FR3154512A1 - Coherent optical combining device and system employing such a device for illuminating a far-field target - Google Patents

Abstract

The invention relates to an optical device (4) for coherently combining a plurality of phase-locked elementary light beams (FLE). It comprises a first stage (4a) comprising a plurality of near-field recombination modules (5), each recombination module (5) coherently combining a group of beams from the plurality of elementary light beams (FLE) to form a plurality of intermediate light beams (FLE'). It also comprises a second stage (4b), arranged downstream of the first stage (4a), the second stage (4b) performing a far-field recombination of the intermediate light beams (FLE') to form an output light beam (FLS). The invention also relates to a system for producing an output light beam (FLS) using such an optical device (4) Figure 5

Description

Coherent optical combining device and system employing such a device for illuminating a far-field target FIELD OF THE INVENTION

The present invention relates to a coherent combining device and a system for producing a light beam using such a device, in particular for illuminating a target in the far field.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

The coherent combination of a plurality of elementary light beams is described in document EP3921695 or in document WO2015181130A1. The latter recalls that the coherent combination aims in particular to produce a high-power laser source, and/or capable of delivering high energy in the case of ultra-short pulsed sources, for example with a pulse width of less than a picosecond.

An overview of this technology is also provided in Fathi, et al “Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers” Photonics 2021, 8 , 566. https://doi.org/10.3390/photonics8120566

Document WO2015181130A1 also recalls that two general approaches are used to obtain the coherent combination of a plurality of beams locked in phase with respect to each other.

The first approach is that of so-called "far-field" or "tiled aperture " beam combining, in which the beams to be combined are placed side by side and pointed in the same direction, so that they propagate as a single beam.

The second approach is that of beam combination known as "near field" or "spatial superposition" ("filled aperture" in its Anglo-Saxon designation) according to which a diffractive optical part is used to combine the beams by superimposing them and so that they propagate as a single beam.

To form a high-power output light beam, for example greater than 100 kW, it is necessary to combine a large number of elementary light beams, several dozen or even a hundred or more. The document “Optical Coherent Combining of High Power Optical Amplifiers for Free Space Optical Communications” by V. BILLAULT et al published on 06/14/2023 (Doc. ID 494908) presents a graph showing the efficiency of the combination according to the number of elementary beams to be combined.

Regardless of the approach adopted, this efficiency tends to decrease with the number of these beams. Beyond 100 beams to be combined, this efficiency is very low, less than 20%. This is due in particular to the complexity of achieving the precise arrangement of the beams with each other when the number of beams increases and the thermal limitations imposed by the high power of the recombined beam.

SUBJECT OF THE INVENTION

One aim of the invention is to propose an optical device for coherently combining a plurality of phase-locked elementary beams that is more efficient than prior art devices. Another aim of the invention is to propose a system for producing an output light beam that takes advantage of this optical device.

BRIEF DESCRIPTION OF THE INVENTION

With a view to achieving one of these aims, the subject of the invention proposes an optical device for coherently combining a plurality of phase-locked elementary light beams.

The optical device comprises a first stage comprising a plurality of recombination modules, each recombination module coherently combining a group of beams of the plurality of elementary light beams to form a plurality of intermediate light beams.

It also includes a second stage, arranged downstream of the first stage, the second stage performing far-field recombination of the intermediate light beams to form an output light beam.

• les modules de recombinaison du premier étage effectuent une combinaison cohérente en champ proche ;
• au moins un module de recombinaison comprend une pièce optique diffractive ;
• au moins un module de recombinaison comprend un dispositif optique de conversion multiplan ou une lanterne photonique ;
• les modules de recombinaison du premier étage effectuent une combinaison cohérente en champ lointain ;
• au moins un module de recombinaison est configuré pour conformer un faisceau lumineux intermédiaire selon une forme choisie ;
• au moins un module de recombinaison comprend au moins une optique de mise en forme ;
• la forme choisie est à plateau plat, le plateau plat formant un polygone, tel qu’un carré, un rectangle ou un hexagone ;
• la forme choisie est circulaire, avantageusement à plateau plat.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• the first stage recombination modules perform near-field coherent combination;
• at least one recombination module comprises a diffractive optical part;
• at least one recombination module comprises a multiplane optical conversion device or a photonic lantern;
• the first stage recombination modules perform coherent far-field combination;
• at least one recombination module is configured to conform an intermediate light beam according to a chosen shape;
• at least one recombination module comprises at least one shaping optic;
• the chosen shape is flat-topped, the flat top forming a polygon, such as a square, rectangle, or hexagon;
• the chosen shape is circular, advantageously with a flat top.

