Optical filter

Abstract

Optical Filter This description concerns an optical filter (100) comprising a superposition of: – at least the first (111a) and second (111b) resonant cavities; and – at least one first polarizing filter (113a). Figure for the abbreviation: Fig. 1

Description

Title of the invention: Optical filter technical field

[0001] This description relates generally to optical filters, and in particular to polarizing spectral filters. A polarizing spectral filter is an optical filter adapted to transmit predominantly radiation within at least a certain range of wavelengths and exhibiting at least a certain polarization. Previous technique

[0002] Numerous optical filters, including polarizing spectral filters, have been proposed. However, existing optical filters have various drawbacks.

[0003] International application WO2018 / 070269 describes an optical device for eliminating a decrease in the extinction ratio of transmitted light. However, this optical device only achieves a weak intensity contrast between s-polarized light, which has linear polarization orthogonal to the plane of incidence, and p-polarized light, which has linear polarization parallel to the plane of incidence. Furthermore, the device requires a significant etching depth and the use of different materials.

[0004] US patent US9601532 describes an optical filter comprising a Fabry-Perot type resonator with a plate-shaped metal grid polarizer. However, this filter exhibits low resilience to the angle of incidence of light: the light intensity transmitted by the filter decreases sharply as the angle of incidence increases.

[0005] European patent EP3839454 describes a polarizing spectral filter comprising an array of bars made of materials with different refractive indices interposed between reflectors, each comprising alternating layers of these materials. However, this filter exhibits a narrow transmission band and high sensitivity to variations in wavelength, thickness, and angle of incidence. Summary of the invention

[0006] There is a need to overcome all or part of the drawbacks of existing optical filters, particularly existing polarizing spectral filters. In particular, it would be desirable to improve the resilience of existing filters to the angle of incidence.

[0007] To this end, one embodiment provides an optical filter comprising a superposition of: - at least first and second resonant cavities; and - at least one first polarizing filter.

[0008] According to one embodiment, the optical filter further comprises a second polarizing filter, the first and second polarizing filters being respectively located in the first and second resonant cavities.

[0009] According to one embodiment, the optical filter further comprises a third resonant cavity interposed between the first and second resonant cavities, each first polarizing filter being located in one of the first, second and third resonant cavities.

[0010] According to one embodiment, the optical filter comprises a single first polarizer filter preferably located in the third resonant cavity.

[0011] According to one embodiment, each resonant cavity is interposed between stacks, each comprising an alternation: - of at least two initial layers made of a first insulating material having a first optical index; and - of at least a second layer of a second insulating material having a second optical index strictly greater than the first optical index.

[0012] According to one embodiment, the first and second insulating materials are chosen from: - oxides, for example silicon oxide, titanium oxide, niobium oxide, tantalum oxide, etc.; - nitrides, for example silicon nitride; and - amorphous silicon.

[0013] According to one embodiment, each first layer and each second layer has a quarter-wave thickness.

[0014] According to one embodiment, each resonant cavity has a half-wave thickness.

[0015] According to one embodiment, each polarizing filter comprises an array of alternating parallel bars including: - the first bars made of a third material with a third optical index; and - second bars made of a fourth insulating material having a fourth optical index strictly greater than the third optical index.

[0016] According to one embodiment, the third and fourth materials are respectively identical to the first and second materials.

[0017] According to one embodiment, the third material is a metallic material, for example silver or aluminum.

[0018] According to one embodiment, the bar arrays of the first and second polarizing filters have an identical pitch.

[0019] According to one embodiment, the bar arrays of the first and second polarizing filters have different pitches.

[0020] One embodiment provides an optical filter as described, intended to be placed opposite a pixel matrix of an image sensor, the optical filter being adapted to transmit incident radiation predominantly in a first range of wavelengths and according to a first polarization to certain pixels of the sensor, and predominantly in at least a second range of wavelengths, different from the first range of wavelengths, and / or according to at least a second polarization, different from the first polarization, to other pixels of the sensor.

[0021] One embodiment provides for a multispectral or hyperspectral sensor comprising an image sensor having a pixel matrix opposite which is located an optical filter as described. Brief description of the drawings

[0022] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0023] [Fig.1] is a schematic and partial perspective view of an optical filter according to one embodiment;

[0024] [Fig.2] is a flowchart illustrating steps of a process for designing and optimizing the optical filter of [Fig.1] according to one embodiment;

[0025] [Fig. 3] is a comparative graph illustrating the resilience to the angle of incidence of different optical filters; and

[0026] [Fig.4] is a schematic and partial perspective view of an optical filter according to one embodiment. Description of the implementation methods

[0027] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0028] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the various applications of the optical filters in this description, including the various optical devices that can incorporate these filters, have not been detailed, as the described embodiments are compatible with all or most common optical applications and devices using at least one optical filter, possibly with adaptations that are within the grasp of a person skilled in the art upon reading this description.

