DE19508754A1 - Avoidance of speckling effects on surface receiving coherent light beam - Google Patents

Abstract

The interference of a coherent light beam on a surface (64) producing an effect known as speckling is reduced by using different polarisation paths. For a video system light from different light sources (30,40,50) is directed through monomode fibres (82,84,86) and into a multimode fibre (74). Located in the paths of the separate light sources are different polarising elements (87,88,89) that produce different conditions for the cells of an LCD matrix. The control voltages applied by the cyclically varying conditions suppress the interference effect.

Description

The invention relates to a method for reducing interference from a coherent light beam. Furthermore, the invention relates to a device in particular to carry out the method for illuminating an area with a coherent light beam by means of a device for changing the Interference ability of the light beam.

For light pointers during lectures with photographs or for illuminating an object in A microscope often uses light beams. In addition, the technical Use of rasterized light beams for displaying a video image or for Creation of a printout with a laser printer known over a larger area. Because of the good parallelism and the possible good imaging properties individual pixels are often used with laser light beams.

Laser light in particular is characterized by high temporal and spatial coherence out. Therefore, when a surface is illuminated, there is interference from, for example, the Illuminated area scattered partial beams of a laser light beam can be observed. Due to the interference, spatially different brightnesses arise also with different directions of observation because of different Interference phase relationships vary. This variation is called glitter perceived and is, for example, in the display of video images undesirable.  

Such glittering brightness maxima are known in the literature as "speckle" designated. In particular, they can be observed in two ways:

• - The light deflected on a scattering surface is at one Measuring method recorded using a measuring probe. The measurement result conveys the so-called objective speckle. These occur, for example, with a laser printer in which a different brightness distribution within a pixel in a photosensitive layer is recorded.
• - The scattered laser light is applied to an image area by an imaging system, for example, the retina of a viewer's human eye. On The disturbing statistical interference pattern appears in this image area in the Brightness intensified picture elements and darker picture elements, in which the Light more or less extinguishes.

Various methods for reducing the number or are known from the literature Brightness of this annoying speckle is known. The procedures can basically be in divide the three following categories:

• - reducing time coherence,
• - reducing spatial coherence,
• - Fast movement of the speckle, therefore no annoying speckle phenomenon.

According to B. Dingel and S. Kawata, "Speckle-free image in a laser diode microscope using the optical feedback effect ", Optics Letters, April 1, (1993), Vol. 18, No. 7, pages 549-551, by modulating the laser pump power To get picked up. One is for gas lasers and for diode-pumped solid-state lasers such a method cannot be implemented.

The spatial coherence is, for example, according to S. Jutamulia and T. Asakura, "Reduction of coherent noise using various artificial incoherent sources", Optik 70, (1985), pages 52 to 57, by a multimode fiber moved with a vibrator for Light transmission reduced. Due to the movement of the light guide Phase shifts of the light beams guided in the light guide due to changes their way, so that temporally different fashion patterns are mapped, which compensate for averaging over a certain time interval. This well known The method for reducing the speckle is through the movement of the light guide  only suitable, the different fashions, which are in the passage of the Adjust laser light through a multimode fiber to vary in time, so that at the brightness on the photosensitive layer is approximately the same result by "blurring" the different interference patterns of the individual modes will. The light emerging from the optical fiber is still partially coherent, so that after imaging on a screen by imaging the eye always can still result in subjective speckle.

Another known rapid movement of the speckle, for example so fast that the eye can no longer grasp the speckle due to its sluggishness by moving the scattering surface, for example the screen a video system. This creates between the scattering centers and the human eye a relative movement. The speckle distribution then varies on the Retina of the eye and is blurred. This is in a video system, in particular with large screens, but can only be carried out with great effort. also this method is not applicable for illumination in a microscope, because the Object should rest during observation.

However, one could reverse the principle of the moving surface Move the laser beam with a small deflection relative to the screen. Surprisingly, this does not affect the speckle. That can be however, if you take into account that the laser has a flat wavefront owns and no changes in the impact of the transverse wavefront spatial or temporal coherence can occur relative to the image surface. On the other hand, the scattered laser light, which creates the image in the eye, comes from a scattering center that is immobile relative to the eye. As a result, it cannot become one Relative movement and therefore no reduction in speckle on the retina come.

Another way to reduce the number or brightness of the speckle consists of inserting a rotating lens into the beam path. Thereby the diffuser the good parallelism of the light beam is lifted and scattered Laser light can practically not be collimated again very high power loss.

