Integrated photonic circuit for demultiplexing a highly multimode incident optical beam and for recombination

Abstract

The invention relates to a planar photonic circuit (PIC) for demultiplexing a multimode incident optical beam into a plurality of single-mode sub-beams comprising a guide layer (GL), the planar photonic circuit comprising: • - an integrated diffractive device (Ddiff) comprising a diffractive grating or two superimposed diffractive gratings (DGO, DG1, DG2, DG3, DG4, DG5, DG6) configured to couple the multimode optical beam in the guide layer of the planar photonic circuit and to generate two or four diffracted beams (Fd1, Fd1', Fd2, Fd2', Fd3, Fd4); • - an integrated conversion device (Dconv) comprising free propagation zones (Zpl), a free propagation zone being associated with each of the diffracted beams, a plurality of collector waveguides arranged downstream of each free propagation zone (OGcol), and a plurality of single-mode waveguides (OGmono), a collector waveguide being coupled to a single-mode waveguide so as to generate the plurality of single-mode sub-beams (Fi).

Description

DESCRIPTION TITLE: Integrated photonic circuit for demultiplexing a highly multimode incident optical beam and recombination. FIELD OF THE INVENTION

[0001] The invention relates to the field of optical communications using a spatial multimode propagation channel. This is particularly the case in free space, where the phase front of the information vector wave is more or less strongly distorted by crossing a disturbed medium, such as an atmospheric layer in an Earth-satellite link. According to another example, propagation in a multimode fiber induces a disturbance of the beam by coupling between the spatial modes.

[0002] More particularly, the invention relates to a planar photonic circuit comprising the functionalities necessary for the spatial demultiplexing of the incident beam, i.e. the collection of the multimode incident beam and its decomposition into a plurality of single-mode optical beams. The invention also relates to a planar photonic circuit integrating the functionalities of recombination and detection / processing of the single-mode beams. STATE OF THE ART

[0003] A problem to be solved for optical communications, when the phase front of an information-carrying optical beam is distorted by a free-space crossing, is to maintain an optimal link budget despite these perturbations.

[0004] If this degraded beam were directly coupled to a single-mode guide, a large majority of the energy on the other spatial modes would be lost and the signal-to-noise ratio would be very degraded. In addition, due to atmospheric disturbances, the energy can quickly pass from one mode to another. In the case of single-mode detection, we arrive at enormous fluctuations in coupled intensity (up to 30 dB for example), whereas by recovering and summing the energy of each of the modes, we greatly reduce these fluctuations in coupled energy. We thus seek to recover the maximum number of spatial modes possible contents in the degraded beam to recover the maximum energy and thus improve the signal-to-noise ratio.

[0005] For this, during reception, a solution consists of collecting and then transforming a multimode beam into multiple single-mode beams, this operation being carried out by a spatial demultiplexing device 14. The different single-mode beams are then recombined, then processed / detected. Thus, the spatial demultiplexing device is inserted into a reception system, three examples of which are given below without limitation.

[0006] The first example illustrated in Figure 1 illustrates a reception system SR1 for the case of a WDM link in which, at transmission, a beam consisting of M multiplexed wavelengths λ, λ2, ...XM, (index j varying from 1 to M) emitted by lasers L1, L2, LM, each carrying a modulation signal (modulators modi, mod2 ... and multiplexer XMUX), is emitted in free space. After propagation in the atmosphere FS the beam has a highly distorted amplitude and phase, forming at reception the incident beam Fid. The reception system SR1 comprises the spatial demultiplexer 14 which decomposes the multimode beam Fid into a plurality of N single-mode beams Fi (i varying from 1 to N). The single-mode beams are then coherently recombined by a recombiner 20, which generates a recombined beam Frecomb.The Frecomb sum beam is then demultiplexed by a wavelength demultiplexing device ÀDEMUX so as to form M beams at M wavelengths Àj, each constituting an information transport channel. Then the M beams are each sent to M detectors PDi, PD2,... PD. M (for M WDM channels). At the output of the detectors, M electrical signals S j are recovered, one per wavelength. This is a coherent recombination reception system with very good recombination efficiency over a wide spectral band or frequency comb (WDM architecture). Document US 2020-0195355 describes such a reception system and a coherent recombiner component architecture 20 that can be integrated into a planar photonic circuit.

[0007] The second example illustrated in Figure 2 concerns an incoherent detection in which the reception system SR2 also includes the spatial demultiplexer 14 which decomposes the multimode beam Fid into a plurality of N single-mode beams Fi. Each single-mode beam Fi is coupled to a demultiplexer ÀDEMUX and the beams at each wavelength Àj are detected by N x M photodetectors PDi,j. Electrical connections perform the summation over i of all the signals from the detectors PDij at the wavelength Àj. At the end of the summation, M electrical signals S^j are recovered, one per wavelength.

