WO2019238881A1

WO2019238881A1 - Device for producing a hologram

Info

Publication number: WO2019238881A1
Application number: PCT/EP2019/065597
Authority: WO
Inventors: Thorsten Alexander Kern
Original assignee: Continental Automotive Gmbh
Priority date: 2018-06-15
Filing date: 2019-06-13
Publication date: 2019-12-19

Abstract

The invention relates to a device for producing a hologram (52) and to a method for producing a hologram of this type. The device for producing the hologram comprises an optical writing unit (42), which produces a lattice structure in a holographic material (56) by illuminating a master hologram (M), and an electrical writing unit (34, 35, SP), which produces a spatially modulated electric field in the holographic material. A basic structure of the lattice can be defined by the master hologram (M), and said basic structure can be distorted by the electric field produced by the electrical writing unit (34, 35, SP).

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.

Masters are required to produce holograms, that is to say gratings or holograms, of which a large number of gratings or holograms are produced by a type of impression. In general, the impression is taken by optical exposure without direct physical contact of the master with the grid or hologram which it has molded. However, an impression due to physical contact or another type of impression can also be used. The masters themselves are manufactured using complex and expensive systems under controlled conditions.

The demand for grids or holograms which contain a specifically adapted correction of the final grating or hologram, for example adapted to differently shaped windshields, requires a large number of masters with correspondingly high costs and long times for the provision. A discount and acceleration is desirable in such cases. It is therefore an object of the present invention to provide an improved device and an improved method for the production of individually adapted holograms.

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, which generates a lattice structure by illuminating a master hologram in a holographic material, and an electrical writing unit, which generates a spatially modulated electric field in the holographic material.

With a single master hologram, also known as an optical master, this enables an adaptation and / or adjustment in a simple and inexpensive manner by varying the electrical field generated by the electrical writing unit, that is to say by additionally using an “electrical master”.

Vary the hologram generated. In particular, the number of different master holograms required can be minimized.

In this case, the master hologram preferably specifies a basic structure of the grating and the electrical field generated by the electrical writing unit distorts the basic structure.

Advantageously, the electrical writing unit for applying the spatially modulated electrical field has at least two electrodes arranged on opposite sides of the holographic material. In this case, a transparent electrode can be arranged in particular between the optical writing unit and the holographic material. In this way it is possible to pass the light of the optical writing unit for the illumination of the holographic material through the electrode made of transparent material.

According to an advantageous embodiment, the electrical writing unit has at least one structured electrode. Such an electrode with a structured height profile of the surface enables a spatially modulated electric field to be generated in the holographic material.

In this case, the structured electrode and the transparent electrode are preferably arranged on opposite sides of the ho lographic material.

According to a further advantageous embodiment, the electrodes arranged on opposite sides of the holographic material are arranged in a wedge shape on one another. In this way, for example, an electric field with a linear gradient can be generated in order to increase or decrease the diffraction efficiency of the generated hologram in the direction of this gradient with respect to a basic grating.

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

In the method according to the invention, different holograms with different diffraction properties can advantageously be produced with a master hologram, depending on the electrical field generated by the electrical writing unit in the holographic material. 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.

LIST OF FIGURES

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;

Fig. 5 shows schematically the alignment of a liquid

crystal droplets without (a) and with (b) electric field; FIG. 1 shows schematically a first embodiment of a device according to the invention with a structured electrode for generating a spatially modulated electric field in the holographic material;

Fig. 7 shows schematically a modification of the first imple mentation form;

Fig. shows schematically an optical fiber with

circularly distorted lattice structure of the Auskop pel hologram;

9 schematically shows a second embodiment with electrodes arranged in a wedge shape relative to one another;

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

11 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. Diffuse during the recording process Then, monomers from the dark areas do not yet polymerize 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 of 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 transmitted to the surroundings by total reflection at the boundary layers passed through the optical fiber. 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 grating 51 and a coupling 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 is likewise similar to many partially transparent ones arranged one behind the other Mirror acts 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.

In addition to this widening or enlarging effect, in the case of a head-up display based on optical fibers, an image distortion caused by the normally curved windshield is also compensated for by means of the decoupling hologram.

