LIGHT DISTRIBUTION DEVICE BASED ON DIFFRACTION NETWORKS

Abstract

A light distribution device (100) configured to distribute light rays from an auxiliary light source onto a scene to be illuminated (S), comprising: - a planar waveguide (110), with a core layer (120) disposed between two cladding layers; and - an extraction assembly, located within the planar waveguide, and consisting of a plurality of diffraction gratings (151) distributed along two dimensions of a plane parallel to the plane of the planar waveguide. Figure for the abbreviation: Fig. 2

Description

Title of the invention: LIGHT DISTRIBUTION DEVICE BASED ON DIFFRACTION GRIDS technical field

[0001] The invention relates to a light distribution device adapted to, in use, receive light rays from a light source and distribute these rays over a scene to be illuminated. Prior art

[0002] Lensless optical imaging systems are known in the prior art, in which a detection module acquires a wide-field image of a sample. The sample and the detection module are placed in close proximity to each other, without any magnifying optics between them. A light source provides the light to illuminate the sample. A light distribution device can be disposed at the output of the light source to recover the light from a narrow beam emitted by the light source and distribute this light over a large area of ​​the sample. The light distribution device advantageously has a reduced thickness, allowing it to be housed in a space with a thickness less than or equal to the distance between the sample and the detection module during use.

[0003] French patent application FR1914926 describes an example of such an optical system for performing infrared spectrometric analysis on a sample. In this example, the light-dispersing device consists of a series of passive extraction structures, each coupled to a respective secondary waveguide, and integrated with the secondary waveguides on the same substrate. The secondary waveguides are themselves coupled to a primary waveguide by evanescent coupling. Each passive extraction structure is formed by a reflective surface located at the output of the corresponding secondary waveguide and inclined at an angle relative to the plane of the substrate. Each reflective surface is formed by a reflective coating deposited on an etched interface of the substrate.

[0004] Such a light distribution device has several disadvantages, including complex manufacturing requiring the creation of a multitude of secondary waveguides and angled facets in the substrate, and a non-homogeneous distribution of light on the surface to be illuminated of the sample.

[0005] An objective of the present invention is to provide a light distribution device suitable for integration into a lensless optical imaging system in order to distribute light rays from a an additional light source, and which does not present the disadvantages of the prior art mentioned above. Presentation of the invention

[0006] This objective is achieved with a light distribution device configured to, in use, distribute light rays from an auxiliary light source onto a scene to be illuminated, and which comprises: - a planar waveguide comprising two cladding layers and a core layer, with two largest faces of the core layer extending parallel to a plane called the plane of the planar waveguide, and with the two cladding layers and the core layer superimposed together along an axis orthogonal to the plane of the planar waveguide, with the core layer disposed between the two cladding layers; and - an extraction set, located in the planar waveguide, and consisting of a plurality of diffraction gratings distributed along the two dimensions of a plane parallel to the plane of the planar waveguide.

[0007] Diffraction gratings are coupling gratings, known in the field of integrated optics for achieving optical coupling between an optical fiber and a waveguide integrated on a photonic chip. In the invention, the diffraction gratings are numerous and distributed over a large surface area within the extraction assembly.

[0008] The extraction assembly is housed within the planar waveguide without increasing its thickness. The thickness of the light distribution device therefore corresponds to the thickness of the planar waveguide. The light distribution device can thus have a reduced thickness, compatible in particular with its integration into a lensless imaging optical system such as that described in the introduction. Specifically, the light distribution device according to the invention can easily have a thickness less than or equal to the distance separating, in operation, the sample and the detection module in the lensless imaging optical system described in the introduction. This thickness is advantageously less than or equal to 1.5 pm.

[0009] The fabrication of a light distribution device according to the invention requires the creation of a simple planar waveguide, within which diffraction gratings are etched. The fabrication does not require the implementation of complex processes and presents few constraints, particularly in terms of alignment. The light distribution device according to the invention can therefore be manufactured simply, quickly, and inexpensively.

[0010] In operation, light is injected into the light distribution device at a transverse face of the latter, at the core layer. The light then travels through the planar waveguide, by successive reflections at the interfaces between the core layer and each of the cladding layers. During its propagation in the planar waveguide, the light is confined only along the thickness axis of the planar waveguide, that is, along an axis orthogonal to the plane of the planar waveguide. In the other two spatial dimensions, propagation in the planar waveguide results in a spatial spreading of the light (unless the injected beam already has a width substantially equal to that of the planar waveguide).

[0011] The light circulating in the planar waveguide is extracted from it by each of the diffraction gratings in the extraction assembly. Preferably, each diffraction grating is configured to extract the light in the form of a beam oriented along an axis orthogonal to the plane of the planar waveguide. In practice, the light is extracted in the direction of the scene to be illuminated as well as in the opposite direction, but one of the directions can be favored by appropriate choices of refractive index in the media in contact with the planar waveguide.

[0012] The light propagating in the planar waveguide does not simultaneously reach all the diffraction gratings. Each diffraction grating extracts only a portion of the light arriving at its level in the planar waveguide, the unextracted light continuing to propagate through the planar waveguide to the next diffraction grating. This allows for light extraction over a large area, especially since this effect is in addition to the free propagation of light in the plane of the planar waveguide. The light distribution device according to the invention can thus recover the light from a narrow beam and distribute this light over at least a large area located outside the planar waveguide.One of the at least one surfaces preferably belongs to a sample to be analyzed, and preferably has a width greater than or equal to 1 mm (the width being the greatest distance separating two points on the surface considered, for example a diameter or a major axis of an ellipse). The distribution of light is relatively homogeneous, since each grating participates in the extraction of light.

[0013] The invention thus makes it possible to distribute light over a large surface, in a homogeneous manner, and with the help of a thin and easy-to-manufacture device.

[0014] Extracting light by diffraction gratings also makes it possible to control the angle of light extraction.

[0015] Advantageously, the extraction assembly is located in one of the two sheath layers.

