WO2001061384A1

WO2001061384A1 - Switchable holograms

Info

Publication number: WO2001061384A1
Application number: PCT/GB2001/000591
Authority: WO
Inventors: Milan Momcilo Popovich; John J. Storey; Jonathan David Waldern; C. David Nabors; Gary Qian
Original assignee: Digilens, Inc.
Priority date: 2000-02-15
Filing date: 2001-02-14
Publication date: 2001-08-23
Also published as: AU3210001A

Abstract

Holographic fringes recorded in a substrate (10) can be activated and de-activated by the application of an electrical signal to electrodes (13, 14) on opposed surfaces (11, 12) of the substrate. At least one of the electrodes (13, 14) is composed of separate portions (17, 18) having interdigitated fingers (20). When an electric signal is applied across the electrodes (13, 14) and electric field is produced generally in a first direction perpendicular to the surfaces (11, 12). In contrast, when an electric signal is applied across the electrode portions (17, 18) an electric field is produced generally in a second direction parallel to the surfaces (11, 12) and orthogonal to the said first direction. In this latter condition, the respective state of the substrate (i.e. activated or de-activated, as the case may be) is reinforced by the electric field.

Description

TITLE: SWITCHABLE HOLOGRAMS

Field of the Invention

This invention relates to switchable holograms.

Description of the Relevant Art

Switchable holograms typically comprise a substrate having a pair of opposed surfaces on which electrodes are respectively disposed. The hologram itself is formed by holographic fringes recorded in the substrate, these fringes being composed of particles whose orientation can be changed by applying an electric field by way of the electrodes. It is by means of this change in orientation of the particles that the hologram can be activated and de-activated. In transmission holograms, the fringes tend to be oriented perpendicularly to the surface of the hologram, whereas in reflection holograms the fringes tend to be oriented parallel to the hologram surface.

In a typical arrangement, the hologram is formed by liquid crystal droplets and a surrounding polymer. In one type of material, the droplets tend to be aligned with their long axes normal to the Bragg surfaces of the holographic fringes, and within each droplet the liquid crystal molecules tend to be aligned parallel to the long axis of the droplet. When an electric field is applied by way of the electrodes, the liquid crystal molecules become re-aligned parallel to the field direction. In another type of material, the liquid crystal molecules tend to be oriented orthogonally to the long axes of the droplets, but become re-oriented perpendicularly to the field direction when an electric field is applied.

In general, the distribution of the orientation of the liquid crystal droplets is different for transmission holograms and reflection holograms due (under current thinking) to polymerisation-induced compression and other factors. Transmission holograms conform better to the ideal requirement than do reflection holograms. Typically, in transmission holograms it is found that most of the liquid crystal droplets tend to be elongated with their long axes lying in a direction normal to the Bragg surfaces. Such elongated droplets will tend to have a preferred molecule alignment direction, which results in a large change in refractive index between the "on" and "off states. On the other hand, reflection holograms tend to have droplets which exhibit a relatively small degree of anisotropy: the liquid crystal molecules in this case are less uniformly aligned and. as a result, the range of refractive index between the on and off states tends to be smaller than for transmission holograms.

In an ideal situation, the liquid crystal molecules would be uniformly oriented in one direction when the electric field is not applied, and in an orthogonal direction when it is. Moreover, this would be irrespective of the type of liquid crystal material used or the Bragg geometry of the holographic fringes. This would result in a maximum change in the average refractive index between the hologram activated and de-activated states, and would be advantageous for achieving optimum diffraction efficiency and a large dynamic range (or contrast) between these states. The latter is a particularly important factor in cases where several holograms are stacked together and are switched on and off in sequence.

In practice, however, the desired uniformity in molecular alignment can only be achieved in one of the two states, i.e. when the hologram is de-activated by the application of a sufficiently large electric field. When there is no applied field, the molecules are aligned in directions determined by the material and the Bragg fringe geometry. Indeed, even within each liquid crystal droplet there is considerable variation in the orientation of the individual molecules. Amongst other things, this tends to render the hologram sensitive to the polarisation state of the incident light. For example, for transmission-type holograms the diffraction efficiency for p-polarised light tends to be somewhat greater than that for s- polarised light.