[0029] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements coupled together, this means that these two elements can be connected or linked through one or more other elements.

[0030] In the following description, when reference is made to absolute positional qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative positional qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientational qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0031] Unless otherwise specified, the expressions "approximately", "about", "substantially", and "in the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0032] In the following description, the terms "insulating" and "conducting" mean, unless otherwise specified, electrically insulating and electrically conductive respectively.

[0033] Unless otherwise specified, the expression "in contact with" means "in mechanical contact with".

[0034] In the following description, the expression "transmission of a filter" refers to a ratio between the intensity of radiation exiting the filter and the intensity of radiation entering the filter.

[0035] In the following description, the expression "central wavelength of a filter" refers to the wavelength that lies at the center of the transmission wavelength range of the filter under consideration.

[0036] In the following description, the expression "resilience to the angle of incidence of a filter" refers to the ability of the filter to transmit radiation inclined with respect to the optical axis, that is to say inclined with respect to a direction orthogonal to a face of the filter intended to be illuminated by the radiation.

[0037] Fig. 1 is a schematic and partial perspective view of an optical filter 100 according to one embodiment.

[0038] The filter 100 is, for example, intended to be placed opposite a pixel matrix of an image sensor, for example to form a multispectral or hyperspectral sensor. The filter 100 is then, for example, adapted to transmit incident radiation predominantly in a first range of wavelengths and with a first polarization to certain pixels of the sensor, and predominantly in at least a second range of wavelengths, different from the first range of wavelengths, and / or with at least a second polarization, different from the first polarization, to other pixels of the sensor.

[0039] As an alternative, the optical filter 100 is adapted to transmit incident radiation predominantly within a single wavelength range and with a single polarization. Furthermore, the filter 100 can be intended for use with devices other than an image sensor.

[0040] In the example shown, the filter 100 is interposed between an input medium 101, from which radiation illuminates the filter 100, and an output medium 103, towards which the filtered radiation is transmitted. If the filter 100 is intended to be positioned above an image sensor, the input medium 101 is, for example, opposite the pixel array of the image sensor, and the output medium 103 is directed towards the pixel array of the image sensor. The input medium 101 is, for example, air.

[0041] In the example illustrated in [Fig. 1], the optical filter 100 comprises alternating dielectric layers 105 and 107. The layers 105 are made of at least one insulating, or dielectric, material having an optical index, or refractive index, ni. In addition, the layers 107 are made of at least one other insulating material having an optical index n2 strictly greater than the optical index ni.

[0042] By way of example, the materials of layers 105 and 107 are chosen from: - oxides, for example silicon oxide, titanium oxide, niobium oxide, tantalum oxide, etc.; - nitrides, for example silicon nitride; and - amorphous silicon.

[0043] Preferably, the layers 105 are all made of the insulating material with optical index n1 and the layers 107 are all made of the insulating material with optical index n2. This simplifies the design and manufacture of the filter 100. By way of example, the dielectric layers 105 and 107 are made of silicon oxide and amorphous silicon, respectively. In this example, the optical indices n1 and n2 are approximately 1.5 and 3.8, respectively, for radiation with a wavelength of approximately 940 nm.

[0044] In the example shown, the filter 100 more precisely comprises, between the inlet media 101 and the outlet media 103, three stacks 109a, 109b, and 109c of alternating dielectric layers 105 and 107. In this example, the stack 109b is located between the stacks 109a and 109c. The stacks 109a, 109b, and 109c each form, for example, a reflector. The stacks 109a, 109b, and 109c exhibit, for example, a Bragg mirror-type structure.

[0045] Figure 1 illustrates an example in which each stacking 109a, 109c comprises two alternating layers 105 and one alternating layer 107, i.e., one layer 107 interposed between two layers 105. Furthermore, in this example, the stacking 109b comprises three alternating layers 105 and two alternating layers 107, i.e., each layer Layer 107 of stacking 109b is interposed between two adjacent layers 105. This example is not limiting, however, and each stacking 109a, 109b, 109c can, as a variant, include different numbers of layers 105 and 107 than those shown.

