 1.486 respectively at 589 nm). In other embodiments where materials other than calcite are used, the minimum particle size can be readily derived from a knowledge of the material's polarisation state modulating properties.

In a modification of the above embodiment, 4-cyano-4'-n-pentyl-p- terphenyl was used in place of calcite.

Referring to figure 2, the depolariser 2 of Figure 1 is shown in an image replication system. The image replication system is that described in our copending British Patent Application No. 0114862.6. Basically, the system comprises a light guide 12 made from a solid block of optical glass having a rectangular cross section and optically flat first and second end faces (12a, 12b) which are each perpendicular to the longitudinal axis of the block. The four sides of the block are each parallel to the longitudinal axis of the block and are also optically flat. A beam splitter 14 is optically coupled to the first end face 12a of the light guide 12 and an electrically addressed spatial light modulator (EASLM) 16 is optically coupled to the second end face 12b of the light guide 12. The EASLM (of per se known type) is a pixelated rectangular liquid display mounted on a reflective silicon layer. Electrically switched pixels induce a 900 rotation in the polarisation state of light incident thereon. It will be understood that since the EASLM 16 is rectangular, it is convenient for the light guide 12 to have a rectangular cross section (in addition, the cross section of the light guide 12 matches the size of the object data area on the EASLM 16).

The system also comprises a laser 18 (in this case an argon laser), a beam steering prism 20, a light spreading element in the form of a diffuser 22 and the depolariser 2 described with reference to figure 1. The beam steering prism 20 directs light from the laser 18 towards the beam splitter 14. The diffuser 22, which in this case is a holographic diffuser of an array of small lenses is positioned between the prism 20 and the beam splitter

14. The depolariser 2 is positioned between the prism 20 and the diffuser 22.

A polariser 24 is disposed between the second end face 12b of the light guide 12 and the EASLM 16, with a quarter-wave plate 26 disposed between the EASLM 16 and the polariser 24. The fast axis of the quarterwave plate 26 is arranged to be at 450 with respect to the polariser 24.

The system also comprises an imaging lens 28 and an imaging screen 30, both of which are arranged on the longitudinal axis of the light guide 12 adjacent the beam splitter 14. The design of the lens 28 determines the required length of the light guide 12.

Such a system delivers light to the EASLM having both spatially uniform intensity and brightness.

Claims

Claims 1. A depolariser comprising a plurality of randomly orientated regions of a polarisation state modulating material in a carrier matrix, whereby the polarisation state of a given light ray after transmission through the depolariser is dependent upon the path of that light ray through the depolariser.
2. A depolariser in accordance with claim 1, wherein said regions are constituted by discrete particles of the polarisation state modulating material.
3. A depolariser in accordance with claim 1, wherein said regions are constituted by discrete volumes of liquid or semi-solid polarisation state modulating material.
4. A depolariser in accordance with any preceding claim, wherein said polarisation state modulating material is a birefringent material.
5. A depolariser in accordance with any preceding claim, wherein the birefringent material is selected from one or more of the group consisting of liquid crystal materials, tourmaline, calcite, quartz, sodium nitrate, rutile titanium oxide and mica.
6. A depolariser in accordance with any preceding claim, wherein at least half of the regions of polarisation state modulating material have a

<Desc/Clms Page number 12>

sufficient diameter to be capable of modulating any given input polarisation state to any other possible polarisation state.
7. A depolariser in accordance with claim 6 when appended to claim 4, wherein the diameter of a majority of the regions of polarisation state modulating material is greater then À/2 (no-np), where À is the wavelength of incident light in use, and n,) and n, are the ordinary and extraordinary refractive indices respectively of the polarisation state modulating material.
8. A depolariser in accordance with any preceding claim wherein each region is below a predetermined size, whereby to minimise localise polarisation effects.
9. A depolariser in accordance with any preceding claim, wherein some of said regions are of sufficiently small size that light scatter is promoted.
10. A depolariser in accordance with any one of claims 1 to 8, wherein substantially all of said regions are of sufficiently large size that light scatter is substantially avoided.
11. A depolariser in accordance with any preceding claim, wherein the carrier matrix is a cured polymer.
12. A depolariser in accordance with any preceding claim comprising one or more additional carrier matrices.

<Desc/Clms Page number 13>
13. A depolariser in accordance with any preceding claim, additionally comprising a substrate, such as a glass plate, on at least one side of which the carrier matrix/matrices is/are supported.
14. A depolariser substantially as hereinbefore described with reference to Figure 1.
15. A process for manufacturing a depolariser in accordance with any one of claims 1 to 14.
16. A process for manufacturing a depolariser, comprising the steps of :- (i) dispersing a polarisation state modulating material in a flowable material to form a dispersion, (ii) applying said dispersion to at least one side of a substrate, and (iii) setting said flowable material whereby to form a carrier matrix for said polarisation state modulating material, wherein discrete regions of said polarisation state modulating material are formed in the carrier matrix.
17. A process for manufacturing a depolariser, substantially as hereinbefore described.