Free-space optical telecommunication system

Abstract

The invention relates to a free-space optical telecommunication system (1) comprising a telescope (T) having an objective lens (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).

Description

OPTICAL TELECOMMUNICATION SYSTEM IN FREE SPACE FIELD OF THE INVENTION

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

[0002] In free-space optical telecommunications, a transmitter modulates light radiation (usually 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 rest 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, 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, whose variation dynamics typically extend between 100Hz 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 (for example from documents WO2022185020 and US20170070289) to provide adaptive optics aimed at compensating for these phenomena. However, 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 coherently via a photonic device. Document WO2016047100 proposes, after a modal decomposition of the received radiation, to electrically convert the elementary radiations in order to process these signals in a digital processing device.

[0006] The coherent recombination of elementary light rays requires perfect control of their polarization. These rays 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 of the light radiation in free space. On the other hand, the propagation of this light radiation in the receiver can affect its polarization, in particular when we want to move the optical processing of this radiation from the collector via an optical fiber.

[0008] Depending on the chosen communication protocol, 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, possible relative movements between the transmitter and the receiver can lead to changes in its characteristics at the receiver. This is particularly the case when the transmitter is placed in a satellite and the latter is likely to rotate on itself.

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

[0010] We therefore understand that the light radiation received by the receiver has a state of polarization which is not always perfectly controlled, but which must absolutely be taken into account 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 involves a coherent combination of elementary beams and when these processing operations are removed from the collector. SUBJECT OF THE INVENTION

[0012] An aim of the invention is to propose an optical communication system that remedies, at least in part, the aforementioned problems. More specifically, an aim 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] In order to achieve 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 splitter comprising a modal decomposition device configured to decompose the first light radiation, the modal splitter producing a plurality of elementary light radiations;a photonic device optically coupled to the modal splitter, the photonic device being configured to coherently recombine at least part of the elementary light radiations 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 modal conversion device preserves at least one polarization state of light radiation which propagates therein, and the modal separator comprises a polarization conditioning device configured to conform the first light radiation to the preserved polarization state of the modal conversion device;the optical telecommunications system comprises a polarizing beam splitter disposed 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 rays and produce a second recombined light radiation from the second plurality of elementary light rays;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 recombined light radiation having a single polarization;the recombination device is configured to form a single recombined light radiation having superimposed polarizations;the optical port comprises a device for static or dynamic control of the polarization of the incident radiation to conform it to a determined polarization before its injection into the multimode waveguide;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 splitter (S) is devoid of optical fibersthe optical telecommunication system comprises an optical receiver for demodulating the optically recombined light radiation coupled to the optical processing device (DR);the modal splitter comprises a shaping device arranged upstream of the modal decomposition device;the second end of the polarization-maintaining multimode waveguide is directly coupled to an input port of the modal splitter.; 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]

[0017] The represents an optical telecommunications system in accordance with the invention;

[0018]

[0019]

[0020]

[0021]

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

[0023]

[0024]

[0025] Figures 3a, 3b represent two examples of a photonic device implemented in an optical telecommunication system of the;

[0026]

[0027] The represents an MPLC device of an optical telecommunications system according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] Elements common to all implementation methods.

[0029] With reference to the, an optical telecommunications 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 can 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 telecommunications system 1, is supplied to an optical receiver OR of a base station, capable of extracting the information from the received radiation.

[0030] In the example shown in this, 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.

[0031] The emitted incident light radiation I 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.

[0032] 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 I1. 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 I1. This telescope T may be orientable in order to point and track the transmitter, here arranged in the satellite SAT.The telescope T may also comprise a TTM guidance device for the incident radiation (such as a tilt 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.

[0033] Returning to the description of the, the optical telecommunication 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.

[0034] To allow this distancing between the telescope T and the rest of the optical telecommunication 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 I1 produced by the optical port P to the optical processing device DR.

[0035] Advantageously, this waveguide has a length of at least one meter, and typically between 1 m and 10 m, thus allowing the optical processing device DR to be sufficiently moved away from the telescope T to, for example, shelter it.

