Coherent beam combination system and method for controlling an output beam

Abstract

The coherent beam recombination (CBC) system (1) provides an output recombined beam (2) and comprises: a laser source (3) providing a source beam (10); a splitter (7) splitting the source beam into a plurality of secondary beams (12I,...,12N); and a propagating body (15) providing a plurality of intermediate beams (21I,...,21N) from the secondary beams (12I,...,12N), wherein the propagating body has a plurality of channels (20I,...,20N), one for each secondary beam, and each channel comprises an optical amplifier (40) and a phase modulator (42). The system further comprises an output optics (17) that receives the intermediate beams (21I,...,21N), provides the output recombined beam (2) from a first portion of each intermediate beam, and provides a sampled beam (57) from a second portion of each intermediate beam; a movable optical sensor (23) that receives the sampled beam (57) and provides a detection signal (INT) indicative of an intensity of the sampled beam; and a control unit (61) coupled to the optical sensor and the propagating body. The control unit is configured to: receive an output-beam control signal including a focusing signal (FDt) indicative of a focusing distance (fb) of the output recombined beam (2) and / or a steering signal (THt) indicative of a steering angle of the output recombined beam; control a displacement (Sx, SY, SZ) of the movable optical sensor (23) based on the output-beam control signal; and provide, based on the detection signal (INT), a plurality of phase control signals (U) to the phase modulators (42) of the plurality of channels, so as to optimize the intensity of the sampled beam.

