A device for optically processing an incident beam propagating in free space.

Abstract

The present invention relates to an optical processing device (DR) for optically processing an incident light beam (I0) that propagates in free space and is prone to phase plane distortion. This optical processing device (DR) processes at least a portion of the incident light beam (I0) into at least two fundamental light beams (R1~R N A mode conversion device (M) configured to decompose into ) and at least two fundamental light beams (R1~R N ) coherently recombines and, via the output optical port (P2), at least one recombined light beam (R c The system comprises an integrated photonic device (C) configured to generate a fundamental light beam (R1~R1). N The light propagates between the input optical port (P1) and the output optical port (P2) along at least two separate optical paths. The optical processing device (DR) is characterized in that at least one of the optical paths comprises at least one static delay element (D1, D2, D3) configured to equalize the propagation time of the fundamental light radiation along the optical path.

Description

[Technical Field]

[0001] This invention relates to a device for optically processing an incident light beam that propagates in free space and whose phase plane is prone to distortion. This distortion can arise from atmospheric disturbances during optical communication in free space. More generally, this distortion is caused by the propagation of the light beam in its medium. This invention has applications in the field of free-space optical communication. [Background technology]

[0002] In free-space optical communication, the transmitter modulates an optical beam (usually generated by a laser) to transmit information, which takes the form of a narrow beam emitted in the direction of the receiver. After propagating through its medium (air is used as an example for the remainder of this description, but the medium can be of any properties, such as water in the case of underwater communication), the optical beam is collected and demodulated to recover the transmitted information.

[0003] As a light beam propagates, it is exposed to atmospheric disturbances. These irregular disturbances, with fluctuating dynamics of around 1 kHz, lead to deformation of the beam, affecting its wavefront. This deformation manifests as "speckle" patterns in spots formed by the projection of the beam onto beam collection devices and by scintillation. This can limit the flow of information on the link between the transmitter and receiver, degrade transmission quality, and even render the link unusable.

[0004] To overcome this limitation, prior art has proposed mode-decomposing an incident light beam into multiple fundamental light beams via a mode-conversion device, coherently recombining these fundamental light beams via an integrated photonic device, and injecting this recombined light beam into a single-mode optical fiber for guidance to a detector / demodulator, for example. For further information, see U.S. Patent No. 7,974,543 or European Patent No. 3,672,109(A1).

[0005] Coherent coupling of light beams naturally requires that these beams are coherent with each other, meaning that the length difference between the different optical paths through which the fundamental light beams propagate (i.e., in a homogeneous medium, the distance covered by the beam multiplied by the refractive index encountered along its path) must be less than the coherence length of these beams. This coherence length is specifically determined by the spectral width of the incident light beam (more precisely, this coherence length varies as the square of the carrier wavelength divided by the spectral width).

[0006] It is well established that the spectral width of a modulated light beam increases with the transmitted data rate. The spectral efficiency of the modulation technique used is measured as the ratio between the transmitted information rate (bits / second) and the spectral width of the modulated light beam (or spectral congestion, expressed in Hz). In practice, this spectral efficiency is approximately 1 / 2 in amplitude modulation of the ASK (amplitude shift keying) type, meaning that the spectral width increases linearly with the information rate at a ratio of 2.

[0007] Therefore, transmission at relatively high rates increases the spectral width of the incident beam and decreases its coherence length, which means that the length of the optical path within the device used to process the incident light beam must be controlled with great precision. For example, an information rate of 50 gigabaud / second may require a coherence length of around 100 micrometers or several hundred micrometers. Providing a processing chain for an incident light beam in which each element is configured to have complete control over the extension of various optical path lengths through which it passes is particularly challenging.

[0008] In the solution proposed by European Patent No. 3672109(A1) and the publication "Free space optical communication receiver based on a spatial demultiplexer and a photonic integrated coherent combining circuit" by Vincent Billault et al., Opt. Express 29, 33134-33143 (2021), a mode conversion device and an integrated photonic device are optically coupled to each other via multiple single-mode fibers. This optical fiber link means that a portion of the device (integrated photonic device) can be offset from the telescope or equivalent incident beam collection means, which may be important for practical reasons when implementing this solution.

