Free Space Optical Communication System

Abstract

The present invention relates to a free-space optical communication system (1) comprising a telescope (T) having an objective lens (O) that collects incident optical radiation (I) and generates a first optical radiation (I1) at an optical port (P), 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

[Technical Field]

[0001] The present invention relates to optical communication systems that seek to compensate for wavefront distortions of incident optical radiation, which may result from atmospheric turbulence during optical communication in free space, or more generally, caused by the propagation of an optical beam in that medium. The present invention has particular application in the field of free-space optical communication. [Background technology]

[0002] In free-space optical communications, a transmitter modulates optical radiation (usually generated by a laser) with the information to be transmitted, and the optical radiation takes the form of a narrow beam emitted in the direction of a receiver. After propagating through the medium (air will be used 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 communications), the optical radiation is collected at the receiver and demodulated to recover the transmitted information. Generally speaking, with the goal of maximizing the data rate, one tries to utilize as much energy as possible present in the optical radiation received by the receiver in order to maximize the transmission rate.

[0003] To multiplex communication channels and maximize throughput, optical radiation can be multiplexed in wavelength and / or polarization.

[0004] The propagation of optical radiation exposes the radiation generated by the transmitter to atmospheric disturbances, particularly temperature and pressure fluctuations that the radiation experiences during its propagation. These irregular disturbances, whose fluctuation dynamics are typically between 100 Hz and a few kHz, result in deformations that affect the wavefront. More precisely, the disturbances tend to spatially redistribute the energy in the radiation, generating random fluctuations in amplitude and phase. These deformations are embodied in the form of a "speckle" pattern in the spot formed by the projection of the beam onto the radiation collection device and by the phenomenon of scintillation. This limits the data rate on the link between the transmitter and the receiver.

[0005] To overcome this limitation, it is known (for example from WO 2022185020 and US 20170070289) to provide adaptive optics that compensate for these phenomena. However, this type of solution only affects the phase of the incident light, and therefore has limited performance. EP 3672109 proposes a receiver that is able to modally decompose the received radiation (at the collector) into elementary rays. These elementary rays are coherently recombined by a photonic device. WO 2016047100 proposes, after modal decomposition of the received radiation, to electrically convert the elementary rays in order to process these signals in a digital processing device.

[0006] Coherent recombination of the fundamental rays requires perfect control of their polarization: these rays must have the same polarization to allow interference mechanisms to produce the desired recombination.

[0007] It should be noted that the optical radiation generated by the transmitter is polarized, and this polarization is not affected by the free-space propagation of the optical radiation. Conversely, the propagation of this optical radiation in the receiver can affect its polarization, especially if the optical processing of this radiation is offset from the collector via an optical fiber.

[0008] Depending on the selected communication protocol, the polarization state of the optical radiation generated by the transmitter may or may not be determined (e.g., linearly polarized, left-handed circularly polarized, or right-handed circularly polarized). In the latter case, its characteristics can freely evolve over time. Even if the polarization state of the optical radiation is determined, any relative displacement between the transmitter and the receiver can lead to a change in its characteristics at the receiver. This is especially true when the transmitter is located within a satellite, which is likely to rotate on its own.

[0009] Furthermore, as previously mentioned, some communication protocols also provide for the generation of polarization multiplexed optical radiation.

[0010] It is therefore clear that the optical radiation received by a receiver has a polarization state that is not always perfectly controlled but that must be taken into account in order to make the most of the transmitted energy and / or to decode the transmitted symbols.

[0011] This is especially true when the receiver processes the received light using coherent combining of the fundamental beams, and when this processing is remote from the collector.

[0012] [Object of the Invention] An object of the present invention is to propose an optical communication system that at least partially overcomes the aforementioned problems. More precisely, an object of the present invention is to provide an optical communication system comprising a collector of incident optical radiation and a photonic device that implements coherent combining of fundamental optical radiation, the photonic device being remote from the collector. Summary of the Invention

[0013] To this end, the subject of the present invention is a telescope having an objective lens for collecting incident optical radiation and generating a first optical radiation at an optical port; an optical processing device, a mode splitter comprising a mode decomposition device configured to decompose the first optical radiation to generate a plurality of fundamental optical radiations; an optical processing device comprising: a photonic device optically coupled to the mode splitter, the photonic device configured to coherently recombine at least a portion of the fundamental optical radiation to generate at least one recombined optical radiation; -Propose a free-space optical communication system comprising: at least one polarization-maintaining multimode waveguide having a first end coupled to an optical port of a telescope and a second end coupled to an optical processing device.