[0016] The device may further comprise a support substrate, transparent over a range of operating wavelengths of the planar waveguide, and superimposed on the planar waveguide along an axis orthogonal to the plane of the planar waveguide. The support substrate may comprise a cavity, located on the side opposite the planar waveguide, and whose extent in a plane parallel to the plane of the planar waveguide is greater than or equal to an extent of the extraction assembly in a plane parallel to the plane of the planar waveguide.

[0017] The diffraction gratings of the extraction assembly can extend along a series of parallel bands, each extending from one edge of the extraction assembly to the other. Alternatively, they can be distributed along the two dimensions of a plane parallel to the plane of the planar waveguide.

[0018] Advantageously, an average value of the filling factor varies monotonically, from one diffraction grating to another in the extraction assembly, and along an axis parallel to the plane of the planar waveguide.

[0019] Preferably, the device according to the invention is configured to, in use, distribute over the scene to be illuminated light beams whose wavelength spectrum is centered on a wavelength called the central wavelength, and the diffraction gratings of the extraction assembly all have the same average pitch value, with said average pitch value adapted to extract out of the planar waveguide, and along an axis orthogonal to the plane of the planar waveguide, a light beam with the central wavelength propagating in the planar waveguide.

[0020] Advantageously, the extraction assembly is located in one of the two sheath layers, and in that the patterns of the diffraction gratings of the extraction assembly each consist of at least one solid area and at least one hollow area, where the at least one solid area is made up of the material of the sheath layer receiving the extraction assembly, and where the at least one hollow area is suitable for being occupied by a surrounding medium.

[0021] The diffraction gratings of the extraction assembly may each consist of a plurality of motifs, with the motifs of said diffraction gratings extending along parallel straight lines. Alternatively, the diffraction gratings of the extraction assembly may each consist of a plurality of motifs, with the motifs of said diffraction gratings extending along convex curved lines, each extending from one edge to the opposite edge of the planar waveguide.

[0022] Advantageously, the device according to the invention is configured to, in use, distribute light rays over the scene to be illuminated, the wavelength spectrum of which extends from a minimum wavelength to a maximum wavelength, and the planar waveguide is single-mode along an axis orthogonal to the plane of the planar waveguide and to said minimum wavelength.

[0023] The invention also covers an infrared imaging system comprising: - a light distribution device according to the invention, configured to, in use, distribute light rays from an auxiliary light source onto a scene to be illuminated; and - a detection module, comprising a matrix of photodetectors configured to receive light rays reflected by the scene to be illuminated; with the photodetector array located opposite the extraction assembly, and on the side of the planar waveguide receiving the extraction assembly, and with an overlap rate between the photodetectors of the photodetector array and the diffraction gratings of the extraction assembly less than or equal to 50%.

[0024] The system may further include an infrared light source forming the auxiliary light source, and the light distribution device is configured to, in use, receive light rays from said infrared light source as input and distribute these rays over the scene to be illuminated, with the input of the light distribution device formed by a transverse face of the planar waveguide.

[0025] The system may further include an imaging assembly comprising at least one refractive lens, disposed between the input of the light distribution device and the infrared light source, and configured to receive as input the light emitted by the infrared light source and to provide as output a beam of parallel light rays.

[0026] The system may further include a spacer element, mounted as a unit with the detection module and provided with a bearing surface intended to come into contact with a sample to be analyzed and located on one side of the spacer element opposite the detection module, and the light distribution device forms all or part of the spacer element. Brief description of the figures

[0027] The present invention will be better understood upon reading the description of exemplary embodiments given by way of illustration only and in no way limiting, with reference to the accompanying drawings in which:

[0028] [Fig.1A] illustrates schematically, and according to a cross-sectional view, a first embodiment of a light distribution device according to the invention;

[0029] [Fig.1B],

[0030] [Fig.lC],

[0031] [Fig.1D], and

[0032] [Fig.1E] schematically illustrate, from a top view, different examples of the distribution of the diffraction gratings of the extraction assembly, in a device according to the invention such as that of [Fig.1A];

[0033] [Fig.2] schematically illustrates the embodiment of [Fig.1A] in use;

[0034] [Fig.3] illustrates schematically, and according to a cross-sectional view, a second embodiment of a light distribution device according to the invention;

[0035] [Fig.4] illustrates schematically, and according to a cross-sectional view, a third embodiment of a light distribution device according to the invention;

[0036] [Fig.5] schematically illustrates a fourth embodiment of a light distribution device according to the invention;

[0037] [Fig.6] illustrates schematically, and according to a cross-sectional view, a first embodiment of an infrared imaging system according to the invention;

[0038] [Fig.7A] and

[0039] [Fig.7B] schematically illustrate a second embodiment of an infrared imaging system according to the invention;

[0040] [Fig.8A] and

[0041] [Fig.8B] schematically illustrate a third embodiment of an infrared imaging system according to the invention; and

[0042] [Fig.9] schematically illustrates an infrared imaging system according to the invention, in use. Description of the implementation methods

[0043] To facilitate reading, the axes of an orthonormal coordinate system (Oxyz) have been represented on the figures.

[0044] Throughout the text, the term “infrared” refers to a part of the light spectrum belonging to a spectral band from 0.78 pm to 50 pm, more preferably from 2 pm to 12 pm.

[0045] Throughout this text, a planar waveguide refers to an optical guiding element capable of guiding the propagation of light by successive reflections on parallel plane surfaces. In a planar waveguide, the light is confined along one axis of three-dimensional space and free to propagate along the other two axes of three-dimensional space. A waveguide consists of a core, in which the light travels, and a cladding, ensuring a desired difference in refractive index between the core and a medium surrounding the core. In the case of a planar waveguide, a core layer is sandwiched between two cladding layers, and the light is guided in the core layer by successive reflections at the interfaces between the core layer and each respective cladding layer. The core layer The core layer is essentially an optical element with a small dimension along one of the three-dimensional axes (here, the (Oz) axis of thickness) and large dimensions along each of the other two three-dimensional axes (here, the (Ox) and (Oy) axes of length and width). Thus, the light is confined along the (Oz) axis and propagates freely along the (Oy) and (Ox) axes. In the core layer, the ratio of thickness to length, or thickness to width respectively, is preferably greater than or equal to 5, or even greater than or equal to 10. The core layer is made of a material that is optically transparent at the wavelengths to be propagated, for example, with a transmission coefficient greater than or equal to 98% at said wavelengths. Advantageously, it has the same refractive index and chemical composition throughout its entire volume.

