Coherent beam synthesis system and control method

Abstract

The coherent beam recombination (CBC) system (1) includes a laser source (3) providing a source beam (8) with a linewidth that provides a recombined output beam (2), a beam expander (5) providing an expanded beam (10) from the source beam, a splitter (7) splitting the expanded beam into a number of secondary beams (12A-12D), and a number of channels (20A-20D) coupled to the splitter. Each channel receives a respective secondary beam (12A-12D) and provides a respective intermediate beam (21A-21D). Each channel includes an optical amplifier (40), a phase modulator (42), an optical delay line (44), and an opto-mechanical element (47). The CBC system further comprises an optical sensor (23, 24) configured to provide detection signals (INT, IMG) representative of the intensities of the received optical beams, and focusing optics (17) configured to receive the intermediate beams (21A-21D), provide a recombined output beam (2) from a first portion of each intermediate beam, and provide a sampled recombined beam (63) from a second portion (63) of each intermediate beam to the optical sensor. The CBC system further comprises a control unit (30) coupled to the optical sensor and the plurality of channels. The control unit comprises a phase locking module (26) configured to provide a plurality of phase control signals (u1, u2, u3, u4) to the phase modulator (42), receive detection signals (INT) from the optical sensor representative of the intensities of the sampled recombined beams, calculate a cost function (J) from the detection signals (INT), perform an optimization algorithm of the cost function, the cost function being a function of the intensities of the sampled recombined beams, the optimization algorithm being configured to maximize the intensities of the sampled recombined beams, and provide a plurality of updated phase control signals based on the results of the optimization algorithm.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This patent application claims priority to European Patent Application No. 22425015.9, filed April 5, 2022, and Italian Patent Application No. 102022000013186, filed June 22, 2022, the disclosures of which are incorporated herein by reference in their entireties.

[0002] The present invention relates to a coherent beam combining (CBC) system and a control method thereof. [Background technology]

[0003] As is well known, coherent beam combining is a technique used to obtain high power laser beams from low power laser sources.

[0004] A known coherent beam combining system comprises a laser source generating a primary laser beam, a splitter splitting the primary laser beam into N secondary beams, an amplifier having N channels, one for each secondary beam, and a recombination unit recombining the N secondary beams to form an output beam focused on a target.

[0005] In fact, the possibility of obtaining high power laser beams from the amplification of a single laser source is limited by nonlinear optical and thermal effects.

[0006] On the other hand, in a coherent beam combining system, the N channels are amplified separately and then recombined with each other.

[0007] Recombination of the amplified beams can then be used to obtain high power outputs.

[0008] If the amplified beams are mutually coherent, then they will interfere with each other, and it is particularly desirable for the amplified beams to constructively interfere with each other.

[0009] In fact, in the theoretical case, if the amplified beams are mutually coherent and have a mutual phase shift equal to zero or a multiple of 2π, the recombined beam will have a size of N 2 where N is the number of channels in the CBC system.

[0010] On the other hand, if the beams are not mutually coherent, the intensity of the recombined beam is simply proportional to N.

[0011] However, maintaining the amplified beams coherent and phase-locked with respect to one another requires precise control of the phase of the amplified beams.

[0012] Applicants have determined that known CBC systems have low efficiency with respect to the theoretical case. Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention is to overcome the drawbacks of the prior art. [Means for solving the problem]

[0014] The present invention relates to a coherent beam combining system and a control method thereof as set forth in the accompanying claims. [Brief description of the drawings]

[0015] [Figure 1] 1 shows a block diagram of a coherent combining system according to one embodiment of the present invention. [Diagram 2] 1 illustrates a detailed block diagram of a portion of the present CBC system according to one embodiment. [Diagram 3] 1 shows a detailed block diagram of different parts of the present CBC system according to one embodiment. [Figure 4] 2 shows a detailed block diagram of another portion of the CBC system of FIG. 1 according to one embodiment. [Diagram 5]2 shows a flow chart of a phase locking method for controlling the CBC system of FIG. [Figure 5A] 6 shows a flowchart of an optimization algorithm for the phase-locking method of FIG. 5 according to one embodiment. [Figure 5B] 2 shows a schematic top view of the intensity sensor of the CBC system of FIG. 1 in use, in two different conditions of use. [Figure 5C] 2 shows a schematic top view of the intensity sensor of the CBC system of FIG. 1 in use, in two different conditions of use. [Figure 6] 2 shows a flow chart of a delay equalization method for controlling the CBC system of FIG. [Figure 7] 2 shows an example of the intensity distribution of the recombined laser beam obtained by the CBC system of FIG. 1. [Figure 8] 2 illustrates a processor module of the CBC system of FIG. 1 according to one embodiment. [Figure 9] 2 shows a flowchart of a simulation method for controlling the CBC system of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The following description is presented to enable any person skilled in the art to make and use the present invention. Various modifications of the embodiments that do not depart from the scope of the invention as set forth in the claims will be readily apparent to those skilled in the art. 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.