[0009] The static and dynamic delays introduced by the differences in fiber lengths and the chromatic dispersion of the incident beam through various media (air, fiber, etc.) are compensated for by multiple variable delay lines combined with multiple adjustable phase shifts located within the integrated photonic device. These variable delay lines and adjustable phase shifts must be dynamically controlled in real time, which can be challenging to achieve. [Overview of the project] [Problems that the invention aims to solve]

[0010] One object of the present invention is to propose an optical communication system that addresses this problem at least partially. Specifically, one object of the present invention is to provide an optical processing device for a free-space communication system that enables a high transmission rate without requiring dynamic control of a large number of variable delay lines. [Means for solving the problem]

[0011] To achieve this objective, the object of the present invention is a device for optically processing an incident light beam that propagates in free space and whose phase plane is prone to distortion, • An optical input port for receiving the incident light beam, A mode conversion device optically coupled to an optical input port by an offset waveguide formed by at least one multimode fiber, configured to decompose at least a portion of an incident light beam into at least two fundamental light beams, The invention proposes a device comprising: an integrated photonic device optically coupled to a mode-decomposing device in free space without the use of fiber intervening, the integrated photonic device being configured to coherently recombine at least two fundamental light beams and generate at least one recombined light beam at an optical output port.

[0012] The fundamental light beam propagates between the device's optical input port and optical output port along at least two separate optical paths.

[0013] According to the present invention, at least one of the optical paths comprises at least one static delay element configured to equalize the propagation time of the fundamental light beam along the optical path, and the device lacks any adjustable delay compensation element.

[0014] According to other advantageous non-limiting features of the present invention, either alone or in any technically feasible combination, - The mode conversion device comprises at least one multiplane conversion device, each comprising a plurality of optical components, each having a reflective surface for guiding the propagation of an incident light beam, wherein at least one of the reflective surfaces is microstructured to decompose the incident light beam into at least two fundamental light beams across multiple reflection paths. -The static delay element is integrated into the integrated photonic device. - The integrated photonic device includes a plurality of waveguides for propagating a basic optical beam to an optical combiner, where two of the plurality of waveguides have different lengths, and the difference in lengths serves as a static delay element. - The optical processing device includes an offset waveguide having at least two guiding modes coupled to an optical input port for guiding an incident optical beam to a mode decomposition device, and the offset waveguide is formed from at least one multimode fiber. - The offset waveguide and the mode decomposition device are configured to introduce a pulse width below a threshold. - The threshold is less than 0.5 ps or 0.2 ps. - The mode decomposition device is configured to decompose an incident beam according to the guiding modes of the offset waveguide. - The offset waveguide is formed by a multimode fiber, the multi-plane conversion device has a plurality of input modes, and the multi-plane conversion device is configured such that each input mode is formed by a linear combination of the modes of the multimode fiber, and these modes have similar group velocities. - The multi-plane conversion device is configured such that each input mode is formed by a linear combination of the modes within the same mode group of the multimode fiber. - The mode decomposition device is directly connected to the integrated photonic device.

[0015] According to another aspect, an object of the present invention is to propose an optical communication system that processes an incident optical beam generated by a transmitter and, by modulation, conveys information to be transmitted, the optical communication system comprising the above-described optical processing device optically coupled to an optical receiver.

[0016] The optical receiver can be configured to demodulate the recombined optical beam when the incident optical beam has a transmission information rate greater than 10 gigabits per second, or preferably greater than 50 gigabits per second.

[0017] The modulation generated by the transmitter can be performed at the operating wavelength. [Brief explanation of the drawing]

[0018] Other features and advantages of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying figures. [Figure 1a] These figures show optical processing devices according to the present invention. [Figure 1b] These figures show optical processing devices according to the present invention. [Figure 1c] These figures show optical processing devices according to the present invention. [Figure 2] This figure shows a photonic device for an optical processing device according to the present invention. [Figure 3a] This figure shows two exemplary embodiments of the present invention. [Figure 3b] This figure shows two exemplary embodiments of the present invention. [Figure 4] This figure shows an exemplary application of the optical processing device according to the present invention. [Figure 5] This figure shows an optical combiner based on the Mac Zehnder architecture. [Modes for carrying out the invention]

[0019] Figures 1a, 1b, and 1c show different embodiments of the optical processing device DR.

[0020] This device is designed to process incident light I0 that propagates in free space and whose phase plane is prone to distortion. This incident beam I0 is collected at the optical input port P1 of the optical processing device DR.

