LIQUID CRYSTAL LIGHT MODULATOR

Abstract: A light modulator liquid crystal cell comprises a first optically transparent wall supporting a first electrode. The device also comprises a second wall supporting a second, reflective electrode, and a cholesteric liquid crystal layer formed within the first and second walls, where the cholesteric liquid crystal forms a diffracting or refracting layer that diffracts or refracts light passing through the structure.

Description: 1 LIQUID CRYSTAL LIGHT MODULATOR This invention relates to the field of liquid crystal devices used for controlling the intensity of spatial distribution of light, either in the visible region or in the infra-red. Specific examples of optical systems where a device according to the invention may find application include projection display systems for use in the visible region, fibre optic transmission or illumination systems, and laser systems and laser marking systems. In many optical systems it is desirable to modulate the intensity of light passing out of the system by diverting the light within it, rather than absorbing it within the modulating component. In this way the light can either be dumped internally in a part of the optical system which is not sensitive to heat, or be diverted to some other part the system where it can be used. Specific examples of such systems include projection display systems, high power laser systems and fibre optic systems. In LCD projection display systems the yield of the LCD manufacturing process and cost of the product is linked to the size of the LCD panel. The smaller the panel, the better the yield in manufacturing, and the lower the volume of materials used in the construction of the projector. There is therefore good reason to use small LCD panels to project an image. In order to produce high screen intensity, and overcome the optical losses, these systems must use high intensity light within the optical system, with consequent heating and UV bleaching problems within the polarisers. In high power laser systems, it is often the case that a laser has its optimum output properties (for example beam divergence, beam mode quality and electrical to optical power efficiency) at the design output power. However, it is sometimes desirable to adjust the output power without modifying the beam distribution (as would be the case if an adjustable iris were to be used) . One example may be annealing a surface without burning, or limited depth cutting. Mechanically driven polarisation controllers can achieve this, but only if the laser is pre-polarised and only at low speed. It would therefore be advantageous to provide means for controlling the intensity without polarisation sensitivity and without modifying the beam shape, which can be electrically controlled in such a way that feedback from the laser process could be used to control the applied power. In laser marking systems it is advantageous to have a mechanism to electrically produce spatial modulation in the laser beam, which can then produce a mark on the substrate carrying information such as lot codes, security codes or other product information that can be readily updated on a product by product or lot by lot basis. In fibre optics systems, such as trunk telephone networks or local area networks, it is often desirable to have a mechanism for controllably adjusting the intensity of the received light, for example to avoid the effects of receiver saturation in short links, or to switch light from one fibre to another for the purpose of providing transmitter or receiver redundancy, network node bypassing, or signal routing. In fibre optics systems, the transmitters used are often semiconductor lasers which are intrinsically polarised. However, the optical fibre in the transmission link is normally not intrinsically polarised and random mechanical and thermal variations along the link scramble the polarisation state of the light passing along it, even over distances of centimetres, rendering polarisation sensitive devices ineffective. According to the present invention there is provided a light modulating liquid crystal cell comprising: a first optically transparent wall supporting a first electrode; 3 a second wall supporting a second, reflective electrode; and a cholesteric liquid crystal layer formed within the first and second walls, wherein the cholesteric liquid crystal forms a diffracting or refracting layer that diffracts or refracts light passing through the structure. The light refracting or diffracting means may be provided by the shape of one or both of the electrodes causing a structure to be apparent in the cholesteric when the electrodes are driven. For example, the electrodes may be inter-digitated to produce a linear grating structure within the liquid crystal, or individual pixels may be driven in a pattern to produce a square or rectangular array of driven areas which optically resembles a binary phase hologram. Alternatively the light refracting or diffracting means may be provided by the provision of surface energy modifying agents on selected areas of at least one of the walls producing domains of different alignment in the liquid crystal layer. Alternatively, the light diffracting or refracting means may be provided by a physical structure modulating the depth of the liquid crystal layer. This physical structure may for example take the form of a simple linear grating, a linear sawtooth grating, or may be a two dimensional array of pillars, made from a transparent dielectric such as an