From this it follows that all previous solutions to the problem of speckle unsatisfactory and especially when generating video images below  Use of large screens cannot be used. In particular, there is none satisfactory solution for reducing subjective speckle that is only on the Retina of the eye of the viewer of a video image are known.

Proceeding from this, the object of the invention is a method and a Finding a device that reduces the contrast in subjective speckle allow an image without significant loss in laser power. In particular, should the method can also be used in a video system and in a light beam rastering video system can be used without significant loss of light.

The object is achieved in a method of the type mentioned solved in that the light beam locally perpendicular to the direction of propagation is polarized differently.

It takes advantage of the fact that different polarization states of light can no longer completely interfere with each other. Assign two bundles of light even two directions of polarization orthogonal to one another, one disappears Interference completely. Orthogonal directions of polarization are linear polarized light, for example, by two perpendicular to each other Vibration levels realized. In the case of circularly polarized light, they are clockwise and left-handed polarization states orthogonal to each other. For one in general elliptically polarized polarization state can be added to this always find the orthogonal state by looking at the vibration components split into linear polarization states and the orthogonal state Applying a rotation of the polarization state by 90 ° wins.

By different local changes in the polarization state Interference capability reduced. This will reduce the speckle Interference ability of the light beam is at least partially suppressed.

A movement of the polarization device as in the example mentioned with the Optical fiber or the rotating lens is no longer necessary. Since one therefore can generally dispense with movement mechanisms is the Solution according to the invention with regard to the maintenance of corresponding devices extremely advantageous since it does not require any mechanically wearing parts.  

Compared to diffusers there is also the advantage that the total light intensity is let through by this polarization device. So there are no losses on. Such losses can heat the diffuser, for example, and destroy thermal stresses. Such adverse effects are also in the solution according to the invention excluded.

According to the invention, even subjective speckle can be reduced since it is according to the invention due to the destruction of the interference ability also interference on the retina can be reduced or even completely avoided.

As already explained above, the interference ability of two side by side partial light beams emanating from the polarization device even completely To get picked up. A preferred further development of the inventive method that the polarization of the light beam discontinuously in adjacent to each other perpendicular to the direction of propagation Areas are polarized so that those of two lying side by side Regions outgoing partial light beams are polarized orthogonally to each other.

So if there are several areas for changing the state of polarization of these outgoing partial light beams are used, you get a variety of Partial light bundles that are no longer connected to one another with different polarization states are capable of interference. The interference ability is practically completely eliminated, so that speckle can theoretically be completely avoided. In practice there are but also transition zones on the adjacent areas to Applying different polarization states. The influence of such Transition zones can, however, be strong through measures described in more detail later be reduced.

The device according to the invention is based on the device of the aforementioned Kind of and is characterized in that the device for changing the Interference capability of the light beam in the beam path of the light beam has located polarization device, through the different partial light beams of the light beam on the input side can be polarized differently.

In the device according to the invention there is therefore only one specially designed Polarization device necessary. Mechanisms of movement, as in the stand the technology are often necessary, in principle omitted in the invention. Thereby  mechanical wear is prevented and frequent replacement of Spare parts can be avoided.

In a device according to the invention, speckles can thus be reliably suppressed without any maintenance for long periods of use.

In a preferred development of the device according to the invention, the Polarization device designed in a special way by being perpendicular to Direction of the light beam has areas in which Partial light beams can be polarized differently, with at least two of the areas generate mutually orthogonal polarization states.

The benefit of reducing speckle due to orthogonal Polarization states have already been carried out during the process. According to the A polarization device can be developed in a simple manner by providing different areas.

According to another preferred development of the invention Polarization device designed so that the partial light beams emanating from it of a polarization state has the same overall intensity as the partial light beam have to this orthogonal polarization state.

Then a uniform mixture of the polarization states, which the Interference ability disturbs to a maximum.

Apart from absorption, all substances usually leave a partial jet of one Beam of light through and reflect a partial beam. A preferred training The invention provides that the polarization device as a reflective Deflection device for the light beam is formed. Another advantageous one The polarization device for is a further development of the method according to the invention Light in the light beam permeable and for transmission in the beam path of the Light beam arranged. In both training courses, the entire Light intensity of the light beam on the input side in the total intensity of all Continue to use partial beams. This will involve a lot of effort for example in a video system when the laser power is increased disproportionately would increase avoided in a device according to the invention.  

In a preferred development of the invention, the polarization device has on at least one of their surfaces through which the light beam passes or from which it is reflected, a polarizing structure in the form of a matrix arranged areas depending on the location of the passage or the reflection the partial light beam has different phase shifts for Generate polarization components in the partial light beam.