[0046] A first embodiment of a light distribution device 100 according to the invention is shown in [Fig.1A], according to a cross-sectional view in a plane (xOz).

[0047] The device 100 comprises a first cladding layer 130i, a core layer 120, and a second cladding layer 1302, superimposed in that order along the (Oz) axis to form a planar waveguide 110. Each of these three layers is optically transparent to the wavelengths to be propagated, for example, with a transmission coefficient greater than or equal to 98% at said wavelengths. The wavelengths to be propagated refer to a range of wavelengths that the device is adapted to receive and then distribute over a scene to be illuminated.

[0048] The optical index of the first cladding layer 130i and the optical index of the second cladding layer 1302 are each lower than the optical index of the core layer 120. The differences between the optical index of the core layer and the optical index of the first and second cladding layers, respectively, are adapted to allow optical guidance of light in the core layer. Here, but not exclusively, the first cladding layer 130i and the second cladding layer 1302 are made of the same material. The core layer 120 is, for example, made of germanium (Ge) or a germanium-silicon alloy (SiGe). The cladding layers 130i and 1302 are, for example, made of a germanium-silicon alloy (SiGe) with a higher silicon content than the core layer. For example, the core layer is made of germanium and the sheath layers are made of germanium-silicon alloy with 40% germanium atoms.

[0049] The core layer 120 is bounded, along the (Oz) axis, by two faces 121, 122 parallel to the (xOy) plane. These faces 121, 122 are the largest faces, or areas, of the core layer 120. A plane of the planar waveguide 110 is defined as a plane parallel to said faces 121, 122. Preferably, the largest external faces of the planar waveguide 110 extend in planes parallel to the faces 121, 122. The core layer 120 and the first, respectively second, cladding layer 130i, 1302, are in direct physical contact with each other. The core layer 120 and the two cladding layers 130i, 1302 are superimposed on each other, along an axis (Oz) orthogonal to the plane (xOy) of the planar waveguide, and with the core layer sandwiched between the two cladding layers 130i, 1302.

[0050] The planar waveguide 110 here has the shape of a rectangular parallelepiped with a square base. Preferably, the length and width (along the (Ox) axis, respectively (Oy)) of the planar waveguide 110 are each between 5 mm and 20 mm, for example, 10 mm. The planar waveguide 110 may also include side claddings in direct physical contact with the core layer 120 at two opposite transverse faces of the latter. The side claddings are made of a material with a lower refractive index than the core layer, preferably the same material as the first and second cladding layers 130i, 1302.

[0051] The device 100 further comprises an extraction assembly 150, located in the planar waveguide 110, and consisting of a plurality of diffraction gratings 151. The diffraction gratings are distributed along the two dimensions of a plane (xOy). Preferably, they are distributed regularly in the plane (xOy).

[0052] In the example illustrated here, the extraction assembly 150 extends more particularly into one of the two cladding layers, here the second cladding layer 1302. In variants not shown, the extraction assembly extends into the core layer 120.

[0053] A diffraction grating refers to a succession of patterns, distributed periodically or quasi-periodically (pitch variation less than or equal to 10% between two consecutive patterns). In the invention, each diffraction grating 151 is adapted to deflect, outwards from the planar waveguide 110, the light circulating in the core layer 120.

[0054] The diffraction gratings 151 are preferably all arranged coplanarly. Here, but not limited to, each of the diffraction gratings 151 extends through the entire thickness of the second cladding layer 1302. Alternatively, each of the diffraction gratings 151 extends only over a portion of the thickness of the second cladding layer 1302. In any case, the motifs forming the diffraction grating 151 are preferably invariant about the (Oz) axis. Each motif comprises a first portion made of the material of the second cladding layer 1302, and a second portion made of another material. The second portion may be solid, made for example of ZnS, or hollow, suitable for being filled by a surrounding gaseous medium such as air.

[0055] Diffraction gratings are advantageously distributed according to a regular arrangement in a plane (xOy). Various examples of distribution are described below.

[0056] Figure 1B schematically illustrates a first example of the distribution of the diffraction gratings 151 in the planar waveguide. Figure 1B schematically represents an extraction assembly in a device according to the invention, in a transparent top view. The diffraction gratings 151 extend here along parallel lines, each running from one edge to the opposite edge of the planar waveguide. More specifically, the diffraction gratings 151 extend along lines parallel to the (Oy) axis and are distributed regularly along the (Ox) axis.

[0057] Figure 1C shows a detailed view of one of the diffraction gratings 151 illustrated in Figure 1B. More specifically, it is a detailed view of the portion of grating 1511 circled in Figure 1B. Each diffraction grating is a one-dimensional grating, consisting of motifs 1510 which extend here along lines parallel to the (Oy) axis. In each diffraction grating 151, a step size of the motif distribution along the (Ox) axis is denoted Px.

[0058] Fig. 1D schematically illustrates another example of the distribution of the 151' diffraction gratings in the planar waveguide. In this variant, the 151' diffraction gratings are distributed along the two dimensions (Ox) and (Oy) of the plane of the planar waveguide. Here, the diffraction gratings are arranged in rows and columns. The rows and columns extend respectively along the (Ox) and (Oy) axes, with the same spacing along both axes. Each 151' diffraction grating has a square cross-section in the (xOy) plane. As in the previous example, each 151' diffraction grating is a one-dimensional grating, here consisting of patterns extending along lines parallel to the (Oy) axis (like the grating portion illustrated in Fig. 1C).