optical polymer or glass. When the physical structure is a sawtooth grating, the physical structure may have an optical depth of one half a wavelength when the liquid crystal is relaxed to give optimum diffraction efficiency in reflection. When the physical structure is a simple linear grating or an array of pits or peaks, the physical structure may have an optical depth of one quarter of a wavelength when the liquid crystal is relaxed to give optimum diffraction efficiency in reflection. The present invention provides a liquid crystal cell which can produce modulation of light without requiring polarised light, with consequent improvements in efficiency of the optical system to which it is applied and a reduction in heating of the optical elements. Some of the advantages of the invention are that there is electrical control of intensity and direction of light, modulation and switching without absorption, low sensitivity to state of polarisation of the light being modulated, high switching speed compared to twisted nematic (TN) liquid crystal devices, and ease of integration with silicon integrated circuit manufacturing techniques. The apparatus of the invention may be utilised in front or back projection apparatus particularly, which may include a projection lens comprising an annular reflecting element to separate the zero and higher diffraction orders produced by the diffraction means. The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram liquid crystal cell according to the present invention; Fig. 2 is a schematic diagram showing the mode of operation of the device; Fig. 3 shows the method for constructing the diffraction means. Fig. 4 shows an alternative method for constructing the diffraction means; Fig. 5 is a schematic diagram of a known liquid crystal projection display system; Fig. 6 is a schematic diagram of a known laser marking system; Fig. 7 is a schematic diagram of a known fibre optic system; Fig. 8 is a schematic diagram of an example of the invention applied to a projection display system; Fig 9 is a schematic diagram of a laser marking system employing the present invention; Fig 10 is a schematic diagram of fibre optic switch employing the present invention; Fig. 11 is a schematic of the components of a fibre optic switch employing the present invention; Figure 12 is a schematic diagram of a reconfigurable fibre optic switch employing the present invention; and Figures 13A and 13B show a further illumination system for use with the present invention. An example of the invention, which can be arranged to be applied to any of the above systems, shown in Fig. 1, consists of a liquid crystal device which contains a cholesteric liquid crystal layer within which is formed a diffractive or refractive structure. In this example, the device comprises the following components: a substantially transparent glass or polymer cell wall 1 coated with an electrically conductive surface layer 2, with a refractive or diffractive structure 3 within the liquid crystal layer 4, the cell being completed by a back substrate 5 (which may be of glass, polymer, or silicon) which has deposited upon it an electrode layer 6 and a reflector 7. Alternatively, the diffractive or refractive structure may be produced within the liquid crystal by appropriate patterning of the electrode structure, or by selective deposition of surface energy modifying agents to selectively produce areas of different alignment . Cholesteric liquid crystals are characterised by the molecular layers within the liquid crystal layer being at a relative angle to each other such that in the direction substantially normal to the cell, the molecular axes form a helix. The pitch of this helix is less than the maximum thickness of the liquid crystal layer, and more than half a wavelength of light at the wavelength of operation. The optical properties of the liquid crystal can be chosen such that the resulting material, when correctly aligned, appears to be only very weakly polarisation sensitive to light passing parallel to the direction of the axis of the molecular helix. Such a liquid crystal can, for example, be made by the addition of the chiral additive CB15 to the nematic liquid crystal E44 in the ratio of 9% to 91% by weight. This particular mixture has an effective refractive index change of 0.13 and a cholesteric pitch of 1.3um. Such liquids are available from E. Merck, of Darmstadt, for example. The refractive or diffractive structure may be one dimensional, for example a linear grating, or two dimensional, for example an egg box type structure, or an array of pillars. In the case of very fine pitch structures, the vector nature of light can result in linear structures made from isotropic materials being polarisation sensitive, so a two dimensional structure, which does not demonstrate polarisation sensitivity, may be advantageous. The operation of the invention can be described with reference to a specific example. The example optical structure shown in Figure 4 is a grating of substantially saw-tooth cross-section 3. The cell consists of substrates with electrode coatings 2, and a grating 3 made from a material with refractive index substantially equal to the ordinary refractive index of the nematic material in the cholesteric mixture 4. It can be demonstrated that when the liquid crystal is relaxed, it forms the helical structure as described above, and the effective refractive index of the cholesteric material is given by the average of the ordinary and extraordinary refractive indices