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 optical radiation (I) and generating 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 means (DR).

Description

Technical Field

[0001] The present invention relates to an optoelectronic system seeking to compensate for the distortion of the wavefront of incident optical radiation. Such distortion may result from atmospheric perturbations during optical communication in free space. More generally, such distortion is caused by the propagation of a light beam in its medium. The present invention has a specific application in the field of free-space optoelectronics. Background Art

[0002] In free-space optoelectronics, a transmitter uses the information to be transmitted to modulate optical radiation (usually generated by a laser), and the optical radiation takes the form of a narrow beam emitted in the direction of a receiver. After propagating in its medium (air will be used as an example for the rest of this specification, but the medium can have any nature, such as water in the case of underwater telecommunications), the optical radiation is collected at the receiver and demodulated to recover the transmitted information. Generally speaking, and in order to maximize the data rate, one seeks to use as much as possible the energy present in the optical radiation received by the receiver in order to maximize the transmission rate.

[0003] In order to multiply communication channels and maximize throughput, optical radiation can be multiplexed in wavelength and / or polarization.

[0004] The propagation of optical radiation subjects the radiation generated by the transmitter to atmospheric perturbations and, in particular, to variations in temperature and pressure that the radiation undergoes during its propagation. These unstable perturbations (whose dynamics typically range between 100 Hz and several kHz) cause the deformation of the optical radiation, which affects the wavefront of the optical radiation. More precisely, the perturbations tend to redistribute the energy in the radiation spatially, generating random fluctuations in amplitude and phase. This deformation appears in the form of a "speckle" pattern in a spot, which is formed by projecting the light beam onto a radiation collection device and through the scintillation phenomenon. 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 documents WO2022185020 and US20170070289) to provide adaptive optical devices to compensate for these phenomena. However, this type of solution is limited in performance because the solution only affects the phase of the incident light. Document EP3672109A1 proposes a receiver that can pattern-decompose the radiation received (at the collector) into elementary rays. These elementary rays are coherently recombined by a photon device. Document WO2016047100 proposes, after the mode decomposition of the received radiation, to perform an electrical conversion of the elementary radiation 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 radiations must have the same polarization to implement the interference mechanism that generates the desired recombination.

[0007] It should be noted that the light radiation generated by the emitter is polarized and this polarization is not affected by the free-space propagation of the light radiation. On the contrary, the propagation of this light radiation in the receiver may affect the polarization of the light radiation, especially when the optical processing of this radiation is to be biased with respect to the collector via an optical fiber.

[0008] Depending on the communication protocol selected, the polarization state of the light radiation generated by the emitter may be determined (e.g., linearly polarized, left-circularly polarized or right-circularly polarized) or undetermined. In the latter case, the characteristics of the light radiation may evolve freely over time. Even when the polarization state of the light radiation has been determined, any relative displacement between the emitter and the receiver may cause the characteristics of the light radiation to change at the receiver. This is especially the case when the emitter is located in a satellite and the satellite may rotate on its own.

[0009] Furthermore, and as previously mentioned, some communication protocols also support the generation of polarization-multiplexed light radiation.

[0010] Thus, it is clear that the light radiation received by the receiver has a polarization state that is not always perfectly controlled, but this polarization state must be taken into account in order to fully exploit the transmitted energy and / or to decode the transmitted symbols.

[0011] This is especially the case when the receiver processes the received light using the coherent combination of elementary light beams and when this processing is remote from the collector.

[0012] Object of the Invention

[0013] 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 that includes a collector of incident light radiation and a photonic device that implements the coherent combination of elementary light radiation, the photonic device being remote from the collector. Summary of the Invention

[0014] To achieve this object, the subject matter of the present invention proposes a free-space optical telecommunications system that includes:

[0015] - a telescope that has an objective lens for collecting incident light radiation and generating first light radiation at an optical port;

[0016] - an optical processing device that includes:

[0017] · a mode separator that includes modes configured to decompose the first light radiation

[0018] A decomposition device, the mode separator generating a plurality of basic optical radiations;

[0019] · A photon device optically coupled to the mode separator, the photon device being configured to coherently recombine at least some of the basic optical radiations and generate at least one recombined optical radiation;

[0020] - 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.

[0021] According to other advantageous non-limiting features of the present invention (individually or according to any technically feasible combination):

[0022] - The mode decomposition device includes at least one multi-plane conversion device;

[0023] - The mode decomposition device includes a bundle of parallelly assembled single-mode optical fibers;

[0024] - The mode conversion device preserves at least one polarization state of the optical radiation propagating therein, and the mode separator includes a polarization adjustment device configured to conform the first optical radiation to the preserved polarization state of the mode conversion device;

[0025] - The optical communication system includes a polarization beam splitter disposed upstream of the photon device, the beam splitter generating a first plurality of basic optical radiations and a second plurality of basic optical radiations having different polarizations, the first plurality of basic optical radiations and the second plurality of basic optical radiations constituting the plurality of basic optical radiations;

[0026] - The polarization beam splitter is coupled to the second end of the multimode waveguide, the polarization beam splitter generating a first polarized optical radiation and a second polarized optical radiation having different polarizations;

[0027] - The polarization beam splitter is disposed in the optical port of the telescope to generate a first polarized optical radiation and a second polarized optical radiation having different polarizations, the polarization beam splitter being disposed in the optical port to inject the first polarized optical radiation into a first polarization-maintaining multimode waveguide and inject the second polarized optical radiation into a second polarization-maintaining multimode waveguide;

