Filter device for an optical sensor

Abstract

A filter device for an optical sensor, including a hologram having a defined number of holographic functions, which are developed in such a way that the filter device blocks optical radiation that impinges upon the filter device from a defined first solid angle and optical radiation that impinges upon the filter device from a defined second solid angle is able to pass through the filter device.

Claims

1. A filter device for an optical sensor, comprising: an anisotropic stray light filter for the optical sensor for monochromatic radiation, wherein to prevent stray light from outside a utilized field of view from reaching a lens system element of the optical sensor, the anisotropic holographic filter provides different filtering in a horizontal field of view and in a vertical field of view, and is situated on a surface of the lens system element, so that optical radiation impinging from a particular solid angle does not reach the lens system element, including: a hologram having a defined number of holographic optical functions, which provide that the filter device blocks optical radiation that impinges upon the filter device from a defined first solid angle, and further provides that optical radiation that impinges upon the filter device from a defined second solid angle is passable through the filter device; wherein the holographic optical functions of the filter device is configured so that an angle for which a reflection, realized by diffraction, takes place differs horizontally and vertically, wherein the fields of view are not disturbed by the holographic optical functions of the filter device, and wherein the optical holographic functions are provided either by holographic multiplexing in a single layer or by producing a layer stack having multiple holographic layers.

2. The filter device as recited in claim 1, wherein the filter device includes a volume hologram.

3. The filter device as recited in claim 2, wherein the volume hologram has a plurality of layers, and each layer has at least one holographic function.

4. The filter device as recited in claim 3, wherein the volume hologram has a defined number of a plurality of holographic functions per layer of the layers.

5. The filter device as recited in claim 1, wherein holographic materials of the filter device includes polymer-based materials.

6. The filter device as recited in claim 1, wherein the optical sensor includes a lidar sensor or a time of flight sensor.

7. A method for producing a filter device for an optical sensor, the method comprising: providing a hologram having a defined number of holographic functions, wherein the hologram includes a hologram anisotropic stray light filter for the optical sensor for monochromatic radiation, wherein to prevent stray light from outside a utilized field of view from reaching a lens system element of the optical sensor, the anisotropic holographic filter provides different filtering in a horizontal field of view and in a vertical field of view, and is situated on a surface of the lens system element, so that optical radiation impinging from a particular solid angle does not reach the lens system element; wherein the holographic functions provide that the filter device blocks radiation that impinges upon the filter device from a defined first solid angle, and further provides that radiation that impinges upon the filter device from a defined second solid angle is passable through the filter device; wherein the holographic optical functions of the filter device are configured so that an angle for which a reflection, realized by diffraction, takes place differs horizontally and vertically, wherein the fields of view are not disturbed by the holographic optical functions of the filter device, and wherein the optical holographic functions are provided either by holographic multiplexing in a single layer or by producing a layer stack having multiple holographic layers.

8. The method as recited in claim 7, wherein the filter device includes a volume hologram.

9. The method as recited in claim 7, wherein the volume hologram has a plurality of layers, and each layer has at least one holographic function.

10. The method as recited in claim 9, wherein the volume hologram has a defined number of a plurality of holographic functions per layer of the layers.

11. The method as recited in claim 7, wherein holographic materials of the filter device includes polymer-based materials.

12. The method as recited in claim 7, wherein the optical sensor includes a lidar sensor or a time of flight sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic illustration of a wavelength selectivity of reflection holograms featuring different embodiments of the holographic diffraction grating.

(2) FIGS. 2 and 3 show an illustration of an angle and wavelength selectivity of a holographic optical element in reflection with a defined recording geometry.

(3) FIG. 4 shows a basic illustration of a ghost image for a 4-lens lidar lens system.

(4) FIG. 5 shows a plan view of a provided filter device.

(5) FIG. 6 shows a side view of a provided filter device.

(6) FIGS. 7 and 8 show optical functions of a provided filter device.

(7) FIGS. 9 and 10 show diffraction characteristics of the provided filter device for a defined useful wavelength, the reconstruction angle θ being varied in a vertical direction.

(8) FIG. 11 shows a filter effect for a complete field of view and a segmentation of the horizontal field of view due to the diffraction characteristic illustrated in FIG. 10, with an oblique incident radiation on the filter device.

(9) FIGS. 12 and 13 show an angle definition of a hologram recording in a vertical and a horizontal direction.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(10) One particular main idea of the present invention is to provide an anisotropic stray light filter for an optical sensor for monochromatic radiation.

(11) An anisotropic holographic stray light filter is provided. Optical radiation that impinges upon the stray light filter from outside the field of view is largely deflected or deflected to a defined extent or blocked by the stray light filter and is therefore unable to reach a lens system of the optical sensor, and if it does, then only in an advantageously heavily attenuated form.

(12) An optical sensor, as schematically illustrated in FIGS. 5 and 6, includes a detection element 10, which is situated on a first surface 21 of a lens system 20. A provided filter device 30 is situated on a second surface 22 of lens system 20, which in essence completely reflects or diffracts optical radiation impinging upon filter device 30 from a solid angle or field of view 40. FIG. 5 shows the system with the optical sensor and filter device 30 in a plan view, and FIG. 6 shows it in a side view.

(13) It can be seen that filter device 30 has an unsymmetrical field of view FOV which has a different development in a plan view (FIG. 5) and in a side view (FIG. 6). Horizontal field of view FOV.sub.hori has a defined angular range (e.g., of approximately 500 to approximately 120°), which is thus considerably greater than an angular range (e.g., of approximately 9° to approximately 16°) of a vertical field of view FOV.sub.vert.

