WO2019238876A1 - Device for producing a hologram - Google Patents

Abstract

The invention relates to a device for producing a hologram (52) and to a method for producing such a hologram. The device for producing the hologram comprises an optical write unit (42) and an electrical write unit (EL1-EL4). The optical write unit (42) and the electrical write unit (EL1-EL4) are arranged with respect to one another such that, during the write action, an electrical field is present adjacent to a region illuminated by the optical write unit (42) and/or in a region illuminated by the optical write unit (42). The electrical write unit (EL1-EL4) can comprise at least two electrodes which, during the write action of the hologram, are arranged on opposite sides of the holographic material and to which electrodes different electrical potentials are applied.

Description

 description

Device for making a hologram

The present invention relates to an apparatus and a method for producing a hologram.

The holography Waveguide Technology, ie the use of optical fibers that have holograms, is based on holograms, which are embedded in a birefringent liquid crystal layer, hereinafter also referred to as LC layer. This LC layer can be switched electrically. A typical application for this is the extensive switching on and off of the LC layer in order to synchronize the coupling in color when using several optical fibers arranged one above the other in accordance with a sequentially operating light source. Head-up displays with holography waveguide technology are explained in more detail at the end of the description below.

For such optical waveguides having a hologram, spatially different values for the diffraction efficiency or diffraction quality of the hologram may be desired. During the manufacture of such optical fibers, it is therefore necessary to actively influence the diffraction efficiency of the hologram locally.

It is an object of the present invention to provide an improved device and an improved method for producing such an optical waveguide.

This object is achieved by a device having the features of claim 1 and by a method having the features of claim 8. Preferred embodiments of the invention are the subject of the dependent claims. An inventive device for producing a hologram has an optical writing unit and an electrical one

Writing unit, wherein the optical writing unit and the electrical writing unit are arranged in relation to one another such that an electrical field is present during the writing process adjacent to an area illuminated by the optical writing unit and / or in an area illuminated by the optical writing unit.

The combination of an optical writing unit with an electrical writing unit according to the invention enables, in particular, a locally limited, high-resolution influencing of the thermal properties or also the basic orientation of the liquid crystals during the writing process. In this way, particularly good control over the diffraction efficiency of the holograms generated is achieved.

The electrical writing unit preferably has at least two electrodes which are arranged on opposite sides of the holographic material during the writing process of the hologram and which are subjected to different electrical potentials.

According to an advantageous embodiment, the optical writing unit comprises a laser, which is directed line by line onto a writing area of the holographic material during the writing process of the hologram. The electrical screaming unit has two electrodes arranged in pairs on opposite sides of the holographic material in front of and behind this writing area.

Here, an alternating voltage can be present between electrodes arranged on opposite sides of the holographic material. A direct voltage can also be present between electrodes arranged on opposite sides of the holographic material.

Furthermore, the electrical writing unit

Advantageously have a structured electrode. This has, for example, a grid of points.

The electrical writing unit preferably has a transparent electrode.

A method according to the invention for producing a hologram uses a device according to the invention.

Preferably, in the method according to the invention, one is generated by an electrical writing unit

Electrical alternating field, the holographic material is thermally activated, and / or by means of a static electrical field generated by the electrical writing unit in the holographic material, an electrical pre-orientation takes place.

A hologram according to the invention is produced by means of a device according to the invention or a method according to the invention.

A head-up display according to the invention has at least one optical waveguide with a hologram according to the invention.

Further features of the present invention will become apparent from the following description and the appended claims in conjunction with the figures. Figure overview:

Fig. 1 shows schematically the replication of a master ter hologram using a Kontaktpro process;

Fig. 2 shows an enlarged section of the replicated

 Hologram with a lattice structure formed from liquid crystal droplets;

Fig. 3 shows schematically an optical waveguide, the three

 Areas with holographic lattice structures;

Fig. 4 shows schematically the deflection and enlargement of

 Light through the optical fiber of Fig. 3;

5 shows schematically for the optical waveguide from FIG.