[0059] Fig. 1E schematically illustrates yet another example of the distribution of 151′ diffraction gratings in the planar waveguide. Here again, the 151′ diffraction gratings are distributed along the two dimensions (Ox) and (Oy) of the plane of the planar waveguide. In this case, the 151′ diffraction gratings are staggered and arranged in a regular pattern. Each 151′ diffraction grating has a square cross-section in the (xOy) plane. As in the previous examples, each 151′ diffraction grating is a one-dimensional grating, here consisting of patterns extending along lines parallel to the (Oy) axis (like the grating portion shown in Fig. 1C).

[0060] As illustrated later, with reference to [Fig.8A], the invention is not limited to a device with diffraction gratings whose patterns extend along straight lines parallel to each other.

[0061] In operation, the device 100 is immersed in a surrounding gaseous or liquid medium, preferably air. Light arrives at the device 100 at a transverse face of the latter, called the entrance face 113, and at the level of the core layer 120. The entrance face 113 extends here in a plane parallel to the (yOz) plane. Inside the planar waveguide 110, the light propagates in the core layer 120 by successive reflections at the interfaces between the core layer 120 and each of the cladding layers 130i and 1302. Here, the light propagates in the planar waveguide along the (Ox) axis. If the light beam injected into the planar waveguide 110 is narrower than the waveguide itself, the light propagates within the planar waveguide 110, spreading out along the (Oy) axis. The light is extracted from the planar waveguide 110 at the extraction assembly 150.Preferably, each of the diffraction gratings in the extraction assembly participates in this light extraction. With the exception of a series of gratings located on the side opposite the entrance face 113, each diffraction grating extracts only a portion of the light arriving at its level. Thus, the unextracted light continues to propagate in the planar waveguide until it reaches a neighboring diffraction grating, which in turn will extract at least a portion of the light arriving at its level. This results in light extraction outwards from the planar waveguide 110, and this light is distributed over a surface area greater than or equal to that of the extraction assembly 150.

[0062] Figure 2 schematically illustrates the device 100 in use. Arrow 20 represents the light beam injected at the inlet of the planar waveguide 110. Beams 21 represent the light beams extracted at each of the diffraction gratings 151, directed towards a scene to be illuminated S. The scene to be illuminated S extends here, with respect to the extraction assembly, in a plane (xOy). The scene to be illuminated S is shown here spaced away from the device 100 according to the invention. In practice, it is usually placed against an external face of the device 100 according to the invention. The scene to be illuminated S has, for example, a width between 1 mm and 15 mm (the greatest distance between two points). For example, it could be a square with sides of 3 mm. In any case, the dimensions of the scene to be illuminated are roughly equal to the dimensions of the extraction unit.

[0063] Each of the diffraction gratings in the extraction assembly extracts light from the plane of the planar waveguide in the form of a diverging beam. This allows the light to be distributed over the scene to be illuminated, without shadows, using diffraction gratings spaced apart from each other. The greater the (solid) divergence angle of the extracted light beams, the greater the spacing between the diffraction gratings.

[0064] In the planar waveguide, the angle between an external face of the planar waveguide, at which the light emerges, and a light ray deflected by a diffraction grating, must remain below a critical threshold beyond which the deflected light is reflected back into the device according to the invention. Here, the light is extracted in the form of light beams each oriented perpendicular to the plane of the planar waveguide, with a half-angle of divergence Δi (where the half-angle of divergence is half the total angle of divergence). Δi must therefore remain below said critical threshold. It is assumed that the external face of the planar waveguide at which the light emerges is parallel to the plane of the planar waveguide, which is generally the case. Preferably, the diffraction gratings are configured so that the half-angle of divergence is equal to, or only slightly less than, the critical threshold.This maximizes the divergence of the arrays, allowing them to be spaced as far apart as possible, and thus limiting the number of blinded photodetectors in the system described below.

[0065] Each diffraction grating in the extraction assembly deflects the light in two opposite directions: towards the scene to be illuminated S and in the opposite direction. Here, the scene to be illuminated is located on the side of the planar waveguide opposite the extraction assembly. As detailed below (see description of [Fig. 6]), it is possible to favor one of these two directions, so that most of the extracted light reaches the scene to be illuminated. Alternatively, the rays emitted in the opposite direction can be blocked, for example, by means of an absorbing or reflective structured layer attached to the extraction assembly. This structured layer has solid and open areas, so that each diffraction grating is covered by a solid absorbing or reflective area on the side opposite the scene to be illuminated.Between the diffraction gratings, the open areas allow light to pass through, including light backscattered by the scene to be illuminated.

[0066] Each diffraction grating can have a constant pattern spacing value over its entire extent. Alternatively, at least one of the diffraction gratings can have a variable pattern spacing value to increase the divergence angle of the extracted light. Advantageously, all the gratings in the extraction assembly are configured to extract the light with the same divergence angle. In all cases, an average pattern spacing value can be defined. When the spacing is constant, this average value is equal to the pattern spacing. When the spacing is variable, this average value is equal to the arithmetic mean of the pattern spacing values.

[0067] Preferably, the diffraction gratings of the extraction assembly all have the same average pattern spacing. This average pattern spacing, or average spacing, is adjusted so that the light extracted by the corresponding grating forms a beam centered on an axis orthogonal to the plane of the planar waveguide. To achieve this, the average spacing is adjusted so that light at a central wavelength is deflected along an axis orthogonal to the plane of the planar waveguide. The central wavelength refers to the central value of a range of wavelengths that the device according to the invention is adapted to receive circulating in the planar waveguide and then extract to a scene to be illuminated via the extraction assembly. This range of wavelengths is called the operating wavelength range.The average value of the step size allows us to verify that the above condition is determined, in a known way, using the lattice formula and considering the orders of refraction 1 and -1.

[0068] Preferably, the planar waveguide 110 is single-mode along the (Oz) axis and over the entire operating wavelength range. For this condition to be satisfied over said wavelength range, it is sufficient for it to be satisfied for the smallest value in this range. This condition sets a maximum value for the core layer thickness, for example, 1.6 pm. To limit losses at longer wavelengths, the core layer thickness is preferably chosen to be as close as possible to this maximum value.

