Multilevel backpropagation device

Abstract

Title: Multilevel Backpropagation Device The invention relates to a multilevel backpropagation device for electromagnetic radiation, comprising at least a first level (n1) and a second level (n2) at least partially superimposed, the first level comprising a first waveguide (11) for guiding the propagation of light radiation in a first direction (s1) and the second level comprising a second waveguide (21) for guiding the propagation of light radiation in a second direction (s2) opposite to the first direction, the device comprising: • a coupling portion (3) in which the first and second waveguides are coupled by bringing them closer together so that the electromagnetic radiation propagates from the first level (n1) to the second level (n2), and • at least one reflector (4) at one end (30) of the coupling portion (3), configured to reflect the electromagnetic radiation. Figure for the abstract: Fig.3A.

Description

Title of the invention: Multi-level back-propagation device Technical field

[0001] The field of the invention is that of integrated photonics. The invention finds for particular application a compact addressing device for a projector, in particular for a retinal projector. STATE OF THE ART

[0002] One of the current challenges in the application fields related to integrated photonics is the ever-increasing need for integration. Advances in optical telecommunications or LiDARs require ever-increasing component densities on limited surfaces. To increase the density of photonic circuits, the most common method is to bring the components closer together while preventing parasitic coupling. To do this, it is possible to optimize the dimensioning of the components or the shape of the components. It is also possible to add structures between the components to act as a barrier to parasitic coupling.

[0003] [Fig.l] illustrates a conventional photonic projector architecture, comprising an addressing part al attached to a projection part pi. The addressing part al typically comprises a plurality of waveguides 11 originating from a main waveguide, for example by means of a balanced tree of multimode interferometers MMI (Multimode Interferometer). The projection part pl typically comprises a dense distribution of waveguides (4096 guides in [Fig.l] which are not visible individually) made of silicon nitride (Si3N4) with rectangular section, spaced regularly.

[0004] Such a photonic architecture occupies a significant surface area of ​​the chip or substrate. In particular, the addressing part al has a non-negligible surface area compared to that of the projection part pl.

[0005] There is therefore a need to reduce the size of such photonic architectures, so as to free up surface area to be able to integrate more components and / or densify the photonic systems. One objective of the invention is to meet this need.

[0006] In particular, one object of the invention is a multi-level back-propagation device for superimposing different waveguide levels, for example an addressing level and a projection level. Another object of the invention is an integrated projector comprising a superimposed addressing level and a projection level.

[0007] Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0008] To achieve this objective, according to one embodiment, a multi-level back-coupling device for light radiation is provided, comprising at least a first level extending along a first plane and a second level extending along a second plane parallel to the first plane, said first and second levels being at least partly superimposed along a direction normal to the first and second planes, the first level comprising at least a first waveguide intended to guide propagation of the light radiation along a first direction and the second level comprising at least a second waveguide intended to guide propagation of the light radiation along a second direction opposite to the first direction.

[0009] Advantageously, the device comprises: - a coupling portion in which the first and second waveguides are coupled by proximity so that the electromagnetic radiation propagates from the first level to the second level, and - at least one reflector at one end of the coupling portion, configured to reflect electromagnetic radiation.

[0010] This makes it possible to superimpose at least two levels comprising waveguides. The conventional photonic projector architecture illustrated in [Fig.l] can thus be “folded” as illustrated in [Fig.2]. The projection level p2 located on the second level is here superimposed on the addressing level al located on the first level. In [Fig.2], the light radiation propagates from left to right in the addressing level al; it is coupled to the projection level p2 in the coupling region 3, and reflected by the reflector 4; the light radiation propagates from right to left in the projection level p2.

[0011] The multi-level back-coupling device has significantly improved compactness. This advantageously makes it possible to reduce the size of a photonic architecture comprising such a multi-level back-coupling device.

[0012] Another aspect of the invention relates to an optical projection system comprising a multi-level back-coupling device as described above, in which the second level corresponds to a projection level. Such a system further comprises an electrode below the projection level. This electrode, called the lower electrode, is typically used to activate extraction elements on the surface of the device. Activation can in particular be achieved by associating an attached counter-electrode with a component arranged above the system. optical projection. Activation is done, for example, with a liquid crystal layer located between the lower electrode and the counter electrode. This allows part of the light radiation guided in the waveguides to be extracted from the projection level, in particular when applying an electrical voltage between the lower electrode and the counter electrode, in order to emit light points and display an image. BRIEF DESCRIPTION OF THE FIGURES

[0013] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0014] [Fig.l] [Fig.l] illustrates a conventional photonic projector architecture, according to the prior art.

[0015] [Fig.2] [Fig.2] illustrates a photonic projector architecture, according to an embodiment of the present invention.

[0016] [Fig.3A][Fig.3B] Figures 3A and 3B schematically illustrate a multi-level back-coupling device according to an embodiment of the present invention, respectively in top view and in cross-section.

[0017] [Fig.4A] [Fig.4A] schematically illustrates in top view a multi-level back-coupling device according to another embodiment of the present invention.

[0018] [Fig.4B][Fig.4C][Fig.4D] Figures 4B, 4C and 4D illustrate simulation results of a propagation of light radiation in the multi-level back-coupling device illustrated in [Fig.4A], respectively in an xyl plane of the first level n1, in an xy2 plane of the second level n2, and in a transverse plane xz.

[0019] [Fig.5A][Fig.5B] Figures 5A and 5B schematically illustrate two variants of reflectors formed by Bragg mirrors, according to embodiments of the present invention.

[0020] [Fig.6] [Fig.6] shows iso values ​​of bandwidth (in nm) of a Bragg mirror as a function of the length L of the mirror (in pm) and the index perturbation An.