In this development, the different polarization of partial light beams by a structure for generating a polarization change due to Phase shifts reached. As from the further training described later can be seen, this can be achieved in various ways. All examples is common, however, that the polarization device by simple Structuring techniques, such as physical or chemical etching can be produced.

This simple production method makes the polarization device special inexpensive and also allows inexpensive mass production. That's why such a polarization device, in particular in video technology for the Consumer area can be used particularly advantageously.

With structures that are, for example, raised and deepened on one birefringent crystal are formed, but can only be in a limited Wavelength range a polarization with respect to the desired orthogonal Reach polarization states. In color video technology, however, you need at least three wavelengths around the range of hues that man can grasp cover at least approximately.

The polarization device for such applications is designed in accordance with a preferred development of the invention in an advantageous manner that the Structure for each wavelength Areas for exposure to a partial light beam has a state of polarization that is the same as that of others Areas outgoing partial light beam is orthogonal. As above are explained, orthogonal states for a particularly effective suppression of the Suitable for speckle.

For use in the color video area, in a preferred development, the Invention provided that the structure on the polarizer for each Wavelength Areas for applying a partial light beam to a  Polarization state has that of the polarization state of other partial light beams is orthogonal. With this training you can do it for everyone in the light beam contained wavelengths the advantageous properties described above of mutually orthogonal polarization states.

As has already been made clear by the above explanations, it is sufficient for complete suppression of the speckle does not stop, only two partial light beams polarize differently, because then there is interference in a partial light beam containing light is still possible. The number of such partial light beams is the larger the smaller the areas emitting the partial light beam are. Also around in terms of area, very small brightness maxima, the speckle, should be reduced Select the partial light beam very small and when structuring the polarization device as many small areas as possible to create different ones Provide polarization states.

In a preferred development, these areas are the same for generating Polarization states of the polarization device locally according to a statistical Pattern distributed. Because of this distribution, regular structures that are Partial light beams of one and the same polarization state cause interference again avoided. Advantageously, the regular Arrangement of expected, emerging speckle suppressed.

To implement such structures, for example by means of physical or chemical etching techniques, you can design the etching mask for each area with a random number generator, whether a certain polarization state or that orthogonal polarization state through this area with transmission or Reflection of a partial light beam should be set and the etching mask accordingly form.

As has been mentioned several times above, the fabrication of the structure is included With the help of physical or chemical etching particularly cheap. Such Structures are particularly simple in a preferred further training realize in which the polarization device made of birefringent material exists and areas for applying different partial light beams with different polarization states due to different thicknesses of the material are realized.  

Because of the birefringent properties of the material, light has a dependency The polarization has different terms, which can be different Refractive indices. That means at certain thicknesses of the material, like is shown later using exemplary embodiments, can be by new phases due to the different phases of polarization states Target polarization states.

The training therefore advantageously allows simple training individual areas for setting a certain polarization state in Dependence on the polarization state of the incoming light beam. In Dependence of its birefringent properties and the wavelength of light the thickness of the material for each area must only be chosen accordingly. For Forming a structure with different thicknesses are common, for example Etching techniques applicable.

Furthermore, this development of the invention has the advantage that the Polarization device does not act as a filter, but the polarization state of the incident light only changes. While a filter is part of the light output would record, there is almost no loss of power in this embodiment fear. Local warming and associated stress thus avoided according to the training, and the lifespan of Polarization device is increased.

The negligible loss of performance when configured according to the training Polarization devices show further advantages when using the invention in Laser video systems. The power of the laser sources can be because of the low Reduce losses to what is necessary to illuminate a pixel. On additional effort required for the lasers and their cooling with increased power or temperature stabilization can then be avoided.

As stated above, there is speckle avoidance in the invention Limits that are essentially due to manufacturing technology. So you can theoretically consider structures in which the speckle is completely suppressed if the areas of equal polarization are statistically distributed and due to only orthogonal states are generated by the polarization device. In practice but there are always transition zones between areas of a polarization state  to an adjacent area of another polarization state. About the influence To keep such transition zones very small, they should be very small.

Therefore, according to a preferred development of the invention, it is provided that the polarization device is structured by ion beam etching. In contrast to chemical etching enables this technique to have particularly steep flanks which essential only from the accuracy of the ion beam direction when removing material are determined. This method of manufacture enables polarization devices for a particularly good suppression of speckle. Ion etching techniques are with today's state of the art also applicable on an industrial scale, such as Semiconductor technology shows where ion etching techniques are progressing Miniaturization also for cost-effective mass production be used as standard.