It is an object of the present invention to obviate or mitigate the above-mentioned difficulties. Summary of the Invention

According to a first aspect of the present invention, there is provided a switchable hologram comprising: a substrate in which there is recorded a hologram comprising fringes which are composed of particles (e.g. molecules) whose orientation can be changed by the application of an electrical stimulus, thereby activating and de-activating the hologram,

first means by which an electric field can be applied to said substrate in a first direction and thereby orient said particles in a first direction corresponding to the hologram being activated, and

second means by which an electric field can be applied to said substrate in a second direction and thereby orient said particles in a second direction corresponding to the hologram being de-activated.

Advantageously, said first and second directions are mutually orthogonal.

Preferably, said first direction is generally parallel to opposed surfaces of the substrate, and said second direction is generally perpendicular to said surfaces.

Conveniently, said second means includes a pair of electrodes disposed on opposite sides of said substrate.

Desirably, said first means includes a pair of electrode portions disposed on one side of said substrate but spaced apart from one another in a direction parallel to said opposed surfaces. According to a second aspect of the present invention, there is provided a switchable hologram comprising: a substrate having a pair of opposed surfaces, said substrate having recorded therein holographic fringes which can be activated and de-activated by the application of an electrical stimulus, and

first and second electrodes disposed respectively on said opposed surfaces of said substrate and by means of which an electrical stimulus can be applied to the holographic fringes, said first electrode being composed of first and second portions spaced apart from each other in a direction parallel to the respective surface of said substrate,

the arrangement being such that, when a first electrical stimulus is applied across said first and second electrodes, an electric field is produced which is directed generally perpendicularly to said opposed surfaces of said substrate, and when a second electrical stimulus is applied across said first and second portions of said first electrode, an electric field is produced which is directed generally parallel to said respective surface of said substrate.

The second electrode can also be composed of first and second portions spaced apart from each other in a direction parallel to the respective surface of said substrate, the arrangement being such that, when an electrical stimulus is applied across said first and second portions of said second electrode, an electric field is produced which is directed generally parallel to said respective surface of said substrate.

Advantageously, said first and second portions of said first electrode are spaced apart from each other in a first direction, and said first and second portions of said second electrode are spaced apart from each other in a second direction which is generally perpendicular to said first direction. In the case where the holographic fringes have Bragg surfaces whose slant angle (as defined herein) varies across the area of said substrate, at least one of said first and second electrodes is preferably, composed of a plurality of independently operable segments, each of which is disposed over an area of said substrate in which said slant angle lies within a respective predetermined range.

Conveniently, each of said segments comprises first and second portions as hereinbefore described.

The term 'slant angle' as used herein means the angle between the said Bragg surfaces and a normal to said opposed surfaces of the substrate.

According to a third aspect of the present invention, there is provided a switchable hologram comprising: a substrate having a pair of opposed surfaces, said substrate having recorded therein holographic fringes which can be activated and de-activated by the application of an electrical stimulus,

first means operative to apply an electrical stimulus across said first and second electrodes, thereby to produce an electric field which is directed generally perpendicularly to said opposed surfaces of said substrate, and

second means operative to apply an electrical stimulus across first and second portions of said first electrode, thereby to produce an electric field which is directed generally parallel to said respective surface of said substrate.

According to a fourth aspect of the present invention, there is provided a switchable optical component comprising: a switchable hologram as defined in any of the preceding paragraphs,

at least one optical input path to said hologram, and

at least one optical output path from said hologram,

wherein at least one of said input and output paths is a guided wave optical path.

In a first arrangement, the switchable optical component comprises first and second guided wave input paths and first and second guided wave output paths, the switchable hologram when activated being operative to direct light from said first guided wave input path to said first guided wave output path and/or to direct light from said second guided wave input path to said second guided wave output path, the switchable hologram when de-activated permitting the passage of light from said first guided wave input path to the second guided wave output path and/or permitting the passage of light from said second guided wave input path to said first guided wave output path.

In a second arrangement, the switchable optical component comprises a guided wave input path and a guided wave output path, the switchable hologram when activated being operative to reflect light from said guided wave input path back along said guided wave output path.