However, since the shape of the windshield is not standardized, there are a large number of differently curved windshields depending on the vehicle manufacturer, vehicle type and vehicle variant, which therefore require individually adapted compensation of the image distortion. Even if the holograms required for this do not differ significantly, this would require an individual master hologram for each of the differently curved windshields because the master holograms cannot be changed.

This is where the invention comes in, by using the holographic layer additionally with a replication process for producing the replicated holograms

electric field. Depending on the electric field, holograms with slightly different lattice structures can then be generated using a single master hologram, which are adapted to differently curved windshields. Based on the diffraction of the light on the grating structure recorded in each case, an additional image with properties similar to the use of corresponding conventional optical elements such as lenses or mirrors can be achieved. On the one hand, there is a diffraction corresponding to a classic optical grating. As already mentioned above, this results from the arrangement of the LC droplets in the holographic layer or the phase separation between regions with a high concentration of directed LC droplets from regions with a low concentration of directed LC droplets. The distance between these areas then determines the grating constant and thus the diffraction angle of the grating.

In the LC droplets, at least some of the individual rod-shaped liquid crystal molecules are oriented in a preferred orientation, preferably parallel to their longitudinal axis. This parallel orientation of the molecular axes leads macroscopically on the one hand to a direction-dependent refractive index and on the other hand that the orientation of the LC droplets can be changed during the writing process by applying an electrical field.

This is shown as an example in FIG. 5 for a single LC droplet. There is no electric field in FIG. 5 a). In this case, the LC droplet is arranged at an angle. If, on the other hand, a voltage is applied by a voltage source SP, the LC droplets are aligned parallel to the electrical field if the electrical field is sufficiently strong, as shown in FIG. 5 b) again as an example for a single LC droplet. If incident light is transmitted through the LC droplets in the holographic layer, an additional deflection of the light can take place depending on the orientation of the LC droplet.

In that an electrical field is now provided in the holographic layer during writing, which locally has different field strengths or directions of the electrical field, a different orientation of the LC droplets can also be achieved locally, which can lead to a local adaptation of the imaging properties , This is made possible by providing an electrical writing unit in addition to an optical writing unit, which generates a spatially modulated electric field in the holographic material. Although it is then still necessary to generate the basic grating by an optical master, it is possible according to the invention to influence the alignment of the LC droplets by using the spatially modulated electric field and thus to carry out the distortion of the straight optical grating electrically.

FIGS. 6 and 7 show first embodiments for a device according to the invention for producing a hologram with such a combination of an optical writing unit with an electrical writing unit.

The optical writing unit has a light source 42, which is indicated schematically here by means of a plurality of lamps. A laser is typically used as the light source, which emits visible or ultraviolet light, for example.

After diffraction at an optical master in the form of a master hologram M, the light from the light source 42 falls on the hologram layer 56 of the optical waveguide 5. In this case, the laser light can also strike the hologram layer 56 at an angle. The hologram can preferably be an outcoupling hologram 52 of an optical waveguide shown schematically in FIGS. 3 and 4, but also, for example, an incoupling hologram 53 or a folding hologram 51.

The hologram in hologram layer 56 is then written line by line. 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 fiber shifted one line relative to each other before the next line is exposed.

During the optical writing process, an electrical field is generated in the region of the hologram layer 56 by means of a voltage source SP and electrodes 34, 35 arranged on both sides of the optical waveguide 5. The voltage source SP can in particular be a power supply unit integrated in the device for producing the hologram, but it can also be a battery or an accumulator. Analogous to the designation of the master hologram as an optical master, the electrodes 34, 35 can also be understood as an "electrical master".

In order to enable the optical writing unit to illuminate the hologram layer 56, there is between the

Optical waveguide 5 and the light source 42 arranged electrode 34 made of transparent material such as glass and is provided with an indium tin oxide (ITO) layer, for example. This electrode 34 is preferably unstructured in order to avoid further diffraction effects.

By contrast, the electrode 35 arranged on the opposite side of the optical waveguide 5 has a structured surface in order to generate a locally modulated electric field. Thus, by changing the distance relative to an unstructured electrode 34, the electric field can be modulated in intensity and, if necessary, also in the direction. The structured electrode 35 preferably has a highly absorbent black and, if necessary, surface-rough conductive material to further increase the absorption, for example a painted steel. However, it can also be provided that the structured electrode 35 is also made of glass, in particular if particularly high accuracy is desired.