Description

[0001]“COHERENT BEAM COMBINATION SYSTEM AND METHOD FOR CONTROLLING AN OUTPUT BEAM” Cross-Reference to Related Applications This Patent Application claims priority from European Patent Application No. 23195783.8 filed on September 6, 2023, and Italian Patent Application No. 102023000026193 filed on December 6, 2023 the entire disclosure of which is incorporated herein by reference. Technical Field of the Invention The present invention relates to a coherent beam combination (CBC) system and to a method for controlling an output beam, in particular for steering and / or changing a focusing distance of the output beam. State of the Art As is known, coherent beam combination is a technique that is used to obtain a high power laser beam from a low power laser source. A known coherent beam combination comprises a laser source that generates a primary laser beam, a splitter that splits the primary laser beam into N secondary beams, an amplifying body having N channels, one for each secondary beam, and a recombination unit that recombines the N secondary beams, thereby forming an output beam focused on a target. In fact, the possibility to obtain a high-power laser beam from the amplification of a single laser source is limited by non-linear optical effects and thermal effects. On the other hand, in a coherent beam combination system, the N channels are individually amplified and then recombined with each other. This allows to use the recombination of the amplified beams to obtain a high-power output. If the amplified beams are coherent one with the other, the amplified beams interfere with each other. In particular, it is desired that the amplified beams interfere constructively with each other. In fact, in a theoretical case, if the amplified beams are coherent with each other and have a mutual phase-shift equal to zero or a multiple of 2π, then the recombined beam has a peak intensity proportional to N2, wherein N is the number of channels of the CBC system. On the other hand, if the beams are not coherent with each other, the intensity of the recombined beam is just proportional to N. However, maintaining the amplified beams coherent with each other and phase- locked with each other require an accurate control of the phase of the amplified beams. Document WO 2023 / 194921 A1 discloses a CBC system comprising a phase- locking module that measures an intensity of the recombined beam, calculates a cost function that depends on said intensity, and performs an optimization algorithm of the cost function so as to maximise the intensity of the recombined beam. Moreover, the need is felt to control the output recombined beam, for example for steering the output recombined beam, that is changing the propagation direction of the output beam, for example for following a target. Known approaches for steering the recombined output beam are based on tip-tilt actuators that are installed on each collimating system of the beam combiner. In the known approaches, the output beam focusing is achieved by means of dedicated multi-lens optical systems or by means of variable focal length systems, e.g. a deformable mirror. These systems increase the whole system complexity, size and weight. Moreover, such systems could exhibit thermal issues at high laser power and some countermeasures (e.g. active cooling) are needed, further increasing system complexity, size and weight. Document WO 2023 / 194921 A1 discloses also a method for steering the output recombined beam. Article by MA PENGFEI et al: “7.1 kW coherent beam combining system based on a seven-channel fiber amplifier array” published in Optics and Laser Technology, Elsevier Science Publishers BV., vol.140, on 28 February 2021, discloses a CBC system that uses a Stochastic Parallel Gradient Descent (SPDG) algorithm to obtain the coherent combination of seven-channel fiber amplifiers. Article by YU C X et al: ”Coherent combining of a 4 kW, eight-elment fiber amplifier array” published in Optics Letters, Optical Society of America, US, vol.36, no. 14, pages 2686-2688, on 15 July 2011, discloses a CBC system made of commercial 0.5 kW Yb-doped fiber amplifiers. US 11735886 B2 discloses a CBC system that uses a mechanism for phase and / or polarization locking by using a reference optical beam and an array of optical detectors, wherein each detector is configured to detect overall intensity of an optical interference signal caused by interfering of the reference beam and a beam of the respective channel. However, the Applicant has verified that the performance of the known CBC systems may be improved. Subject and Summary of the Invention The aim of the present invention is to overcome the disadvantages of the prior art. The present invention relates to a coherent beam combination system, a method, a control unit and a computer program, as claimed in the appended claims. Brief Description of the Drawings Figure 1 shows a block diagram of a coherent combination system, according to an embodiment of the present invention. Figures 2 and 3 show in detail block diagrams of a beam combiner of the present CBC system, according to embodiments of the invention. Figure 4 shows a portion of the CBC system of Figure 1, in use, for steering the output recombined beam, in response to a movement of a target. Figure 5 shows a portion of the CBC system of Figure 1, in use, for changing the focusing distance of the output recombined beam, in response to a movement of the target. Figure 6 shows an example of the intensity distribution of the interference pattern formed by a recombined laser beam obtainable by the CBC system of Figure 1. Figure 7 shows a schematic top plan view of an optical sensor of the CBC system of Figure 1, in use. Figure 8 shows a flow chart of a method for controlling steering angle and focusing distance of the output recombined beam, according to an embodiment. Figure 9 shows the schematic top plan view of Figure 7, wherein the optical sensor has been displaced to achieve steering of the output recombined beam of the CBC system. Figures 10A, 11A and 12A show examples of continuous wavefronts that are needed at the output of the beam combiner of Figure 2 for achieving a specific beam steering, for a number of channels of the CBC system equal to 7, 19 and 61, respectively. Figures 10B, 11B and 12B show examples of discrete wavefronts obtainable by the CBC system of Figure 1 at the output of the beam combiner of Figure 2 for achieving the specific beam steering, for a number of channels of the CBC system equal to 7, 19 and 61, respectively. Figures 13A, 14A and 15A show examples of continuous wavefronts that are needed at the output of the beam combiner of Figure 2 for achieving a specific focusing distance of the output recombined beam, for a number of channels of the CBC system equal to 7, 19 and 61, respectively. Figures 13B, 14B and 15B show examples of discrete wavefronts obtainable by the CBC system of Figure 1 at the output of the beam combiner of Figure 2 for achieving the specific focusing distance of the output recombined beam, for a number of channels of the CBC system equal to 7, 19 and 61, respectively. Figure 16 shows a detailed flow chart of a phase-locking step of Figure 8, according to an embodiment. Figure 17 shows a detailed flow chart of an optimization algorithm of the phase- locking method of Figure 16, according to an embodiment. Detailed Description of Preferred Embodiments of the Invention The following description is provided to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the claimed invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments disclosed belongs. In the case of conflict, the present specification, including definitions, will control. In addition, the examples are illustrative only and not intended to be limiting. For the purposes of promoting understanding of the embodiments described herein, reference will be made to certain embodiments and specific language will be used to describe the same. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present disclosure. Figure 1 shows a block diagram of a coherent beam combination (CBC) system 1 configured to provide an output recombined beam 2 having a high peak intensity, for example up to tens of kW, directed onto a target T. The CBC system 1 may be used for example for space debris removal, spectroscopy and laser-shaping applications, Point-to-Point communication in air (such as air-air, ground-air, air-space, ground-space), as a counter for Unmanned Aerial System, improvised explosive devices or as a dazzling system. The CBC system 1 comprises a laser source (or master oscillator) 3 and a splitter 7, optically coupled with each other, in particular through an optical fibre. The laser source 3 generates a source beam 10 and the splitter 7 splits the source beam 10 into N secondary beams 12. In the following, the N secondary beams 12 may also be indicated individually by 12i to refer to any of the secondary beams 121,…,12N. The laser source 3 may be a fibre laser and the source beam 10 may be a substantially monochromatic laser, for example having a wavelength of 1064 nm. However, the wavelength of the source beam 10 generated by the laser source 3 may be different, depending on the specific application. According to an embodiment, the laser source 3 may be configured to provide a variable-wavelength beam. The laser source 3 may be a continuous wave laser source or a pulsed laser source, here a continuous wave laser source. The source beam 10 may have a narrow or broad linewidth, preferably a broad linewidth for example of about tens of GHz, in particular up to 50 GHz. According to an embodiment, the laser source 3 may comprise a source having a narrow linewidth, for example below 20 kHz, coupled to a beam broadener that is configured to enlarge the linewidth thereof, thereby generating the source beam 10. For example, the beam broadener may be a chirp modulator, a sinusoidal modulator or a noise modulator. For example, the beam broadener may be the beam broadener that has been described in document WO 2023 / 194921 A1 with reference to Figure 2 and indicated therein by the reference numeral 5. The CBC system 1 further comprises a main propagating body 15, optically coupled with the splitter 7, and an output optics 17, optically coupled with the main propagating body 15. The main propagating body 15 receives the secondary beams 121,…,12N and provides N intermediate beams 21, one for each