[0046] Each dielectric layer 105, 107 has for example a thickness El called "quarter wave", or "X / 4n", that is to say a thickness substantially equal to the central wavelength X of the optical filter 100 divided by four times the optical index ni, n2 of the layer.

[0047] According to one embodiment, the optical filter 100 further comprises resonant cavities 11a and 111b. In the example illustrated in [Fig.1], the resonant cavity 111a is interposed between the stacks 109a and 109b, and the resonant cavity 111b is interposed between the stacks 109b and 109c.

[0048] Each resonant cavity 111a, 111b includes, for example, a polarizing filter 113a, 113b. In the example shown, the resonant cavities 11a and 111b have a thickness greater than or equal to "2*X / 4n", that is, a thickness greater than or equal to twice the central wavelength X of the optical filter 100 divided by four times the optical index ni, n2 of the cavity 111a, 111b. As an alternative, each resonant cavity 111a, 111b has a thickness equal to at least k times the thickness El of the material constituting the dielectric layer 107, with k an integer greater than or equal to two. In the illustrated example, each polarizing filter 113a, 113b extends vertically across the entire thickness of the resonant cavity 111a, 111b and laterally across the entire surface of the resonant cavity 111a, 111b. In this example, each polarizing filter 113a, 113b occupies the entire internal volume of the resonant cavity 111a, 111b.

[0049] In the illustrated example, each polarizing filter 113a, 113b comprises a periodic structure having alternating parallel bars 115 and 117, the bars 115 and 117 extending laterally along a substantially horizontal direction. The bars 115 are, for example, made of an insulating material having a refractive index n3 and the bars 117 are, for example, made of another insulating material having a refractive index n4 strictly greater than the refractive index n3.

[0050] Preferably, the bars 115 are made of the same material as the dielectric layers 105, i.e. the insulating material with optical index n1, and the bars 117 are made of the same material as the dielectric layers 107, i.e. the insulating material with optical index n2. This simplifies the design and construction of the optical filter 100.

[0051] Although [Fig. 1] illustrates an example in which the optical filter 100 comprises two resonant cavities 11a and 11b, this example is not limiting and the optical filter 100 may, as an alternative, comprise a number of resonant cavities strictly greater than two, each resonant cavity being then, for example, interposed between stacks of alternating layers 105 and 107 analogous to the stacks 109a, 109b and 109c. The increase in the number of resonant cavities in the structure leads to a broadening of the range of wavelengths transmitted by the optical filter 100.

[0052] Each resonant cavity 111a, 111b has, for example, a thickness E2 referred to as "half-wave" or "X / 2n", that is, a thickness substantially equal to approximately the central wavelength X of the optical filter 100 divided by twice an effective optical index neff of the resonant cavity. Alternatively, each resonant cavity 111a, 111b has a thickness equal to k times the thickness E2.

[0053] One advantage of the optical filter 100 is that it combines a wavelength interference filtering function with a polarization filtering (polarizer) function without loss of light intensity and while exhibiting significantly greater resilience to the angle of incidence than existing polarizing filters. Furthermore, another advantage of the optical filter 100 is that, when only two materials with different refractive indices, Ni and n2, are used to create the dielectric layers and resonant cavities of the filter structure, manufacturing the optical filter 100 requires etching only one of these two materials. This simplifies the manufacturing of the optical filter 100.

[0054] The [Fig.2] is a flowchart illustrating steps of a process 200 of designing and optimizing the optical filter 100 of the [Fig.1] according to an embodiment.

[0055] In the case where the optical filter 100 is intended to be integrated into a multispectral or hyperspectral sensor, the steps below are, for example, implemented for each part of the filter intended to be placed opposite at least one pixel of an image sensor and adapted to transmit, towards this or these pixels, incident radiation predominantly in a certain wavelength range and with a certain polarization. Alternatively, for example where the filter 100 is intended to perform a global filtering function, the steps below can be implemented only once for the entire filter.

[0056] During a step 201, a central X wavelength is selected. For example, the X wavelength is chosen to be approximately 940 nm.

[0057] In another step 203, subsequent to step 201, a selection of dielectric materials is made. For example, dielectric materials transparent at the central X wavelength and exhibiting the greatest possible contrast in optical indices are chosen in step 203. For simplicity, the case where only two insulating materials with different optical indices—in this instance silicon oxide (SiO2, with optical index n1) and amorphous silicon (aSi, with optical index n2 > n1)—are used to produce the dielectric layers is taken as an example below. 105 and 107 and bars 115 and 117 of polarizing filters 113a and 113b. However, a person skilled in the art is of course able, from the indications in this description, to transpose this example to cases in which more than two different insulating materials are used to manufacture the optical filter 100.