[0028] - The mode separator includes: a first mode decomposition device arranged to receive the first polarized optical radiation and generate a first plurality of basic optical radiations; and a second mode decomposition device arranged to receive the second polarized optical radiation and generate a second plurality of basic optical radiations, the first plurality of basic optical radiations and the second plurality of basic optical radiations constituting the plurality of basic optical radiations generated by the mode separator;

[0029] - The mode decomposition device is coupled to the second end of the multimode waveguide to generate a plurality of decomposed optical radiations, and the polarization beam splitter is optically arranged downstream of the mode decomposition device to receive the plurality of decomposed optical radiations and generate the first plurality of fundamental optical radiations and the second plurality of fundamental optical radiations;

[0030] - The photon device is configured to generate a first recombined optical radiation from the first plurality of fundamental optical radiations and a second recombined optical radiation from the second plurality of fundamental optical radiations;

[0031] - The photon device includes: a first photon device optically coupled to the mode separator for receiving the first plurality of fundamental optical radiations and generating the first recombined optical radiation; and a second photon device optically coupled to the mode separator for receiving the second plurality of fundamental optical radiations and generating the second recombined optical radiation;

[0032] - The photon device includes a recombination device configured to recombine the first and second recombined radiations to form a single recombined optical radiation;

[0033] - The recombination device is configured to form a single recombined optical radiation having a single polarization;

[0034] - The recombination device is configured to form a single recombined optical radiation having a superimposed polarization;

[0035] - The optical port includes means for statically or dynamically controlling the polarization of the incident radiation to match the polarization with a determined polarization before the incident radiation is injected into the multimode waveguide;

[0036] - The photon device is optically coupled to the mode separator via a plurality of single-mode optical fibers;

[0037] - The single-mode optical fibers in the plurality of single-mode optical fibers are polarization-maintaining;

[0038] - The coupling between the photon device (C) and the mode separator (S) lacks an optical fiber;

[0039] - The optical communication system includes an optical receiver for demodulating the optically recombined optical radiation and coupled to the optical processing device (DR);

[0040] - The mode separator includes a shaping device arranged upstream of the mode decomposition device;

[0041] - The second end of the polarization-maintaining multimode waveguide is directly coupled to the input port of the mode separator. Description of the Drawings

[0042] Other features and advantages of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings, in which:

[0043] Figure 1

[0044] Figure 1 shows an optical telecommunications system according to the present invention;

[0045] Figure 2a

[0046] Figure 2b

[0047] Figure 2c

[0048] Figure 2d

[0049] Figure 2a 、 Figure 2b 、 Figure 2c 、 Figure 2d shows Figure 1 different embodiments of the optical telecommunications system shown;

[0050] Figure 3a

[0051] Figure 3b

[0052] Figure 3a and Figure 3b shows two examples of photonic devices used in the optical telecommunications system of Figure 1 ;

[0053] Figure 4

[0054] Figure 4 shows the MPLC device of the optical telecommunications system according to the present invention. Detailed Description

[0055] Elements common to all embodiments 。

[0056] Referring to Figure 1 , an optical telecommunications system 1 in accordance with the present invention is designed to process incident optical radiation I generated by a transmitter and carry information to be transmitted by modulation. The incident optical radiation I may have several wavelengths (as is usually the case with WDM-type transmission) and / or may use several polarizations. Once the incident optical radiation is processed by the optical telecommunications system 1, it is supplied to the optical receiver OR of the base station, which is capable of extracting information from the received radiation.

[0057] In Figure 1 ​​​​​​​​​​​​​​​​​​​In the example shown here, the transmitter is arranged in a satellite SAT, but the telecommunication system of the invention is by no means limited to this particular application. In general, the transmitter can be located on land, at sea or in space and propagate in any free space, in the atmosphere in the case of terrestrial communications, or in water in the case of maritime communications. The transmitter and the telecommunication system 1 can be stationary or mobile relative to each other.

[0058] The emitted incident optical radiation I takes the form of a narrow beam directed towards the telecommunication system 1. During the propagation of the incident optical radiation in free space, the emitted radiation is subject to atmospheric disturbances of the atmospheric environment, causing the incident optical radiation I arriving at the base station to undergo spatial and temporal amplitude and phase fluctuations. This phenomenon affects the shape of this radiation, which presents an unstable and irregular variable shape over time. The optical telecommunication 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. To this end, the receiver OR may include an amplification function and / or a spectral demultiplexing function, in particular for WDM transmissions.

[0059] The optical telecommunication system 1 comprises a telescope T having an objective lens O for collecting incident optical radiation I and producing a first optical radiation I1 at an optical port P. As is well known per se, the objective lens may comprise a concave mirror that focuses the received radiation at an image focus. The convergent radiation may be reflected back to the optical port P using a second mirror of the objective lens O, which may be flat or convex. When present, the second mirror causes the formation of a central region with ultra-low intensity in the optical radiation propagating 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 turned to point to and track a transmitter located in the satellite SAT. The telescope T may also comprise means for directing the incident radiation TTM, such as a top tilt mirror, so as to optimally direct the optical radiation towards the optical port P, for example to concentrate the radiation in the port P, and more generally to correct any pointing deviation of the telescope T.

[0060] Return to [ Figure 1In accordance with the description in [reference], the optical telecommunications system 1 further includes an optical processing device DR located downstream optically of the telescope T, which is designed to at least partially compensate for the distortion of the incident light radiation I collected. This device DR, which will be described in detail in subsequent sections of the present disclosure, is composed of a plurality of precisely assembled optical or photonic elements (which may be particularly sensitive to their operating environment). Therefore, it is advantageous to offset this system of the telescope T by several meters to several tens of meters, for example, to place the system in a cabinet, a room, or any other shelter in a vehicle or a base station operation center. The positioning of the telescope T is generally determined by the reception quality of the incident radiation I. In this way, the optical processing device DR can be protected, its operating environment (temperature, atmosphere, exposure to dust, vibration, movement, etc.) can be controlled, and its operation and maintenance can be made easier.