According to a further preferred development of the invention, it is provided that for the polarization of a partial light beam in the polarization device Generating the respective polarization state are provided, which are each in any direction perpendicular to the beam path of the partial light beam between one Wavelength and twenty times the wavelength of the light beam.

Because of these features, such areas are kept very small. How already noted above, the more effectively suppressing the speckle, the smaller such areas are. On the other hand, they shouldn't be that small either be that the light beam is too strong due to diffraction at the small structures is expanded. The dimensioning of the areas between the one and the Twenty times the wavelength has proven particularly favorable in this regard exposed.

In visible light, for example around wavelengths of 400 nm, this is due to the Twenty times the upper limit is approximately 8 micrometers, i.e. in one The order of magnitude known from semiconductor technology Microstructuring techniques can be easily implemented.

It has already been mentioned above that a moving device In principle, use of the polarization device according to the invention is not necessary, if it is designed so that the interference ability is completely disturbed. A  However, an advantageous further development sees a coupling coupled to the polarization device Movement device for movement perpendicular to the light beam in front.

This has the advantage of causing small, remaining speckle may be that the structure is not mechanically arbitrary to a theoretical form Elimination of speckle can be adjusted, blurred in time. With help of the movement mechanism, the polarization device is displaced and different partial light beams take the different ones during the movement Polarization states so that the remaining speckle on the retina of the eye hike. If the movement is so fast that the sluggishness of the eye does not can capture more, these speckle are "blurred".

In another advantageous development of the invention, the Polarization device is an active component that is used to generate the various Polarization states uses electro-optical or magneto-optical effects. With electro-optical or magneto-optical components can polarization through Setting a voltage or a magnetic field can be changed. Such Polarization device can therefore be easily adapted to the desired wavelength will. The setting option also enables optimization for suppression from Speckles.

In particular, for a rapid temporal variation of the polarization according to one preferred development of the invention provided that to achieve different Polarization states for partial light beams on a structured Polarizing LCD matrixes are arranged or the Polarizing device electro-optical or magneto-optical crystals, such as DKDP, Contains LiNbO₃, optical ceramics or liquid crystals and that the setting of Polarization states of different partial light beams through an electrical or magnetic excitation is feasible.

By switching different areas of the above described Polarization device via an LCD matrix or if the polarization device itself is an LCD matrix, the polarization states can also be quickly change so that one of the above described with respect to the movement Can make use of advantage without wear from one Movement mechanism associated parts is expected. The same applies if the  Different polarization takes place through electrical or magnetic excitation for example, the Kerr effect is used to change the polarization.

The specified materials are commercially available with high purity to achieve high degrees of polarization available. That is why they are particularly suitable for use regarding the invention.

If one forms the polarization device itself as an LCD matrix, it can be local different polarization of the liquid crystals due to different Tensions on the matrix elements to different polarization of the Light beam can be used. You save the previously mentioned Structuring steps. Through the mass production of LCD matrixes as a display these can be used particularly cost-effectively in the consumer sector.

In a preferred further development of the invention is also one of the Optical system that detects outgoing light beams intended.

With such an optical system, light is also detected from the diffraction pattern, that on the edges of the areas to be loaded with a certain one The state of polarization expands the light beam. With the help of the optical system the loss of performance is therefore reduced even further, or it still allows smaller areas for applying a polarization state without a Loss of intensity due to diffraction must be feared. This training is thus also advantageously applicable for reducing speckle.

The further developments of the invention specified below relate primarily to the Use with a video device.

The speckle is perceived as particularly annoying in video systems. The In contrast to the cited prior art, application of the method allows even a complete wiping out of the speckle without the rotating speckle Diffusion disc substantial loss of light occur. In a device for production A video image is a raster device for raster and image rastering a light beam modulated with a video signal on a screen provided and the polarization device in the beam path of the light beam and at Providing an optical system arranged in front of this.  

In a color video system, at least three light beams with different Wavelengths combined into a common light beam. With such a The system provides an advantageous development of the device according to the invention, that they have several individual light beams of different wavelengths and one Unification device for generating the light beam from these contains and that a polarization device for changing the wavelength for each wavelength Polarization states of different partial light beams in the beam path of each Single light beam is provided in front of the union.

Because of these features, the interference ability for light of any wavelength disturbed regardless of the other wavelengths. This makes it possible for everyone Wavelength cancel the interference ability separately, what compared to the previously described polarization device that operates on three different Wavelengths adjusted, a better optimization regarding the suppression of Speckles allows.