[0036] The F waveguide can be passive or active and in this case integrate an additional amplification function.

[0037] By "polarization maintenance" we mean that the waveguide has, along 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.

[0038] For example, this waveguide F may be formed from at least one multimode optical fiber, active or passive, comprising at least one elliptical core and having a parabolic index gradient or a step-varying index. The dimension of the core is chosen to allow the propagation of a plurality of modes, for example at least 10 modes or at least 50 modes. The multimode optical fiber may be spun. Such a fiber is created by rotating a polarization-maintaining preform while it is being drawn. It allows the propagation of light radiation having circular polarization, while preserving this polarization. The multimode optical fiber may also comprise stress bars, integrated into the preform before it is drawn, to correspond to a fiber of the Panda, “Bowtie” (bow tie-shaped) or “Elliptical stress layer” type.

[0039] The waveguide F can be formed from a bundle of polarization-maintaining multimode optical fibers.

[0040] Polarization maintenance may involve two specific linear polarization states of the waveguide, for example along two orthogonal axes arranged in a plane transverse to the propagation direction. Polarization maintenance may alternatively involve two circular polarization states, as has been presented in relation to spun optical fiber.

[0041] In some 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 there.

[0042] A polarization conditioning device refers to any means (passive or static or dynamic control device) for transforming the nature (linear, circular) and / or orientation of the polarization of incident light radiation. This may include a simple half-wave or quarter-wave phase plate (or delay plate).

[0043] As an example, which can be applied to each of the implementation modes that will be set out in the present description, 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.

[0044] Generally speaking, one can refer to the chapter "Polarization Measurement" by Soe-Mie F. Nee, from the book "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.

[0045] In all cases, and whether or not a polarization conditioning device is provided, the first light radiation I1 has a controlled polarization after its propagation in the waveguide F, when it appears at the optical processing device DR. By "controlled" we mean 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 I1 are preserved during this propagation.

[0046] In general, 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 I1 provided by the waveguide F at the first input port and produce, at its output port, a plurality of elementary light radiations R1-R N . Elementary radiations R1-R N are advantageously single-mode and all have the same state of polarization.

[0047] It is possible to provide for placing in the modal separator S, between the waveguide F and the modal decomposition device, a polarization conditioning device in order to conform, if this is not already the case, the polarization of the radiation supplied by the waveguide F to the polarization preserved by the modal decomposition device. It is also possible to provide another polarization conditioning device, in the modal separator S, optically downstream of the modal decomposition device, in order to conform the polarization of the elementary light radiations R1-R N to the polarization expected by the devices arranged downstream of the separator S.

[0048] 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 radiation R1-R N and produce at least one single-mode recombinant light radiation R c .

[0049] The optical processing device DR therefore allows, by modal decomposition of the first radiation I1 and the coherent recombination of the elementary radiations R1-R N products, 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 R1-R Nhaving identical polarization states, they can be efficiently recombined coherently with each other, i.e. form a single-mode recombined light radiation Rc having a maximum energy.

[0050] The modal splitter 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 R1-R N or affecting them in the same way.

[0051] These two devices can thus be separated from each other and the elementary radiations R1-R N propagate in free space between these two devices.

[0052] Alternatively, a bundle of fibers, for example a bundle of single-mode fibers, can be provided to respectively guide the propagation of the elementary radiations R1-R NAdvantageously, these single-mode fibers are polarization-maintaining.

[0053] 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.

[0054] As already specified, a polarization conditioning device can be provided, arranged in the modal separator S or between the modal separator S and the photonic device C so as to adapt the polarization of the elementary radiations R1-R N , or some of them, to the polarization expected by the photonic device C or to the polarization expected of the single-mode fibers, when such fibers are used to respectively guide the propagation of the elementary radiations R1-R Nto the photonic device C.