[0017] 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 disclosed embodiments belong. In case of conflict, the present specification, including definitions, will control. In addition, the examples are merely illustrative and are not intended to be limiting.

[0018] For purposes of facilitating an 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 certain embodiments only and is not intended to limit the scope of the present disclosure.

[0019] FIG. 1 shows a block diagram of a coherent beam combining (CBC) system 1 configured to provide a recombined output beam 2 directed at a target T with a high peak intensity, for example up to tens of kW.

[0020] The CBC system 1 may be used, for example, for space debris removal, spectroscopy and laser shaping applications, for point-to-point communications in the air (air-to-air, ground-to-air, airspace, ground space, etc.), as an interceptor for unmanned aerial systems and improvised explosive systems, or as a dazzling system.

[0021] The CBC system 1 comprises, inter alia, a laser source 3, a beam expander 5 and a splitter 7 which are optically coupled to each other through optical fibers.

[0022] A laser source 3 generates a primary laser beam 8 having a narrow linewidth, for example below 20 kHz, a beam expander 5 generates an expanded beam 10 from the primary laser beam 8, and a splitter 7 splits the expanded beam 10 into N secondary beams, of which first, second, third and fourth secondary beams 12A, 12B, 12C and 12D are shown.

[0023] The CBC system 1 further comprises a main section 15 optically coupled to the splitter 7 and a focusing optic 17 optically coupled to the main section 15 .

[0024] The main section 15 comprises a number of channels, one for each secondary beam. In particular, and with reference to Figure 1, the main section 15 has first, second, third and fourth channels 20A, 20B, 20C, 20D, each receiving a respective secondary beam 12A, 12B, 12C, 12D and providing a respective intermediate beam 21A, 21B, 21C, 21D.

[0025] The focusing optics 17 are configured to receive the intermediate beams 21A-21D and recombine the intermediate beams 21A-21D to generate a recombined output beam 2 that is directed towards the target T.

[0026] Focusing optics 17 , discussed in detail below with respect to FIG. 4, direct portions of intermediate beams 21 A- 21 D to an intensity sensor, here a photodiode 23 , and to an image sensor, here a CCD camera 24 .

[0027] In this embodiment, the photodiode 23 is coupled to a motor 25, e.g. a piezoelectric actuator, configured to move the photodiode 23 along one or more axes, in particular here along three orthogonal axes X, Y and Z.

[0028] The CBC system 1 further comprises a control unit 26 which includes a phase locking unit or module 30 and a delay compensation unit or module 31 .

[0029] In this embodiment, the laser source 3 is a fiber laser, in particular having low intensity noise and high beam quality, e.g. M 2 It is a single-mode DFB fiber laser with a factor |

[0030] The laser source 3 is substantially a monochromatic laser, for example having a wavelength of 1064 nm. However, the wavelength of the primary beam 8 generated by the laser source 3 may vary depending on the particular application.

[0031] According to an embodiment, the laser source 3 may be configured to provide a primary beam 8 of variable wavelength.

[0032] The laser source 3 can be a continuous wave laser source or a pulsed laser source, here a continuous wave laser source.

[0033] A beam expander 5, for example a chirp, sinusoidal or noise modulator, expands the linewidth of the primary beam 8. For example, the expanded beam 10 may have a linewidth of about several tens of GHz, in particular up to 50 GHz.

