FIELD

The present invention is directed to a device and a method for recording a hologram.

BACKGROUND INFORMATION

As a result of holographic materials newly available on the market and the miniaturization of laser sources, new markets and fields of application are becoming increasingly available for holographic optical elements, HOE for short. Potential areas of application such as data goggles or head-up displays are becoming increasingly attractive and could be implemented or improved by the use of holographic optical elements.

To record a hologram, two laser sources (reference wave and object wave) coherent relative to one another are caused to interfere on a photosensitive holographic layer. The resulting interference pattern is then written into the holographic layer. By illuminating this interference structure using the reference wave, it is possible to reconstruct the object wave. The optical function of a volume hologram may, for example, be written into the holographic layer using two different recording methods. On the one hand, the two laser beams may be expanded in a large-scale or analogous recording to the point that the entire holographic layer may be illuminated in one working step. The resultant interference pattern is thus written immediately onto the entire surface of the hologram. An alternative recording method is based on the fact that laser beams having very small beam diameters are caused to interfere, and thus sub-holograms, also referred to as hogels or voxels, are written sequentially into the photosensitive layer, as is shown inFIG.1. Sequentially exposed or printed (digital) holograms have the advantage that each hogel may be assigned a different optical function. Completely novel optical systems may be manufactured as a result. In addition, manufacturing sequentially printed holograms generally requires significantly lower laser powers as compared to large-scale recorded holograms. The power available when recording large-scale holograms frequently represents a limiting factor for the size of the hologram.

Holographic printers presently available on the market are able to create holograms having a minimal hogel size of between approximately 200 μm and 500 μm. The optical function of the individual hogels in this case is defined, for example, via a spatial modulator for light, also referred to as a spatial light modulator or SLM for short. These holographic printers are mostly used for creating image holograms without having to holograph a real three-dimensional object. A so-called holographic stereogram printer is therefore mostly used. The holograms produced with such a printer use the stereoscopic effect in order to achieve a three-dimensional representation of the printed content. A classic hologram, which is able to reconstruct the original wavefront of the holographed object, is usually not able to be produced using such holographic stereogram printers. For this purpose, so-called holographic wavefront printers are required which, unlike stereogram printers, are also suitable for printing holographic-optical elements. Previously known wavefront printers may, for example, print hogels having a minimum size of approximately 400 μm at maximum image angles of +/−40°.

SUMMARY

In accordance with the present invention, an example exposure device for recording a hologram, an example method for recording a hologram, an example method for controlling an exposure device for recording a holograph, as well as an example control unit that uses this method, are provided.

Advantageous refinements of and improvements on the example devices and methods are possible with the measures described herein.

In accordance with an example embodiment of the present invention, an exposure device for recording a hologram is provided, the exposure device including the following features:

at least one modulation unit, which is designed to produce a modulation beam representing a reference beam and/or an object beam by impressing a modulation representing at least one holographic element of the hologram onto a laser beam;

at least one reduction unit, which is designed to produce a modified modulation beam using the modulation beam, the modified modulation beam having a smaller beam diameter than the modulation beam; and

at least one objective lens unit, which is designed to direct the modulated modulation beam through an immersion medium onto a recording material in order to record the hologram by exposing the recording material to the modified modulation beam.

A modulation unit may, for example, be understood to mean a spatial modulator for light, also referred to as a spatial light modulator, such as, for example, an LCoS display, which is designed to code pieces of information relating to at least one optical function represented by the holographic element into the laser beam. A holographic element may be understood to mean a three-dimensional grid element of the hologram, also referred to as a voxel or hogel, having a particular optical function. The modulation unit may, for example, be designed to produce the modulation beam by modulating an amplitude or phase of a wavefront of the laser beam. Depending on the specific embodiment, the modulation unit may be electronically or optically activatable. The hologram may include a plurality of holographic elements, which may be situated in a grid pattern or overlapping one another and may fulfill different optical functions depending on the specific embodiment. For example, the modulation unit may be designed by modulating the laser beam accordingly, to produce a modulation beam representing a rectangular or Gaussian holographic element. A hologram may, for example, be understood to mean a reflection hologram or transmission hologram or a waveguide.

A reference beam may be understood to mean a wave field, which represents a portion of a laser beam reflected and scattered by an object. A reference beam may be understood to mean a wave field, which represents an unscattered portion of the same light beam. The modulation unit may, for example, be designed to produce the reference beam and the object beam independently of one another via corresponding modulation of the laser beam.

A reduction unit may, for example, be understood to mean an optical system made up of one or of multiple lenses for reducing a beam diameter of the modulation beam, for example, by a reduction factor of 30 to 100.