The invention will now be described in more detail by way of example with reference to the drawing explained. Show it:

Fig. 1 shows a schematic representation of a polarization device usable in the invention;

FIG. 2 is a side view of the polarization device shown in FIG. 1;

Fig. 3 is a schematic diagram for explaining the change in polarization with a polarizing device according to Figures 1 and 2.

Figure 4 is a polarizing device for an embodiment in which this is constructed for reflecting a light beam.

Fig. 5 shows an inventive embodiment of a video system;

Fig. 6 shows a further embodiment of this invention for a video system.

In Figs. 1 to 3, the inventive method is schematically illustrated, and is a polarization device 1 illustrated, which can be used in an inventive device.

In Fig. 1 is a plan view is shown on the polarizing means 1 with respect to the direction of an incident light beam. The polarization device 1 shown as an example is subdivided into uniform regions 2 , which in the exemplary embodiment are designed as square fields. These areas 2 serve to divide the incident light beam into partial light beams emanating from each area 2 . Each area 2 acts on the partial light bundle emanating from it with a defined polarization state for a given polarization state of the incident light bundle. The method for suppressing the speckle uses the property that two light beams with mutually orthogonal polarization states do not interfere with one another. By changing the polarization states of a coherent light beam, the interference ability of the light is reduced, that is, bacon intensities are reduced.

In Fig. 1, two areas 4 and 6 are shown by way of example, the outgoing partial light beam are acted upon by different linear polarizations. The direction of the linear polarization of the partial light beams is indicated by the arrows drawn in the regions 4 and 6 .

The different polarization states of the partial light beams originating from regions 4 and 6 are orthogonal, that is to say the product of the two polarization vectors is zero. For this reason, the partial light beams originating from areas 4 and 6 do not interfere with one another.

For the destruction of the interference capability, it is particularly advantageous if regions 4 and 6 , which each cause different polarization states of the outgoing partial light beams, are distributed unevenly over the surface of the polarization device 1 . A statistical distribution in particular prevents a regular pattern for the emission of partial beams, which could again lead to sharp interference or speckle.

How the different polarization of different regions 2 can be realized can be seen from the schematic side view of the polarization device 1 in FIG. 2.

FIG. 2 shows that the light of the light bundle has to pass through a greater material thickness in the areas 4 than in the areas 6 . These different material thicknesses lead to different polarization states in the outgoing partial light bundles for a given polarization state of the incident light bundle. This will become more apparent in connection with the description of FIG. 3.

Furthermore, the irregular arrangement of regions 4 and 6 is also indicated in FIG. 2. A statistical arrangement with different areas 4 and 6 is achieved, for example, in the case of etching through a mask, the free areas of which are determined by means of a random generator.

The material used for the polarization device 1 in the exemplary embodiment is a birefringent crystal. Depending on the linear polarization state of the incident light beam to a crystal direction caused by the material and the cutting direction, two components of different polarization of a light beam are given, which are assigned different refractive indices n 1 and n 2 for the propagation in the material.

The terms of the two components of light with different States of polarization are as by the different refractive indices given, different from each other, and result from passing through the material different phases of the individual components to each other. Based on these Change of phases depending on the thickness of the material passed through different polarization states are generated when the light emerges.

This is illustrated in more detail in FIG. 3. The coordinate cross marked by broken lines 10 and 12 designates the axes in which the material allows the two linear polarization states of a light bundle belonging to the refractive indices n 1 and n 2 to pass without a change in polarization. The vector 14 pointing upward denotes, by way of example, a linear polarization state of the incident light beam. This has the same components with respect to the coordinate cross marked with lines 11 and 12 . If the components of the light bundle again have the same phase to one another as in the incident light bundle after passing through them, the components for the outgoing light bundle add up again to the same polarization state denoted by the vector 14 .

If the component plotted along line 12 is shifted by half a wavelength due to the different transit times compared to the component plotted on line 10 , a reversal of direction results for this component, as indicated in FIG. 3 by the white arrow 16 . The components for the outgoing partial light beam then add up to the polarization vector identified by the white arrow 18 . FIG. 3 firstly makes it clear that orthogonal polarization states in the polarization device 1 can be achieved by different material thicknesses. Secondly, the dependencies of the polarization changes on the relative phase of the ordinary and the extraordinary beam can also be obtained from the discussions mentioned.

The same considerations as for linear polarization in the incident light beam can also be applied to circularly polarized light with the aid of FIG. 3. However, in the case of circularly polarized light, the components in the coordinate system indicated by lines 10 and 12 are imaginary, so that a calculation is more suitable than a graphic representation. Such a calculation leads to the result that the birefringent material on which FIG. 3 is based, with the coordinate cross determined by the crystal direction and represented by lines 10 and 12, is also suitable, circularly polarized light due to the different thicknesses shown in FIG to divide the areas 4 and 6 into partial beams of right and left polarized light.