[0014] According to other advantageous, non-limiting features of the present invention, taken alone or in any technically feasible combination, - the mode decomposition device comprises at least one multiplane transformation device; the mode decomposition device comprises a bundle of single-mode optical fibers assembled in parallel; the mode converting device preserves at least one polarization state of optical radiation propagating therethrough, and the mode splitter comprises a polarization adjusting device configured to adapt the first optical radiation to the preserved polarization state of the mode converting device; the optical communication system comprises a polarizing beam splitter arranged upstream of the photonic device, the optical splitter generating a first plurality of fundamental optical radiations and a second plurality of fundamental optical radiations having distinct polarizations, the first plurality of fundamental optical radiations and the second plurality of fundamental optical radiations constituting a plurality of fundamental optical radiations; a polarizing beam splitter coupled to a second end of the multimode waveguide, the polarizing beam splitter generating first polarized radiation and second polarized radiation having distinct polarizations; a polarizing beam splitter disposed within an optical port of the telescope to generate first and second polarized radiation having distinct polarizations, the polarizing beam splitter disposed within the optical port to couple the first polarized radiation into the first polarization-maintaining multimode waveguide and couple the second polarized radiation into the second polarization-maintaining multimode waveguide; the mode splitter comprises a first mode decomposition device arranged to receive radiation of a first polarization and to generate a first plurality of fundamental optical radiations, and a second mode decomposition device arranged to receive radiation of a second polarization and to generate a second plurality of fundamental optical radiations, the first plurality of fundamental optical radiations and the second plurality of fundamental optical radiations constituting the plurality of fundamental optical radiations generated by the mode splitter; a mode decomposition device coupled to the second end of the multimode waveguide to generate a plurality of decomposed optical radiations, and a polarizing beam splitter optically positioned downstream of the mode decomposition device to receive the plurality of decomposed optical radiations and generate a first plurality of fundamental optical radiations and a second plurality of fundamental optical radiations; the photonic device is configured to generate a first recombined optical radiation from the first plurality of fundamental optical radiations and to generate a second recombined optical radiation from the second plurality of fundamental optical radiations; the photonic device comprises: a first photonic device optically coupled to the mode splitter to receive the first plurality of fundamental optical radiations and generate a first recombined optical radiation; and a second photonic device optically coupled to the mode splitter to receive the second plurality of fundamental optical radiations and generate a second recombined optical radiation; the photonic device comprises a recombination device configured to recombine the first and second recombined radiations to form a single recombined optical radiation; the recombination device is configured to form a single recombined optical radiation having a single polarization; the recombination device is configured to form a single recombined optical radiation with superimposed polarizations; the optical port comprises a device for statically or dynamically controlling the polarization of the incident radiation to adapt the incident radiation to a determined polarization before it is launched into the multimode waveguide; The photonic device is optically coupled to a mode splitter via a plurality of single-mode optical fibers; the single-mode optical fiber of the plurality of single-mode fibers is polarization-maintaining; - the coupling between the photonic device (C) and the mode splitter (S) is devoid of optical fiber, - an optical communication system comprising an optical receiver for demodulating optically recombined optical radiation, coupled to an optical processing device (DR); the mode splitter comprises a shaping device arranged upstream of the mode decomposition device; The second end of the polarization-maintaining multimode waveguide is directly coupled to the input port of the mode splitter. [Brief explanation of the drawings]

[0015] Other features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings. [Figure 1] 1 shows an optical communication system according to the present invention. [Figure 2a] 2 illustrates a different embodiment of the optical communication system shown in FIG. [Figure 2b] 2 illustrates a different embodiment of the optical communication system shown in FIG. [Figure 2c] 2 illustrates a different embodiment of the optical communication system shown in FIG. [Figure 2d] 2 illustrates a different embodiment of the optical communication system shown in FIG. [Figure 3a] 2 shows two examples of photonic devices used in the optical communication system of FIG. 1. [Figure 3b] 2 shows two examples of photonic devices used in the optical communication system of FIG. 1. [Figure 4] 1 shows an MPLC device of an optical communication system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Elements common to all embodiments 1, an optical communication system 1 compatible with the present invention is designed to process incident optical radiation I, which is generated by a transmitter and carries information to be transmitted by modulation. The incident optical radiation I may have several wavelengths and / or use several polarizations, as is usually the case in WDM type transmissions. Once processed by the optical communication system 1, the incident optical radiation is supplied to an optical receiver OR at a base station, which is able to extract the information from the received radiation.

[0017] In the example shown in Figure 1, the transmitter is located within a satellite SAT, but the communication system of the present invention is not limited to this particular application. Generally speaking, the transmitter can be located on land, sea, or in space and can propagate in any free space, the atmosphere for terrestrial communications, or underwater for marine communications. The transmitter and communication system 1 can be stationary or moving relative to each other.

[0018] The emitted incident light I takes the form of a narrow beam directed towards the communication system 1. During its propagation through free space, the emitted radiation is subject to atmospheric disturbances in the atmosphere, so that the incident optical radiation I arriving at the base station has spatial and temporal variations in amplitude and phase. This phenomenon affects the shape of this radiation, which takes on a shape that changes irregularly and irregularly over time. The optical communication system 1 is designed to at least partially compensate for this distortion, so that the optical receiver OR can process the radiation and decode the transmitted message by direct or coherent detection. For this purpose, the receiver OR can incorporate amplification and / or spectral demultiplexing functions, especially for WDM transmission.

[0019] The optical communication system 1 comprises a telescope T, which has an objective lens O for collecting incident optical radiation I and generating a first optical radiation I1 at an optical port P. As is known per se, this objective lens can comprise a concave mirror that focuses the received radiation at an image focus. This focused radiation can be reflected back to the optical port P using a second mirror of the objective lens O, which can be flat or convex. If present, the second mirror results in the formation of a central zone of very low intensity in the optical radiation propagating towards the optical port P. Thus, in all cases, this optical port P of the telescope T generates the first radiation I1. This telescope T can be rotated to point and track a transmitter located on the satellite SAT. The telescope T can also comprise a device (such as a tilt-tip mirror) for directing the incident radiation TTM in order to best direct the optical radiation towards the optical port P, for example to center this radiation within the port P, or more generally to correct any pointing deviations of the telescope T.