 3 shows the spatial course of the initial intensity with constant diffraction efficiency;

FIG. 6 schematically shows an optical waveguide with heating elements arranged thereon for generating an increasing temperature profile and a resulting course of temperature, diffraction efficiency and output intensity;

Fig. 7 shows schematically a first arrangement for applying an electric field during the generation of the holographic lattice structure in the optical waveguide; Fig. 8 schematically shows a second arrangement for applying an electric field during the generation of the holographic lattice structure in the optical waveguide;

9 schematically shows a head-up display with several of the optical fibers described;

10 shows a head-up display with optical fibers in a motor vehicle.

figure description

For a better understanding of the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. The same reference characters are used in the figures for the same or equivalent elements and are not necessarily described again for each figure. It is understood that the invention is not limited to the illustrated embodiments and that the described features can also be combined or modified without departing from the scope of the invention as defined in the appended claims.

The inventive production or writing of a liquid crystal-based hologram is based on already known production processes, which are therefore briefly explained below.

If a larger amount of copies is required from a hologram, a master hologram is first produced, on the basis of which the required number of copies is then generated by a replication process. The master hologram is typically manufactured using a classic two-beam holographic recording system that includes an object beam and a reference beam. The replication can then take place by copying the master hologram into another holographic recording material using a contact process.

This is shown schematically in FIG. 1 for the volume holograms considered here, which are present in a thin hologram layer 56. The recording is carried out by the photopolymerization of a mixture of suitable monomers with a liquid crystal material. For this purpose, the mixture is introduced into a cell, which is formed, for example, by parallel glass plates or plastic substrates 54, 55. An interference pattern is then generated in the cell by superimposing two laser beams L, L '. In this case, in the case of a master hologram M designed as a transmission hologram, this master hologram can be illuminated by a single laser beam and the copying process can then be based on an interference of the first-order beams generated by a lattice structure of the master hologram M and the undeflected zeroth-order beams ,

In the light areas I of the interference pattern, the monomers polymerize faster than in the dark areas of the interference pattern. During the recording process, monomers which have not yet polymerized then diffuse from the dark areas into the light areas and form further polymers there and at the same time liquid crystals LC diffuse into the dark areas and form microdroplets D, so-called

LC droplets, as shown schematically in Figure 2. This results in phase separation in the form of regions which have a large number of such LC droplets D and are separated from regions in which there are essentially polymers. These alternating regions then form a lattice structure in the form of a modulated refractive index profile in the hologram layer 56, which is based on the different refractive indices the polymers on the one hand and the liquid crystals on the other. Incident light L ″ can then be diffracted at the grating structure.

Transparent electrodes E1 and E2, for example made of indium tin oxide, can be provided on the glass plates or substrates of the cell in order to be able to apply an electrical field above the hologram layer 56 during operation of the hologram and thus to influence the diffraction behavior. Here, the orientation of the liquid crystals LC of the LC droplets D changes due to the electric field, as a result of which the refractive index modulation of the lattice structure is changed. The refractive index modulation can be reduced in this way and, with a suitable choice of materials and the electric field, can even disappear completely by an index adaptation between liquid crystals and polymers, and then have the consequence that the incident light is no longer deflected. By switching the electric field on and off, the deflection by the lattice structure can also be modulated, with short switching times being achieved.

If, as provided here, the hologram layer 56 is embedded in an optical waveguide, the coupling in or out of light can also be controlled by means of the electrical field. In the case of the optical waveguide, the incident light is guided through the optical waveguide by total reflection at the boundary layers to the surroundings. The light can then be decoupled by the switchable grating of the hologram layer diffracting the light in such a way that the light strikes the boundary layer at an angle that is smaller than the critical angle, so that total reflection no longer takes place.

In such an optical waveguide, several such holograms can also be provided. This is the case, for example, for fiber optic cables that are installed in head-up displays. Such an optical fiber is exemplified in Figure 3 Darge. Here, the optical waveguide 5 has a coupling hologram or coupling grating 53, a folding hologram or

Grid 51 and a coupling-out hologram or grating 52, which are arranged in different areas of the hologram layer 56.