• au moins une source produisant une pluralité de faisceaux lumineux initiaux ;
• un étage d’asservissement de phase de la pluralité de faisceaux initiaux, l’étage d’asservissement produisant une pluralité de faisceaux lumineux élémentaires asservis en phase ;
• un dispositif optique de combinaison cohérente tel que défini précédemment.

According to another aspect, the invention provides a system for producing an output light beam intended to illuminate a far-field target, comprising:

• at least one source producing a plurality of initial light beams;
• a phase-locking stage for the plurality of initial beams, the phase-locking stage producing a plurality of phase-locked elementary light beams;
• a coherent combining optical device as defined above.

• la source comprend une source laser et au moins un séparateur optique disposé optiquement en aval de la source laser pour former la pluralité de faisceaux lumineux initiaux ;
• la source produit au moins 50 faisceaux lumineux initiaux, et de préférence entre 100 et 1000 faisceaux lumineux initiaux ;
• l’étage d’asservissement comprend une pluralité d’actuateurs de phase respectivement couplés à une pluralité d’amplificateurs optiques pour établir la pluralité de faisceaux lumineux élémentaires asservis en phase ;
• l’étage d’asservissement comprend au moins un dispositif de commande de la pluralité d’actuateurs de phase ;
• l’étage d’asservissement comprend une pluralité de dispositifs de commande, chaque dispositif de commande recevant en entrée des signaux produits par un module de recombinaison donné et chaque dispositif de commande produisant des signaux de commande pour les actuateurs de phases associés au groupe de faisceaux recombinés par le module de recombinaison donné ;
• la source comprend une pluralité d’actuateurs de phase complémentaires et le système de production comprend en outre un dispositif de commande complémentaire recevant en entrée des signaux produits par le deuxième étage du dispositif optique et produisant des signaux de commande pour les actuateurs de phases complémentaires ;
• la pluralité d’actuateurs de phase complémentaires est optiquement disposée entre un séparateur optique complémentaire et le séparateur optique ;
• le système de production comprend un capteur de lumière configuré pour collecter un rayonnement lumineux réfléchi par une cible vers laquelle le faisceau lumineux de sortie est dirigé et relié à l’étage d’asservissement.

According to other advantageous characteristics of this aspect of the invention, taken alone or in any technically feasible combination:

• the source comprises a laser source and at least one optical splitter optically disposed downstream of the laser source to form the plurality of initial light beams;
• the source produces at least 50 initial light beams, and preferably between 100 and 1000 initial light beams;
• the servo stage comprises a plurality of phase actuators respectively coupled to a plurality of optical amplifiers to establish the plurality of phase-locked elementary light beams;
• the servo stage comprises at least one device for controlling the plurality of phase actuators;
• the servo stage comprises a plurality of control devices, each control device receiving as input signals produced by a given recombination module and each control device producing control signals for the phase actuators associated with the group of beams recombined by the given recombination module;
• the source comprises a plurality of complementary phase actuators and the production system further comprises a complementary control device receiving as input signals produced by the second stage of the optical device and producing control signals for the complementary phase actuators;
• the plurality of complementary phase actuators is optically disposed between a complementary optical splitter and the optical splitter;
• the production system comprises a light sensor configured to collect light radiation reflected by a target towards which the output light beam is directed and connected to the servo stage.