[0061] To achieve this distance between the telescope T and the rest of the optical telecommunications 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. The first end of the waveguide F is coupled to the optical port P of the telescope T, and the second end of the waveguide F is coupled to the optical processing device DR. The waveguide propagates the first light radiation I1 generated by the optical port P to the optical processing device DR.

[0062] Advantageously, the waveguide has a length of at least one meter and typically between 1 m and 10 m, such that the optical processing device DR can be sufficiently offset from the telescope T to, for example, accommodate the waveguide.

[0063] The waveguide F can be passive or active. In this case, the waveguide incorporates additional amplification functions.

[0064] "Polarization-maintaining" means that the waveguide has a polarization extinction ratio (PER) greater than 7 dB, advantageously greater than 10 dB, and even more advantageously greater than 20 dB with respect to its length.

[0065] For example, this waveguide F can be formed by at least one multimode optical fiber (active or passive), which includes at least one elliptical core and has a parabolic refractive index gradient or a step refractive index gradient. The size of the core is selected such that multiple modes can be propagated, for example, at least 10 modes or at least 50 modes. The multimode optical fiber can be a spun fiber. Such fibers are formed by rotating a polarization-maintaining preform during drawing. The fiber can be used to propagate circularly polarized light while retaining this polarization. The multimode optical fiber can also include stress bars (which are integrated into the preform before drawing) corresponding to Panda, Bowtie, or Elliptical stress layer fibers.

[0066] The waveguide F can be formed by a bundle of polarization-maintaining multimode optical fibers.

[0067] Maintaining polarization can involve two specific linear polarization states of a waveguide, for example along two orthogonal axes arranged in a plane transverse to the propagation direction. Maintaining polarization can alternatively involve two circular polarization states, as already shown in connection with coiled optical fibers.

[0068] In some cases, it may be advantageous to provide a device for adjusting the polarization of the optical radiation injected into the waveguide, the device being arranged in the optical port P and coupled to the first end of the waveguide. The device is designed to match the polarization of the light collected by the objective lens of the 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 thus it is not possible to directly inject this radiation collected by the telescope T into the waveguide F without the risk of affecting the polarization of the radiation propagating through the waveguide.

[0069] The "polarization adjustment device" is understood to mean any member (passive device or static or dynamic control device) for transforming the nature (linear, circular) and / or orientation of the polarization of the incident optical radiation. In particular, the polarization adjustment device can be a simple half-wave or quarter-wave phase plate (or retarder).

[0070] By way of example, applicable to each of the embodiments to be set forth in this specification is that a polarimeter can be used to measure in whole or in part the polarization state of the incident optical radiation, and accordingly an electrically actuated or static birefringent retarder or a liquid crystal or Pockels birefringent cell can be used to correct the polarization state. For an example of such a device for controlling the polarization of optical radiation, reference is made to the article "Polarization stabilizer using liquid crystal rotatable waveplates" by T. Chiba, Y. Ohtera, and S. Kawakami in Journal of Lightwave Technology, Vol. 17, No. 5, pp. 885 - 890, May 1999.

[0071] In general, for the basic principles of light polarization, reference is made to the chapter "Polarization Measurement" by Soe-Mie F. Nee in "Measurement, Instrumentation, and Sensors Handbook" edited by John G. Webster and Halit Eren and published by CRC Press (ISBN 9781315217444).

[0072] In all cases and regardless of whether a polarization adjustment device is provided, when the first light radiation I1 reaches the optical processing device DR, the first light radiation exhibits a controlled polarization after its propagation in the waveguide F. "Controlled" is understood to mean that the propagation in the waveguide is not affected by crosstalk and that the energy present in the different polarization states of the first light radiation I1 is preserved during this propagation.

[0073] Generally speaking, the optical processing device DR includes a mode separator S optically coupled to the waveguide F. The mode separator S has an input port and an output port. The mode separator includes at least one mode decomposition device configured to decompose the first light radiation I1 supplied by the waveguide F at the first input port and generate a plurality of elementary light radiations R1 to R at the output port of the mode separator. N Advantageously, the elementary radiations R1 to R N are single-mode and all have the same polarization state.

[0074] The polarization adjustment device can be placed in the mode separator S between the waveguide F and the mode decomposition device so as to match the polarization of the radiation supplied by the waveguide F with the polarization retained by the mode decomposition device (if this is not already the case). Additional polarization adjustment devices can also be provided optically downstream of the mode decomposition device in the mode separator S so as to match the polarization of the elementary light radiations R1 to R N with the polarization desired by the devices arranged downstream of the separator S.

[0075] The optical processing device DR further includes at least one photon device C located optically downstream of the mode separator S and coupled to the mode separator. The photon device C is configured to coherently recombine at least some of the elementary light radiations R1 to R N among the elementary light radiations and generate at least one single-mode recombined light radiation R c .

[0076] By decomposing the mode of the first radiation I1 and coherently recombining the generated elementary radiations R1 to R N , the optical processing device DR makes it possible to utilize the maximum energy of the incident light radiation I collected and at least partially compensate for the distortions experienced by the radiation during its propagation in free space. Since the elementary radiations RI to RN have the same polarization state, the elementary radiations can be effectively and coherently recombined with each other, that is, to form a single-mode recombined light radiation R c with the maximum energy.