[0058] In a further step 205, subsequent to step 203, a choice of the number of dielectric layers is made. From the wavelength λ and the choice of dielectric materials, a Bragg mirror-type stack is designed, comprising, between the inlet media 101 and the outlet media 103, a multiple N of bilayers (with N a non-zero integer, for example equal to 4) of silicon dioxide and amorphous silicon, plus one layer of silicon dioxide. In other words, this amounts to forming a stack consisting of alternating 2*N layers of material with refractive index n2 and 2*N + 1 layers of material with refractive index ni, the bottom and top layers of the stack being of the material with refractive index ni (i.e., silicon dioxide, in this example).The choice of the number N is, for example, a function of the contrast of optical indices (the difference between ni and n2, in this example): the lower the contrast of optical indices, the larger the number N in order to optimize the rejections of the filter.

[0059] The table below details an example of such a stacking, the dielectric layers being numbered in ascending order from the input medium 101 to the output medium 103. In this table, the thickness of each layer is expressed in multiples of X / 4n and in nanometers (nm), and the letter "n" represents the optical index at the considered wavelength X.

[0060] [Tables 1] Layer No. Material n Thickness (X / 4n) Thickness (nm) 1 SiO2 1.46 1162 2 aSi 3.78 162 3 SiO2 1.46 1162 4 aSi 3.78 162 5 SiO2 1.46 1162 6 aSi 3.78 162 7 SiO2 1.46 1162

[0061] In the example above, the stacking comprises two aSi / SiO2 alternations located on either side of an aSi layer (layer no. 4) of thickness X / 4n.

[0062] During another step 207, subsequent to step 205, the central dielectric layer of the stack (i.e., layer No. 5, in the example above) is replaced by a half-wave resonant cavity. This allows for the creation of a filter transmitting a range of wavelengths centered on wavelength X.

[0063] The table below details the structure obtained at the end of step 207 in the case of the previous example:

[0064] [Tables2] Layer No. Material n Thickness (X / 4n) Thickness (nm) 1 SiO2 1.46 1162 2 aSi 3.78 162 3 SiO2 1.46 1162 4 aSi 3.78 2124 5 SiO2 1.46 1162 6 aSi 3.78 162 7 SiO2 1.46 1162

[0065] In the example above, the stack comprises two aSi / SiO2 alternations located on either side of an aSi layer (layer no. 4) of thickness 2*X / 4n (= X / 2n). Although the example above details a case in which the resonant cavity has a half-wave thickness, this example is not limiting and the cavity may, alternatively, have a thickness equal to an integer multiple greater than two of the half-wave. The choice of the thickness of the resonant cavity is, for example, a function of the contrast of optical indices (the difference between ni and n2, in this example): the lower the contrast, the greater the thickness of the resonant cavity in order to obtain a high transmission contrast between an s polarization, i.e. a linear polarization substantially orthogonal to the plane of incidence, and a p polarization, i.e. a linear polarization substantially parallel to the plane of incidence and substantially orthogonal to the s polarization.

[0066] In another step 209, subsequent to step 207, the resonant cavity, formed by the central layer of the stack, is filled by a network forming a polarizing filter. This allows the s and p polarizations to be separated. By way of example, the network comprises an alternation of parallel bars in the low-index material ni and parallel bars in the high-index material n2 extending in the plane of the resonant cavity orthogonally to the stack.

[0067] The thickness of the central layer, and therefore of the resonant cavity, can be adjusted during step 209 so that, for the desired s or p polarization, the structure exhibits a transmission peak at the central X wavelength chosen in step 201.

[0068] The table below details the structure obtained at the end of step 209 in the case of the previous example, by choosing to transmit the polarization s at the wavelength X:

[0069] [Tables3] Layer No. Material n Thickness (X / 4n) Thickness (nm) 1 SiO2 1.46 1 162 2 aSi 3.78 1 62 3 SiO2 1.46 1 162 4 aSi 3.78 - 156 5 SiO2 1.46 1 162 6 aSi 3.78 1 62 7 SiO2 1.46 1 162

[0070] In the above example, the transmission peak of the p-polarization is obtained at a wavelength of approximately 850 nm.

