METHOD OF OPERATING AN OPTICALLY ADDRESSED SPATIAL LIGHT MODULATOR AND

HOLOGRAPHIC DISPLAY SYSTEM

Field

The present invention relates to a method of operating an OASLM, and to a system for projecting images using an OASLM.

An embodiment of the method and system uses a C60-doped nematic liquid crystal; other embodiments use liquid crystal materials doped with materials that exhibit space charge properties or liquid crystal materials that exhibit enhanced polarisability.

Background

OASLMs have been known since the early 1970s. An example, in this case of a reflective OASLM, is shown in US-A-3592527. In this example, writing light is used to apply information to a photoconductive layer that in turn applies an electric field to a liquid crystal material associated with the photoconductive layer, the field varying according the illumination applied. Reading light is then applied to the liquid crystal material from the side of the OASLM opposite to that to which the writing light is applied, and reflected from a reflecting surface to provide an image.

EASLMs, for example LCOS SLMs with active backplanes, are also known. Many of these, latter devices are pixellated, and the pixels are typically addressed via circuitry in a silicon backplane. High reading and writing speeds can be achieved, but the size of the SLM is likely to be limited for example on economic grounds.

It has thus been proposed to use an OASLM in concert with one or more LCOS

SLMs, taking information on the LCOS SLM and optically applying it to a first zone of the OASLM. Then fresh information is written to the LCOS SLM and this is applied to a different zone of the OASLM, building up until information is tiled across the OASLM. This may be achieved, for example, by using fan-out optics to reproduce beams from the LCOS SLM onto the different zones of the OASLM, and then activating the respective zones synchronously with the appearance on the LCOS SLM of the image information for the respective zone. Once data is written to the

OASLM, a reading beam of light is applied so that the contents of the OASLM is projected, for example via Fourier optics to a screen for viewing.

Recently OASLMs, having no separate photosensing layer, have been proposed. (LC. Khoo et al, Proc IEEE vol 87(11) (1999) pp. 1897-1911). One type has a single electro-optic layer of dye-doped liquid crystal. Another uses C60 doped liquid crystal material.

Summary In one aspect there is provided a method of operating an OASLM, the method comprising: applying a dc field across a liquid crystal material; terminating the dc field and thereafter applying a first light beam to the liquid crystal material with a first light beam, whereby information is written to the liquid crystal material; terminating application of the first light beam and thereafter applying a dc field across the material; and while the dc field is applied, applying a second light beam to the liquid crystal material, whereby information held in the liquid crystal material is read by the second light beam.

The material may comprise a liquid crystal material doped with a dopant that provides a space charge effect.

The material may comprise a liquid crystal material doped with a dopant comprising a fullerene that provides a space charge effect.

The material may comprise a liquid crystal material doped with C60.

The method may further comprise applying the first light beam to the liquid crystal material with a predetermined spatial frequency.

In another aspect there is provided a method of providing an image comprising pre- charging liquid crystal material doped with C60 by applying a dc field across the material; terminating the applied field, thereafter illuminating the material with a first light beam, whereby information is written to the liquid crystal material; terminating the illumination; and thereafter applying a dc field across the material and while the field is applied, illuminating the material with a second light beam, whereby information held in the liquid crystal material is read by the second light beam, wherein the information written to the liquid crystal material is hologram information, and the information read out by the second light beam is applied to a Fourier lens to provide a replay field in the Fourier plane of the lens.

In yet another aspect there is provided a method of providing an image comprising pre-charging liquid crystal material doped with C60 by applying a dc field across the material; terminating the applied field, thereafter illuminating the material with an interference pattern derived from a first light beam from a reflective LCOS SLM and a second light beam disposed to interfere with the first light beam, whereby information from the LCOS SLM is written to the liquid crystal material; terminating the illumination; and thereafter applying a dc field across the material and while the field is applied, illuminating the material with a third light beam, whereby information held in the liquid crystal material is read by the third light beam, wherein the information written to the liquid crystal material is hologram information, and the information read out by the third light beam is applied to a Fourier lens to provide a replay field in the Fourier plane of the lens.

