Free space optical telecommunications system

Abstract

The invention relates to a free-space optical telecommunication system (1) comprising a telescope (T) having an objective (O) for collecting incident light radiation (I) and producing, at an optical port (P), a first light radiation (I1); an optical processing device and at least one polarization-maintaining multimode waveguide (F) having a first end coupled to the optical port (P) of the telescope (T) and a second end coupled to the optical processing device (DR). Figure to be published with the abstract: Fig. 1

Description

Title of the invention: Free space optical telecommunication system FIELD OF THE INVENTION

[0001] The invention relates to an optical telecommunications system intended to compensate for the distortion of the wavefront of incident light radiation. This distortion may originate from atmospheric disturbances during optical communication in free space. More generally, this distortion is caused by the propagation of light radiation in its medium. The invention finds particular application in the field of optical telecommunications in free space. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] In free-space optical telecommunications, a transmitter modulates light radiation (generally produced by a laser) using information to be transmitted, the light radiation taking the form of a narrow beam that is emitted towards a receiver. After propagation in its medium (air will be taken as an example in the remainder of this description, but the medium can be of any nature, such as water in the case of underwater telecommunications), the light radiation is collected at a receiver and demodulated to recover the transmitted information. Generally speaking, and in order to maximize the information flow rate, we seek to exploit as much of the energy present in the light radiation received by the receiver as possible, in order to maximize the transmission rate.

[0003] To multiply the communication channels and maximize this flow rate, the light radiation can be multiplexed in wavelength and / or polarization.

[0004] The propagation of light radiation subjects the radiation produced by the transmitter to disturbances in the atmosphere, and in particular to variations in temperature and pressure that the radiation undergoes during its propagation. These erratic disturbances, the variation dynamics of which typically extend between 100 Hz and a few kHz, lead to its deformation, which affects its wavefront. More precisely, the disturbances tend to spatially redistribute the energy in the radiation, producing random fluctuations in amplitude and phase. This deformation materializes in the form of so-called "speckle" figures in the spot formed by the projection of the beam onto the radiation collection device and by a scintillation phenomenon. It leads to limiting the information rate of the link between the transmitter and the receiver.

[0005] To overcome this limitation, it is known to provide adaptive optics aimed at compensating for these phenomena. But such a solution has limited performance, because it only acts on the phase of the received light radiation. Document EP3672109A1 proposes a receiver capable of modally decomposing the received radiation (at a collector) in the form of elementary radiation. These elementary radiations are recombined in a coherent manner via a photonic device.

[0006] The coherent recombination of elementary light radiation requires perfect control of their polarization. These radiations must have the same polarization in order to allow the interference mechanism producing the desired recombination.

[0007] It is noted that the light radiation produced by the transmitter is polarized, and that this polarization is not affected by the propagation in free space of the light radiation. On the other hand, the propagation of this light radiation in the receiver can affect its polarization, in particular when it is desired to move the optical processing of this radiation from the collector via an optical fiber.

[0008] Depending on the communication protocol chosen, the polarization state of the light radiation produced by the transmitter can be determined (for example, linear, left circular or right circular polarization) or not determined. In the latter case, its characteristics can evolve freely over time. Even when the polarization state of the light radiation is determined, any relative movements between the transmitter and the receiver can lead to its characteristics evolving at the receiver. This is the case in particular when the transmitter is placed in a satellite and the latter is likely to rotate on itself.

[0009] Furthermore, and as indicated previously, certain communication protocols also provide for the production of polarization multiplexed light radiation.

[0010] It is therefore understood that the light radiation received by the receiver has a polarization state which is not always perfectly controlled, but which must absolutely be taken into account in order to exploit a maximum of the energy transported and / or to allow the decoding of the symbols transmitted.

[0011] This is particularly true when the processing carried out by the receiver on the received light radiation uses a coherent combination of elementary beams and when these processing operations are removed from the collector. OBJECT OF THE INVENTION

[0012] An object of the invention is to propose an optical communication system which remedies, at least in part, the aforementioned problems. More specifically, an object of the invention is to propose an optical communication system comprising a collector of incident light radiation and a photonic device implementing a coherent combination of elementary light radiation, the photonic device being distant from the collector. BRIEF DESCRIPTION OF THE INVENTION

[0013] With a view to achieving this aim, the subject of the invention proposes a free space optical telecommunication system comprising: - a telescope having an objective for collecting incident light radiation and producing, at an optical port, a first light radiation; - an optical processing device comprising: • a modal separator comprising a modal decomposition device configured to decompose the first light radiation, the modal separator producing a plurality of elementary light radiations; • a photonic device optically coupled to the separator modal, the photonic device being configured to coherently recombine at least part of the elementary light radiation and produce at least one recombined light radiation; - at least one polarization-maintaining multimode waveguide having a first end coupled to the optical port of the telescope and a second end coupled to the optical processing device.