In a third arrangement, the switchable optical component comprises a guided wave input path, a guided wave output path and a free space output path, the switchable hologram when activated being operative to direct light from said guide wave input path to said free space output path, and when de-activated permitting the passage of light from said guided wave input path to said guided wave output path.

In a fourth arrangement, the switchable optical component comprises a free space input path, a free space output path and a guided wave output path, the switchable hologram when activated being operative to direct light from said free space input path to said guided wave output path, and when de-activated permitting the passage of light from said free space input path to said free space output path.

Brief Description of the Drawings

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic side view of a first embodiment of a switchable hologram according to the present invention;

Figure 2 is a schematic, exploded perspective view of the switchable hologram shown in Figure 1 but also illustrating the electrical connections thereto;

Figure 3 is a similar exploded perspective view of a second embodiment of a switchable hologram according to the present invention;

Figures 4 and 5 are respective schematic sectional views of two different electrode arrangements for the switchable hologram;

Figure 6 is a schematic exploded perspective view of a third embodiment of a switchable hologram according to the present invention; and

Figures 7 to 12 show various applications of the switchable hologram to telecommunications switching systems. Detailed Description

Referring first to Figures 1 and 2, the switchable hologram shown therein comprises generally a substrate layer 10 in which holographic fringes are recorded. The substrate layer 10 has a pair of opposed surfaces 11 and 12 on which are respectively disposed a pair of electrodes 13 and 14, each electrode being composed of a support substrate 15 and an electrically conductive coating 16. As will be explained in detail later, the hologram can be de-activated by applying an electrical stimulus across (i.e. between) the electrodes 13 and 14. The hologram recorded in the substrate layer 10 can be a thin phase hologram (that is, one which conforms to the Raman Nath regime) or can be a volume hologram, also known as a thick or Bragg hologram. Use of the latter is preferred, because it offers high diffraction efficiencies for incident beams whose wavelengths are close to the theoretical wavelength satisfying the Bragg diffraction condition, and which are within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition.

To facilitate switching of the hologram, the medium in which it is recorded is typically a polymer-dispersed liquid crystal mixture which undergoes phase separation during the hologram recordal process, creating fringes comprising regions densely populated by liquid crystal micro-droplets interspersed with regions of clear polymer. When an electric field is applied by way of the electrodes 13 and 14, the natural orientation of the liquid crystal molecules is changed, causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to a very low level, thereby effectively erasing the hologram. Using such a system, it is possible to achieve very fast switching rates, typically with a switching time of less than 150 microseconds, and perhaps as low as a few microseconds.

The support substrates 15 of the electrodes 13 and 14 can be composed of glass, plastics or composite materials which can be flexible or rigid, and flat or curved. In cases where the hologram is used in the free space propagation of light, the coatings 16 can be composed of transparent conducting material, such as ITO or electrically-conducting polymers, and can be provided with anti-reflection coatings. It is also possible for the switching circuitry for the electrodes 13 and 14 to be deposited on the support substrates 15 as well.

Referring now particularly to Figure 2 (where the substrate layer 10 has been omitted for the sake of clarity), the electrode 13 is composed of first and second portions 17 and 18 each of which comprises a shank 19 and a series of fingers 20 extending perpendicularly from the shank 19 in equidistant, laterally-spaced relation. The fingers 20 of the two portions 17 and 18 are interleaved, such that the portions 17 and 18 are interdigitated with each finger 20 of the portion 17 being spaced from the adjacent finger(s) 20 of the portion 18 in a direction parallel to the plane of the surface 11 of the substrate layer 10. In contrast, the electrode 14 is of substantially planar configuration and extends over the area of the surface 12 of the substrate layer 10.

A voltage source 21 is connected between the electrode 14 and the portion 17 of the electrode 13, whilst a further voltage source 22 is connected between the electrode 14 and the portion 18 of the electrode 13. When one or both of the voltage sources 21 and 22 is activated, an electric field is applied across the thickness of the layer 10 (i.e. perpendicularly to the surfaces 11 and 12) in the direction of arrow A (see Figure 1). This causes the liquid crystal molecules to re-orientate as described above and thereby de-activate the hologram.