[0034] FIG. 2 shows an rf input signal RF coupled between a laser source 3 and a splitter 7. in 1 shows an embodiment of a beam expander 5, here a noise modulation expander, comprising a phase modulator 35, in particular an electro-optic modulator, driven by

[0035] In detail, the beam expander 5 comprises a noise generator 36 for generating a noise signal NS, e.g. a noise waveform or a pseudorandom binary sequence (PRBS), an rf amplifier 37 for amplifying the noise signal NS, and an rf input signal RF in and a low pass filter 38 which generates

[0036] The low pass filter 38 filters the rf input signal RF in , and therefore the optical bandwidth modification of the primary beam 8.

[0037] According to one embodiment, the low pass filter 38 filters the rf input signal RF depending on the particular application. in A signal shaping module is also included that changes the spectral shape of the

[0038] The beam expander 5 receives an rf input signal RF in The rf output signal RF generated by the phase modulator 35 based on out The termination load 39 is for impedance matching and for coupling the rf input signal RF to the phase modulator 35. in The device can be used as a heat sink to dissipate heat generated by the device.

[0039] Each of the channels 20A-20D of the main portion 15 specifically comprises an amplifier 40, a phase modulator 42, an optical delay line 44 and an aperture combiner 46 optically coupled to one another, here through respective optical fibers.

[0040] The amplifier 40 of each channel 20A-20D is coupled to a respective optical fiber extending from the splitter 7 and carrying a respective secondary beam 12A-12D. The amplifier 40 of each channel 20A-20D amplifies a respective secondary beam 12A-12D.

[0041] Each of the amplifiers 40 may be, for example, 10 and 10 9 , and each of the gains may be, for example, constant or variable.

[0042] The control unit 26 , and here in particular the delay equalization unit 31 , may provide a signal S to the amplifier 40 that controls one or more parameters of the amplifier 40 .

[0043] In particular, signal S includes a number of beam control signals s1, s2, s3, s4, one for each amplifier 40.

[0044] For example, each of the beam control signals s1, s2, s3, and s4 may control the gain of the amplifier 40 in a respective channel 20A-20D.

[0045] According to one embodiment, each of the beam control signals s1, s2, s3, s4 may command the activation and deactivation of a respective channel 20A-20D by commanding the switching on and off of a respective amplifier 40. The phase modulators 42, each formed for example by an electro-optic modulator or a fiber stretcher, receive a phase control signal U, for example a voltage signal, from the phase locking unit 30.

[0046] In particular, the phase modulators of each channel 20A-20D receive a respective phase control signal u 1 , u 2 , u 3 , u 4 , eg, a voltage signal, from the phase lock unit 30 .

[0047] The phase control signals u 1 , u 2 , u 3 , u 4 control the phase change experienced by the secondary beams 12 A- 12 D as they propagate through their respective phase modulators 42 after being amplified by their respective amplifiers 40 .

[0048] To obtain high stability even at high optical powers, the phase modulator 42 can be fabricated, for example, as a waveguide, by using a photon exchange process.

[0049] The optical delay lines 44 are variable delay lines that set the optical path length of each of the channels 20A-20D and receive a delay control signal D from the delay equalization unit 31. In particular, each of the optical delay lines 44 receives a respective delay control signal d1, d2, d3, d4 from the delay equalization unit 31.

[0050] In effect, each of the delay control signals d1, d2, d3, d4 may adjust the physical length of the respective optical delay line 44 and / or may vary the refractive index of the respective optical delay line 44.

[0051] For example, the optical delay line 44 can be a fiber stretcher or a folded delay line.

[0052] According to one embodiment, each of the optical delay lines 44 may comprise a fiber input coupled to a respective phase modulator 42, a fiber output coupled to the beam combiner 45, and a movable opto-mechanical element, such as a retroreflector, disposed between the fiber input and the fiber output to reflect the respective secondary beams 12A-12D from the fiber input to the fiber output. By moving the movable retroreflector, it is possible to vary the path length traveled by each secondary beam 12A-12D, and therefore the optical path length of each channel 20A-20D.

[0053] For example, where the optical delay line 44 comprises a moveable opto-mechanical element, the delay control signals d1, d2, d3, d4 may control an actuator, such as a piezoelectric actuator, configured to move the moveable opto-mechanical element.

[0054] The beam combiner 45 has a rear combining section 45A that receives the secondary beams 12A-12D propagating from the optical delay line 44, and a forward combining section 45B having a plurality of apertures 46 that each provide a respective intermediate beam 21A-21D.