[0069] Figure 3 schematically illustrates, in cross-section, a second embodiment of a light distribution device 300 according to the invention. The device 300 differs from the first embodiment only in that it further comprises a support substrate 360, superimposed on the planar waveguide along the (Oz) axis, on the side opposite the extraction assembly 350. The support substrate 360 ​​extends in a (xOy) plane. Here, the support substrate 360 ​​is attached to the planar waveguide 310, in direct physical contact with it at the level of the first sheath layer 330i.

[0070] In an orthogonal projection of the device 300 in a plane parallel to the (xOy) plane, the projection of the extraction assembly 350 is located inside the projection of the support substrate 360. In other words, the support substrate 360 ​​covers the entire extraction assembly 350.

[0071] The support substrate 360 ​​is transparent over the operating wavelength range. It preferably has a lower refractive index than the first cladding layer 330p. The support substrate 360 ​​is preferably made of silicon. In particular, it ensures the proper mechanical strength of the planar waveguide 310. With this support substrate 360, the thickness e (along the (Oz) axis) of the device 300 is preferably between 100 pm and 1.5 mm, preferably between 100 µm and 1.0 mm, for example equal to 725 µm. The thickness of the support substrate is preferably greater than or equal to 200 µm.

[0072] Here, the highest refractive index jump is located at the interface between the 360° support substrate and the surrounding medium. The critical threshold, as described above, beyond which light deflected by a diffraction grating is reflected back into the device according to the invention, therefore depends on the refractive indices in the 360° support substrate and in the surrounding medium. For a silicon 360° support substrate, in a surrounding medium consisting of air, the critical threshold is approximately 17° in absolute value.

[0073] A face of the support substrate 360, located on the side of said substrate opposite the planar waveguide, forms a support surface 361 for a sample to be observed. In use, the support surface 361 is positioned against the sample. The scene to be observed is formed by a surface of the sample in contact with the support face 361, located opposite the extraction assembly 350.

[0074] Figure 4 schematically illustrates, in cross-section, a third embodiment of a light distribution device 400 according to the invention. The device 400 differs from the device of [Fig. 3] only in that the support substrate 460 has a cavity 462, open on the side opposite the planar waveguide 410. The cavity 462 has a bottom 463, which here extends in a plane (xOy), and lateral faces 464. The depth of the open cavity 462, measured along the (Oz) axis, is, for example, between 100 pm and 400 pm.

[0075] In an orthogonal projection of the device 400 in a plane parallel to the (xOy) plane, the projection of the extraction assembly 450 is located inside the projection of the cavity 462. In other words, the cavity 462 covers the entire extraction assembly 450.

[0076] The support substrate 460 extends laterally relative to the cavity 462, and in all directions of a plane (xOy). A face of the support substrate 460, located on the side of said substrate opposite the planar waveguide and around the cavity 462, forms the support surface 461 for a sample to be observed. In use, the support surface 461 is positioned against the sample, and the scene to be observed S is formed by a surface of the sample located opposite the extraction assembly 450 and the cavity 462.

[0077] This embodiment allows for the introduction of an intercalated layer between the scene to be observed and the support substrate 460, formed by the material of the surrounding medium in the cavity 462 (preferably air). Since the optical index of the surrounding medium is much lower than the optical index of the support substrate 460, the divergence of the extracted light beams increases sharply when light enters the cavity 462 (see beams 21'). This increase in divergence makes it possible, in particular, to reduce the thickness of the device according to the invention while maintaining unchanged its other characteristics. As a complement or alternative, it allows the number of diffraction gratings in the extraction assembly to be reduced, which reduces the number of blinded photodetectors in the system illustrated below.

[0078] An advantageous feature of the extraction assembly of the device according to the invention is then described. As explained above, in operation, each of the diffraction gratings in the extraction assembly extracts only a portion of the light arriving at its level, and the light not extracted by said grating continues to propagate in the planar waveguide to a neighboring grating. Consequently, if all the diffraction gratings in the extraction assembly have the same light extraction rate, they will extract a progressively smaller quantity of light as the light propagates along the extraction assembly. Indeed, the gratings reached first by the light circulating in the planar waveguide will extract a percentage Pc from a large quantity of light, while the gratings reached last will extract the same percentage Pc but from a residual quantity of light.It is therefore advantageous for the extraction system to have a variable extraction rate from one diffraction grating to another, so as to ensure good homogeneity of the light distribution on the scene to be illuminated. The extraction rate of the diffraction gratings advantageously varies monotonically along lines parallel to a light propagation axis in the planar waveguide, and from a light entry face in the planar waveguide. The variation may include steps or be strictly increasing.

[0079] The extraction rate of a diffraction grating is a function of the average value of its filling factor, here a ratio between the total volume occupied by the lowest index material and the total volume of the diffraction grating. When the spacing is constant, the average value of the filling factor can be calculated on a single unit cell, or pattern of the grating. The higher the filling factor, the higher the extraction rate, and vice versa. Preferably, the average value of the filling factor does not exceed 0.5 in any of the diffraction gratings in the extraction set.

[0080] Figure 5 schematically illustrates, from a top view, the distribution of filling factors in the diffraction gratings of an extraction assembly 550 of a device according to a fourth embodiment of the invention. The value of the filling factor in a diffraction grating is represented by a gray level value (scale on the right).

[0081] All diffraction gratings in the extraction assembly have the same depth (dimension along the (Oz) axis). The distribution of filling factors varies monotonically along lines parallel to the (Ox) axis, increasing from an entrance face to an exit face of the planar waveguide. Here, the distribution of fill factors is adapted to a case in which light propagates in the planar waveguide as a plane wave. The corresponding optical power follows a Gaussian distribution as a function of position along the (Oy) axis. The fill factor of the diffraction gratings therefore follows this Gaussian distribution. Thus, we have diffraction gratings with a fill factor that increases along lines parallel to the (Ox) axis, and with a constant fill factor along convex curved lines. Throughout this text, the term "convex" refers to lines curving in a direction opposite to the entrance face of the device according to the invention.