[0071] The structure obtained at the end of steps 201 to 209 constitutes a so-called "sharp" optical filter, that is to say, one whose transmission decreases sharply as the angle of incidence increases. For example, when the filter is illuminated by radiation with a wavelength X equal to 940 nm, the transmission is, for angles of incidence greater than 10°, less than 20% of the maximum transmission obtained for a zero angle of incidence (radiation directed along the optical axis).

[0072] In a subsequent step 211, after step 209, the previously described stacking is doubled, or duplicated. Step 211a results in an increase in the number of resonant cavities 111a, 111b, thereby widening the maximum transmission window of the filter 100.

[0073] The two stacks are superimposed such that the bottom layer of the upper stack (layer 7, in the example above) coincides with the top layer of the lower stack (layer 1, in this example). Furthermore, the top layer of the upper stack (layer 1, in the example above) and the bottom layer of the lower stack (layer 7, in this example) are, for example, removed. The optical filter 100 previously described in relation to [Fig. 1] is thus obtained.

[0074] The table below details the structure obtained at the end of step 211 in the case of the previous example, by choosing to transmit the polarization s at the wavelength X:

[0075] [Tables4] Layer No. Reference Material Thickness (X / 4n) 1 105 SiO2 1 2 107 aSi 1 3 105 SiO2 1 4 111a aSi / SiO2 2 5 105 SiO2 1 6 107 aSi 1 7 105 SiO2 1 8 107 aSi 1 9 105 SiO2 1 10 111b aSi / SiO2 2 11 105 SiO2 1 12 107 aSi 1 13 105 SiO2 1

[0076] Following steps 201 to 211, a quarter-wave stack comprising two resonant cavities 11la and 111b is obtained. In this stack, the greater the number of bilayers interposed between the resonant cavities 11la and 111b, the narrower the range of wavelengths transmitted by the optical filter 100.

[0077] The polarizing filters 113a and 113b of the resonant cavities 11la and 111b of the structure are adapted to filter the same polarization. In the case where the resonant cavities 11la and 111b each comprise a bar array forming a polarizing filter, the bars of the two arrays are, for example, substantially parallel to each other.

[0078] In an optional step 213, subsequent to step 211, the thicknesses of the layers of the optical filter 100 are optimized, for example using a computer program product comprising program code instructions leading to the implementation of a method for optimizing the filter 100 when executed by a computer.

[0079] By way of example, the thickness of each layer in the optical filter stack 100 constitutes an adjustment parameter. The initial thicknesses taken into account for optimization are, for example, those in Table 4 above (quarter-wavelength layers and half-wavelength resonant cavities). The boundary conditions are set, for example, by: - the central X wavelength of the optical filter 100; - a range of angles of incidence for which it is desired that the optical filter 100 be functional; and - minimum and maximum thicknesses.

[0080] The optimization step 213 advantageously allows for further improvement of the angle resilience of the optical filter 100. As an example, step 213 is carried out using computer-implemented multilayer optical calculation software.

[0081] The table below details an example of optimization of the structure obtained at the end of step 213 in the case of the previous example, by choosing to transmit the polarization s at the wavelength X:

[0082] [Tables5] Layer No. Material Thickness (X / 4n) 1 SiO2 2.254 2 aSi 0.884 3 SiO2 0.664 4 aSi / SiO2 2.407 5 SiO2 0.729 6 aSi 0.895 7 SiO2 0.804 8 aSi 1.407 9 SiO2 0.504 10 aSi / SiO2 2.435 11 SiO2 0.679 12 aSi 0.926 13 SiO2 1.740

[0083] Fig. 3 is a comparative graph illustrating the resilience to the angle of incidence (expressed in degrees, °) of different optical filters.

[0084] Graph 300 comprises three curves 301, 303 and 305 illustrating variations in transmission, expressed as a percentage of the maximum transmission obtained for a zero angle of incidence (radiation directed along the optical axis), as a function of the angle of incidence for, respectively: - an optical filter comprising a single resonant cavity (curve 301), for example the optical filter in table 3; - the unoptimized 100 optical filter from table 4 (curve 303); and - the optimized optical filter 100 from table 5 (curve 305).

[0085] Figure 300 shows that using an optical filter with two resonant cavities, such as optical filter 100, increases angular resilience compared to an optical filter with only one resonant cavity. Furthermore, optimizing the thicknesses of the layers and resonant cavities of optical filter 100 significantly increases angular resilience compared to the unoptimized optical filter 100.