(14) To prevent stray light from outside the utilized field of view FOV from reaching lens system element 20, an anisotropic holographic filter device 30, i.e. one providing different filtering in horizontal field of view FOV.sub.hori and in vertical field of view FOV.sub.vert, is situated on a surface 22 of lens system element 20. This makes it possible that optical radiation impinging from solid angle 40 does not reach lens system 20. The holographic optical function of filter device 30 used for this purpose is developed in such a way that the angle for which the reflection (realized by diffraction) takes place differs horizontally and vertically. Volume holograms, which are able to achieve a very high diffraction efficiency (theoretically up to 100%), are preferably used for filter device 30. Field of view FOV.sub.hori, FOV.sub.vert is not disturbed by the holographic optical function of filter device 30.

(15) FIGS. 7 and 8 show the optical function or the optical path of anisotropic holographic filter device 30. A deflection of the light outside the vertical field of view FOV.sub.vert requires two optical holographic functions HOE1, HOE2 for filter device 30. They are able to be provided either by holographic multiplexing in a single layer or by producing a layer stack having multiple holographic layers. In the illustrated example of FIG. 7, stray radiation ST above vertical field of view FOV.sub.vert is guided back in a similar direction by holographic function HOE1 of filter device 30. For the angular range underneath vertical field of view FOV.sub.vert, this is accomplished by optical holographic function HOE2 of filter device 30.

(16) The calculated diffraction characteristic of anisotropic holographic filter device 30 is shown using the example of holographic function HOE1 of filter device 30 in FIG. 9. At a horizontal reconstruction or reflection angle of 0°, the figure shows a characteristic of the reconstruction wavelength of the stray radiation in nm across an extension of the vertical reconstruction angle in degrees. An efficiency of the reflection or diffraction due to holographic function HOE1 is sketched using a grey schema. The angles in holographic function HOE1 have been defined in such a way that a stray radiation beam having an angle of incidence of 50° with respect to the normal is diffracted at an angle of 40° with respect to the normal (and vice versa). An exemplary recording wavelength of the radiation of 970 nm was defined for the calculation, the angle of incidence with respect to the normal in the vertical direction and the reconstruction wavelength being varied.

(17) In FIG. 9, a range or band may be seen in which holographic function HOE1 diffracts radiation of a defined wavelength with very high efficiency. The position of the band is predefined by the recording wavelength or the wavelength of the electromagnetic useful radiation. The width of the band depends on the material parameters of the holographic layer (e.g., the layer thickness and diffraction index modulation). It can be seen that holographic function HOE1 is inactive within the vertical field of view FOV.sub.vert between 0° and approximately 9°.

(18) In order to cover the solid angle of the stray radiation both in the vertical and horizontal directions, field of view FOV of filter device 30 is preferably subdivided into a plurality of segments, which is schematically illustrated in FIG. 11. FIG. 11 shows field of view FOV from the front, and regions 40 outside field of view FOV are processed by a total of six holographic functions HOE1.1, HOE1.2, HOE1.3, HOE2.1, HOE2.2 and HOE2.3 (“sub-holograms”).

(19) Each of the mentioned six holographic functions is disposed across the entire surface of filter device 30. It can be seen that the anisotropic filter function of filter device 30 becomes more selective the more holographic functions are developed therein. It has been shown that filter device 30 should have a minimum of four different holographic functions HOE1 . . . HOE4 in order to effectively block optical stray radiation in regions 40 outside field of view FOV.

(20) When the holograms are recorded, a reference wave is brought to interference with an object wave. This is schematically illustrated in the side view of FIG. 12 and the plan view of FIG. 13, where it can be seen that the rays are tilted both in the vertical direction relative to the normal θ.sub.Ref, θ.sub.Obj, and in the horizontal direction relative to the normal φ.sub.Ref, φ.sub.Obj. In an oblique incidence (i.e., φ.sub.reconstruction≠φ.sub.Ref, φ.sub.Obj), the diffraction characteristic of the volume holograms changes.

(21) For a reconstruction or reflection angle φ.sub.reconstruction=φ.sub.Ref+30°, the diffraction characteristic illustrated in FIG. 10 results for vertical field of view FOV.sub.vert. It can be seen that in comparison with the diffraction characteristic shown in FIG. 9, the band shifts to smaller wavelengths at φ.sub.reconstruction=φ.sub.Ref. If the demands on the system are high (e.g., with regard to the temperature range, accuracy, etc.), then the number of sub-holograms of filter device 30 increases. These sub-holograms may either be written in a holographic volume by holographic multiplexing or else by forming a stack of multiple holographic layers that are laminated on top of one another.

(22) Holographic polymer materials or polymer-based materials are preferably used for filter device 30; these have advantageous properties when used in the motor vehicle field because they are very robust with respect to the environmental influences prevailing there (e.g., temperature, humidity fluctuations, etc.).

(23) With the aid of multiplexing, a plurality of optical functions is able to be written in a layer of a holographic material. How many holograms are able to be written in a holographic material depends on the material, but the efficiency of the individual holograms decreases with the number of stored optical functions. For this reason, it may also be provided to store the optical functions in a plurality of holographic layers that are laminated on top of one another. By realizing this stack, higher efficiency is able to be achieved in a single layer in comparison with the multiplex hologram.

(24) In an advantageous manner, the optical sensor provided with the proposed filter device for detecting monochromatic radiation may be developed as a lidar sensor or as a time of flight sensor.

(25) It is of course understood that all previously mentioned numerical values (e.g., in connection with angles, field of view, etc.) are merely of an exemplary nature.

(26) One skilled in the art will understand that a multitude of modifications of the present invention is possible without departing from the core of the invention.