In a further aspect there is provided image projection apparatus comprising a blue light source, a reflective LCOS SLM, a transmissive OASLM , a reference light source, control circuitry, a red light source and a Fourier lens, wherein the blue light source is disposed to be capable of illuminating the LCOS SLM, wherein the transmissive OASLM is disposed to be capable of receiving an interference pattern derived from the blue light from the blue light source after reflection by the LCOS SLM interfered with a reference beam from the reference light source; wherein the red light source is disposed to be capable of illuminating the transmissive OASLM

with a similar angle of incidence to an angle of incidence of the blue light from the blue light source after reflection by the LCOS SLM, wherein the Fourier lens is disposed to receive red light from the red light source after passing through the transmissive OASLM; wherein the transmissive OASLM comprises a liquid crystal material doped with C60; wherein the control circuitry is adapted to apply a pre- charge pulse across the liquid crystal material doped with C60 of the transmissive OASLM and then to terminate the precharge pulse, thereafter to activate the blue light source whereby the blue light source irradiates the reflective LCOS SLM, wherein the control circuitry is adapted to thereafter terminate the blue light source, thereafter to apply a read pulse across the liquid crystal material and while the field is applied, to activate the red light source, whereby information held in the liquid crystal material is read by light from the red light source, wherein the information written to the liquid crystal material is hologram information, and the information read out by the light from the red light source is applied to the Fourier lens to provide a replay field in a Fourier plane of the Fourier lens.

Brief description of the drawings

The invention will be more clearly understood after referring to the accompanying description of an embodiment together with the drawings, in which: Fig. 1 shows an embodiment of a homeotropic transmissive single layer

OASLM 10;

Fig. 2 is a timing diagram of an embodiment;

Fig. 3 shows a diagram illustrating tiling;

Fig. 4 shows a projection system embodying the invention in a write mode; Fig. 5 shows the projection system of Fig. 4 in a projection mode;

Fig. 6 shows the structure of a reflective device;

Fig. 7 shows the structure of a multilayer device;

Fig. 8 shows the structure of a resonant device;

Figs. 9a and 9b show devices having planar and homeotropic alignment respectively;

Fig. 10 is a graph showing current and voltage behavior of three devices;

Fig. 11 shows a two-beam interference arrangement; and

Fig. 12 shows an imaging arrangement.

Detailed Description

Fig. 1 shows an embodiment of a homeotropic transmissive single layer OASLM 10. This consists of two glass plates 11,12 separated by spacers (not shown). On the inner surface of each glass plate 11,12 there is deposited a respective ITO electrode layer 13, one of which is connected to a first contact 16 and the other of which is connected to a second electrical contact 17. Respective layers 14 of a surfactant [Merck (ZLI3344)] are disposed on the ITO electrodes 13 and these form alignment layers for a C60-Doped liquid crystal material 15 that forms the active material of the device, being disposed between the two alignment layers 14.

In making an experimental device a variety of materials were used for making the liquid crystal layer, based on a nematic host, C60, and carbon nanotube materials. A mixture of these materials was dispersed by sonification at a temperature close to the phase transition of the nematic host, and was left for sufficient time to allow maturing to take place.

A method of operating the OASLM 10 will now be described with respect to Fig. 2. The mechanism by which the effect is achieved is highly likely to work for fullerenes other than C60 and indeed for dopants other than fullerenes that have a space-charge effect. The method has three stages:

1. DC pulse application This may be viewed as a precharge step

2. After termination of the DC pulse, apply optical beam containing information in the form of amplitude variations.

This may be viewed as a writing step.

3. After termination of the optical pulse, application of DC pulse and simultaneously application of light beam

This may be viewed as a reading step.

Referring to Fig. 2, at a time tl a precharge pulse, here 5 volts was applied between contacts 16 and 17 for a precharge period until time t2. Ih experiments this period was varied between Is and lmin. After terminating the precharge pulse the voltage 30 between the contacts 16, 17 (and hence the ITO electrodes 13) was allowed to float.

At time t3, an optical write beam 41 - (18 see Fig. 1) was applied. The period t2-t3 was not found to be critical. In experiments this period t2-t3 was up to 10 s. The duration t3-t4 of the optical write beam was varied successfully between 17 ms (l/60th s) up to a duration of more than 1 second - in the experiment, optical power was adjusted to take into account pulse duration. As an example, the optical energy density in the 17ms optical pulse was ~60mJ/cm ² . Then, after terminating the optical write beam 41, light was no longer applied 42.