[0055] The modal decomposition device M of the modal separator S can be implemented by a multiplane optical conversion device, referred to as an “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 propagation of the radiation in free space. At least some of the optical parts on which the reflections and / or transmissions take place, and which guide the propagation of the incident radiation, have microstructured zones which modify the incident light radiation.

[0056] By "microstructured zone" we mean 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 or transmitted there.

[0057] Thus, light radiation propagating 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 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 microstructured 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.

[0058] Theoretical foundations and practical implementation examples of an MPLC device can be found in the documents “Programmable unitary spatial mode manipulation”, Morizuret Al, J.Opt. Soc. Am. A / Vol. 27, No. 11 / November2010; N. Fontaine et Al, (ECOC, 2017), “Design of High Order Mode-Multiplexers using Multiplane Light Conversion”; US9250454 and US2017010463.

[0059] As presented in detail in the aforementioned documents, the microstructured 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 certain states of polarization of the light radiation passing through it.

[0060] An MPLC device is generally produced in the form of optical parts having main surfaces parallel to each other on which the multiple reflections and / or transmissions occur. In such a configuration, the polarization states preserved during the propagation of the radiation in the device concern:the s state of polarization, i.e. a state perpendicular to the axis of propagation of the radiation and parallel to the planes defined by the main reflection and / or transmission surfaces;the p state of polarization, i.e. a state also perpendicular to the axis of propagation and perpendicular to the planes defined by the main reflection and / or transmission surfaces.

[0061] We will therefore seek to ensure that the radiation injected into such an MPLC device has an s and / or p polarization.

[0062] As an illustration, there is shown such an MPLC device M formed of two optical parts Ma, Mb arranged opposite each other, these optical parts having surfaces parallel to each other on which the first radiation I1 is reflected (at the level of microstructured zones z), a plurality of times, to decompose it into elementary radiations R1-R N. In the illustration of the, only one of these optical parts Ma carries these microstructured zones z, but it is quite possible to have them carried by the two optical parts Ma, Mb or by any other combination of optical parts. Also shown on the, two polarization conditioning devices Pol1 and Pol2. The first Pol1 of these devices makes it possible to conform the polarization of the first radiation I1 to conform its polarization to that, maintained, by the MPLC device M. It can be for example a half-wave or quarter-wave delay plate as previously mentioned. Similarly, a second polarization conditioning device Pol2 is arranged at the output of the MPLC device M, to intercept the elementary radiations R1-R N and adjust their polarization to that expected by the optical device located further downstream.

[0063] 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 I1. The family of output modes may be formed of N Gaussian modes, spatially separated, these modes defining the elementary radiations R1-R N . 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 I1 received at the input port is decomposed according to the modes of the input basis and transported into the MPLC device to distribute and conform to the output Gaussian modes with which the modes of the input basis are associated.

[0064] 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.

[0065] 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 I1, decompose it spatially via the N fibers of the bundle and thus produce the elementary light radiations R1-R N .

[0066] 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 disk or inscribed in a disk, to best decompose the first light radiation I1. 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 I1, when this comes from a telescope T having a second mirror, as previously presented. Alternatively, the ends of the fibers of the bundle may be arranged in a line.In this case, an optical device can be provided, arranged between the second end of the waveguide F and the bundle of fibers, 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 bundle.

[0067] 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 by 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 I1. Each of the waveguides can be provided with a microlens in order to promote the decomposition of the first light radiation I1 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, arranged in a grid onto which the first radiation I1 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.

[0068] We can naturally predict 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.

[0069] In all cases, care is taken to ensure that the polarization of the light radiation propagating in the modal separator S is not excessively affected. In particular, by providing a polarization conditioning device, it will be sought to ensure that the radiation injected into the modal conversion device has a polarization state which conforms to the polarization state preserved by this device.

[0070] The person skilled in the art will be able to choose the appropriate polarization conditioning device according to the nature of the radiation I1 coming from the waveguide and according to the nature of the polarization preserved by the modal conversion device. For example, it could be a half-wave plate to adjust the orientation of a linear polarization by 45°, or a quarter-wave plate to transform a circular polarization (typically from a polarization-maintaining waveguide of the "spun fiber" type) into a linear polarization.