Such a structured electrode can be produced, for example, by subtractive methods such as machining with a Micro milling machine or laser ablation process or also be produced by suitable additive processes.

As shown in FIG. 6, the transparent electrode 34 can be arranged behind the master hologram M from the direction of the light source 42. However, it is also possible, as shown in FIG. 7, to arrange the master hologram between the electrode 34 and the optical waveguide, even if this may require a higher voltage Uo to generate the electric field.

Various combinations of structured and unstructured electrodes 34, 35 can be used to spatially influence the electrical field and to bias it in the active plane of the liquid crystal material. On the one hand, large-area electrodes are possible that partially or completely cover the holographic layer in the area of the hologram to be produced. Where appropriate, the electrodes in the optical waveguide from FIGS. 3 and 4 in the area of the coupling-out hologram 52 can have a structured surface or a structure which in some other way causes a modulated electric field. This can then be used to generate the circularly distorted lattice structure shown in FIG. 8 in the area of the coupling-out hologram 52, with the aid of which optical distortion through a windshield with a known curvature can be compensated. The schematically illustrated circular distortion is to be understood purely as an example; depending on the electrical fields present, various other complex patterns can also be generated in the coupling hologram 52 of the optical waveguide 5.

An adaptation to different curved windshields can then take place, for example, by exchanging the structured electrode 35. However, it is also conceivable to use a large number of individual electrodes which, if appropriate, are isolated from one another at different electrical potentials and can be controlled separately. In this case, the hologram generated can be adapted to different ones Disc curvatures are caused by changes in the electrical potentials of the individual electrodes.

Likewise, according to an embodiment shown in FIG. 9, the electrodes 34, 35 arranged on opposite sides of the optical waveguide 5 can be arranged in a wedge shape with respect to one another. In this way, for example, a static, flat electric field with a linear gradient can be generated in the holographic material 56. This can be used, for example, to spatially vary the diffraction efficiency and thus the degree of coupling out in the direction of this gradient, in particular to increase or decrease it continuously, in the exposure for reproducing a coupling out hologram.

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 10 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 optics unit 2 has a mirror 20, by means of which the light generated by the image generator 1 and shaped by the optics 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 is one in Compared to their light-emitting surface has a low overall height.

11 shows a head-up display in a motor vehicle in a spatial representation and with an optical waveguide 5. The schematically indicated image generator 1 can be seen, 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

34, 35 electrodes

42 Laser beam from the optical 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 Liquid crystal droplets

El, E2 electrodes

I bright areas of interference

L, L ', L "laser beams

LI - L4 light beam

LC liquid crystal

M master hologram

SB1 beam

SP voltage source

VB virtual image

Claims

claims

1) Device for producing a hologram (52), comprising an optical writing unit (42) which generates a lattice structure by illuminating a master hologram (M) in a holographic material (56) and an electrical writing unit (34, 35, SP), which generates a spatially modulated electric field in the holographic material.

2) Device according to claim 1, wherein a basic structure of the grid is predetermined by the master hologram (M) and a distortion of the basic structure is made by the electrical field generated by the electrical writing unit (34, 35, SP).

3) Device according to claim 1 or 2, wherein the electrical

Writing unit for applying the spatially modulated electric field has at least two electrodes (34, 35) arranged on opposite sides of the holographic material (56).

4) Device according to claim 3, wherein between the optical

Writing unit (42) and the holographic material (56) a transparent electrode (34) is arranged.

5) Device according to claim 3 or 4, wherein the electrical

Writing unit has at least one structured electrode (35).

6) Device according to claim 5, wherein the structured electrode (35) and the transparent electrode (34) are arranged on opposite sides of the holographic material (56).

7) Device according to one of claims 3 to 6, wherein the electrodes (34, 35) arranged on opposite sides of the holographic material (56) are arranged in a wedge shape with respect to one another.

8) Method for producing a hologram using a device according to one of the preceding claims.

9) Method according to claim 8, wherein with a master hologram (M) depending on the by the electrical writing unit (34, 35,

SP) different holograms with different diffraction properties are produced in the electrical field generated in the holographic material. 10) Hologram produced using a device according to one of the

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.