At a time period t5, a 5 volt pulse 32 was applied between contacts 16 and 17 for a read period until time t6. In an experimental set up this period was 1 second.

Substantially at the instant t5, an optical read pulse (19- see Fig. 1) was applied. This gave rise to an output beam 20- see Fig. 1, having a characteristic approximately as shown in Fig. 2c.

When the read voltage is applied, the optical output builds up in 1 s, and remains at or around its max level for at least 40ms, before relaxing in around 3 s. A diffraction efficiency of around 10% was achieved in experiment. Reflective OASLMs may be operated in a like fashion.

Turning now to Fig. 3, an EASLM 50 is illuminated so as to provide an output beam 52, directed via suitable optics (not shown) to an OASLM 55. As can be seen from the drawing, the OASLM has, in this case, four zones of equal size and as shown the light beam 52 is being applied to the upper left quarter.

Tiling may be achieved by disposing fan-out or other replication optics between the EASLM 50 and the OASLM 55 so that versions of the beam 52 are present on each quarter, and then only activating the quarter to which the information currently on the

EASLM pertains. Once writing has occurred, fresh information is written to the EASLM and a new quarter of the OASLM is activated for writing. Clearly more than four zones may be provided.

Referring now to Figs 4 and 5, an image projection system using a transmissive OASLM 113, for example an OASLM similar to OASLM 10 of Fig. 1 will be described. Fig4 shows the system in the mode where writing to the OASLM is occurring and Fig. 5 shows the mode where reading from the OASLM 113 onto a screen 125 is occurring. Beam paths having pecked lines are inactive in the respective figure.

The system 100 has a laser diode 101 at the focus of a collimating lens 105 so that light 102 from the laser diode 10 is formed into a collimated beam 106. The collimated beam 106 is applied off normal to a reflective LCOS SLM 108 to which has been written an amplitude hologram via its back plane (not shown). In this embodiment, the laser diode 101 provides blue light.

Rather than imaging the hologram onto the OASLM 113, as has been proposed in the past, a light beam 109, hereinafter referred to as an illuminating beam 109 from the LCOS SLM is caused to interfere with a reference beam 110 at the OASLM 113. The illuminating beam 109 has information derived from the amplitude hologram information of the LCOS SLM 108, leaving the LCOS SLM 108 to be incident upon the OASLM 113. At the same time the reference beam 110, a collimated beam having a known wavelength and at a known angle to the illuminating beam 109 is incident upon the OASLM 113, so that the two beams 110, 109 interfere.

To implement this method, a control device 120 provides operating voltages to the OASLM 113, for example the pulses described with respect to Fig. 2, over a control connection shown as 121. The control device 120 has a further control link 122.

The result is a phase hologram having a spatial frequency determined by the wavelengths of the two beams and their mutual angle, in known fashion.

It is known that transmissive, photorefractive OASLMs operate optimally over a limited spatial frequency range where the grating spacing of the hologram formed on the device is approximately double the thickness of the active portion of the OASLM. Hence by selecting the parameters of the system bearing in mind the dimensions of the OASLM, good diffraction efficiency can be achieved.

For devices having a liquid crystal layer of 20 microns thickness, an effective spatial frequency is 25 line pairs per mm, and this is the proposed carrier frequency for one embodiment of the hologram.

Where it is desired to increase the information content of the hologram, the carrier frequency is increased. For such an embodiment a 5 micron cell is used with a carrier frequency of 100 line pairs/mm.

Turning now to Fig. 5, a read beam 130 is caused to be incident upon the OASLM at substantially the same angle of incidence as the illuminating beam 109. hi this embodiment, the read beam 130 is red and the illuminating beam 109 is blue.

The OASLM is activated for reading, e.g. following the method described above, and a collimated output beam 124 having information derived from the phase hologram data previously written to the OASLM 113 is provided along an optical axis to a Fourier lens 123. A projection plane 125 forms the replay field of the hologram

The control link 122 is operative to control the application of the reference beam 110 and the illuminating beam 130 in concert with driving the OASLM 113. The control device 120 may also control scanning and other operating circuitry, including operating the laser diode 101, and passing synchronizing pulses or the like to drive circuitry for the LCOSSLM 1OS, to allow writing to the pixels of this, where it is pixellated.