secondary beam 12i, as for example shown in detail in Figure 2. In the following, the intermediate beams 21 may also be indicated individually by 21i to refer to any of the intermediate beams 211,…, 21N. In detail, the main propagating body 15 is formed by a plurality of channels 20, one for each secondary beam 12i, each receiving a respecting secondary beam 12iand providing a respective intermediate beam 21i. In the following, the channels 20 may also be indicated individually by 20i to refer to any of the channels 201,…, 20N. The output optics 17 receives the intermediate beams 21 and directs part of the intermediate beams 21 onto the target T, thereby allowing the recombination thereof and forming the recombined output beam 2, and part of the intermediate beams 21 towards an optical sensor 23 that is configured to measure the intensity of an incoming beam. The channels 201, …, 20Nof the main body 15 comprise each a respective amplifier 40 (indicated as AMP1,…, AMPn in Figure 1) and a respective phase modulator 42 (indicated as PM1,…, PMn in Figure 1) optically coupled with each other, in particular here through a respective optical fibre. Preferably, in each channel 20i, the amplifier 40 may be arranged downstream of the respective phase modulator 42, so that the phase modulators 42 may operate on lower-power beams. The phase modulators 42, for example electro-optical modulators or fibre stretchers, are configured to modify each the optical phase of the beam propagating through the respective channel 20i, based on a received phase control signal U, for example a voltage signal. In detail, the phase modulator 42 of each channel 201, …, 20N may receive each a respective phase control signal u1,…,un. According to a preferred embodiment, the phase modulators 42 are each configured to introduce a 0th-order phase offset to the respective beam 121,…,12N. In detail, the phase modulators 42 may be coupled each to a respective optic fibre extending from the splitter 7 and carrying the respective secondary beam 12i. In practice, the phase control signals u1,…,uncontrol the phase variations undergone by the secondary beams 121, …, 12N propagating through the respective phase modulators 42. The phase modulators 42 may for example be manufactured as a waveguide, by using a proton-exchanged process, in order to obtain a high stability even at high optical power. The amplifiers 40 are each configured to amplify the optical power of the beam propagating through the respective channel 201,…,20N. The amplifiers 40 each have a respective gain, for example fixed or variable, comprised, for example, between 10 and 109. Functioning of the amplifiers 40 may be controlled by a received electrical signal, here not shown, for example for controlling gain, switching on / off, etc. In this embodiment, the channels 20 provide the intermediate beams 21 through a beam combiner 45, which is an opto-mechanical device that is configured to deliver N collimated laser beams, one for each channel 20i, propagating each along its own optical axis and parallel with each other. In detail, with reference to the embodiment of Figure 2, the beam combiner 45 has a back coupling portion 45A receiving the secondary beams 121,…,12N propagating from the amplifiers 40, and a front coupling portion 45B having a plurality of apertures 46, each providing a respective intermediate beam 211,…,21N. The apertures 46 may be arranged, on the front coupling portion 45B, in a tiled- aperture configuration, in particular in a honeycomb configuration, which allows a high scalability in the number of channels. However, the apertures 46 may be arranged, on the coupling portion 45B, in a different configuration, depending on the specific application and / or on the desired filling factor. By way of example only, Figure 3 shows an example of the coupling portion 45B, in a case wherein the CBC system 1 has nineteen apertures 46 arranged in a honeycomb configuration. The apertures 46 have a circular cross section having a diameter d defining the beam waist of the intermediate beams 211,…,21N. In the honeycomb configuration, two adjacent apertures 46A, 46B are arranged at a distance l, for example measured between the centres of the two adjacent apertures 46A, 46B. Still with reference to the exemplificative configuration of Figure 3, the apertures 46 are arranged along an axis X in order to form a plurality of rows mutually spaced along an axis Y perpendicular to the axis X. In this embodiment, two rows that are adjacent along the axis Y are arranged at a distance h measured in a direction parallel to the axis Y. For example, the distance h may be measured between the centres of two apertures of two adjacent rows. The distance h may be expressed as a function of the distance l by the formula: h=l⋅√3 / 2. According to an embodiment, again with reference to Figure 2, the beam combiner 45 comprises a plurality of opto-mechanical elements 47, one for each channel 201,…,20N, which couples the respective secondary beam 121,…,12N towards the aperture 46. In other words, according to an embodiment, the channels 201,…,20Nmay further comprise each a respective aperture 46, arranged downstream of the respective amplifier 40, that provides at output the respective intermediate beam 211,…,21N. Preferably, each opto-mechanical device 47 is fed by the respective amplifier 40. In detail, each opto-mechanical element 47 may comprise a fibre connector 48 and an optical element 49, arranged at the respective aperture 46. In this configuration, the fibre connector 48 is coupled to the optical fibre extending from the output of the respective amplifier 40. The secondary beams 121,…,12Ncoming from the amplifiers 40 propagate in free space between the fibre connector 48 and the respective optical element 49. The optical element 49, for example a converging lens, collimates the respective secondary beam 121,…,12N propagating from the fibre connector 48, thereby generating the respective intermediate beam 211,…,21N. For example, the fibre connector 48 may be placed at the focal plane of the respective optical element 49. Moreover, each opto-mechanical element 47 may further comprise an intensity mask 50 arranged between the respective fibre connector 48 and the respective optical element 49. The intensity mask 50 may reduce the beam waist of the beam propagating from the fibre connector 48 and the optical element 49, so that only a portion of the secondary beam 121,…,12N, for example comprised between 95% and 100%, in particular of about 95%, is transmitted and forms the respective intermediate beam 211,…,21N, while the remaining portion, for example comprised between 5% and 0%, in particular of about 5%, of the respective secondary beam 211,…,21Nis blocked. The intensity mask 50 may allow to optimise the filling factor of the apertures 46 on the front coupling portion 45B of the beam combiner 45. Again with reference to Figure 1, in this embodiment, the output optics 17 comprises a mirror 55 and a lens 56. The mirror 55 receives the intermediate beams 21, reflects a first portion of the intermediate beams 21 towards the target T and allows transmission of a second portion of the intermediate beams 21. The mirror 55 may be a high-reflectivity mirror, for example having a reflectivity higher than 99.9%. In practice, the second portion of the intermediate beams 21 is a sample of the intermediate beams 21, indicated as sampled beam 57. The lens 56, here a positive convex lens, is a focusing element that receives the sampled beam 57 and focuses the sampled beam 57 onto the optical sensor 23. The lens 56 has an optical axis OA (shown by a dash-dot line in Figures 4 and 5) that is parallel to an axis Z of a Cartesian reference system XYZ and has a focal length f measured along the optical axis OA. The focal plane of the lens 56 lies transversal (in particular, perpendicular) to the optical axis OA of the lens 56, at a distance f from the lens 56, and is indicated in Figure 4 by a dash-dot line FP. The sampled beam 57 is therefore focused by the lens 56 on the focal plane FP. Therefore, on the focal plane FP, the sampled beam 57 is recombined thereby forming an interference pattern of the intermediate beams 211,…,21N. An example of the interference pattern is shown in Figure 6. The interference pattern of the sampled beam 57 has a main lobe ML and one or more secondary lobes SL. In other words, according to an embodiment, the sampled beam 57 may be a sampled recombined beam. The optical sensor 23 is a movable sensor that may be displaced, in use, with respect to the output optics 17. In detail, the optical sensor 23 may be movable with respect to the optical axis OA, in particular along three orthogonal axis X, Y, Z. In this embodiment, the optical sensor 23 may be moved longitudinally, that is along the optical axis OA (parallel to the axis Z), and transversally, that is along directions (parallel to axis X and Y) transversal to the optical axis OA of the lens 56. In particular, the transversal movements lie in a plane transversal to the optical axis OA, i.e., parallel to the focal plane FP. In detail, the movable optical sensor 23 may comprise a photodiode 59 mounted on a movable stage controlled by an actuator or motor 60, for example a piezoelectric actuator. For example, the motor 60 may be configured to cause a displacement of the photodiode 59 that is comprised between 10 µm and 25 mm. According to an embodiment, the motor 60 may be configured to move the photodiode 59 along one or more axis, preferably along the three orthogonal axis X, Y, Z. In practice, the optical sensor 23 may be mounted on a motorized linear stage. In greater detail, as shown in the top plan view of Figure 7, the optical sensor 23 has an active area 63, which is the sensitive area configured to measure the intensity of the impinging beam (here the sampled beam 57). Still with reference to Figure 7, the optical sensor 23 is configured to receive a beam spot 64 of the sampled beam 57 focused by the lens 56. The optical sensor 23 provides a detection signal INT that is indicative of the intensity of the impinging beam 57; in particular, indicative of the beam spot 64 of the impinging beam 57 falling within the active area 63. Therefore, the detection signal INT is also indicative of the intensity of the sampled beam 57. Moreover, by knowing the splitting characteristics of the mirror 55, the detection signal INT is also indicative of the intensity of