[0071] This polarization conditioning device is not, however, imperative. This is the case, for example, when the first radiation from the waveguide F has at least one of the s and p polarization states of an MPLC device conforming to that shown in the figure. In this case, it is simply ensured that the natural axes of the waveguide F are well arranged according to these two orientations s and p of the MPLC device.

[0072] In general, and whatever the mode of implementation chosen for the modal decomposition device M, it is possible to provide an optical device for shaping the first radiation I1 which comes from the waveguide F, this device being arranged in the modal separator S upstream of the modal decomposition device M, between the second end of the waveguide F and the modal decomposition device M itself. This optical device may comprise or be exclusively constituted of at least one optical element passive in transmission or in reflection, for example one or a plurality of free-form optics.

[0073] Advantageously, the second end of the polarization-maintaining multimode waveguide F is directly coupled to an input port of the modal splitter S, i.e. there is no other element between this second end and the modal splitter. The light radiation from the waveguide F, after a possible polarization shaping or conditioning treatment (as just explained), is injected directly into the modal decomposition device M.

[0074] As already stated, the photonic device C is configured to coherently recombine at least part of the elementary light rays R1-R N 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.

[0075] The photonic device C can 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 R1-R N which are presented on its input port and phase actuators allowing the relative phase of these radiations to be adjusted and ensuring their recombination as accurately as possible to produce the recombined radiation Rc. An example of such a chip is for example described in document EP3672109A1.

[0076] The photonic device C may take other forms than an integrated photonic chip PIC or include other components to achieve the coherent recombination of the elementary radiations R1-R N. 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

[0077] The present invention is 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.

[0078] 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.

[0079] 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 I1 which propagates there. 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.

[0080] 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 the 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 R c. A conditioning device or a plurality of such devices may be placed in the DR treatment device to adjust the polarization of the radiation propagating therein and to ensure that before each treatment carried out on this radiation, its polarization is in accordance with that expected, that is to say in accordance with a polarization which will not be excessively affected by the treatment in question. Second mode of implementation.

[0081] This implementation mode illustrated in 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.

[0082] 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 I1 propagating in the waveguide has the same characteristics as those of the incident radiation I.

[0083] 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, for example integrated into a polarization conditioning device arranged in the splitter upstream of the modal decomposition device M. 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 I1 are found.

[0084] The polarizing beam splitter PBS produces a first polarized light radiation Rlp and a second polarized light radiation Rls, the first and second polarized light radiation Rlp,Rls having distinct, generally orthogonal, polarizations. It has been possible to provide in the polarization conditioning device, preferably upstream of the PBS splitter device, optical elements (delay plate for example) making it possible to prepare the first radiation so that it has this orthogonal polarization if this radiation does not naturally occur in this form.

[0085] In this configuration, the two polarized light rays Rlp, Rls propagate towards separate inputs of the input port of the modal decomposition device M. This is configured to decompose these two rays Rlp, Rls and provide the elementary light rays R1-R N. More specifically, the modal decomposition device M is configured to decompose the first polarized light radiation Rlp into a first plurality of elementary light radiations R p 1-R p P and to decompose the second polarized light radiation Rls into a second plurality of elementary light radiations R s 1-R s P . These two pluralities of elementary light radiations R p 1-R p P ,R s 1-R s Qhave distinct polarizations, respectively identical to those of the first and second polarized light radiation Rlp,Rls. It is therefore possible to provide, before the photonic device C, or even before the MPLC device, delay plates or any other polarization conditioning device, making it possible to modify the polarization state of certain radiations which propagate in order to facilitate their processing by the photonic device or their injection into single-mode fibers. The first plurality and the second plurality of elementary light radiations R p 1-R p P ,R s 1-R s P constitute, in combination, the plurality of elementary light rays R1-R N produced by the modal separator S.

[0086] 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 rays Rlp, Rls. But this is not an essential characteristic, and the two polarized light rays Rlp, Rls may also be decomposed by a single modal decomposition device, for example a single MPLC device M.