i.e. n,effective = (n 0 + n e ) / 2 (1) If the optical depth of the structure is chosen to be one half of a wavelength (or an integer number of wavelengths plus one half of a wavelength) , light is diffracted away from the original specular reflection substantially into a single diffraction order. The grating depth at which this happens is given by the equation: h x (n effective - n s ) = ( 2m + 1 ) x 8 / 2 ( 2 ) Where h is the grating depth, and 8 is the operating wavelength of the device, n s is the refractive index of the substrate and m is an integer. As an example, using E44 (refractive index values n 0 = 1.528 and n e = 1.790), with the device operating at a wavelength designed to be at the peak of visual intensity (green, = 530) , and m = 0, the height of the structure needs to be 1.8 microns. The angle of separation of the diffraction orders is given by the conventional Bragg equation: n x 8 = d x sin (Θ) (3) where n is an integer, d the grating pitch, and 2 the angle of the diffracted order number n. In an alternative embodiment the cell structure may be in the form of a rectangular grating structure. In this case for optimum efficiency the structure height needs to be one quarter of a wavelength (or a multiple of half wavelengths and one quarter of a wavelength) . In this case the diffraction efficiency at the design wavelength approaches 80% in the two first orders. It is clear from the form of equation (2) that the effect is colour dependent. If equation (2) is satisfied for green (8 = 530nm) , a value of h can be chosen such that the equation is not satisfied for red and green. In this way, green light is diverted into the first orders and red and blue are transmitted undeviated, and the device becomes coloured in operation. Figure 2 illustrates schematically the operation of the device. When the liquid crystal is relaxed the diffraction efficiency is maximised 8; when the liquid crystal is switched on its effective refractive index matches that of the polymer substrate and the device 8 becomes water clear 9. In this way an optical switching mechanism is produced. Constructing the device to operate in reflection offers significant advantages over transmission mode devices. The diffractive structure is thinner, resulting is structures that are easier to manufacture, and the reduced cell thickness also allows operation at reduced cell drive voltages and produces a faster switch-off speed (switch off speed being dominated by surface alignment forces which effectively propagate only a small distance into the bulk liquid crystal material) . The polymer grating structure can be made by replication from a metal master structure by two techniques, illustrated in Figures 3 and 4. In the first technique, an electroform of the original master structure 10 is applied to an embossing roller and a continuous film replica made 11. This is then used as a sub-master by rolling onto a flat substrate pre-coated with a UV-curing resin 12. The resin is cured and the sub master is then peeled away to leave a thin replica 13. In the second technique, a silicon rubber copy 15 is made of the original master structure 14 and pressed onto a substrate pre- coated with a UV curing resin 16. The resin is cured and the silicone is then peeled away to leave the thin replica 17. The invention may be used in a variety of optical systems where high efficiency and low polarisation sensitivity are required. Below we give examples of applications where the invention can be utilised. The first example is in projection display systems. A schematic of a known LCD projection display system is shown in Figure 5. A lamp housing 18 illuminates a pre-polariser 19. This produces polarised light which then passes through a TN LCD cell 20 to the post polariser 21. A projection lens 22 images the cell on to a screen (not shown) . The TN LCD cell is divided into pixels which form the image. In a colour display each pixel is further sub- divided into red, green and blue areas which can be individually addressed. The image is produced by electrical control of polarisation using a twisted nematic (TN) liquid crystal within the cell 3. In this type of projector, more than half the light from the source is wasted, since the source produces naturally unpolarised light. This must be pre-polarised for the projector to operate, and light is additionally absorbed by the pre- and post- polarisers. Figure 6 illustrates one way in which a device according to the invention may operate within an optical projection system. With a cell 23 constructed as shown in Figure 1 placed in a projector, light from the projector lamp 24 converges on to the cell. A stop is placed such that the light is blocked if it is reflected from the cell undeviated 25, but the first order is transmitted by the projection lens 26 to form the image on a screen 27. Therefore when relaxed the cell allows most of the illumination to reach the screen 27. If for example, the diffraction structure is a saw-tooth, only the first order is utilised, and the transmission of the system becomes close to 100%. This compares to a transmission of 30% for a conventional polariser based liquid crystal projector. The image is formed by dividing the area of the display into pixels and driving these independently. In order to operate with useful efficiency the pixels have a size that is a multiple of the period of the diffraction structure. This multiple must be at least two for the device to operate correctly in this mode, i.e. the diffraction that results from