Resolution target for infrared imaging

Abstract

The invention relates to a resolution target for infrared imaging, comprising a plurality of patterns, resting on the front face of a support (22), spaced laterally two by two in a main plane, intended to form dark patterns on a light background when the resolution target is illuminated by the infrared beam. Each pattern is formed of a Fabry-Pérot type resonant cavity (CR) comprising a lower reflector (23) and an upper reflector (25) between which an intermediate layer (24) is located; the lower reflector (23), located on the side of the support (22), being less reflective than the upper reflector (25). The intermediate layer (24) extends continuously from one resonant cavity (CR) to the other, and therefore also in an intermediate zone (ZI) surrounding the resonant cavities in the main plane. Figure for abstract: Fig.1A

Description

Title of the invention: Resolution target for infrared imaging Technical field

[0001] The field of the invention is that of resolution targets for imaging, in particular for infrared imaging. STATE OF THE PRIOR ART

[0002] Resolution targets are devices for evaluating the performance of an imaging system, such as the spatial resolution of the matrix photodetector, and possibly the geometric distortions of the associated optical system, in particular for the purpose of carrying out a calibration.

[0003] Spatial resolution expresses the ability of an imaging system to distinguish details. Furthermore, distortions are geometric aberrations, induced by the optical system associated with the matrix photodetector, which can be of different types, for example radial, prismatic, decentering distortions, etc.

[0004] A resolution target is usually formed of several patterns, for example dark patterns on a light background or light patterns on a dark background, arranged according to a predefined spatial frequency. The patterns can be lines of different widths, points of different diameters and / or different shapes (circular, square, etc.), or others, usually arranged in a regular manner. As an example, we can cite the resolution target known by the code "1951 USAF".

[0005] For example, such a resolution target can be formed from a transparent support coated with an opaque layer. This layer has through openings which define the patterns. Light patterns are then obtained on a dark background. Alternatively, the resolution target can be formed from a transparent support on which opaque metal studs form the patterns: here, dark patterns are obtained on a light background.

[0006] In a method of using the resolution target, the resolution target is illuminated and one or more images are acquired by the imaging system. The spatial resolution of the matrix photodetector can then be determined. In the case where several images are acquired, the resolution target can be moved in rotation and / or translation in the field of view of the imaging system, which then makes it possible to determine any geometric distortions.

[0007] However, there is a need to have a resolution target particularly suited to infrared imaging, in particular in the mid-infrared, the spectral band of which extends between approximately 5 and 100 μm. Statement of the invention

[0008] The invention aims to propose a resolution target for infrared imaging, in particular in the mid-infrared. For this, the subject of the invention is a resolution target for infrared imaging, intended to be illuminated on the rear face by an infrared beam, comprising: • a support transparent to the infrared beam, having a rear face intended to receive the infrared beam, and an opposite front face; • a plurality of patterns, resting on the front face of the support, spaced laterally two by two in a main plane, intended to form dark patterns on a light background when the resolution target is illuminated by the infrared beam.

[0009] According to the invention, each pattern is formed from a Fabry-Pérot type resonant cavity comprising a lower reflector and an upper reflector between which an interlayer is located; the lower reflector, located on the support side, being less reflective than the upper reflector.

[0010] Furthermore, the interlayer extends continuously from one resonant cavity to the other, and therefore also in an intermediate zone surrounding the resonant cavities in the main plane.

[0011] Some preferred but non-limiting aspects of this resolution target are as follows.

[0012] The interlayer may have a thickness in the resonant cavities between, and different from, Xc / (4nci) and Xc / (2nci), where Xc is the wavelength of the infrared beam and nci is the optical index of the interlayer.

[0013] The interlayer may have a thickness, in the intermediate zone, greater than or equal to Xc / (4nci) and less than Xc / (2nci).

[0014] The interlayer may have an absorption rate of less than 10% at the wavelength Xc of the infrared beam. The lower and upper reflectors may have non-zero absorption.

[0015] The interlayer may have an optical index equal to that of the support.

[0016] The lower reflectors may be made of a metal and have a thickness less than their skin thickness.