[0077] The mode separator S and the photon device C can be coupled together in different ways using optical fibers or without using any optical fibers at all, while preserving the polarization state of the elementary radiations R1 to R N will affect both in the same way.

[0078] Thus, the two devices can be separated from each other, and the fundamental radiations R1 to R N propagate in the free space between the two devices.

[0079] Alternatively, an optical fiber bundle, such as a single-mode optical fiber bundle, can be used to guide the propagation of the fundamental radiations R1 to R N . Advantageously, these single-mode optical fibers are polarization-maintaining.

[0080] Alternatively, the two devices can be configured for mechanical assembly, in which the output port of the mode separator device S faces the input port of the photonic device C, such that the fundamental radiation can propagate from one device to the other.

[0081] As already mentioned, the polarization adjustment device can be arranged in the mode separator S or between the mode separator S and the photonic device C so that when a single-mode optical fiber is used to guide the propagation of the fundamental radiations R1 to R N respectively to the photonic device C, the polarization of some or all of the fundamental radiations R1 to R N is adapted to the polarization desired by the photonic device C or to the polarization desired for such an optical fiber.

[0082] The mode decomposition device M of the mode separator S can be implemented by a multi-plane light converter device, which is referred to as an "MPLC device" in the remainder of this specification. For completeness, it is recalled that in such an MPLC device, the incident light radiation undergoes a series of reflections and / or transmissions, followed by free-space propagation of the radiation after each reflection and / or transmission. At least some of the optical components (on which the reflections and / or transmissions occur and which guide the propagation of the incident radiation) have microstructured regions that modify the incident light radiation.

[0083] The term "microstructured region" means that the surface of the optical component has ridges in this region, which can be decomposed, for example, in the form of "pixels", the size of which can be comprised between a few micrometers and a few hundred micrometers. The microstructured region can be a metasurface. The ridges or each pixel of the ridges have a variable height of at most a few micrometers or at most a few hundred micrometers relative to the average plane defining the surface in question. Regardless of the nature of the microstructuring of the region, the optical component having such a region forms a phase mask that introduces a local phase shift in the transverse cross-section of the light beam that is reflected or transmitted at this phase mask.

[0084] Thus, the optical radiation propagating within the MPLC device undergoes a series of local phase shifts separated by propagation. These elementary conversions of the series (e.g., at least four successive conversions such as 8, 10, 12, 14 or even at least 20 conversions) establish a global conversion of the spatial distribution of the incident radiation. Thus, a microstructured reflective or transmissive surface can be configured to convert a first optical radiation, which particularly has a specific shape, into a second radiation with a different shape.

[0085] The following documents: "Programmable unitary spatial mode manipulation", Morizur et al., J. Opt. Soc. Am. A / Vol. 27, No. 11 / November 2010; N. fontaine et al., (ECOC, 2017), "Design of High Order Mode-Multiplexers using Multiplane Light Conversion"; US9250454; and US2017010463, contain the theoretical basis and examples of the practical implementation of MPLC devices.

[0086] As shown in detail in the aforementioned documents, the microstructured regions carried by the optical components forming the MPLC device are designed and configured to operate on a mode conversion aimed at decomposing the optical radiation received at the input port under a mode family called the "input" mode. At the output port of the MPLC device, the energy in the modes present in the input family is transmitted and shaped accordingly into modes in the "output" mode family. The MPLC device is configured to match the input fundamental mode and the output fundamental mode accordingly. It is a passive device with a particularly stable and robust transfer function that has little or no effect on certain polarization states of the optical radiation passing through it.

[0087] The MPLC device generally consists of optical components having parallel main surfaces on which multiple reflections and / or transmissions occur. In such a configuration, the polarization states retained during the propagation of the radiation in the device involve:

[0088] - The s polarization state, i.e., the state perpendicular to the radiation propagation axis and parallel to the plane defined by the main reflection surface and / or the main transmission surface;

[0089] - The p polarization state, i.e., the state also perpendicular to the propagation axis and perpendicular to the plane defined by the main reflection surface and / or the main transmission surface.

[0090] Therefore, the aim is to ensure that the radiation injected into such an MPLC device exhibits s polarization and / or p polarization.

[0091] For purposes of illustration, Figure 4 shows such an MPLC device M which includes two optical parts Ma, Mb arranged opposite to each other, these optical parts having surfaces parallel to each other, on which the first radiation I1 is reflected (at the microstructured region z) multiple times to decompose the first radiation into elementary radiations R1 to R N . In Figure 4 's illustration, only one of these optical parts Ma carries these microstructured regions z, but it is entirely possible for the microstructured regions to be carried by both optical parts Ma, Mb or by any other combination of optical parts. Figure 4 also shows two polarization adjustment devices Pol1 and Pol2. The first of these devices, Pol1, matches the polarization of the first radiation I1 so that the polarization of the first radiation matches the polarization maintained by the MPLC device M. This first polarization adjustment device can be, for example, a retardation half-wave or quarter-wave plate, as previously mentioned. Similarly, the second polarization adjustment device Pol2 is located at the output of the MPLC device M to intercept the elementary radiations R1 to R N and adjust the polarization of this elementary radiation to the polarization desired by an optical device located further downstream.

[0092] In the context of this specification and by way of example, the input mode family may include a Hermite-Gaussian basis consisting of N Hermite-Gaussian modes, which is arranged spatially relative to the first optical radiation I1. The output mode family may be formed by N spatially separated Gaussian modes, which define the elementary radiations R1 to R N . The MPLC device is configured to associate the Hermite-Gaussian modes of the input basis with the Gaussian modes of the output basis. The energy of the first radiation I1 received at the input port is decomposed according to the input elementary modes and transmitted in the MPLC device to be distributed to and matched with the output Gaussian mode (which is associated with the input elementary mode).