[0008] The third example illustrated in Figure 3 illustrates a single-wavelength coherent detection system SR3, for example used in the field of telecommunications, in the case of coherent optical communications to provide very high data transmission rates. The beam emitted by the laser L1 to A1 is modulated both in amplitude and in phase using two modulators mod,l and mod,Q. The reception system SR3 comprises the spatial demultiplexer 14 and the coherent recombiner 20. The recombined signal is then mixed by a mixer 24 with a signal from a local oscillator OL. The mixer is typically a component of the MMI type for "Multi Mode Interference coupler" in English. For conventional coherent detection, by mixing the signal with a local oscillator, the amplitude of the electric field is accessed by beating on a balanced photodiode.In the field of coherent optical communications, both in-phase and quadrature field (I and Q) encoded information is needed. In this case, illustrated in Figure 3, there is coherent detection for the in-phase signal (I) on a balanced detector PDb (output electrical signal Si) and coherent detection for the quadrature signal (Q) detection on another balanced detector (output electrical signal SQ). The 2 to 4 MMI (two inputs: the signal and the local oscillator) directly generates the mixture of the 2 signals with the appropriate phases. This particular modulation format has the largest data transmission capacity, but is even more sensitive to atmospheric disturbances.

[0009] All the reception systems described above require a spatial demultiplexer 14 which is available in several architectures known to those skilled in the art.

[0010] According to a first example described in document EP3100095, spatial sampling of the incident beam is carried out according to a pre-established spatial mode base (for example Hermite-Gauss) by successively crossing phase plates and free propagation sections. This demultiplexer is therefore made up of discrete optical elements, is passive, and does not include components for detection. A commercial product is derived from this patent (TILBA) and has been associated with an integrated photonic circuit for coherent recombination and detection of single-mode beams (V. Billault, et al. "Free space optical communication receiver based on a spatial demultiplexer and a photonic integrated coherent combining circuit," Opt. Express 29, 33134-33143 (2021)).

[0011] According to a second example described in document US20150086157, the spatial demultiplexer performs an adiabatic fusion of single-mode fibers into a multi-mode core fiber. This fully fibered solution is interesting, but there are only a few commercial products, and they only offer a few spatial modes (3 or 6 for the product from the company OPTOSCRIBE).

[0012] The main drawback of these two approaches is that they are relatively bulky and cannot be integrated into a photonic circuit. They must be combined, with the associated connection problems, with other external components to achieve more complex functions (recombination, detection), which makes the final system less compact and less robust to the environment.

[0013] A third example described in the publication by Watanabe et al. "Coherent few mode demultiplexer realized as a 2D grating coupler array in silicon." (Optics Express 28.24 (2020): 36009-36019) describes an integrated photonics device for generating and receiving low-order spatial modes for coupling to a weakly multimode fiber FMF shown in Figure 4. Low-order spatial modes are understood to mean only LP modes O i, LPn a and LPn b . It is rated E O i, E 11a , E 11 b the projections of the incident field onto each of these modes. A weakly multimode optical fiber is a fiber in which a number of spatial modes (by x or y polarization) less than or equal to 4 propagate.

[0014] The technique presented consists of juxtaposing 4 two-dimensional coupling networks G1, G2, G3 and G4 in a 2x2 matrix (see figure 4 A). This decomposition into 4 gratings thus makes it possible to process separately and simultaneously 4 spatial zones of the light incident on the demultiplexer. All non-Gaussian modes having spatial lobes symmetrical with respect to one or two axes (x and / or y) and of opposite amplitude, the overlap integral between a non-Gaussian mode and a diffraction grating towards a single-mode guide is generally close to zero: the non-Gaussian modes are not coupled in the guiding layer. By processing 4 spatial zones separately, this structure makes it possible to collect the intensity of 4 lobes of an optical wave incident on the integrated circuit. This structure thus makes it possible to process the projections E O i, In a, Ei-ib (see Figure 4 B). This system cannot be extrapolated to a higher number of spatial modes. Each of the 4 gratings is also the superposition of 2 orthogonal gratings. Each grating Gi is coupled respectively to 2 perpendicular single-mode guides MC (“Mode Coupler”), to collect the corresponding rectilinear and orthogonal polarizations. The incident beam emerging from the multimode fiber is thus coupled to 8 single-mode guides via the 4 gratings G1 to G4. The signal propagating in each of the 8 single-mode guides is then mixed with a LO signal from a local oscillator by means of an MMI, to perform a coherent l,Q detection on each of the channels. This device has the advantage of being integrated on a photonic circuit but it is adapted to a weakly multimode fiber and does not allow processing a number of spatial modes greater than 4.

[0015] An aim of the present invention is to overcome the aforementioned drawbacks by proposing a planar photonic circuit comprising an integrated spatial demultiplexer capable of processing numerous spatial modes of a highly multimode incident beam. The integration of the demultiplexer also makes it possible to carry out the entire processing on the same planar photonic circuit, including the recombination and detection of single-mode beams. DESCRIPTION OF THE INVENTION

[0016] The present invention relates to a planar photonic circuit for demultiplexing a multimode incident optical beam into a plurality of single-mode sub-beams, comprising a guiding layer, the planar photonic circuit comprising: - an integrated diffractive device comprising one or two superimposed diffractive coupling gratings, configured to couple the multimode optical beam into the guiding layer of the planar photonic circuit and to generate two or four diffracted beams, - an integrated conversion device comprising: o free propagation zones, a free propagation zone being associated with each of the diffracted beams, the conversion device being further configured so that a diffracted beam diverges during its propagation in the associated free propagation zone, o a plurality of collector waveguides arranged downstream of each free propagation zone and o a plurality of single-mode waveguides, a collector waveguide being coupled to a single-mode waveguide and being configured to adiabatically transform a fraction of the multimode optical beam collected locally by said collector guide into an optical beam according to a fundamental mode of said single-mode waveguide, so as to generate said plurality of single-mode sub-beams (Fi).