the pixel periodicity must be half the angle of diffraction due to the diffraction structure. When an electric field is applied to the cell 23, the liquid crystal becomes orientated parallel to the field lines in the cell and its effective refractive index becomes the same as that of the substrate material. All the light from the lamp 24 is transmitted by the cell undeviated and absorbed by the beam stop 25. No light is 10 transmitted by the projection lens 26 to the screen 27. This effect is substantially independent of the wavelength of the light and depends only on the refractive index match between the substrate and the liquid crystal and not on the form of the structure in the cell. The cell therefore produces a dark area on the screen with high contrast . In addition, the surface reflection from the device is specular and therefore the contrast is not limited by the first surface reflection as it is in other reflective LCD devices. In this implementation, the device can be classified as a first order read-out light valve. In an alternative embodiment, the device can be configured as a multiple order readout. In this case the profile of the diffractive structure may for example be an array of pits or peaks, which might, for example, be square or round. The illumination is normal to the plane of the display. The diffractive structure is constructed from a transparent dielectric such as a polymer or glass. The pits are filled with cholesteric liquid crystal. In this form of device, the periodicity of the pits can be equal to the periodicity of the pixels of the display (i.e. one pit per pixel) . When the device is switched off the optical depth of the pits is one quarter of a wavelength and destructive interference takes place in the back-reflected beam, resulting in first and higher order diffraction. When the pixel is switched on, the refractive index becomes uniform across the structure and the beam is directly reflected. In this embodiment, the device is analogous to an electrically addressable compact disc. The operation of the device is substantially independent of polarisation state. There are a number of possible mechanisms that can allow illumination of the device . An efficient illumination system for a device of this type is shown in Figures 13A and 13B. 11 The illumination passes from a lamp condenser system 70 through the open centre of a toroidal mirror 71, to the device. In the off state (Fig. 13A) , the light is scattered into the first and higher diffraction orders of the structure, to the toroidal mirror and out to a projection lens 72. When the liquid crystal is switched on (Fig. 13B) , the diffraction structure is index matched and the illumination is reflected directly back to the light source . In this implementation, the advantages of high contrast, high efficiency and high switching speed are combined with very high pixel resolution. Typically the maximum practical size for an integrated circuit is 20mm square; a device of this sort constructed on such an IC can allow 100 million pixels. When projected onto a front or rear projection screen, this allows full colour to be displayed at 80 dots per cm on a 75-150 cm diagonal screen size, allowing a true large area paper like display. Between the on and off end states there is a continuum of grey states where the transmission of the cell is related directly to the driving voltage. An image is formed by adding to the cell an array of driving electrodes corresponding to the pixels of the desired image, as in a conventional active matrix display. It is clear that, with appropriate modification to the structure height, the structure could take many other forms, for example sinusoidal, rectangular, saw-tooth etc. in one or two dimensions. The device operates in reflection, and this allows conventional silicon transistors to be used to drive the liquid crystal and the drive and control electronics to be integrated onto the same display substrate. A further advantage of the invention is that the forces acting to hold the liquid crystal molecules in their helical structure are relatively large. Although this means that the driving voltage is larger than a normal TN 12 liquid crystal (typically 6-7 Volts for a cholesteric compared to 2V for the corresponding undoped nematic) the switching time is correspondingly higher. Switching speeds in the order of milliseconds are readily achievable. This is a considerable improvement over TN type displays, where switching speeds are typically 100ms, and is more than fast enough for video applications. The switching speed is fast enough to allow so-called field sequential imaging, in which red, green and blue images are projected in sequence. Field sequential imaging allows the pixel density on the display to be reduced by a factor of three over conventional LCD panels, reducing the complexity of the device and increasing the yield in production. Above is described, as an example, application of the invention to projection displays. A further example is application to laser marking systems. In known laser marking systems, such as that shown schematically in Figure 7, the mark to be made is often formed by illuminating a mask 28 with a high power pulsed laser 29 via a beam delivery system 30 and imaging this mask using a fold mirror 31 and a projection lens 32 on to the substrate to be marked 33. The substrate is typically moving on a production line so that repetitive marks can be made. Conventionally, the mask 28 is made from a metal plate which must be capable