[0093] Of course, the Hermite-Gaussian and Gaussian modes used as examples are only for illustrative purposes, and other modes can be selected to perform the decomposition.

[0094] The mode decomposition device M of the mode separator S can be implemented by other components in addition to the MPLC device described above as an example and in detail. 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 (if necessary) collimated with micro-lenses. This bundle of optical fibers is arranged opposite to the waveguide F to receive the first optical radiation I1, and the first optical radiation is spatially decomposed via the N optical fibers in the bundle, and thus the elementary optical radiations R1 to R N are generated.

[0095] The bundle of optical fibers can be arranged in a matrix, or more generally, the ends of the fibers in the bundle can be arranged in a plane, for example, arranged in the shape of a disk or inscribed in a disk, so as to optimally decompose the first light radiation I1. The fiber bundle can be arranged such that the ends of the fibers are arranged in a ring, and the central part of the ring is not provided with fibers and corresponds to the ultra-low-intensity central region that may exist in the first light radiation when the first light radiation I1 comes from a telescope T having a second mirror, as presented previously. Alternatively, the ends of the fibers of the bundle can be arranged in a line. In this case, an optical device can be arranged between the second end of the waveguide F and the fiber bundle to shape the first light radiation from the waveguide and make the first light radiation conform to the line defined by the ends of the fibers in the bundle.

[0096] Alternatively, the mode decomposition device M (and the mode separator S) can be integrated into the photon device C itself, which has an input port formed, for example, by a bundle of single-mode waveguides (e.g., belonging to the Gaussian type, close to the Gaussian type, or belonging to any other type), and these single-mode waveguides are arranged closely to each other in a line or in a matrix to perform spatial decomposition of the first light radiation I1. Each waveguide in the waveguide can be equipped with a microlens to facilitate the decomposition of the first light radiation I1 and the coupling of the first light radiation to the waveguides of the photon device C. When the photon device C uses a photon chip, the latter can be equipped with a plurality of grating couplers on one of its surfaces, and the plurality of grating couplers are arranged in a grid onto which the first radiation I1 is projected. Each coupler decomposes a part of the first radiation to inject the part into the waveguide, which is buried in the chip, and this part forms the end of the waveguide.

[0097] Naturally, the mode separator S can be designed to combine these different implementation schemes of the mode decomposition device M, for example, by combining the MPLC device with the fiber bundle.

[0098] In all cases, attention is paid to ensuring that the polarization of the light radiation propagating in the mode separator S is not overly affected. In particular, by providing a polarization adjustment device, the aim is to ensure that the radiation injected into the mode conversion device has a polarization state that conforms to the polarization state retained by the device.

[0099] Those skilled in the art will know how to select the correct polarization adjustment device according to the nature of the radiation I1 from the waveguide and the nature of the polarization retained by the mode conversion device. For example, this polarization adjustment device can be a half-wave plate that adjusts the orientation of linear polarization by 45°, or a quarter-wave plate that converts circular polarization (usually from a rotating fiber polarization-maintaining waveguide) into linear polarization.

[0100] However, this polarization adjustment device is not mandatory. For example, when the first radiation from the waveguide F has an MPLC device (as Figure 4This is the case when the s - polarization state and / or the p - polarization state of the MPLC device as shown is considered. In this case, it is only necessary to ensure that the eigenaxis of the waveguide F is arranged according to these two s - orientations and p - orientations of the MPLC device.

[0101] Generally speaking, and regardless of the implementation chosen for the mode - decomposition device M, an optical device for shaping the first radiation I1 from the waveguide F can be provided, which is arranged in the mode separator S upstream of the mode - decomposition device M, between the second end of the waveguide F and the mode - decomposition device M itself. The optical device can comprise or consist solely of: at least one passive optical element for transmission or reflection, such as one or more free - form optical devices.

[0102] Advantageously, the second end of the polarization - maintaining multimode waveguide F is directly coupled to the input port of the mode separator S, i.e., there are no other elements between this second end and the mode separator. The optical radiation from the waveguide F is directly injected into the mode - decomposition device M after any processing for shaping or polarization adjustment (as described above).

[0103] As already stated, the photon device C is configured to coherently recombine at least some of the elementary optical radiations R1 to R N to produce at least one recombined optical radiation Rc. The recombined optical radiation Rc is single - mode and is supplied to the optical receiver OR, for example, by simple free - space propagation (or preferably via a single - mode optical fiber).

[0104] The photon device C can take the form of a photon integrated circuit PIC (or multiple photon integrated circuits). The circuit then comprises: waveguides for guiding the elementary radiations R1 to R arriving at the input port of the circuit N ; and phase actuators for adjusting the relative phase of the radiation and ensuring that the radiation is recombined as accurately as possible to produce the recombined radiation Rc. Examples of such circuits are described, for example, in the document EP3672109A1.

[0105] The photon device C can take other forms than the photon integrated circuit PIC or include other components for achieving the coherent recombination of the elementary radiations R1 to R N . In particular, it can be configured for this recombination by one or more multi - plane light - conversion devices configured to perform such recombination, as illustrated, for example, in the application FR2111490.