[0021] [Fig.7] [Fig.7] schematically illustrates in top view a multi-level feedback device according to another embodiment of the present invention.

[0022] [Fig.8][Fig.9][Fig.lO][Fig.ll][Fig.l2] Figures 8 to 12 illustrate backcoupling rate curves for the device illustrated in [Fig.7], as a function of the coupling length and the approach distance ds.

[0023] [Fig. 13] [Fig. 13] illustrates the optical losses by absorption induced by an ITO layer as a function of the gap between said ITO layer and a waveguide.

[0024] [Fig. 14A] [Fig. 14B] Figures 14A and 14B schematically illustrate the positioning of an electrode in an optical system comprising a multi-level back-coupling device, according to an embodiment of the present invention.

[0025] [Fig.l5A][Fig.l5B] Figures 15A and 15B schematically illustrate the positioning of an electrode in an optical system comprising a multi-level back-coupling device, according to another embodiment of the present invention.

[0026] [Fig.l6A][Fig.l6B] Figures 16A and 16B schematically illustrate the positioning of an electrode in an optical system comprising a multi-level back-coupling device, according to another embodiment of the present invention.

[0027] [Fig. 17] [Fig. 17] illustrates a photonic projector architecture, according to an embodiment of the present invention.

[0028] [Fig. 18] [Fig. 18] illustrates a multi-level back-coupling device called RGB, comprising sub-levels dedicated to light radiation of different wavelengths, according to an embodiment of the present invention.

[0029] [Fig.l9A][Fig.l9B] Figures 19A and 19B schematically illustrate an RGB multi-level feedback device according to an embodiment of the present invention, respectively in top view and in cross-section.

[0030] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular, on the schematic diagrams, the thicknesses and / or dimensions of the different layers, patterns and reliefs are not necessarily representative of reality. DETAILED DESCRIPTION

[0031] Before beginning a detailed review of embodiments of the invention, optional features which may possibly be used in combination or alternatively are set out below:

[0032] According to one example, the reflector is formed by an interruption of the first and second waveguides in a plane normal to the first and second directions of propagation.

[0033] According to one example, the reflector is formed by a block interrupting the first and second waveguides, said block being based on a reflective material such as aluminum. Such a reflector is not wavelength selective. It is advantageously broadband. Such a reflector can therefore be used for a large number of applications, for example in the visible and infrared ranges.

[0034] According to one example, the block is formed by a metal layer deposited on the surface of a plane which intersects the first and second waveguides perpendicular to the direction of propagation. This can be done by creating a trench with one wall forming this plane. This trench, which has the shape of a cavity, is typically covered with a metal layer by deposition, typically between 30 and 200 nm of aluminum. The cavity is not necessarily filled with metal. The metal layer on the cavity wall has a thickness typically between 30 and 200 nm. This provides good reflectivity for the reflective block. In another example, the block is formed from the following steps: - Cut the device at the end of the coupling portion, - Polish this end, - Make a metal deposit on this end.

[0035] According to one example, the coupling portion has a length Lc / 2 along a direction of propagation of the electromagnetic radiation, Le being the characteristic coupling length for a transfer of a propagation mode of the electromagnetic radiation from the first waveguide to the second waveguide. Thus, the coupling portion is folded back on itself thanks to the reflector and has a length which advantageously corresponds to half the characteristic coupling length. The coupling is thus more compact.

[0036] According to one example, the reflector is formed by a first Bragg mirror attached to the first waveguide and by a second Bragg mirror attached to the second waveguide, said first and second Bragg mirrors being superimposed in the normal direction. Such a reflector can be easily manufactured by microelectronics processes and technologies. It is also wavelength selective. This makes it possible, for example, to filter light radiation and / or to create sub-levels dedicated to light radiation of different wavelengths.

[0037] According to one example, the coupling portion has a length Lc / 2 - ô along a direction of propagation of the electromagnetic radiation, Le being the characteristic coupling length for a transfer of a propagation mode of the electromagnetic radiation from the first waveguide to the second waveguide, and ô > 0 being a penetration length of the mode in the first and second Bragg mirrors. The length of the coupling portion is here reduced, due to the penetration of the propagation mode in the Bragg mirror(s).

[0038] According to one example, the first level corresponds to an addressing level and the lower electrode is located below the addressing level. This makes it possible to limit or eliminate absorption phenomena linked to the lower electrode.

[0039] According to one example, the first level corresponds to an addressing level and the lower electrode is located between the projection level and the addressing level. This makes it possible to bring the lower electrode closer to the projection level. This makes it possible to reduce the voltage to be applied to extract the light radiation.

[0040] According to one example, the first level corresponds to an intermediate level and the lower electrode is located at the same level as the intermediate level, and the system further comprises an addressing level located below the lower electrode and the intermediate level. This makes it possible to reduce optical losses while reducing the voltage to be applied to extract the light radiation. The use of an intermediate level between the addressing level and the projection level also makes it possible to optimize the coupling and to reduce the length of the coupling portion.

[0041] According to an example, the at least one first level comprises a first addressing level comprising waveguides configured to guide a propagation of a first light radiation having a first wavelength X1, a second addressing level comprising waveguides configured to guide a propagation of a second light radiation having a second wavelength X2, a third addressing level comprising waveguides configured to guide a propagation of a third light radiation having a third wavelength X3. This makes it possible to form, for example, a multi-level RGB (acronym for “Red Green Blue”) backcoupling device or an RGB projector.