the recombined output beam 2 directed towards the target T. Size and shape of the beam spot 64 may be modified depending on the specific application. For example, the optical sensor 23 may comprise a pinhole 62 (Figures 4 and 5) arranged between the lens 56 and the photodiode 59. The pinhole 62 has an aperture that may be chosen depending on the size of the main lobe approximately equal to the size of the main lobe of the sampled beam 57. For example, the aperture of the pinhole 62 may be comprised between 10 µm and 100 µm, in particular of about 100 µm. In practice, in this way, the photodiode 23 may measure only the intensity of the main lobe of the sampled beam 57. With reference to figure 1, the CBC system 1 further comprises a control unit 61 that is coupled to the optical sensor 23 and to the channels 201,…,20N. The control unit 61 is configured to receive the detection signal INT from the optical sensor 23, to provide a displacement signal MVT to the actuator 60 for moving the optical sensor 23, and to provide the phase control signal U to the phase modulators 42 of the channels 201,…,20N. The control unit 61 is configured, in use, to perform a method for controlling the output recombined beam 2, that is for steering and / or changing the focusing distance of the output recombined beam 2. Figure 4 shows a flowchart of a method 70 for both steering and changing the focusing distance of the output recombined beam 2. The method 70 may be started by a user of the CBC system 1. For example, at the start of the method 70, the beam spot 64 may fall completely within the active area 63 of the optical sensor 23, as shown in Figure 7. At a step 71, the control unit 61 receives a steering signal THt and a focusing signal FDt, for example from a user of the CBC system 1. The steering signal THtindicates an intended change of the current propagation direction of the output recombined beam 2. In detail, the steering signal THt is indicative of a steering angle θ of the output recombined beam 2, i.e. of the angle θ at which the output recombined beam 2 is desired to be steered, for example for following the target T if the target T has moved along a direction transversal to the current propagation direction of the output recombined beam 2 (as for example shown in Figure 4). The steering angle θ may be decomposed into an x-steering angle θx measured with respect the axis X and into a y-steering angle θymeasured with respect the axis Y. The focusing signal FDt indicates an intended change of the distance, along the propagation direction of the output recombined beam 2, at which the output recombined beam 2 is focused. In detail, the focusing signal FDtis indicative of a focusing distance fb of the output recombined beam 2 along the propagation direction; i.e., measured along the optical path of the output recombined beam 2. In other words, the focusing distance fbis indicative of a new distance along the propagation direction of the output recombined beam 2 at which it is desired to focus the output recombined beam 2, for example for following the target T if the target T has moved along the current propagation direction of the output recombined beam 2 (as schematically shown in Figure 5, wherein the current and new positions of the target T are indicated by a solid and, respectively, a dash line). In Figure 5, the new focusing distance fbof the output recombined beam 2 is indicated, by way of example, as measured from the mirror 55 to the target T. However, different definitions of the focusing distance fbmay be employed, for example it may be measured along the optical path of the output beam and starting from the beam combiner 45, depending on the specific application. At a step 72, the control unit 61 calculates a displacement of the optical sensor 23, based on the steering signal THtand the focusing signal FDt. The displacement may be determined and calibrated based on the formulas discussed in detail hereinbelow. The calculated displacement may be configured to cause a change in the intensity of the beam spot 64 falling within the active area 63 of the optical sensor 23. For steering the output recombined beam 2, the control unit 61 calculates transversal displacements SXand SYof the optical sensor 23, wherein the transversal displacements SX, SYare parallel to axis X and Y, respectively (examples of which are shown in Figures 4 and 9). For example, the transversal displacements SX, SY may cause the beam spot 64 to fall partially outside the active area 63 (as for example shown in Figure 9 for a steering angle θx). In particular, given the intended steering angle θ, the transversal displacement SX may be calculated based on the x-component θx and the transversal displacement SY may be calculated based on the y-component θy. In other words, for steering the output recombined beam 2, the control unit 61 calculates displacements that are transversal to the optical axis OA of the lens 56. In particular, the transversal displacements SX, SY lie in a plane parallel to the focal plane FP. The transversal displacements may be calculated as: SX = f / tan(θx) and SY = f / tan(θy), wherein f is the focal length of the lens 56 and θx, θy the x and y components, respectively, of the intended steering angle θ. However, the transversal displacements SX, SY may be a more general function of the respective quantities f / tan(θx) and f / tan(θy), for example including proportional or scaling factors or other functions, depending on the specific setup and calibration. For changing the focusing distance of the output recombined beam 2, the control unit 61 calculates a longitudinal displacement SZ of the optical sensor 23 along the optical axis OA of the lens 56 (Figure 5). For example, the longitudinal displacement SZ may cause a change (increase or decrease) of the portion of the main lobe MB of the sampled beam 57 that impinges on the active area 63, thereby changing the measured intensity. The longitudinal displacement SZmay be calculated as SZ= f2 / fb, wherein f is the focal length of the lens 56 and fb is the desired focusing distance indicated by the focusing signal FDt. However, the longitudinal displacements SZmay be a more general function of the quantity f2 / fb, for example including proportional or scaling factors or other functions, depending on the specific setup and calibration. At a step 73, the control unit 61 provides the displacement signal MVT to the actuator 60, which in turn moves the photodiode 59. The displacement signal MVT is indicative of the longitudinal displacement SZand the transversal displacements SX, SY. In other words, the displacement signal MVT causes a displacement of the optical sensor 23. Figure 9 shows an example wherein the optical sensor 23 has been moved by the transversal displacement SX (towards right in Figure 9) with respect to the beam spot 63, thereby causing the beam spot 64 to fall partly outside the active area 63. Accordingly, the optical sensor 23 detects a reduction in the intensity of the impinging beam 57. In other words, at steps 72 and 73, the control unit 61 causes a displacement of the optical sensor 23, based on the steering signal THt and the focusing signal FDt. At a step 74, the control unit 61 performs a phase-locking algorithm for optimizing the intensity of the sampled beam 57 impinging on the optical sensor 23, based on the detection signal INT. In other words, the control unit 61 provides the phase control signal U, based on the detection signal INT, so as to optimize the intensity of the sampled beam 57 impinging on the optical sensor 23. In particular, the control unit 61 is configured to provide the phase control signal U, based on the detection signal INT, so as to maximise the intensity of the sampled beam 57 impinging on the optical sensor 23. In practice, the control unit 61 performs a closed-loop optimization algorithm that modifies the phase-control signal U so to maximise the intensity measured by the optical sensor 23. According to an embodiment, at step 74, the control unit 61 may perform the method described in WO 2023 / 194921 A1 with reference to Figures 5 and 5A, as also discussed in detail hereinbelow with respect to Figures 16 and 17. Method 70 may end when the control unit 61 verifies that the intensity of the sampled beam 57 impinging on the optical sensor 23 has been maximised, for example following verification of an optimization or convergence condition. At the end of method 70, the output recombined beam 2 has been steered by the angle θ and has changed the focusing distance to fb. In fact, the control unit 61 is configured to provide the phase control signal U so as to maximise the intensity of the recombined sampled beam 57 impinging on the active area 63 of the optical sensor 23. If the position of the optical sensor 23 is not symmetrical with respect to the combined beam 75 (i.e., the beam spot 64 does not fall completely within the active area 63) and / or the active area 63 of the optical sensor 23 does not lie on the focal plane FP of the lens 56, then the intensity measured by the optical sensor 23 is less than the optimal (max) value. Therefore, the control unit 61 updates the phase control signal U so as to add a phase offset to the beams in order to find the optimal phase values that maximize the measured intensity. For this reason, a displacement in the Fourier plane of the lens 56 (transversal plane) is equivalent to provide a phase tilt, while a displacement along the optical axis OA of the lens 56 is equivalent to a quadratic (i.e. parabolic) phase contribution. In turn, the combined beam intensity pattern changes. Hence, the phase can be mapped into the beam intensity for a position of the optical sensor 23. By going into further mathematical details, the following may be verified. At a given position of the optical sensor 23 with respect to the optical axis OA of the lens 56, the N intermediate beams 21 have a wavefront error Δ ^^(^^, ^^)with respect to the far field that can be expressed as: Δ ^^(^^, ^^)= ^^(^^)⋅ ^^2+ ^^ℎ ^^ ^^⋅ ^^ ⋅ ^^ ^^ ^^(^^)+ ^^^^ ^^ ^^ ^^( ^^) ⋅ ^^ ⋅ ^^ ^^ ^^(^^), wherein ^^ = √ ^^2+ ^^2is the radial coordinate of the optical sensor 23 in the transversal plane (i.e. a plane parallel to focal plane FP) with respect to the optical axis OA; ^^ is the angle of the optical sensor 23 in the transversal plane, with respect to the optical axis OA; ^^( ^^) is the defocusing coefficient due to a certain longitudinal displacement ^^ (i.e., SZ) of the optical sensor 23; and ^^ℎ ^^ ^^, ^^ ^^ ^^ ^^is the tilt coefficient due to a horizontal / vertical displacement ^^ (i.e., SXand SY) in the transversal plane. The exact values of A and B can be retrieved by decomposing Δ ^^( ^^, ^^) in terms of Zernike polynomials up to 2ndorder. In order to coherently combine the N intermediate beams 21 and maximize the intensity of the beam spot 64 on the optical sensor 23, the wavefront error Δ ^^( ^^, ^^) should be removed in order to keep the relative phase difference equals to 0 or multiple of 2 ^^. Therefore, the control unit 61 provides a correction such that the equivalent phase map will be Φ = − Δ ^^, thereby removing the wavefront error. This means that it is possible to indirectly provide a specific (discrete) wavefront at the output of the beam combiner 45 and thus on the target T, in terms of 1stand 2ndphase order, without knowing the exact optical phase value of each intermediate beam 211,…,21N. It may be verified that the desired focal distance fband steering angle θ (with the respective components θx, θy) of the output recombined beam 2 may be related to the position of the optical sensor 23 by the following formulas: ^^^^= tan−1( ^^ / ^^^^), ^^^^= ^^ ^^ ^^−1( ^^ / ^^^^), ^^^^≈ ^^2 / ^^^^, from which the formulas discussed with reference to step 73 of figure 8 may be derived. The Applicant has also verified that the minimum steering angle θx,min, θy,min achievable by moving the optical sensor 23 may be limited by the minimum transversal displacements SX,min, SY,minachievable by the optical sensor 23. In other words, the minimum steering angle may be given by the resolution of the motor 60 moving the photodiode 59. Moreover, the maximum steering angle θx,max, θy,maxachievable by moving the optical sensor 23 may be calculated by the angular displacement that the optical sensor 23 may do within the envelope of each intermediate beam 211,…,21N. For example, by considering a gaussian beam, the envelope of a single beam may be ± ^^^^, wherein ^^^^is the radius, at the focal plane FP of the lens 56, of the main lobe ML as measured at1⁄ ^^2the peak intensity. For example, each of the maximum steering angles θx,max, θy,maxmay be calculated, approximately, as: ^^[ ^^ ^^] ^^^^ ^^ ^^≅ ± ^^ ^^ ^^ wherein ^^0is the beam at the lens 56 and ^^ the wavelength of the intermediate beams 211,…,21N. For example, for ^^ = 1.064 ^^ ^^ and ^^0= 2 ^^ ^^, the maximum steering angle is ± 0.675 ^^ ^^ ^^ ^^. The maximum focal distance fb,maxachievable by moving the optical sensor 23 may be limited by the minimum longitudinal displacement SZ achievable by the optical sensor 23. In other words, the maximum focal distance angle may be given by the resolution of the motor 60 moving the photodiode 59. The minimum focal distance fb,min achievable by moving the optical sensor 23 may be calculated as the focal induced in the system by moving the optical sensor 23 by a quantity equal to the Rayleigh length with respect to the focal plane FP of the lens 56, that is the longitudinal extension of the focal zone of a gaussian beam. For example, the minimum focal distance fb,min may be calculated, approximately, as: ^^2( ^^ [ ^^ ^ ] 2 ^^ ^^ ^0^ ) ^^^^^=≅ wherein ^^0is the beam 211,…,21N at the lens 56 and ^^ the wavelength of the intermediate beams 211,…,21N. For example, for ^^ = 1.064 ^^ ^^ and ^^0= 2 ^^ ^^, the minimum focal distance fb,minis approximately 0.25 m. In other words, with the CBC system 1 and the method 70: • a 1storder phase offset (i.e., tilt) may be introduced to the intermediate beams 21 by moving the optical sensor 23 in the transversal plane, perpendicular to the lens 56 optical axis OA, so as to achieve beam steering; • a 2ndorder phase offset (i.e., defocus) may be introduced to the intermediate beams 21 by moving the optical sensor 23 longitudinally along the lens 56 optical axis OA, so as to achieve a change in the focusing distance. Described in other words, the method 70 relies on sampling the wavefront needed for steering and focusing by using only the 0thorder of the optical phase of each laser source present in the array, the same phase term that is used to coherently combine all the laser beams. The present method does not need knowing the exact phase value of each intermediate beam 21ifor steering and changing the focusing distance of the recombined beam, thereby simplifying the overall system architecture. Indeed, the 1stand 2ndphase orders, needed to perform optical beam steering and focusing, are provided indirectly by properly adjusting the position of the optical sensor 23, which is also employed to close the phase locking loop, with respect to the lens 56 focusing the beam sample 57 on its active surface 63. Moreover, the correlation between the position of the optical sensor 23 and the desired wavefront can be easily calibrated, allowing to have a well-known map between the position of the optical sensor 23 with respect to the optical axis OA and the optical phase map. In practice, the CBC system 1 allows to achieve laser beam pointing (steering) and focusing only by adjusting the optical phase of each beam propagating in the channels 20, without recurring to extra and usually heavy optomechanical devices (e.g. deformable mirror) or requiring any disruptive change in the setup of the CBC system, such as using specific devices and methods like Shark-Hartmann, Heterodyne optical detection, interference methods. For example, for a lens 56 having a focal length ^^ = 0.5 ^^ , a transversal displacement Δ^^, ^^= 50 ^^ ^^ of the optical sensor 23 will determine a steering angle ^^ ^^^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^≈ = 100 ^^ ^^ ^^ ^^. steering and changing the focusing distance of the output recombined beam 2 may be employed also when the number N of channels 20 is high. In fact, the higher the number of channels, the higher the accuracy of the spatial sampling of the beams 21 wavefront. Indeed, an accurate spatial sampling allows to improve the reconstruction of the desired wavefront and produce the wanted effect on the combined beam, since the mean wavefront value may be provided to each beamlet. Therefore, the present system and method are highly scalable. By way of example only, Figures 10A and 10B show the continuous (theoretical) and, respectively, the reconstructed (discrete) wavefronts of the intermediate beams 211,…,21Nat the output of the beam combiner 45 that are needed to achieve ^^^^= 100 ^^ ^^ ^^ ^^ steering in the horizontal direction (with respect to axis X), in a case with N=7 channels. Figures 11A and 11B show the continuous and, respectively, the reconstructed wavefronts in a case with N=19; and Figures 12A and 12B show the continuous and, respectively, the reconstructed wavefronts in a case with N=61. Figures 13A and 13B show the continuous (theoretical) and, respectively, the reconstructed (discrete) wavefronts of the intermediate beams 211,…,21N at the output of the beam combiner 45 that are needed to achieve a focusing distance fb of about 10 ^^, in a case with N=7 channels. Figures 14A and 14B show the continuous and, respectively, the reconstructed wavefronts in a case with N=19; and Figures 15A and 15B show the continuous and, respectively, the reconstructed wavefronts in a case with N=61. Figure 16 shows a detailed flow chart of the step 74 of Figure 8, according to an embodiment, wherein the control unit 61 performs a phase-locking algorithm to maximise the intensity measured by the optical sensor 23. At a step 91, the control unit 61 receives the detection signal INT from the optical sensor 23. At a step 92, the control unit 61 calculates a cost function from the detection signal, wherein the cost function is a function of the intensity detected by the optical sensor 23. At a step 93, the control unit 61 performs an optimisation algorithm for maximising the intensity measured by the optical sensor 23. At a step 94, the control unit 61 provides a plurality of updated phase control signals to the phase modulators 42, based on an output of the optimization algorithm. According to an embodiment, as shown in Figure 17, the control unit 61 performs, as optimization algorithm, a method 100 based on a Stochastic Parallel Gradient Descent (SPGD) algorithm. In an initialization step 103, the control unit 61 initializes the phase values of the secondary beams 121, …, 12Nat an initial phase. In detail, the control unit 61 provides an initial phase signal U0to the phase modulators 42. For example, the initial phase signal U0 may provide the same phase signal to the modulator 42 of each channel 201,…,20N, i.e. U0= {u0, u0, …., u0, u0}. However, the control unit 61 may apply a different phase values to the phase modulators 42, depending on the specific application. The method 100 is an iterative method. Each iteration will be indicated by the index k. Moreover, in the following, the index j will be used to identify any one of the channels 201,…,20N. At each iteration k, the control unit 61 generates, step 105, a phase-perturbation vector δu(k) = {δu1, δu2, …., δuN-1, δuN} comprising a plurality of perturbation voltages δu1, δu2, …., δuN-1, δuN, one for each phase modulator 42. In detail, the perturbation voltages δu1, δu2, …, δuN-1, δuN are generated according to a Bernoulli distribution having values v1and v2wherein v2is different from v1and wherein P(δuj= v1) = p and P(δuj= v2) = 1-p. Therefore, each perturbation voltage δuj may have either the value v1 or v2 with a probability p and, respectively, 1-p. According to an embodiment, the values v1and v2have the same modulus and opposite sign, i.e. v1 = -v2. According to an embodiment, p=0.5, so that P(δuj = v1) = P(δuj = v2) = 0.5. According to an embodiment, v1= -v2and p=0.5. Then, step 107, the control unit 61 provides a phase control signal U = U(k-1) + δu(k) = {u1(k-1) +δu1, u2(k-1)+δu2, …., uN-1(k-1)+δuN-1, uN(k-1)+δuN} to the phase modulators 42. In practice, the control unit 61 sums the phase-perturbation vector δu(k) to the phase control signal U(k-1) = {u1(k-1), u2(k-1), …, uN-1(k-1), uN(k-1)} that has been determined in the previous iteration k-1. At the first iteration, i.e. for k=1, the phase-perturbation vector δu(k) is summed to the initial phase signal U0. Therefore, at step 107 the phase modulators 42 change the phase values of the secondary beams 121,…,12Npropagating in the respective channels 201,…,20N, with respect to the phase values provided in the previous