[0014] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: - the modal decomposition device comprises at least one multi-plane conversion device; - the modal decomposition device comprises a bundle of single-mode optical fibers assembled in parallel with each other; - the optical telecommunications system comprises a polarizing beam splitter arranged upstream of the photonic device, the optical splitter producing a first plurality of elementary light rays and a second plurality of elementary light rays having distinct polarizations, the first plurality and the second plurality of elementary light rays constituting the plurality of elementary light rays; - the polarizing beam splitter is coupled to the second end of the multimode waveguide, the polarizing beam splitter producing a first polarized light radiation and a second polarized light radiation having distinct polarizations; the polarizing beam splitter is arranged in the optical port of the telescope to produce first polarized light radiation and second polarized light radiation having distinct polarizations, the polarizing beam splitter being arranged in the optical port to inject the first polarized light radiation into a first polarization-maintaining multimode waveguide and to inject the second polarized light radiation into a second polarization-maintaining multimode waveguide; the modal separator comprises a first modal decomposition device arranged to receive the first polarized light radiation and produce a first plurality of elementary light radiations and a second modal decomposition device arranged to receive the second polarized light radiation and produce a second plurality of elementary light radiations, the first plurality and the second plurality of elementary light radiations constituting the plurality of elementary light radiations produced by the modal separator; the modal decomposition device is coupled to the second end of the multimode waveguide to produce a plurality of decomposed light rays and the polarizing beam splitter is optically disposed downstream of the modal decomposition device to receive the plurality of decomposed light rays and produce the first plurality of elementary light rays and the second plurality of elementary light rays; the photonic device is configured to produce a first recombined light radiation from the first plurality of elementary light radiations and produce a second recombined light radiation from the second plurality of elementary light radiations; the photonic device comprises a first photonic device optically coupled to the modal splitter to receive the first plurality of elementary light rays and produce the first recombined light radiation and a second photonic device optically coupled to the modal splitter to receive the second plurality of elementary light rays and produce the second recombined light radiation; - the photonic device comprises a recombination device configured to recombine the first and second recombined radiation and form a single recombined light radiation; - the recombination device is configured to form a single recombinant light radiation having a single polarization; - the recombination device is configured to form a single recombinant light radiation having superimposed polarizations; - the optical port includes a static or dynamic polarization control device; - the photonic device is optically coupled to the modal splitter via a plurality of single-mode optical fibers; - the single-mode optical fibers of the plurality of single-mode fibers are polarization-maintaining; - the coupling between the photonic device (C) and the modal separator (S) is devoid of optical fibers - the optical telecommunication system comprises an optical receiver for demodulating the optically recombined light radiation coupled to the optical processing device (DR). BRIEF DESCRIPTION OF THE FIGURES

[0015] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0016] [Fig.l]

[0017] [Fig.l] represents an optical telecommunication system according to the invention;

[0018] [Fig.2a]

[0019] [Fig.2b]

[0020] [Fig.2c]

[0021] [Fig.2d]

[0022] [Fig.2a], 2b, 2c, 2d represent different modes of implementation of the optical telecommunication system of [Fig.l];

[0023] [Fig.3a]

[0024] [Fig.3b]

[0025] Figures 3a, 3b represent two examples of a photonic device implemented work in an optical telecommunication system of [Fig.l]. DETAILED DESCRIPTION OF THE INVENTION

[0026] Elements common to all implementation methods.

[0027] With reference to [Fig.l], an optical telecommunication system 1 according to the invention aims to process incident light radiation I produced by a transmitter and carrying, by modulation, information to be transmitted. The incident light radiation I may have several wavelengths, as is usually the case for WDM type transmissions, and / or exploit several polarizations. The incident light radiation, once processed by the optical telecommunication system 1, is supplied to an optical receiver OR of a base station, capable of extracting the information from the received radiation.

[0028] In the example shown in this [Fig.l], the transmitter is arranged in a satellite SAT, but the telecommunications system of the invention is in no way limited to this particular application. Generally speaking, the transmitter can be arranged indifferently on land, in the sea or in space, and propagate in any free space, the atmosphere in the case of terrestrial communication, water in the case of marine communication. The transmitter and the telecommunications system 1 can both be stationary, or move relative to each other.

[0029] The incident light radiation I emitted takes the form of a narrow beam directed towards the telecommunications system 1. During its propagation in free space, the emitted radiation is subject to atmospheric disturbances in the atmosphere, so that the incident light radiation I arriving at the base station exhibits spatial and temporal fluctuations in amplitude and phase. This phenomenon affects the shape of this radiation, which takes a form that varies over time, in an erratic and irregular manner. The optical telecommunications system 1 aims to compensate, at least in part, for this distortion to allow the exploitation of the radiation by the optical receiver OR and the decoding of the transmitted message, by direct or coherent detection. For this purpose, it may be provided that the receiver OR integrates amplification and / or spectral demultiplexing functions, in particular in the context of WDM transmission.