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

FIG. 1

There FIG. 1 represents a system for producing an output light beam intended to illuminate a target in the far field;

FIG. 2

There FIG. 2 represents an embodiment of a first stage of a coherent optical combining device according to the invention;

FIG. 3

There FIG. 3 represents another mode of implementation of a first stage of a coherent optical combining device according to the invention;

FIG. 4 FIG. 4 FIG. 4

Figures 4a, 4b and 4c represent different arrangements of the intermediate light beams in a second stage of a coherent combining optical device according to the invention;

FIG. 5

There FIG. 5 represents a particular mode of implementation of the invention;

FIG. 6

There FIG. 6 represents an example of implementation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of clarity, a light beam is defined in the present application as radiation formed from at least one mode of the electromagnetic field, each mode forming a spatio - frequency distribution of the amplitude, phase, and polarization of the field. Consequently, the modification or transformation of the phase of the light radiation designates the spatio - frequency modification or transformation of each of the modes of the radiation.

The "shape" of a beam will be defined as the transverse distribution of the amplitude and phase of the mode or the combination of the transverse distributions of amplitude and phase of the modes composing this beam.

For the sake of simplification, in this description it will be considered that the radiation composing a light beam is polarized in a single direction and has a single frequency. However, the principles set out are entirely applicable to radiation having more than one direction of polarization or more than one single frequency.

There FIG. 1 represents a system 1 for producing an FLS output light beam intended to illuminate a target in the far field.

In a very general manner, such a system 1 comprises at least one source 2 producing a plurality of initial light beams FLI. A servo-control stage 3 is arranged downstream of the source. This stage is configured to equalize the phases of the initial light beams FLI, that is to say so that they have between them a predetermined phase difference, generally chosen to be zero. Stage 3 produces a plurality of elementary light beams FLE, locked in phase when the servo-control is actually in action. Finally, the production system 1 comprises an optical device 4 for coherent combination of the plurality of elementary beams FLE locked in phase, this optical device 4 producing the output light beam FLS.

In the example shown in the FIG. 1 , the source 2 comprises a laser source 2a producing a master light radiation, preferably single-mode. The master light radiation preferably has a narrow linewidth, i.e. having low phase noise and therefore high spectral purity. The laser source 2a may be of a continuous or pulsed nature, the duration of the pulses may be of any length, typically of the order of a few hundred picoseconds to a few hundred nanoseconds. The laser source 2a is connected to an optical splitter 2b, optically arranged downstream of this laser source 2a, to establish the plurality of initial light beams FLI from the master light radiation. The optical splitter 2b can produce a large number of initial light beams FLI, for example at least 50 initial light beams FLI and typically between 100 and 1000 initial light beams FLI, which ultimately makes it possible to produce a high-power output light beam FLS. To make the most of the optical device 4 which will be the subject of a subsequent section of this description, it will be preferable to produce more than 10 initial light beams FLI. The laser source 2a can produce a master light radiation having, for example, a power of between 10 mW and 100 mW. The source 2 may have an intermediate amplification stage, for example arranged between the laser source 2a and the optical separator 2b, to increase this power between a few Watts and several tens of Watts, so that each initial light beam FLI has a power for example between 10 mW and 1 W. This may be, for example, a doped fiber amplifier.

Regardless of their number, the initial light beams FLI are guided to the servo stage 3 preferably via waveguides, such as optical fibers.

The servo stage 3 comprises a plurality of phase actuators 3a respectively coupled to a plurality of optical amplifiers 3b to establish the plurality of phase-locked elementary light beams FLE. Each input light beam FLI is optically coupled to a phase actuator 3a and to an optical amplifier 3b. By phase actuator, we mean any device capable of introducing a controlled phase shift or a controlled delay in the radiation forming the elementary light beam FLE. These actuators can also be used to temporally align the pulses of each initial light beam FLI, when the laser source 2a is of the pulse type.

By way of illustration, the power of an elementary light beam FLE may be of the order of 1 kW, and typically between 0.5 kW and 2 kW, depending on the power of the laser source 2a, the number of initial light beams FLI produced by the optical splitter 2b and the gain of the optical amplifiers 3b, and the possible presence of an intermediate amplification stage in the source 2. The optical amplifiers 3b may comprise a plurality of amplification stages, to facilitate the high-gain amplification of the initial light beams FLI.