iteration k-1. Since the phases of the secondary beams 121,…,12N have been changed with respect to the previous iteration k-1, also the interference pattern formed by the recombination of the intermediate beams 211,…,21N changes. Accordingly, the intensity of the main lobe measured by the optical sensor 23 changes. The optical sensor 23 measures a positive intensity I+, k, which is indicative of the intensity change in the sampled beam 57 caused by the phase control signal U(k-1) + δu(k). The control unit 61, step 109, receives the detection signal INT from the optical sensor 23. The control unit 61 calculates, step 111, a positive cost function J+,k given by I+,k / Imax, wherein Imaxis the maximum intensity that may be achieved if the intermediate beams 211,…,21Nare perfectly matched, i.e. if the mutual phase difference among the intermediate beams 211,…,21N is 0 or an integer multiple of 2π. Then, step 113, the control unit 61 provides a phase control signal Uc= U(k-1) – δu(k) = {u1(k-1)-δu1, u2(k-1)-δu2, …, uN-1(k-1)-δuN-1, uN(k-1)-δuN} to the phase modulators 42. In practice, the control unit 61 subtracts the phase-perturbation vector δu(k) from the phase control signal U(k-1) that has been determined in the previous iteration k-1. At the first iteration, i.e. for k=1, the phase-perturbation vector δu(k) is subtracted to the initial phase signal U0. Therefore, at step 113 the phase modulators 42 change the phase values of the secondary beams 121,…,12N propagating in the respective channels 201,…,20N, with respect to the phase values provided in the previous iteration k-1 and with respect to the phase provided at step 107. The optical sensor 23 measures a negative intensity I-, k, which is indicative of the intensity change in the recombined signal caused by the phase control signal U(k-1) – δu(k). The control unit 61, step 115, receives the detection signal INT from the photodiode 23. The control unit 61calculates, step 117, a negative cost function J-,k given by I-,k / Imax. Then, step 119, the control unit 61 calculates a new phase control signal U(k) by updating the phase control signal U(k-1) based on the phase-perturbation vector δu(k) and the positive and negative cost functions J+,k, J-,k. In detail, in this embodiment, the new phase control signal U(k) is calculated as U(k) = U(k-1) + δu(k) ∙ γ ∙ δJ(k), wherein γ is a gain value and δJ(k) is the difference between the positive and the negative cost functions J+,k, J-,k e.g. δJ(k) = J+,k - J-,k. The gain value γ may be chosen by a user of the CBC system 1, for example during the calibration of the CBC system 1. The control unit 61, step 121, provides the new phase control signal U(k) to the phase modulators 42. Therefore, at step 121 the phase modulators 42 update the phase of the secondary beams 121,…,12N, based on the new phase control signal U(k). The optical sensor 23 measures a corrected intensity Icorr, k, which is indicative of the change of intensity of the sampled beam 57 caused by the new phase control signal U(k). The control unit 61, step 123, receives the detection signal INT from the optical sensor 23. The control unit 61 then calculates, step 125, a corrected cost function Jcorr,kas Icorr,k / Imax. The control unit 61 verifies, step 127, a convergence condition of the optimization method 100. In detail, in this embodiment, the control unit 61 checks if the corrected cost function Jcorr,k is equal to or higher than a convergence threshold Jth, which may be for example chosen by a user during a calibration step of the CBC system 1. If the convergence condition is not verified, i.e. here if the corrected cost function Jcorr,k is lower than the convergence threshold Jth (branch N output from step 127), the control unit 61 returns to step 105 and generate a new random perturbation vector δu(k+1) for the next iteration k+1. The control unit 61 then repeats all steps from 107 to 127. On the other hand, if the convergence condition is verified, branch Y output from step 127, the control unit 61 returns to step 123 and repeats steps 123, 125 and 127. In practice, if the convergence condition is verified, the control unit 61 keeps monitoring the cost function by acquiring the detection signal INT (step 123) and by calculating the associated cost function (step 125), until the convergence condition is not verified anymore. For example, in response to the convergence condition being verified (branch Y output from step 127), the control unit 61 may immediately return to step 123 or may wait a time interval, which may be chosen depending on the specific application, before returning to step 123. In practice, the phase-locking method of Figures 16 and 17 performed by the control unit 61 may be summarized as follows: - providing a plurality of phase control signals (u1, …, uN), each to the phase modulator (42) of a respective channel (201,…,20N); - receiving the detection signal (INT) from the movable optical sensor, wherein the detection signal is indicative of an intensity of the sampled beam; - calculating a cost function (J) from the detection signal (INT), wherein the cost function is a function of the intensity of the sampled beam; - performing an optimization algorithm of the cost function, wherein the optimization algorithm is configured to maximise the intensity of the sample recombined beam; and - providing a plurality of updated phase control signals, based on a result of the optimization algorithm. Moreover, according to an embodiment, the phase-locking method may comprise: - acquiring (123) an updated detection signal; - calculating (125) an updated cost function; - verifying (127) if the updated cost function satisfies a convergence condition; - if the convergence condition is verified, acquiring a new detection signal and calculating a new updated cost function while the convergence condition is verified; and - if the convergence condition is not verified, performing a new iteration of the optimization algorithm and providing a plurality of new updated phase control signals based on a result of the new iteration. According to a further embodiment, the optimization algorithm is an iterative algorithm, in particular a Stochastic Parallel Gradient Descent algorithm, and the phase- locking method may comprise, for each iteration: - generating (105) a plurality of random dephasing signals (δu(k)), one for each channel; - providing (107) a plurality of positive phase control signals, each to a respective phase modulator, each positive phase control signal being the sum between a respective actual phase control signal (U(k-1)) determined in a previous iteration and the respective random dephasing signal; - acquiring (109) a positive detection signal; - calculating (111) a positive cost function from the positive detection signal; - providing (113) a plurality of negative phase control signals, each to a respective phase modulator, each negative phase control signal being the difference between the respective actual phase control signal and the respective random dephasing signal; - acquiring (115) a negative detection signal; - calculating (117) a negative cost function from the negative detection signal, wherein the updated phase control signals are each a function of the respective current phase control signal, the respective random dephasing signal and a difference between the positive and the negative cost function. Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. For example, the present method and system may be only for changing the focusing distance the output recombined beam, or only for steering the output recombined beam, or else for both steering and changing the focusing distance the output recombined beam. For example, the phase-locking algorithm performed by the control unit 61 at step 74 of Figure 8 may be different from what has been described for Figures 16 and 17, for example may be a hill-climbing algorithm different from the SPGD algorithm or else an optimization algorithm of different kind, for example a LOCSET algorithm. For example, the channels 20 of the CBC system 1 may comprise each also a respective optical delay line, as described in WO 2023 / 194921 A1 with reference to the elements indicated by 44. In this case, the CBC system 1 may comprise also another optical sensor, such as a CCD sensor, and the control unit 61 may be configured also to perform a method for optimising the delay lines, as described with reference to Figure 6 of WO 2023 / 194921 A1. The delay lines and the corresponding optimisation method may be useful in an initial calibration step of the CBC system 1. For example, the control unit 61 may comprise a plurality of units or modules (hardware and / or software) each configured for performing one or more of the steps of the present method. For example, the optical sensor 23 may comprise an element other than a photodiode, for example a CCD or other device, that is configured to detect the intensity of the impinging beam. The output optics 17 may comprise other elements than the mirror 55 and lens 56 for splitting and directing the intermediate beams 21, depending on the specific configuration and application. For example, the lens 56 may be a different focusing element for focusing the sampled beam 57 towards (onto) the optical sensor 23, or may be a more complex focusing system having an equivalent focal length and comprising one or more optical elements, for example may be a two-lens focusing system or a different focusing system. For example, the mirror 55 may be a different optical element configured to sample part of the incoming radiation, for example a beam splitter. For example, the output optics 17 may be the same as the focusing optics 17 of Figure 4 of WO 2023 / 194921 A1. The sample beam 57 may be split directly from the intermediate beams 211,…,21N, before or after being recombined. For example, splitting of the intermediate beams 21 may be performed by each opto-mechanical element of the beam combiner, which may also comprise a respective fibre splitter that extracts a small portion of laser power, for example below 0.1%. In this case, each intermediate beam comprises a first portion that propagates through the aperture and is focused on the target, and a second portion extracted by the fibre splitter that is focused on the optical sensor. For example, the main propagating body 15 may comprise an element for providing at output the intermediate beams 211,…,21Nother than the beam combiner 45.