[0017] The upper reflectors may be made of a metal and may have a thickness greater than their skin thickness, or may each be formed by a Bragg mirror.

[0018] The upper reflectors may be regions of the same continuous metal layer. This is particularly the case when the upper reflectors are Bragg mirrors.

[0019] The upper reflectors may be separate pads, spaced two by two in the main plane.

[0020] The target may include an anti-reflection layer covering the resonant cavities and the intermediate zone.

[0021] The interlayer may have a thickness in the intermediate zone (ZI) equal to Xc / (4nci).

[0022] The interlayer may have a first thickness for a first group of adjacent resonant cavities, and a second thickness, different from the first thickness, for a second group of adjacent resonant cavities.

[0023] The invention also relates to a calibration system comprising: an optical source adapted to emit an infrared beam of wavelength Xc; the resolution target according to any one of the preceding characteristics, adapted to receive the infrared beam via a rear face of the support opposite the patterns; and an imaging system, adapted to receive the infrared beam and to acquire an image of the resolution target.

[0024] Furthermore, the invention also relates to a resolution target for infrared imaging, intended to be illuminated on the rear face by an infrared beam, comprising: a support transparent to the infrared beam; a plurality of patterns, resting on the support and joined two by two in a main plane, intended to form more or less dark patterns when the resolution target is illuminated by the infrared beam; each pattern being formed of a Fabry-Pérot type resonant cavity comprising a lower reflector and an upper reflector between which an interlayer is located; the lower reflector, located on the side of the transparent support, being less reflective than the upper reflector; the interlayer extends continuously from one resonant cavity to the other, and has a different thickness between any two adjacent resonant cavities. Brief description of the drawings

[0025] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0026] [Fig.1A] is a schematic and partial view, in cross-section, of a resolution target according to one embodiment, here in transmission;

[0027] [Fig.lB] illustrates the spectral response of a test pattern of resolution similar to that of [Fig.lA];

[0028] [Fig.2A] is a schematic and partial view, in cross-section, of a resolution target according to another embodiment, here in reflection;

[0029] [Fig.2B] illustrates the spectral response of a test pattern of resolution similar to that of [Fig.2A];

[0030] [Fig.3A] is a schematic and partial view, in cross-section, of a resolution target according to an alternative embodiment;

[0031] [Fig.3B] is a schematic and partial view, in cross-section, of a resolution target according to another variant embodiment;

[0032] [Fig.3C] is a schematic and partial view, in cross-section, of a resolution target according to another variant embodiment.

[0033] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0034] In the figures and in the remainder of the description, the same references represent identical or similar elements. Furthermore, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the limits are included, unless otherwise indicated.

[0035] The invention relates to a resolution target, which can also be called a calibration target (the terms calibration, calibration and calibration are considered here as being synonymous). As indicated previously, such a resolution target makes it possible to evaluate the performance, in terms of spatial resolution, of an imaging system, and possibly, in the case where the imaging system comprises an optical system associated with the matrix photodetector, to determine any geometric distortions. The resolution target is suitable for infrared imaging, and in particular for mid-infrared MWIR (for Middle Wave InfraRed, in English) ranging from approximately 5 to 10 μm.

[0036] Generally speaking, the resolution target comprises, when illuminated by a predefined infrared beam, dark patterns on a light background. The dark patterns are formed by resonant cavities, called asymmetric, which absorb at least partially, or even totally, the incident infrared beam, and the light background is formed by an intermediate zone which surrounds the resonant cavities in the main plane of the resolution target. Each resonant cavity is formed by a stack of two reflectors between which is located an interlayer, which extends continuously from one resonant cavity to the other and therefore also extends into the intermediate zone. Thanks to this structural configuration and the use of asymmetric resonant cavities, the resolution target has dark patterns without them being obtained by opaque metal pads or by the use of a semiconductor material which strongly absorbs in the infrared.In addition, it has a low topology in the main plane (low thickness variation), thus limiting diffractive effects at the edge of the resonant cavities, which helps preserve optical performance.

[0037] [Fig.1A] is a schematic and partial view, in cross-section, of a resolution target 20 according to one embodiment, here in transmission, within the framework of a calibration system 1.