The servo stage 3 also comprises at least one control device 3c for controlling the plurality of phase actuators 3a. This control device 3c develops the control signals for the phase actuators 3a to equalize the phases of the elementary light beams FLE and lock them together. To develop the control signals, the control device 3c has at least one input or a plurality of inputs connected to at least some of the light beams from the production system 1. As is well known per se, semi-reflecting blades or any other type of optical element will be provided to take a small portion of these light beams and supply them to the control device 3c. The control device 3c comprises converters or detectors allowing it to process its inputs digitally or analogically. It can be implemented by a signal processing computer, by a programmable logic component (FPGA according to the generally used English acronym) or by any other device having sufficient computing power. The document WO2015181130A1 cited in the introduction to this application lists the algorithms well known to the person skilled in the art which can be implemented by the control device 3c to equalize the phases of the elementary light beams FLE.

The phase-locked elementary FLE light beams produced by the servo stage 3 are guided, for example via waveguides such as optical fibers, towards the coherent combining optical device 4. This optical device 4 can be provided with a fiber network (“fiber array” according to the English expression used in the field) to collect the elementary FLE light beams and inject them with precise positioning at an input port of this device 4. It is recalled that in such fiber propagation, the beams generally take a Gaussian shape.

In a particularly advantageous manner, the coherent optical combination device 4 is composed of two stages optically coupled to each other. A first stage 4a implements recombinations, which can be indifferently in the near field or in the far field, of groups of elementary light beams FLE, each group comprising a relatively small number of elementary light beams, to produce intermediate light beams FLE’. A second stage 4b, arranged optically downstream of the first stage 4a, implements a far-field recombination of the intermediate light beams FLE’ to form the output light beam FLS.

By thus splitting the coherent combination of elementary light beams FLE into two stages, a reduced number of elementary light beams FLE are combined in each stage, with reduced complexity and thermal losses and therefore with high efficiency. This is particularly true for the far-field recombination of the second stage 4b, the intermediate light beams FLE' to be recombined being fewer in number and therefore more easily placed side by side than in the solutions of the state of the art in which all the elementary light beams are recombined in a single stage. This two-stage configuration also avoids concentrating all the energy of the light beams on a single optical part, which would undergo excessive heating that could degrade the proper functioning of the optical recombination device, unless the targeted power of the output beam is limited.

According to a first particularly advantageous approach, the first stage 4a is formed of a plurality of near-field recombination modules 5. Each recombination module 5 coherently combines a group of beams from the plurality of elementary light beams FLE to form a plurality of intermediate light beams FLE’. This group of beams is composed of a reduced number of beams, for example between 4 and 61. The groups of beams may have the same number of beams or distinct numbers of beams. Due to the relatively reduced number of beams to be recombined at each module 5 (compared to the number of elementary light beams), this near-field recombination can be carried out with high efficiency, of the order of 70%, 80% or even 90% or more.

According to a second approach, the first stage 4a is formed from a plurality of far-field recombination modules 5.

Whatever the nature of the recombination, the number of beams in each group will be chosen (in the preferred range mentioned above) according to the number of elementary light beams FLE to be recombined, so as to produce a chosen plurality of intermediate light beams FLE’, recombined in the far field by the second stage 4b. The number of beams in each group will also be chosen so that each recombination module 5 is exposed to a reasonable optical power, for example 30 kW maximum, in order to limit its heating. It is thus possible to choose and configure these recombination modules 5 so that they are capable of receiving an optical power of 5 kW, 10 kW, 15 kW, 20 kW or 25 kW.

The choice of the number of intermediate light beams FLE’ is advantageously made to facilitate the coherent far-field combination carried out by the second stage 4b, for example by allowing a tiling of a pupil of this second stage. It is preferably greater than 4. It can thus be a centered hexagonal number (i.e. included in the list formed by the numbers 7, 19, 37, 61, 91, 127, 169, etc.), in particular when each intermediate light beam FLE’ has a hexagonal shape.

Thus, and preferably, we will seek to compose between 4 and 61 groups of beams, each group of beams being processed by a recombination module 5 to therefore provide between 4 and 61 intermediate light beams FLE’. Such a number is sufficient to control the shape of the output light beam (by far-field combination of the intermediate light beams FLE’) while maintaining good far-field recombination efficiency, which can be of the order of 70% or more.