[0017] According to a first embodiment, the diffractive device comprises a single diffraction grating having plane fringes and generating two diffracted beams propagating in two opposite directions, and the conversion device comprises two diverging lenses positioned on the respective paths of the two diffracted beams upstream of the free propagation zones.

[0018] According to a second embodiment, the diffractive device comprises two diffraction gratings having perpendicular plane fringes, and configured to each generate two diffracted beams propagating in two opposite directions, and the conversion device comprises four diverging lenses positioned on the respective paths of the four diffracted beams upstream of the free propagation zones.

[0019] According to a third embodiment, the diffractive device comprises two diffraction gratings having planar fringes and configured to each generate a single diffracted beam and so that said two diffracted beams propagate in two non-collinear and non-orthogonal directions, and the conversion device comprises two diverging lenses positioned on the respective paths of the two diffracted beams upstream of the free propagation zones.

[0020] According to a fourth embodiment, the diffractive device comprises two diffraction gratings having curved fringes and configured to each generate a single diffracted beam and so that said two diffracted beams propagate in two non-collinear and non-perpendicular directions.

[0021] According to one embodiment, the planar photonic circuit according to the invention further comprises an integrated device for combining and detecting said single-mode sub-beams comprising at least one photodetector.

[0022] According to one embodiment, the integrated combination and detection device further comprises a plurality of delay lines configured to equalize the optical or mixed optical / electrical paths of the single-mode sub-beams, considered from the position of the superimposed diffractive grating(s) and up to a location where a signal is generated carrying information associated with a single wavelength and integrating the contribution of all the single-mode sub-beams.

[0023] According to one embodiment, the integrated combination and detection device comprises: -a plurality of wavelength demultiplexers respectively coupled to the plurality of single-mode waveguides, -a plurality of photodetectors coupled to the outputs of said demultiplexers, - electrical tracks configured to sum, for each wavelength, the signals from the photodetectors.

[0024] According to one embodiment, said location is that for which the electrical summation includes all the signals from the photodetectors associated with the same wavelength.

[0025] According to one embodiment, the combination and detection device comprises: - a plurality of elementary coherent recombination devices arranged in cascades so as to obtain a recombined beam, the single-mode waveguides being coupled 2 by 2 to the elementary coherent recombination devices of the first stage of said cascade, - a wavelength demultiplexer, a plurality of photodetectors coupled to the outputs of the demultiplexer.

[0026] According to one embodiment, the elementary coherent recombination device comprises an integrated interferometer comprising a first and a second phase shifter element.

[0027] According to one embodiment, said location is the output of the wavelength demultiplexer.

[0028] According to one embodiment, the combining and detecting device further comprises a mixer configured to mix the recombined beam with a local oscillator, the photodetector being a balanced photodetector.

[0029] According to one embodiment, the mixer has 4 outputs coupled to two balanced photodetectors so as to perform in-phase and quadrature detection.

[0030] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered.

[0031] The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which:

[0032] Figure 1 already cited illustrates a first example of transmission / reception of a WDM link according to the state of the art, in which at transmission a beam, consisting of M multiplexed wavelengths each carrying a modulation signal, is emitted in free space, and on reception there is coherent recombination.

[0033] Figure 2 already cited illustrates a second example of transmission / reception of a WDM link according to the state of the art in which on reception the recombination is incoherent.

[0034] Figure 3 already cited illustrates a single wavelength transmission / reception system with coherent detection in phase and in quadrature according to the state of the art.

[0035] Figure 4 already cited illustrates an integrated photonics device for generating and receiving spatial modes for coupling to a weakly multimode fiber according to the state of the art.

[0036] Figure 5 illustrates the planar photonic circuit (PIC) according to the invention.

[0037] Figure 6 illustrates a first variant of the PIC according to the invention in which the diffractive device comprises a single diffraction grating having plane fringes and generating two diffracted beams propagating in two opposite directions.

[0038] Figure 7 illustrates the unidirectional diffraction of an incident beam spatially structured along the so-called transverse direction Y perpendicular to the diffraction direction X and incident at an oblique incidence.

[0039] Figure 8 illustrates the unidirectional diffraction of an incident beam spatially structured along the so-called longitudinal direction X (diffraction direction X) and incident at an oblique incidence.

[0040] Figure 9 illustrates a double diffraction in opposite directions of a beam structured along X and incident at normal incidence on the grating.