An objective lens unit may be understood to mean an immersion objective lens directly upstream from the recording material or from a carrier for holding the recording material. In this case, the objective lens unit may be optically coupled or couplable to the recording material via the immersion medium, which may be, for example, a liquid or a gel. The recording material may be understood in general to mean a photosensitive layer, which may be suited, in particular, for manufacturing a holographic optical element. The exposure device may, for example, be designed to record the hologram using the modified modulation beam by sequential, in particular, overlapping exposure of the recording material.

In accordance with the present invention, a holographic wavefront exposure device for holographic optical elements that includes a phase shifter for spatial light modulation, a high-quality reduction optical system, for example, having a magnification of approximately 1 to 60, and an immersion objective lens, for example, having a numerical aperture of greater than 1.1 and an operating distance of greater than 600 μm, which includes an immersion solution having a diffraction index adapted to a holographic material, may be implemented. The immersion solution in this case may, for example, fill a space between the immersion objective lens and a glass plate, which serves as carrier material for the holographic material. The phase shifter may, for example, be designed in order to generate a reference wave or an object wave or both a reference wave as well as an object wave having diffraction angles of greater than 2.5°.

Hogels in the range of between 100 and 200 may advantageously be printed via the three aforementioned points, which allow for angles of incidence and angles of reflection of greater than +/−90°, i.e., over the total reflection angle.

Reference wave and object wave may, in particular, be manipulated independently of one another by such an exposure device.

With the optional use of a phase-shifting modulator unit having a large diffraction angle in the reference beam path and object beam path, it is possible, for example, to produce holographic optical elements compounded of sub-holograms, which include variously definable optical functions. Furthermore, it is possible in combination with the reduction optical system to manufacture holographic optical elements having very large angles of incidence or angles of reflection up to and including waveguide structures—in contrast to previous holographic wavefront printers, in which angles of incidence and angles of reflection are generally severely limited and the reference beam is usually a flat wave, in which at most the angle of incidence may be varied.

A further advantage of an exposure device according to one specific embodiment of the present invention is that instead of rectangular hogels, Gaussian hogels may optionally also be exposed and superimposed on one another.

With the softer transitions between the individual hogels, it is possible to avoid or at least significantly reduce a grid structure of the hologram, as it may form, in particular, when using rectangular apertures for shaping hogels, and which may possibly adversely affect the image quality. Thus, by overlapping the Gaussian hogels, it is possible to improve the image quality. This effect may be still further optimized by an optional subsequent copying process.

One optional specific embodiment of the present invention enables the recording of reflection holograms if two separate beam paths including one modulation unit each are used for exposing. If only one beam path is used, the object wave and reference wave may then also be produced by only one modulation unit. Transmission holograms may therefore also be printed.

In summary, the advantages of example embodiments of the present invention may include the very large optical angles or angular ranges in the direction of incidence and direction of radiation to beyond the total reflection angle, the selectable reference wave and object wave in each hogel through the possible use of a phase modulator in both partial beams, the improved image quality as a result of Gaussian, overlapping hogels, and the possibility of producing reflection holograms as well as transmission holograms and holographic optical elements having variously definable optical functions to and including coupling and uncoupling holograms for waveguide structures.

With the optional incorporation of two CMOS cameras into beam paths of the exposure device assigned to one partial beam each of a laser beam, it is also possible to ensure an exact and simple adjustment of the two partial beams while simultaneously optimizing the exposure time.

According to one specific embodiment of the present invention, the modulation unit may be designed to impress the modulation by shifting a phase of the laser beam. In this way, the hologram may be produced having preferably large angles of incidence and angles of reflection.

The modulation unit in this case may be designed as a LCoS display. In this way, a propagation direction and a divergence of the modulation beam may be very precisely modified across a large adjustment range.

According to a further specific embodiment of the present invention, the modulation unit may have a pixel structure made up of pixels having a size of smaller than 4 μm. In addition or alternatively, the modulation unit may be designed to radiate the modulation beam at a diffraction angle of greater than 2.5 degrees into a beam path to the reduction unit. Optical functions of the hologram may be flexibly defined and simply implemented with this specific embodiment as well.

It is advantageous if the objective lens unit includes a numerical aperture of greater than 1.1 or, in addition or alternatively, an operating distance to the recording material of greater than 600 μm. In this way, the hologram may be recorded having very small hogels, in particular, having a size of smaller than 400 μm.

It is also advantageous if the reduction unit is designed to produce the modified modulation beam having a beam diameter of between 100 μm and 200 μm. The size of the hogels may be significantly reduced with this specific embodiment as well.