Claims

CLAIMS 1. A coherent beam recombination system (1) configured to provide an output recombined beam (2), comprising: - a laser source (3) configured to provide a source beam (10); - a splitter (7) configured to split the source beam into a plurality of secondary beams (121,…,12N); - a propagating body (15) coupled to the splitter and configured to provide a plurality of intermediate beams (211,…,21N) from the secondary beams (121,…,12N), the propagating body comprising a plurality of channels (201,…,20N), one for each secondary beam, wherein each channel comprises an optical amplifier (40) and a phase modulator (42); - an output optics (17) configured to receive the intermediate beams (211,…,21N), to provide the output recombined beam (2) from a first portion of each intermediate beam, and to provide a sampled beam (57) from a second portion of each intermediate beam; - a movable optical sensor (23) configured to receive the sampled beam (57) and to provide a detection signal (INT) indicative of an intensity of the sampled beam; and - a control unit (61) coupled to the movable optical sensor and the propagating body, wherein the control unit is configured to: - receive an output-beam control signal including a focusing signal (FDt) indicative of a focusing distance (fb) of the output recombined beam (2) and / or a steering signal (THt) indicative of a steering angle of the output recombined beam; - control a displacement (SX, SY, SZ) of the movable optical sensor (23) based on the output-beam control signal; and - provide, based on the detection signal (INT), a plurality of phase control signals (U) to the phase modulators (42) of the plurality of channels, so as to optimize the intensity of the sampled beam.