[0038] Here and for the remainder of the description, a direct three-dimensional orthogonal reference frame XYZ is defined, where the X and Y axes form a main plane of the resolution target 20, and where the Z axis is oriented along the illumination direction. In the remainder of the description, the terms “lower” and “upper” are understood as relating to an increasing positioning when moving away from the transparent support 22 along the +Z direction.

[0039] The optical source 10 is adapted to emit a predefined infrared beam, here called the calibration infrared beam, so as to illuminate all the resonant cavities CR (patterns) of the resolution target 20. The calibration infrared beam has a spectral band AXC centered on the wavelength Xc. Preferably, the spectral band AXC belongs to the mid-infrared and is here between 5 and 10 pm. The wavelength Xc is, for example, equal to 7 or 8 pm. The infrared beam may be monochromatic or not, coherent (spatially and temporally coherent) or not, and may be continuous or be formed of pulses. In this example, the calibration infrared beam comes from a laser source: it is then a monochromatic and coherent beam, and here continuous.

[0040] By way of example, the optical source 10 may be a laser source, for example of the vertical cavity surface emitting laser (VCSEL) type, or even of the edge emitting laser (EEL) type. As a variant, for example, the optical source 10 may comprise one or more light-emitting diodes.

[0041] The optical source 10 may also comprise optical elements for shaping the infrared calibration beam, for example a beam expander associated with a collimation lens (not shown). Thus, the lateral dimension of the infrared calibration beam is widened so as to illuminate all the resonant cavities CR. Of course, these different optical elements may be distinct or combined. In this example, a collimation lens (free space optical configuration) has been shown, but this lens and these different optical elements may have a guided optical configuration, and may be, for example, a network of diffraction matrices, integrated on a substrate and optically coupled to the laser source.

[0042] The resolution target 20 comprises a plurality of resonant cavities CR resting on a transparent support 22. The resonant cavities CR form dark patterns and the surrounding intermediate zone ZI forms a light zone. It is intended to be illuminated on the rear face by the optical source 10. Also, the infrared beam of ca- libration illuminates the resolution target in the +Z direction.

[0043] The support 22 has a rear face and an opposite front face. It is made of a material transparent to the wavelength Xc of the infrared calibration beam, insofar as the resolution target is illuminated on the rear face. In other words, the material of the support 22 has a transmission rate at the wavelength Xc at least equal to 50%, preferably at least equal to 70%, or even 90%, or even more. For example, it may be a semiconductor plate (wafer, in English) having for example a thickness of one to several hundred microns. It may here be made of a crystalline semiconductor compound, for example here silicon.

[0044] An antireflection layer 21 may be arranged on the rear face of the support 22 (as illustrated in [Fig.lA]), or may be located between the support 22 and the resonant cavities CR. It has a thickness of the order of Xc / 4ncar, where ncar is the optical index of the material of the antireflection layer 21. For example, it may be made of ZnS, with an optical index ncar equal to 2.2 at the wavelength of 7pm, and a thickness equal to approximately 800nm.

[0045] Each resonant cavity CR is a Fabry-Pérot type optical cavity formed by a stack of two reflectors opposite each other, a lower reflector 23 and an upper reflector 25, between which is located an interlayer 24 which is weakly absorbent at the wavelength Xc or non-absorbent. The reflectors, on the other hand, have a power of partial absorption of infrared light (the resonant absorption of the cavity is linked to the partial absorption of the reflectors). The resonant cavities CR are said to be asymmetrical, in the sense that the lower reflectors 23 are less reflective than the upper reflectors 25. They rest on the support 22. The resonant cavities CR have an absorption resonance thus making it possible to form the dark patterns of the resolution target 20.

[0046] The lower reflectors 23 have partial transmission and reflection (and partial, non-zero absorption), so as to allow the infrared calibration beam to enter by transmission into the resonant cavity CR, then to reflect the infrared calibration beam present in the resonant cavity CR.

[0047] These are pads that are spatially distinct from each other in the XY plane, which participate in defining the lateral dimensions of the resonant cavities CR. They are located on the side of the support 22, and here rest on and in contact with the latter. They are preferably identical from one resonant cavity CR to another, in terms of materials and thickness. Preferably, they are made of a thin metallic material, such as for example titanium Ti with a thickness of a few tens of nanometers.