[0041] Figure 10 illustrates a second variant of the PIC according to the invention in which the diffractive device comprises two superimposed diffraction gratings having planar and perpendicular fringes and configured to each generate two diffracted beams propagating in two opposite directions.

[0042] Figure 11 illustrates a third variant of the PIC according to the invention in which the diffractive device comprises two diffraction gratings having planar fringes and configured to each generate a single diffracted beam and so that the two diffracted beams propagate in two non-collinear and non-orthogonal directions.

[0043] Figure 12 illustrates a fourth variant of the PIC according to the invention in which the diffractive device comprises two diffraction gratings each having curved fringes and configured to each generate a single diffracted beam and so that said two diffracted beams propagate in two non-collinear and non-orthogonal directions.

[0044] Figure 13 illustrates an embodiment of the PIC according to the invention in which the planar photonic circuit further comprises an integrated device for combining and detecting said single-mode sub-beams.

[0045] Figure 14 illustrates an embodiment for which incoherent recombination is performed.

[0046] Figure 15 illustrates the cascade architecture of coherent recombination.

[0047] Figure 16 illustrates an elementary coherent recombination device according to the state of the art.

[0048] Figure 17 illustrates a planar photonic circuit according to the invention which performs spatial demultiplexing on 16 single-mode beams and coherent recombination with interferometers as illustrated in Figure 16. DETAILED DESCRIPTION OF THE INVENTION

[0049] The invention relates to a planar photonic circuit PIC for demultiplexing a highly multimode incident optical beam into a plurality of single-mode sub-beams.

[0050] The complex multimode beam to be processed is arbitrary. For example, it comes from a degradation by propagation in the atmosphere of an initially Gaussian emitted beam or from an optical fiber, as explained above. In addition, the optical beam incident on the device typically comes initially from an optical source modulated by a communication signal, and has crossed a non-homogeneous free-space propagation channel or was injected into a multimode fiber and propagated therein. Propagation in the atmosphere or in the fiber distorts the phase and amplitude profile of the beam. The beam incident on the PIC according to the invention is therefore strongly spatial multimode and is preferentially collimated or focused to illuminate the diffractive device Ddiff. Strongly multimode means a beam that comprises at least N=5 spatial modes per polarization, typically between 5 and 30 spatial modes. The PIC according to the invention comprises a guiding layer GL, parallel to the substrate Sub, in which the light propagates.

[0051] The principle of the PIC according to the invention is illustrated in Figure 5. It comprises a diffractive device Ddiff integrated into the circuit which comprises a diffractive grating or two superimposed diffractive coupling gratings, the grating(s) being configured to couple the multimode incident optical beam Fine into the guiding layer GL of the planar photonic circuit and to generate two or four diffracted beams. Only one diffracted beam Fd is illustrated in Figure 5 and the following Figures 6, 10, 11 and 12 illustrate different variants of generation of two or four diffracted beams. The diffracted beams from the incident beam are spatial multimode. The incident beam may have one or more wavelengths or a given spectral band. The demultiplexer according to the invention operates provided that the wavelength(s) are diffracted with sufficient efficiency by the diffractive grating(s).

[0052] The PIC, also called a photonic chip, also comprises an integrated conversion device Dconv comprising free propagation zones Zpl of the diffracted beams, a free propagation zone being associated with each of the diffracted beams. The PIC also comprises a plurality of collector waveguides OGcol arranged downstream of each free propagation zone, and a plurality of single-mode waveguides OGmono, each collector waveguide being coupled to a single-mode waveguide (and vice versa), so as to generate the plurality of single-mode sub-beams Fi, i varying from 1 to N. Figure 5 illustrates only one free propagation zone, associated with the beam Fd.

[0053] By way of non-limiting example, four variants of the PIC according to the invention are described below.

[0054] The diffractive grating(s) are Bragg gratings with alternating refractive index fringes of the guiding layer. These gratings are usually used to couple a single-mode beam, typically Gaussian, from free space to a waveguide on a photonic chip. They diffract the incident beam in the guiding layer GL of the PIC and in one or more directions given by the wave vector(s) of the grating(s). The diffraction of the 2-dimensional electromagnetic field incident on the diffractive device leads to one or more one-dimensional projections of the 2D Fine field in the plane of the integrated circuit. Each of these 1D projections is spatially multimode.

[0055] The conversion device Dconv converts the 1D multimode projections of Fine in the plane of the integrated circuit into a plurality of single-mode guided beams Fi, of different phases and amplitudes. The one or two Bragg gratings couple the incident multimode beam towards several multimode guides (the collector guides). To do this, the diffracted beam is diverged in the plane of the photonic chip (free propagation zones Zpl) and collector guides OGcol are placed on the path of the diverging diffracted beam to sample the beam spatially. There is therefore a zone Zpl and a plurality of guides OGcol associated with each diffracted beam. Thus the conversion device is configured so that a diffracted beam diverges during its propagation in the associated free propagation zone.This divergence is achieved, for example, with a sub-wavelength structure (called a metamaterial) configured to achieve a variation in effective index, allowing a diverging lens function to be obtained. For example, holes or grooves of variable dimensions are dug to create the variation in effective index. The term "diverging lens" is therefore understood to mean any structure or element allowing a divergence of the diffracted beam to be achieved.