[0087] It is noted that this second mode of implementation remains compatible with non-polarization multiplexed light radiation, this polarization being able to be indeterminate. In fact, a part of the energy present in the incident radiation can be collected on each of the polarization axes of the multimode waveguide F. This energy is recombined in the processing device.

[0088] 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).

[0089] 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 (Rld1-Rld P), as detailed in a previous passage. This modal separator M can thus be implemented by an MPLC device. The modal separator S can comprise, upstream of the modal decomposition device M, a polarization conditioning device.

[0090] The modal splitter S also comprises a polarizing beam splitter PBS arranged, in this variant, optically downstream of the modal decomposition device M. This splitter may be part of a polarization conditioning device implementing the other functions already described of such a device. The polarizing beam splitter PBS therefore receives the plurality of decomposed light rays Rld1-Rld P and produces a first plurality of elementary light rays R p 1-R p P and a second plurality of elementary light rays R s 1-R s Q, the first plurality and the second plurality of elementary light rays R p 1-R p P ,R s 1-R s Q having distinct polarizations. They constitute, in combination, the plurality of elementary light rays R1-R N produced by the modal separator S.

[0091] Whether the variant of or that of 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 R p 1-R p P and a second plurality of elementary light rays R s 1-R s Q. These two pluralities of elementary light radiation naturally have distinct polarizations. As already indicated, wave plates or any other form of polarization conditioning device can be provided to place these elementary light radiations (or a part of them) in a chosen polarization state, for example a polarization state expected by the photonic device C. The elementary light radiations 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.

[0092] Advantageously, and as illustrated in Figures 3a and 3b, the photonic device C is configured to produce a first recombinant light radiation Rc1 from the first plurality of elementary light radiations R p 1-R p Pand producing a second recombinant light radiation Rc2 from the second plurality of elementary light radiations R s 1-R s Q 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 rays R p 1-R p P , R s 1-R s Q . 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 rays R p 1-R p Pand produce the first recombined light radiation Rc1. 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 radiations R s 1-R s P and produce the second recombined light radiation Rc2. Wave plates or any other polarization conditioning 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).

[0093] As seen previously, these optical couplings can be achieved in free space, via optical fibers, for example single-mode polarization-maintaining fibers, or even by mechanical assembly of the photonic devices to the modal splitter S.

[0094] In a first embodiment shown in the, the photonic device C comprises a recombination device R configured to recombine the first and second recombined radiation Rc1, Rc2 and form a single recombined light radiation R C having a single polarization. In this configuration, the recombination device can be formed by a Mach-Zehnder device or a polarizing beam splitter in reverse assembly. In this second case, the polarization of the single recombined light radiation R C can be unstable and fluctuate over time. This variant of execution 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 R C presenting a single polarization.

[0095] In a second embodiment shown in the, 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.

[0096] In each of the embodiments of Figures 3a and 3b, the recombination device is completely optional. It can be provided that the photonic device C (and therefore the optical communication system 1) delivers the first and second recombined radiation Rc1, Rc2 separately, 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 Rc1, Rc2 supplied by the optical communication system 1. Third mode of implementation.

[0097] The third implementation method is illustrated in the and is a variant of the second implementation method. It therefore applies under the same conditions of use and benefits from the same advantages.

[0098] 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 Rls 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 radiations R p 1-R p P. It is also configured to decompose the second polarized light radiation Rls provided by the second multimode waveguide F2 into a second plurality of elementary light radiations R s 1-R s Q . The separator S may have a polarization conditioning device or a plurality of such devices, as detailed in a previous section of this description.

[0099] 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 figures 3a and 3c.

[0100] In the embodiments just described, the incident light radiation I has a polarization that may need 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 linked to the optical communication system 1.

[0101] 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 in order to impose a determined polarization on it.

[0102] 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.