[0048] The lower reflectors 23 have a (physical) thickness less than its skin thickness ô. Let us recall that the skin thickness is such that ô=Xc / (2irk), where k is the extinction coefficient of the material. Thus, in the mid-infrared, we can choose for example titanium or platinum. Indeed, titanium has an extinction coefficient k of 15.8 at the wavelength Xc of 8pm, so that its skin thickness is equal to approximately 80nm. The thickness of the lower reflectors 23 is therefore chosen to be less than 80nm, for example equal to 15nm or 25nm.

[0049] The interlayer 24 is made of a non-absorbent or weakly absorbent material at the wavelength Xc of the infrared calibration beam, for example having an absorption rate of less than 10%, so that in the surrounding intermediate zone ZI, the infrared beam is transmitted efficiently by the interlayer 24.

[0050] The interlayer 24 is located between and in contact with the two reflectors. It is preferably made of a dielectric material, amorphous or crystalline, for example an oxide, a nitride or a type IV, III-V or ILVI material. For example, it may be made of amorphous silicon, here not intentionally doped, or even CdTe. It has a thickness eci>cav and an optical index nci such that the absorption in the resonant cavities CR is at least equal to 80%, or even 90% at the wavelength Xc. The thickness eci>cav in the resonant cavities CR is between Xc / (4nci) and Xc / (2nci) but different from these values, so as to optimize the absorption by resonance.

[0051] Furthermore, the optical index nci is preferably substantially equal to that of the support 22, so as to limit reflections at the interface. Of course, the interlayer 24 can be made of the same material (and therefore in a single layer), or be formed from a stack of different materials. In the case where the interlayer 24 is made of several materials (without an interface with a significant index jump), the refractive index nci can be an average index defined by an average (e.g. arithmetic) of the indices of the different sub-layers weighted by their thicknesses.

[0052] For example, in the case of an interlayer 24 made of amorphous silicon, with an optical index nci of 3.4 for a wavelength Xc of 7 pm, the thickness may be equal to 800 nm, therefore between and different from Xc / (4nci)=515 nm and (Xc / (2nci)=1030 nm.

[0053] The upper reflectors 25 have a high reflection, preferably total, so as to reflect the infrared calibration beam present in the resonant cavities CR, and have a partial (non-zero) optical absorption. Here, they are pads spatially distinct from each other in the XY plane, and participate in defining the lateral dimensions of the resonant cavities CR. They may have lateral dimensions in the XY plane substantially identical to those of the lower reflectors 23. Alternatively, they may be zones of the same continuous layer (see [Fig.2A]).

[0054] The upper reflectors 25 are located on the opposite side to the support 22 with respect to the interlayer 24, and here rest on and in contact with it. They are preferably identical from one resonant cavity CR to another, in terms of materials and thickness. Preferably, they are made of a metallic material, preferably a noble metal, whose (physical) thickness is greater than its skin thickness δ. Thus, in the mid-infrared, it can be chosen from gold, aluminum, copper, etc., with a thickness of a few tens of nanometers.

[0055] Indeed, by way of example, gold has an extinction coefficient k of 48.3 at the wavelength Xc of 8 pm, so that its skin thickness δ is equal to approximately 26 nm. The thickness of the upper reflectors 25 is therefore chosen to be greater than 26 nm, for example equal to 50 nm. Note that, as a variant and in particular in the case of [Fig.2A], the upper reflectors 25 may be Bragg mirrors, thus formed from an alternation of quarter-wave dielectric or semiconductor layers having respectively a high refractive index and a low refractive index.

[0056] The CR resonant cavities have lateral dimensions in the XY plane which define the shape of the dark patterns. They can thus extend in the form of lines, rectilinear or not, preferably parallel in pairs. The CR resonant cavities can have other shapes, for example circular, square, or other. The CR resonant cavities can have different lateral dimensions, from one cavity to another, or from one group of cavities to another group of cavities. The resonant cavities can have the same thickness for all the cavities, or can have the same thickness for a group of adjacent cavities, and a different thickness for another group of adjacent cavities. These lateral dimensions can be, for example, at least equal to 1 pm or even 10 pm, and typically of the order of a few tens of microns.