[0020] Returning to the description of FIG. 1 , the optical communication system 1 also includes an optical processing device DR optically downstream of the telescope T, designed to at least partially compensate for distortions in the collected incident optical radiation I. This device DR, described in detail in subsequent sections of this disclosure, is composed of multiple precisely assembled optical or photonic elements, which may be particularly sensitive to their operating environment. Therefore, it is advantageous to offset this system from the telescope T by several meters to several tens of meters, and the positioning of the telescope T is often determined by the reception quality of the incident radiation I, for example, to place the telescope T in a cabinet, a room, a vehicle, or any other shelter of a base station operations center. In this way, the optical processing device DR can be protected and its operating environment (temperature, atmosphere, exposure to dust, vibration, movement, etc.) can be controlled, making its operation and maintenance easier.

[0021] To enable this distance between the telescope T and the rest of the optical communication system 1, the optical port P of the telescope is optically connected 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, which propagates the first optical radiation I1 generated by the optical port P to the optical processing device DR.

[0022] Advantageously, this waveguide has a length of at least 1 meter, typically between 1 m and 10 m, allowing the light processing device DR to be sufficiently offset from the telescope T, e.g. to accommodate it.

[0023] The waveguide F may be passive or active, in which case it incorporates an additional amplification function.

[0024] By "polarization-maintaining" it is meant that the waveguide has a polarization extinction ratio (PER) over its length that is greater than 7 dB, preferably greater than 10 dB, and more preferably greater than 20 dB.

[0025] By way of example, this waveguide F can be formed by at least one active or passive multimode optical fiber including at least one elliptical core and having a parabolic or step refractive index gradient. The core size is selected to allow multiple modes, for example at least 10 or at least 50 modes, to be propagated. The multimode optical fiber can also be a spun optical fiber. Such a fiber is created by rotating a polarization-maintaining preform during drawing. It can be used to propagate circularly polarized light while preserving this polarization. The multimode optical fiber can also include stress bars incorporated into the preform before drawing, corresponding to panda, bowtie, or elliptical stress layer fibers.

[0026] The waveguide F may be formed by a bundle of polarization-maintaining multimode optical fibers.

[0027] Preserving polarization can involve, for example, two specific linear polarization states of the waveguide along two orthogonal axes arranged in a plane transverse to the direction of propagation. Preserving polarization can alternatively relate to two circular polarization states, as shown in connection with spun optical fiber.

[0028] In some cases, it may be advantageous to provide a device for adjusting the polarization of the optical radiation injected into the waveguide, this device being located at optical port P and coupled to the first end of the waveguide. This device is designed to adapt the polarization of the light collected by the objective lens of telescope T to the polarization state maintained by the waveguide. This is particularly useful when the polarization of the incident radiation I is not perfectly controlled, and therefore it is not possible to directly inject this radiation collected by telescope T into waveguide F without risking affecting the polarization of the radiation propagating through waveguide F.

[0029] "Polarization adjusting device" is understood to mean any means (passive device or static or dynamic control device) for converting the nature (linear, circular) and / or orientation of the polarization of incident optical radiation. In particular, it may be a simple half-wave or quarter-wave phase plate (or retarder).

[0030] As an example applicable to each of the embodiments described herein, a polarimeter can be used to measure the polarization state of incident optical radiation in whole or in part and correct it accordingly using electric or static birefringent retarders, or liquid crystal or Pockels birefringent cells. For an example of such a device for controlling the polarization of optical radiation, see the article "Polarization stabilizer using liquid crystal rotatable waveplates" by T. Chiba, Y. Ohtera and S. Kawakami, Journal of Lightwave Technology, May 1999, Vol. 17, No. 5, pp. 885-890.

[0031] In general, for the basic principles of light polarization, reference can be made to the chapter "Polarization Measurement" by Soe-Mie F. Nee in "Measurement, Instrumentation, and Sensors Handbook" (ISBN 9781315217444), edited by John G. Webster and Halit Eren and published by CRC Press.

[0032] In all cases, whether or not a polarization adjusting device is provided, the first optical radiation I1 exhibits a controlled polarization when it reaches the optical processing device DR after propagating in the waveguide F. "Controlled" is understood to mean that the propagation in the waveguide is free from crosstalk and that the energy present in different polarization states of the first optical radiation I1 is conserved during this propagation.

[0033] Generally speaking, the optical processing device DR comprises a mode splitter S optically coupled to a waveguide F. The mode splitter S has an input port and an output port. It splits at its first input port a first optical radiation I1 supplied by the waveguide F and at its output ports splits a plurality of fundamental optical radiations R1 to R2. N Advantageously, the at least one modal decomposition device is configured to generate the fundamental radiation R1 to R N are single mode and all have the same polarization state.

[0034] If not already the case, a polarization adjusting device can be placed in the mode splitter S between the waveguide F and the mode decomposition device in order to adapt the polarization of the radiation supplied by the waveguide F to the polarization preserved by the mode decomposition device. N A further polarization adjusting device may be provided within the mode splitter S, optically downstream of the mode decomposition device, to adapt the polarization of the light beam to the polarization expected by a device located downstream of the splitter S.

[0035] The optical processing device DR also comprises at least one photonic device C optically coupled downstream to the mode splitter S. This photonic device C generates fundamental optical radiation R1 to R N coherently recombine at least some of the light components into at least one single-mode recombined optical radiation R c is configured to generate

[0036] Modal decomposition of the first radiation I1 and the generated fundamental radiations R1 to R N The coherent recombination of the fundamental radiations R1 to R2 allows the optical processing device DR to utilize the maximum energy of the collected incident optical radiation I and at least partially compensate for the distortions that this radiation undergoes during its propagation in free space. N have identical polarization states, they can be efficiently and coherently recombined with each other, i.e., the single-mode recombined optical radiation R with maximum energy ccan be formed.