In this way, deflection and expansion of the incident light can be achieved in a compact manner, as shown in FIG. For reasons of better clarity, the representation of the optical waveguide 5 does not show the separate substrates with the hologram layer in between. In the lower left area of the

Optical waveguide 5 is a coupling hologram 53, by means of which light LI coming from an imaging unit (not shown) enters the optical waveguide 5

is coupled. In it, it spreads to the top right in the drawing, according to arrow L2. In this area of the optical waveguide 5 there is a folding hologram 51, which acts similarly to many partially transparent mirrors arranged one behind the other, and generates a light beam that is widened in the Y direction and propagates in the X direction. This is indicated by three arrows L3. In the part of the optical waveguide 5 which extends to the right in the illustration, there is a large-area coupling-out hologram 52, which likewise acts like many partially transparent mirrors arranged one behind the other and, indicated by arrows L4, light in

Coupling the Z direction upwards out of the optical waveguide 5. A broadening takes place in the X direction, so that the original incident light bundle LI leaves the optical waveguide 5 as a light bundle L4 enlarged in two dimensions.

Do the folding hologram 51 or the coupling-out hologram 52 have a spatially constant diffraction efficiency which, according to the above image, corresponds to a plurality of partially transparent mirrors arranged one behind the other, each with the same reflection or If the degree of transmittance corresponded, the generated light beams would have spatially different intensities. This is indicated schematically in FIG. 5. The reason for this is that after the first partial reflection of the light that originally entered the respective hologram with maximum intensity, only the non-reflected portion of the light with a lower intensity is still present. The same percentage is then deflected by the latter in the second partial reflection, so that the light advancing in the light guide again has a lower intensity. In this way, the intensity of the light in the light guide continues to decrease with increasing diffraction efficiency with increasing spread in the optical waveguide and thus also leads to a corresponding decrease in the intensity of the outcoupled light.

For example, for a fixed Y value yi with diffraction efficiency remaining constant for this Y value in the X direction, the intensity IL3 of the light propagating in the X-direction in the optical waveguide and thus also the intensity IL4 of the light coupled out of the optical waveguide in X- Decrease direction exponentially.

In the case of applications of the optical fibers in head-up displays, however, a uniform illumination of the so-called eyebox is desirable. This can be done by spatially adapting the diffraction efficiency of the hologram used in each case during the duplication process for producing the replicated holograms.

The quality of the generated lattice structure and thus the diffraction efficiency of the hologram depends on various parameters during the exposure phase. Making the lattice structure as precise as possible requires that the rates for photopolymerization, the diffusion processes and the nucleation of the LC droplets are in the correct relationship to one another. The formation of an error-free lattice can be ensured here if the diffusion time for liquid crystals between adjacent strips of the lattice structure is significantly less than the time for the nucleation of the LC droplets and this in turn occurs significantly faster than the polymerization. The polymerization rate can be influenced in particular by the intensity of the laser beam used for exposure and increases with increasing intensity.

Furthermore, the diffraction efficiency of the hologram is dependent on the diffusion time, which in turn is influenced by the viscosity, which indicates the viscosity of the liquid mixture. If suitable temperature values are selected, then a viscosity with a diffusion time or diffusion rate of the liquid crystals and monomers required for a perfect lattice structure can be present. If, on the other hand, temperatures that are too low are selected, for example, the monomer / liquid crystal mixture has a high viscosity, which results in a long diffusion time or low diffusion rates of the liquid crystals and monomers. This can lead to the fact that a part of the liquid crystal molecules cannot diffuse into the dark areas of the interference pattern during the exposure, since the solidification of the mixture by the polymerization already sets in beforehand. The same applies to the diffusion of the monomers. As a result, only an incomplete phase separation is achieved, which can result in an inadequate grating structure and thus a lower diffraction efficiency.

As shown schematically in FIG. 6, this temperature dependency can be used, for example, to reduce the degree of coupling out or to vary it spatially in the exposure for reproducing a coupling out hologram, by influencing or changing the temperature of the monomer / liquid crystal mixture during the exposure.