A further voltage source 23 is connected between the portions 17 and 18 of the electrode 13. When the source 23 is activated, an electric field is created between the parallel interleaved fingers 20 of the electrode portions 17 and 18, acting in a direction as indicated generally by arrows B in Figure 2. According to the basic principles of electrostatics, the resultant electric field distribution will penetrate into the layer 10 and will act such that the electric field vectors within that layer are generally parallel to the direction of arrows B, i.e. generally parallel to the plane of the surface 1 1.

Just as the application of an electric field across the thickness of the hologram will cause the liquid crystal molecules to align uniformly and erase the hologram, the application of a field in a direction parallel to the hologram surface will similarly cause the liquid crystal molecules to align uniformly, but in an orthogonal direction. It is therefore possible to achieve a near-ideal state where the molecules are uniformly aligned in both the activated and de-activated states of the hologram. Not only does this increase the diffraction efficiency of the hologram in the activated state, but it also renders the hologram substantially insensitive to the polarisation state of the incident light. Accordingly, even a transmission-type hologram will be equally efficient for both the p- and s- polarised light components.

Figure 3 shows a modified arrangement in which the electrode 14 has a similar structure to the electrode 13. More particularly, the electrode 14 is composed of first and second portions 24 and 25 each comprising a shank 26 having a plurality of fingers 27 extending laterally therefrom in equi-spaced relation. The fingers 27 of the two portions 24 and 25 are interdigitated as before, but are spaced apart from one another in a direction perpendicular to that of the fingers 20 of the electrode 13.

An additional voltage source 28 is connected between the two portions 24 and 25 of the electrode 14. When this source 28 is activated, an electric field is created between the parallel interdigitated fingers 27, which acts in a direction as indicated generally by arrows C in Figure 3. Again, the resultant electric field distribution will penetrate into the layer 10 and will act such that the field vectors within that layer are generally parallel to the direction of arrows C, i.e. generally parallel to the plane of the surface 12. However, this direction is orthogonal to that of the field created by applying an electrical stimulus between the two portions 17 and 18 of the electrode 13 (i.e. as indicated by the arrows B in Figure 3).

Under this arrangement, an electric field can be applied to the hologram in any one of three orthogonal directions, i.e. (1) across the thickness of the hologram by activating the voltage source 21 and/or the voltage source 22, (2) parallel to the hologram surface by activating the voltage source 23, and (3) parallel to the hologram surface but in an orthogonal direction by activating voltage source 28. By this means, it is possible to align the liquid crystal molecules uniformly and selectively in three orthogonal directions, giving even more control over the optical properties of the hologram.

In a modified arrangement, the fingers 27 of the portions 24 and 25 of the electrode 14 are oriented parallel to (rather than perpendicularly to) the fingers 20 of the portions 17 and 18 of the electrode 13. One such arrangement is shown in cross-section in Figure 4, where the fingers 20 are shown as being mutually staggered with respect to the fingers 27 in a direction parallel to the plane of the hologram. A further arrangement is shown in cross-section in Figure 5, where the fingers 20 are shown as being mutually aligned with the fingers 27.

A further arrangement is shown in Figure 6, wherein the substrate layer 10 has again been omitted for the sake of clarity. In this embodiment, the electrode 13 is divided into a plurality of regions or segments 13a, 13b, etc. each of which extends over a respective part of the surface of the substrate, whilst the electrode 14 takes essentially the same form as in the embodiment of Figure 1. Within each of its segments 13a, etc. the electrode 13 is composed of two portions with interdigitated fingers, again in the same manner as in the embodiment of Figure 1. These are shown schematically in the segment 13 a, but for clarity are not specifically shown in the other segments. The reason for dividing the electrode 13 into such segments will now be explained.