[0030] The optical telecommunications system 1 comprises a telescope T, this telescope T having an objective O for collecting the incident light radiation I and producing, at an optical port P, a first light radiation II. As is well known per se, this objective may comprise a concave mirror focusing the received radiation into an image focus. This convergent radiation may be returned to the optical port P using a second mirror of the objective O, this second mirror being able to be planar or convex. The second mirror, when present, leads to the formation of a central zone of very low intensity in the light radiation which propagates towards the optical port P. In all cases, this optical port P of the telescope T therefore produces the first radiation II. This telescope T may be orientable in order to point and follow the transmitter, here arranged in the satellite SAT. The telescope T may also include a TTM guidance device for the incident radiation (such as a tilting mirror or “tip tilt mirror” according to the English expression usually used in the field) in order to best guide the light radiation towards the optical port P, for example to center this radiation in the port P and, more generally, to correct any deviations in the pointing of the telescope T.

[0031] Returning to the description of [Fig.l], the optical telecommunications system 1 also comprises, optically downstream of the telescope T, an optical processing device DR intended to compensate, at least in part, for the distortions of the collected incident light radiation I. This DR device, which will be the subject of a detailed description in a following section of this presentation, is composed of a plurality of optical or photonic elements assembled with precision and which may be particularly sensitive to their operating environment. It is therefore advantageous to move this system away from the telescope T, the positioning of which is often dictated by the reception quality of the incident radiation I, by a few meters to several tens of meters, for example to place it in a cabinet, a room, in a vehicle or any other shelter of an operations center of the base station.This allows the DR optical processing device to be protected, its operating environment to be controlled (temperature, atmosphere, exposure to dust, vibrations, movements, etc.) and its operation and maintenance to be facilitated.

[0032] To allow this distancing between the telescope T and the rest of the optical telecommunications system 1, provision is made to optically connect the optical port P of the telescope to the optical processing device DR via a polarization-maintaining multimode waveguide F. A first end of this waveguide F is coupled to the optical port P of the telescope T and a second end of the waveguide F is coupled to the optical processing device DR. It propagates the first light radiation II produced by the optical port P to the optical processing device DR.

[0033] Advantageously, this waveguide has a length of at least one meter, and typically between 1 m and 10 m, thus making it possible to sufficiently offset the optical processing device DR from the telescope T to, for example, shelter it.

[0034] By "polarization maintenance" is meant that the waveguide has, over its length, a polarization extinction ratio (or PER for "Polarization Extinction Ratio" according to the English expression usually used in the field) greater than 7 dB, and advantageously greater than 10 dB and even more advantageously greater than 20 dB.

[0035] By way of example, this waveguide F may be formed from at least one multimode optical fiber comprising at least one elliptical core and having a parabolic index gradient or an index varying in steps. The dimension of the core is chosen for allow a plurality of modes to be propagated, for example at least 10 modes or at least 50 modes. The multimode optical fiber can be spun. Such a fiber is created by rotating a polarization-maintaining preform while it is being drawn. It allows light radiation with circular polarization to be propagated, while preserving this polarization. The multimode optical fiber can also include stress bars, integrated into the preform before it is drawn, to correspond to a Panda, Bowtie or Elliptical stress layer fiber.

[0036] The waveguide F can be formed from a bundle of polarization-maintaining multimode optical fibers.

[0037] The polarization maintenance may concern two determined linear polarization states of the waveguide, for example along two orthogonal axes arranged in a plane transverse to the propagation direction. The polarization maintenance may alternatively concern two circular polarization states, as has been presented in relation to the spun optical fiber.

[0038] In certain cases, it may be advantageous to provide a device for conditioning the polarization of the light radiation which is injected into the waveguide, this device being arranged in the optical port P and coupled to the first end of the waveguide. This device aims to conform the polarization of the light radiation collected by the objective of the telescope T to the maintained polarization states of the waveguide. It is particularly useful when the polarization of the incident radiation I is not perfectly controlled and it is therefore not possible to directly inject this radiation collected by the telescope T into the waveguide F without risking affecting the polarization of the radiation which propagates therein.

[0039] In all cases, and whether or not a polarization conditioning device is provided, the first light radiation II has a controlled polarization after its propagation in the waveguide F, when it appears at the optical processing device DR. By "controlled" is meant that the propagation in the waveguide is not very subject to crosstalk, and that the energies present in the different polarization states of the first light radiation II are preserved during this propagation.

[0040] Generally speaking, the optical processing device DR comprises a modal splitter S optically coupled to the waveguide F. The modal splitter S has an input port and an output port. It comprises at least one modal decomposition device configured to decompose the first light radiation II provided by the waveguide F at the first input port and produce, at its output port, a plurality of elementary light radiations. RrRN. These elementary radiations RrRN are advantageously single-mode and all have the same polarization state. The optical processing device DR also comprises, optically downstream and coupled to the modal splitter S, at least one photonic device C. This photonic device C is configured to coherently recombine at least part of the elementary light radiations Rr Rn and produce at least one single-mode recombined light radiation Rc.

[0041] The optical processing device DR therefore makes it possible, by modal decomposition of the first radiation II and the coherent recombination of the elementary radiations Ri-Rn produced, to exploit a maximum of the energy of the incident light radiation I collected and to compensate, at least in part, the distortion undergone by this radiation during its propagation in free space. The elementary radiations RrRN having identical polarization states, they can be efficiently recombined in a coherent manner with each other, that is to say to form a single-mode recombined light radiation Rc having a maximum of energy.