The shape of the FLS output light beam can, for example, correspond to a Gaussian shape or a flat-plateau shape (known as a “top-hat” shape according to the commonly used English expression) in the far field or even be chosen to compensate for its deformation induced by atmospheric turbulence during its propagation towards the target, as will be illustrated in a later passage of this description.

The efficiency of the coherent combining optical device 4 can therefore reach 63% or more overall. For illustration, a source 2 produces 160 initial light beams FLI, phased and amplified by the servo stage 3 to provide 160 elementary light beams FLE, each having a power of 1 kW. The first stage 4a of the coherent combining optical device 4 comprises 10 near-field recombination modules 5, each module recombining 16 elementary light beams FLE. The first stage therefore provides 10 intermediate light beams FLE' recombined with each other in the far field by the second stage and thus forming the output light beam FLS. The efficiency of the optical device 4, thus decomposed into two stages, can be of the order of 63%, which leads to the production of an output light beam FLS having a power of approximately 100 kW, which is very significant.

Higher powers can be obtained by increasing the number of elementary FLE light beams. For example, 1000 elementary FLE light beams of 1 kW each can be recombined by 40 recombination modules 5 each processing 25 elementary FLE light beams. The 40 intermediate FLE light beams produced are recombined in the far field by the second stage 4b. By estimating the efficiency of such an assembly around 50%, an FLS output light beam of the order of 500 kW can be produced. It is noted that it is generally technically easier to increase the number of elementary FLE light beams than to seek to amplify a given number of these beams with greater intensity, which makes the proposed approach particularly interesting for producing high powers.

In addition to its recombination function, a recombination module 5 can also have the function of shaping the intermediate light beam FLE’ that it produces. It is thus possible to choose to shape this beam so that it has a circular, triangular, hexagonal, rectangular or square flat plate.

For this purpose, a recombination module 5 may comprise at least one shaping optic for conforming the intermediate light beam FLE' to the chosen shape. The shaping of the intermediate light beams FLE' has a great advantage, because it makes it possible, by choosing an appropriate shape for each intermediate light beam FLE', to improve the quality of the output beam FLS, in particular when this quality is measured by the so-called PIB measurement, acronym for "Power in the Bucket" according to English terminology, as will be explained in more detail in the section of this description relating to the second stage 4b of the coherent optical combination device 4.

The recombination modules 5 which collectively form the first stage 4a of the coherent combining optical device 4 can implement any technique which is suitable for achieving the coherent combination of the groups of elementary light beams FLE. These techniques can be identical for each module 5 or different.

When implementing near-field recombination, a recombination module 5 may comprise, as is well known per se, at least one diffractive optical element 6 (or “DOE” acronym for the English expression Diffractive Optical Element) illustrated in the FIG. 2 . This element 6 (designated DOE for simplicity in the remainder of this description) has a surface structure defining a spatial period. The elementary light beams FLE are incident on the DOE 6 at an angle defined by the spatial period. When the elementary light beams are well locked in phase, these beams interfere constructively on the main order of the DOE 6 to form the intermediate light beam FLE', and destructively on the higher orders, represented in dotted lines on the FIG. 2 . The light intensity present in these higher orders (just like that present in the main order) can be used as input to the control device 3c in order to facilitate or enable the phase locking of the elementary light beams FLE. In a variant, a photonic lantern can be provided instead of the diffractive optical element.

When it is also desired to conform the intermediate light beam FLE' to a chosen shape, it is possible, as already mentioned, to provide, upstream or downstream of the DOE or the photonic lantern 5, optical shaping parts carrying out this change of shape. This may be any optical part or arrangement of optical parts comprising, for example, at least one lens, for example lenses for collimating the elementary light beams FLE, an axicon, at least one non-spherical and non-planar optical element, such as an aspherical or freeform optical element (from the translation of the English expression "freeform optics"). For reasons of management of the thermal losses which occur during the propagation of the light beam in the recombination module 5, it will be preferable to use reflective optical parts, in particular when these optical parts are arranged optically downstream of the recombination module 5.

