WO2002048801A1

WO2002048801A1 - Switchable holographic device

Info

Publication number: WO2002048801A1
Application number: PCT/US2001/048294
Authority: WO
Inventors: John E. Gunther
Original assignee: Digilens Inc.
Priority date: 2000-12-15
Filing date: 2001-12-14
Publication date: 2002-06-20
Also published as: AU2002230825A1

Abstract

A switchable holographic device (20) comprises a substrate (22) in which there is recorded a hologram comprising fringes fromed by particles (e.g. molecules) whose orientation can be changed between first and second conditions by the application of an external stimulus. The particles when in their first condition are orientated generally in a common direction with respect to a direction of progragation of radiation through the hologram. The particles when in their second condition are orientated in a direction determined by said external stimulus. The external stimulus being arranged so as to orientate the particles in different regions of the hologram into respectively different directions, such that the particles are orientated essentially randomly with respect to said direction of radiation propagation.

Description

Title: Switchable Holographic Device

Field of the invention

This invention relates to a switchable holographic device, such as is used (for example) in optical displays or in optical telecommunications systems.

Background to the invention

Switchable holographic devices are typically composed of a holographic diffraction grating recorded in a polymer-dispersed liquid crystal (PDLC) material. During the hologram recordal process, the material undergoes a phase separation to create regions densely populated by liquid crystal microdroplets interspersed by regions of clear polymer. When the device is constructed as a waveguide and the PDLC material utilises nematic liquid crystal, the molecules of liquid crystal within the droplets tend to align normal to the grating fringes and parallel to the waveguide axis, such that the grating will exhibit very low levels of diffraction efficiency in respect of radiation propagating in a direction parallel to the axis of the waveguide. When an electric field is applied to the hologram by way of electrodes, the orientation of the liquid crystal molecules is changed to a direction other than parallel with the waveguide axis increasing the diffraction efficiency of the holographic fringes and effectively turning the hologram on. In the case where the holographic diffraction grating is in the form of a Bragg grating, the resulting hologram can exhibit very high diffraction efficiencies and fast switching times. Various switchable waveguide devices are described in our pending International patent application PCT/GB01/03169.

However, a problem with known switchable holographic devices is that the PDLC material exhibits polarisation sensitivity when the hologram is turned on. That is to say, the diffraction efficiency for radiation polarised (say) in the p direction is significantly greater than that for radiation polarised in the orthogonal (s) direction. This difference in diffraction efficiency can be as much as a factor of 50 to 100. One possible reasomor this will be explained below.

Figure 1 of the accompanying drawings is a cross-section through a portion of PDLC material taken in a direction perpendicular to the direction of propagation of a beam through the material and showing the droplets (referenced 15) in a rest state of the PDLC material. Within each droplet, there is a tendency for the liquid crystal molecule directors to exhibit a bipolar alignment pattern, as indicated schematically by lines 16. One of the droplets 15 is shown in detail in Figures 2 and 3, with Figure 3 being a view perpendicular to the polar axis Q and Figure 2 being a viewed parallel to that axis. Because of the way in which the molecule directors 16 are orientated, the droplet will exhibit birefringence. More particularly, for radiation incident in a direction parallel to the polar axis Q (i.e. as viewed in Figure 2) the droplet will have an ordinary refractive index n₀ irrespective of the polarisation state of the radiation. However, for radiation incident perpendicularly to the polar axis (i.e. as viewed in Figure 3) the molecule with have an extraordinary refractive index n_e, with n_e generally being greater than n₀, for radiation in one polarisation state (say in the p direction) and an ordinary refractive index for radiation polarised in the othagonal (s) direction.

As can be seen from Figure 1 , in the rest state, the molecule directors 16 within each of the droplets 15 are generally aligned parallel with the direction of propagation of radiation, which is indicated by the arrow E (i.e. into the plane of the paper). In this condition, radiation will see only the normal refractive index n₀ regardless of its polarisation state.

