METHOD FOR PRODUCING A HOLOGRAPHIC OPTICAL ELEMENT, CONTROL DEVICE AND EXPOSURE DEVICE

Abstract

A method for producing a holographic optical element. The method includes a step of exposing a recording material to a phase pattern which is provided by a first modulated light beam with a first phase portion. Furthermore, the method includes a step of an additional exposure of the recording material to the phase pattern, which is provided by a second modulated light beam with a second phase portion, wherein the second phase portion has a phase offset with respect to the first phase portion in order to produce a holographic optical element.

Claims

1. A method for producing a holographic optical element, the method comprising the following steps: exposing a recording material to a phase pattern which is provided by a first modulated light beam having a first phase portion; and additionally exposing the recording material to the phase pattern which is provided by a second modulated light beam having a second phase portion, wherein the second phase portion has a phase offset with respect to the first phase portion, to produce the holographic optical element.

2. The method according to claim 1, further comprising: further exposing the recording material to the phase pattern which is provided by a third modulated light beam having a third phase portion, wherein the third phase portion has a phase offset with respect to the first phase portion and with respect to the second phase portion.

3. The method according to claim 1, wherein the steps of the method are performed sequentially one after another.

4. The method according to claim 1, wherein the steps of the method are carried out at least partially simultaneously or at the same time.

5. The method according to claim 1, wherein, in the additional exposure step, the phase offset has a magnitude of maximally 10%.

6. The method according to claim 1, wherein the phase offset is adjustable in or for the additional exposure step.

7. The method according to claim 1, wherein, in the exposure step, the first modulated light beam is provided as an object beam, wherein a reference beam is used which is provided by the same light source as the object beam.

8. The method according to claim 7, wherein, in the step of exposing, the recording material is exposed to the reference beam and to the object beam on the same or alternatively on different sides.

9. The method according to claim 1, further comprising: adjustably focusing the first and/or second light beam onto a desired focal point of the recording material.

10. A control device configured to produce a holographic optical element, the control device configured to: expose a recording material to a phase pattern which is provided by a first modulated light beam having a first phase portion; and additionally expose the recording material to the phase pattern which is provided by a second modulated light beam having a second phase portion, wherein the second phase portion has a phase offset with respect to the first phase portion, to produce the holographic optical element.

11. An exposure device, comprising: a control device configured to produce a holographic optical element, the control device configured to: expose a recording material to a phase pattern which is provided by a first modulated light beam having a first phase portion; and additionally expose the recording material to the phase pattern which is provided by a second modulated light beam having a second phase portion, wherein the second phase portion has a phase offset with respect to the first phase portion, to produce the holographic optical element.

12. A non-transitory machine-readable storage medium on which is stored a computer program for producing a holographic optical element, the computer program, when executed by a computer, causing the computer to perform the following steps: exposing a recording material to a phase pattern which is provided by a first modulated light beam having a first phase portion; and additionally exposing the recording material to the phase pattern which is provided by a second modulated light beam having a second phase portion, wherein the second phase portion has a phase offset with respect to the first phase portion, to produce the holographic optical element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a diagram of an exemplary embodiment of a relationship between diffraction efficiency, coupling strength and the Kogelnik factor.

[0025] FIG. 2 is a flow chart of a method for producing a holographic optical element according to one exemplary embodiment of the present invention.

[0026] FIG. 3 is a flow chart of a method for producing a holographic optical element according to one exemplary embodiment of the present invention.

[0027] FIG. 4 is a diagram of an exemplary embodiment of a diffraction-efficiency-independent bandwidth adaptation of a holographic optical element, according to the present invention.

[0028] FIG. 5 is a block diagram of a control device according to one exemplary embodiment of the present invention.

[0029] FIG. 6 is a block diagram of a system with an exposure device according to one exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0030] In the following description of advantageous exemplary embodiments of the present invention, the same or similar reference signs are used for the elements shown in the various figures and acting similarly, a repeated description of these elements being dispensed with.