[0055] The apertures 46 are arranged in the front joining portion 45B in a tiled aperture configuration, particularly a honeycomb configuration, that allows high scalability in terms of the number of channels.

[0056] However, depending on the particular application and / or desired fill factor, the openings 46 may be arranged in different configurations in the mating portion 45B.

[0057] By way of example only, FIG. 3 shows an example of a coupling portion 45 where the CBC system 1 has 19 openings 46 arranged in a honeycomb configuration.

[0058] Aperture 46 has a circular cross-section with a diameter d that defines the beam waists of intermediate beams 21A-21D.

[0059] In the honeycomb configuration, two adjacent openings 46A, 46B are disposed at a distance 1, for example measured between the centers of the two adjacent openings 46A, 46B.

[0060] Still referring to the exemplary configuration of FIG. 3, the openings 46 are arranged on an X-axis to form a plurality of rows spaced apart from one another on a Y-axis perpendicular to the X-axis.

[0061] In this embodiment, two adjacent rows on the Y axis are spaced a distance h measured in a direction parallel to the Y axis. For example, the distance h may be measured between the centers of two apertures of two adjacent rows.

[0062] The distance h can be expressed as a function of distance by the formula: h=l·√3 / 2.

[0063] Referring again to FIG. 1, the beam combiner 45 comprises a plurality of optomechanical elements 47, one for each channel 20A-20D, that combine the respective secondary beams 12A-12D propagating from the delay line 44 to an aperture 46.

[0064] In particular, each opto-mechanical element 47 includes a fiber connector 48 and an optical element 49 disposed in a respective aperture 46 .

[0065] Fiber connector 48 is coupled to the optical fiber extending from optical delay line 44. Secondary beams 12A-12D entering optical delay line 44 propagate in free space between fiber connector 48 and respective optical elements 49.

[0066] An optical element 49, such as a converging lens, generates respective intermediate beams 21A-21D by collimating respective secondary beams 12A-12D propagating from fiber connector 48.

[0067] For example, the fiber connector 48 may be placed in the focal plane of each optical element 49 .

[0068] Also, in this embodiment, each opto-mechanical element 47 further comprises an intensity mask 50 disposed between the respective fiber connector 48 and the respective optical element 49 .

[0069] The intensity mask 50 can reduce the beam waist of the beam propagating from the fiber connector 48 and the optical element 49 so that only a portion of the secondary beams 21A-21D, for example between 95% and 100%, in particular about 95%, is transmitted to form the respective intermediate beams 21A-21D, while the remaining portion of each secondary beam 21A-21D, for example between 5% and 0%, in particular about 5%, is blocked.

[0070] The intensity mask 50 may optimize the fill factor of the aperture 46 in the pre-combining portion 45 B of the beam combiner 45 .

[0071] FIG. 4 shows a CBC system, in which a detailed embodiment of the focusing optics 17 is illustrated.

[0072] The focusing optics 17 defines a primary optical path 53 that directs a first portion of the intermediate beams 21A-21D towards the target T, and a secondary optical path 54 that directs a second portion of the intermediate beams 21A-21D towards the photodiode 23 and the CCD camera 24.

[0073] In detail, the primary optical path 53 of the focusing optics 17 forms a two-lens optical system comprising a convex lens 55 having a focal length f1 and positioned in front of the forward coupling surface 45B of the beam combiner 45, and a concave lens 56 having a focal length f2 and optically coupled to the first lens 55.

[0074] The first and second lenses 55, 56 enable the intermediate beams 21A-21D to be recombined with one another to form a recombined beam 75.

[0075] The recombined beam 75 is generated by interference of the intermediate beams 21A-21D, and therefore the wavefront of the recombined beam 75 forms an interference pattern (eg, as shown in FIG. 7) having a main lobe and one or more secondary lobes.

[0076] Also, the position of concave lens 56 on primary optical path 53 can be altered in use so that the two-lens system formed by convex lens 55 and concave lens 56 has a variable focal length.

[0077] For example, the focal length of the two-lens system may be changed depending on the distance from the CBC system 1 to the target T.

[0078] For example, concave lens 56 may be coupled to a DC actuator, not shown here, configured to move concave lens 56 to decrease or increase the distance between convex lens 55 and concave lens 56 .

