No assignee for this patent application has been made.
News editors obtained the following quote from the background information supplied by the inventors: "For holographic displays, as well as for other applications, fast phase modulators as well as fast amplitude modulators are required. For LC (Liquid Crystal) based modulators in many LC modes, a relationship between the thickness of the LC layer and the switching time of modulator is known. Approximately, the switching of the modulator slows quadratically with increasing thickness of the LC layer. This is because, in general, the LC molecules react to a change in the electric field more rapidly in contact with a surface than at a distance therefrom. On the other hand, however, in order to achieve a predetermined maximum value of the amplitude or phase modulation, a particular product of LC layer thickness and birefringence is required. For this reason, the layer thickness parameter can only be varied within narrow limits--for instance by selection of an LC material having a high birefringence. The layer thickness thus cannot be reduced arbitrarily when wishing to reduce the switching time of the modulator.
"In order to achieve faster switching times in the case of LC based non-pixelated shutters, there are for example applications in which an LC layer thickness required for the modulation is distributed between a plurality of individual layers, with glass substrates arranged between the individual layers. For example, a fast shutter is known in the form of a sandwich of 3 LC layers, each with a thickness of 1.5 .mu.m, embedded in glass substrates. This shutter achieves the same optical function as a single 4.5 .mu.m thick LC layer, but has considerably shorter switching times than this individual layer. This sandwich approach, however, could not be applied in this way to a pixelated light modulator having pixels whose dimensions are small in comparison with the thickness of the glass substrates. Owing to the glass substrates, undesired diffraction effects would then occur during the light propagation between the individual LC layers, which would entail crosstalk between the individual pixels. For example, a typical pixel pitch in a light modulator for a holographic display is around 30 micrometers, while the typical thickness of a glass substrate, such as is used in the display industry, is 700 micrometers.
"Polymerizable LC structures (PDLC: Polymer Dispersed LC Structures), in which a polymer network stabilizes a particular orientation of the LC molecules, are also known, which can likewise have a positive effect on the speed of a switching process. In general, however, such crosslinking leads to problems in relation to scattering during the light transmission.
"On the other hand, switchable volume gratings which have a grating structure consisting of a regular polymer network and LC layers lying in-between are known. Such an arrangement is described, for example, in the publication by
"Both publications describe a type of switchable Bragg grating, which is used for light deviation and, depending on the switching state, transmits a smaller or larger incident light fraction either deviated or undeviated. It is also described that this grating can be switched in a pixelated fashion. In this way, the incident light would thus also be locally transmitted either deviated or undeviated, depending on the switching state of the pixel. The arrangement then corresponds to a Bragg grating driven in a pixelated fashion.
"The Bragg condition is given by
"with n--number of the diffraction order, .lamda.--light wavelength, d--distance between the grating planes, .theta.--angle between the incident light beam and the grating planes.
"For example, such a pixelated arrangement could be used as a spatial amplitude modulator when, for example, the deviated light is filtered out and only the undeviated light is allowed to pass through, or vice versa. However, an application of such a pixelated arrangement as a spatial phase modulator, i.e. one in which the phase of the light interacting with the arrangement can be modified at the pixel level, would not be possible in this form. Furthermore, such an arrangement is subject to restrictions which are due to the known properties of Bragg gratings, namely a particular angle and wavelength selectivity. Although Bragg gratings have a high diffraction efficiency of close to 100 percent in a single diffraction order, they have this efficiency only for a small angle range of the incident light and only for a small wavelength range. It is therefore to be expected, for example, that such a switchable Bragg grating cannot readily be operated uniformly with a high efficiency of close to 100 percent for red, green and blue light."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "It is therefore an object of the present invention to provide a spatial light modulator with which, in the region of the respectively activated pixel, the phase of the incident light can be modulated according to the applied voltage. In this case, the increased switching speed compared with light modulators not having such a grating structure is intended to be maintained, and wavelength selectivity is intended to be substantially suppressed.
"This object is achieved according to the invention by the means of claim 1. Further advantageous configurations and refinements of the invention may be found in the dependent claims.