In Fig. 2 two dimensions h and d are entered, which determine the transit time of partial light beams through the polarization device 1 and thus the polarization state of the outgoing partial light beam. On the basis of the considerations given above, if, according to FIG. 1 and FIG. 3, a relative phase change of an integral multiple is to take place after the passage of a light bundle through the region 6, and in the region 4 a relative phase change by a half-number multiple of the wavelength is to take place, the relationships:

in which the sizes d and h, the sizes shown in Fig. 2, n₁ and n₂ the refractive indices for ordinary and extraordinary beam, m₁ and m₂ freely selectable integers and λ mean the wavelength of light in the light beam.

For λ / [[2 (n₁-n₂)] results in a quartz plate used in the exemplary embodiment a size of 23 µm at a wavelength of 434 nm and 508 nm in light Wavelength a size of 27 µm. These are the minimum sizes of d and h. Other suitable sizes for d and h result from these by the given Values are multiplied by any odd number.

If a light beam is linearly polarized on the input side in partial beams with circular Decomposition of polarization states is instead of shifting by half Wavelength for h or d a further phase shift by a quarter of the Wavelength to be considered. The size d is then compared to the specified Values chosen half smaller.

A birefringent material is used that is perpendicular to its optical Is cut, you can also see the specific polarization rotation of the Use material. For quartz, this results, for example, at one wavelength of 404 nm a thickness of 1.84 mm for a rotation of the linear polarization around 90 °.

While a polarization device 1 for the transmission of a light bundle was shown in the example of FIG. 2, a polarization device suitable for reflecting the incident light bundle is shown by way of example in FIG. 4.

In order to apply different polarization states to an incident light bundle 20 , a layer 22 of optically active material is provided, which is provided on its rear side with a reflective layer 24 . On the top there is a structure made up of further reflecting surfaces 26 . The reflecting surfaces 24 and 26 can be produced, for example, by vapor deposition of aluminum and subsequent structuring of the vapor-deposited aluminum layer using a mask.

A light beam that strikes one of the reflecting surfaces 26 changes its polarization by reflection in the same way as a light beam reflected on the reflecting layer 24 . However, the light bundle reflected by the reflecting layer 24 additionally undergoes a change in polarization as it passes through the layer 22 of optically active material, in accordance with the principle shown in FIG. 3.

In this example, the change in polarization cannot only be caused by Phase shift of the two components in a birefringent material but also with the help of electro-optical or magneto-optical effects will. DKDP crystals, LiNbO₃ or are particularly suitable for the material optical ceramics.

You can also use LCD matrices instead of structured matrices. Liquid crystals are polarized in LCD matrixes when electrical fields are applied. The liquid crystals thus aligned can, in the same way as described for the polarization device 1 according to FIG. 1, break down an incident light bundle into partial light bundles with different polarization states, so that speckle formation is reduced or even completely avoided.

In the absence of electrical control, the crystal directions are Liquid crystals already in a statistical arrangement, so that even with these effective suppression of speckle without a defined orientation by a Suspense is expected.

Fig. 5 shows an embodiment in which the invention is used within a color video system. In the type of video devices already described in the introduction, three light sources 30 , 40 , 50 of different wavelengths, that is to say different colors, are usually used for the color display, the intensity of which is controlled with respect to the color tone and brightness for each pixel of a video image. The three light beams generated in this way are combined via mirrors 32 , 42 and 52 . In particular, if the mirrors 32 , 42 and 52 are designed as dichroic mirrors, all three light beams can be combined in a simple manner, as shown in FIG. 5, into a common parallel light bundle 60 without losses.

The light beam 60 is rasterized by means of a raster device 62 in terms of image and lines on a screen 64 in order to display a video image. By controlling the intensity of the three light sources 30 , 40 and 50 and by rastering, the video image is generated analogously to the known television, in which, however, electron sources are controlled in terms of intensity and are deflected by magnetic or electrical fields onto a screen which displays the energy of the electrons converted into photons of different colors.

In a video system that works with light sources 30 , 40 , 50 , lasers are usually used because of the internal emission of a highly parallel light beam. However, due to the coherence of such laser beams, the speckles mentioned, which are considered by the viewer to be disturbing, arise.

In order to suppress such speckle, the polarization device 1 from FIG. 1 is arranged in the beam path of the light beam 60 . However, as described, this is structured by statistical arrangement of the regions for different polarization for all three wavelengths of the light sources 30 , 40 , 50 .