of absorbing or reflecting to an optical dump the unused optical power. This system has the disadvantage of inflexibility in that the mask must be mechanically changed in order to change the printed message . In the past electrically adjustable masks have been made for Nd:YAG laser marking systems using TN liquid crystal cells, but these have the disadvantage of requiring a pre-polarised source laser and a co-linear polariser or polarising beam splitter to remove the unwanted light. Lasers used for mask marking are generally unpolarised pulsed lasers. The present invention provides a mechanism which diverts the unwanted light from the optical path, and 13 which is not sensitive to the polarisation state of the light passing through it. Fig. 8 illustrates schematically how the invention can be used to provide electrical control of the laser power from the laser 34 via the beam delivery 35 and a fold mirror 36. The cell 37 operates by adjustment of the diffracted power between the substrate 40 via the imaging lens 38 and an optical dump 39. In marking systems, the cell can be electrically addressed to provide a pattern that can be adjusted at high speed for production line marking purposes . The present invention may also be arranged to provide a switching or modulation mechanism in a fibre optic system which can be electrically controlled and provide low polarisation sensitivity and low intrinsic loss. A known example fibre optic network is shown in Figure 9. A fibre optic link 41 has at one end a master node 42 and along the length a series of network slave nodes 43. Inside each of the slave nodes 44 there is a transmitter 46 and receiver 47. In normal operation the transmitted signal from the master node 42 is received by the slave node receiver 47. The slave node then decodes the signal, adds its own information, and recodes it. The signal is then regenerated by the slave transmitter 46 and passed to the next slave node. In the event of a failure in the slave node 44 a switch 45 is provided to lock out the transmitter and receiver of the slave node and bypass the node. Often this switching function is achieved with mechanical switches which physically move the fibres. Mechanical switches of this type are slow and suffer poor reliability. Fixed splitters or couplers cannot be used, because they do not allow discrimination between weak incoming signals and strong local signals. The current invention may be used to produce controllable attenuation or switching from one fibre to another . 14 An application of the invention in fibre optics systems is shown in Figure 10. Light from the incoming optical fibre 48 is collimated by a lens 52 and passes through to the cell 53. When the cell is switched on and is effectively water clear, the incoming light is reflected to the local receiver fibre 51. The fail-safe power off condition is that the incoming light is sent to the outgoing fibre 50 and the local node is bypassed. Figure 11 shows the components of such a device. A four fibre array is butted against one face of a graded index rod lens. The cell is glued to the other end. The grating periodicity is chosen such that the displacement of the spot at the fibres caused by the grating is one fibre width. In this way the fibres may be spaced by their own diameters. The height of the grating is chosen to maximise the coupling efficiency at the operating wavelength, which for example may be in the near infra-red at 820nm, 1300nm or 1500nm. This application of the present invention is a network bypass switch, in which a local transmitter/receiver in a loop network may be bypassed. In a more sophisticated device, the microstructure may be addressed by an array of pixels. By controlling the driving voltage on each pixel, an analogue phase array is produced which will allow the beam to be steered into one or a number of different diffraction orders. These diffraction orders may correspond to one or more output fibres, or may correspond to different optical detectors. In a specific example the liquid crystal cell of the present invention may be driven by an array of pixels each 12um square on 13um period, which is an industry standard pixel size used for silicon backplane display panels and CCD devices. In Figure 12, nine fibres are shown 57-65, illuminating a collimating lens 66 and silicon backplane driven cell 67. If the silicon backplane is configured as shown in the sketch 68, the light from the input fibre 61 is distributed equally amongst fibres 57, 59, 63, and 65. 15 If the silicon backplane is configured as shown in the sketch 69, the light is distributed from the input fibre 61 to output fibres 60 and 62. More complex patterns allow other combinations of inputs and outputs to be addressed. Naturally the device can be configured so that a multiplicity of transmitters can be coupled to a single output, or a variety of inputs and outputs can be connected. In this way an electronically reconfigurable optical switch can be made. The angle between the diffracted orders is given again by the diffraction equation 3, so that if the pixels are on a 13um period, for a fibre optic device operating at 820nm the diffraction angle is 3.92 degrees. If a collimating lens of focal length 1.82mm is used, the diffraction orders are focused to spots in the focal plane of the lens with a periodicity of 125um, which is the width of a single fibre. In this way a fibre array may be constructed to take inputs and outputs, with the optical fibres conveniently spaced by their own diameters .