[0106] First embodiment

[0107] Figure 2a ​Shows a first embodiment particularly suitable for the case where the polarization of the incident light radiation I is not multiplexed and is perfectly determined. For example, it can be transmitted from a fixed transmitter according to a protocol that imposes a specific polarization state (such as linear or circular) on the incident radiation I. Alternatively, it can be transmitted from a transmitter that is movable in a reference frame associated with the optical communication system, and the communication protocol imposes circular polarization on the incident radiation I.

[0108] In this first embodiment, the multimode waveguide F can be formed by a polarization-maintaining multimode optical fiber, which is selected and coupled to the optical port P of the telescope to propagate the incident radiation I without affecting its polarization. As already seen, the multimode optical fiber can be selected to maintain the linear or circular polarization of the incident radiation I propagating through it. In other words, the multimode optical fiber F is selected such that the maintained polarization state corresponds to the determined polarization state of the incident radiation I. In the case of linear polarization of this incident radiation, the optical fiber is coupled to the optical port in such a way that the polarization-maintaining axis of the multimode optical fiber is aligned with the polarization axis of the incident radiation I.

[0109] In this first embodiment, the optical port P of the telescope T may lack means for adjusting the polarization of the incident light radiation I. The incident light radiation is directly injected into the waveguide F and constitutes the first light radiation I propagating at the waveguide. However, if the polarization of the incident light radiation I is not perfectly determined, adjusting means can be placed in the optical port P of the telescope T, as will be described in detail in another section of this specification.

[0110] The processing device DR of this first embodiment is fully consistent with the general description above. For example, as Figure 2a shown, although this is not the only way to implement this first embodiment, the mode separator S can include a single-mode decomposition device M, such as a multi-plane conversion device that generates a plurality of fundamental light radiations (such as 10, 20, 50, or 100 radiations). All of these fundamental radiations have the same polarization as the incident radiation I and are recombined by a single-photon device C to generate at least one single-mode recombined light radiation R c . Adjusting means or a plurality of such means can be placed in the processing device DR to adjust the polarization of the radiation propagating through the device and ensure that before each processing operation performed on the radiation, the polarization of the radiation conforms to the expected polarization, that is, the polarization that will not be overly affected by the processing in question.

[0111] Second embodiment

[0112] Figure 2b The embodiment shown is particularly suitable for the case of polarization multiplexing (polarization is perfectly determined) of the incident light radiation I. For example, the two multiplexed polarizations can be linear or circular and orthogonal to each other. ​

[0113] As in the first embodiment, the multimode waveguide F can be formed of a polarization-maintaining multimode optical fiber, which is selected and coupled to the optical port P of the telescope to propagate the polarization-multiplexed incident radiation I without affecting its polarization. The first radiation I1 propagating in the waveguide has the same characteristics as the incident radiation I.

[0114] In a second embodiment, a polarization beam splitter PBS is optically arranged between the waveguide F and the mode separator S of the optical processing device DR. The polarization beam splitter PBS can be integrated into the mode separator S, for example, integrated into a polarization adjustment device, which is located upstream of the mode decomposition device M in the separator. Thus, the polarization beam splitter PBS is coupled to the second end of the multimode waveguide F and is aligned with the waveguide such that two polarized radiations of the first optical radiation I1 appear at the output of the beam splitter.

[0115] The polarization beam splitter PBS generates a first polarized optical radiation R1p and a second polarized optical radiation R1s, and the first and second polarized optical radiations R1p, R1s have different substantially orthogonal polarizations. In the polarization adjustment device, preferably upstream of the beam splitter device PBS, optical elements (such as retardation plates) can be provided to prepare the first radiation for this orthogonal polarization if the radiation does not naturally occur in this form.

[0116] In this configuration, the two polarized optical radiations R1p, R1s propagate towards separate inputs of the input port of the mode decomposition device M. The mode decomposition device is configured to decompose these two radiations R1p, R1s and supply the fundamental optical radiations R1 to R N . More precisely, the mode decomposition device M is configured to decompose the first polarized optical radiation R1p into a first plurality of fundamental optical radiations R p 1 to R p P , and decompose the second polarized optical radiation R1s into a second plurality of fundamental optical radiations R s 1 to R s P . These two sets of a plurality of fundamental optical radiations R p 1 to R p P , R s 1 to R s Q have different polarizations, which are the same as the polarizations of the first and second polarized optical radiations R1p, R1s respectively. Therefore, before the photon device C, or even before the MPLC device, a retardation plate or any other polarization adjustment device can be provided so as to be able to change the polarization state of certain propagating radiations in order to facilitate the processing of these propagating radiations by the photon device or to inject these propagating radiations into a single-mode optical fiber. The first plurality of fundamental optical radiations R p 1 to R pP and the second plurality of elementary optical radiations R s from 1 to R s P which, combined, form the plurality of elementary optical radiations R1 to R produced by the mode separator S N .

[0117] The mode decomposition device M can be formed by two separate devices (for example, two secondary MPLC devices M1, M2), which are independent of each other, i.e., equipped with separate optical components for processing the first and second polarized optical radiations R1p, R1s respectively. However, this is not an essential feature, and the two polarized optical radiations R1p, R1s can also be decomposed by a single mode decomposition device (for example, a single MPLC device M).

[0118] It should be noted that this second embodiment remains compatible with non-polarization-multiplexed optical radiation, and this polarization can also be undetermined. A part of the energy present in the incident radiation can be collected on each of the polarization axes of the polarization axis of the multimode waveguide F. This energy is recombined in the processing device.

[0119] The ability to process multiplexed and non-multiplexed radiation with undetermined polarization is a particular advantage of this design, which enables the optical communication system 1 to decode the messages transmitted by the transmitter SAT for multiple communication protocols. It should be noted that this advantage is obtained at the cost that the mode decomposition involves more than twice as many elementary optical radiations (all other conditions being equal).