In the field of switchable holograms, experimental evidence shows that the magnitude of the electric field required to switch the hologram between its activated and de-activated states is dependent upon the so-called slant angle of the holographic fringes, that is the angle between the Bragg surface of the fringes and the normal to the hologram surface. In certain applications where a switchable hologram is required to operate over a wide range of incidence angles, there is likely to be a significant variation in the slant angle over the area of the hologram. When an electric field of a predetermined magnitude is applied to the hologram, there may be areas in which the slant angle is such as to prevent the hologram from switching, i.e. where the switching threshold due to the slant angle is higher than the applied field. Under these circumstances, the hologram will only be partially erased. If an attempt is made to overcome this problem by increasing the magnitude of the electric field, there may be other areas which would then be exposed to excessively high voltages, i.e. where the switching threshold due to the slant angle is significantly below the applied voltage. This can give rise to electrical breakdown, de-lamination or other effects causing irreversible damage to the hologram.

In order to overcome this problem, each of the segments 13 a, etc. is arranged to extend over an area of the hologram in which the slant angle of the fringes lies within a respective predetermined range. Moreover, the voltage source 23 is now provided with a voltage- stepping arrangement 30 which causes a respective different voltage to be applied to each of the segments 13a, etc. For each segment, the magnitude of the respective applied voltage is (having regard to the range of slant angles contained in the hologram portion covered by that segment) large enough to cause full erasure of the hologram whilst not being so large as to risk causing damage to the hologram.

This particular concept is described in more detail in our International Patent Application No. PCT/US00/22773 filed 18 August 2000.

The present invention has many and varied applications. For example, in the field of image display, the switchable hologram can be incorporated into a colour sequential filter, such as is used to illuminate a monochrome display to produce a full-colour image therefrom. In such an arrangement, the display shows sequentially (in monochrome) red, green and blue colour components of the final image. The display is illuminated by red light when the red component is being displayed, by green light when the green component is being displayed, and by blue light when the blue component is being displayed. The colour sequential filter essentially comprises a stack of switchable holograms which are "tuned" respectively to red, green and blue wavelengths. The holograms are switched on and off in sequence, such that when the "red" hologram is activated, the "green" and "blue" holograms are de-activated, and so on. The large contrast achieved between the activated and de-activated states of the switchable hologram according to the present invention is an important factor to be considered when stacking holograms together in this manner.

The invention also has important applications in the telecommunications sector. More particularly, granted US patent No. 5937115 describes an arrangement in which switchable holograms are used to control the propagation of light in telecommunications switching systems, both in guided mode (i.e. in waveguides) and in free space propagation mode. For example, the switchable holograms are used to change the direction in which an optical signal passes through a waveguide, to filter optical signals (especially multi-wavelength or multichannel signals), and to couple optical signals along paths out of the plane of the waveguide (so-called in-couplers and out-couplers).

US patent No. 5937115 does recognise that conventional switchable holograms give rise to losses due to their polarisation sensitivity, and attempts to overcome this by utilising raised channel waveguides whose aspect ratio can be designed to achieve a desired difference in propagation vectors between the different polarisation components. However, the present invention provides a much more simple solution to this problem, by essentially rendering the switchable hologram insensitive to the polarisation state of the incident light.

Particular examples of the application of the invention to telecommunications switching systems are shown in Figures 7 to 12, wherein the switchable hologram is designated generally at H and the electrodes at E. Figure 7 illustrates the switchable hologram utilised as a free space 2 x 2 switch, where there are two free space optical input paths I, and I₂ to the hologram and two free space optical output paths O, and 0₂ from the hologram. Light reaching the hologram H along input path I, is deflected into output path O, when the hologram is activated, but passes substantially undeflected into output path O₂ when the hologram is de-activated. Light reaching the hologram along input path I₂ is deflected into output path O₂ when the hologram is activated, but passes substantially undeflected into output path O, when the hologram is de-activated.

Figure 8 shows the switchable hologram in the form of a waveguide W, wherein the Bragg surfaces of the holographic fringes (indicated generally at F) are oriented perpendicularly to the direction of propagation of the light within the waveguide W. Light propagating forwardly along the waveguide along a guided input path I₃ passes substantially undeflected through the hologram H when the latter is de-activated, to emerge along a guided output path O₃ When the hologram is activated the light is reflected by the hologram back along the waveguide W along a guided output path O₄. Under these circumstances, it will be manifest that the hologram H acts as a reflection hologram.