"The spatial light modulator according to the invention is used to modulate light, from at least one light source, which interacts with the spatial light modulator, the spatial light modulator (in analogy with a switchable volume grating) being configured in the form of a periodic structure of polymer grating layers arranged essentially at equal distances and intermediate spaces, filled with an active optical medium, of the polymer grating layers, wherein the surfaces bounding the periodic grating structure are provided with electrodes with which the refractive index of the active optical medium can be influenced by an electric field, wherein the electrodes have a pixelated arrangement in a regular pattern and can be driven independently of each other with an electrical voltage, and wherein the orientation of the polymer grating layers, the layer thickness and the grating period are configured in such a way that they do not correspond to the Bragg condition for the light from the at least one light source, so that for light from the at least one light source incident on the spatial light modulator the light fraction deviated owing to Bragg diffraction is less by a predeterminable value than the undeviated transmitted light fraction and the fractions of the deviated and undeviated transmitted light respectively remain essentially unchanged when the drive voltage changes.
"The angle of incidence of the light from the light source, with respect to the surface of the periodic grating structure, is in this case selected in such a way that it does not correspond to the Bragg angle of the periodic grating structure, so that the light from the at least one light source passes almost fully undeviated through the spatial light modulator, in order to influence the light in terms of its phase as a function of the respectively driven pixels.
"By virtue of the regularly arranged polymer grating layers of the periodic grating structure, a layer structure of the spatial light modulator is produced which has a shorter switching time in comparison with light modulators having a single active layer. This is due to the fact that the switching time of an LC based light modulator increases with the square of the thickness of the active LC layer. This suggests that the active layer should be subdivided into a plurality of sublayers. However, subdivision by glass substrates as separating layers leads to undesired diffraction effects at the separating layers, which cannot be tolerated for example in the case of a phase modulator for a holographic display.
"In comparison with light modulators having a plurality of active layers, which are separated by glass substrates, by virtue of the present invention undesired diffraction effects between the individual layers, and therefore crosstalk between neighboring pixels, are avoided.
"Advantageously, for the production of such a periodic grating structure, it is possible to use known methods for the production of switchable volume gratings, such as described in the aforementioned publications by
"The layer thickness of the grating structure can likewise be adapted to the requirements of a light modulator for phase or amplitude, for example by using spacers of suitable size.
"The grating planes of the periodic grating structure may in this case selectively be arranged perpendicularly or parallel (or in the general case even inclined) to the surface of the recording medium by suitable orientation of the recording medium and the lasers.
"In conjunction with at least one polarizer arranged before and/or after the modulator layer, depending on the intended use, amplitude modulation or a phase modulation of the incident light can then be produced.
"The light modulator may also be used to modulate light from a plurality of light sources of different wavelengths, for example at least one red, one green and one blue light source. In this case, the period and inclination angle of the periodic grating structure are selected in such a way that they do not correspond to the Bragg condition for the angle of incidence of any of the three light sources, so that the light from the at least three light sources passes almost fully undeviated through the light modulator, in order to influence the light in terms of its phase as a function of the respectively driven pixels. In particular, this can be achieved well when using narrowband LED or laser light sources, such is the case for example for a holographic display.
"Advantageously, the grating planes of the periodic grating structure are arranged perpendicularly to the surface of the light modulator, and the grating period is selected to be less than the wavelengths of the light sources. The walls and intermediate spaces of the polymer grating layers of the periodic grating structure may in this case have different widths.
"In the light modulator according to the invention, instead of the conventional ITO based electrodes (ITO: Indium Tin Oxide), it is also possible to use WGP based electrodes (WGP: Wire Grid Polarizer), which, besides the function as an electrode, also act as a polarizer, or as an analyzer for polarized light. This has the advantage that separate polarizers are not necessary when using the light modulator according to the invention as an amplitude modulator. Further details of this are given in the figure description of FIG. 7. In this regard, not only can the light modulator according to the invention be equipped with WGP based electrodes, but in principle any type of light modulator can be equipped with WGP based electrodes.
"Very generally, WGP electrodes may also be used as electrodes in light modulators which are not formed according to the light modulator according to the invention.
"Such displays having image diagonals of more than 8 inches have electrode structures with structure widths .gtoreq.1 .mu.m. These structure widths can still be produced by contact copy. In this case, so far as is known, amplitude gratings are exclusively used. With a currently used UV exposure wavelength of, for example, .lamda..sub.exp.=365 nm (i-line), the resolution limit is therefore reached. The minimum structure width is referred to as a CD (critical dimension). When using the light modulator according to the invention for holographic displays and synthetic, i.e. inscribed periods .LAMBDA..sub.synth..gtoreq.1 .mu.m, an electrode period of .LAMBDA..sub.E=0.5 .mu.m is required. With a mark-space ratio of TV=0.5, this corresponds to an electrode width of 0.25 .mu.m. This is significantly below the resolution limit of the contact copy method currently used by display manufacturers.