[0082] The extraction rate of a diffraction grating is also a function of the depth of that grating, here along the (Oz) axis. Thus, according to a variant not shown, the diffraction gratings of the extraction set have a depth that varies monotonically along lines parallel to the axis of light propagation in the planar waveguide.

[0083] The extraction rate of a diffraction grating is also a function of the grating's dimensions along a light propagation axis in the planar waveguide. Thus, according to an unshown variant, the diffraction gratings in the extraction assembly each have an extent that varies along lines parallel to a light propagation axis in the planar waveguide.

[0084] At least two of the three parameters mentioned above can be combined to vary an extraction rate in the diffraction gratings of the extraction set.

[0085] In each of the embodiments and variants of the invention, the filling factor may be slightly variable within the same diffraction grating, in order to symmetrize the far-field radiation. In this case, a variation in the average value of the filling factor is considered, from one grating to another in the extraction assembly.

[0086] The invention provides a solution for distributing light over a scene to be illuminated, preferably under normal or near-normal lighting. Advantageously, the scene to be illuminated is parallel to the planar waveguide, and the light emerges from the latter as a plurality of elementary beams, each oriented along an axis normal to the planar waveguide, with each beam having a divergence angle of approximately 17°. The device according to the invention exhibits low losses and good flux resistance (particularly in comparison with a light distribution device based on optical fibers). It also exhibits high compactness, and in particular a reduced thickness, enabling it to serve as a spacer between the sample and the detection module in the lensless optical imaging system described in the introduction.

[0087] Figure 6 schematically illustrates, in cross-section, a first embodiment of an infrared imaging system 1000 according to the invention. The system 1000 here comprises a light distribution device 600 and a detection module 10 (or imaging module). The device 600 is identical to that of Figure 4.

[0088] The detection module 10 comprises an array of photodetectors 11 sensitive in the infrared, and more particularly over the operating wavelength range. For example, it may be an array of semiconductor photodiodes or an array of bolometers. The photodetector array extends over a square or rectangular surface, preferably with sides between 1 mm and 10 mm. Here, it extends in a plane (xOy). The detection module 10 may further comprise an electronic circuit, not shown, for reading electrical signals provided by the photodetector array. The photodetector array extends, with respect to the extraction assembly 650 of the light distribution device 600, on the side of the planar waveguide receiving the extraction assembly (and therefore here on the side opposite the cavity 662 formed in the support substrate 660).

[0089] In operation, light is distributed over the scene to be illuminated by the light distribution device 600. The scene to be illuminated reflects some of the received light by backscattering. The backscattered light passes through the light distribution device 600 until it reaches the detection module 10, in which the array of photodetectors acquires an image of the light backscattered by the scene to be illuminated.

[0090] The light distribution device 600 and the detection module 10 are not in direct physical contact with each other, but are separated by a low-index interlayer 13, in particular an air layer. The low-index interlayer 13 has a refractive index strictly lower than that of the support substrate 660, thus favoring extraction in the direction of the scene to be illuminated rather than in the opposite direction. As detailed above, an absorbing or reflective structured layer, as described above, can also be attached to the extraction assembly.

[0091] Despite this, the photodetectors 11 located directly facing a diffraction grating of the extraction assembly can be dazzled by the light extracted directly towards the detection module, and consequently be blinded and inoperative. It is therefore advantageous to maximize the divergence of the light beams reflected by each of the diffraction gratings, in order to maximize the spacing between the diffraction gratings, and thus minimize the number of blinded photodetectors. As detailed above, this divergence can be increased using a variable grating pitch within the same diffraction grating, and / or using a cavity as described with reference to [Fig. 4]. The half-angle of divergence must nevertheless remain below or equal to a critical threshold as mentioned above. Furthermore, the distribution of diffraction gratings along the two dimensions of the (xOy) plane is more advantageous than the distribution along parallel bands, as it allows fewer photodetectors to be masked.

[0092] Preferably, the overlap between the photodetectors 11 of the detection module and the diffraction gratings of the extraction assembly 650 is less than or equal to 50%. More preferably, at least half of the photodetectors 11 are not covered, even partially, by at least part of a diffraction grating of the extraction assembly. When the diffraction gratings are distributed along the two dimensions of the (xOy) plane, there are preferably at least twice as many photodetectors as diffraction gratings.

[0093] Figure 7A illustrates a second embodiment of an infrared imaging system 2000 according to the invention. This system differs from that of Figure 6 only in that it further comprises an infrared light source 14, as well as a refractive optic 15.

[0094] The infrared light source 14 is configured to emit at least one light beam at an infrared wavelength. The light emitted by the infrared light source 14 has a wavelength spectrum that extends over the range of wavelengths of use as mentioned above. The light source 14 may comprise one or more elementary sources, including at least one laser source (such as a quantum cascade laser (QCL), an interband cascade laser (ICL), an external or internal cavity laser), at least one LED, at least one blackbody source, etc. The infrared light source 14 may include a broadening element, such as an auxiliary planar waveguide, to transform a narrow light beam into a light beam of the same width as the light spreading device.

[0095] The refractive optic 15 is interposed between the light source 14 and the entrance face 713 of the light-splitting device 700. It is configured to receive an input of a light beam from the light source 14 and to provide an output of a collimated light beam which then propagates to the entrance face 713 of the device 700, at the level of the core layer. The light wave propagating in the planar waveguide of the device 700 therefore has a planar wavefront. The diffraction gratings 751 forming the extraction assembly each consist of a plurality of motifs, which extend along lines parallel to the wavefront, here lines parallel to the (Oy) axis (see [Fig. 7B]).

[0096] Figure 8A illustrates a third embodiment of an imaging system infrared 3000 according to the invention. This system differs from that of [Fig. 7A] only in that the infrared light source 14 is arranged directly at the inlet of the light-splitting device 800, without refractive optics between the two. The light beam propagating in the planar waveguide of the device 800 is therefore a diverging beam, with a curved wavefront of convex shape. Each diffraction grating 851 then consists of a plurality of patterns that extend along lines parallel to the wavefront arriving at said grating. These lines are convex, each extending from one edge to the opposite edge of the extraction assembly (see [Fig. 8B]).