Figure 4 shows a cross-section through a portion of the PDLC material in the state where an electric field is applied in the direction of arrow F, the cross-section again being taken in a direction perpendicular to the direction of propagation of a beam through the material. The molecule directors 16 within each droplet have now become re-orientated such that they are aligned with the electric field vector. In this condition, radiation propagating through the material will see either the extraordinary refractive index n_e or the ordinary refractive index n₀ depending on its polarised state. Hence, with all the molecules in the PDLC material orientated in the same direction, the diffractive efficiency of the fringes formed by the droplets will vary in dependence upon tne polarisation state of the radjajtip

In an ideal situation, the device should be capable of being switched between two conditions in which the holographic fringes are respectively switched on and off, and in both of which the material shows no sensitivity to the polarisation state of the radiation. For example, in a first condition, the molecule directors can all be oriented in a direction parallel to the direction of radiation propagation, such that the radiation sees only the ordinary refractive index of the droplets. In a second condition the liquid crystal molecule directors 16 can be randomly oriented, such that the polarisation sensitivity of the individual liquid crystal droplets is evened out because of their overall random orientation. However, it is not possible using an external electric field to achieve a random orientation of the molecules of a nematic liquid crystal.

It is an object of the present invention to obviate or mitigate the above-described problems and disadvantages.

Summary of the invention

According to a first aspect of the present invention, there is provided a switchable holographic device comprising:

a substrate in which there is recorded a hologram comprising fringes formed by particles (e.g. molecules) whose orientation can be changed between first and second conditions by the application of an external stimulus,

the particles when in their first condition being orientated generally in a common direction with respect to a direction of propagation of radiation through the hologram,

the particles when in their second condition being orientated in a direction determined by said external stimulus,

the external stimulus being arranged so as to orientate the particles in different regions~δf the hologram into

particles are orientated essentially randomly with respect to said direction of radiation propagation.

In a switchable holographic device in accordance with the invention, radiation propagating through the hologram will be incident upon droplets in which the molecules of crystal material are orientated in different directions at different regions in the hologram. Consequently, the polarisation sensitivity of the individual droplets is evened out in a manner similar to that with material in which the molecules are orientated randomly.

Preferably, the external stimulus is arranged such that the direction into which the particles are orientated varies progressively along a direction of propagation of radiation through the hologram.

Preferably, the external stimulus comprises an electric field which is applied byway of electrodes and, when in their second condition, the particles in the immediate vicinity of each electrode are generally orientated in one direction, and the particles in regions between electrodes are generally orientated in a different direction.

More preferably, the substrate is elongate, and the electrodes extend generally parallel to the substrate but at a small angle to the axis of the substrate. In particular, the electrodes may be arranged to extend at an angle of 10 degrees or less to the axis of the substrate.

Preferably, the electrodes are arranged to provide an effective axis of field rotation that substantially coincides with the axis of propagation through the device of a beam of radiation.

Preferably, the electrodes are spaced laterally from the substrate by a distance substantially equal to 4/10 of their width, or alternatively by a distance substantially equal to 4/10 of their separation. In a particularly preferred embodiment, the electrodes have a width of substantially 10μ and/or a separation of substantially 10μ. Advantageousi rtne substrate comprises

More preferably, the polymer-dispersed liquid crystal material comprises nematic liquid crystals.

Preferably, the device comprises a waveguide.

According to a second aspect of the invention, there is provided a switchable holographic device comprising:

a polymer-dispersed liquid crystal material in which there is recorded a hologram comprising fringes formed by molecules of liquid crystal,

an electrode arrangement by means of which an electric field can be applied to the hologram to change the orientation of the liquid crystal molecules between first and second conditions,

the liquid crystal molecules when in their first condition being orientated generally in a common direction with respect to a direction of propagation of radiation through the hologram,

the liquid crystal molecules when in their second condition being orientated in a direction determined by said electric field,

the electrode arrangement being designed so that the electric field causes the liquid crystal molecules in different regions of the hologram to orient into respectively different directions, such that the liquid crystal molecules are oriented essentially randomly with respect to said direction of radiation propagation.