[0057] The resonant cavities CR are distinct from each other, insofar as at least the lower reflectors 23 are distinct (spaced) from each other. On the other hand, the intermediate zone ZI which surrounds each resonant cavity CR in the XY plane forms a clear continuous zone. The interlayer 24 extends continuously in the resonant cavities CR as well as in the intermediate zone ZI. Also, in the intermediate zone ZI, there are no lower reflectors 23, but the interlayer 24 is. In addition, the interlayer 24 completely fills the intermediate zone ZI: it continuously surrounds each resonant cavity CR in the XY plane, and also fills the intercavity space located between two adjacent resonant cavities CR.

[0058] In the case of a target with a resolution of 20 in transmission ([Fig. 1 A]), there are also no upper reflectors 25 in the intermediate zone ZI. On the other hand, in the case of a target with a resolution of 20 in reflection (cf. [Fig.2A]) where the upper reflectors 25 are regions of the same continuous reflective layer 41, the inter zone median ZI also includes a region of this continuous reflective layer 41.

[0059] The interlayer 24 has a thickness eci>zi in the intermediate zone which is substantially constant. It can be constant around all the resonant cavities CR of the target, or be constant with a first value around a first group of adjacent cavities, and constant with another value for another group of adjacent cavities (see [Fig.3B] described below).

[0060] The thickness eci>zi can be substantially equal to the thickness eci>cav in the resonant cavities CR, or even be equal to the sum of the thickness eci>cav and the thickness eri of the lower reflectors 23 (see for example [Fig.3A]), in the case where the interlayer has a flat upper face, for example following mechanical-chemical planarization.

[0061] In the case of a target with a resolution of 20 in transmission, this may comprise an upper anti-reflection layer 26, which continuously covers the resonant cavities CR and the intermediate zone ZI. This may be a layer of ZnS with a thickness of approximately 800 nm.

[0062] Alternatively, in the case where such an antireflection layer is not present, it is possible to reduce the thickness ecizi of the interlayer 24 in the intermediate zone ZI so that it is substantially equal to Xc / 4nci (see [Fig. 3A] described below). Thus, the interlayer 24 has, in the resonant cavities CR, a thickness eci>cav such that Xc / (4nci) < eci>cav < Xc / (2nci), and, in the intermediate zone ZI, a thickness eci>zi ~ Xc / (4nci).

[0063] The imaging system 30 comprises a matrix photodetector, and may also comprise an associated optical system located between the resolution target and the matrix photodetector. In this example, such an optical system is absent (application for example in lensless imaging).

[0064] In operation, the optical source 10 emits the infrared calibration beam so as to illuminate the resolution target 20, and more precisely so as to illuminate all the resonant cavities CR. The infrared beam is transmitted by the transparent support 22. In the patterns, it is partially transmitted by the lower reflector 23 and is reflected at least in part, and preferably totally, by the upper reflector 25. It is then completely absorbed here in the resonant cavities CR by resonance absorption. In the intermediate zone ZI, the infrared beam is transmitted by the interlayer 24 while being weakly absorbed. Thus, the infrared beam transmitted by the intermediate zone ZI is received and detected by the imaging system 30. The image of the resolution target 20 then shows dark patterns on a light background.

[0065] Thus, the patterns of the 20 resolution target are not produced by opaque metal pads, but by asymmetric Fabry-Pérot resonant cavities. CR resonant cavities operating by absorption resonance, this avoids parasitic reflections that can be induced by opaque metal patterns, which are likely to degrade the performance of the target. It also avoids using materials that are highly absorbent in the targeted infrared range (particularly the mid-infrared), such as semiconductor materials deposited by epitaxy, which would make the resolution target particularly expensive.

[0066] In addition, the resolution pattern 20 has a low topology, i.e. a low variation in thickness, between the dark patterns (resonant cavities) and the light background (intermediate zone), thus limiting the diffractive effects that could be generated by the edge of the resonant cavities CR. Also, the resolution pattern 20 has good optical performance, which makes it possible to be able to precisely determine the spatial resolution of the dark patterns, and, where appropriate, the geometric distortions.