[0021] Referring to Figures 1a, 1b, and 1c, the optical processing device DR comprises a mode-decomposing device M optically connected to the optical input port P1. The mode-decomposing device converts at least a portion of the incident light beam I0 into at least two fundamental light beams R1~R N It is configured to decompose into these. Advantageously, each of these fundamental light beams consists of a single optical mode.

[0022] For the sake of simplicity in expression and notation, only two fundamental light beams are described and illustrated, but the optical processing device DR according to the present invention uses an incident light beam I0 at least partially with any number of fundamental light beams R1~R N For example, a dozen, several dozen, or even hundreds of basic light beams R1~R N It allows for disassembly using [this method].

[0023] The mode-decomposition device M can be implemented by a multiplane optical conversion device, which will be referred to as “MPLC device” for the remainder of this specification. In such an MPLC device, it is conceivable that an incident light beam undergoes a series of reflections and / or transmissions, followed by free-space propagation of the beam after each reflection and / or transmission. At least some of the optical components in which the reflections and / or transmissions occur and which guide the propagation of the incident beam have microstructured zones that modify the incident light beam.

[0024] The term "microstructured zone" means that the surface of an optical component has a relief on this zone, which can be broken down into "pixels," for example, which may have dimensions ranging from a few microns to several hundred microns. These may be metasurfaces. Each raised portion or pixel of this raised portion has a variable elevation angle of up to a few microns or up to several hundred microns with respect to the mean plane defining the surface. Regardless of the nature of the microstructure of the zone, an optical component having such a zone forms a phase mask that introduces a local phase shift into the cross-section of the beam reflected therein or transmitted therein.

[0025] As a result, the light beam propagating within the MPLC device undergoes a series of local phase shifts separated by propagation. A sequence of these fundamental transformations (e.g., at least four consecutive transformations such as 8, 9, 10, 12, 14, or, for example, at least 20 more transformations) establishes an overall transformation of the spatial profile of the incident beam. This makes it possible to configure a microstructured reflective or transmission surface, specifically, to transform a first light beam having a particular shape into a second beam having a different shape.

[0026] The literature "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. 9,250454 and U.S. Patent Application Publication No. 2017010463, contain the theoretical basis and practical implementation examples of MPLC devices.

[0027] As detailed in the aforementioned literature, the microstructured zones supported by the optical components forming the MPLC device are designed and configured to perform mode conversion, which aims to decompose the first light beam received at the input port into a family of modes called the "input" mode. The energy present in the modes of the input family is transferred to the modes of the "output" mode family at the output port of the MPLC device, where they are shaped. The MPLC device is configured to match the input fundamental mode and the output fundamental mode, respectively. This is a particularly stable and robust passive device with little to no effect on the polarization state of the light beam passing through it.

[0028] In the context of this specification, as an example, the family of input modes may include a Hermitian Gaussian base consisting of N Hermitian Gaussian modes positioned spatially opposite to the incident beam I0 when the incident beam I0 is collected at the input port P1. The family of output modes may consist of N spatially separated Gaussian modes, these modes of the fundamental beam R1~R N The MPLC device is configured to associate an input-based Hermitian Gaussian mode with an output-based Gaussian mode. The energy of the incident beam I0 received at input port P1 is decomposed according to the input-based mode, distributed to the output Gaussian mode to which the input-based mode is associated, and transferred into the MPLC device to match.

[0029] Of course, the Hermitian-Gaussian and Gaussian modes used as examples are for illustrative purposes only, and other modes could be chosen to perform the decomposition.

[0030] The mode-decomposition device M may include elements other than the MPLC device detailed above. Specifically, it may include the incident beam I0 or the fundamental light beams R1~R N This may include at least one transmission optical element or reflective optical element for shaping the surface, such as one or more free-form surface optical systems.

[0031] Regardless of how the mode splitter M is configured, the basic light beams R1~R N The light propagates along separate optical paths, which may have different lengths. This phenomenon introduces a deviation in the propagation time of the incident light beam I0 in the optical processing device DR, which imposes a long coherence length and / or limits the transmission rate, as described in the introduction of this application.