Advantageously, the recombination module 5 comprises, instead of or in addition to the elements which have just been described, at least one multiplane conversion device designated “MPLC device” in the remainder of this description. Such a device makes it possible to properly control the spatial parameters of the elementary light beams FLE in order to recombine them and, at the same time, conform the intermediate light beam FLE’ to a chosen shape. In particular, it is possible, thanks to an MPLC, to compensate for the geometric imperfections of a fiber network, when such a network is used to optically couple the servo stage 3 to the coherent optical combination device 4. Of course, it is perfectly possible to use an MPLC device to only implement the recombination of the elementary light beams FLE, without applying any particular shaping (and therefore preserving the naturally Gaussian shape of these beams).

It is recalled that in an MPLC device, incident light radiation undergoes a succession of reflections and/or transmissions, each reflection and/or transmission being followed by propagation of the radiation in free space. At least some of the optical parts on which the reflections and/or transmissions take place, and which guide the propagation of the incident radiation, have microstructured zones which modify the incident light radiation.

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

Thus, light radiation propagating within an MPLC device undergoes a succession of local phase shifts separated by propagations. The succession of these elementary transformations establishes a global transformation of the spatial profile of the incident radiation. It is thus possible to configure the microstructuring of the microstructured reflection or transmission zones to recombine a plurality of incident beams. It is also possible to configure the microstructuring to transform a first light radiation, which in particular has a specific shape, into a second radiation whose shape is different.

The theoretical foundations and examples of practical implementation of an MPLC device can be found in the documents “Programmable unitary spatial mode manipulation”, Morizur et Al, J. Opt . Soc. Am. A/Vol. 27, No. 11/ November 2010 ; N. Fontaine et Al, (ECOC, 2017), “Design of High Order Mode-Multiplexers using Multiplane Light Conversion”; US9250454 and US2017010463. The document EP3921695, cited in the introduction, also describes in detail the implementation of an MPLC device within the framework of a system for producing an output light beam by coherent combination.

We have thus represented on the FIG. 3 an illustration of the implementation of a recombination module 5 by an MPLC device 7. This comprises a plurality of reflective optical parts 7a having a microstructured surface, here two of these parts. It also comprises a non-microstructured reflective optical part 7b. The elementary light beams FLE are projected successively, during their propagation, onto the reflective optical parts 7a, 7b, which leads to combining them in a coherent manner, or even to conforming the intermediate light beam produced according to a chosen shape.

When implementing far-field recombination, a recombination module 5 may comprise a device juxtaposing the elementary light beams FLE, similar to that implemented in the second stage 4b, the description of which follows.

So, and to return to the general description of the FIG. 1 , the second stage 4b is configured to juxtapose the intermediate light beams FLE' and thus recombine them in the far field. For this purpose, the second stage may comprise optical parts, such as mirrors, aimed at guiding the intermediate light beams to arrange them side by side, all pointing in the same direction so that they have parallel propagation directions.

The intermediate light beams FLE’ may be Gaussian in shape. However, as mentioned in a previous passage, the recombination modules 5 may be configured to shape the intermediate light beams FLE’ in order to facilitate their juxtaposition, by limiting the spaces separating them. In particular, a juxtaposition of Gaussian intermediate light beams FLE’ (i.e. not exhibiting any particular shaping), has a theoretical PIB measurement of the order of 50% to 70%, in particular because of the spaces of low light intensity which separate the juxtaposed beams.

Advantageously therefore, the recombination modules 5 are configured to conform the intermediate light beams FLE' according to a flat plate mode (called "top-hat" according to the English expression usually used). This flat plate generally has a circular shape, but it can also, within the general framework of the present description, have a rectangular, square, triangular or hexagonal shape. Advantageously, the flat plate has a polygon shape making it possible to juxtapose the beams and thus best pave a pupil of this second stage 4a. FIG. 4 thus presents a configuration of a second stage 4b taking advantage of intermediate light beams FLE' presenting a square flat plate shape, and the FIG. 4 a second stage configuration taking advantage of FLE' intermediate light beams having a hexagonal flat plate shape.