Figure 9 shows a similar arrangement, but wherein the hologram H (which is again of the reflection type) is separate from and does not form part of the waveguide W.

Figure 10 illustrates the switchable hologram H used as a planar 2 x 2 switch, in which the hologram itself acts as a waveguide. More particularly, there are two guided input paths I₅ and I₆ to the hologram, and two guided output paths O₅ and O₆ from the hologram. Light reaching the hologram along input path I₅ is deflected into the out-put path O₆ when the hologram is activated, but passes substantially undeflected into the output path O₅ when the hologram is de-activated. Light reaching the hologram along input path I₆ is deflected into the output path O₅ when the hologram is activated, but passes substantially undeflected into the output path O₆ when the hologram is de-activated.

Figure 11 shows the switchable hologram 10 utilised as a waveguide -to-free-space out- coupler, wherein the hologram again itself acts as an optical waveguide. Light reaching the hologram H along a guided input path I₇ is deflected out of the plane of hologram into a free space output path O₇ when the hologram is activated. When the hologram is de-activated, the light passes substantially undeflected into a guided output path O₈.

Figure 12 shows a generally similar arrangement, but where the switchable hologram is employed as a free-space-to-waveguide in-coupler. In this arrangement, light is incident on the hologram H along a free-space input path I₉ When the hologram is de-activated, the light passes substantially undeflected through the hologram H to emerge along a free-space output path O₉. When the hologram H is activated, the light is deflected into the plane of the waveguide and propagates therealong in a guided output path O₁₀

In the embodiment of Figure 7. the switchable hologram acts essentially as a simple transmission hologram, with the light passing across the thickness of the hologram from one of its opposed surfaces to the other. Accordingly, the Bragg surfaces of the holographic fringes (indicated generally at F) are arranged generally perpendicularly to the plane of the hologram and are activated and de-activated by the application of electric fields in the directions of arrows X, and X₂ In the embodiment of Figure 10, however, although the hologram still functions as a transmission hologram, it also acts as a waveguide and the direction of propagation of the light is essentially parallel to the said opposed surfaces. Under these circumstances, the Bragg surfaces of the holographic fringes extend generally perpendicularly to the plane of the hologram, and are activated and de-activated by the application of electric fields in the directions of arrows Y, and Y₂.

In the embodiments of Figures 1 1 and 12, the hologram again acts as a waveguide but light can be deflected into or out of the plane of the latter. In these cases, the Bragg surfaces of the holographic fringes F are oriented relative to the plane of the hologram at a suitable angle, having regard to the desired angle of free space emission in Figure 11 or the desired angle of entry from free space in Figure 12. The fringes are activated and de-activated by the application of electric fields in the directions of arrows Z, and Z₂.

In the embodiment of Figure 9, the hologram acts essentially as a simple reflection hologram and therefore the Bragg surfaces of the holographic fringes F extend generally parallel to the plane of the hologram. The hologram is activated and de-activated by applying electric fields in the directions of arrows W, and W₂. In the embodiment of Figure 8, although the hologram again acts as a reflection hologram, it also functions as a waveguide. Accordingly (as already described) in this arrangement the Bragg surfaces of the holographic fringes F extend perpendicularly to the plane of the hologram, and the hologram is activated and deactivated by the application of electric fields in the directions of arrows W₃ and W₄ Whereas the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention. For example, where the switchable hologram is used in the free space propagation of light, it has been stated that the conductive coatings 16 of the electrodes 13 and 14 will normally be transparent. However, the electrodes can instead be composed of very narrow, non- transparent conducting bars, although the ratio of bar width to bar spacing would have to be sufficiently large to minimise transmission losses.

Also, the above description refers to holograms embodying liquid crystal material of a type wherein the molecules align themselves parallel to the applied electric field. Thus, when an electric field is applied across the thickness of the hologram, the liquid crystal molecules become aligned perpendicularly to the surface of the hologram, and this corresponds to the hologram being de-activated. On the other hand, when an electric field is applied parallel to the hologram surface, the liquid crystal molecules become aligned parallel to the hologram surface, corresponding to the hologram being activated.