[0037] The mode splitter S and the photonic device C affect the fundamental radiation R1 to R2 identically. N can be coupled to each other in different ways, using optical fiber or without optical fiber at all, while preserving the polarization state of the light.

[0038] Therefore, the two devices can be separated from each other, and the fundamental radiation R1 to R N propagates in free space between the two devices.

[0039] Alternatively, a fiber bundle, e.g., a single-mode fiber bundle, may be used to transmit the fundamental radiation R1 to R N Advantageously, these single-mode fibers are polarization-maintaining.

[0040] Alternatively, the two devices can be configured to be mechanically assembled with the output port of the mode splitter device S facing the input port of the photonic device C so that the fundamental radiation can propagate from one device to the other.

[0041] As already mentioned, the fundamental radiation R1 to R N to the polarization expected by the photonic device C, or the single-mode fiber to convert the fundamental radiation R1 to R2 N A polarization adjusting device can be placed within the mode splitter S or between the mode splitter S and the photonic device C to match the expected polarization of such a fiber when it is used to guide the propagation of

[0042] The mode decomposition device M of the mode splitter S can be implemented by a multi-plane optical conversion device, referred to in the remainder of this specification as an "MPLC device." For completeness, recall that in such an MPLC device, the incident optical radiation undergoes a series of reflections and / or transmissions, each of which is followed by free-space propagation of the radiation. At least some of the optical components through which the reflections and / or transmissions occur and which guide the propagation of the incident radiation have microstructured zones that modify the incident optical radiation.

[0043] The term "microstructured zone" means that the surface of an optical component has a relief on this zone, which can be resolved into the form of "pixels" whose dimensions can be, for example, from a few microns to hundreds of microns. It can also be a metasurface. Each pixel has a height of up to a few microns or up to hundreds of microns relative to the average plane defining the face or surface in question. Regardless of the microstructured nature of the zone, an optical component with such a zone forms a phase mask that introduces a local phase shift in the cross section of the beam reflected or transmitted therefrom.

[0044] Thus, optical radiation propagating within an MPLC device undergoes a series of local phase shifts separated by propagation. A succession of these elementary transformations (e.g., at least four successive transformations, e.g., 8, 10, 12, 14, or even at least 20 transformations) establishes an overall transformation of the spatial profile of the incident radiation. Thus, it is possible to configure a microstructured reflective or transmissive surface to transform a first optical radiation having a particular shape into a second radiation having a different shape.

[0045] The publications "Programmable unitary spatial mode manipulation", Morizur et al., J. Opt. Soc. Am. A / Vol. 27, No. 11 / November 2010, and N. Fontaine et al. (ECOC, 2017), "Design of High Order Mode-Multiplexers using Multiplane Light Conversion", U.S. Patent No. 9250454 and U.S. Patent No. 2017010463 contain theoretical foundations and examples of practical implementation of MPLC devices.

[0046] As detailed in the aforementioned publications, the microstructured zones carried by the optical components forming the MPLC device are designed and configured to operate a mode conversion process aimed at decomposing the optical radiation received at the input port into a family of modes called "input" modes. The energy present in the input family of modes is transferred and shaped, respectively, at the output port of the MPLC device into a family of "output" modes. The MPLC device is configured to match the input fundamental mode and the output fundamental mode, respectively. It is a passive device with a particularly stable and robust transfer function, which has little or no effect on the specific polarization state of the optical radiation passing through it.

[0047] MPLC devices typically consist of optical components with parallel major surfaces where multiple reflections and / or transmissions occur. In such a configuration, the polarization state preserved during radiation propagation within the device is: -s polarization state, i.e. perpendicular to the radiation propagation axis and parallel to the plane defined by the primary reflecting and / or transmitting surfaces. -p polarization state, i.e., perpendicular to the propagation axis and perpendicular to the plane defined by the primary reflecting and / or transmitting surfaces.

[0048] The aim is therefore to ensure that radiation incident on such an MPLC device exhibits s-polarisation and / or p-polarisation.

[0049] For illustrative purposes, FIG. 4 shows such an MPLC device M comprising two optical components Ma, Mb arranged opposite each other, with their surfaces parallel to each other, and a first radiation I1 which is divided into fundamental radiations R1 to R2. N 4, only one of these optical components Ma carries these microstructured zones z, but it is entirely possible for them to be carried by both optical components Ma, Mb or any other combination of optical components. Also shown in FIG. 4 are two polarization adjusting devices Pol1 and Pol2. The first of these devices, Pol1, adapts the polarization of the first radiation I1 to match the polarization carried by the MPLC device M. It may for example be a retardation, half-wave or quarter-wave plate, as mentioned above. Similarly, the second polarization adjusting device Pol2 is placed at the output of the MPLC device M and adjusts the polarization of the fundamental radiations R1 to R2. N and adjusts their polarization to the expected polarization by an optical device located further downstream.

[0050] In the context of this specification, by way of example, the family of input modes can include a Hermite-Gaussian basis composed of N Hermite-Gaussian modes spatially arranged opposite the first optical radiation I1. The family of output modes can be formed by N spatially separated Gaussian modes, which are the fundamental radiations R1 to R2. N The MPLC device is configured to associate Hermite-Gaussian modes in the input basis with Gaussian modes in the output basis. The energy of the first radiation I1 received at the input port is decomposed according to the input fundamental mode and transported within the MPLC device to distribute and match the output Gaussian modes with which the input fundamental mode is associated.

[0051] Of course, the Hermite-Gaussian and Gaussian modes used as examples are for illustrative purposes only, and other modes can be chosen to perform the decomposition.