There FIG. 4 represents a particularly interesting arrangement of intermediate light beams FLE' which makes it possible to form an output light beam FLS whose general shape approaches a Gaussian beam. This shape being globally preserved (in the absence of atmospheric disturbance) during the far-field propagation of the output light beam FLS, it makes it possible to reach a target with a high energy density at the center of the beam. In this arrangement, each intermediate light beam FLE' has the shape of a flat polygonal plateau, here a square. These beams are arranged around a central elementary light beam FC. The dimension of the flat plateau of the light beams FLE' tends to increase as it moves radially away from the central beam FC. All other things being equal, the power density present in each of the intermediate light beams FLE' tends to decrease as it moves radially away from the central elementary light beam FC, which tends to give a general Gaussian shape to the output light beam FLS.

Returning to the general description of the FIG. 1 , it is optionally possible to provide a light sensor 8, connected to the servo stage 3 and more particularly to the control device 3c, making it possible to collect the light radiation reflected by a target towards which the output light beam FLS is directed. The information provided by this sensor 8 can be processed by the control device 3c in order to adjust the relative phases of the elementary light beams FLE via the plurality of phase actuators 3a, with a view to maximizing the power of the output light beam which is reflected on the target (and the reflection of which is received by the sensor 8). This approach, which leads to modifying the shape of the output light beam FLS, makes it possible in particular to compensate for the deformation of the output beam FLS which occurs during its propagation towards the target, in the presence of atmospheric disturbance. Advantageously, the light sensor 8 is positioned in the vicinity of the stage for producing the output light radiation FLS.

There FIG. 5 represents a particular embodiment of the invention. In this embodiment, we find the source 2 producing a plurality of initial light beams FLI, the phase control stage 3 of the plurality of initial beams FLI, this control stage 3 producing a plurality of elementary light beams FLE locked in phase and the optical device 4 for coherent combination with double stage 4a, 4b which has just been presented.

According to a first particular aspect of this embodiment, the servo stage 3 comprises a plurality of control devices 3c, the control devices 3c being respectively associated with the recombination modules 5 of the first stage of the optical device 4. More precisely, each control device 3c receives as input signals produced by a given recombination module 5. This may be a portion taken from the intermediate light beam FLE' produced by this module 5 or light signals indicating the phase shift existing between the elementary light beams FLE incident on this module 5, as previously explained. Each control device 3c produces control signals for the phase actuators associated with the group of beams recombined by the given recombination module 5. In this way, local phase regulation loops of the elementary light beams are formed which are specific to a given recombination module.

To enable the phase locking of all the elementary light beams FLE with each other, a plurality of complementary phase actuators 2c have been provided, here arranged in the source 2, but which could just as easily have been integrated into the servo stage 3. The production system 1 further comprises a complementary control device 3c' receiving as input signals produced by the second stage 4b of the optical device 4. The complementary control device 3c' produces control signals for the complementary phase actuators 2c.

It is not necessary for the number of complementary phase actuators 2c to correspond to the number of elementary light beams FLE. In particular, one complementary phase actuator can be provided per recombination module 5 by optically placing this plurality of complementary phase actuators between a complementary optical separator 2b' and the optical separator 2b, as shown in the FIG. 5 . In this case the complementary optical splitter 2b' produces as many light beams as recombination module 5 in the first stage 4a of the optical device, and the optical splitter 2b divides each of these beams to provide the group of elementary light beams incident to each of the recombination modules 5.

There FIG. 6 represents, by way of illustration, a practical embodiment of an optical device 4 according to the invention. 7 recombination modules 5 are here all identically formed from a hollow main body 5a in which the elementary light beams FLE propagate to be recombined in the near field. For this purpose, optical fiber networks 5a have been provided making it possible to propagate these beams on an input port arranged on the main body 5a and to inject them with great geometric precision inside this body. On one face of this main body 5a are also fixed reflective optical parts 7a having a microstructured surface, here two of these parts. On an opposite face of the main body 5a have also been fixed two non-microstructured reflective optical parts 7b. The elementary light beams FLE injected at the optical port of the recombination module 5 are successively reflected by the reflective optical parts 7a, 7b to recombine and emerge from the module 5 at an output optical port.