[0056] Each of the diffracted beams propagating in the associated free propagation zone Zpl is multimode. The single-mode extractions then take place, via the association of a collector guide OGcol, which each collects a fraction of the multimode beam propagating in the Zpl zone to send it to an associated single-mode guide OGmono.

[0057] On the upstream side of the OGcol guide, each collector guide collects a fraction of the multimode wavefront (amplitude / phase) propagating in Zpl. Each of the OGcol collector guides adiabatically transforms this fraction of the multimode optical beam collected locally by the collector guide, called multimode light, into an optical beam propagating according to the fundamental mode (downstream of the guide). The fundamental mode is understood to be the lowest order mode. The number of collector guides defines the spatial sampling resolution of the multimode light. The minimum resolution required depends on the number of modes of the incident light. Thus, to efficiently sample the multimode wavefront, the number of OGcol guides is chosen to be at least as large as the number of modes in the 1D multimode projections in the free propagation zone Zpl.

[0058] In the collector guide, the energy that was in the different modes initially present in the upstream part of the guide has been transferred to a single mode present in the downstream part, the fundamental mode. There is therefore a high modal rejection rate on the downstream side. A single-mode waveguide is arranged in the extension (downstream side) of the associated collector waveguide and is configured to transport this fundamental mode, which constitutes the single-mode sub-beam Fi.

[0059] In other words, the collector guide transforms the collected multimode light into light propagating according to the fundamental mode of the associated single-mode waveguide.

[0060] According to a preferred embodiment, the collector waveguide has a width decreasing according to the direction of propagation (so-called “taper” mode, funnel shape), illustrated in Figure 5.

[0061] According to another embodiment, the collector waveguide has a flared shape, i.e. a width increasing according to the direction of propagation (“reverse taper”, tip shape on the upstream side).

[0062] Thus it is not the physical width of the collector guide section that counts, but it is the size of the fundamental mode which must be decreasing in the direction of propagation.

[0063] According to one embodiment, the collector guides are joined (upstream side, i.e. on the side of the propagation zone in free space) so as to recover all the light from the diverging diffracted beam. Joined collector guides are preferably in “taper” mode (decreasing width) as illustrated in Figure 5.

[0064] According to one embodiment, the OGmono single-mode guides all have the same width. The modes carried by these guides are intended to be recombined, so it is preferable that the single-mode guides all have the same width for simplicity of combination.

[0065] According to one embodiment, the initial width / geometry of the collector guides is adapted to the statistics of the modes most present in the incident beam.

[0066] With the PIC according to the invention, we do not seek to multiplex and / or demultiplex the spatial modes of a weakly multimode fiber, but we collect on a photonic circuit the greatest quantity of energy from a spatially distorted beam, with a high modal content.

[0067] The advantage of the invention is to offer a PIC that integrates a spatial mode demultiplexer. For this purpose, an original spatial demultiplexer architecture comprising Ddiff and Dconv is proposed. The multimode incident beam is decomposed into a plurality of single-mode components that are integrated into the PIC and available for processing also in an integrated manner, without any connection or addition of external components, the processing being carried out according to a preferred mode in the same photonic circuit (see below).

[0068] A first variant of the PIC circuit according to the invention is illustrated in Figure 6. In this variant the diffractive device Ddiff comprises a single diffraction grating DG0 having plane fringes and generating two diffracted beams Fd1, Fd1', propagating in two opposite directions (bidirectional diffraction). There are therefore here two free propagation zones Zpl, one per diffracted beam. To allow spatial sampling by the collector guides, the conversion device Dconv further comprises two diverging lenses LD1, LD1' positioned on the respective paths of the two diffracted beams Fd1 and Fd1' upstream of the free propagation zones ZpL. Let N be the number of single-mode guides OGmono; here we have N / 2 guides per direction. In this variant, the incident beam preferentially illuminates Ddiff at a normal incidence so that the Bragg condition is verified for the two opposite propagation directions.

[0069] The advantage of having two opposite diffraction directions for a DG grating is illustrated in Figures 7 to 9. Figure 7 illustrates the monodirectional diffraction only (diffracted beam Fd) of an incident beam 70 spatially structured according to the so-called transverse direction Y perpendicular to the diffraction direction X and incident according to an oblique incidence for which the Bragg condition is verified. The PIC circuit has a guiding layer GL of index ng, a substrate Sub of index nO and an intermediate layer IL of intermediate refractive index n int <ng. Le faisceau diffracté Fd couplé dans la couche guidante GL du circuit intégré PIC reproduit la structuration du faisceau.

[0070] Figure 8 illustrates the monodirectional diffraction only (diffracted beam Fd) of an incident beam 80 spatially structured along the so-called longitudinal direction X (diffraction direction X) and incident at an oblique incidence for which the Bragg condition is verified. The diffracted beam Fd coupled into the guiding layer GL of the PIC integrated circuit has an energy proportional to the overlap integral between the beam structuring profile in the diffraction direction, and the diffraction profile of the grating. The coupling of the beam Fd into the "average" guiding layer thus structuring the beam, and part of the energy associated with this mode is lost. Monodirectional diffraction only is therefore suboptimal for beams structured along the diffraction direction, or even totally ineffective for certain types of modes.