[0032] Returning to the descriptions of Figures 1a, 1b, and 1c, the illustrated optical processing device DR further comprises an integrated photonic device C, which comprises (at least) two fundamental light beams R1~R NIn order to receive it, it is optically coupled to the mode decomposition device M. The integrated photonic device C coherently recombines these two fundamental optical beams R1 to R N and is configured to generate a recombined optical beam R c at the optical output port P2. This recombined optical beam Rc preferably has a single mode or a reduced number of modes that is sufficiently lower than the number N of the fundamental optical beams R1 to R N . This reduced number of modes is, for example, less than 3 or 5. The recombined optical beam Rc can be supplied to an optical receiver or any other downstream device, for example, by simple free space propagation or, preferably, via a fiber Fs as shown in FIGS. 1a, 1b, and 1c. If the recombined optical beam Rc itself is in a single mode, the fiber can be a single mode Fs. If the recombined beam Rc has two or more modes, it can be a "few-mode fiber".

[0033] The photonic device C can take the form of a photonic integrated circuit PIC (or multiple optical integrated circuits). The chip is composed of a waveguide WG for guiding the two fundamental beams R1 to R N given to the optical combiner Co at its input port, enabling them to be coherently recombined to generate the recombined beam Rc. The optical combiner Co can be implemented by any suitable technique. Specifically, it can be an optical combiner Co having a Mach-Zehnder architecture as shown in FIG. 5.

[0034] The optical combiner Co shown in this figure consists of two arms connecting splitters BS arranged in series. A controllable phase shifter PS is attached to each arm. At the input, the optical combiner Co has two fundamental beams R1 to R respectively coupled on the two arms NThe optical combiner receives the recombined beam Rc at the output stage. At the output stage, the optical combiner supplies the recombined beam Rc to one of its two arms. The other arm is connected to a photodetector PD to control a controllable phase shifter PS and to form the recombined beam Rc at the output stage of the optical combiner Co. This control of the phase shifter PS is intended to minimize, or more generally, optimize, the signal measured on the photodetector PD. This can be implemented by a computer M integrated into or connected to the photonic device.

[0035] Photonic device C has three or more basic beams R1~R N When designed to recombine beams, the device is provided with several cascaded stages of optical combiners Co, each stage consisting of an optical combiner that recombines two beams from the previous stage. An embodiment of such a photonic device is schematically shown in Figure 2. A computer (not shown), integrated into or connected to the photonic device C, operates the optical combiners and supplies all fundamental beams R1-R1 to the input of device C. N It forms a coherent recombination.

[0036] The present invention is, of course, not limited to the exemplary embodiments of the integrated photonic device C presented as an example. Generally speaking, this integrated photonic device is used with fundamental light beams R1~R N The optical beam R coherently recombines these fundamental beams and, at the optical output port P2, recombines at least a portion of these fundamental beams. c It can be implemented using any technology to generate it.

[0037] In the embodiment shown in Figure 1a, the mode-resolving device M is optically coupled to the photonic device C by a plurality of waveguides, and each waveguide is connected to the fundamental beams R1~R N This induces one of the following. This type of coupling can be achieved, for example, by a bundle of single-mode optical fibers. In some cases, these fibers can be polarization-retaining.

[0038] However, these waveguides are disadvantageous because they introduce uncontrollable deviations in optical path length, as mentioned in prior art literature. These length deviations are related to the variability of waveguide length, which cannot actually be selected or shaped to be exactly the same. Thus, the uncertainty surrounding the length deviation of a given optical processing device DR makes it impossible to know in advance the extent of the various optical path lengths passing through it. This necessitates providing a line with an adjustable delay, as proposed in the prior art literature.

[0039] Furthermore, and more advantageously, the optical processing device DR lacks any waveguide between the mode-decomposing device M and the photonic device C. This coupling can be achieved by free-space propagation. Figure 1b shows such an embodiment, in which the mode-decomposing device M and the photonic device C are assembled directly without any fiber intervening. The mode-decomposing device M and the photonic device C can also be precisely positioned relative to each other by assembling them on a common support, thereby enabling their coupling by free-space propagation.

[0040] In the embodiment shown in Figure 1c, where the mode-decomposing device M and the photonic device C are optically coupled by free-space propagation, an offset waveguide Fd is integrated between the optical input port P1 of the optical processing device DR and the mode-decomposing device. This offset guide Fd, so to speak, compensates for the lack of a fiber link between the mode-decomposing device M and the photonic device C, allowing the mode-decomposing device M and the photonic device C to be offset from the point where the incident beam I0 is collected. The offset waveguide Fd can consist of a multimode optical fiber or a bundle of multimode optical fibers.