For rough temperature profiles you can

Several resistance or ceramic heating plates HP can be provided during the writing process, which are in contact with the optical waveguide and can be controlled separately. In this way, a thermal profile can be applied to the optical waveguide during the manufacturing process, for example in order to generate increasing temperature values in the X and possibly also in the Y direction. This is sufficient to spatially influence the phase separation and to create a gradient for the diffraction efficiency. With a suitable choice of the temperature values, an increase in the diffraction efficiency BE in the X direction can then be achieved in the example shown, which enables a constant intensity IL4 of the light coupled out of the optical waveguide. However, the use of resistance or ceramic heating plates HP only allows rough temperature profiles and no exact spatial control.

On the other hand, this is made possible by providing an electrical writing unit in addition to an optical writing unit, in which electrodes are provided above and below the optical waveguide, by means of which electrical fields can be generated in the region of the hologram layer. If an AC voltage is applied between the electrodes, an electrical current flows in an alternating direction, similar to a capacitor. This leads to an electrical power loss and thus local heating of the optical waveguide.

FIGS. 7 and 8 show a device according to the invention with such an electrical writing unit in different operating states. In this case, two half-bar-shaped electrodes ELI and EL3 are arranged above the optical waveguide and two half-bar-shaped electrodes EL2 and EL4 below the optical waveguide. The electrodes ELI to EL4 are at electrical potentials U1 to U4, so that there is a voltage between them at different values of the electrical potentials.

The hologram is written with a laser 42 of an optical writing unit, which is, for example, visible or emits ultraviolet light and falls on the hologram layer at an angle. Here, the hologram is written line by line by moving the laser beam 42 for exposure of a line through a suitable deflection device in the Y direction between the electrodes. Depending on the illuminance required to photopolimerize the holographic material in the hologram layer 56 and the power of the laser, this can take place in a fraction of a second. After a line has been written, the arrangement and the optical waveguide are then shifted relative to one another by one line in the X direction before the next line is then exposed.

During the optical writing process, a voltage, in particular an alternating voltage, is applied across the electrodes in front of and behind the writing area, so that the liquid crystal material in the optical waveguide 5 is thermally activated in front of and behind the writing area.

Various temperature profiles can be generated with the device according to the invention, including the thermal profile shown in FIG. 6. However, it is particularly advantageous that very local heating can take place in this way. This enables sharper edges of the generated lattice structure and cross-talk effects to be reduced. Furthermore, the diffraction efficiency can be varied locally, for example by providing two directly adjacent grating lines with a high diffraction efficiency and the adjacent one with a low diffraction efficiency.

As shown in FIG. 8, oblique fields are also possible by applying the electrodes accordingly. These also enable localized, short thermal heating via an alternating field. Furthermore, a static potential can directly influence the writing area in the form of an electrical pre-orientation of the

LC droplets take place, which then in combination with the

Writing laser 42 enables alignment of the liquid Burning in crystal droplets that would otherwise only be possible by modulating the laser light.

The shape, spacing and number of electrodes are not limited to the embodiments shown in FIGS. 7 and 8. For example, only one pair of electrodes can be provided in front of the writing area. Provision can also be made to provide only one instead of two electrodes on the side opposite the writing laser 42.

Furthermore, instead of the non-transparent, in particular metallic electrodes used in FIGS. 7 and 8, transparent electrodes can also be provided, which then, if necessary, also enable optical writing by means of these electrodes. The electrodes can thus be designed as transparent glass plates with a conductive coating, for example made of indium tin oxide. If these electrodes are also structured, local pixel-like influencing of the liquid crystal material during writing is also possible, which is only limited by the diffractive properties of the structuring of the transparent electrodes and any active components, such as transistors, located on the glass.

The optical waveguides produced according to the invention can be particularly advantageous for a head-up display in one

Motor vehicle are used. Here, three light waveguides can be arranged one above the other for each elementary color red, green and blue.