[0031] FIG. 1 is a diagram of an exemplary embodiment of a relationship between the diffraction efficiency η and the coupling strength C.sub.c and the Kogelnik factor C.sub.d in accordance with the Kogelnik coupled-wave model. In the illustration shown here, a first modulated light beam 100 with a first phase portion, a second modulated light beam 105 with a second phase portion and a third modulated light beam 110 with a third phase portion are shown only by way of example. Only by way of example, the first light beam 100 has a coupling strength C.sub.c/Π of 0.25, the second light beam 105 has a coupling strength C.sub.c/Π of 0.50 and the third light beam 110 has a coupling strength C.sub.c/Π of 0.75. The width of the respective peak of the light beams 100, 105, 110 is considered a measure of the bandwidth of a holographic optical element (HOE). In this case, the second modulated light beam 105 has a phase offset 120 from the first modulated light beam 100, likewise as the third modulated light beam 110 in this exemplary embodiment has a further phase offset 125 with respect to the second light beam 105 and thus also with respect to the first light beam 100.

[0032] The coupling strength of an HOE is considered a measure of the achievable diffraction efficiency. The equation shown below represents the coupling strength C.sub.c in accordance with the Kolgelnik coupled-wave theory.

[00001]$C_c=\frac{\pi n_{1}d}{{\lambda }_0\cos \vartheta }$

[0033] Here the coupling strength depends on the amplitude of the refractive index modulation n.sub.1, the thickness of the holographic film d as well as the wavelength λ.sub.0 and the angle ϑ of the incident light. The bandwidth of an HOE can be illustrated by means of the Kogelnik factor C.sub.d. In this case, the deviation of the playback angle ϑ and the playback wavelength λ.sub.0 from the ideal Bragg condition of the imprinted grating structure is taken into account. The deviation can correspond to the following form.

[00002]$C_d=\left[\cos \left({\vartheta }_K-\vartheta \right)-\frac{.Math.K.Math.{\lambda }_0}{4\pi n_0}\right].Math.\frac{d.Math.K.Math.}{2\cos \vartheta }$$\mathrm{Here}$$.Math.K.Math.=\frac{2\pi }{{\lambda }_0}$

describes the imprinted grating structure, K describes the angle of the grating normal and n.sub.0 describes the mean background refraction index. The bandwidth can thus be described with the aid of the diffraction efficiency as a function of the playback angle ϑ and the playback wavelength λ.sub.0. The diffraction efficiency can correspond to the following term:

[00003]$\eta \left(,{\lambda }_0\right)={\left\{1+\left(1-\frac{C_d^2}{C_c^2}\right){\sinh }^{-2}\left[{\left(C_c^2-C_d^2\right)}^{1/2}\right]\right\}}^{-1}$

[0034] In the production of HOEs with conventional variants, there is in particular a trade-off between the coupling strength C.sub.c or the diffraction efficiency η and the bandwidth of an HOE. For conventional holography recording methods, for an increase in the bandwidth the factor C.sub.c must be increased. This inevitably also leads to an increase in the diffraction efficiency. The diffracting structure of a hologram can, corresponding to the imprinted interference pattern, be designed as a modulation of the refractive index in the holographic film. In this case, the interference pattern can be created by the interference of two coherent HOE recording waves with an intensity distribution that can correspond to the following form.

I.sub.int(r)=I.sub.1+I.sub.2+2(I.sub.1I.sub.2).sup.1/2.Math.cos((k.sub.1−k.sub.2).Math.r+δ)

[0035] The form of the interference function can be described by the ideal cosine function. I.sub.1 and I.sub.2 describe the intensities of the interfering waves with the location vector r as well as the spatial frequencies k.sub.1 and k.sub.2 of the recording wavefronts.