6 of the recombination modules 5 are angularly distributed, in a star shape, along a circumference. The last module faces them. The intermediate light beams FLE' are combined in the far field by means of the second optical stage formed here by 6 mirrors arranged in a circle and intended to receive the intermediate light beams FLE' emitted by the recombination modules 5 arranged in a star shape. The arrangement of the 6 mirrors in a circle preserves a central passage in which the intermediate light beam FLE' emitted by the recombination module 5 facing those arranged in a star shape propagates.

It is very apparent in this figure that none of the optical parts making up the optical device 4 entirely receives the output light beam FLS, which makes it possible to produce a high-power beam without risk of damaging these optical parts.

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

Claims (19)

Optical device (4) for coherent combination of a plurality of phase-locked elementary light beams (FLE), the optical device being characterized in that it comprises:

• a first stage (4a) comprising a plurality of recombination modules (5), each recombination module (5) coherently combining a group of beams of the plurality of elementary light beams (FLE) to form a plurality of intermediate light beams (FLE');
• a second stage (4b), arranged downstream of the first stage (4a), the second stage (4b) carrying out a far-field recombination of the intermediate light beams (FLE') to form an output light beam (FLS).

Optical device (4) according to the preceding claim in which the recombination modules (5) of the first stage (4a) carry out a coherent combination in the near field. Optical device (4) according to the preceding claim in which at least one recombination module (5) comprises a diffractive optical part. Optical device (4) according to one of the preceding claims in which at least one recombination module (5) comprises a multiplane conversion optical device. Optical device (4) according to one of the preceding claims in which at least one recombination module (5) comprises a photonic lantern. Optical device (4) according to claim 1 wherein the recombination modules (5) of the first stage (4a) perform coherent far-field combination. Optical device (4) according to one of the preceding claims in which at least one recombination module (5) is configured to conform an intermediate light beam (FLE’) according to a chosen shape. Optical device (4) according to the preceding claim in which the at least one recombination module (5) comprises at least one shaping optic. Optical device (4) according to one of the two preceding claims in which the chosen shape is a flat plate, the flat plate forming a polygon, such as a square, a rectangle or a hexagon. Optical device (4) according to one of claims 7 and 8 in which the chosen shape is circular, advantageously with a flat plate. System for producing (1) an output light beam (FLS) intended to illuminate a target in the far field, comprising:

• at least one source (2) producing a plurality of initial light beams (FLI);
• a phase-locking stage (3) for the plurality of initial beams (FLI), the phase-locking stage (3) producing a plurality of phase-locked elementary light beams (FLE);
• an optical coherent combining device (4) according to one of the preceding claims.

Production system (1) according to the preceding claim wherein the source (2) comprises a laser source (2a) and at least one optical separator (2b) optically arranged downstream of the laser source (2a) to form the plurality of initial light beams (FLI). Production system (1) according to one of the two preceding claims in which the source produces at least 50 initial light beams (FLI), and preferably between 100 and 1000 initial light beams (FLI). Production system (1) according to one of the three preceding claims in which the servo stage (3) comprises a plurality of phase actuators (3a) respectively coupled to a plurality of optical amplifiers (3b) to establish the plurality of phase-locked elementary light beams (FLE). Production system (1) according to claim 14 wherein the servo stage (3) comprises at least one control device (3c) for the plurality of phase actuators (3a). Production system (1) according to the preceding claim in which the servo stage comprises a plurality of control devices (3c), each control device (3c) receiving as input signals produced by a given recombination module (5) and each control device (3c) producing control signals for the phase actuators associated with the group of beams recombined by the given recombination module (5). Production system (1) according to the preceding claim in which the source (2) comprises a plurality of complementary phase actuators (2c) and the production system further comprises a complementary control device (3c') receiving as input signals produced by the second stage (4b) of the optical device (4) and producing control signals for the complementary phase actuators (2c). Production system (1) according to the preceding claim wherein the plurality of complementary phase actuators (2c) is optically arranged between a complementary optical separator (2b') and the optical separator (2b). Production system (1) according to one of claims 11 to 18 comprising a light sensor (8) configured to collect light radiation reflected by a target towards which the output light beam (FLS) is directed and connected to the servo stage (3).