[0042] According to one example, the projection level comprises waveguides configured to respectively guide the propagations of the first, second and third light rays. The projection is thus done from a single level. According to one example, the first, second and third addressing levels are located at different depths dpi, dp2, dp3 with respect to the projection level in the normal direction. This makes it possible to optimize the propagation of light rays of different wavelengths, by limiting optical losses by crossing between the different levels.

[0043] According to an example, X3 > X2 > XI, and dp3 > dp2 > dpi.

[0044] According to one example, the reflector comprises at least one first Bragg mirror configured to reflect the first wavelength X1, at least one second Bragg mirror configured to reflect the second wavelength X2, and at least one third Bragg mirror configured to reflect the third wavelength X3, said first, second and third Bragg mirrors being located at the same depth dpO with respect to the projection level in the normal direction.

[0045] According to one example, the waveguides of the first, second and third addressing levels are optically connected to the first, second and third Bragg mirrors by at least one portion of intermediate depth guide dpmi (i=l.. .3) such that dp3 < dpmi < dpO.

[0046] According to one example, the waveguides of the first addressing level are attached to the at least one first Bragg mirror without an inter-depth guide portion. intermediate and dpi = dpO, the waveguides of the second addressing level are connected to the at least one second Bragg mirror by a portion of intermediate depth guide dpml = dpO, the waveguides of the third addressing level are connected to the at least one third Bragg mirror by a portion of intermediate depth guide dpm2 = dp2 and by a portion of intermediate depth guide dpm3 = dpO.

[0047] Unless incompatibility exists, it is understood that all of the above optional features may be combined to form an embodiment that is not necessarily illustrated or described. Such an embodiment is obviously not excluded from the invention. The features and advantages of one aspect of the invention, for example the device or the method, may be adapted mutatis mutandis to the other aspect of the invention.

[0048] The invention relates generally to a multi-level back-coupling photonic device. The term "back-coupling" designates the association of the optical coupling functions of a propagation mode of light radiation, typically between two waveguides located on levels of different altitude, and of back-propagation of this light radiation.

[0049] In the following, the term "absorption" or its equivalents refers to the phenomenon by which the energy of an electromagnetic wave is transformed into another form of energy, for example by thermal dissipation. In the present description, a material is considered to be absorbent when it absorbs at least 20% of light radiation.

[0050] In the following, the term "reflection" or its equivalents refers to the phenomenon of re-emission of incident light radiation from an element or a surface. In the present description, an element is considered to be reflective when it re-emits at least a portion of incident light radiation, this portion being greater than or equal to 50%, preferably this portion being greater than or equal to 80% of the incident light radiation.

[0051] It is specified that, in the context of the present invention, the terms "on", "overcomes", "covers", "underlying", "facing" and their equivalents do not necessarily mean "in contact with". Thus, for example, the deposition, transfer, bonding, assembly or application of a first layer on a second layer does not necessarily mean that the two layers are in direct contact with each other, but means that the first layer at least partially covers the second layer by being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element.

[0052] A layer may also be composed of several sub-layers of the same material or of different materials.

[0053] A substrate, a film, a layer, a structural element “based” on a material A is understood to mean a substrate, a film, a layer, a structural element comprising this material A only or this material A and possibly other materials, for example doping elements or alloying elements.

[0054] A preferably orthonormal reference frame, comprising the x, y, z axes, is shown in the attached figures. When a single reference frame is shown on the same sheet of figures, this reference frame applies to all the figures in this sheet.

[0055] In the present patent application, the thickness, height or depth are taken along z. The relative terms "on", "overcomes", "under", "underlying" refer to positions taken along the z direction. The length is taken along x and the width is taken along y. A "lateral" dimension corresponds to a dimension along a direction of the xy plane. A "lateral" or "laterally" extension is understood to mean an extension along one or more directions of the xy plane.

[0056] An element located "perpendicular to" or "in line with" another element means that these two elements are both located on the same line perpendicular to a plane in which a lower or upper face of a substrate mainly extends, that is to say on the same line oriented vertically in the cross-sectional figures.

[0057] The separation distance ds is typically taken from "center to center", between the central axes of the waveguides.

[0058] The terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and..." and equivalents mean that the limits are included, unless otherwise stated.

[0059] A principle of the present invention consists in implementing an optical coupling between two levels ni, n2 of different altitude along z, this coupling being combined with a reflection so as to change the direction of propagation of the light radiation. It is thus possible to stack and superimpose different photonic levels in which the light radiation propagates. Typically, a second projection level p2 located at an altitude n2 can be superimposed along z on a first addressing level al located at an altitude ni, as illustrated in [Fig.2].

[0060] As illustrated in Figures 3A and 3B, the device typically comprises a first waveguide 11 configured to guide the propagation of light radiation in a first direction s1, and a second waveguide 21 configured to guide the propagation of the light radiation in a second direction s2 opposite to the first direction s1. The first waveguide 11 is located on a first level ni having an altitude ni along z. The second waveguide 21 is located on a second level n2 having an altitude n2 along z. The device comprises a coupling portion 3 at which the first and second waveguides 11, 21 are brought together and have a minimum separation distance ds allowing optical coupling, typically evanescent coupling, between the first waveguide 11 and the second waveguide 21. This coupling portion 3 has a length L along x. At the end 30 of the coupling portion 3, the device further comprises a reflector 4 configured to reflect the light radiation. In the example illustrated in FIGS. 3A, 3B, the reflector 4 is formed by a layer 40 based on a reflective material, for example aluminum.