[0071] Figure 9 illustrates the case of double diffraction (beams Fd and Fd') in opposite directions of a 90 beam structured along X and incident at normal incidence on the DG grating. The diffraction profile of the grating, exponentially decreasing, is represented on the left curves respectively for the backward propagation direction of the grating Ar and on the right for the forward diffraction Av. Diffraction by the grating towards both forward and backward directions gives an additional degree of freedom to decompose the spatial structuring of the incident beam. The overlap integral between the beam structuring profile and the diffraction profile of the grating in both directions generates two beams Fd and Fd' with a total energy much higher than a one-way diffraction.

[0072] According to a second variant illustrated in Figure 10, the diffractive device Ddiff comprises two superimposed diffraction gratings DG1, DG2 having planar fringes and configured to each generate two diffracted beams, respectively (Fd1, Fd1') and (Fd2, Fd2') propagating in two opposite directions. The respective fringes of the two gratings are perpendicular to each other. Here we have 4 free propagation zones and the conversion device comprises four diverging lenses LD1, LD1', LD2, LD2' positioned on the respective paths of the four diffracted beams upstream of the free propagation zones. Let N be the total number of single-mode guides OGmono, here we have N / 4 guides per direction. In this variant, the incident beam preferentially illuminates Ddiff according to a normal incidence so that the Bragg condition is verified for the 4 diffracted beams.The advantage of this configuration is that due to the presence of two perpendicular gratings, the Y-structure of the incident beam is reproduced by diffraction on DG1 (diffraction along X and -X) and the X-structure of the incident beam is reproduced by diffraction on DG2 (diffraction along Y and -Y). The modal structure along both X and Y directions is present in the beams diffracted by this architecture. In addition, the architecture with two perpendicular gratings can potentially treat both polarization directions of the incident beam.

[0073] According to a third variant illustrated in Figure 11, the diffractive device Ddiff comprises two diffraction gratings DG3, DG4 having plane fringes and configured to each generate a single diffracted beam and so that the two diffracted beams propagate in two non-collinear and non-orthogonal directions. The plane fringes therefore have an angle between them different from 90°. The conversion device comprises two diverging lenses LD3, LD4 positioned on the respective paths of the two diffracted beams Fd3 and Fd4 upstream of the free propagation zones. There are N / 2 OGmono guides per direction. The advantage of this configuration is that the Bragg condition can be simultaneously respected for the two gratings with an incident beam along the bisector plane defined by the line B and the Z axis. This configuration is thus compatible with an oblique incidence in this plane.

[0074] According to a fourth variant illustrated in Figure 12, the diffractive device Ddiff comprises two diffraction gratings DG5, DG6 each having curved fringes and configured to each generate a single diffracted beam and so that said two diffracted beams propagate in two non-collinear and non-orthogonal directions. The curved fringes therefore have an angle between them other than 90°. This configuration is compatible with an oblique incidence in the bisector plane.

[0075] According to an embodiment illustrated in Figure 13, the planar photonic circuit according to the invention further comprises an integrated device for combining and detecting Dcd said single-mode sub-beams Fi, Dcd comprising at least one photodetector, a plurality of photodetectors in most cases. The objective is the recovery of the information which modulates the incident beam. At the output of Dcd, electrical signals are recovered, a signal SÀj per wavelength Àj in the case of a WDM link (coherent or incoherent recombination), a single electrical signal for mono-À coherent detection, or two electrical signals at the output of the two balanced detectors in the case of l / Q coherent detection.

[0076] The PIC achieves a monolithic integration of the spatial mode demultiplexer and the detector(s). The integration in a single circuit of all the reception functionalities (coupling / spatial demultiplexing / recombination / detection) gives the PIC according to the invention an extreme compactness going well beyond the approaches of the state of the art, and entirely relevant for embedded applications. One field of exploitation of the PIC according to the invention is that of free space communications in the space domain (earth-satellite), or that of communication between carriers in the land, naval or air domains. The compactness of the reception system according to the invention makes the solution usable for communications between drones.

[0077] On the other hand, the direct integration on a single photonic chip of the multimode reception function avoids optical coupling losses from the receiver to the detector.

[0078] Furthermore, the coherent or incoherent recombination of modulated sub-beams makes it possible to drastically reduce the fading of the communication signal induced by rapid energy exchanges between spatial modes, typically due to atmospheric turbulence.

[0079] Finally, another advantage of the PIC integrating the entire reception is to allow integrated balancing of the optical paths of each sub-beam up to the combination, which makes it possible to process a significant bandwidth, and in particular authorizes the use of the PIC according to the invention for wavelength division multiplexing formats. For this, the combination and detection device Dcd further comprises a plurality of delay lines LR configured to equalize the optical or mixed optical / electrical paths of the single-mode sub-beams, considered from the position of the superimposed diffractive grating(s), and up to a location where the signal carrying information associated with a single wavelength and integrating the contribution of all the single-mode sub-beams is generated. A delay line is for example a spiral "delay line" or "delay line" in English.