The invention can, however, also be applied to holograms embodying other types of liquid crystal material, such as those wherein the molecules become orientated perpendicularly to the applied electric field. In this case, when an electric field is applied across the thickness of the hologram, the molecules will become oriented parallel to the hologram surface, corresponding to the hologram being activated. When an electric field is applied parallel to the hologram surface, the molecules will become oriented perpendicularly to that surface, corresponding to the hologram being de-activated.

It will also be manifest that the invention is applicable to holograms of both the transmission and reflection types.

Claims

1. A switchable hologram comprising a substrate in which there is recorded a hologram comprising fringes which are composed of particles (e.g. molecules) whose orientation can be changed by the application of an electrical stimulus, thereby activating and de-activating the hologram,

2. A switchable hologram as claimed in claim 1 , wherein said first and second directions are mutually orthogonal.

3. A switchable hologram as claimed in claim 1 or 2, wherein said first direction is generally parallel to opposed surfaces of the substrate, and said second direction is generally perpendicular to said surfaces.

4. A switchable hologram as claimed in claim 1, 2 or 3, wherein said second means includes a pair of electrodes disposed on opposite sides of said substrate.

5. A switchable hologram as claimed in any preceding claim, wherein said first means includes a pair of electrode portions disposed on one side of said substrate but spaced apart from one another in a direction parallel to said opposed surfaces.

6. A switchable hologram comprising a substrate having a pair of opposed surfaces, said substrate having recorded therein holographic fringes which can be activated and de-activated by the application of an electrical stimulus, and

7. A switchable hologram as claimed in claim 6, wherein the second electrode is also composed of first and second portions spaced apart from each other in a direction parallel to the respective surface of said substrate, the arrangement being such that, when an electrical stimulus is applied across said first and second portions of said second electrode, an electric field is produced which is directed generally parallel to said respective surface of said substrate.

8. A switchable hologram as claimed in claim 7, wherein said first and second portions of said first electrode are spaced apart from each other in a first direction, and said first and second portions of said second electrode are spaced apart from each other in a second direction which is generally perpendicular to said first direction.

9. A switchable hologram as claimed in claim 6, 7 or 8, wherein the holographic fringes have Bragg surfaces whose slant angle (as defined herein) varies across the area of said substrate, and at least one of said first and second electrodes is composed of a plurality of independently operable segments, each of which is disposed over an area of said substrate in which said slant angle lies within a respective predetermined range.

10. A switchable hologram as claimed in claim 9, wherein each of said segments comprises first and second portions as hereinbefore claimed.

1 1. A switchable hologram comprising a substrate having a pair of opposed surfaces, said substrate having recorded therein holographic fringes which can be activated and de-activated by the application of an electrical stimulus,

12. A switchable optical component comprising a switchable hologram as claimed in any preceding claim, at least one optical input path to said hologram, and at least one optical output path from said hologram, wherein at least one of said input and output paths is a guided wave optical path.

13. A switchable optical component as claimed in claim 12, comprising first and second guided wave input paths and first and second guided wave output paths, the switchable hologram when activated being operative to direct light from said first guided wave input path to said first guided wave output path and/or to direct light from said second guided wave input path to said second guided wave output path, the switchable hologram when de-activated permitting the passage of light from said first guided wave input path to the second guided wave output path and/or permitting the passage of light from said second guided wave input path to said first guided wave output path.

14. A switchable optical component as claimed in claim 12, comprising a guided wave input path and a guided wave output path, the switchable hologram when activated being operative to reflect light from said guided wave input path back along said guided wave output path.

15. A switchable optical component as claimed in claim 12. comprising a guided wave input path, a guided wave output path and a free space output path, the switchable hologram when activated being operative to direct light from said guide wave input path to said free space output path, and when de-activated permitting the passage of light from said guided wave input path to said guided wave output path.

16. A switchable optical component as claimed in claim 12, comprising a free space input path, a free space output path and a guided wave output path, the switchable hologram when activated being operative to direct light from said free space input path to said guided wave output path, and when de-activated permitting the passage of light from said free space input path to said free space output path.