[0052] The mode decomposition device M of the mode splitter S can be implemented by means other than the MPLC device taken as an example and detailed above. For example, this mode decomposition device M can be formed by a bundle of N single-mode optical fibers assembled parallel to each other and collimated by microlenses if necessary. This fiber bundle is arranged opposite the waveguide F and receives the first optical radiation I1 and spatially decomposes it through the N fibers of the bundle, thus dividing it into the fundamental optical radiations R1 to R2. N Generate.

[0053] To best resolve the first optical radiation I1, the fiber bundles may be arranged in a matrix, or more generally, the ends of the fibers in the bundle may be arranged in a plane, for example, in the shape of a disk or inscribed in a disk. The fiber bundle may also be arranged so that the ends of the fibers are arranged in a ring, with the central part of the ring being free of fibers, corresponding to the central zone of very low intensity that may be present in the first optical radiation I1 coming from the telescope T with the second mirror, as described above. Alternatively, the ends of the fibers in the bundle may be arranged in a line. In this case, an optical device may be placed between the second end of the waveguide F and the fiber bundle to shape the first optical radiation coming from this waveguide to fit the line defined by the ends of the fibers in the bundle.

[0054] Alternatively, the mode decomposition device M (and the mode splitter S) can be integrated into the photonic device C itself, which device has, for example, an input port formed by a bundle of single-mode waveguides (for example of Gaussian, near-Gaussian or any other type) arranged relative to each other in a line or matrix in order to spatially decompose the first optical radiation I1. A microlens can be attached to each of the waveguides to facilitate the decomposition of the first optical radiation I1 and its coupling into the waveguides of the photonic device C. If the photonic device C uses a photonic chip, the photonic chip can comprise on one of its surfaces a plurality of grating couplers arranged in a grid onto which the first radiation I1 is projected, each coupler decomposing a portion of the first radiation to enter a waveguide embedded in the chip and forming the end of the waveguide.

[0055] Of course, the mode splitter S can be designed to combine these different embodiments of the mode decomposition device M, for example by combining an MPLC device with a fiber bundle.

[0056] In all cases, care is taken to ensure that the polarization of the optical radiation propagating within the mode splitter S is not unduly affected. In particular, by providing a polarization adjusting device, the aim is to ensure that the radiation incident on the mode converting device has a polarization state that matches the polarization state preserved by this device.

[0057] Those skilled in the art will know how to select the right polarization adjusting device depending on the nature of the radiation I1 coming from the waveguide and the nature of the polarization to be preserved by the mode converting device. For example, this can be a half-wave plate to adjust the orientation of linear polarization by 45°, or a quarter-wave plate to convert circular polarization (typically from spun fiber polarization-maintaining waveguides) to linear polarization.

[0058] However, this polarization adjusting device is not necessary, as is the case, for example, if the first radiation from waveguide F has at least one p polarization state of an MPLC device as shown in Figure 4. In this case, one must simply ensure that the eigenaxes of waveguide F are aligned according to these two s and p orientations of the MPLC device.

[0059] Generally speaking, regardless of the embodiment chosen for the modal decomposition device M, an optical device for shaping the first radiation I1 emerging from the waveguide F can be provided, this device being arranged upstream of the modal decomposition device M, between the second end of the waveguide F and the modal decomposition device M itself, in the mode splitter S. This optical device may comprise or consist solely of at least one passive optical element for transmission or reflection, for example one or more freeform optical elements.

[0060] Advantageously, the second end of the polarization-maintaining multimode waveguide F is directly coupled to the input port of the mode splitter S, i.e. there are no other elements between this second end and the mode splitter. The optical radiation from the waveguide F, after any processing for shaping or polarization adjustment (as described above), is directly incident on the mode decomposition device M.

[0061] As already mentioned, the photonic device C generates fundamental optical radiation R1 to R N to generate at least one recombined optical radiation Rc, which is single-mode and is fed to an optical receiver OR, for example by simple free-space propagation, or preferably via a single-mode fiber.

[0062] The photonic device C can take the form of a photonic integrated circuit PIC (or multiple photonic integrated circuits). In this case, the circuit generates the fundamental radiation R1 to R2 arriving at its input ports. Nand a phase actuator for adjusting the relative phase of this radiation to ensure that it recombines as accurately as possible to produce the recombined radiation R. An example of such a circuit is described, for example, in EP 3 672 109 A1.

[0063] The photonic device C may take a form other than a photonic integrated circuit PIC, or may be a fundamental radiation R1 to R N In particular, this recombination can be implemented by one or more multi-plane photoconversion devices configured to perform this recombination, as illustrated for example in patent application FR 2 111 490 A1.

[0064] [First embodiment] 2a shows a first embodiment that is particularly suited to situations in which the polarization of the incident optical radiation I is not multiplexed but is completely determined. For example, the transmission can be from a fixed transmitter according to a protocol that imposes a particular polarization state, such as linear or circular, on the incident radiation I. Alternatively, the transmission can be from a transmitter that moves within a reference frame linked to an optical communication system, and the communication protocol imposes a circular polarization of the incident radiation I.

[0065] In this first embodiment, the multimode waveguide F may be formed by a polarization-maintaining multimode optical fiber, which optical fiber is selected to propagate the incident radiation I without affecting its polarization, and is coupled to an optical port P of the telescope. As mentioned above, this multimode optical fiber may be selected to preserve the linear or circular polarization of the incident radiation I propagating through it. In other words, the multimode optical fiber F is selected so that the polarization state it preserves corresponds to the determined polarization state of the incident radiation I. In the case of a linear polarization of this incident radiation, this fiber is coupled to the optical port so that the polarization-maintaining axis of the multimode fiber is aligned with the polarization axis of the incident radiation I.