2. The coherent beam recombination system according to claim 1, wherein the output optics (17) comprises a focusing system (56) for focusing the sampled beam (57) towards the movable optical sensor (23), the focusing system having an optical axis (OA) and a focal length (f), the control unit being configured to control the displacement of the movable optical sensor with respect to the focusing system.

3. The coherent beam recombination system according to the preceding claim, the control unit being configured to control a longitudinal displacement (SZ) of the movableoptical sensor (23) along the optical axis, the longitudinal displacement being a function of the focusing signal (FDt).

4. The coherent beam recombination system according to the preceding claim, wherein the longitudinal displacement (SZ) is a function of f2 / fb, wherein f is the focal length of the focusing system (56) and fb is the focusing distance indicated by the focusing signal.

5. The coherent beam recombination system according to any of claims 2-4, the control unit (61) being configured to control a transversal displacement (SY, SX) of the movable optical sensor (23) along a direction (Y, X) that is transversal to the optical axis (OA), the transversal displacement being a function of the steering signal (THt).

6. The coherent beam recombination system according to the preceding claim, wherein the transversal displacement (SY, SX) is a function of f / tan(θ), wherein f is the focal length of the focusing system (56) and θ is a steering angle indicated by the steering signal (THt).

7. The coherent beam recombination system according to any of the preceding claims, wherein the output-beam control signal includes the focusing signal (FDt), or the focusing signal (FDt) and the steering signal (THt).

8. The coherent beam recombination system according to any of the preceding claims, wherein for optimizing the intensity of the sampled beam, the control unit is configured to: - provide a plurality of first phase control signals, each to the phase modulator (42) of a respective channel (201,…,20N); - receive the detection signal (INT) from the optical sensor; - calculate a cost function (J) from the detection signal (INT), the cost function being a function of the intensity of the sampled beam; - perform an optimization algorithm of the cost function, the optimization algorithm being configured to maximise the intensity of the sample recombined beam; and - provide a plurality of updated phase control signals, based on a result of the optimization algorithm.

9. The coherent beam recombination system according to any of the preceding claims, wherein the control unit is configured to perform a Stochastic Parallel Gradient Descent (SPDG) algorithm for optimising the intensity of the sampled beam.

10. The coherent beam recombination system according to any of the preceding claims, wherein each channel (201,…,20N) further comprises an opto-mechanical element(47) and an aperture (46) providing the respective intermediate beam (211,…,21N).

11. The coherent beam recombination system according to any of the preceding claims, wherein the sampled beam (57) forms an interference pattern having a main lobe (ML), the movable optical sensor comprising a pinhole (62) having a size equal to or smaller than a size of the main lobe.

12. The coherent beam recombination system according to any of the preceding claims, the movable optical sensor having an active area (63) configured to receive a spot (64) of the sampled beam (57), the control unit being configured to control the displacement of the movable optical sensor based on the output-beam control signal, so as to change the intensity of the spot impinging on the active area.

13. A method for controlling an output recombined beam (2) of a coherent beam recombination system (1), comprising: - providing, by a laser source (3), a source beam (10); - splitting the source beam into a plurality of secondary beams (121,…,12N); - providing a plurality of intermediate beams (211,…,21N) from the secondary beams (121,…,12N), comprising adjusting the phase of each secondary beam by a respective phase modulator (42) and amplifying each secondary beam by a respective optical amplifier (40); - by an output optics (17), providing the output recombined beam (2) from a first portion of each intermediate beam and providing, to a movable optical sensor, a sampled beam (57) from a second portion of each intermediate beam; - providing, by the movable optical sensor (23), a detection signal (INT) indicative of an intensity of the sampled beam, the method further comprising, by a control unit (61): - receiving an output-beam control signal including a focusing signal (FDt) indicative of a focusing distance (fb) of the output recombined beam (2) and / or a steering signal (THt) indicative of a steering angle of the output recombined beam; - controlling a displacement (SX, SY, SZ) of the movable optical sensor (23) based on the output-beam control signal; and - providing, based on the detection signal (INT), a plurality of phase control signals (U) to the phase modulators (42) of the plurality of channels, so as to optimize the intensity of the sampled beam.

14. A control unit (61) couplable to a coherent beam recombination system that is configured to provide an output recombined beam (2) and comprises: - a laser source (3) configured to provide a source beam (10);- a splitter (7) configured to split the source beam into a plurality of secondary beams (121,…,12N); - a propagating body (15) coupled to the splitter and configured to provide a plurality of intermediate beams (211,…,21N) from the secondary beams (121,…,12N), the propagating body comprising a plurality of channels (201,…,20N), one for each secondary beam, wherein each channel comprises an optical amplifier (40) and a phase modulator (42); - an output optics (17) configured to receive the intermediate beams (211,…,21N), to provide the output recombined beam (2) from a first portion of each intermediate beam, and to provide a sampled beam (57) from a second portion (63) of each intermediate beam; and - a movable optical sensor (23) configured to receive the sampled beam (57) and to provide a detection signal (INT) indicative of an intensity of the sampled beam, the control unit being configured to, when coupled to the coherent beam combination system: - receive an output-beam control signal including a focusing signal (FDt) indicative of a focusing distance (fb) of the output recombined beam (2) and / or a steering signal (THt) indicative of a steering angle of the output recombined beam; - control a displacement (SX, SY, SZ) of the movable optical sensor (23) based on the output-beam control signal; and - provide, based on the detection signal (INT), a plurality of phase control signals (U) to the phase modulators (42) of the plurality of channels, so as to optimize the intensity of the sampled beam.

15. A computer program comprising instructions that, when executed by a control unit according to claim 14, cause the control unit to: - receive an output-beam control signal including a focusing signal (FDt) indicative of a focusing distance (fb) of the output recombined beam (2) and / or a steering signal (THt) indicative of a steering angle of the output recombined beam; - control a displacement (SX, SY, SZ) of the movable optical sensor (23) based on the output-beam control signal; and - provide, based on the detection signal (INT), a plurality of phase control signals (U) to the phase modulators (42) of the plurality of channels, so as to optimize the intensity of the sampled beam.

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