[0067] Furthermore, by choosing the thickness of the intercalary layer 24 in the resonant cavities CR, it is possible to simply adjust the absorption resonance of the resonant cavities CR, and therefore the contrast between the dark patterns and the light background.

[0068] [Fig.lB] illustrates an example of spectral response in terms of absorption A, reflection R and transmission T, of the resolution target 20 similar to that of [Fig.lA], in the dark zones formed by the resonant cavities CR and in the intermediate light zone ZI which surrounds the dark zones.

[0069] In this resolution target, the transparent support 22 is a crystalline silicon substrate with a thickness of, for example, several hundred microns. An anti-reflection layer 21 here covers the rear face of the transparent support 22 and makes it possible to minimize optical losses by reflection at the lower face. It is made of ZnS, insofar as its optical index of 2.2 at the wavelength Xc of 7 pm allows good adaptation of the optical index between the surrounding air (optical index of 1) and the transparent silicon support (optical index of 3.4). Its thickness is of the order of Xc / 4nZnS, or here approximately 800 nm.

[0070] The lower reflectors 23 are 30nm thick titanium pads and are distinct from each other in the XY plane. The interlayer 24 is here amorphous silicon, with a thickness between Xc / nSi and Xc / 4nSi, here about 800nm ​​(nSi = 3.4). It extends continuously from one resonant cavity to the other, and also extends into the intermediate zone, which makes it possible to maximize the transmission of the infrared beam through the stack. The upper reflectors 25 are here also pads distinct from each other, here made of gold with a thickness of 50nm. A second antireflection layer 26 made of ZnS with a thickness of 800nm ​​covers the stack.

[0071] As a result, the resolution target presents, in the CR resonant cavities, an absorption Acav of the order of 90% and a reflection Rcav of the order of 10%. The transmission of the infrared beam in the patterns is almost zero. Dark patterns are thus obtained when the 20 resolution target is illuminated by the infrared beam with a wavelength Xc of 7 pm.

[0072] On the other hand, the 20 resolution target has, in the intermediate zone ZI, a transmission Tzi of the order of 90%, a reflection Rzi of the order of 10% and a quasi-zero absorption Azi. This gives a light background which surrounds the dark patterns. It is noted that the 20 resolution target actually operates in resonance absorption.

[0073] Finally, the topology of the resolution pattern, defined as the variation in thickness between the resonant cavities CR and the intermediate zone ZI, is equal to the sum of the thicknesses of the lower 23 and upper 25 reflectors, i.e. here equal to 80nm. This value is very far from Xc / (4nSi), i.e. approximately 550nm, so that optical edge phenomena (diffraction by the flanks of the patterns) are avoided. The resolution pattern 20 thus has good performance for determining the spatial resolution of the imager and / or, where appropriate, geometric distortions of the optical system of the imager.

[0074] [Fig.2A] is a schematic and partial view, in cross-section, of a resolution target suitable for infrared imaging, here in the mid-infrared, according to another embodiment.

[0075] In this example, the calibration system 1 differs from that of [Fig. 1A] essentially in that the optical source 10 comprises an integrated optical illumination system, and in that the resolution target 20 operates in reflection and not in transmission.

[0076] The calibration system 1 comprises: an optical source 10 which illuminates the resolution target 20 on the rear face; the resolution target 20 in reflection, where the infrared calibration beam is reflected, the intermediate zone ZI of which follows the direction -Z; and an imager 30 located on the opposite side of the resolution target 20 with respect to the optical illumination system.

[0077] The optical source 10 is formed here of a laser source 11 coupled to an optical illumination system 12 made in integrated optics. This makes it possible to illuminate all the patterns of the resolution target 20. Thus, the infrared calibration beam emitted by the laser source 11 is optically coupled to an integrated waveguide 13 located in a photonic substrate 14, and propagates along an axis contained in the XY plane, until it is extracted, for example by means of a diffraction grating, in the direction of the resolution target. The photonic substrate 14 is, moreover, made so as to be at least partly transmissive to the infrared beam reflected by the resolution target.