[0079] In this embodiment, the focusing optics 17 further includes a first mirror 58 positioned in the primary optical path 53 between the first lens 55 and the second lens 56, and a second mirror 59 positioned in the primary optical path 53 between the concave lens 56 and the target T.

[0080] In practice, the first and second mirrors 58, 59 are arranged such that the primary optical path 53 is folded, thereby reducing the footprint of the CBC system 1.

[0081] The focusing optics 17 also comprises a beam splitter 60 disposed on the primary optical path 53 , specifically here between the concave lens 56 and the second mirror 59 .

[0082] Beam splitter 60 splits recombined beam 75 propagating from concave lens 56 to form a sample beam 63 that propagates along secondary optical path 54 .

[0083] Beam splitter 60 samples a small portion of recombined beam 75, for example 1% or less, depending on the power of recombined beam 75 and the maximum optical power maintained by photodiode 23 and CCD camera 24.

[0084] In particular, the secondary optical path 54 includes a beam splitter 65 , a CCD lens 66 , a mirror 67 and a photodiode lens 68 .

[0085] A beam splitter 65 further splits the sample beam 63 so that a first portion is focused by a CCD lens 66 of the CCD camera 24 and a second portion is directed by a mirror 67 to the photodiode 23 and focused thereon by a photodiode lens 68 .

[0086] CCD lens 66 and photodiode lens 68 may be selected depending on the desired size of the main lobe of the portion of recombined beam 75 that is directed to CCD camera 24 and photodiode 24, respectively. For example, CCD lens 66 and photodiode lens 68 may be selected such that the size of the main lobe of recombined beam 75 is equal to or smaller than the active areas of photodiode 23 and CCD camera 24, respectively.

[0087] In this embodiment, the secondary optical path 54 also includes a pinhole 71 disposed between the photodiode lens 68 and the photodiode 23. The pinhole 71 has an aperture approximately equal to the size of the main lobe of the beam propagating from the photodiode lens 68. For example, the aperture of the pinhole 71 may be comprised between 10 μm and 100 μm, and in particular about 100 μm.

[0088] In reality, only the intensity of the main lobe of the beam propagating from the photodiode lens 68 can be measured by the photodiode 23 .

[0089] Photodiode 23 provides an intensity signal INT representative of the intensity of recombined beam 75, and in this case the main lobe of recombined beam 75. In fact, by knowing the splitting characteristics of first and second beam splitters 60, 65, intensity signal INT can be used to obtain the intensity of the main lobe of recombined beam 75.

[0090] In use, the phase-locked unit 30 receives the intensity signal INT from the photodiode 23 and provides phase control signals U={u1, u2, u3, u4} to the phase modulators 42 of the channels 20A-20D.

[0091] The phase lock unit 30 implements a closed-loop optimization algorithm that varies the phase control signal U to maximize the intensity measured by the photodiode 23 .

[0092] FIG. 5 shows a flow chart of a method 90 performed by phase-locking module 30 to maximize the intensity measured by photodiode 23.

[0093] In step 91 , the phase-lock module 30 receives a detection signal, here the intensity signal INT from the photodiode 23 .

[0094] In step 92 , the phase-locked module 30 calculates a cost function from the detected signal, where the cost function is a function of the intensity detected by the photodiode 23 .

[0095] In step 93 , the phase-locking module 30 performs an optimization algorithm that is configured to maximize the intensity measured by the photodiode 23 .

[0096] At step 94, the phase locking module 30 provides a number of updated phase control signals to the phase modulator 42 based on the output of the optimization algorithm.

[0097] According to one embodiment, as shown in FIG. 5A, the phase-locking unit 30 performs a method 100 based on the Stochastic Parallel Gradient Descent (SPGD) algorithm as the optimization algorithm.

[0098] In the initialization step 103, the phase-locking unit 30 initializes the phase values ​​of the secondary beams 12A-12D in an initial stage. In detail, the phase-locking unit 30 provides an initial stage phase signal U0 to the phase modulator 42. For example, the initial stage signal U0 may provide the same phase signal, i.e., U0={u0,u0,u0,u0}, to the modulator 42 of each channel 20A-20D.