[0041] The fibers or multiple fibers constituting the offset waveguide Fd can be of any suitable type. These may be circular graded-index multimode fibers (e.g., 50 micrometer or 62.5 micrometer core and 125 micrometer cladding, types OM1, OM2, OM3, OM4, OM5), circular step-index multimode fibers, elliptic core fibers (graded or graded or step-index), spanned fibers (graded or step-index), graded or step-index fibers with stress rods ("panda" or "bowtie" type), photonic crystal or hollow core fibers that utilize their low dispersion, or multicore fibers (graded or step-index for each core, each core being elliptic or circular with or without stress rods). In the case of multicore fibers, each core can be optically associated with a mode from a family of input modes of the MPLC device forming the mode conversion device M.

[0042] This fiber or a set of fibers can be polarity-retaining if it is important to control the polarization during optical processing by the device DR. In this case, a device for adjusting the polarization of the incident light beam can also be attached to the optical input port P1. This device aims to match the polarization of the incident light beam to the polarization state maintained by the Fd offset waveguide. This is particularly useful when the polarization of the incident beam I0 is not perfectly controlled, and therefore it is impossible to directly inject the incident beam into the offset waveguide Fd without the risk of affecting the polarization of the beam propagating through the offset waveguide Fd.

[0043] It should be noted that the offset waveguide Fd affects the propagation of the incident beam I0 through mode dispersion. As a result, the optical paths of some modes may differ from those of other modes, and these paths may have different lengths. This phenomenon affects the required coherence length of the beam propagating through it. The propagation deviation affecting the modes propagating through waveguide Fd is added to that potentially introduced by the mode-decomposing device M. However, unlike the case of the waveguide link between the mode-decomposing device M and the photonic device C, the extension of different optical path lengths caused by this dispersion is fully known and therefore controllable in advance, as it depends on the intrinsic properties of the offset waveguide Fd, specifically its refractive index profile, its length, and the properties of the material from which it is fabricated.

[0044] Mode dispersion induced by the offset waveguide Fd tends to distort the symbols carried by the incident beam propagating through the offset waveguide Fd and the mode-converting device M through pulse widening. This widening can be characterized by measuring the duration of the optical pulses propagated to each output section of the mode-converting device M after injecting an excitation optical pulse at the free end of the offset waveguide Fd. For each of these output sections, a widening value can be set corresponding to the difference between the measured duration of the pulse on that output section and the duration of the optical excitation pulse. The pulse widening considered is the maximum widening value across all output sections.

[0045] Furthermore, as a general rule, it is desirable that the pulse widening be below a threshold. For example, to enable wavelength division multiplexing transmission, it is desirable to limit the pulse widening to below a threshold of 0.5 ps, and the transmission wavelength is selected from a 20 nm wavelength range centered on 1550 nm. The purpose is to limit the pulse widening to below a threshold of 0.2 ps for a 40 nm wavelength range centered on 1550 nm (C band).

[0046] Such results can be achieved when the offset waveguide Fd is relatively short, typically less than 5m, or exhibits little to no modal dispersion. This can be an OMx multimode graded index optical fiber, which may also have an elliptic core.

[0047] Alternatively or additionally, if the mode conversion device M is formed by an MPLC device, the latter may be configured during the design of the device such that each mode in the input mode family is selected as a linear combination of modes having similar group velocities, such that this mode is slightly distorted as it propagates through the offset waveguide Fd. Similar means that these velocities are identical within 10%, and preferably within 5%.

[0048] For example, each mode in an input mode family can be selected as a linear combination of modes from the same multimode fiber mode group. It should be noted that modes from the same mode group in a fiber propagate at the same speed. This configuration avoids the formation of a fundamental beam generated by the MPLC device from multimode fiber modes propagating at different speeds. As a further example of such a configuration, the mode-decomposing device M can be configured to decompose the incident beam I0 into the guiding modes of the offset waveguide Fd.

[0049] As seen in the introduction of this application, the extension of various optical path lengths through the optical processing device DR between the optical input port P1 and the optical output port P2 leads to a relatively large coherence length. As a result, the transmission rate is limited when the optical processing device DR operates in free-space optical communication applications.