[0097] In any event, in each diffraction grating, the grating patterns preferably extend along lines that are perpendicular at every point to a light ray propagating in the planar waveguide at the level of a central area of ​​said grating.

[0098] Figure 9 schematically illustrates an infrared imaging system 4000 according to the invention in use. The infrared imaging system 4000 is of the type shown in Figure 7A, with a light distribution device 900 according to the invention, receiving light from an infrared light source 14 with a refractive optic 15 between the two. The light is distributed onto a scene to be illuminated, here belonging to a sample 96.

[0099] In [Fig. 9], the 10A photodetector matrix and the reading circuit 10B together forming the detection module 10.

[0100] The infrared imaging system 4000 here includes a spacer element 95, or spacer, mounted as a unit with the detection module 10 on the side of the photodetector matrix 10A.

[0101] The spacer element 95 has a bearing surface 97 on the side opposite the detection module 10. The bearing surface 97 extends in a plane parallel to the (xOy) plane, which is parallel to the plane of the photodetector array 10A. The spacer element 95 has a thickness W, measured along the (Oz) axis. During operation, the bearing surface 97 is pressed against the sample 96. A region of the sample 96 facing the photodetector array 10A forms the scene to be illuminated. The spacer element 95 ensures a predetermined fixed distance between the scene to be illuminated and the photodetector array 10A, called the working distance. The working distance is preferably between 100 µm and 1.5 mm. The infrared light source 14 is located here on the spacer element 95, on the opposite side to the support surface 97. The light distribution device 900 is here combined with the spacer element 95.

[0102] In unrepresented variants, the light-splitting device 900 forms only a part of the spacer element 95. For example, the spacer element 95 may be formed by superimposing the light-splitting device 900 and an additional shim. In any case, the small thickness of the light distribution device according to the invention, advantageously between 100 pm and 1.5 mm, allows it to form all or part of the spacing element 95.

[0103] The detection module 10 and the light distribution device 900 together can be considered to form a lensless imaging system, capable of acquiring an image of the scene to be illuminated, without refractive image-forming optics (except possibly for a microlens array upstream of the photodetector array 10A). The images obtained are wide-field, reflected images.

[0104] The 4000 infrared imaging system is advantageously formed in a photonic chip. For example, an infrared imaging system according to the invention has been implemented with the following characteristics: - range of wavelengths of light to be distributed over the scene to be illuminated: from 5.9 pm to 8.1 pm; - Ge core layer, of a thickness adapted so that the planar waveguide is single-mode along the (Oz) axis over the entire range of wavelengths considered, i.e. a thickness of 1.6 pm; - extraction assembly which extends over an area of ​​2.72 mm*2.72 mm; - SiGe sheath layers, 3 pm thick; - photodetector matrix formed of a bolometer matrix with 80*80 micro-sensors, each square with a side of 25 pm, and distributed according to a distribution step of 34 pm, i.e. an active area of ​​3.28 mm*3.28 mm (slightly larger than the extraction assembly); - the diffraction gratings of the extraction assembly have a length of 34 pm, extend along parallel bands as in [Fig.1B], and are aligned with some of the micro-sensors of the photodetector array; - the pitch of the diffraction gratings is adapted to obtain a zero extraction angle at the median value of the considered wavelength range (7 pm), equal here to 1.81 pm; - the depth of the networks is 2.5 pm.

[0105] It can then be shown that a support substrate with a thickness of 300 pm and an open cavity with a depth of 250 pm offer a good compromise between the number of blinded microsensors and compactness.

[0106] In a second configuration which differs only in that the diffraction gratings are distributed along the (Ox) and (Oy) axes, only 1 / 8th of the micro-sensors are blinded.

[0107] The light distribution device according to the invention is advantageously made from a crystalline silicon substrate, which can be thinned to obtain the desired working distance. The planar waveguide is obtained by successive epitaxies, to successively form the first cladding layer, the core layer and the The second cladding layer is then polished on the side opposite the silicon substrate. The diffraction grouts are then etched into the second cladding layer using partial anisotropic etching. The resulting assembly can then be flipped over and partially anisotropically etched on the back side to create a cavity in the silicon substrate.

[0108] The invention therefore relates to a device forming a passive extraction structure, advantageously coupled to one or more quantum cascade lasers, and for use at very short working distances. The device according to the invention preferably has an entrance pupil smaller than the exit pupil. The invention makes it possible to distribute the light from an infrared beam, preferably belonging to the spectral range from 2 pm to 12 pm. It finds application particularly in the field of active multispectral imaging and active hyperspectral imaging, for obtaining biochemical information easily and quickly. In particular, the chemical composition of a sample can be determined from its infrared light absorption signature.

[0109] The invention is not limited to the examples detailed above, and many other examples can be implemented without departing from the scope of the invention. For example, the extraction assembly can be located in the first cladding layer, on the side of the scene to be illuminated, or in the core layer of the planar waveguide. The gratings of the extraction assembly can extend through all or part of the thickness of the core layer, or the first cladding layer, respectively. If they are formed in the core layer, they advantageously consist of portions of material from the core layer and portions of material from the first and second cladding layers, respectively. The invention is also not limited to one-dimensional diffraction gratings, and also covers devices in which the extraction assembly comprises or is made up of two-dimensional diffraction grating(s).A two-dimensional diffraction grating is preferably in the form of a grid of dots. Similarly, the invention is not limited to the examples of materials mentioned. For example, the support substrate can be made of any material having at least one infrared transparency window and a refractive index lower than that of the cladding layers. Likewise, the detailed system examples above can be combined with each of the embodiments and variants of the light-spreading device according to the invention. According to other variants, the planar waveguide can include a spatial spreading region upstream of an extraction region, with the spatial spreading region intended to achieve spatial spreading of the light and the extraction region receiving the extraction assembly.A grayscale diaphragm can be positioned upstream of the light distribution device according to the invention, to homogenize the illumination on the scene to be lit.