[0120] Figure 2c shows a variant of the second embodiment just described. According to this variant, the mode decomposition device M of the mode separator S is coupled to the second end of the multimode waveguide F. This mode decomposition device produces a plurality of decomposed optical radiations (R1d1 to R1d P ), as detailed in the previous paragraph. Thus, this mode separator M can be implemented by an MPLC device. Upstream of the mode decomposition device M, the mode separator S can include a polarization adjustment device.

[0121] The mode separator S also includes a polarization beam splitter PBS, which, in this variant, is arranged optically downstream of the mode decomposition device M. This beam splitter can be part of the implementation of the polarization adjustment device for performing other functions already described for such devices. Thus, the polarization beam splitter PBS receives the plurality of decomposed optical radiations R1d1 to R1d P and produces a first plurality of elementary optical radiations R p from 1 to R p P and a second plurality of elementary optical radiations R s from 1 to R S Q, the first plurality of elementary optical radiations R p from 1 to R p ​P and a second plurality of elementary optical radiations R S from R1 to R S Q have different polarizations. The first plurality of elementary optical radiations and the second plurality of elementary optical radiations together constitute the plurality of elementary optical radiations R1 to R generated by the mode separator S N .

[0122] Whether the variant shown in Figure 2b or the variant shown in Figure 2c is implemented in the optical processing device DR of the second embodiment, the mode separator S thus generates a first plurality of elementary optical radiations R p from R1 to R p P and a second plurality of elementary optical radiations R s from R1 to R s Q . These two sets of pluralities of elementary optical radiations naturally have different polarizations. As already mentioned, a wave plate or any other form of polarization adjustment device can be provided to place these elementary optical radiations (or a part thereof) in a selected polarization state, for example, the polarization state desired by the photon device C. The elementary optical radiations propagate from the mode separator S to the photon device C in free space or guided by an optical fiber, which is, for example, single-mode (and advantageously polarization-maintaining).

[0123] Advantageously, and as Figure 3a and Figure 3b shown, the photon device C is configured to generate a first recombined optical radiation Rc1 from the first plurality of elementary optical radiations R p from R1 to R p P and to generate a second recombined optical radiation Rc2 from the second plurality of elementary optical radiations R S from R1 to R S Q . This configuration can be achieved by a single photon device C, such as a single photonic integrated circuit PIC, or via two independent photon devices (such as two photonic integrated circuits PIC1, PIC2) that respectively receive the first and second pluralities of elementary optical radiations R p from R1 to R p P , R s from R1 to R S Q . More precisely, the first photon device (such as the first photonic integrated circuit PIC1) is optically coupled to the mode separator S to receive the first plurality of elementary optical radiations R p from R1 to R p Pand generate a first recombined optical radiation Rc1. A second photon device (e.g., a second photon circuit PIC2) is optically coupled to the mode separator S to receive a second plurality of elementary optical radiations R s 1 to R s P and generate a second recombined optical radiation Rc2. A wave plate or other polarization adjustment device may be provided upstream and / or downstream of the photon circuit PIC or the photon circuits PIC1, PIC2 to adjust the polarization of the elementary radiation to the polarization desired by the circuit.

[0124] As we have seen, these optical couplings can be achieved in free space via optical fibers such as polarization-maintaining single-mode fibers, or even by mechanically connecting the photon devices to the mode separator S.

[0125] In Figure 3a shown in the first alternative embodiment, the photon device C includes a recombination device R that is configured to recombine the first and second recombined radiations Rc1, Rc2 to form a single recombined optical radiation R with a single polarization c . In this configuration, the recombination device can consist of a Mach-Zehnder device or a polarization beam splitter assembled in reverse. In the second case, the polarization of the single recombined optical radiation R c may be unstable and fluctuate over time. This variant is of particular interest when the polarization of the incident optical radiation I is not multiplexed, since all the beam energy is placed in a single recombined optical radiation R c with a single polarization.

[0126] In Figure 3b shown in the second embodiment (which is of interest when the polarization of the incident optical radiation I is multiplexed), the photon device C includes a recombination device R that is configured to form a single recombined optical radiation with two superimposed polarizations. In this configuration, the recombination device can consist of a polarization beam splitter assembled in reverse.

[0127] In Figure 3a and Figure 3b In each of the alternative embodiments shown in the alternative embodiments, the recombination device is completely optional. It may be desirable for the photon device C (and thus the resulting optical communication system 1) to deliver the first and second recombined radiations Rc1, Rc2 separately, i.e., without recombining them with each other. In this case, the optical receiver OR can be equipped with two separate input terminals for processing the radiations Rc1, Rc2 emitted by the optical communication system 1.

[0128] Third embodiment

[0129] The third embodiment is shown in Figure 2dand is a variant of the second embodiment. Thus, the third embodiment can be applied under the same usage conditions and has the same beneficial effects.

[0130] In this third embodiment, the polarization beam splitter PBS is arranged in the optical port P of the telescope T upstream of the multimode waveguide F. The polarization beam splitter PBS is arranged to inject the first polarized light radiation R1p into the first polarization-maintaining multimode waveguide F1 and the second polarized light radiation R1s into the second polarization-maintaining multimode waveguide F2. As in the second embodiment, the mode decomposition means M of the mode separator S is configured to decompose the first polarized light radiation R1p provided by the first multimode waveguide F1 into a first plurality of elementary light radiations R p 1 to R p P . The mode separator is also configured to decompose the second polarized light radiation R1s provided by the second multimode waveguide F2 into a second plurality of elementary light radiations R s 1 to R s Q . The separator S may feature polarization adjustment means or a plurality of such means, as detailed in earlier sections of this specification.