By using the DC pulse drive method described above, and where the beam 109 is arranged to be writeable to selected zone of the OASLM an embodiment envisages a novel projection technique. Each image zone of plural zones may be written sequentially or otherwise to the OASLM. This is achieved in one embodiment by precharging a respective zone, then illuminating that zone with the information, for example the carrier frequency hologram, for that zone from the EASLM. Then once all of the data are loaded into the OASLM, a single read pulse may be applied to the whole OASLM, and the read beam applied across its surface to provide a replay field from the entire tiled hologram information

Where this method is used, in an embodiment, the sequence is as follows: a) precharge pulse is applied to a zone of the OASLM to which writing is to occur; no precharge to other zones. b) both the blue light source and the reference beam are turned on substantially simultaneously after termination of precharge. The blue light source and the reference beam may be allowed to be incident on zones that have not been precharged, i.e. must not be incident on zones that have already been written to. Shutters or the like may be used to block light flow to zones that have already been written to. c) this is repeated until all desired zones have been written to. d) after all zones are written, the "read" electrical pulse is applied to the whole OASLM and illumination light is incident on the whole OASLM.

It is of course not essential to the invention in its broadest aspects to operate in this way.

To allow for writing information to the invention to use a LCOSSLM; other devices may be used. Further, the light beam 109 may be provided from a target object itself, rather than from an amplitude hologram.

In the above description, an embodiment using a homeotropically aligned LC material is described. Other embodiments include planar aligned material.

As will be exemplified below, alternative device structures may be employed to improve the phase shift produced by the OASLM. It has been found that the positioning of the device in the focal plane is tolerant to misalignment. Further, resolution of the device depends on device thickness. Writing can be done at normal incidence, and is polarisation insensitive, and reading can be done at oblique incidence (substantially between 30° and 45°, although other angles are envisaged). The device is sensitive to the reading beam polarisation.

A reflective device structure is shown in Fig. 6. This comprises a single layer device 61 as described above with reference to Fig. 1, but with a mirror surface 65 provided. The mirror surface 65 may be provided on a surface of either glass plate 11, or 12, on a surface of the glass plate opposite the ITO electrodes 13. The mirrored surface 65 may be provided by another layer arranged adjacent to either glass plate 11 or 12.

In the device according to Fig. 6, the reading beam pass length is doubled, which doubles the phase shift provide by the device. The mirror surface 65 may be a conventional mirror or a "hot" mirror, thus enabling writing and reading from different sides. As described above in relation to figure 5, the read beam is red and the illuminating beam is blue. A hot mirror reflects long wavelength light e.g. red light, but is substantially transparent for short wavelength light, e.g. blue light. Where mirror surface 65 is a hot mirror, the red read beam is reflected, so improving the phase modulation. The hot mirror allows the blue illuminating beam to pass through. Accordingly, the blue illuminating beam may be incident from behind the mirror such that there is more space available for the red read optics.

Such an arrangement may require a trade off of reduced resolution for improved phase shift. For example, at a 20μm resolution the critical angle for reading is 49°. Beyond this critical angle, crosstalk becomes a significant effect.

A multilayer structure is shown in Fig. 7. This comprises a plurality of single layer devices (71, 72 and 73), as described above with reference to Fig. 1. Adjacent glass

plates, 12 and 13 of adjacent layers may be arranged adjacent each other. Alternatively, adjacent glass plates, 12 and 13 of adjacent layers may comprise a single common glass plate with the ITO electrodes 13 and contacts 16, 17 for adjacent layers arranged on opposite sides of the glass plate.

In the device according to Fig. 7, a multiple gain in phase is provided with no, or at least negligible, loss of resolution. This occurs because the device thickness is increased. The positioning of the device in the focal plane is not crucial, so in use no problems arise directly from the thicker form factor of the device. The multilayer device according to Fig. 7 may be used in both transmission mode (as per Fig. 1) and reflection mode (as per Fig. 6). For reflection mode an optional mirror surface 75 is provided. The critical angle for a device according to Fig. 7 having 2 layers and for 20μm resolution is 13° in transmission mode, and 6.9° in reflection mode.