[0078] The resolution target 20 operates here in reflection and not in transmission. For this reason, it differs from that of [Fig.lA] in that the upper reflectors 25 are not not distinct plots from each other, but regions of the same continuous reflective layer 41. This reflective layer 41 thus extends continuously in all the patterns and in the intermediate zone. Thus, the infrared calibration beam is transmitted inside the resonant cavities CR where it is almost totally absorbed. On the other hand, in the intermediate zone ZI, the infrared beam is transmitted by the intermediate layer 24 in the +Z direction, is reflected in the -Z direction by the regions of the layer 41, and reaches the imager after having passed through the intermediate layer 24 once again.

[0079] [Fig.2B] illustrates an example of spectral response, in terms of absorption and reflection, of the resolution target 20. Here, the infrared calibration beam has a wavelength Xc of approximately 8 pm. The transparent support 22 is made of CaF2 and has an optical index of approximately 1.5. The lower reflectors 23 are made of titanium with a thickness of 15 nm; the intermediate layer 24 is made of amorphous silicon with a thickness of approximately 750 nm; and the upper reflectors 25 are formed by the continuous reflective layer 41 of gold with a thickness of approximately 50 nm. Under these conditions, it appears that the absorption Acav of the infrared calibration beam by the resonant cavities is of the order of approximately 97% (reflection less than 3%). On the other hand, in the intermediate zone, the reflection Rzi of the calibration infrared beam is of the order of 95% (absorption of the order of 5%).Thus, the 20 resolution target shows dark patterns on a light background with maximum contrast for a wavelength Xc located in the 7.5-8.5pm range. If you want to reduce the contrast, you can change the wavelength of the infrared calibration beam, for example by placing it in the 6.5-7.5pm range.

[0080] Note also that the resolution target can operate in transmission as illustrated in [Fig.lA], in reflection as in [Fig.2A], but also in “transflection”, that is to say that the target is identical or similar to that of [Fig.lA] (transmission target), but the imager is located on the side of the transparent support. A mirror is then arranged on the upper side of the resolution target, to reflect the infrared beam transmitted by the intermediate zone.

[0081] [Fig. 3A] is a schematic and partial view, in cross-section, of a resolution target 20 operating in transmission according to an alternative embodiment. This example illustrates the fact that, as mentioned previously, the interlayer 24 may have a thickness eci>cav different in the resonant cavities from the value eci zi in the intermediate zone. The resolution target 20 operates here in transmission: it is therefore useful to limit the reflection at the level of the upper face of the interlayer 24. For this, an antireflection layer may be present, as illustrated in [Fig. 1A], or the interlayer 24 may have a thickness e zi substantially equal to Xc / (4nci), and therefore less than the thickness eci>cav in the cavities resonant.

[0082] [Fig.3B] is a schematic and partial view, in cross-section, of a resolution pattern 20 according to another embodiment variant. Here, the resolution pattern operates in transmission but it could also operate in reflection. Here, several groups of patterns are defined which differ from each other essentially by the thickness eci>cav in the resonant cavities. Thus, in a first group of adjacent patterns, the intermediate layer 24 has a first thickness value eci>cavi, constant in each resonant cavity CR1 of the first group. On the other hand, in a second group of adjacent patterns, the intermediate layer 24 has a second thickness value eci>cav2, different from the first value and here greater than it. This thickness is here also constant in each resonant cavity CR2 of the second group.

[0083] [Fig.3C] is a schematic and partial view, in cross-section, of a resolution target 10 according to another embodiment variant. Here, the resolution target 20 operates in reflection, and does not include dark patterns separated two by two by a uniform light background, but includes patterns attached to each other, which are differentiated from each other by a different reflection.

[0084] The resolution target 20 comprises a transparent support 22 for illumination on the rear face. It also comprises a continuous reflective layer 42, located between the intermediate layer 24 and the transparent support 22. This reflective layer 42 extends continuously on the transparent support 22, and forms the lower reflectors 23. The latter are formed by regions of the reflective layer 42. It can be considered that the reflectors 23 are joined two by two and are not spatially separated by an intermediate transmissive zone. The resolution target 20 comprises a continuous reflective layer 41, which covers the intermediate layer 24. It extends here continuously, so that the upper reflectors 25 are, in a similar manner to the reflectors 23, regions joined to each other of this same continuous reflective layer 4L.