[0066] In this first embodiment, the optical port P of the telescope T may lack a device for adjusting the polarization of the incident optical radiation I. The latter constitutes the first optical radiation I that is directly injected into the waveguide F and propagates therein. However, if the polarization of the incident optical radiation I is not perfectly determined, an adjustment device can be arranged at the optical port P of the telescope T, as will be detailed in another section of this description.

[0067] The processing device DR of this first embodiment is fully compliant with that described in the general description above. For example, as shown in FIG. 2a, although this is not the only way to implement this first embodiment, the mode splitter S may comprise a single-mode decomposition device M, e.g., a multi-plane conversion device that generates multiple fundamental optical radiations, e.g., 10, 20, 50, or 100. All of these fundamental radiations have the same polarization as the incident radiation I and are recombined by a single photonic device C to generate at least one single-mode recombined optical radiation Rc. An adjustment device or multiple such devices can be arranged within the processing device DR to adjust the polarization of the radiation propagating therethrough and ensure that, before each processing operation performed on this radiation, its polarization matches the expected polarization, i.e., a polarization that is not excessively affected by the processing in question.

[0068] Second Embodiment This embodiment shown in Figure 2b is particularly suitable for situations where the incident optical radiation I is polarization multiplexed and the polarization is fully determined. For example, the two multiplexed polarizations can be linear or circular and orthogonal to each other.

[0069] As in the first embodiment, the multimode waveguide F may be formed from a polarization-maintaining multimode optical fiber, the optical fiber being selected to propagate the polarization-multiplexed incident radiation I without affecting its polarization, and is coupled to the telescope's optical port P. The first radiation I1 propagating in the waveguide has the same properties as the incident radiation I.

[0070] In a second embodiment, a polarizing beam splitter PBS is optically arranged between the waveguide F of the optical processing device DR and the mode splitter S. The polarizing beam splitter PBS can be integrated into the mode splitter S, for example into a polarization adjustment device arranged upstream of the splitter of the mode decomposition device M. The polarizing beam splitter PBS is thus coupled to the second end of the multimode waveguide F in alignment with this waveguide, such that two polarized radiations of the first optical radiation I1 are present at the splitter output.

[0071] The polarizing beam splitter PBS generates a first polarized radiation R1p and a second polarized radiation R1s, the first and second polarized radiation R1p, R1s having separate and approximately orthogonal polarizations. In the polarization adjustment device, preferably upstream of the splitter device PBS, an optical element (e.g. a retarder) can be provided to prepare the first radiation for this orthogonal polarization if it does not naturally occur in this form.

[0072] In this configuration, the two polarized radiations R1p, R1s propagate towards separate inputs of the input ports of the mode decomposition device M. The mode decomposition device M decomposes these two radiations R1p, R1s into fundamental optical radiations R1 to R N More precisely, the mode decomposition device M is configured to decompose the first polarized radiation R1p into a first plurality of elementary optical radiations R p 1~R p P and decomposing the second polarized radiation R into a second plurality of elementary optical radiations R s 1~R s P These two multiple elementary light radiations R p 1~R p P , R s 1~R s Qhave distinct polarizations identical to the polarizations of the first and second polarized radiations R1p, R1s, respectively. Thus, a retarder or any other polarization adjusting device can be provided before the photonic device C, or even before the MPLC device, to change the polarization state of certain propagating radiations in order to facilitate their processing by the photonic device or their injection into a single-mode fiber. The first plurality of fundamental optical radiations and the second plurality of fundamental optical radiations R p 1~R p P , R s 1~R s P are combined to form the multiple fundamental optical emissions R1 to R2 generated by the mode splitter S. N Configure.

[0073] The mode decomposition device M can be formed by two independent devices, for example two second-order MPLC devices M1, M2 that are independent of each other, i.e. can comprise separate optical components that process the first and second polarized radiation R1p, R1s separately. However, this is not an essential feature, and the two polarized radiation R1p, R1s can also be decomposed by a single mode decomposition device, for example a single MPLC device M.

[0074] It should be noted that this second embodiment is compatible with non-polarization multiplexed optical radiation, whose polarization may also be indeterminate. A portion of the energy present in the incident radiation may be collected in each of the polarization axes of the multimode waveguide F. This energy is then recombined in the processing device.

[0075] The ability to handle multiplexed and non-multiplexed radiation of uncertain polarization is a particular advantage of this design, which enables the optical communication system 1 to decode messages transmitted by the transmitter SAT for multiple communication protocols. Note that this advantage comes at the expense of a modal decomposition that includes twice the number of elementary optical radiation (all else being equal).

[0076] 2c shows a variant of the second embodiment described above. According to this variant, a mode decomposition device M of a mode splitter S is coupled to the second end of a multimode waveguide F, which generates a plurality of decomposed optical radiations (R1d1 to R1dP), as detailed in the previous section. This mode splitter M can therefore be implemented by an MPLC device. Upstream of the mode decomposition device M, the mode splitter S can include a polarization adjustment device.

[0077] The mode splitter S also comprises in this variant a polarizing beam splitter PBS arranged optically downstream of the mode decomposition device M. This splitter may be part of a polarization adjustment device that implements other functions already described for such devices. The polarizing beam splitter PBS therefore splits a plurality of decomposed optical radiations R1d1 to R1d P and receiving a first plurality of elementary light radiation R p 1~R p P and a second plurality of fundamental light emissions R s 1~R s Q and generating a first plurality and a second plurality of fundamental light radiation R p 1~R p P , R s 1~R s Q have distinct polarizations, which are combined to form the multiple fundamental optical emissions R1 to R2 generated by the mode splitter S. N Configure.