A multiple reflection structure is shown in Fig. 8. This comprises a small mirror on the front surface glass. In use, the device according to Fig. 8 provides gain in phase. Such an arrangement is relatively simple to assemble but increases the "pixel" size - resulting in a loss in resolution. The nature of this arrangement makes it suitable for use in reflection mode.

A Gires Tournois etalon arrangement of the device improves the phase response. In such an arrangement, two reflective surfaces are provided, either side of a single layer device 81, as described above with reference to Fig. 1. A first reflective surface 61, is partially reflective. A second reflective surface 65, shown on the bottom in Fig. 6, is highly reflective.

Turning now to the liquid crystal material used in the device, this comprises a host and a dopant. The following dopants may be used, this list is not exclusive, and the use of other materials will be readily apparent to the skilled person. . C ₆₀ , 99.5% purity (Aldrich)

• Single- Walled Carbon NanoTube (SWCNT) (as produced by Thomas Swan)

• SWCNT Octadecylamine functionalised (Product 652482 by Aldrich) (easier to disperse in liquid crystal hosts compared to non-functionalised CNTs)

• Multi-Walled Carbon NanoTube (MWCNT) 95% purity, from Nanocyl

• SH and NH ₂ functionalised MWCNT, from Nanocyl

For dispersion in liquid crystalline host materials dopants may need hydrophobic covalent functionalisation or an aid of surfactants. Generally, hydrophilic covalent functionalisation does not aid dispersion.

Host materials that may be used comprise, but are not limited to, nematic materials, in particular cyanobiphenyls, e.g. 5CB, E7, BLO48, BLO37. Where C ₆₀ is the dopant, it has been found that this disperses well in 5CB. In contrast, nanotubes prefer E7. Ferroelectric LCs may also be used as the host material.

The alignment layers 14 described above in relation to Fig. 1 may comprise non-ionic alignment agents. Such agents prevent the devices from receiving electric damage and so extend the operational lifetime of the device. Examples of possible alignment agents are: Planar (such as PI, rubbed ITO, SiOx, and nylon), and Homeotropic (such as ZLIl 134, and lecithin).

The incorporation of dopants into the liquid crystal host materials has been made by sonification in an ultrasonic bath or with high power ultrasonic Bioruptor or Nanoruptor systems. A vibrating tip may be used to improve sonification results in volumes larger than 5ml. In preparation of the liquid crystal layer, care is taken to prevent moisture contaminating the LC solutions.

Dopants are dispersed in the nematic phase or at (not above) the nematic-isotropic temperature of the LC host. Dopants are dispersed in the host by sonification of the mixture, setting out, and then filtering.

During preparation, the sonified mixture is allowed to settle out. This process may take between 2 days and 3 weeks. Once settled out, the mixture is filtered using a

pore filter. A 0.2μm pore filter is used where C ₆₀ is the dopant. A 5μm pore filter is used where CNT is the dopant.

Doped liquid crystal (host plus dopant, combined as described above) is filled into a cell. The cell comprises a glass slide with a transparent ITO electrode, and an alignment layer. The alignment layer may introduce planar or homeotropic alignment. The cell thickness is defined by spacers and is usually within a range of 2- lOOμm. The structure of single layer optically addressed spatial light modulator is shown in Fig. 9. The same reference numerals are used for like elements as Fig.l above. Fig. 9a shows a planar aligned device (LC molecules parallel to substrate). Fig. 9b shows a homeotropically aligned device (LC molecules perpendicular to substrate).

The devices of Figs. 9a and 9b have two glass plates 11, 12 separated by spacers 20. On the inner surface of each glass plate 11, 12 there is deposited a respective ITO electrode layer, one of which is connected to a first contact 16 and the other of which is connected to a second electrical contact 17. Respective layers of an alignment material are disposed on the ITO electrodes 13 and these form alignment layers 14 for the doped liquid crystal material 15 that forms the active material of the device. The doped liquid crystal material 15 is held within the space between the two alignment layers 14 by glue seals 21.

During manufacture of the device, strict clean room conditions are provided to ensure that there are no impurities in the samples. Special care should be taken to avoid contamination of materials by ions, for reasons that will be explained below.