[0042] The modal separator S and the photonic device C can be coupled to each other in different ways, using optical fibers or without any optical fiber, while preserving the polarization states of the elementary radiations RrRNou affecting them in an identical manner.

[0043] These two devices can thus be separated from each other and the elementary RrRN radiation can propagate in free space between these two devices.

[0044] Alternatively, a bundle of fibers, for example a bundle of single-mode fibers, may be provided to respectively guide the propagation of the RrRN elementary radiation. Advantageously, these single-mode fibers are polarization-maintaining.

[0045] Alternatively, the two devices can be configured to allow their mechanical assembly, placing the output port of the modal splitter device S and the input port of the photonic device C opposite each other, so that the elementary radiations can propagate from one device to the other.

[0046] Wave plates, or any other polarization modification device, may be provided, arranged between the modal separator S and the photonic device C so as to adapt the polarization of the elementary radiations Ri-Rn, or some of them, to the polarization expected by the photonic device.

[0047] The modal decomposition device M of the modal separator S can be implemented by a multiplane optical conversion device, designated “MPLC device” in the remainder of this description. It is recalled that in such an MPLC device, incident light radiation undergoes a succession of reflections and / or transmissions, each reflection and / or transmission being followed by a propagation free space radiation. At least some of the optical parts on which reflections and / or transmissions take place, and which guide the propagation of the incident radiation, have micro-structured zones which modify the incident light radiation.

[0048] By "microstructured zone" is meant that the surface of the optical part has a relief on this zone, which can for example be broken down into the form of "pixels" whose dimensions can be between a few microns and a few hundred microns. These can be metasurfaces. The relief or each pixel of this relief has a variable elevation relative to a mean plane defining the surface in question, of at most a few microns or at most a few hundred microns. Whatever the nature of the microstructuring of the zones, an optical part having such zones forms a phase mask introducing a local phase shift within the transverse section of the radiation which is reflected there or which is transmitted there.

[0049] Thus, light radiation which propagates within an MPLC device undergoes a succession of local phase shifts separated by propagations. The succession of these elementary transformations (for example at least four successive transformations such as for example 8, 10, 12, 14, or even at least 20 transformations) establishes a global transformation of the spatial profile of the incident radiation. It is thus possible to configure the micro-structured reflection or transmission zones to transform a first light radiation, which in particular has a specific shape, into a second radiation whose shape is different.

[0050] The theoretical foundations and examples of practical implementation of an MPLC device can be found in the documents “Programmable unitary spatial mode manipulation”, Morizur et Al, J. Opt. Soc. Am. A / Vol. 27, No. 11 / November 2010; N. Fontaine et Al, (ECOC, 2017), “Design of High Order Mode-Multiplexers using Multiplane Light Conversion”; US9250454 and US2017010463.

[0051] As presented in detail in the aforementioned documents, the micro-structured zones carried by the optical part(s) forming the MPLC device are designed and configured to carry out a modal conversion aimed at decomposing the first light radiation received at the input port according to a family of modes called "input". The energies present in the modes of the input family are transported and respectively conformed to the modes of a family of "output" modes at the output port of the MPLC device. The MPLC device is configured to respectively match the modes of the input base and the modes of the output base. It is a passive device whose transfer function is particularly stable and robust and which, moreover, does not affect or very little affects the state of polarization of the light radiation passing through it.

[0052] In the context of the present description, and by way of example, the family of input modes may comprise a Hermite-Gauss basis formed of N Hermite-Gauss modes, arranged spatially opposite the first light radiation II. The family of output modes may be formed of N Gaussian modes, spatially separated, these modes defining the elementary radiations Ri-Rn. The MPLC device is configured to associate a Hermite-Gauss mode of the input basis with a Gaussian mode of the output basis. The energy of the first radiation II received at the input port is decomposed according to the modes of the input basis and transported into the MPLC device to be distributed and conform to the output Gaussian modes with which the modes of the input basis are associated.

[0053] Of course, the Hermite-Gauss and Gaussian modes taken as examples are given only for illustration purposes and other modes could be chosen to carry out the decomposition.

[0054] The modal decomposition device M of the modal separator S can be implemented by means other than the MPLC device taken as an example and detailed above. For example, it is possible to provide that this modal decomposition device M is formed of a bundle of N single-mode optical fibers assembled in parallel with each other and, possibly, collimated with microlenses. This bundle of fibers is arranged opposite the waveguide F in order to receive the first light radiation II, decompose it spatially via the N fibers of the bundle and thus produce the elementary light radiation Ri-Rn. In addition to or as a replacement for the microlenses, it is possible to provide an optical device for shaping the first radiation II which comes from the waveguide F, this device being arranged between the second end of the waveguide F and the bundle of fibers.This optical device may comprise at least one optical element in transmission or reflection, for example one or a plurality of free-form optics.