[0061] [Fig.4A] illustrates an embodiment of the backcoupling device comprising guides 11, 21 made of silicon nitride (Si3N4) having a refractive index n=1.97 for light radiation of wavelength X = 532 nm. The guides 11, 21 have a width along y of 300 nm and a thickness along z of 200 nm. The guides 11, 21 are thus single-mode for light radiation of wavelength X = 532 nm. The light radiation thus propagates according to the fundamental optical mode within the guides 11, 21. The backcoupling principle according to the invention can however be implemented for different propagation modes, for example for secondary coupling modes.

[0062] The guides 11, 21 are superimposed along z at the coupling portion 3. Along the coupling portion 3, the fundamental optical mode can be transferred from one guide 11 to the other guide 21 by evanescent coupling, if the two guides 11, 21 are sufficiently close to each other over a sufficiently long length. The “sufficiently close” distance is the minimum separation distance ds. The “sufficiently long length” is called the coupling length Le, and corresponds to the characteristic length necessary for a mode, typically the fundamental mode, to be transferred from the first guide 11 to the second guide 21 while retaining its direction of propagation, i.e. in the absence of disturbances along the guide taken by the light radiation. This mode transfer without modification of the direction of propagation can be conventionally carried out by a directional coupler.According to the so-called super-mode approach, the transferred mode can be seen as the combination of a symmetric mode and an antisymmetric mode on the two guides 11, 21. The coupling length is then written: .

[0063] £ =_To_ c 2An

[0064] with An = lneffi— neff2l which corresponds to the difference in effective indices between the symmetric and antisymmetric modes. For a separation distance ds of 550 nm between the center of the first guide and the center of the second guide, the coupling length Le is of the order of 60 pm.

[0065] In the present case, a reflector 4, here a block 40 in the form of an aluminum layer with a thickness of 100 nm along x, interrupts the guides 11, 21 at one end. of the coupling portion 3. The light radiation thus passes through the coupling portion 3 a first time in the direction si, then a second time in the direction s2. For this embodiment, the coupling portion 3 has a length L = Lc / 2. For a separation distance ds of 550 nm between the center of the first guide and the center of the second guide, the length L of the coupling portion 3 is therefore of the order of 30 pm. The coupling portion 3 of this device is thus twice as compact as a conventional directional coupler.

[0066] Figures 4B, 4C and 4D illustrate FDTD simulation results (acronym for “Finite Difference Time Domain” designating a method for calculating finite differences in the time domain) obtained for propagation of light radiation of wavelength X = 532 nm in the device illustrated in [Fig.4A].

[0067] [Fig.4B] is a simulated mapping of the electromagnetic field in an xyl plane corresponding to the first level n1. It appears that the incident light radiation propagates in the first guide 11 of the first level n1. [Fig.4C] is a simulated mapping of the electromagnetic field in an xy2 plane corresponding to the second level n2. It appears that the reflected light radiation propagates in the second guide 21 of the second level n2. [Fig.4D] is a simulated mapping of the electromagnetic field in a transverse xz plane. It appears that the light radiation is split between the first guide 11 and the second guide 21 as it approaches the reflector. This shows that coupling and back-propagation are possible in combination.

[0068] Based on these numerical simulations carried out on the device of [Fig.4A], a maximum backcoupling of approximately -0.68 dB (i.e. approximately 87%) is obtained.

[0069] Such a device can be produced by creating a cavity in the middle of a directional coupler and making a metal deposit there (of aluminum for example or a stack of titanium / aluminum deposits).

[0070] The coupling portion 3 shown in [Fig.4A] can be manufactured by conventional microelectronics processes, typically by photolithography then by etching a layer of Si3N4 deposited on an oxide layer created by oxidation of a silicon substrate. A mask and a photolithography / etching step are implemented for each guide level.

[0071] The metal mirror 40 can be made in different ways: - by etching a cavity through the guides and the oxide and then metallizing to cover the cavity wall corresponding to the end of the coupling portion. Another photolithography mask is then used to form the cavity. This last mask does not require high dimensional resolution. It is therefore advantageously inexpensive. - by creating a cavity by focused ion beam FIB (acronym for “Focused Ion Beam”) - by forming the end surface of the guides by cutting then polishing, then depositing a metal on said surface

[0072] The advantage of using a block 40 of metal as reflector 4 is that it is not very selective in wavelength. Such a reflector 4 can therefore be used for a large number of applications in the visible and infrared ranges.

[0073] The reflector 4 may also simply consist of an interruption of the guides 11, 21 by a transverse cut of the substrate. This cut is carried out as previously by etching a cavity or cutting the chip. The contrast of refractive indices An = ng - na between the guide material of the guides 11, 21 (typically ng = 2) and the air (index na = 1) leads to a reflection close to 10%. This value is low for a projection device type application but may be sufficient for other applications (for example for sensors). It offers the advantage of not requiring metal deposition and of being dependent on the surrounding environment (useful for a sensor application).

[0074] The reflector 4 can also be made by a stack of dielectric layers with different refractive indices. The layers can be SiO2 and TiO2 with an index contrast that approaches unity. The dimensioning of this stack is similar to that of the Bragg mirrors described below. The high index contrast makes it possible to greatly reduce the length of the equivalent Bragg mirror, thus reducing the number of layers in the stack (typically 10). The formation of such a stack is less expensive.

[0075] Figures 5A and 5B illustrate alternative reflectors 4 formed by Bragg mirrors. Bragg reflectors are very widely used in integrated photonics on silicon. They are easily produced by conventional microelectronics processes.

[0076] [Fig.5A] illustrates a first embodiment of a Bragg mirror 41a, 42a comprising two materials 411, 412 of indices n', n” arranged alternately along the direction of propagation of the light radiation. The interfaces between these two materials cause successive reflections of the wave which penetrates it. At each interface, a part of the wave is reflected, and the other transmitted. The mode of propagation of the light radiation thus penetrates the Bragg mirror 41a, 42a according to a penetration length δ > 0.