[0036] FIG. 2 is a flow chart of a method 200 for producing a holographic optical element according to one exemplary embodiment. The method 200 comprises a step 205 of exposing a recording material to a phase pattern which is provided by a first modulated light beam with a first phase portion. The first modulated light beam is only an example of a laser beam that was split by means of a beam splitter into a reference beam and an object beam. In this exemplary embodiment, the first modulated light beam is thus provided as an object beam on one side of the recording material, while the recording material is simultaneously only by way of example exposed to the reference beam from another side.

[0037] Furthermore, the method 200 comprises a step 210 of an additional exposure of the recording material to the phase pattern, which is provided by a second modulated light beam with a second phase portion, wherein the second phase portion has a phase offset from the first phase portion in order to produce a holographic optical element. Only by way of example, in this additional exposure step 210 the phase offset is adjustable and has a magnitude of maximally 5%. In this exemplary embodiment, steps 205, 210 of the method 200 are performed sequentially. In another exemplary embodiment, the steps can even be carried out simultaneously and the phase offset can have a variable magnitude of maximally 10%.

[0038] The method 200 described here represents a new type of recording process of a holographic optical element (HOE), which, in comparison to a conventional exposure process, comprises a plurality of independent exposure steps which occur sequentially one after another. For this purpose, for each step, an individual adaptation of the phase function of the recording wavefront is performed, which is designed so as overall to produce an HOE with a specifically influenced bandwidth without influencing the desired target function of the HOE. In conventional recording methods of holography, an increase in the bandwidth is necessarily accompanied by an increase in the diffraction efficiency. A high diffraction efficiency is, however, in particular counterproductive for applications focusing on high transparency, because the influence of interference light or interference holograms is also amplified by the increased diffraction efficiency. These include in particular applications for holograms of data eyeglasses and for head-up displays. The decisive advantage of the method presented is therefore the increase in the bandwidth of a hologram without the need to tolerate an increase in diffraction efficiency.

[0039] FIG. 3 is a flow chart of a method 200 for producing a holographic optical element according to one exemplary embodiment. The method shown here corresponds to or is similar to the method described in the preceding FIG. 2, with the difference that it has additional steps. Thus, in this exemplary embodiment, an adjustable focusing step 300 precedes the exposure step 205. In this step 300, the light beams provided in the following steps are, only by way of example, focused on a desired focal point of the recording material.

[0040] In addition, in this exemplary embodiment, step 210 of the additional exposure is followed by a further exposure step 305. The recording material is exposed to the phase pattern which is provided by a third modulated light beam with a third phase portion, wherein the third phase portion has a phase offset with respect to the first and with respect to the second phase portion.

[0041] The diffracting structure of a hologram is formed as a modulation of the refractive index in the holographic film according to the imprinted interference pattern. The interference pattern is generated by the interference of two coherent HOE recording waves with an intensity distribution that can have the following form.

I.sub.int(r)=I.sub.1+I.sub.2+2(I.sub.1I.sub.2).sup.1/2.Math.cos((k.sub.1−k.sub.2).Math.r+δ)

[0042] The form of the interference function is described by the ideal cosine function. I.sub.1 and I.sub.2 describe the intensities of the interfering waves with the location vector r as well as the spatial frequencies k.sub.1 and k.sub.2 of the recording wavefronts.

[0043] In this exemplary embodiment, the spatial frequencies k.sub.1 and k.sub.2 of the recording wavefronts are precisely set. Furthermore, the introduction of a controllable but constant additive angular contribution or phase contribution 5 is made possible independently of the recording wavefronts. Applied to the Kogelnik factor C.sub.d, this results in a change to the parameters K or |K|, which corresponds to a changed exposure angle or an additive phase contribution.