[0086] The influence of different structural parameters on the performance of optical filter 100 is detailed below.

[0087] The following description takes as an example the case where each polarizing filter 113a, 113b of the optical filter 100 comprises parallel bars 115 of the low index material ni and parallel bars 117 of the high index material n2 totally filling all the free spaces extending laterally between the bars 115. As an example, the bars 115 each have a width L1 and the bars 117 each have a width L2.

[0088] In the following description, the term "form factor" refers to a ratio between the width L1 and the sum of the widths L1 and L2 (F = L1 / (L1 + L2), where F denotes the form factor of the bar array 115 and 117). Furthermore, the term "array period" denotes the sum of the widths L1 and L2 (T = L1 + L2, where T denotes the array period).

[0089] The resilience to the angle of incidence of the optical filter 100 is not modified, or is very slightly modified, as a function of the form factor F for form factor values ​​F ranging from 0.1 to 0.9.

[0090] Furthermore, in a case where the polarizing filters 113a and 113b of the optical filter 100 have a substantially identical structure and dimensions, apart from manufacturing variations, the filter's transmission is not affected by a lateral offset of one polarizing filter relative to the other. In other words, the fact that the bars 115 and 117 of one of the polarizing filters 113a, 113b are not directly above the bars 115 and 117 of the other polarizing filter 113b, 113a does not impact the transmission of the optical filter 100. One advantage is that this simplifies the manufacturing of the optical filter 100, for example, by relaxing the constraints on the alignment of the etching masks used to produce the polarizing filters 113a and 113b.

[0091] With a constant physical, or geometric, thickness of the optical filter 100, the choice of the form factor F influences the position of the respective transmission wavelength ranges of the s and p polarizations. Furthermore, with a constant optical thickness of the optical filter 100, the choice of the form factor F influences the position of the transmission wavelength range of the p polarization without altering, or only slightly altering, the position of the transmission wavelength range of the s polarization. In the case of a multispectral or hyperspectral sensor comprising The optical filter 100, by modifying the aspect ratio (F) in front of different pixels of the image sensor, allows the transmission of radiation in different wavelength ranges to these pixels while maintaining a constant filter thickness. This simplifies the manufacturing of the optical filter 100.

[0092] Another way to modify the transmission wavelength range of the optical filter 100 is to adjust the overall thickness of the layer stack and / or the thickness of the resonant cavities 11la and 111b. The thicker the resonant cavities 11la and 111b are, the more the transmission wavelength range of the optical filter 100 is shifted towards higher wavelengths. Furthermore, the thicker the resonant cavities 11la and 111b are, the further apart the transmission wavelength ranges of the s and p polarizations become. Thus, increasing the thickness of the resonant cavities 11la and 111b, for example, makes it possible to move the transmission wavelength ranges of the s and p polarizations further apart in a case where the contrast between the optical indices n1 and n2 is low.

[0093] The period T of the bar array of each polarizing filter allows for adjustment of the position of the transmission wavelength range of the s-polarization relative to the transmission wavelength range of the p-polarization. In particular, increasing the period T tends to bring these ranges closer together. Furthermore, as the period T of the array increases, the width of the transmission wavelength range of the s-polarization remains substantially constant, while the width of the transmission wavelength range of the p-polarization decreases. The aspect ratio F has a similar influence on the transmission wavelength ranges of the s- and p-polarizations to that of the period T of the array.

[0094] Furthermore, it is possible to provide that the polarizing filter 113a has a different form factor F than the polarizing filter 113b, for example, a form factor Fa of about 0.45 for the polarizing filter 113a and a form factor Fb of about 0.9 for the polarizing filter 113b. This makes it possible to reduce the transmission of p-polarization without reducing the transmission of s-polarization.

[0095] Moreover, the greater the difference between the indices ni and n2, the further the transmission wavelength range of the s polarization is from the transmission wavelength range of the p polarization.

[0096] By way of example, the optical filter 100 comprises groups of four regions adapted to filter radiation according to four different polarization orientations, for example, linear polarizations along four directions forming angles of 0°, 90°, 45°, and 135° with respect to a reference direction. Each group of four regions is, for example, intended to be placed opposite a group of four adjacent pixels of an image sensor. In this example, the bars 115 and 117 of the polarizing filters 113a and 113b of the four regions of the optical filter 100 extend laterally in four directions respectively, forming angles of 0°, 90°, 45° and 135° with respect to the reference direction.