[0050] Furthermore, in the device according to the present invention, the static delay element is the two fundamental light beams R1~R NIt is located in at least one of the optical paths through which the light beam passes. For clarity, these optical paths extend from the input port P1 of the DR optical processing device to its output port P2. The static delay introduced by this element equalizes the propagation time of the fundamental light beam along the optical path, i.e., the fundamental light beam R1~R propagating along these paths at the operating wavelength, i.e., wavelengths included within the target wavelength range. N It is selected to reduce the propagation time difference. This limits the need for coherence length, thereby maintaining optical transmission throughput. "Static delay" means a fixed and unchangeable delay. To avoid doubt, it is specified that the static delay introduced by the static delay element is related to the optical path deviation due to the beam propagation speed in the DR optical processing device. In practice, to enable high-speed transmission of more than 10 gigabits / second at a single operating wavelength, it may be desirable to make the lengths of various optical paths equal so that the lengths of the various optical paths differ by no more than 3000 microns at most.

[0051] When the transmission involves several operating wavelengths, as in WDM transmission, it is desirable to more precisely reduce the distance between different optical paths. This means that, in order to enable transmission at two wavelengths, for example, 1553.3 nm and 1536.6 nm, it is desirable to make the lengths of the different optical paths equal, so that they differ by only 120 microns or less. More generally, the transmission operated by the device according to the present invention may use a single wavelength, two wavelengths, or more.

[0052] It should be noted that the differences in the lengths of multiple optical paths through the offset waveguide Fd or within the mode-conversion device M are particularly stable over time. These can be easily characterized and measured, for example, by measuring the propagation deviation of the beam propagating along these different paths, and this delay can be compensated for by a static delay element without the need to adjust for it over time. Characterization techniques may include "time-of-flight" measurements using white light interferometry, tunable laser interferometry, or even ultrashort pulse or sinusoidal modulated light sources. As already mentioned, the differences in the lengths of multiple optical paths are also stable for each optical processing device, specifically because the fiber link between the mode-conversion device M and the integrated photonic device is eliminated. Therefore, it is entirely possible to compensate for the length differences with a fixed, unchangeable static delay element.

[0053] According to the first method, the static delay element D1 is integrated into the integrated photonic device C. This allows the two fundamental light beams R1~R, which are supplied to the optical combiner Co by the mode-decomposed device M, to be connected. N The two waveguides WG of the integrated photonic device C, each propagating a different signal, are designed to have different lengths, and the difference in length becomes a static delay element D1.

[0054] Specifically, the lengths of the waveguides WG are made different so that the optical path lengths between the optical input port P1 and the optical output port P2 of the optical processing device DR are equal.

[0055] Integrated photonic device C has three or more fundamental light beams R1~R N When recombining, it has been previously shown that this device can be fitted with multiple optical combiner stages Co, and the stages are arranged in a cascade. In this case, it is naturally necessary to make the optical paths extending between the optical input port P1 and each optical combiner Co equal in order to enable this coherent recombination.

[0056] According to the first solution shown in Figure 3a, the lengths of all waveguides in the first stage (the so-called "input" stage) of the optical combiner Co are adjusted (except in some cases, the one identified as the longest path). This adjustment affects the fundamental optical beams R1~R along the optical path extending from the optical input port P1 to the first stage of the optical combiner Co. N This equalizes the propagation times of all signals. Therefore, the static delay element D1 is placed in each waveguide leading to the input stage's optical combiner Cos (except for those that form part of the longest optical path, in some cases). In this case, all waveguides interconnecting the stages are the same length and therefore do not need to be adjusted.

[0057] The alternative solution shown in Figure 3b involves adjusting the waveguide length in each optical combiner Co. This adjustment is performed along the optical path extending from the optical input port P1 to each optical combiner Co, affecting the elemental optical beams R1~R N Make the propagation times equal.

[0058] In the second method, the static delay element D2 is integrated into the mode-decomposition device M. This is the two fundamental light beams R1~R N This can be achieved by forming an elevation angle deviation (from the principal plane) of one of the reflecting surfaces to which one of the beams is reflected. This difference in elevation angle leads to a change in the optical path length of the fundamental light beam reflected there. More generally, a mode-decomposing device M uses the fundamental light beams R1~R N It can have multiple elevation gaps (with respect to the main plane forming the reflective surface) in which at least a portion is reflected. These elevation differences are along the optical path extending between the optical input port P1 and the optical output port P2, along the fundamental optical beam R1~R N The propagation times are configured to be equal.