[0044] The diffraction efficiency results according to the Fourier theory of diffraction. A diffracting structure is described here by an ideal cosine function and generates a discrete diffraction direction in the case of exposure to monochromatic light. The spectral or angular bandwidth of the diffracting structure is then determined by its spatial extent. This means that the discrete diffraction direction is convoluted with a split function, for example, a cardinal sine function. By imprinting a plurality of wavefronts with a different phase offset at short intervals into the same position of the holographic film, the resulting interference grating results in an envelope consisting of the functional components of the individual contributions. For each exposure step of a hologram the angle or the magnitude of the grating vector is adapted in a targeted manner and independently of one another in order to superimpose a plurality of contributions and thus achieve the desired change in the hologram bandwidth. With regard to the bandwidth of the structure produced by the method described, the convolution of the cosine function in the reciprocal frequency domain takes place via a continuum of several cosine functions with different phase angles. The diffraction characteristic of the generated structure thus has an enlarged spectral or angular bandwidth due to the convolution across a continuum of split functions.

[0045] FIG. 4 is a diagram of an exemplary embodiment of a diffraction-efficiency-independent bandwidth adaptation of a holographic optical element. A first modulated light beam 100 with a first phase portion, a second modulated light beam 105 with a second phase portion and a third modulated light beam 110 with a third phase portion are shown in this embodiment, similar to the diagram described in the preceding FIG. 1. Only by way of example, the first light beam 100 has a coupling strength C.sub.c/Π of 1.05.Math./K/, the second light beam 105 has a coupling strength C.sub.c/Π of 1.0.Math./K/ and the third light beam 110 has a coupling strength C.sub.c/Π of 0.95.Math./K/. In this exemplary embodiment, the diffraction efficiency η is independent of the bandwidth adaptation, because the imprinted grating structure is adapted by applying a method as described in the preceding FIGS. 2 and 3. Accordingly, an adjustment is made possible by introducing an additive angle contribution or phase contribution.

[0046] FIG. 5 is a block diagram of an exemplary embodiment of a control device 500 for controlling a method for producing a holographic optical element according to a variant presented here. The control device 500 comprises an exposure device 505 for controlling an exposure of a recording material to a phase pattern which is provided by a first modulated light beam with a first phase portion. In addition, the control device 500 comprises an additional exposure unit 510 for controlling an additional exposure of the recording material to the phase pattern, which is provided by a second modulated light beam with a second phase portion, wherein the second phase portion has a phase offset with respect to the first phase portion in order to produce a holographic optical element.

[0047] FIG. 6 is a block diagram of a system 600 with an exposure device 605 according to one exemplary embodiment. In this exemplary embodiment, the system 600 comprises a light source 610, which only by way of example is formed to provide a laser beam 615. By means of a beam splitter 620, the laser beam 615 can be split into an object beam 625 and a reference beam 630 in order to expose a recording material 635 from opposite sides, for example. In this case, the object beam 625 can be modulated by an exposure device 605 to form a first light beam 100, wherein the exposure device 640 in this exemplary embodiment comprises a control device 500 which corresponds to or is similar to the control device described in the preceding FIG. 5. In this exemplary embodiment, the exposure device 605 can be used for controlling wavefronts or for precisely setting the spatial frequencies of the recording wavefronts. In addition, it is possible to introduce a controllable but constant additive angular contribution or phase contribution independently of the recording wavefronts. A method for producing a holographic optical element can therefore be realized as described in the preceding FIGS. 2 and 3 by means of the exposure device, which can also be referred to as a holographic wavefront exposure device or spatial light modulator (SLM). In other words, the wavefronts of the two recording waves can be adapted individually and independently of one another by means of the wavefront exposure device. This enables the realization of a novel HOE recording process, which makes it possible to specifically increase the angular and wavelength bandwidth of a holographically-inserted volume hologram, in particular for a specific target value for the diffraction efficiency.

[0048] If an exemplary embodiment has an “and/or” link between a first feature and a second feature, this is to be understood to mean that the exemplary embodiment according to one specific embodiment has both the first feature and the second feature and, according to a further specific embodiment, has either only the first feature or only the second feature.