Claims

Claims

1. Light distribution device (100; 300; 400; 600; 700; 800; 900) configured to, in use, distribute light rays from an ancillary light source (14) over a scene to be illuminated (S), characterized in that it comprises: - a planar waveguide (110; 310; 410) which comprises two cladding layers (130i, 1302) and a core layer (120), with two larger faces (121, 122) of the core layer extending parallel to a plane called the plane of the planar waveguide, and with the two cladding layers (130i, 1302) and the core layer (120) being superimposed together along an axis orthogonal to the plane of the planar waveguide with the core layer disposed between the two cladding layers; and - an extraction assembly (150; 350; 450; 550; 650), located in the planar waveguide, and consisting of a plurality of diffraction gratings (151; 151'; 151”; 751;851) distributed along the two dimensions of a plane parallel to the plane of the planar waveguide, and - in that an average value of a filling factor varies monotonically, from one diffraction grating to another of the extraction assembly (550), and along an axis parallel to the plane of the planar waveguide.;

2. Device (300; 400; 600; 700; 800) according to claim 1, characterized in that it further comprises a support substrate (360; 460; 660), transparent over a range of wavelengths of use of the planar waveguide, and superimposed on the planar waveguide (310; 410) along an axis orthogonal to the plane of the planar waveguide.

3. Device (400; 600; 700; 800) according to claim 2, characterized in that the support substrate (460; 660) comprises a cavity (462; 662), located on the side opposite the planar waveguide (410), and of which an extent in a plane parallel to the plane of the planar waveguide is greater than or equal to an extent of the extraction assembly (450; 650) in a plane parallel to the plane of the planar waveguide.

4. Device (100; 300; 400; 600; 700; 800; 900) according to any one of claims 1 to 3, characterized in that the diffraction gratings (151) of the extraction assembly extend according to a series of parallel strips, each extending from one edge to the other of the extraction assembly.

5. Device (100; 300; 400; 600; 700; 800; 900) according to any one of claims 1 to 3, characterized in that the diffraction gratings (151'; 151”; 751; 851) of the extraction assembly are distributed along the two dimensions of a plane parallel to the plane of the planar waveguide.

6. Device (100; 300; 400; 600; 700; 800; 900) according to any one of claims 1 to 5, characterized in that it is configured to, in use, distribute over the scene to be illuminated (S) light rays whose wavelength spectrum is centered on a wavelength called the central wavelength, and in that the diffraction gratings (151; 151'; 151"; 751; 851) of the extraction assembly (150; 350; 450; 550; 650) all have the same average value of a pitch, with said average value of a pitch adapted to extract from the planar waveguide, and along an axis orthogonal to the plane of the planar waveguide, a light beam at the central wavelength propagating in the waveguide planar (110; 310; 410).

7. Device (100; 300; 400; 600; 700; 800; 900) according to any one of claims 1 to 6, characterized in that the extraction assembly (150; 350; 450; 550; 650) is located in one of the two cladding layers (130i, 1302), and in that the patterns of the diffraction gratings (151; 151'; 151”; 751; 851) of the extraction assembly (150; 350; 450; 550; 650) each consist of at least one solid zone and at least one hollow zone, where the at least one solid zone is made of the material of the cladding layer (1302) receiving the extraction assembly, and ... the less a hollow area is suitable for being occupied by a surrounding environment.

8. Device (100; 300; 400; 600; 700; 900) according to any one of claims 1 to 7, characterized in that the diffraction gratings (151; 151'; 151”; 751; 851) of the extraction assembly each consist of a plurality of patterns (1510), and in that the patterns of said diffraction gratings extend along straight lines parallel to each other.

9. Device (800) according to any one of claims 1 to 7, characterized in that the diffraction gratings (851) of the extraction assembly each consist of a plurality of patterns, and in that the patterns of said diffraction gratings extend along convex curved lines which each extend from one edge to the opposite edge of the planar waveguide.

10. Device (100; 300; 400; 600; 700; 800; 900) according to any one of claims 1 to 9, characterized in that it is configured to, in use, distribute over the scene to be illuminated (S) light rays whose wavelength spectrum extends from a minimum wavelength to a maximum wavelength, and in that the planar waveguide (110; 310; 410) is single-mode along an axis orthogonal to the plane of the planar waveguide and to said minimum wavelength.

11. An infrared imaging system (1000; 2000; 3000; 4000) which comprises: - a light distribution device (600; 700; 800; 900) according to any one of claims 1 to 10, configured to, in use, distribute light rays coming from an additional light source over a scene to be illuminated; and - a detection module (10), comprising a matrix (10A) of photodetectors (11) configured to receive light rays returned by the scene to be illuminated; with the matrix (10A) of photodetectors (11) located opposite the extraction assembly (650), and on the side of the planar waveguide receiving the extraction assembly, and with a superposition rate between the photodetectors (11) of the matrix of photodetectors and the diffraction gratings of the extraction assembly less than or equal to 50%.

12. The system (2000; 3000; 4000) of claim 11, which further comprises an infrared light source (14) forming the ancillary light source, and wherein the light distribution device (700; 800; 900) is configured to, in use, receive as input light rays from said infrared light source and distribute these rays over the scene to be illuminated, with the input of the light distribution device formed by a transverse face (713) of the planar waveguide.

13. The system (700; 900) of claim 11 or 12, which further comprises an imaging assembly (15) comprising at least one refractive lens, disposed between the inlet (713) of the light distribution device and the infrared light source (14), and configured to receive as input the light emitted by the infrared light source and to provide as output a beam of light rays parallel to each other.

14. System (4000) according to any one of claims 11 to 13, characterized in that it comprises a spacing element (95), mounted integral with the detection module (10) and provided with a bearing surface (97) intended to come into contact with a sample to be analyzed (96) and located on one side of the spacing element (95) opposite the detection module (10), and in that the light distribution device (900) forms all or part of the spacing element.

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