[0099] However, the phase locking unit 30 may apply different phase values ​​to the phase modulator 42 depending on the particular application.

[0100] The method 100 is an iterative method, each iteration being represented by an index k, and in the following, the index j is used to identify any one of the channels 20A-20D.

[0101] In each iteration k, the phase-locking unit 30 generates, in step 105, a phase perturbation vector δu(k)={δu1, δu2, δu3, δu4} that contains a number of perturbation voltages δu1, δu2, δu3, δu4, one for each phase modulator 42.

[0102] In detail, the perturbation voltages δu1, δu2, δu3, δu4 are generated according to a Bernoulli distribution with values ​​v1 and v2, where v2 is different from v1, and P(δu j = v1) = p and P(δu j =v2)=1-p.

[0103] Therefore, each perturbation voltage δu j can have either value v1 or v2 with probabilities p and 1-p, respectively.

[0104] According to one embodiment, the values ​​v1 and v2 have the same coefficient but opposite signs, i.e. v1=-v2.

[0105] According to one embodiment, p=0.5, so P(δu j =v1)=P(δu j =v2) = 0.5.

[0106] According to one embodiment, v1=-v2 and p=0.5.

[0107] Then, in step 107, the phase-locked unit 30 provides a phase control signal U=U(k-1)+δu(k)={u1(k-1)+δu1,u2(k-1)+δu2,u3(k-1)+δu3,u4(k-1)+δu4} to the phase modulator 42.

[0108] In fact, the phase locking unit 30 sums the phase perturbation vector δu(k) to the phase control signal U(k-1)={u1(k-1),u2(k-1),u3(k-1),u4(k-1)} determined in the previous iteration k-1.

[0109] In the first iteration, ie, for k=1, the phase perturbation vector δu(k) is summed to the initial phase signal U0.

[0110] Therefore, in step 107, the phase modulator 42 varies the phase values ​​of the secondary beams 12A-12D propagating in the respective channels 20A-20D with respect to the phase values ​​provided in the previous iteration k-1.

[0111] Because the phases of the secondary beams 12A-12D have changed with respect to the last iteration k-1, the interference pattern formed by the recombination of the intermediate beams 21A-21D also changes, and therefore the intensity of the main lobe measured by the photodiode 23 changes.

[0112] Photodiode 23 detects a positive intensity I +,k Measure.

[0113] The phase-lock unit 30 receives the intensity signal INT from the photodiode 23 in step 109 .

[0114] In step 111, the phase lock unit 30 +,k / I max A positive cost function J given by +,k Calculate I max is the maximum intensity that can be achieved when the intermediate beams 21A-21D are perfectly aligned, i.e. when the mutual phase difference between the intermediate beams 21A-21D is 0 or an integer multiple of 2π.

[0115] Then, in step 113, the phase lock unit 30 outputs the phase control signal U c=U(k-1)-δu(k)={u1(k-1)-δu1, u2(k-1)-δu2, u3(k-1)-δu3, u4(k-1)-δu4} is provided to the phase modulator 42.

[0116] In effect, the phase locking unit 30 subtracts a phase perturbation vector δu(k) from the phase control signal U(k−1) determined in the previous iteration k−1.

[0117] In the first iteration, ie, for k=1, a phase perturbation vector δu(k) is subtracted from the initial phase signal U0.

[0118] Therefore, in step 113, the phase modulator 42 changes the phase values ​​of the secondary beams 12A-12D propagating in each channel 20A-20D relative to the phase values ​​provided in the previous iteration k-1 and relative to the phase provided in step 107.

[0119] The photodiode 23 detects a negative intensity I -,k Measure.

[0120] The phase-lock unit 30 receives the intensity signal INT from the photodiode 23 in step 115 .

[0121] The phase lock unit 30, in step 117, -,k / I max The negative cost function J obtained by -,k Calculate.

[0122] Then, in step 119, the phase locking unit 30 calculates the phase perturbation vector δu(k) and the positive and negative cost functions J +,k ,J -,k A new phase control signal U(k) is calculated by updating the phase control signal U(k-1) based on

[0123] Specifically, in this embodiment, the new phase control signal U(k) is calculated as U(k)=U(k-1)+δu(k)·γ·δJ(k), where γ is the gain value and δJ(k) is the positive and negative cost function J +,k ,J -,k For example, δJ(k)=J +,k -J -,k It is.

