AN INTERFERENCE FILTER, OPTICAL DEVICE AND METHOD OF MANUFACTURING AN INTERFERENCE FILTER

Abstract

An interference filter comprises a substrate, a filter stack and at least one absorption layer. The filter stack comprises alternating layers of optical coatings with different refractive indices arranged on the substrate. The at least one absorption layer is comprised of an optically absorbing material which is arranged on the substrate.

Claims

1. An interference filter, comprising: a substrate, a filter stack comprising alternating layers of optical coatings with different refractive indices arranged on the substrate, and at least one absorption layer of an optically absorbing material arranged on the substrate.

2. The interference filter according to claim 1, wherein the at least one absorption layer is arranged on a surface of the filter stack, and/or the at least one absorption layer is arranged between layers of the filter stack.

3. The interference filter according to claim 1, wherein the layers of optical coatings are characterized by an overall transmission spectrum and an overall reflection spectrum, the overall transmission spectrum and the overall reflection spectrum are coupled such that a course of the overall transmission spectrum depends on a course of the overall reflection spectrum and vice versa, and the at least one absorption layer has an absorption spectrum which is arranged to decouple the overall transmission spectrum and the overall reflection spectrum.

4. The interference filter according to claim 3, wherein the overall transmission spectrum and the overall reflection spectrum are complementary such that reflection and transmission are inversely entangled.

5. The interference filter according to claim 3, wherein the overall transmission spectrum and the overall reflection spectrum are arranged to give the interference filter a long-pass spectrum having at least one spectral cut-on wavelength, or the overall transmission spectrum and the overall reflection spectrum are arranged to give the interference filter a short-pass spectrum having at least one spectral cut-off wavelength, and at least one absorption peak of the absorption spectrum of the at least one absorption layer matches said cut-off or cut-on wavelength.

6. The interference filter according to claim 3, wherein the overall transmission spectrum and the overall reflection spectrum are arranged to give the interference filter a band-pass filter spectrum having at least one spectral cut-off or more spectral cut-offs, and at least one absorption peak of the absorption spectrum of the at least one absorption layer matches said one or more cut-offs and/or one or more cut-ons.

7. The interference filter according to claim 1, wherein the optically absorbing material comprises an Indium tin oxide compound, denoted ITO, comprising a ternary composition of indium, tin and oxygen in a given relative proportion.

8. The interference filter according to claim 1, wherein the substrate comprises a glass, plastic or film substrate.

9. The interference filter according to claim 1, wherein the substrate has an absorption spectrum different from the optically absorbing material of the at least one absorption layer (AL).

10. The interference filter according to claim 8, wherein the substrate is a single colored glass, and the single colored glass has a cut-off in the infrared, IR.

11. The interference filter according to claim 1, wherein the substrate facing the filter stack has a finite distance to the at least one absorption layer.

12. An optical device comprising an optical sensor, and at least one interference filter according to claim 1.

13. The optical device according to claim 12, wherein the optical sensor comprises an image sensor.

14. A method of manufacturing an interference filter, comprising: providing a substrate, arranging a filter stack on the substrate, the filter stack comprising alternating layers of optical coatings with different refractive indices arranged, and arranging at least one absorption layer of an optically absorbing material on the substrate.

15. The method according to claim 14, wherein the at least one absorption layer is arranged on a surface of the filter stack, and/or the at least one absorption layer is arranged between layers of the filter stack.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In the following, the concept presented above is described in further detail with respect to drawings, in which examples of embodiments are presented. In the embodiments and Figures presented hereinafter, similar or identical elements may each be provided with the same reference numerals. The elements illustrated in the drawings and their size relationships among one another, however, should not be regarded as true to scale, rather individual elements, such as layers, components, and regions, may be exaggerated to enable better illustration or a better understanding.

[0041] FIGS. 1A to 1C show back-reflection for an example hybrid interference filter,

[0042] FIGS. 2A to 2B show back-reflection under varying angle of incidence for an example hybrid interference filter,

[0043] FIGS. 3A to 3D show further example embodiments of an interference filter,

[0044] FIG. 4 shows an optical sensor arrangement with an interference filter from the prior art,

[0045] FIGS. 5A to 5C show shows an example of back-reflection for a prior art interference filter at normal incidence, and

[0046] FIGS. 6A to 6B show an example of back-reflection in a prior art interference filter at various angles of incidence.

DETAILED DESCRIPTION

[0047] FIGS. 1A to 1C show back-reflection in an example interference filter. FIG. 1A depicts a dichroic filter comprising a substrate SB and a filter stack FS of alternating layers of optical coatings with different refractive indices arranged in a distance to an optical sensor OS, e.g. an image sensor. Furthermore, an absorption layer AL of an optically absorbing material is arranged on the substrate.