[0078] Therefore, regardless of whether the variant shown in FIG. 2b or the variant shown in FIG. 2c is implemented in the optical processing device DR of the second embodiment, the mode splitter S splits the first plurality of fundamental optical radiation R p 1~R p P and a second plurality of fundamental light emissions R s 1~R s QThese two fundamental optical radiations will naturally have distinct polarizations. As already mentioned, wave plates or any other form of polarization adjusting device can be provided to bring these fundamental optical radiations (or parts of them) into a selected polarization state, for example the polarization state expected by the photonic device C. The fundamental optical radiations are either propagated in free space or are, for example, single-mode and are advantageously guided from the mode splitter S to the photonic device C by a polarization-maintaining fiber.

[0079] Advantageously, as shown in Figures 3a and 3b, the photonic device C emits a first plurality of fundamental optical radiation R p 1~R p P and generating a first recombination optical radiation Rc1 from the second plurality of fundamental optical radiations R s 1~R s Q This arrangement can be implemented by a single photonic device C, e.g. a single photonic circuit PIC, or by a plurality of first and second fundamental optical radiations R p 1~R p P , R s 1~R s Q More precisely, a first photonic device (e.g., a first photonic circuit PIC1) is optically coupled to a mode splitter S to receive a first plurality of fundamental optical radiation R p 1~R p P A second photonic device (e.g., a second photonic circuit PIC2) is optically coupled to the mode splitter S to generate a second plurality of fundamental optical radiations R s 1~R s Pand generates a second recombined optical radiation Rc2. Wave plates or other polarization adjusting devices may be provided upstream and / or downstream of the photonic circuit PIC or the photonic circuits PIC1, PIC2 to adjust the polarization of the fundamental radiation to that expected by the circuit.

[0080] As we have seen, these optical couplings can be achieved in free space via optical fibers, such as polarization-maintaining single-mode fibers, or by mechanically splicing the photonic device to a mode splitter S.

[0081] In a first alternative embodiment shown in FIG. 3a, the photonic device C recombines the first and second recombined radiation Rc1, Rc2 to produce a single recombined optical radiation R having a single polarization. c In this configuration, the recombining device may consist of a Mach-Zehnder device or a polarizing beam splitter in an inverse assembly. In this second case, a single recombined optical radiation R c The polarization of R is unstable and can vary with time. This variation results in a single recombined optical radiation R, where all beam energies have a single polarization. c , and is therefore of particular interest when the polarization of the incident optical radiation I is not multiplexed.

[0082] In a second embodiment shown in Fig. 3b, which is of interest when the polarization of the incident optical radiation I is multiplexed, the photonic device C comprises a recombination device R configured to form a single recombined optical radiation with two superimposed polarizations. In this configuration, the recombination device can be formed by a polarizing beam splitter in a disassembled arrangement.

[0083] 3a and 3b, the recombination device is entirely optional. The photonic device C (and thus the optical communication system 1) can be expected to deliver the first and second recombined radiation Rc1, Rc2 separately, i.e., without recombining them with each other. In this case, the optical receiver OR can be equipped with two separate inputs for processing the radiation Rc1, Rc2 emitted by the optical communication system 1.

[0084] Third Embodiment The third embodiment is shown in Figure 2d and is a variant of the second embodiment, and can therefore be applied under the same conditions of use with the same benefits.

[0085] In this third embodiment, a polarizing beam splitter PBS is arranged upstream of a multimode waveguide F at an optical port P of the telescope T. The polarizing beam splitter PBS is arranged to couple a first polarized radiation R1p into a first polarization-maintaining multimode waveguide F1 and a second polarized radiation R1s into a second polarization-maintaining multimode waveguide F2. As in the second embodiment, a mode decomposition device M of the mode splitter S couples the first polarized radiation R1p provided by the first multimode waveguide F1 into a first plurality of fundamental optical radiation R p 1~R p P It is also configured to decompose the second polarized radiation R1s provided by the second multimode waveguide F2 into a second plurality of fundamental optical radiation R s 1~R s Q The splitter S may feature a polarization adjusting device or multiple such devices, as detailed in the previous section of this specification.

[0086] The modal decomposition device M can also be formed by two independent devices, for example two second-order MPLC devices M1, M2 independent of each other, or as in the second embodiment by a single modal decomposition device M. The photonic device C can be implemented according to any of the variants presented in connection with the description of Figures 3a and 3c.

[0087] In the above embodiment, the incident optical radiation I has a polarization that may need to be determined. However, this situation is not common. In particular, this is not the case when the communication protocol does not impose a specific polarization on the incident radiation I or when the transmitter SAT is movable in the reference frame linked to the optical communication system 1.

[0088] To address this situation, and as already mentioned in the general presentation of this document, the optical port P of the telescope T may be equipped with a device for adjusting the polarization of the incident optical radiation I. This adjusting device may in particular be integrated into the optical port P of the telescope T in order to process the incident optical radiation I before it enters the waveguide F in order to impose a predetermined polarization on it.