[0059] Regardless of the solution chosen, the static delay element or multiple static delay elements introduced into the optical processing device DR maintain the appropriate coherence length of the incident beam I0 propagating through it, thereby facilitating the coherent recombination of the fundamental light beam. The optical paths extending through the optical processing device DR have equal lengths, i.e., any length differences that may exist are smaller than in the case where the static delay element or multiple such elements are absent.

[0060] Figure 4 shows an optical communication system 1 using the optical processing device DR according to the present invention.

[0061] The optical communication system 1 in Figure 4 is designed to process an incident light beam I0 that carries the transmitted information, as generated by the transmitter and modulated. The incident light beam I0 may have one or more operating wavelengths and / or may use multiple polarizations, as is typical in WDM transmission. The recombined light beam Rc is supplied to an optical receiver OR, which can extract information from the received beam. Specifically, this optical receiver OR is configured to demodulate the recombined light beam Rc when the transmission (modulation) is at a very high bit rate, e.g., 10 gigabits / second, 50 gigabits / second, or even several hundred gigabits / second.

[0062] In the embodiment shown in Figure 1, the transmitter is located on a satellite (SAT), but generally speaking, the transmitter can be located on land, at sea, or in space, and can propagate in any free space, in the atmosphere in the case of land communications, or underwater in the case of sea communications. The transmitter and communication system 1 may be stationary or may be moving relative to each other.

[0063] The emitted incident light beam I0 takes the form of a narrow beam directed toward the communication system 1. While propagating in free space, the emitted beam is subjected to atmospheric disturbances, and as a result, the incident light beam I0 reaching the base station has spatial and temporal variations in amplitude and phase. This phenomenon affects the shape of the beam, which takes on a shape that changes irregularly and intermittently over time. The optical communication system 1 is designed to compensate for this distortion at least partially, so that the optical receiver OR can process the beam and decode the transmitted message by direct detection or coherent detection. For this purpose, the receiver OR can incorporate amplification and / or spectral demultiplexing functions, particularly for WDM transmission.

[0064] The optical communication system 1 comprises a telescope T, which has an objective lens O for collecting an incident light beam I0 and generating it at the optical input port P1 of an optical processing device DR. As is well known, this objective lens may be equipped with a concave mirror for focusing the beam received at the image focal point. This focused beam can be reflected back to the optical input port P1 using a second mirror of the objective lens O, which can be flat or convex. The telescope T can be rotated to point at and track a transmitter located here on the satellite SAT. The telescope T may also be equipped with a device TTM (such as a tip-tilting mirror) for guiding the incident beam in order to best guide the light beam toward the optical port P, for example, to center this beam in port P1, or more generally, to correct any directional deviation of the telescope T.

[0065] Finally, the optical communication system 1 further comprises the optical processing device DR described above, located optically downstream of the telescope T. This device is designed to at least partially compensate for the distortion of the incident light beam I0.

[0066] Mode decomposition of the incident beam I0 and the generated fundamental beams R1~R NThrough coherent recombination, the optical processing device DR can utilize the maximum energy of the collected incident beam I0 and at least partially compensate for the distortion this beam experiences during its propagation in free space. (Base beam R1~R) N Since it propagates along an optical path of almost the same length, the coherence distance of the incident beam I0 is hardly affected. As a result, the fundamental beam R1~R N These can recombine coherently with each other, meaning that even when the incident beam I0 arises from high-rate modulation, it can form a single-mode recombined optical beam Rc with maximum energy.

[0067] Naturally, the present invention is not limited to the embodiments described, and alternative embodiments may be added without departing from the scope of the invention as defined by the claims.

[0068] Finally, it should be noted that the processing device according to the present invention arises from an approach entirely opposite to that presented in the prior art. According to the present invention, the objective is to create a processing device in which the optical path length difference is completely controlled. Since this difference is controlled on a per-device basis, especially in mass production, it is possible to incorporate at least one static delay element into the device to equalize the optical path lengths and limit their extension. This eliminates the need for dynamic or calibration compensation of the path length by an adjustable delay compensation element, as required in devices of the prior art. Furthermore, and highly advantageously, the processing device according to the present invention lacks any adjustable delay compensation element.

[0069] As mentioned above, reducing the spread to less than 3,000 micrometers makes it possible to achieve transmission rates exceeding 10 gigabits. The smaller the spread, the higher the transmission rate and / or the wider the wavelength range used in WDM transmission.