[0085] The interlayer 24 extends continuously from one resonant cavity to the other. The resonant cavities are adjacent to each other, so that there is no intermediate zone forming a uniform light background as in the examples of [Fig. 1A] and [Fig. 2A]. The interlayer 24 has a thickness eci>cav that is different from one resonant cavity to the other. Thus, it has a thickness eci>cavi in ​​a first cavity CR1, and a thickness eci>cav2 that is different from eci>cavi in ​​a second cavity CR2 adjacent to the first cavity CR1. In this example, the first resonant cavity CRI presents, around Xc, a weak reflection RI while the second resonant cavity CR2 presents a reflection R2 greater than RL. Thus, patterns of different optical intensities are formed, the contrast of which is defined by the difference between RI and R2.

[0086] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

Claims

1. Resolution target (20) for infrared imaging, intended to be illuminated on the rear face by an infrared beam, comprising: • a support (22) transparent to the infrared beam, having a rear face intended to receive the infrared beam, and an opposite front face; • a plurality of patterns, resting on the front face of the support (22), spaced laterally two by two in a main plane, intended to form dark patterns on a light background when the resolution target is illuminated by the infrared beam; • characterized in that: • each pattern is formed of a resonant cavity (CR) of the Fabry-Pérot type comprising a lower reflector (23) and an upper reflector (25) between which an interlayer (24) is located; the lower reflector (23), located on the side of the support (22), being less reflective than the upper reflector (25);• the interlayer (24) extends continuously from one resonant cavity (CR) to the other, and therefore also in an intermediate zone (ZI) surrounding the resonant cavities in the main plane.;

2. Resolution target (20) according to claim 1, in which the interlayer (24) has a thickness (eci>cav) in the resonant cavities (CR) between, and different from, Xc / (4nci) and Xc / (2nci), where Xc is the wavelength of the infrared beam and nci is the optical index of the interlayer (24).

3. Resolution target (20) according to claim 1 or 2, in which the interlayer (24) has a thickness (eci zi), in the intermediate zone (ZI), greater than or equal to Xc / (4nci) and less than Xc / (2nci).

4. Resolution target (20) according to any one of claims 1 to 3, in which the interlayer (24) has an absorption rate of less than 10% at the wavelength Xc of the infrared beam.

5. A resolution target (20) according to any one of claims 1 to 4, wherein the interlayer (24) has an equal optical index to that of the support (22).

6. A resolution target (20) according to any one of claims 1 to 5, wherein the lower reflectors (23) are made of a metal and have a thickness less than their skin thickness.

7. Resolution target (20) according to any one of claims 1 to 6, in which the upper reflectors (25) are made of a metal and have a thickness greater than their skin thickness, or are each formed by a Bragg mirror.

8. A resolution target (20) according to any one of claims 1 to 7, wherein the upper reflectors (25) are regions of a single continuous metal layer (41).

9. Resolution target (20) according to any one of claims 1 to 7, in which the upper reflectors (25) are separate pads, spaced two by two in the main plane.

10. Resolution target (20) according to claim 9, comprising an anti-reflection layer (26) covering the resonant cavities (CR) and the intermediate zone (ZI).

11. Resolution target (20) according to any one of claims 1 to 10, in which the interlayer (24) has a thickness (ecizi) in the intermediate zone (ZI) equal to Xc / (4nci).

12. Resolution target (20) according to any one of claims 1 to 11, in which the interlayer (24) has a first thickness (eci>cavi) for a first group of adjacent resonant cavities, and a second thickness (eci>cav2), different from the first thickness, for a second group of adjacent resonant cavities.

13. Calibration system (1), comprising: • an optical source (10) adapted to emit an infrared beam of wavelength Xc; • the resolution target (20) according to any one of the preceding claims, adapted to receive the infrared beam via a rear face of the support (22) opposite the patterns; • an imaging system (30), adapted to receive the infrared beam and to acquire an image of the resolution target (20).

Images