Claims

Optical telecommunication system (1) in free space 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 (DR) comprising: a modal splitter (S) comprising a modal decomposition device (M) configured to decompose the first light radiation (I1), the modal splitter (S) producing a plurality of elementary light radiations (R1-R N ); a photonic device (C) optically coupled to the modal separator (S), the photonic device (C) being configured to coherently recombine at least part of the elementary light radiation (R1-R N ) and produce at least one recombinant light radiation (R c);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). Optical telecommunication system (1) according to claim 1 wherein the modal decomposition device (M) comprises at least one multi-plane conversion device. Optical telecommunications 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. Optical telecommunication system (1) according to one of the preceding claims in which the modal conversion device (M) preserves at least one polarization state of a light radiation which propagates therein, and the modal separator (S) comprises a polarization conditioning device configured to conform the first light radiation (I1) to the preserved polarization state of the modal conversion device (M). Optical telecommunications system (1) according to one of the preceding claims comprising a polarizing beam splitter (PBS) arranged upstream of the photonic device (C), the optical splitter (S) producing a first plurality of elementary light rays (R p 1-R p P ) and a second plurality of elementary light rays (R s 1-R s Q) having distinct polarizations, the first plurality and the second plurality of elementary light rays (R p 1-R p P , R s 1-R s Q ) constituting the plurality of elementary light rays (R1-R N ). Optical telecommunication system (1) according to claim 5 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 (Rls) having distinct polarizations. Optical telecommunication system (1) according to claim 5 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 (Rls) 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 (F1) and to inject the second polarized light radiation (Rls) into a second polarization-maintaining multimode waveguide (F2). Optical telecommunication system (1) according to claim 6 or 7 wherein the modal separator (S) comprises a first modal decomposition device (M1) arranged to receive the first polarized light radiation (Rlp) and produce a first plurality of elementary light radiations (Rp 1-R p P ) and a second modal decomposition device (M2) arranged to receive the second polarized light radiation (Rls) and produce a second plurality of elementary light radiations (R s 1-R s Q ), the first plurality and the second plurality of elementary light rays (R p 1-R p P , R s 1-R s Q ) constituting the plurality of elementary light rays (R1-R N ) produced by the modal separator (S). Optical telecommunication system (1) according to claim 5 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 (Rld1-Rld P) and the polarizing beam splitter (PBS) is optically arranged downstream of the modal decomposition device (M) to receive the plurality of decomposed light rays (Rld1-Rld P ) and produce the first plurality of elementary light rays (R p 1-R p P ) and the second plurality of elementary light rays (R s 1-R s Q ). Optical telecommunication system (1) according to one of claims 5 to 9 in which the photonic device (C) is configured to produce a first recombined light radiation (Rc1) from the first plurality of elementary light radiations (R p 1-R p P ) and produce a second recombinant light radiation (Rc2) from the second plurality of elementary light radiations (R s 1-R s P ). Optical telecommunication system (1) according to claim 10 wherein the photonic device (C) comprises a first photonic device (C1) optically coupled to the modal splitter (S) for receiving the first plurality of elementary light rays (R p 1-R p P ) and produce the first recombined light radiation (Rc1) and a second photonic device (C2) optically coupled to the modal splitter (S) to receive the second plurality of elementary light radiations (R s 1-R s P ) and produce the second recombined light radiation (Rc2). Optical telecommunication system (1) according to claim 10 or 11 wherein the photonic device (C) comprises a recombination device (R) configured to recombine the first and second recombined radiation (Rc1, Rc2) and form a single recombined light radiation (R C ). Optical telecommunications system (1) according to one of the preceding claims in which the optical port (P) comprises a device for static or dynamic control of the polarization of the incident radiation to conform it to a determined polarization before its injection into the multimode waveguide (F). 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. Optical telecommunication system (1) according to one of the preceding claims in which the modal separator (S) comprises a shaping device arranged upstream of the modal decomposition device (M). Optical telecommunication system (1) according to one of the preceding claims wherein the second end of the polarization-maintaining multimode waveguide (F) is directly coupled to an input port of the modal splitter (S).

Images