[0077] [Fig.5B] illustrates a second embodiment of a Bragg mirror 41b, 42b obtained by corrugation of the sides of a waveguide 11, 21. In this case, the sections 410 of width wl have a first effective index and the narrow sections of width w2 have a second effective index. This makes it possible to recreate a succession of interfaces due to the succession of two different effective indices. propagation mode of the light radiation also penetrates the Bragg mirror 41b, 42b at a penetration length ô > 0.

[0078] The penetration length ô can be evaluated according to:

[0079] - JL. argcosh ( \jë )

[0080] Where X is the wavelength, and An the index contrast An = n' - n”.

[0081] The reflectivity R of the Bragg mirror depends in a known manner on the strength of the grating K and the length of the grating 1 defining the Bragg mirror, according to:

[0082] R = tanh2Kl

[0083] A reflectivity R of 99.9% corresponds to a product k1 of the order of 4.15 rad.

[0084] The bandwidth AX of the Bragg mirror also depends on these parameters K and 1. [Fig.6] illustrates different bandwidth isovalue curves (18 nm, 36 nm, 54 nm...) as a function of the length of the grating 1 and the difference in indices An (k being directly proportional to An). The continuous line curve corresponds to the values ​​of 1 and An giving Kl = 4.15 rad for a theoretical reflectivity of 99.9%.

[0085] For an index contrast An ~ 0.22 (corresponding to a silicon nitride waveguide encapsulated in silica) with corrugations of width w2 = 75 nm and period A ~ 170 nm, a reflectivity of approximately 99.9% is achieved for a grating length 1 of 5.13 pm. The bandwidth is estimated in this case to be approximately 54.5 nm.

[0086] [Fig.7] illustrates an embodiment of the backcoupling device comprising guides 11, 21 made of silicon nitride (Si3N4) having a width along y of 300 nm and a thickness along z of 200 nm, and separated by a separation distance ds of 550 nm, as previously.

[0087] In this embodiment, the reflector 4 is formed by two identical superimposed Bragg mirrors 41b, 42b. Here, the coupling portion 3 has a length L = Le / 2 - ô which takes into account the penetration length ô of the light radiation in the Bragg mirrors 41b, 42b. [Fig. 8] shows different curves of backcoupled optical power, as a function of the coupling length L, for different separation distances ds and for a wavelength of 532 nm. For each curve, the second maximum of backcoupling has been retained. The curve Cl is obtained for a separation distance ds of 400 nm between the center of the first guide and the center of the second guide. The length L of the coupling portion 3 corresponding to the maximum of backcoupled power (-0.77 dB) is of the order of 12 pm. Curve C2 is obtained for a separation distance ds of 450 nm.The length L of the coupling portion 3 corresponding to the maximum backcoupled power (-0.76 dB) is of the order of 23 pm. The curve C3 is obtained for a separation distance ds of 500 nm. The length L of the coupling portion 3 corresponding to the maximum of . backcoupled power (-0.80 dB) is of the order of 41 pm. Curve C4 is obtained for a separation distance ds of 550 nm. The length L of the coupling portion 3 corresponding to the maximum backcoupled power (-0.74 dB) is of the order of 70 pm.

[0088] The same type of curves is illustrated in Figures 9 to 12 for different wavelengths.

[0089] [Fig.9] and curves C1 1.. .C15 illustrate the backcoupled optical power, as a function of the coupling length L, for different separation distances ds and for a wavelength of 450 nm. Curve C11 is obtained for a separation distance ds of 350 nm. Curve C12 is obtained for a separation distance ds of 400 nm. Curve C13 is obtained for a separation distance ds of 450 nm. Curve C14 is obtained for a separation distance ds of 500 nm. Curve C15 is obtained for a separation distance ds of 550 nm. The multilevel backcoupling ratio is between -1.33 dB (-73.6%) and at least -0.72 dB (-84.7%) at X = 450 nm.

[0090] [Fig. 10] and curves C21.. .C25 illustrate the backcoupled optical power, as a function of the coupling length L, for different separation distances ds and for a wavelength of 633 nm. Curve C21 is obtained for a separation distance ds of 400 nm. Curve C22 is obtained for a separation distance ds of 450 nm. Curve C23 is obtained for a separation distance ds of 500 nm. Curve C24 is obtained for a separation distance ds of 550 nm. Curve C25 is obtained for a separation distance ds of 600 nm. The multilevel backcoupling ratio is between -1.05 dB (-78.5%) and -0.87 dB (-81.9%) at X = 633 nm.

[0091] [Fig.11] and curves C31...C35 illustrate the backcoupled optical power, as a function of the coupling length L, for different separation distances ds and for a wavelength of 1320 nm. Curve C31 is obtained for a separation distance ds of 800 nm. Curve C32 is obtained for a separation distance ds of 900 nm. Curve C33 is obtained for a separation distance ds of 1000 nm. Curve C34 is obtained for a separation distance ds of 1100 nm. Curve C35 is obtained for a separation distance ds of 1300 nm. The multilevel backcoupling ratio is between -4.37 dB (-36.6%) and at least -0.83 dB (-82.6%) at X = 1320 nm. These results are given for information purposes only. Some values, notably the value -4.37 dB, may be underestimated, particularly in view of the sampling of the different curves.