[0048] The filter stack FS in this example is configured as a long-pass filter which has a cut-on wavelength λ.sub.cut-on at 412 nm. This cut-on is the result of a dedicated filter design, e.g. based on simulation of the alternating layers of optical coatings with different refractive indices and material properties.

[0049] The absorption layer AL is coated on a surface of the filter stack FS. This surface faces the substrate SB, and, thus, the optical sensor OS. The optically absorbing material comprises an indium tin oxide compound, denoted as ITO. The ITO compound comprises a ternary composition of indium, tin and oxygen in a given relative proportion. ITO is a high index of refraction material and has absorption in the ultraviolet, UV. Furthermore, as can be seen from FIG. 1B the ITO compound has an absorption peak close to the cut-on wavelength λ.sub.cut-on at 412 nm. Thus, the cut-on wavelength and the absorption peak can be considered to match.

[0050] The substrate SB is arranged as an absorption filter. In this embodiment the substrate is colored glass which comprises optically absorbing material. The colored glass in this example cuts in the IR. For example, the absorption layer AL and filter stack FS are coated onto the glass using multilayer coatings. This way the colored glass contributes to the overall spectral properties of the interference filter IF, which can be considered a hybrid filter.

[0051] For this filter design only one type of substrate, e.g. colored glass needs to be used which reduces overall cost of the filter. The multilayer coatings of the filter stack acts both as an anti-reflection coating, ARC, and as a UV cut-off.

[0052] Light is incident with an intensity denoted I.sub.in. The incident light is partly reflected at the filter stack FS. The reflected light has an intensity of I.sub.r=R.Math.I.sub.in, wherein R denotes a reflection coefficient, or relative reflection contribution. Some light, however, eventually gets transmitted through the filter stack FS, according to its transmission spectrum. The transmitted intensity is given as I.sub.out=T.Math.I.sub.in, wherein T denotes a transmission coefficient, or relative transmission contribution. The transmitted light may then be reflected at the optical sensor OS, e.g. its substrate, which is given as T.Math.R.sub.sub.Math.I.sub.in, wherein R.sub.sub denotes a reflection coefficient, or relative reflection contribution of the optical sensor. Some of this reflected light may be transmitted back into the filter stack, and be subject to additional reflection (see doted arrow in the drawing). Moreover, there may be another reflection at the filter stack FS. This back-reflected light has an intensity of I.sub.br=BR.Math.t.Math.R.sub.sub.Math.I.sub.in, wherein BR denotes a back-reflection coefficient, or relative back-reflection contribution. However, due to the absorption layer AL the back-reflection coefficient BR is also depending on the absorption in the absorption layer AL. As a consequence, the intensity I.sub.br of the back-reflected light is reduced compared to the situation where no absorption layer AL is present.

[0053] FIG. 1B shows transmission and reflection contributions for the interference filter of FIG. 1A. All graphs assume normal incidence. Furthermore, the substrate reflection is assumed to be 100%. The graphs g1 and g2 show the transmission and reflection coefficient introduced above as functions of wavelength, respectively. Graph g3 indicates absorption of the filter stack as a function of wavelength. Finally, graph g4 shows the back-reflection coefficient as a function of wavelength.

[0054] The graphs g1 and g2 intersect at the cut-on wavelength. However, it is quite apparent that these graphs are decoupled in the sense that they are no longer inverse for wavelengths larger than the cut-on, as was the case in prior art filters without the absorption layer AL. Furthermore, graph g3 shows a peak which is matched or close to the cut-on wavelength. As a consequence absorption counteracts the transmission and reflection contributions and leads to an overall intensity reduction of back-reflection.

[0055] FIG. 1C shows transmission and back-reflection contributions for the interference filter of FIG. 1A. This graph represents a product of back-reflection coefficient BR and transmission coefficient T as functions of wavelength. The peak indicates that the back-reflected intensity is ˜5% of the incoming light at the cut-on wavelength. Thus, T and R can be decoupled from each other by introducing the absorption layer and optically absorbing material arranged therein. By shifting the BR to shorter wavelength and lower levels the back-reflected intensity can be reduced by a factor of ˜5 compared to an all dielectric filter, e.g. as shown FIGS. 5A to 5C.