[0131] The mode decomposition means M may also be formed by two independent means (e.g., two secondary MPLC means M1, M2 independent of each other) or by a single mode decomposition means M, as in the second embodiment. The photon device C may be implemented according to any of the variants shown in relation to Figure 3a and the description of FIG. 3c.

[0132] In the above embodiments, the incident light radiation I has a polarization that may need to be determined. However, this is not common. In particular, this is not the case when the communication protocol does not impose any specific polarization on the incident radiation I, or when the transmitter SAT is movable in the reference frame associated with the optical communication system 1.

[0133] To address this situation, and as already mentioned in the general introduction of this specification, the optical port P of the telescope T may be equipped with means for adjusting the polarization of the incident light radiation I. This adjustment means may be particularly integrated into the optical port P of the telescope T in order to process the incident light radiation I before injecting it into the waveguide F, in order to impose a determined polarization.

[0134] Of course, the present invention is not limited to the described embodiments, and alternative embodiments can be added without departing from the scope of the present invention as defined by the claims.

Claims

1. A free-space optical telecommunications system (1), the free-space optical telecommunications system comprising: - A telescope (T), the telescope having an objective lens (O) for collecting incident light radiation (I) and generating first light radiation (I1) at an optical port (P); - An optical processing device (DR), the optical processing device comprising: · A mode separator (S), the mode separator including being configured to process the first light radiation (I1) A mode decomposition device (M) for performing decomposition, and the mode separator (S) generates a plurality of basic light radiations (R1 to R N ); · A photon device (C), which is optically coupled to the mode separator (S), and the photon device (C) is configured to coherently recombine at least some of the fundamental light radiations (R1 to R N ) in the fundamental light radiation and generate at least one recombined light radiation (R c ); - At least one polarization-maintaining multimode waveguide (F), the polarization-maintaining multimode waveguide 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. The optical communication system (1) according to claim 1, wherein, The mode decomposition device (M) includes at least one multi-plane conversion device.

3. The optical communication system (1) according to claim 1 or 2, wherein, The mode decomposition device (M) includes a bundle of single-mode optical fibers assembled parallel to each other.

4. The optoelectronic system (1) according to one of claims 1 to 3, wherein, The mode conversion device (M) retains at least one polarization state of the light radiation propagating therein, and the mode separator (S) includes a polarization adjustment device configured to match the first light radiation (I1) with the retained polarization state of the mode conversion device (M).

5. The optical telecommunications system (1) according to one of claims 1 to 4, the optical telecommunications system comprising a polarization beam splitter (PBS) arranged upstream of the photon device (C), the splitter (S) generating a first plurality of elementary optical radiations (R p 1 to R p P ) and a second plurality of elementary optical radiations (R s 1 to R s Q ), the first and second pluralities of elementary optical radiations (R p 1 to R p P , R s 1 to R s Q ) constituting the plurality of elementary optical radiations (R1 to R N ).

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

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

8. The optical communication system (1) according to claim 6 or 7, wherein, The mode separator (S) comprises: a first mode decomposition device (M1) arranged to receive the first polarized light radiation (R1p) and to generate a first plurality of elementary light radiations (R p 1 to R p P ); and a second mode decomposition device (M2) arranged to receive the second polarized light radiation (R1s) and to generate a second plurality of elementary light radiations (R s 1 to R s Q ), the first and second pluralities of elementary light radiations (R p 1 to R p P , R s 1 to R s Q ) constituting the plurality of elementary light radiations (R1 to R N ) generated by the mode separator (S).

9. The optical communication system (1) according to claim 5, wherein, The mode decomposition device (M) is coupled to the second end of the multimode waveguide (F) to generate a plurality of decomposed optical radiations (Rld1 to Rld P ), and the polarization beam splitter (PBS) is optically disposed downstream of the mode decomposition device (M) to receive the plurality of decomposed optical radiations (Rld1 to Rld P ) and generate the first plurality of fundamental optical radiations (R p 1 to R p P ) and the second plurality of fundamental optical radiations (R s 1 to R s Q ).

10. The optoelectronic system (1) according to one of claims 5 to 9, wherein, The photon device (C) is configured to generate a first recombined light radiation (Rc1) from the first plurality of elementary light radiations (R p 1 to R p P ) and to generate a second recombined light radiation (Rc2) from the second plurality of elementary light radiations (R s 1 to R s P ).

11. The optical communication system (1) according to claim 10, wherein, The photon device (C) includes: a first photon device (C1) optically coupled to the mode separator (S) for receiving the first plurality of elementary light radiations (R p 1 to R p P ) and generating the first recombined light radiation (Rc1); and a second photon device (C2) optically coupled to the mode separator (S) for receiving the second plurality of elementary light radiations (R s 1 to R s P ) and generating the second recombined light radiation (Rc2).

12. The optoelectronic system (1) according to claim 10 or 11, wherein, The photon (C) includes a recombination device (R) configured to recombine the first and second recombined radiations (Rc1, Rc2) and form a single recombined optical radiation (R C ).

13. The optoelectronic system (1) according to one of claims 1 to 12, wherein, The optical port (P) includes a device for statically or dynamically controlling the polarization of the incident radiation to match the polarization with a determined polarization before the incident radiation is injected into the multimode waveguide (F).

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

15. The optoelectronic system (1) according to one of claims 1 to 14, wherein, The mode separator (S) includes a shaping device arranged upstream of the mode decomposition device (M).

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

Images