[0092] [Fig. 12] and curves C41.. .C45 illustrate the backcoupled optical power, as a function of the coupling length L, for different separation distances ds and for a wavelength of 1550 nm. Curve C41 is obtained for a distance ds separation distance of 900 nm. Curve C42 is obtained for a ds separation distance of 1000 nm. Curve C43 is obtained for a ds separation distance of 1100 nm. Curve C44 is obtained for a ds separation distance of 1200 nm. Curve C45 is obtained for a ds separation distance of 1400 nm. The multilevel backcoupling ratio is between -1.85 dB (-36.6%) and at least -0.47 dB (-89.7%) at X = 1550 nm.

[0093] Generally speaking, the backcoupling rate tends to increase when the separation distance ds increases. Indeed, two Bragg mirrors positioned too close to each other can interact and generate optical losses. It is preferable to use relatively large distances ds, to facilitate manufacturing (robustness of the design). An inaccuracy on the coupling length in the design also has little impact because the rate curves show a significant plateau at their maximum. For all the wavelengths presented, a simulated backcoupling rate greater than 80% can be obtained for at least one separation distance ds. The optimal results obtained by simulations are summarized in the following table:

[0094] [Tables 1] X = 450 nm X = 633 nm X = 1320 nm X = 1550 nm distance ds (nm) 450 550 1000 1200 coupling length L (qm) 45 51.4 36.3 48.4 backcoupling ratio (dB) -0.72 -0.87 -0.83 -0.5

[0095] Single-mode waveguides and Bragg mirrors are typically resized for each wavelength. Optimized Bragg mirrors (R = 99.9%) at wavelengths 450 nm, 633 nm, 1320 nm, and 1550 nm typically have the parameters listed in the following table:

[0096] [Tables2] X (nm) 450 633 1320 1550 Thickness (nm) 200 200 500 600 Width wl (nm) 220 400 750 900 Period A (nm) 143 204 420 490 Length 1 (qm) 4.4 6.4 12.0 13.8 Width w2 (nm) 55 100 260 330 Estimated AX (nm) 45.6 62.4 143.3 171.9

[0097] Bragg mirrors can be further optimized by increasing the grating strength. Increasing the grating strength increases the reflectivity of the Bragg mirror and broadens the bandwidth, making the Bragg mirror even more robust in wavelength.

[0098] The multi-level back-coupling device can advantageously be implemented within the framework of an optical projection system, for example an integrated display for retinal projection. To produce such a projection system, a projection level is superimposed on an addressing level. The projection level here corresponds to the second level of the multi-level back-coupling device, and the addressing level corresponds to the first level of the multi-level back-coupling device. To extract the light from the projection level, the system further comprises an electrode below the projection level, called the lower electrode. This lower electrode is typically associated with a counter-electrode or upper electrode, generally in the form of a grid, attached by a second device, for example a liquid crystal device.

[0099] The lower electrode and / or the counter-electrode are preferably based on indium tin oxide (ITO). The lower electrode is typically positioned neither too far from the counter-electrode grid to avoid excessively high potential drops, nor too close to the counter-electrode grid to avoid optical losses by absorption. To evaluate the optical losses linked to the position of the lower electrode, it will be considered that an ITO-based electrode located 300 nm from a waveguide induces an absorption loss of -1.35 dB / cm. These losses decrease exponentially when the gap between the ITO layer and the waveguide increases, as illustrated in Fig. 13.

[0100] Figures 14A, 14B illustrate a first embodiment of the projection system in which the lower electrode 50 is positioned below the second projection level n2 p2 and below the first addressing level ni al. This makes it possible to limit or eliminate optical losses by absorption within the ITO.

[0101] The addressing level al comprises waveguides 11 typically subdivided by means of multimode interferometers MMI 60. According to one example, the light radiation is thus distributed in a balanced manner over a total of 4096 guides. The waveguides 11 open onto Bragg reflectors 41a. The projection level p2 comprises waveguides 21 partly in line with the waveguides 11 in the coupling portion 3. The waveguides 21 are attached to Bragg reflectors 42a. The Bragg reflectors 42a are superimposed along z on the Bragg reflectors 41a. When the light radiation is injected into the waveguides 11 in the direction of the Bragg reflectors 41a, in the direction SI, it is reflected and injected by coupling into the waveguides 21, in the direction S2 opposite to SL II is then extracted from the projection level by applying a voltage between the extraction electrodes, i.e. between the lower electrode and the upper electrode. By superimposing the addressing and projection levels, this projection system is advantageously compact. The first embodiment of the projection system typically makes it possible to limit optical losses.

[0102] The greater the distance between the two extraction electrodes, the greater the voltage to be applied to extract the light radiation. This affects the energy efficiency of an integrated projector. The embodiments of the projection system illustrated in Figures 15A, 15B and 16A, 16B aim to reduce the voltage necessary to apply to the electrodes to extract the light radiation. The elements described and referenced for the first embodiment are also transposable to the following two embodiments.

[0103] Figures 15A, 15B illustrate a second embodiment of the projection system in which the lower electrode 50 is positioned between the second projection level n2 p2 and the first addressing level ni al. This makes it possible to bring the lower electrode 50 closer to the surface of the projector in the vicinity of which the counter-electrode is located. The extraction voltage is thus reduced. To limit the optical losses by absorption due to the lower electrode 50, the separation distance ds between the first and second levels ni, n2 is increased. According to one example, ds > 850 nm. The coupling length of the coupling portion 3 is increased accordingly. This affects the compactness of the system. The second embodiment of the projection system typically allows moderate power consumption.