Preferably, the electrode arrangement is designed so that the direction into which the liquid crystal molecules are orientated by the electric field varies progressively along a direction of propagation of radiation through the hologram.

Advantageously, the substrate is elongate and the electrode arrangement comprises at least two electrodes having interdigitated fingers, which extend generally longitudinally ofTfie substrate but at a small angl^tqlhje-^ ^otlSiS^ ^atgitoa , particularly preferred embodiment, the interdigitated fingers extend at an angle of 10 degrees or less to the axis of the substrate.

Preferably, the electrodes are arranged to provide an effective axis of field rotation which coincides with the axis of propagation through the device of a beam of radiation.

Preferably, the device is a waveguide device having a waveguide core and cladding material arranged about the core. Preferably, the hologram is recorded in the waveguide core. Alternatively, the hologram is recorded in a layer of cladding material close to the core.

Preferably, the hologram comprises a diffraction grating and in particular a Bragg grating.

Brief description of the drawings

An embodiment of the invention will now be described, byway of example only, with reference to the remaining Figures of the accompanying drawings in which:

Figure 5 is a schematic plan view of a switchable holographic device in accordance with the invention;

Figure 6 is a simplified schematic exploded view of the device of Figure 5; and

Figure 7 is cross-sectional view through four interdigitated electrode fingers showing the direction of an electric filed generated by the electrodes.

Detailed description

With reference to Figures 5 and 6, a switchable holographic device, indicated generally at 20, comprises a waveguide substrate 22 having a waveguide core 24 and cladding material arranged about the core. The waveguide core comprises PDLC material Wo which is recorded a

a Bragg grating. The waveguide core 24 is adapted to receive a beam of radiation

(not shown), which propagates along the core in the direction indicated by arrow Y.

The device 20 further comprises a cover plate 26, which may be made of glass, onto which is attached two electrodes 28, 30. Each electrode has a plurality of elongate fingers, with the fingers of one of the electrodes being interdigitated with the fingers of the other electrode. The electrodes may be transparent ITO electrodes and each also comprises an electrode pad 32, 34 by means of which the electrodes can be connected to a suitable source of electrical power in order to apply an electric field in the region of the waveguide core 24.

The PDLC material in the waveguide core utilises nematic crystal molecules, which tend to align normal to the grating fringes and parallel to the axis Z of the waveguide 24. Thus without any field applied, the grating will exhibit minimum, essentially zero, diffraction efficiency. In order to increase the diffraction efficiency of the grating, effectively turning the hologram on, an electric field is applied to the waveguide core by the electrodes to rotate the liquid crystal molecules to some direction other than parallel to the waveguide core. The direction in which the molecules rotate being dependent on the direction of the electric field applied.

As discussed previously, rotating the molecules into a uniform orientation will cause the grating diffraction efficiency to vary with the polarisation axis of the radiation propagating along the core. In order to reduce or eliminate this polarization sensitivity, advantage is taken of localised variations in the electric field generated by the electrodes, by aligning the electrode fingers generally parallel to the waveguide core 24 but at a small angle α to the axis Z of the waveguide 24. Typically, the angle α will be 10° or less. As will be explained in more detail below, this arrangement causes the direction of the electric field applied to the waveguide core to vary progressively along its axis (i.e. along the direction of propagation) such that the direction into which the liquid crystal molecules are rotated when the field is applied also changes progressively along the direction of propagation. Consequently, radiation propagating along the waveguide core 24 will encounter droplets whose molecules are orientated in different directions at different points along the lengtrroT the core, giving an effect

orientated molecules. This reduces or eliminates polarisation sensitivity of the device when the hologram is in the on state, as the polarisation sensitivity of the individual droplets are evened out.