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

Claims

1. A free space optical communication system (1), comprising: a telescope (T) having an objective lens (O) for collecting the incident optical radiation (I) and producing a first optical radiation (I1) at an optical port (P); a light processing device (DR), a mode splitter (S) comprising a mode decomposition device (M) configured to decompose said first optical radiation (I1) into a plurality of fundamental optical radiations (R １ ~R Ｎ a mode splitter (S) that generates a a photonic device (C) optically coupled to said mode splitter (S), said photonic device (C) being adapted to transmit said fundamental optical radiation (R １ ~R Ｎ ) coherently recombining at least a portion of the recombined optical radiation (R ｃ a photonic device (C) configured to generate a light beam; - a free-space optical communication system (1) comprising 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).

2. 2. The optical communication system of claim 1, wherein the modal decomposition device (M) comprises at least one multi-plane transform device.

3. 3. An optical communication system (1) according to claim 1 or 2, wherein the modal decomposition device (M) comprises a bundle of single-mode optical fibres assembled in parallel with one another.

4. 4. The optical communication system (1) according to claim 1, wherein the mode converting device (M) preserves at least one polarization state of the optical radiation propagating therethrough, and the mode splitter (S) comprises a polarization adjusting device configured to adapt the first optical radiation (I1) to the preserved polarization state of the mode converting device (M).

5. A polarizing beam splitter (PBS) is arranged upstream of the photonic device (C), the optical splitter (S) splitting a first plurality of fundamental optical radiations (R ｐ １ ~R ｐ Ｐ ) and a second plurality of fundamental light emissions (R ｓ １ ~R ｓ Ｑ ), and generating said first and second plurality of fundamental light radiations (R ｐ １ ~R ｐ Ｐ , R ｓ １ ~R ｓ Ｑ ) is a function of the plurality of fundamental light radiations (R １ ~R Ｎ 5. The optical communication system (1) according to any one of claims 1 to 4, comprising:

6. 6. The optical communication system (1) of claim 5, wherein the polarizing beam splitter (PBS) is coupled to the second end of the polarization-maintaining multimode waveguide (F) and generates first polarized radiation (R1p) and second polarized radiation (R1s) having distinct polarizations.

7. 6. The optical communication system (1) of claim 5, wherein the polarizing beam splitter (PBS) is arranged at an optical port (P) of the telescope (T) to generate first polarized radiation (R1p) and second polarized radiation (R1s) having distinct polarizations, and is arranged at the optical port (P) to inject the first polarized radiation (R1p) into a first polarization-maintaining multimode waveguide (F1) and to inject the second polarized radiation (R1s) into a second polarization-maintaining multimode waveguide (F2).

8. The mode splitter (S) receives the first polarized radiation (R1p) and splits a first plurality of fundamental optical radiations (R ｐ １ ~R ｐ Ｐ a first modal decomposition device (M1) arranged to receive the second polarized radiation (R1s) and generate a second plurality of fundamental optical radiations (R ｓ １ ~R ｓ Ｑ a second modal decomposition device (M2) arranged to generate a first and second plurality of fundamental optical radiation (R ｐ １ ~R ｐ Ｐ , R ｓ １ ~R ｓ Ｑ ) is the plurality of fundamental optical radiations (R １ ~R Ｎ 8. An optical communication system (1) according to claim 6 or 7, comprising:

9. The mode decomposition device (M) is configured to generate a plurality of decomposed optical radiations (Rld １ ~Rld Ｐ ) coupled to the second end of the polarization-maintaining multimode waveguide (F), and the polarizing beam splitter (PBS) １ ~Rld Ｐ ), and receiving the first plurality of elementary light radiations (R ｐ １ ~R ｐ Ｐ ) and the second plurality of fundamental light radiations (R ｓ １ ~R ｓ Ｑ 6. The optical communication system (1) according to claim 5, wherein the optical communication system (1) is optically arranged downstream of the modal decomposition device (M) to generate a modal decomposition signal.

10. The photonic device (C) emits the first plurality of fundamental optical radiations (R ｐ １ ~R ｐ Ｐ ) to the first recombination optical radiation (Rc １ ), and generating said second plurality of elementary light radiations (R ｓ １ ~R ｓ Ｐ ) to the second recombination optical radiation (Rc ２ 10. The optical communication system (1) according to any one of claims 5 to 9, configured to generate a

11. The photonic device (C) emits the first plurality of fundamental optical radiations (R ｐ １ ~R ｐ Ｐ ), and receiving the first recombined optical radiation (Rc １ a first photonic device (C1) optically coupled to said mode splitter (S) to generate a second plurality of fundamental optical radiations (R ｓ １ ~R ｓ Ｐ ), and the second recombined optical radiation (Rc ２ and a second photonic device (C2) optically coupled to the mode splitter (S) to generate a mode splitter (S).

12. The photonic device (C) is configured to generate the first and second recombination radiation (Rc １ , Rc ２ ) into a single recombined optical emission (R Ｃ 12. An optical communication system (1) according to claim 10 or 11, comprising a recombining device (R) configured to form a recombination signal.

13. 13. The optical communication system (1) according to any one of claims 1 to 12, wherein the optical port (P) comprises a device for statically or dynamically controlling the polarization of the incident radiation to align it with a predetermined polarization before it is launched into the polarization-maintaining multimode waveguide (F).

14. 14. The optical communication system (1) according to any one of claims 1 to 13, wherein the photonic device (C) is optically coupled to the mode splitter (S) via a plurality of single-mode optical fibers.

15. 15. The optical communication system (1) according to any one of the preceding claims, wherein the mode splitter (S) comprises a shaping device arranged upstream of the mode decomposition device (M).

16. 16. An optical communication system (1) according to any one of claims 1 to 15, wherein the second end of the polarization-maintaining multimode waveguide (F) is directly coupled to an input port of the mode splitter (S).

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