Claims

1. A device for optical processing (DR) of an incident light beam (I0) that propagates in free space and whose phase plane may be distorted, wherein the optical processing device (DR) is - An optical input port (P1) for receiving the incident light beam (I0), - A mode conversion device (M) optically coupled to the optical input port (P1) by an offset waveguide (Fd) formed by at least one multimode fiber, wherein at least a portion of the incident light beam (I0) is converted into at least two fundamental light beams (R １ ~R Ｎ A mode conversion device (M) is configured to be disassembled into, - An integrated photonic device (C) optically coupled to the mode-resolving device (M) in free space without the use of fiber intervening, wherein the at least two fundamental light beams (R １ ~R Ｎ ) are coherently recombined, and at the optical output port (P2), at least one recombined light beam (R ｃ The system comprises an integrated photonic device (C) configured to generate, The aforementioned fundamental light beam (R １ ~R Ｎ An optical processing device (DR) characterized in that the light propagates between the optical input port (P1) and the optical output port (P2) along at least two separate optical paths, the optical processing device (DR) comprises at least one static delay element (D1, D2, D3) configured such that at least one of the optical paths equalizes the propagation time of the fundamental light beam along the optical path, and the optical processing device (DR) lacks any adjustable delay compensation element.

2. The mode conversion device (M) consists of at least one multi-plane conversion device comprising a plurality of optical components each having a reflective surface for guiding the propagation of the incident light beam (I0), and at least one of the reflective surfaces is structured in a fine pattern so as to decompose the incident light beam (I0) into at least two basic light beams (R １ ~R Ｎ ) over a plurality of reflection paths. The optical processing device (DR) according to claim 1.

3. The optical processing device (DR) according to claim 1 or 2, wherein the static delay element (D2) is integrated with the mode-decomposition device (M).

4. The static delay element (D2) controls the fundamental light beam (R １ ~R Ｎ The optical processing device (DR) according to claims 2 and 3, wherein one of the ) is the elevation angle deviation from the main plane forming one of the reflective surfaces to be reflected.

5. The optical processing device (DR) according to claim 1 or 2, wherein the static delay element (D1) is integrated within the integrated photonic device (C).

6. The integrated photonic device (C) uses a fundamental light beam (R １ ~R Ｎ The optical processing device (DR) according to claim 5, comprising a plurality of waveguides for propagating a to an optical combiner (C0), wherein two of the plurality of waveguides have different lengths, and the difference in lengths constitutes the static delay element (D1).

7. An optical processing device (DR) according to any one of claims 1 to 6, comprising an offset waveguide (Fd) having at least two guidance modes, coupled to the optical input port (P1) for guiding the incident light beam (I0) toward the mode-decomposition device (M), the offset waveguide (Fd) being formed from at least one multimode fiber.

8. The optical processing device (DR) according to claim 7, wherein the offset waveguide (Fd) and the mode-decomposition device (M) are configured to introduce pulse widening below a threshold.

9. The optical processing device (DR) according to claim 8, wherein the threshold is less than 0.5 ps or 0.2 ps.

10. The optical processing device (DR) according to any one of claims 7 to 9, wherein the mode-decomposition device (M) is configured to decompose the incident beam (I0) into the induction modes of the offset waveguide (Fd).

11. The optical processing device (DR) according to any one of claims 7 to 9 in combination with claim 2, wherein the offset waveguide (Fd) is formed of a multimode fiber, the multiplane converter has a plurality of input modes, and the multiplane converter is configured such that each input mode is formed by a linear combination of modes of the multimode fiber, and these modes have similar group velocities.

12. The optical processing device (DR) according to claim 11, wherein the multiplane conversion device is configured such that each input mode is formed by a linear combination of the modes from the same group of modes of the multimode fiber.

13. The mode-decomposition device (M) is directly assembled to the integrated photonic device (C), as described in any one of claims 1 to 12, for the optical processing device (DR).

14. An optical communication system for processing an incident light beam (I0) generated by a transmitter and transporting transmitted information by modulation, comprising an optical processing device (DR) according to any one of claims 1 to 13, which is optically coupled to an optical receiver OR.

15. The optical communication system according to claim 14, wherein the optical receiver (OR) is configured to demodulate the recombined optical beam (Rc) when the incident optical beam (I0) has a transmitted information rate greater than 10 gigabits per second, or preferably greater than 50 gigabits per second.

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