The reasons for the variation in the direction of the electric field applied to the core by the electrodes can best be described with reference to Figure 7, which shows in cross-section the direction of an external electric field generated by four interdigitated electrode fingers 28a, 30a, 28b, 30b. In the example illustrated, the electrode finger width and the gap width between adjacent electrode fingers are 10μ. The 1st and 3rd electrode fingers 28a, 28b are at a potential of 320 volts; the 2nd and 4th electrode fingers 30a, 30b are at ground potential.

In Figure 7, the direction of the electric field generated by the electrodes is indicated by the arrows. As can be seen, the field direction between adjacent electrode fingers is horizontal, whilst the field direction directly under any of the electrode fingers is vertical. Hence a waveguide core positioned directly underneath one of the electrode fingers will be subjected to an electric field having essentially a vertical field direction whilst a core positioned between two adjacent electrode fingers will be subjected to an electric field having essentially a horizontal field direction.

With reference now to Figure 5, it can be seen that because the fingers of the electrodes 28, 30 are arranged at a slight angle α relative to the axis Z of the core, the position of the core relative to the electrode fingers varies along its length. Thus, at the position indicated by line A-A, the waveguide core 24 is located directly underneath a finger of a first of the electrodes 28, whereas, at the position indicated by the line B-B, the core 24 is located between the finger of the first electrode 28 and an adjacent finger of the second electrode 30. Furthermore, at the position indicated by the line C-C the core 24 is located underneath the finger of the second electrode 30. It will be understood, therefore, that at the positions indicated by the lines A-A and C-C, where the core 24 is directly underneath one the electrode fingers, it will be subjected to an electric field having essentially a vertical direction, whilst the at the position indicated by the line B-B, where the core is located between two of the electrode fingers, it will be subjected to an electric field having essentially a horizontal field direction.

Since the orientation into which the liquid crystal molecules are rotated is dependant on the direction of the electric field, the molecules in the core at the positions A-A and C-C will be orientated in a first direction, and the molecules in the core at the position B-B will be orientated in a second direction generally orthogonal to the first. Furthermore, it will be understood that the direction of the electric field applied to the core 24 changes progressively from a vertical direction at the position A-A to a horizontal position at point B-B and then to a vertical direction again at point C-C. This progressively changing field direction results in a progressively changing direction of orientation of the liquid crystal molecules along the axis of the core (i.e. along the direction of propagation of the radiation), when a current is applied to the electrodes to switch the holographic fringes "on".

To ensure that the PDLC material is subjected to a continuously varying field direction, the electrodes and the core should preferably be arranged such that the electrodes provide an effective "axis" of field rotation, which coincides with the axis of propagation through the waveguide of a beam of radiation.

It should be noted that the size and spacing of the electrodes 28, 30 have been exaggerated in Figures 5 and 6 for clarity.

In an alternative arrangement, rather than the PDLC material being located in the waveguide core, the PDLC material is provided as a switchable cladding layer close to the waveguide core. The PDLC layer may be arranged immediately adjacent to the waveguide core or may be separated from the core by a further thin cladding layer.

In such an arrangement, the refractive index of the cladding layer can be varied by the application of an electrical stimulus such that the device is switchable between first and second conditions in which the refractive indices of the core and cladding portions are respectively substantially matched or substantially unmatched.

By controlling the refractive index of the cladding relative to that of the core, it is possible to control the characteristics of the radiati r^ r«^a^'^i®§3^jnit p®iE^iln . particular, it is possible to control coupling of the radiation propagation between the core and the cladding. For example, when the reflective indices of the core and the cladding are matched, radiation can propagate from the core into the cladding to create a loss path.

A number of switchable holographic devices in accordance with the invention have been produced in the form of waveguide attenuators. These devices show controlled attenuation from a low value to more than 50db, with very small dependence on the polarisation state of the incoming light. The cover glass had 10μ wide transparent ITO electrodes with 10μ spaces between adjacent centres. The PDLC cell thickness proved to be an important factor, with a 4μ thickness providing the best performance as this positioned the core in a plane below the electrodes in which the field direction changes continuously and in which the field strength is relatively constant at about 16volts per micron, which is sufficient to result in about 90% rotation of the molecules in the PDLC layer. This being the plane which contains the effective axis of rotation of the electric field produced by the electrodes.