According to a further specific embodiment of the present invention, the reduction unit may include at least one first imaging unit representing a first Kepler telescope and a second imaging unit representing a second Kepler telescope. In this case, the first imaging unit and the second imaging unit may be optically connected in series. A Kepler telescope may be understood in general to mean an imaging optical system made up of a large, slightly curved convergent lens as the objective lens and a relatively small, more sharply curved convergent lens as the ocular. The two convergent lenses may, for example, be positioned opposite one another. With this specific embodiment, it is possible to precisely reduce the beam diameter with little technical effort. In addition, a significant increase of deflection angles produced by the modulation unit may be achieved as a result.

The exposure device may optionally include at least one adjustment unit for adjusting the modulation beam and/or the modified modulation beam. The adjustment unit may be designed, in particular, as a CMOS element. Exposure errors may be avoided as a result.

According to a further specific embodiment of the present invention, the exposure device may further include a positioning unit, which may be designed to position the recording material relative to the objective lens unit, in order to enable a sequential exposure of the recording material using the modified modulation beam. A positioning unit may, for example, be understood to mean a movable table or the like. As a result, it is possible to record holograms including sub-holograms that differ from one another in their optical functions.

It is particularly advantageous if the exposure device includes an additional modulation unit, an additional reduction unit and an additional objective lens unit, the modulation unit, the reduction unit and the objective lens unit being able to be assigned to a first beam path for exposing the recording material, and the additional modulation unit, the additional reduction unit and the additional objective lens unit being able to be assigned to a second beam path for exposing the recording material. The additional modulation unit may be designed to produce an additional modulation beam by impressing an additional modulation representing the holographic element onto an additional laser beam. An additional laser beam may, for example, be understood to mean a partial beam of the laser beam produced by beam splitting. The modulation beam in this case may represent the reference beam and the additional modulation beam may represent the object beam. The additional reduction unit may be designed to produce an additional modified modulation beam using the additional modulation beam. The additional modified modulation beam may have a smaller beam diameter than the additional modulation beam. The additional objective lens unit may be designed to direct the additional modified modulation beam through an additional immersion medium onto the recording material and to cause it to interfere with the modified modulation beam on the recording material in order to record the hologram. In this way, reflection holograms may be produced with the aid of the exposure device.

According to a further specific embodiment of the present invention, the objective lens unit may be designed to direct the modified modulation beam onto a first side of the recording material. The additional objective lens unit may be designed to direct the additional modified modulation beam onto a second side of the recording material opposite the first side. In this way, the recording material may be exposed on both sides simultaneously.

The present invention also provides an example method for recording a hologram with the aid of an exposure device according to one of the preceding specific embodiments, the method including the following steps:

impressing the modulation representing the holographic element onto the laser beam in order to produce the modulation beam;

reducing a beam diameter of the modulation beam in order to generate the modified modulation beam; and

directing the modified modulation beam through the immersion medium onto the recording material in order to record the hologram.

The present invention further provides an example method for controlling an exposure device according to one of the preceding specific embodiments, the method including at least the following step:

outputting an activation signal to an interface to the modulation unit in order to impress the modulation representing the holographic element onto the laser beam.

This example method may, for example, be implemented in software or hardware or in a mixed form made up of software and hardware, for example, in a control unit.

The present invention further provides an example control unit, which is designed to carry out, activate or implement a variant of a method presented herein in corresponding units. With this embodiment variant of the present invention in the form of a control unit as well, it is possible to quickly and efficiently achieve the object underlying the present invention.

For this purpose, the control unit may include at least one processing unit for processing signals or data, at least one memory unit for storing signals or data, at least one interface to a sensor or to an actuator for reading in sensor signals from the sensor or for outputting control signals to the actuator and/or at least one communication interface for reading in or outputting data, which are embedded in a communication protocol. The processing unit may, for example, be a signal processor, a microcontroller or the like, the memory unit capable of being a flash memory, an EPROM or a magnetic memory unit. The communication interface may be designed to read in or output data wirelessly and/or in a hardwired manner, a communication interface, which is able to read in or output hardwired data, being capable, for example, of reading in these data electrically or optically from a corresponding data transmission line or outputting these data into a corresponding data transmission line.

A control unit in the present case may be understood to mean an electrical device, which processes sensor signals and, as a function thereof, outputs control signals and/or data signals. The control unit may include an interface, which may be designed in hardware and/or in software. In a hardware design, the interfaces may, for example, be part of a so-called system ASIC, which contains a wide variety of functions of the control unit. It is also possible, however, for the interfaces to be dedicated integrated circuits or to be made at least partly of discrete components. In a software design, the interfaces may be software modules, which are present, for example, on a microcontroller alongside other software modules.