The nonlinear optical properties of the LC material improve with time. Electrical conditioning such as DC application may also improve device performance. For example, a BLO48 and C ₆₀ planar device with polyimide as the alignment material may be electrically conditioned by the application of 10V DC for 10 minutes. This electrical conditioning improves the performance of this cell from a barely visible diffraction to 3.1% diffraction efficiency (the ratio of the power in the first diffracted

order to the power of the reading beam; 34% max for the given geometry). If the voltage is applied for too long, eg a further 2 hours, then the efficiency drops 5 times (to 0.6%).

Voltammetry reveals that when a voltage is applied to the LC device, a small current (usually of the order of 10 ^"9 A) is produced. This current is due to ionic impurities or dopants, and charge injection from the electrodes which grows as the voltage is increased. Large currents are detrimental to liquid crystalline material. This is why contamination by ions during manufacture must be kept to a minimum.

Figure 10 is a graph showing current and voltage behaviour of pure and doped BLO48 devices. A freshly made C60 doped device (dark diamonds) has a higher conductivity in the charge injection regime than the older doped device (light triangles). It should also be noted that the older doped device exhibits a stronger non- linear effect than the freshly made doped device. The electrical conditioning (see above) may reduce the conductivity in the charge injection regime, i.e. where the older doped device has been electrically conditioned but the freshly made device has not been conditioned.

The measurements of the current for this arrangement were performed using a

Hewlett Packard 4140B PicoAmmeter (precision 10 ^'12 A - picoAmps). The current is measured through the sample, as the external electric field is applied by the voltage source. The data is collected by computer. This methodology is good for: measuring contamination by ionic impurities; and identifying devices (devices producing good nonlinear effect have specific FV behaviour which may be used as a signature).

Optical nonlinearity studies have proved to be the best tool for characterisation of the device performance. These allow straightforward definition of the dynamics and the efficiency of the device. Such studies may be performed using two-beam interference arrangement or imaging arrangement.

In the two-beam interference arrangement, interference fringes are created on the sample using two coherent laser beams propagated through the sample and intersecting at a small angle within the sample. An example of such an arrangement is shown in Fig. 11. The interference pattern formed in this way is a sinusoidal intensity grating with a period λ, which depends on the angle of intersection φ and wavelength of optical radiation λ.

m is an optical intensity modulation factor and n\\ is the refractive index seen by the writing beam.

A non-polarising beamsplitter 201 is used to obtain two coherent beams of equal intensity. These beams are overlapped on the sample 202 with the. help of mirror 204. The resulting diffraction of a reading beam 203 may be either observed on the screen 205 or the first diffracted order is recorded dynamically (as the writing beams are switched on and off) with the aid of a photodiode and an oscilloscope.

In the imaging arrangement, an optical pattern is created on the sample. This is done by imaging a hologram or a Ronchi grating onto it. Such an arrangement is shown in Fig. 12. The writing beam 301 of any polarisation is expanded by the collimator 303 and may be switched on and off with the aid of the electronically controlled shutter 304. The expanded beam is spatially modulated by a diffraction grating 305 (a set of square- wave amplitude gratings with spacing ranging from 15 to 120 line pairs per millimetre). The resulting intensity profile is imaged onto the sample 307 using a lens 306 (^/ ^" system) at normal incidence. The profile created in the sample is a phase profile.

A He-Ne laser beam 302 (λ=633 nm, power 1 mW) is used to read the recorded information. The reading laser beam may be applied at an angle to the device (which provides higher efficiency), or brought in at normal incidence using a beam splitter (for planar devices only). The reading process is polarisation sensitive.

The resulting diffraction of the reading beam can be viewed/imaged onto a screen. The filter 308 cuts out light at the writing wavelength. If using a grating, the intensity of the 1 ^st diffracted order may be measured by a photodiode 309. hi the case of a hologram, 0 ^th order may be monitored in a similar fashion. The signal from the photo sensor is read by the oscilloscope (TDS 2200) 310, as the writing light is switched on and off. DC and AC voltage of different frequencies and with different waveforms can be applied to the sample during testing.

This technique may be readily implemented and does not suffer from coherence problems, which is an advantage over a comparable setup using interference.

Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described without departing from the scope of the present invention.