[0097] Fig. 4 is a schematic and partial perspective view of an optical filter 400 according to one embodiment.

[0098] Similar to filter 100, filter 400 is, for example, intended to be placed opposite a pixel array of an image sensor, for example to form a multispectral or hyperspectral sensor. As an alternative, optical filter 400 is adapted to transmit incident radiation predominantly within a single wavelength range and with a single polarization.

[0099] In the example illustrated in [Fig. 4], the optical filter 400 comprises alternating dielectric layers 405 and 407. The layers 405 are made of at least one insulating, or dielectric, material having an optical index, or refractive index, n5. In addition, the layers 407 are made of at least one other insulating material having an optical index n6 strictly greater than the optical index n5.

[0100] By way of example, the materials of layers 405 and 407 are chosen from: - oxides, for example silicon oxide, titanium oxide, niobium oxide, tantalum oxide, etc.; - nitrides, for example silicon nitride; and - amorphous silicon.

[0101] Preferably, the layers 405 are made of the same insulating material with optical index n5 and the layers 407 are made of the same insulating material with optical index n6. This simplifies the design and manufacture of the filter 400. By way of example, the dielectric layers 405 and 407 are made of silicon oxide and silicon nitride, respectively. In this example, the optical indices n5 and n6 are approximately 1.5 and 2.02, respectively, for radiation with a wavelength of approximately 550 nm.

[0102] In the example shown, the filter 400 more precisely comprises, between the inlet media 101 and outlet media 103, four stacks 409a, 409b, 409c and 409d of alternating dielectric layers 405 and 407. In this example, stack 409b is located between stacks 409a and 409c and stack 409c is located between stacks 409b and 409d. Stacks 409a, 409b, 409c and 409d each form, for example, a reflector.

[0103] Figure 4 illustrates an example in which each stacking 409a, 409b, 409c, 409d comprises two alternating layers 405 and one layer 407, i.e., a layer 407 interposed between two layers 405. This example is not, however, limiting and Each stacking 409a, 409b, 409c, 409d may, as a variant, include different numbers of 405 and 407 layers than those shown.

[0104] Each dielectric layer 405, 407 has for example a thickness E3 called "quarter wave", or "X / 4n", that is to say a thickness substantially equal to the central wavelength X of the optical filter 400 divided by four times the optical index n5, n6 of the layer.

[0105] According to one embodiment, the optical filter 400 further comprises resonant cavities 411a, 411b and 411c. In the example illustrated in [Fig.4], the resonant cavity 41la is interposed between the stacks 409a and 409b, the resonant cavity 411b is interposed between the stacks 409b and 409c, and the resonant cavity 41le is interposed between the stacks 409c and 409d.

[0106] Each resonant cavity 41la, 41le is, for example, made of a layer of the material with optical index n6 having a thickness equal to at least k times twice the thickness E3 of the layers 407. In addition, the resonant cavity 411b includes, for example, a polarizing filter 413. In the illustrated example, the polarizing filter 413 extends vertically over the entire thickness of the resonant cavity 411b and laterally over the entire surface of the resonant cavity 411b. In this example, the polarizing filter 413 occupies the entire internal volume of the resonant cavity 411b. Although [Fig.4] illustrates an example in which the polarizer filter 413 is integrated into the resonant cavity 411b, this example is not limiting and the polarizer filter 413 can, as an alternative, be integrated into the resonant cavity 411a or into the resonant cavity 411c.

[0107] In the illustrated example, the polarizing filter 413 comprises a periodic structure having alternating parallel bars 415 and 417, the bars 415 and 417 extending laterally along a substantially horizontal direction. The bars 415 are, for example, made of a metallic material having a refractive index n7, and the bars 417 are, for example, made of an insulating material having a refractive index n8 strictly greater than the refractive index n7. By way of example, the bars 415 are made of a metal, for example aluminum, silver, etc., or of a metallic alloy. The bars 417 are, for example, made of one of the materials listed above for layers 405 and 407.

[0108] By way of example, bars 415 are made of aluminium and bars 417 are made of silicon oxide.

[0109] Although [Fig.4] illustrates an example in which the optical filter 400 comprises two resonant cavities 411a and 411c without a polarizing filter and a single resonant cavity 411b comprising a polarizing filter, this example is not limiting and the optical filter 400 may, alternatively, comprise numbers of cavities resonant cavities different from those shown, it being understood that at least one of the resonant cavities includes a polarizing filter.