[0104] Figures 16A, 16B illustrate a third embodiment of the projection system in which the lower electrode 50 is positioned at the same level as an intermediate level designated ai. The lower electrode 50 is positioned above an addressing level a0, and below the projection level p2. In this embodiment, the addressing level a0 comprises waveguides 31 without reflectors. The intermediate level ai comprises waveguides 11 opening onto Bragg reflectors 41a. The projection level p2 comprises waveguides 12 attached to Bragg reflectors 42a. The light radiation is here injected into the waveguides 31 of the addressing level aO, in the direction SL II is then injected by coupling 13 into the waveguides 11, in the direction SL II is then reflected and injected by coupling into the waveguides 21, in the direction S2 as previously.It is then extracted from the projection level by applying a voltage between the extraction electrodes.

[0105] The intermediate level ai makes it possible to make an optical bridge between the addressing level aO and the projection level p2. The separation distance ds can therefore example be reduced to 450 nm. This makes it possible to drastically reduce the length of the coupling portions 13, 3. A finite element simulation of the eigenmodes FDE (Finite Difference Eigenmode) indicates that a coupling length Le for the coupling portion 3 of the order of 20.6 pm is suitable. The compactness of the system is preserved. Thanks to the intermediate level ai, the guides 31 can be moved away from the lower electrode 50. This makes it possible to limit or eliminate optical losses by absorption. The guides 31 are also relatively further away from the guides 21. This makes it possible to limit or eliminate optical losses by multilevel crossings. The optical losses for this third embodiment are comparable to those induced in the first embodiment. The extraction voltage is also reduced with this positioning of electrode 50, compared to the first embodiment.The third embodiment of the projection system is a good compromise allowing to combine low optical losses and moderate power consumption. Its implementation is more expensive than the implementations of the first and second embodiments. In Figures 16A, 16B, the Bragg mirrors 41a, 42a are associated respectively with the intermediate level ai and the projection level p2. Other associations or configurations are conceivable. The Bragg mirrors 41a, 42a can in particular be associated respectively with the addressing level aO and the intermediate level ai.

[0106] [Fig. 17] illustrates a photonic projector architecture comprising two addressing or routing levels a1, a2, and a projection level p3. This makes it possible, from a few light sources (S), to address emission points distributed arbitrarily on the surface of a retinal projector whose guides 21 have “zigzag” shapes to give the emission points a random distribution.

[0107] The routing part comprises waveguides 11, 12 respectively on two levels, to avoid guide crossings. This makes it possible to limit optical losses. The projection part is located on a third level. The backcoupling device 3, 4 advantageously makes it possible to superimpose the projection level p3 on the addressing levels a1, a2.

[0108] [Fig. 18] illustrates a multi-level back-coupling device called RGB (acronym for “Red Green Blue”), comprising sub-levels dedicated to light radiation of different wavelengths (for example red, green, blue). This device typically comprises 1 IR waveguides configured to guide a first wavelength XI (for example XI = 633 nm), 11V waveguides configured to guide a second wavelength X2 (for example X2 = 532 nm), and 1 IB waveguides configured to guide a third wavelength X3 (for example X3 = 450 nm). The 1 IR, 1IV, 1 IB waveguides are typically located on different levels. The device further comprises waveguides 21R, 21V, 21B respectively configured to guide the first, second and third wavelengths XI, X2, X3. The waveguides 21R, 21V, 21B are typically located on the same level, for example a projection level of an RGB projector.

[0109] The different light rays guided by the waveguides 1 IR, 1 IV, 1 I B propagate by backcoupling in the waveguides 21 R, 21 V, 21 B. The zone Z0 illustrated in [Fig. 18] corresponds to this backcoupling zone.

[0110] Figures 19A and 19B illustrate this backcoupling zone in more detail. The waveguides 1 IR, 21R configured to guide the wavelength X1 are typically sized differently from the waveguides 11V, 21V configured to guide the wavelength X2, and differently from the waveguides 1 IB, 21B configured to guide the wavelength X3. The backcouplers are thus specific to the different wavelengths X1, X2, X3. The guides 11R, 21R have a coupling portion 3R and a reflector 41 R, 42R. The guides 1IV, 21V have a coupling portion 3V and a reflector 41V, 42V. The guides 1 IB, 21B have a coupling portion 3B and a reflector 41B, 42B.

[0111] The smaller the wavelength, the more the propagation mode of the corresponding light radiation is confined within the guide. Thus, for effective coupling, the separation distance along z between the guides 11B, 21B is relatively smaller than the separation distance along z between the guides 1IV, 21V, and the separation distance along z between the guides 1IV, 21V is relatively smaller than the separation distance along z between the guides 11R, 21R. Thus, the addressing level a30 comprising the guides 1IR is the deepest with respect to the projection level p2 comprising the guides 21R. The addressing level a10 comprising the guides 11B is the shallowest with respect to the projection level p2 comprising the guides 21B. The addressing levels a30, a20, a10 have respectively depths dp3, dp2, dpi with respect to the projection level p2, such that dp3 > dp2 > dpi.

[0112] According to one possibility, the deepest addressing levels a30, a20 may use guide portions of intermediate depths to route the light radiation to the projection level p2.

[0113] As illustrated in [Fig.l9B], the guide 1IV of the addressing level a20 can be first coupled to a portion of guide 11IV of intermediate depth dpml. A vertical coupling bridge 12V thus allows the light radiation of wavelength X2 to pass from the guide 1IV to the guide portion 11IV, then to be injected into the guide 21V by backcoupling via the coupling portion 3V and the reflectors 41V, 42V.