"One solution to this problem consists, for example, in producing the small electrode structures with significantly shorter light wavelengths than is currently the case. For example, light with a wavelength of 193 nm may be used, as well as an immersion liquid during the exposure of the electrode structures.
"Another possible solution consists in producing the electrode structures of the displays and of the light modulator according to the invention by means of phase shift masks and contact copy as is known for example for reduced imaging lithography systems.
"Further details of this are given in the figure description of FIGS. 11 and 12. In this regard, electrode structures not only of the light modulator according to the invention can be produced with such mask exposure, but in principle electrode structures or other structures for any type of light modulators can be produced with the aid of such mask exposure.
"There are furthermore various possibilities for advantageously configuring and refining the teaching of the present invention. In this regard, reference is to be made on the one hand to the patent claims dependent on patent claim 1 and, on the other hand, to the following description of preferred exemplary embodiments of the invention with the aid of the drawing. In connection with the explanation of the preferred exemplary embodiments of the invention with the aid of the drawing, generally preferred configurations and refinements of the teaching will also be explained.
BRIEF DESCRIPTION OF THE DRAWINGS
"In the drawing, respectively in a schematic representation:
"FIG. 1 shows an experimental structure for the recording of switchable volume gratings according to the prior art,
"FIG. 2 shows the periodic grating structure of the active layer of a first configuration of the light modulator according to the invention,
"FIGS. 3 a and b show the reorientation of the LC molecules in the intermediate spaces of the polymer grating as a function of the electric field,
"FIG. 3a showing a periodic grating structure according to the prior art as a switchable volume grating,
"FIG. 3b showing a periodic grating structure according to the invention with adapted parameter for layer thickness, refractive index modulation and period of the walls of the polymer grating,
"FIG. 4a shows, in a diagrammatic representation, an example of the dependency of the intensity of the light fraction deviated and not deviated by the switchable volume grating as a function of the applied voltage in the case of a switchable volume grating according to the prior art,
"FIG. 4b shows in comparison therewith, in a diagrammatic representation, an example of the transmitted undiffracted and diffracted light intensities as a function of the applied voltage for a light modulator according to the invention,
"FIG. 5 shows, in a diagrammatic representation, the reaction profile as a function of time of a switchable volume grating according to the prior art for a change in the electric field,
"FIG. 6 shows the structure of the active layer of a second configuration of the light modulator according to the invention,
"FIG. 7 shows the use of WGPs as electrodes in a light modulator according to the invention,
"FIG. 8 shows the use of WGPs with structured in-plane electrodes E11-12, E21-22 and E31-32 and in-plane back electrode E01-02 likewise configured in the form of a comb,
"FIG. 9 shows a slightly tilted back electrode in comb form, in order to produce a rapid switch-off time toff for modulators having in-plane LCs,
"FIG. 10 shows the use of WGP segments over two primary in-plane electrodes in the region of a pixel,
"FIG. 11 shows the intensity profile I(x,z) for the contact copy of a grating structure behind a pure amplitude mask AM and behind a phase shift mask PSM for comparison,
"FIG. 12 shows the intensity profile of the exposure light for a grating structure behind a phase shift mask for the exposure wavelength 365 nm,
"FIG. 13 shows a Barker code of length 11 (first from the top) and codes generated therefrom by inversion and reflection,
"FIG. 14 shows a Barker code of length 11 (top left, counting from the inside outward) and codes generated therefrom by inversion and reflection as an axisymmetric 2D distribution. The left-hand distributions are inverted distributions with respect to the right-hand distributions, and form a pairing therewith during alignment,
"FIG. 15 shows a Barker code of length 11 (top left, counting counterclockwise starting at 0.degree.) and codes generated therefrom by inversion and reflection as a radially symmetrical 2D distribution. The left-hand distributions are inverted distributions with respect to the right-hand distributions, and form a pairing therewith during alignment and
"FIG. 16 shows the combination of an eleven-digit Barker code with two four-digit Barker codes.
"In the figures, components which are the same or similar are denoted by the same references."
For additional information on this patent application, see: LEISTER,
Keywords for this news article include: Patents.
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