In order to suppress any residual speckle due to possible manufacturing tolerances to be taken into account in the manufacture of the polarization device, a movement device 66 is also provided in the exemplary embodiment of FIG. 5, which rapidly moves the polarization device 1 oscillating perpendicular to the beam path of the light beam 60 . Due to the movement of the polarization device 1 thus generated, temporally different partial light beams are polarized, which leads to a temporally different location of possible residual speckles on the screen 64 or the retina of an observer. The speckle is thus blurred in time. If the movement occurs so quickly that it is no longer perceived due to the sluggishness of the eye, residual speckle can no longer be detected.

In the exemplary embodiment of FIG. 5, two deflection mirrors 68 can also be seen in the beam path of the light bundle, but these are only useful if the spatial dimensions of the video system are to be reduced.

In this exemplary embodiment, the polarization device 1 was designed with very small regions 2 , on the order of a few wavelengths, so that as many partial light beams as possible are created. Therefore, a diffraction of the light beam 60, so an unwanted expansion of the light beam 60 can be made to the limits of such areas. In order to parallelize the light bundle again without major light losses, the optical system 70 shown in FIG. 5 was arranged in the exemplary embodiment with respect to the propagation of the light bundle 60 behind the polarization device 1 .

In the simplest embodiment, the optical system 70 consists of a single lens, whose focal point facing the polarization device 1 is close to the location at which the light bundle 60 emerges again after passing through it.

Another video system is shown in FIG . Above all, it also illustrates the many possible embodiments for a color video system when compared with FIG. 5.

With FIG. 5 in particular, the light sources 30, 40, 50 together to form the common light beam 60 and the rasterizer 62 to the deflection on the screen 64 are. An optical system 72 can also be seen, but here it only images the light bundle 60 coming from a multimode fiber 74 into the raster device. The optical system 72 again parallelizes the light beam for the raster device 62 , similar to the optical system 70 in FIG. 5. It can be realized by a lens easiest whose focal point is situated 74 in the direction of the multimode fiber to the core of the multimode fiber 74th

The light from the three light sources 30 , 40 , 50 is coupled into the multimode fiber 74 via single-mode fibers 82 , 84 , 86 via leakage field coupling (evanescent field coupling).

To cancel the interference capability, there are polarization devices 87 , 88 and 89 between the light sources 30 , 40 and 50 and the single-mode fibers 82 , 84 , 86 , which are adapted to the respective wavelength of the light sources 30 , 40 , 50 . These can be designed like the polarization device 1 . In this exemplary embodiment, however, the movement mechanism 66 shown in FIG. 5 for reducing residual speckles was dispensed with. In this exemplary embodiment, however, residual speckles are also blurred in time, since LCD matrices were used for the polarization devices 87 , 88 and 89 , which are driven in time for different polarization states of the regions given by the LCD matrix.

In the case of LCD matrices, the polarization takes place by aligning liquid crystals using a voltage. The cells of the LCD matrix are therefore subjected to voltages differently, so that a matrix is formed, as has already been described for the polarization device 1 in connection with FIGS. 1 to 3. The voltages for driving are varied so that temporally different polarization matrices result. This polarization, which changes over time, makes it unnecessary to use an additional movement mechanism to suppress residual speckles.

Optical systems 90 , 92 , 94 are located between the polarization devices 87 , 88 and 89 . In the simplest case, these optical systems 90 , 92 , 94 can be implemented with two lenses. The first lens parallelizes the light bundle, as has already been described with regard to the optical system 70 from FIG. 5. The second lens couples the parallelized light bundle into the respective single-mode fiber 82 , 84 or 86 .

In particular, 5 and FIG. 6 falls in the embodiments of FIG. On, with which by simple means the suppression may be performed speckle according to the invention in a color video system. Before the invention, the experts had a much higher effort, such as the movement of the extremely large screen required, but this does not affect subjective speckle at all. On the other hand, the invention succeeds in suppressing or even eliminating even such speckle with justifiable effort.