[0080] The location of the end of the path to be equalized with the other paths is different depending on the type of recombination. We consider a WDM link in which in the incident beam we have M modulated wavelengths Àj. The different wavelengths Àj (carriers) are for example separated by a frequency Av and modulated to an RF signal of bandwidth A.

[0081] Each recombination type is compatible with all spatial demultiplexer variants [Ddiff+Dconv].

[0082] According to a first embodiment illustrated in figure 14, an incoherent recombination is carried out, the generic architecture of which is illustrated in figure 2. This example integrates the first variant of Ddiff and performs demultiplexing on 6 (2x3) spatial modes. In incoherent detection, an electrical detection is performed on each of the optical carriers and then the electrical signals (photocurrents) are summed. The combination and detection device Dcd thus comprises a plurality (N) of wavelength demultiplexers DEMUX respectively coupled to the plurality (N) of single-mode waveguides Fi. It also comprises a plurality of photodetectors PD coupled to the M outputs of each of the N demultiplexers, i.e. M x N photodetectors and electrical tracks 7 configured to sum, for each wavelength, the signals from the photodetectors. We end up at the output with M electrical signals SÀj integrating the contributions of each of the spatial modes.

[0083] The paths to be equalized are in this case mixed optical / electrical and the electrical summation location is the point noted Pf in the figure, which includes all the contributions of the signals from the photodetectors associated with the same wavelength. Equalization is carried out with the LR delay lines.

[0084] In the incoherent case, the equalization of the paths must be carried out with a precision related to the modulation frequency A. Let c be the speed of light in vacuum, n the index of the propagation medium (guiding layer) and AL the maximum length difference between two paths, we have:

[0085]

[0086] Taking for example A = 10 GHz and Av = 300 GHz we need an AL precision of 3 cm.

[0087] According to a second embodiment, a coherent recombination is carried out, the generic architecture of which is illustrated in Figure 1. In this recombination, the respective phases and amplitudes of the single-mode beams are adjusted to make a constructive Frecomb optical sum, then, after wavelength demultiplexing, the optical signal associated with each wavelength is recovered on a photodetector. The constructive optical sum is typically carried out according to a cascade architecture as illustrated in Figure from an elementary coherent recombination device 15 which recombines the beams two by two. The optical lengths of the paths connecting the successive coherent combination stages (E1, E2, ...) are equalized. The output of the last combination element (Frecomb beam) is connected to a wavelength multiplexer Demux, a detector PDj being connected to each of the outputs for conversion into the electrical domain of the modulation signal.

[0088] According to one embodiment, the device 15 is an integrated interferometer as illustrated in Figure 16 and described in the aforementioned document US 2020-0195355. The optical beams Fi of the single-mode channels OGmono are optically summed 2 by 2 by means of the integrated interferometer 15, which comprises a first controllable phase shifter element PS on at least one of the arms, and a second controllable phase shifter element DL upstream of at least one of the inputs. More precisely, the elementary device 15 comprises a variable coupler VC comprising a first 2x2 Comb combiner, the phase modulator PS and a second 2x2 Comb' combiner, according to a Mach Zehnder or MZI interferometer architecture. The first phase shifter PS can be positioned indifferently on the 2 arms of the MZI because the two inputs Fi1 and Fi2 are mixed following the first Comb combiner.The second combiner Comb' has a main output constituting the output Out of the elementary device 15 and delivering the output beam and a complementary output Sc delivering a beam complementary to the output beam. A control detector Det is connected to the complementary output Sc.

[0089] Figure 17 illustrates a planar photonic circuit according to the invention which performs spatial demultiplexing on 16 single-mode beams (N=16) and coherent recombination with a plurality of interferometers 15 as illustrated in Figure 16. The PIC circuit of the figure therefore comprises a plurality of elementary coherent recombination devices 15 arranged in cascades so as to obtain a Frecomb recombined beam, the single-mode waveguides being coupled 2 by 2 to the elementary coherent recombination devices of the first stage E1 of said cascade. It also comprises a demultiplexer in Demux wavelength and a plurality of PDj photodetectors (M photodetectors) coupled to the outputs of the demultiplexer.

[0090] The path equalization for coherent recombination is performed with the end point of each path being the output of the demultiplexer. The phase matching constraint on the different paths is expressed as follows:

[0091] Av. n. L 2. - “1 c

[0092] Taking for example Av = 300 GHz we need a precision:

[0093] AL « c / 2.Av.n =1 mm.

[0094] Typically to have an effective combination you need AL of the order of c / 2O.Av.n

[0095] According to one embodiment, the PIC according to the invention performs single-wavelength detection and coherent recombination for an application, for example of the coherent optical communication type (where the two quadratures of the electric field are modulated) or lidar. The PIC according to the invention comprises in this case a mixer configured to mix the Frecomb recombined beam with a local oscillator, the photodetector being a balanced photodetector. Preferably, the mixer has 4 outputs coupled to two balanced photodetectors so as to perform in-phase and quadrature detection (see generic architecture figure 3).