[0055] The fiber bundle may be arranged in a matrix, or more generally the ends of the fibers of the bundle may be arranged in a plane, for example in the shape of a disc or inscribed in a disc, to best decompose the first light radiation II. The fiber bundle may be arranged so that the ends of the fibers are arranged in a ring, the central part of the ring not being provided with fibers and corresponding to the central zone of very low intensity which may be present in the first light radiation II, when the latter comes from a telescope T having a second mirror, as presented previously. Alternatively, the ends of the fibers of the bundle may be arranged in a line. In this case, an optical device may be provided, arranged between the second end of the waveguide F and the fiber bundle, in order to shape the first light radiation coming from this waveguide, and conform it to the line defined by the ends of the fibers of the beam.

[0056] Alternatively, the modal decomposition device M (and the modal separator S) can be integrated into the photonic device C itself, this device having for example an input port formed of a bundle of single-mode waveguides (for example of Gaussian type, close to a Gaussian type or of any other type), arranged against each other, in line or in a matrix, in order to spatially decompose the first light radiation II. Each of the waveguides can be provided with a microlens in order to promote the decomposition of the first light radiation II and its coupling to the waveguides of the photonic device C.When the photonic device C implements a photonic chip, the latter may be provided on one of its surfaces with a plurality of grating couplers ("grating couplers" in English terminology), arranged in a grid onto which the first radiation II is projected, each coupler decomposing a part of the first radiation to inject it into a waveguide buried in the chip and of which it forms the end.

[0057] It can naturally be provided that the modal separator S mixes these different modes of implementation of the modal decomposition device M, for example by combining an MPLC device with a fiber bundle.

[0058] As already stated, the photonic device C is configured to coherently recombine at least part of the elementary light radiation RrRN and produce at least one recombined light radiation Rc. The recombined light radiation Rc is single-mode and is supplied to the optical receiver OR, for example, by simple propagation in free space or, preferably, via a single-mode fiber.

[0059] The photonic device C may be in the form of an integrated photonic chip PIC (or a plurality of integrated photonic chips). The chip is then formed of waveguides making it possible to guide the elementary radiations Ri-Rn which appear on its input port and of phase actuators making it possible to adjust the relative phase of these radiations and ensure their recombination as accurately as possible to produce the recombined radiation Rc. An example of such a chip is for example described in the document EP3672109A1.

[0060] The photonic device C may take other forms than an integrated photonic chip PIC or comprise other components to carry out the coherent recombination of the elementary radiations RrRN. It may in particular be provided that this recombination is implemented by one or a plurality of multiplane light conversion devices configured to carry out this recombination, as is for example illustrated in application FR2111490. First mode of implementation

[0061] [Fig. 2a] presents a first embodiment particularly suited to a situation in which the polarization of the incident light radiation I is not multiplexed and is perfectly determined. It may for example be a transmission taking place from a fixed transmitter and according to a protocol imposing a determined polarization state of the incident radiation I, for example linear or circular. Alternatively, the transmission may take place from a mobile transmitter in the reference frame linked to the optical communication system and the communication protocol imposes a circular polarization of the incident radiation I.

[0062] In this first embodiment, the multimode waveguide F may be formed from a polarization-maintaining multimode optical fiber, the optical fiber being chosen and coupled to the optical port P of the telescope to propagate the incident radiation I without affecting its polarization. As seen, this multimode optical fiber may be chosen to maintain the linear or circular polarization of the incident radiation I propagating therein. In other words, the multimode optical fiber F is chosen so that the polarization state maintained corresponds to the determined polarization state of the incident radiation I. In the case of a linear polarization of this incident radiation, care will be taken to couple this fiber to the optical port so that the polarization-maintaining axis of the multimode fiber is well aligned with the polarization axis of the incident radiation I.

[0063] In this first embodiment, the optical port P of the telescope T may be devoid of a device for conditioning the polarization of the incident light radiation I. This is injected directly into the waveguide F and constitutes the first light radiation II which propagates therein. If, however, the polarization of the incident light radiation I is not perfectly determined, provision may be made to place, in the optical port P of the telescope T, a conditioning device, as will be detailed in another section of this description.

[0064] As regards the processing device DR of this first embodiment, it is perfectly consistent with that described in the general description above. Thus, and as illustrated in [Fig.2a] without this forming the only way of implementing this first embodiment, the modal separator S may comprise a single modal decomposition device M, for example a multiplane conversion device producing a plurality of elementary light radiations, for example 10, 20, 50 or 100 radiations. These elementary radiations all have the same polarization as that of the incident radiation I and are recombined by a single photonic device C to produce at least one single-mode recombined light radiation Rc. Second mode of implementation.

[0065] This embodiment illustrated in [Fig.2b] is particularly suitable for a situation in which the incident light radiation I is polarization multiplexed, the polarizations being perfectly determined. For example, two multiplexed polarizations can be linear or circular, and orthogonal to each other.

[0066] Just as in the first embodiment, the multimode waveguide F may be formed from a polarization-maintaining multimode optical fiber, the optical fiber being chosen and coupled to the optical port P of the telescope to propagate the polarization-multiplexed incident radiation I without affecting its polarizations. The first radiation II which propagates in the waveguide has the same characteristics as those of the incident radiation I.