[0114] The 1 IR guide of the addressing level a30 can first be coupled to a portion of 11 IR guide of intermediate depth dpm2. This portion of 11 IR guide can be also coupled to a portion of guide 112R of intermediate depth dpm3. Two vertical coupling bridges 12R, 13R thus allow the light radiation of wavelength XI to pass from guide 1 IR to the portion of guide 112R, then to be injected into the guide 21R by backcoupling via the coupling portion 3R and the reflectors 41 R, 42R. The different levels of guides and guide portions can thus form stairs to optimize the coupling and / or limit optical losses. According to one possibility, dpm2 = dp2 and dpm3 = dpml = dpi. The different levels of guides and guide portions are thus easier to produce by depositing and structuring successive layers.

[0115] In view of the above description, it appears clearly that the proposed device and system offer a particularly effective solution for improving the compactness of an integrated photonics system. The invention is not limited to the embodiments previously described.

Claims

Claims

1. Multi-level back-coupling device for light radiation, comprising at least a first level (ni) extending along a first plane (xyl) and a second level (n2) extending along a second plane (xy2) parallel to the first plane (xyl), said first and second levels being at least partly superimposed along a direction (z) normal to the first and second planes (xyl, xy2), the first level comprising at least a first waveguide (11) intended to guide a propagation of the light radiation in a first direction (si) and the second level comprising at least a second waveguide (21) intended to guide the propagation of the light radiation in a second direction (s2) opposite to the first direction,the device comprising: • a coupling portion (3) in which the first and second waveguides are coupled by approximation so that the light radiation propagates from the first level (ni) to the second level (n2) by evanescent coupling, and • at least one reflector (4) at one end (30) of the coupling portion (3), configured to reflect the light radiation.,

2. Device according to the preceding claim in which the reflector (4) is formed by an interruption of the first and second waveguides (11, 21) in a plane (xz) normal to the first and second directions (S1, S2) of propagation.

3. Device according to claim 1 in which the reflector (4) is formed by a block (40) interrupting the first and second waveguides (11, 21), said block (40) being based on a reflective material such as aluminum.

4. Device according to any one of the preceding claims in which the coupling portion (3) has a length L = Lc / 2 in a direction of propagation of the light radiation, Le being the characteristic coupling length for a transfer of a mode of propagation of the light radiation from the first waveguide (11) to the second waveguide (21).

5. Device according to claim 1 in which the reflector (4) is formed by a first Bragg mirror (41a, 41b) attached to the first waveguide (11) and by a second Bragg mirror (42a, 42b) attached to the second waveguide (21), said first and second Bragg mirrors being superimposed in the normal direction (z).

6. Device according to the preceding claim in which the coupling portion (3) has a length L = Lc / 2 - ô along a direction of propagation of the light radiation, Le being the characteristic coupling length for a transfer of a propagation mode of the light radiation from the first waveguide (11) to the second waveguide (21), and ô > 0 being a penetration length of the mode in the first and second Bragg mirrors.

7. Optical projection system comprising a multi-level back-coupling device according to any one of the preceding claims, in which the second level (n2) corresponds to a projection level (p2), said system further comprising an electrode (50), called the lower electrode, below the projection level (p2).

8. System according to the preceding claim in which the first level (ni) corresponds to an addressing level (al) and the lower electrode (50) is located below the addressing level (al).

9. System according to claim 7 in which the first level (ni) corresponds to an addressing level (al) and the lower electrode (50) is located between the projection level (p2) and the addressing level (al).

10. System according to claim 7 in which the first level (ni) corresponds to an intermediate level (ai) and the lower electrode (50) is located at the same level (ni) as the intermediate level (ai), the system further comprising an addressing level (aO) located below the lower electrode (50) and the intermediate level (ai).

11. System according to any one of claims 7 to 10 wherein the at least one first level (ni) comprises a first addressing level (alO) comprising waveguides (11B) configured to guide a propagation of a first light radiation having a first wavelength X1, a second addressing level (a20) comprising waveguides (1IV) configured to guide a propagation of a second light radiation having a second wavelength X2, a third addressing level (a30) comprising waveguides (1 IR) configured to guide a pro-

12.

13.

14. paging a third light radiation having a third wavelength X3, and in which the projection level (p2) comprises waveguides (21B, 21V, 21R) configured to respectively guide the propagations of the first, second and third light radiations, said first, second and third addressing levels (a10, a20, a30) being located at different depths dpi, dp2, dp3 with respect to the projection level (p2) in the normal direction (z). System according to the preceding claim in which X3 > X2 > XI, and dp3 > dp2 > dpi. System according to either of the two preceding claims, in which the reflector (4) comprises at least one first Bragg mirror (41B) configured to reflect the first wavelength X1, at least one second Bragg mirror (41V) configured to reflect the second wavelength X2, and at least one third Bragg mirror (41R) configured to reflect the third wavelength X3, said first, second and third Bragg mirrors (41B, 41V, 41R) being located at the same depth dpO with respect to the projection level (p2) in the normal direction (z), and in which the waveguides (11B, 1IV, 1IR) of the first, second and third addressing levels (a10, a20, a30) are optically connected to the first, second and third Bragg mirrors (41B, 41V, 41R) by at least one portion (111V, 111 R, 112R) of intermediate depth guide dpmi (i=l.. .3) such that dp3 > dpmi > dpO. System (10) according to the preceding claim in which the waveguides (11B) of the first addressing level (alO) are attached to the at least one first Bragg mirror without intermediate depth guide portion and dpi = dpO, the waveguides (1IV) of the second addressing level (a20) are connected to the at least one second Bragg mirror by a portion (11IV) of intermediate depth guide dpmmi = dpO, the waveguides (11R) of the third addressing level (a30) are connected to the at least one third Bragg mirror by a portion (11IR) of intermediate depth guide dpm2 = dp2 and by a portion (112R) of intermediate depth guide dpm3 = dpO.

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