Claims

CLAIMS Planar photonic circuit (PIC) for demultiplexing a multimode incident optical beam into a plurality of single-mode sub-beams comprising a guiding layer (GL), the planar photonic circuit comprising: - an integrated diffractive device (Ddiff) comprising one or two superimposed diffractive gratings (DGO, DG1, DG2, DG3, DG4, DG5, DG6) for coupling, configured to couple the multimode optical beam into the guiding layer of the planar photonic circuit and to generate two or four diffracted beams (Fd1, Fd1 Fd2, Fd2', Fd3, Fd4), - an integrated conversion device (Dconv) comprising: o free propagation zones (Zpl), a free propagation zone being associated with each of the diffracted beams, the conversion device being further configured so that a diffracted beam diverges during its propagation in the associated free propagation zone, o a plurality of collector waveguides arranged downstream of each free propagation zone (OGcol) and o a plurality of single-mode waveguides (OGmono), a collector waveguide being coupled to a single-mode waveguide and being configured to adiabatically transform a fraction of the multimode optical beam collected locally by said collector guide into an optical beam according to a fundamental mode of said single-mode waveguide, so as to generate said plurality of single-mode sub-beams (Fi). Planar photonic circuit according to the preceding claim wherein: - the diffractive device comprises a single diffraction grating (DGO) having plane fringes and generating two diffracted beams (Fd1, Fd1') propagating in two opposite directions, the conversion device comprises two diverging lenses (LD1, LD1') positioned on the respective paths of the two diffracted beams upstream of the free propagation zones. 3 Planar photonic circuit according to claim 1 in which: - the diffractive device comprises two diffraction gratings (DG1, DG2) having perpendicular plane fringes, and configured to each generate two diffracted beams (Fd1, Fd1', Fd2, Fd2') propagating in two opposite directions, - the conversion device comprises four diverging lenses (LD1, LD1', LD2, LD2') positioned on the respective paths of the four diffracted beams upstream of the free propagation zones. 4 Planar photonic circuit according to claim 1 in which: - the diffractive device comprises two diffraction gratings (DG3, DG4) having planar fringes and configured to each generate a single diffracted beam (Fd3, Fd4) and so that said two diffracted beams propagate in two non-collinear and non-orthogonal directions, - the conversion device comprises two diverging lenses (LD3, LD4) positioned on the respective paths of the two diffracted beams upstream of the free propagation zones. 5 Planar photonic circuit according to claim 1 in which: - the diffractive device comprises two diffraction gratings (DG5, DG6) having curved fringes and configured to each generate a single diffracted beam and so that said two diffracted beams propagate in two non-collinear and non-perpendicular directions - the conversion device comprises two diverging lenses positioned on the respective paths of the two diffracted beams upstream of the free propagation zones. 6 Planar photonic circuit according to one of the preceding claims further comprising an integrated device for combining and detecting (Dcd) said single-mode sub-beams comprising at least one photodetector. 7 Planar photonic circuit according to the preceding claim in which the integrated combining and detection device (Dcd) further comprises a plurality of delay lines (LR) configured to equalize the optical or mixed optical / electrical paths of the single-mode sub-beams, considered from the position of the superimposed diffractive grating(s) and up to a location where a signal is generated carrying information associated with a single wavelength and integrating the contribution of all the single-mode sub-beams. 8 Planar photonic circuit according to one of claims 6 or 7 in which the integrated combination and detection device comprises: -a plurality of wavelength demultiplexers (Demux) respectively coupled to the plurality of single-mode waveguides, - a plurality of photodetectors coupled to the outputs of said demultiplexers, - electrical tracks configured to sum, for each wavelength, the signals from the photodetectors. 9 Planar photonic circuit according to claims 7 and 8 wherein said location is that for which the electrical summation (Pf) includes all the signals from the photodetectors associated with the same wavelength. 10 Planar photonic circuit according to claim 7 wherein the combining and detecting device comprises: - a plurality of elementary coherent recombination devices (15) arranged in cascades so as to obtain a recombined beam (Frecomb), the single-mode waveguides being coupled 2 by 2 to the elementary coherent recombination devices of the first stage of said cascade, - a wavelength demultiplexer (Demux), a plurality of photodetectors (PDj) coupled to the outputs of the demultiplexer. Planar photonic circuit according to claim 10 wherein the elementary coherent recombination device (15) comprises an integrated interferometer comprising a first (PS) and a second (DL) phase shifter element. Planar photonic circuit according to claims 7 and 10 wherein said location is the output of the wavelength demultiplexer. Planar photonic circuit according to claims 10 or 11 wherein the combining and detecting device further comprises a mixer (MMI) configured to mix the recombined beam with a local oscillator, the photodetector being a balanced photodetector. Planar photonic circuit according to the preceding claim wherein the mixer has 4 outputs coupled to two balanced photodetectors so as to perform in-phase and quadrature detection.

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