Figure 9 shows such an arrangement in a spatial representation. The holograms 51, 52, 53 present in the optical fibers 5 are wavelength-dependent, so that one optical fiber 5R, 5G, 5B is used for each of the elementary colors. An image generator 1 and an optical unit 2 are shown above the optical waveguide 5. Both together are often referred to as an imaging unit or PGU. The optical unit 2 has a mirror 20, by means of which the light generated by the image generator 1 and shaped by the optical unit 2 is deflected in the direction of the respective coupling hologram 53. The light generated by the imaging unit is coupled into the optical waveguide 5 in the coupling region 53, in which the respective coupling holograms 53 are located. The image generator 1 has three light sources 14R, 14G, 14B for the three elementary colors. It can be seen that the entire unit shown has a low overall height compared to its light-emitting surface.

10 shows a head-up display in a motor vehicle in a spatial representation and with an optical waveguide 5. One recognizes the schematically indicated image generator 1, which generates a parallel beam SB1, which is coupled into the optical waveguide 5 by means of the mirror plane 523. The optical waveguide 5 is shown here in a greatly simplified manner for the sake of clarity, and the optical unit is also not shown for the sake of simplicity. A plurality of mirror planes 522 each reflect part of the light impinging on them in the direction of the windshield 31. The light is reflected by the latter in the direction of the eye 61. The viewer sees a virtual image VB above the bonnet or even further away from the motor vehicle.

LIST OF REFERENCE NUMBERS

1 image generator

 14, 14R, 14G 14B light source

 2 optics unit

 20 mirrors

 31 windshield

 42 Laser beam from the optical writing unit

ELI - EL4 electrode of the electrical writing unit

5, 5R, 5G, 5B optical fiber

 51 folding hologram

 52 decoupling hologram

 522 mirror plane

 523 mirror plane

 53 coupling hologram

54, 55 substrates

56 hologram layer

61 eye

D Drops of liquid crystal

 El, E2 electrodes

 HP heating plate

 I bright areas of interference

 L, L ', L "laser beams

 LI - L4 light beam

 LC liquid crystal

 M master hologram

 SB1 beam

 Ul - U4 electrical potential

 VB virtual image

Claims

claims 1) Device for producing a hologram (52), comprising an optical writing unit (42) and an electrical one  Writing unit (ELI - EL4), the optical writing unit (42) and the electrical writing unit (ELI - EL4) being arranged with respect to one another such that during the  Writing process adjacent to an area illuminated by the optical writing unit (42) and / or in an area illuminated by the optical writing unit (42) there is an electric field generated by the electric writing unit (ELI-EL4). 2) Apparatus according to claim 1, wherein the electrical writing unit (ELI - EL4) has at least two electrodes (ELI - EL4) which are arranged during the writing process of the hologram (52) on opposite sides of the holographic material (56) and with different electrical Potentials (Ul - U4) are applied. 3) Apparatus according to claim 2, wherein the optical writing unit comprises a laser (42), which is directed line by line during the writing process of the hologram onto a writing area of the holographic material and the electrical writing unit (ELI - EL4) two pairs on opposite sides of the holographic material arranged electrodes in front of and behind this writing area. 4) Apparatus according to claim 2 or 3, wherein an AC voltage is present between electrodes arranged on opposite sides of the holographic material. 5) Apparatus according to claim 2 or 3, wherein a DC voltage is present between electrodes arranged on opposite sides of the holographic material. 6) Device according to one of the preceding claims, wherein the electrical writing unit (ELI - EL4) has at least one structured electrode. 7) Device according to one of the preceding claims, wherein the electrical writing unit (ELI - EL4) has at least one transparent electrode. 8) Method for producing a hologram using a device according to one of the preceding claims. 9) The method of claim 8, wherein  - The holographic material is thermally activated by means of an electrical alternating field generated by the electrical writing unit (ELI-EL4), and / or  - By means of a static electrical field generated by the electrical writing unit (ELI-EL4) in the ho lographic material, an electrical pre-orientation takes place. 10) hologram produced by means of a device according to one of claims 1 to 7 or a method according to claim 8 or 9th 11) head-up display with at least one optical waveguide having a hologram according to claim 10.