[0110] Each resonant cavity 41la, 411b, 41le has, for example, a thickness E4 referred to as "half-wave" or "X / 2n", that is, a thickness substantially equal to approximately the central wavelength X of the optical filter 400 divided by twice an effective optical index neff of the resonant cavity. As an alternative, each resonant cavity 41la, 411b, 41le has a thickness equal to k times the thickness E4.

[0111] One advantage of the 400 optical filter is that it combines a wavelength interference filtering function with a polarization filtering function (polarizer) without loss of light intensity and while exhibiting resilience to the angle of incidence much higher than that of existing filters.

[0112] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0113] In particular, a person skilled in the art is able to foresee, from the indications in this description, to make the bars 115 of the optical filter 100 out of a metallic material, for example a material similar to that of the bars 415 of the optical filter 400. Furthermore, a person skilled in the art is able to foresee, from the indications in this description, to make the bars 415 of the optical filter 400 out of a dielectric material, for example a material similar to that of the bars 115 of the optical filter 100.

[0114] Finally, the practical implementation of the described embodiments and variants is within the reach of a person skilled in the art, based on the functional indications given above. In particular, the described embodiments are not limited to the specific examples of materials and dimensions mentioned in this description.

Claims

Demands

1. Optical filter (100; 400) comprising a superposition of: - at least the first (11a; 41a) and second (111b; 41le) resonant cavities; and - at least a first polarizing filter (113a; 413).

2. Optical filter (100) according to claim 1, further comprising a second polarizing filter (113b), the first (113a) and second (113b) polarizing filters being respectively located in the first (11a) and second (111b) resonant cavities.

3. Optical filter (400) according to claim 1, further comprising a third resonant cavity (411b) interposed between the first (41la) and second (41le) resonant cavities, each first polarizer filter (413) being located in one of the first (411a), second (41le) and third (411b) resonant cavities.

4. Optical filter (400) according to claim 3, comprising a single first polarizing filter (413) preferably located in the third resonant cavity (411b).

5. Optical filter (100; 400) according to any one of claims 1 to 4, wherein each resonant cavity (111a, 111b; 411a, 411b, 411c) is interposed between stacks (109a, 109b, 109c; 409a, 409b, 409c, 409d) each comprising an alternation of: - at least two first layers (105; 405) of a first insulating material having a first optical index; and - at least one second layer (107; 407) of a second insulating material having a second optical index strictly greater than the first optical index.

6. Optical filter (100; 400) according to claim 5, wherein the first and second insulating materials are selected from: - oxides, for example silicon oxide, titanium oxide, niobium oxide, tantalum oxide, etc.; - nitrides, for example silicon nitride; and - amorphous silicon.

7. Optical filter (100; 400) according to claim 5 or 6, wherein each first layer (105; 405) and each second layer (107; 407) has a quarter-wave thickness.

8. Optical filter (100; 400) according to any one of claims 1 to 7, wherein each resonant cavity (11la, 111b; 41la, 411b, 41le) has a half-wave thickness.

9. Optical filter (100; 400) according to any one of claims 1 to 8, wherein each polarizing filter (113a, 113b; 413) comprises an array of alternating parallel bars having: - first bars (115; 415) of a third material having a third optical index; and - second bars (117; 417) of a fourth insulating material having a fourth optical index strictly greater than the third optical index.

10. Optical filter (100; 400) according to claim 9, in its dependence on claim 5, wherein the third and fourth materials are respectively identical to the first and second materials.

11. Optical filter (100; 400) according to claim 9, wherein the third material is a metallic material, for example silver or aluminium.

12. Optical filter (100; 400) according to claim 9, in its dependence on claim 2, wherein the bar arrays of the first (113a, 113b) and second polarizing filters have an identical pitch.

13. Optical filter (100; 400) according to claim 9, in its dependence on claim 2, wherein the bar arrays of the first (113a, 113b) and second polarizing filters have different pitches.

14. Optical filter (100; 400) according to any one of claims 1 to 13, intended to be placed opposite a pixel array of an image sensor, the optical filter (100; 400) being adapted to transmit incident radiation predominantly in a first range of wavelengths and according to a first polarization to certain pixels of the sensor, and predominantly in at least a second range of wavelengths, different from the first range of wavelengths, and / or according to at least a second polarization, different from the first polarization, to other pixels of the sensor.

15. Multispectral or hyperspectral sensor comprising an image sensor having a pixel array opposite which is located an optical filter (100; 400) according to any one of claims 1 to 14.

Images