[0067] In the second embodiment, a polarizing beam splitter PBS is provided, optically arranged between the waveguide F and the modal splitter S of the optical processing device DR. The polarizing beam splitter PBS can be integrated into the modal splitter S. The polarizing beam splitter PBS is therefore coupled to the second end of the multimode waveguide F, in alignment with this waveguide so that at the output of the splitter, the two polarized radiations of the first light radiation II are indeed found.

[0068] The polarizing beam splitter PBS produces a first polarized light radiation Rlp and a second polarized light radiation Ris, the first and second polarized light radiation Rlp,Ris having distinct, generally orthogonal, polarizations.

[0069] In this configuration, the two polarized light rays Rlp, Ris propagate towards separate inputs of the input port of the modal decomposition device M. The latter is configured to decompose these two rays Rlp, Ris and provide the elementary light rays Ri-Rn. More precisely, the modal decomposition device M is configured to decompose the first polarized light radiation Rlp into a first plurality of elementary light rays RprRpP and to decompose the second polarized light radiation Ris into a second plurality of elementary light rays RsrRsP.These two pluralities of elementary light radiations Rpi-Rpp,Rsi-Rsq have distinct polarizations, respectively identical to those of the first and second polarized light radiation Rlp,Ris because the modal decomposition device M, in particular when it is produced in the form of an MPLC device, does not affect the polarization of the treated radiations. It is therefore possible to provide, before the photonic device C, or even before the MPLC device, wave plates or any other. polarization modification device, making it possible to modify the polarization state of certain propagating radiation in order to facilitate their processing by the photonic device. The first plurality and the second plurality of elementary light radiations Rpi-RpP,RsrRsP constitute, in combination, the plurality of elementary light radiations Ri-Rn produced by the modal separator S.

[0070] The modal decomposition device M may be formed of two independent devices, for example two secondary MPLC devices M1, M2 independent of each other, i.e. provided with distinct optical parts separately processing the first and second polarized light radiations Rlp, Rls. But this is not an essential characteristic, and the two polarized light radiations Rlp, Rls may also be decomposed by a single modal decomposition device, for example a single MPLC device M.

[0071] It is noted that this second mode of implementation remains compatible with non-polarization multiplexed light radiation, this polarization being able to be indeterminate. It is indeed possible to collect on each of the polarization axes of the multimode waveguide F a part of the energy present in the incident radiation. This energy is recombined in the processing device.

[0072] The ability to process multiplexed radiation and non-multiplexed radiation of undetermined polarization forms a very particular advantage of this embodiment, which makes the optical communication system 1 capable of decoding the messages transmitted by the SAT transmitter for a plurality of communication protocols. It is noted that this advantage is obtained at the cost of a modal decomposition implementing twice as many elementary light rays (all other things being equal).

[0073] [Fig.2c] illustrates a variant of the second embodiment which has just been explained. According to this variant, the modal decomposition device M of the modal separator S is coupled to the second end of the multimode waveguide F. It produces a plurality of decomposed light rays (RldrRldP), as detailed in a previous passage. This modal separator M can thus be implemented by an MPLC device.

[0074] The modal splitter S also comprises a polarizing beam splitter PBS arranged, in this variant, optically downstream of the modal decomposition device M. The polarizing beam splitter PBS therefore receives the plurality of decomposed light rays RldrRldP and produces a first plurality of elementary light rays RprRpP and a second plurality of elementary light rays RsrRsQ, the first plurality and the second plurality of elementary light rays Rpi-Rpp,Rsi-Rsq having polarizations distinct. They constitute, in combination, the plurality of elementary light rays Ri-Rn produced by the modal separator S.

[0075] Whether the variant of [Fig.2b] or that of [Fig.2c] is implemented in the optical processing device DR of the second embodiment, the modal splitter S therefore produces a first plurality of elementary light rays Rpi-RpP and a second plurality of elementary light rays RsrRSQ. These two pluralities of elementary light rays naturally have distinct polarizations. As already indicated, wave plates can be provided to place these elementary light rays (or a part of them) in a chosen polarization state, for example a polarization state expected by the photonic device C. The elementary light rays propagate, in free space or guided by fibers for example single-mode, and advantageously polarization-maintaining, from the modal splitter S to the photonic device C.

[0076] Advantageously, and as illustrated in Figures 3a and 3b, the photonic device C is configured to produce a first recombined light radiation Rci from the first plurality of elementary light radiations Rpi-RpP and to produce a second recombined light radiation Rc2 from the second plurality of elementary light radiations RsrRSQ. This configuration can be implemented by a single photonic device C, for example a single photonic chip PIC, or by means of two independent photonic devices (for example two photonic chips PIC1, PIC2) respectively receiving the first and second plurality of elementary light radiations RprRpP, RsrRSQ.More specifically, the first photonic device (for example a first photonic chip PIC1) is optically coupled to the modal splitter S to receive the first plurality of elementary light radiation RprRpP and produce the first recombined light radiation Rcp. The second photonic device (for example a second photonic chip PIC2) is optically coupled to the modal splitter S to receive the second plurality of elementary light radiation RsrRsP and produce the second recombined light radiation Rc2. Wave plates or any other polarization modification device may be provided upstream and / or downstream of the photonic chip PIC or the photonic chips PIC1, PIC2 in order to adjust the polarization of the elementary radiation to that expected by the chip(s).