Claims: 16 CLAIMS 1. A light modulating liquid crystal cell comprising: a first optically transparent wall supporting a first electrode; a second wall supporting a second, reflective electrode; and a cholesteric liquid crystal layer formed within the first and second walls, wherein the cholesteric liquid crystal forms a diffracting or refracting layer that diffracts or refracts light passing through the structure. 2. A cell according to claim 1, in which the electrodes formed on either one or both of the walls comprises an array of independently driven pixels. 3. A cell according to claim 1 or claim 2, wherein the light diffracting or refracting means is provided by a physical structure modulating the depth of the liquid crystal layer. 4. A cell according to claim 3, wherein the physical structure is made from a transparent dielectric. 5. A cell according to claim 3, wherein physical structure modulating the depth of the liquid crystal layer comprises a linear sawtooth grating. 6. A cell according to any of claims 3 to 5, wherein the physical structure has an optical depth of one half a wavelength when the liquid crystal is relaxed to give optimum diffraction efficiency in reflection. 7. A cell according to claim 3, wherein the physical structure modulating the depth of the liquid crystal layer comprises a rectangular linear grating. 17 8. A cell according to claim 3, wherein the physical structure is an array of pits or peaks. 9. A cell according to claim 7 or claim 8, wherein the physical structure has an optical depth of one quarter of a wavelength when the liquid crystal is relaxed to give optimum diffraction efficiency in reflection. 10. A cell according to any of claims 7 to 9, wherein there is one period of the diffractive structure per pixel. 11. A cell according to any of the claims 3 to 8 , wherein the structure refractive index is matched to the ordinary refractive index of the liquid crystal, so that in the switched on state the device appears clear. 12. A cell according to claim 1, wherein the light diffracting or refracting means is provided by the shape of one or both electrodes causing a structure to appear in the cholesteric layer when the electrodes are driven. 13. A cell according to claim 1, wherein one or both of the electrodes comprises coplanar interdigitated electrodes, such that when driven in antiphase the electrodes produce a diffractive structure in the liquid crystal . 14. A cell according to claim 1, where the pixels are driven in a pattern to produce a square or rectangular array of driven areas which optically resemble a binary phase hologram that acts as the diffracting means. 15. A cell according to claim 1, wherein the light diffracting or refracting means is a structure formed in the cholesteric liquid crystal by the provision of surface energy modifying agents on at least one of the walls of the cell. 18 16. A front -projection display apparatus incorporating a cell according to any of claims 1 to 15. 17. A back-projection display apparatus incorporating a cell according to any of claims 1 to 15. 18. An apparatus corresponding to claim 16 or claim 17, including a projection lens comprising an annular reflecting element to separate the zero and higher diffraction orders produced by the diffraction means.

Inventors: LARGE TIMOTHY ANDREW, LARGE, TIMOTHY, ANDREW
Applicants: LARGE TIMOTHY ANDREW, TECHNOLOGY PARTNERSHIP, LARGE, TIMOTHY, ANDREW, THE TECHNOLOGY PARTNERSHIP PLC
Application year: 1999
Main classification: G02F1
IPC codes: G02F1