Claims (20)

1. A method for reducing interference of a coherent light beam ( 20 ; 60 ), characterized in that the light beam ( 20 ; 60 ) is polarized locally differently perpendicular to the direction of propagation. 2. The method according to claim 1, characterized in that the polarization of the light bundle ( 20 ; 60 ) is polarized discontinuously in regions ( 4 , 6 ) which are locally adjacent to one another perpendicular to the direction of propagation, such that the regions ( 4 , 6 ) lying next to one another each emanate Partial light beams are polarized orthogonally to each other. 3. A device, in particular for performing the method according to claim 1, for illuminating a surface ( 64 ) with a coherent light beam ( 20 ; 60 ) by means of a device for changing the interference ability of the light beam ( 20 ; 60 ), characterized in that the device has a polarization device ( 1 ; 87 , 88 , 89 ) located in the beam path of the light beam, by means of which different partial light beams of the light beam ( 20 ; 60 ) on the input side can be polarized differently. 4. The device according to claim 3, characterized in that the polarization device ( 1 ) perpendicular to the direction of the light beam extending areas ( 2 , 4 , 6 ) in which partial light beams can be polarized differently, at least two of the areas ( 4 , 6 ) generate mutually orthogonal polarization states. 5. Apparatus according to claim 3 or 4, characterized in that the polarization device ( 1 ; 87 , 88 , 89 ) is designed such that the partial light beams of a polarization state emanating from it have the same overall intensity as the partial light beams of a polarization state orthogonal to this. 6. Device according to one of claims 3 to 5, characterized in that the polarization device as a reflective deflection device for the Beam of light is formed. 7. Device according to one of claims 3 to 5, characterized in that the polarization device ( 1 ; 87 , 88 , 89 ) for light in the light beam ( 20 ; 60 ) is transparent and arranged for transmission in the beam path of the light beam ( 20 ; 60 ) is. 8. Device according to one of claims 3 to 7, characterized in that the polarization device ( 1 ) on at least one of its surfaces through which the light beam ( 20 ; 60 ) passes or from which it is reflected, a polarizing structure with in shape areas ( 2 , 4 , 6 ) arranged in a matrix, which generate different phase shifts for polarization components in the partial light beam depending on the location of the passage or the reflection of the partial light beams. 9. The device according to claim 8 for light beams with a plurality of wavelengths, characterized in that the structure on the polarization device for each wavelength has areas ( 4 , 6 ) for applying a partial light beam with a polarization state which is orthogonal to the polarization state of other partial light beams. 10. The device according to claim 8 or 9, characterized in that areas ( 4 , 6 ) for generating the same polarization states of the polarization device are locally distributed according to a statistical pattern. 11. Device according to one of claims 3 to 10, characterized in that the polarization device ( 1 ) consists of birefringent material and areas for applying different partial light beams with different polarization states are realized by different thicknesses of the material. 12. The apparatus according to claim 11, characterized in that the polarization device ( 1 ) is structured by ion beam etching. 13. Device according to one of claims 3 to 12, characterized in that for the polarization of a partial light beam in the polarization device ( 1 ) areas ( 4 , 6 ) are provided for generating the respective polarization state, which are in each direction perpendicular to the beam path of the partial light beam extend between a wavelength and twenty times the wavelength of the light beam ( 20 ; 60 ). 14. Device according to one of claims 3 to 13, characterized in that a movement device ( 66 ) coupled to the polarization device ( 1 ) is provided for movement perpendicular to the light beam ( 60 ). 15. Device according to one of claims 3 to 14, characterized in that the polarization device ( 87 , 88 , 89 ) is an active component which uses electro-optical or magneto-optical effects to generate the different polarization states. 16. The apparatus according to claim 3 to 15, characterized in that to achieve different polarization states for partial light beams on a structured polarization device ( 1 ; 87 , 88 , 89 ) LCD matrices are arranged or the polarization device ( 87 , 88 , 89 ) electro-optical or magneto-optical Contains crystals such as DKDP, LiNbO₃, optical ceramics or liquid crystals, and that the setting of the polarization states of various partial light beams can be carried out by an electrical or magnetic excitation. 17. Device according to one of claims 3 to 16, characterized by an optical system ( 70 ; 90 , 92 , 94 ) which detects the light bundle ( 60 ) emanating from the polarization device ( 1 ). 18. Device according to one of claims 1 to 17, characterized in that for generating a video image, a raster device ( 62 ) for image and line screening of a light beam modulated with a video signal ( 60 ) is provided on a screen ( 64 ) and the polarization device ( 1 ; 87 , 88 , 89 ) is arranged in the beam path of the light bundle ( 60 ) and if an optical system ( 70 ; 72 ) is provided in front of it. 19. The apparatus according to claim 18, characterized in that it contains a plurality of individual light beams of different wavelengths and a combining device ( 32 , 42 , 52 ; 74 , 82 , 84 , 86 ) for generating the light beam ( 60 ) therefrom and that for each wavelength Polarization device ( 87 , 88 , 89 ) for changing the polarization states of different partial light beams in the beam path of each individual light beam is provided in front of the combining device ( 82 , 84 , 86 ). 20. Application of the method or the device according to one of the previous claims in a video system.