[0077] As seen previously, these optical couplings can be achieved in free space, by means of optical fibers, for example single-mode polarization-maintaining fibers, or even by the mechanical assembly of the photonic devices to the modal separator S.

[0078] In a first embodiment variant shown in [Fig. 3a], the photonic device C comprises a recombination device R configured to recombine the first and second recombined radiation Rch Rc2 and form a single recombined light radiation Rc having a single polarization. In this configuration, the recombination device may be formed from a Mach-Zehnder device or a polarizing beam splitter in reverse assembly. In this second case, the polarization of the single recombined light radiation Rc may be unstable and fluctuate over time. This embodiment variant is naturally of interest when the polarization of the incident light radiation I is not multiplexed, since all the beam energy is placed in the single recombined light radiation Rc having a single polarization.

[0079] In a second variant embodiment shown in [Fig.3b], which is of interest when the polarization of the incident light radiation I is multiplexed, the photonic device C comprises a recombination device R configured to form a single recombined light radiation having two superimposed polarizations. In this configuration, the recombination device can be formed from a polarizing beam splitter in reverse assembly.

[0080] In each of the variant embodiments of Figures 3a and 3b, the recombination device is perfectly optional. It can be provided that the photonic device C (and therefore the optical communication system 1) delivers separately the first and second recombined radiation Rcb Rc2, that is to say without recombining them with each other. In this case, the optical receiver OR can be provided with two separate inputs making it possible to process the radiation Rcb Rc2 supplied by the optical communication system 1. Third mode of implementation.

[0081] The third mode of implementation is illustrated in [Fig.2d] and forms a variant of the second mode of implementation. It therefore applies under the same conditions of use and benefits from the same advantages.

[0082] In this third embodiment, the polarizing beam splitter PBS is arranged in the optical port P of the telescope T, upstream of the multimode waveguide F. The polarizing beam splitter PBS is arranged to inject the first polarized light radiation Rlp into a first polarization-maintaining multimode waveguide F1 and to inject the second polarized light radiation Ris into a second polarization-maintaining multimode waveguide F2. Just as in the second embodiment, a modal decomposition device M of the modal splitter S is configured to decompose the first polarized light radiation Rlp provided by the first multimode waveguide F1 into a first plurality of elementary light rays RprRpP. It is also configured to decompose the second polarized light radiation Ris provided by the second multimode waveguide F2 into a second plurality of elementary light rays RsrRSQ.

[0083] The modal decomposition device M can also be formed of two independent devices, for example two secondary MPLC devices M1, M2 independent of each other, or of a single modal decomposition device M, as in the second embodiment. The photonic device C can be implemented according to one or other of the variants set out in relation to the description of FIGS. 3a and 3c.

[0084] In the embodiments which have just been described, the incident light radiation I has a polarization which needs to be determined. This situation is not, however, common. This is not the case in particular when the communication protocol does not impose any particular polarization on the incident radiation I or when the transmitter SAT is mobile in a frame of reference linked to the optical communication system 1.

[0085] In order to deal with this situation, and as has already been mentioned in the general presentation of this description, the optical port P of the telescope T can be provided with a device for conditioning the polarization of the incident light radiation I. This conditioning device can in particular be integrated into the optical port P of the telescope T, in order to process the incident light radiation I before its injection into the waveguide F.

[0086] The polarization conditioning device may correspond to a static control device or a dynamic polarization control device.

[0087] As an example, which can be applied to each of the embodiments set out above, it is possible to measure totally or partially the state of polarization of the incident light radiation by means of a polarimeter and to correct it accordingly by using motorized or static birefringent delay plates, or else liquid crystal or birefringent Pockels cells. Reference may be made to the article by T. Chiba, Y. Ohtera and S. Kawakami, "Polarization stabilizer using liquid crystal rotatable waveplates," in Journal of Lightwave Technology, vol. 17, no. 5, pp. 885-890, May 1999, for an example of such a device for controlling the polarization of light radiation.

[0088] Generally speaking, one can refer to the chapter “Polarization Measurement” by Soe-Mie F. Nee, from the work “Measurement, Instrumentation, and Sensors Handbook” edited by John G. Webster, Halit Eren and published by CRC Press (ISBN 9781315217444) for the basic principles of polarization of light radiation.

[0089] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

[0090] A communication system according to the present invention may comprise other elements or devices than those presented in detail in the present description. In particular, wave plates, or any other polarization modification device, may be provided, arranged in the propagation path of the different radiations, so as to adapt the polarization of these radiations to the different treatments implemented.