Whereas the invention has been described in relation to what is 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, any suitable electrode arrangement which produces a change in the direction of the electric filed applied to the hologram along the direction of propagation can be used. Furthermore, whilst the invention has been described principally in relation to waveguide devices, the invention be equally applied to other types of switchable holographic devices.

Claims

1. A switchable holographic device comprising:

the external stimulus being arranged so as to orientate the particles in different regions of the hologram into respectively different directions, such that the particles are orientated essentially randomly with respect to said direction of radiation propagation.

2. A switchable holographic device in accordance with claim 1 , in which the external stimulus is arranged such that the direction into which the particles are orientated varies progressively along a direction of propagation of radiation through the hologram.

3. A switchable holographic device in accordance with claim 1 or claim 2, in which the external stimulus comprises an electric field which is applied by way of electrodes and, when in their second condition, the particles in the immediate vicinity of each electrode are generally orientated in one direction, and the particles in regions between electrodes are generally orientated in a different direction.

4. A switchable holographic device in accordance with claim 3, in which the substrate is elongate, and the electrodes extend SΘiϊ§ra1ly^*^f®l ej|Qj;he|^Qftgtiiffc^.. but at a small angle to the axis of the substrate.

5. A switchable holographic device in accordance with claim 4, in which the electrodes are arranged to extend at an angle of 10 degrees or less to the axis of the substrate.

6. A switchable holographic device in accordance with any one of claims 3 to 5, in which the electrodes are arranged to provide an effective axis of field rotation that substantially coincides with the axis of propagation through the device of a beam of radiation.

7. A switchable holographic device in accordance with any one of claims 3 to 6, in which the electrodes are spaced laterally from the substrate by a distance substantially equal to 4/10 of their width, or alternatively by a distance substantially equal to 4/10 of their separation.

8. A switchable holographic device in accordance with claim 7, in which the electrodes have a width of substantially 10μ.

9. A switchable holographic device in accordance with claim 7 or claim 8, in which the electrodes have a separation of substantially 10μ.

10. A switchable holographic device in accordance with any previous claim, in which the substrate comprises polymer-dispersed liquid crystal material.

11. A switchable holographic device in accordance with claim 10, in which the polymer-dispersed liquid crystal material comprises nematic liquid crystals.

12. A switchable holographic device in accordance with any previous claim, in which the device comprises a waveguide.

13. A switchable holographic device comprising: a polyme dispersed liquid crystal mater βf

"BP- hologram comprising fringes formed by molecules of liquid crystal,

14. A switchable holographic device as claimed in claim 13, the electrode arrangement being designed so that the direction into which the liquid crystal molecules are orientated by the electric field varies progressively along a direction of propagation of radiation through the hologram.

15. A switchable holographic device as claimed in claim 13 or claim 14, in which the substrate is elongate and the electrode arrangement comprises at least two electrodes having interdigitated fingers which extend generally longitudinally of the substrate but at a small angle to the axis of the substrate.

16. A switchable holographic device as claimed in claim 15, in which the interdigitated fingers extend at an angle of 10 degrees or less to the axis of the substrate.

17. A switchable holographic device as claimed in any one of claims 13 to 16, in which the electrσαes are arranged to provide an effective a ^ofif eld/ptatio jwhiεh», coincides with the axis of propagation through the device of a beam of radiation.

18. A switchable holographic device as claimed in any one of claims 13 to 17, in which the device is a waveguide device having a waveguide core and cladding material arranged about the core.

19. A switchable holographic device as claimed in claim 18, in which the hologram is recorded in the waveguide core.

20. A switchable holographic device as claimed in claim 18, in which the hologram is recorded in a layer of cladding close to the core.

21. A switchable holographic device as claimed in any previous claim, in which the hologram comprises a diffraction grating.

22. A switchable holographic device as claimed in claim 21 , in which the diffraction grating is in the form of a Bragg grating.