[0056] FIGS. 2A and 2B show T, R, and T*BR under varying angle of incidence for an example hybrid interference filter. In fact, the drawings correspond to those in FIGS. 1B and 1C, respectively. The graphs have been established based on the same interference filter as in FIG. 1A. However, the same graphs g1 to g4 are modeled for different angles of incidence, AOI, i.e. 0°, 15°, 30°, and 40°, and are indexed with those angles for easy reference. The situation is similar to normal incidence, i.e. for higher AOIs the maximum back-reflected intensity essentially remains at the low level of about 5%. There is only low spectral broadening.

[0057] FIGS. 3A to 3D show further example embodiments of an interference filter. The proposed dichroic filters are based on the one shown in the previous Figures. The filters comprise a substrate SB and a filter stack FS of alternating layers of optical coatings with different refractive indices which are to be arranged in a distance to an optical sensor OS, e.g. an image sensor.

[0058] FIG. 3A depicts a dichroic filter based on the one shown in FIG. 1A. The filter comprises a substrate SB and a filter stack FS of alternating layers of optical coatings with different refractive indices arranged in a distance to an optical sensor OS, e.g. an image sensor. Furthermore, an absorption layer AL of an optically absorbing material is arranged on the substrate. The absorption layer AL is coated on a surface of the filter stack FS. This surface faces the substrate SB. As will be shown below, the absorption layer AL can be anywhere in the filter stack, i.e. on a top surface, bottom surface or in-between. In this embodiment, the filter stack FS is arranged on a bottom surface of the substrate, i.e. on a surface facing the optical sensor OS. The layers are not to scale in order to emphasize the relative position of the absorption layer in the filter stack.

[0059] As discussed above, light may be incident with an intensity denoted I.sub.in. The incident light is partly reflected at the filter stack FS. The reflected light has an intensity of I.sub.r=R.Math.I.sub.in, wherein R denotes a reflection coefficient, or relative reflection contribution. Some light, however, eventually gets transmitted through the filter stack FS, according to its transmission spectrum. The transmitted intensity is given as I.sub.out=t.Math.I.sub.in, wherein T denotes a transmission coefficient, or relative transmission contribution. The transmitted light may then be reflected at the optical sensor OS, e.g. its substrate, which is given as T.Math.R.sub.sub.Math.I.sub.in, wherein R.sub.sub denotes a reflection coefficient, or relative reflection contribution of the optical sensor. Some of this reflected light may be transmitted back into the filter stack, and be subject to additional reflection (see doted arrow in the drawing). Moreover, there may be another reflection at the filter stack FS. This back-reflected light has an intensity of I.sub.br=BR.Math.T.Math.R.sub.sub.Math.I.sub.in, wherein BR denotes a back-reflection coefficient, or relative back-reflection contribution. However, due to the absorption layer AL the back-reflection coefficient BR is also depending on the absorption in the absorption layer AL. As a consequence, the intensity I.sub.br of the back-reflected light is reduced compared to the situation where no absorption layer AL is present.

[0060] Furthermore, in FIG. 3B one absorption layer AL of an optically absorbing material is arranged between layers of the filter stack FS, i.e. the alternating layers of optical coatings with different refractive indices. In FIG. 3C there are several absorption layers AL arranged between layers of the filter stack. In FIG. 3D the absorption layer AL is on an outer surface (top or bottom) of the filter stack FS. In fact, one or absorption layers AL can be placed anywhere in the filter stack.

[0061] The absorption layers (one or more) are associated with the filter stack in the sense that the absorption layers can be considered an integral part of the filter stack, e.g. as one of the alternating layers or an additional layer of the filter stack. In other words, the alternating layers of optical coatings and the absorption layer make up the interference filter. In terms of filter design the overall transmission spectrum and the overall reflection spectrum are no longer coupled or entangled in the sense that they inverse functions. Rather the contribution of the one or more absorption layers need to be accounted for. Transmission, reflection and absorption need to meet the general requirement that the product of transmission, reflection and absorption equals 1.

[0062] Including the absorption layer(s) as integral part of the filter stack allows to avoid the need for these additional absorbing materials by carefully selecting the optical constants of the materials in the interference filter itself. In other words, the interference filter multilayer structure no longer only relies on the typical T and R, but incorporates materials with an absorbing function. The layers not only interfere constructively and destructively, they also absorb at specific wavelengths according to the intended application. This is quite a different approach compared to prior art, where typically optical filter design for interference filters targets using materials with no absorption. However, here the argument is put forward that for applications, such as image sensors, some absorption in the interference filter can be advantageous, e.g. in reducing back reflection. The proposed concept simplifies the overall design by avoiding the need for additional absorbing materials, for example colored glass, colored resins or dyes.