Claims

Claims

1. Optical telecommunication system (1) in free space comprising: - a telescope (T) having an objective (0) for collecting incident light radiation (I) and producing, at an optical port (P), a first light radiation (II); - an optical processing device (DR) comprising: • a modal splitter (S) comprising a modal decomposition device (M) configured to decompose the first light radiation, the modal splitter (S) producing a plurality of elementary light radiations (RrRN); • a photonic device (C) optically coupled to the modal splitter (S), the photonic device (C) being configured to coherently recombine at least part of the elementary light radiations (RrRN) and produce at least one recombined light radiation (RJ;- at least one polarization-maintaining multimode waveguide (F) having a first end coupled to the optical port (P) of the telescope (T) and a second end coupled to the modal splitter (S) of the optical processing device (DR).;

2. Optical telecommunication system (1) according to claim 1 wherein the modal decomposition device (M) comprises at least one multi-plane conversion device.

3. Optical telecommunication system (1) according to one of the preceding claims in which the modal decomposition device (M) comprises a bundle of single-mode optical fibers assembled in parallel with each other.

4. Optical telecommunication system (1) according to one of the preceding claims comprising a polarizing beam splitter (PBS) arranged upstream of the photonic device (C), the modal splitter (S) producing a first plurality of elementary light rays (RprRpP) and a second plurality of elementary light rays (Rsi-Rsq) having distinct polarizations, the first plurality and the second plurality of elementary light rays (RprRpP, Rsi-Rsq) constituting the plurality of elementary light rays (Rr ü 1

5. An optical telecommunications system (1) according to claim 4 wherein the polarizing beam splitter (PBS) is coupled to the second end of the multimode waveguide (F), the polarizing beam splitter (PBS) producing a first polarized light radiation (Rlp) and a second polarized light radiation (Ris) having distinct polarizations.

6. An optical telecommunication system (1) according to claim 4 wherein the polarizing beam splitter (PBS) is arranged in the optical port (P) of the telescope (T) to produce a first polarized light radiation (Rlp) and a second polarized light radiation (Ris) having distinct polarizations, the polarizing beam splitter (PBS) being arranged in the optical port (P) to inject the first polarized light radiation (Rlp) into a first polarization-maintaining multimode waveguide (Fl) and to inject the second polarized light radiation (Ris) into a second polarization-maintaining multimode waveguide (F2).

7. Optical telecommunication system (1) according to claim 5 or 6 wherein the modal splitter (S) comprises a first modal decomposition device (Ml) arranged to receive the first polarized light radiation (Rlp) and produce a first plurality of elementary light radiations (RprRpP) and a second modal decomposition device (M2) arranged to receive the second polarized light radiation (Ris) and produce a second plurality of elementary light radiations (Rsr Rsq), the first plurality and the second plurality of elementary light radiations (RprRpP, Rsi-Rsq) constituting the plurality of elementary light radiations (RrRN) produced by the modal splitter (S).

8. Optical telecommunication system (1) according to claim 4 wherein the modal decomposition device (M) is coupled to the second end of the multimode waveguide (F) to produce a plurality of decomposed light rays (RldrRldp) and the polarizing beam splitter (PBS) is optically arranged downstream of the modal decomposition device (M) to receive the plurality of decomposed light rays (Rldr Rldp) and produce the first plurality of elementary light rays (RprRpp) and the second plurality of elementary light rays (Rsi-Rsq).

9. Optical telecommunication system (1) according to one of claims 4 to 8 wherein the photonic device (C) is configured to produce a first recombined light radiation (Rci) from the first plurality of elementary light radiations (RprRpP) and produce a second recombined light radiation (Rc2) from the second plurality of elementary light radiations (RsrRsP).

10. Optical telecommunication system (1) according to claim 9 wherein the photonic device (C) comprises a first photonic device (Cl) optically coupled to the modal splitter (S) to receive the first plurality of elementary light rays (RprRpP) and produce the first recombined light radiation (RcO and a second photonic device (C2) optically coupled to the modal splitter (S) to receive the second plurality of elementary light rays (RsrRsp) and produce the second recombined light radiation (Rc2).

11. Optical telecommunication system (1) according to claim 9 or 10 wherein the photonic device (C) comprises a recombination device (R) configured to recombine the first and second recombined radiation (Rcb Rc2) and form a single recombined light radiation (Rc).

12. Optical telecommunication system (1) according to claim 11 wherein the recombination device (R) is configured to form a single recombined light radiation having a single polarization.

13. Optical telecommunication system (1) according to claim 11 wherein the recombination device (R) is configured to form a single recombined light radiation having superimposed polarizations.

14. Optical telecommunication system (1) according to one of the preceding claims in which the optical port (P) comprises a static or dynamic polarization control device.

15. Optical telecommunication system (1) according to one of the preceding claims wherein the photonic device (C) is optically coupled to the modal splitter (S) via a plurality of single-mode optical fibers.

16. An optical telecommunications system (1) according to claim 15 wherein the plurality of single-mode optical fibers are polarization-maintaining.

17. Optical telecommunication system (1) according to one of claims 1 to 14 in which the coupling between the photonic device (C) and the modal splitter (S) is devoid of optical fibers.

18. Optical telecommunication system (1) according to one of the preceding claims comprising an optical receiver (RO) for demodulating the recombined light radiation (Rc) optically coupled to the optical processing device (DR).

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