[0124] The gain value γ may be chosen by a user of the CBC system 1, for example during calibration of the CBC system 1.

[0125] The phase locking unit 30 provides a new phase control signal U(k) to the phase modulator 42 in step 121 .

[0126] Therefore, in step 121, phase modulator 42 updates the phase of secondary beams 12A-12D based on the new phase control signal U(k).

[0127] Photodiode 23 detects a corrected intensity I(k) which represents the intensity change of recombined signal 75 caused by the new phase control signal U(k). corr,k Measure.

[0128] The phase-lock unit 30 receives the intensity signal INT from the photodiode 23 in step 123 .

[0129] Then, in step 125, the phase lock unit 30 corr,k / I max As the corrected cost function J corr,k Calculate.

[0130] The phase-locking unit 30 checks the convergence conditions of the optimization method 100 in step 127 .

[0131] In detail, in this embodiment, the phase locking unit calculates a correction cost function J corr,k is a convergence threshold J, which may be selected, for example, by a user during a calibration step of the CBC system 1. th Check if it is equal to or higher than.

[0132] If the convergence condition is not confirmed, i.e., the corrected cost function J corr,k is the convergence threshold J th If it is lower (branch N output from step 127), the phase-locked unit 30 returns to step 105 and generates a new random perturbation vector δu(k+1) for the next iteration k+1.

[0133] Then the phase lock unit 30 repeats all steps from 107 to 127.

[0134] On the other hand, if the convergence condition is verified, the phase-locked unit 30 returns to step 123 with a branch Y output from step 127 and repeats steps 123, 125 and 127.

[0135] Indeed, when the convergence condition is confirmed, the phase locking unit 30 continues to monitor the cost function by acquiring the intensity signal INT (step 123) and by calculating the associated cost function (step 125) until the convergence condition is no longer confirmed.

[0136] For example, upon confirmation of the convergence condition (branch Y output from step 127), phase-locked unit 30 may immediately return to step 123 or may wait a time interval that may be selected depending on the particular application before returning to step 123.

[0137] During use, the phases of the secondary beams 12A-12D may undergo undesired changes caused by external factors, for example temperature drift may change the length of the optical fiber through which the secondary beams 12A-12D propagate, causing undesired phase shifts between the secondary beams 12A-12D, which may degrade their mutual phase locking.

[0138] The method 100 can adjust the phases of the secondary beams 12A-12D in a closed loop so that the intensity of the main lobe of the recombined beam 75 is kept at a maximum value.

[0139] Applicants have also determined that method 100 can reduce power noise in recombined beam 75 caused by optical phase perturbations, particularly in the frequency range of the perturbations comprised between 0 Hz and 2000 Hz.

[0140] Furthermore, due to the fact that the phase-locking unit 30 continues to monitor the cost function even after the convergence condition is met (branch Y from step 127), the method 100 is able to achieve fast convergence while at the same time maintaining high performance of the CBC system 1.

[0141] The method 100 may also be used as a method for steering the recombined output beam 2, for example, to track a target T as it moves to different positions (shown by way of example in dashed lines in FIG. 4).

[0142] 5B shows an example of a schematic top view of photodiode 23, where beam spot 130 of the portion of recombined beam 75 focused by photodiode lens 68 is located entirely within the active area of ​​photodiode 23. In this case, by assuming that phase-locking unit 30 has verified the focusing condition, the mutual phase shift between secondary beams 12A-12D is optimized and the main lobe of recombined beam 75 has maximum intensity.

[0143] As shown in FIG. 5C, when the photodiode 23 is moved on the first and second X and Y axes, the beam spot 130 may be located on only a portion of the active area 131.

[0144] The movement of the photodiode 23 is controlled by a motor 25, for example a piezoelectric actuator with high precision, making it possible to produce a displacement of the photodiode 23 comprised, for example, between 10 μm and 25 mm.

[0145] The motor 25 may be controlled by a control unit 26 .

[0146] Upon displacement of the photodiode 23, the photodiode 23 detects a decrease in the measured intensity.

[0147] Therefore, when the phase-locked unit 30 obtains the intensity signal INT (step 123) and calculates the cost function J (step 125), the convergence condition is no longer verified (step 127). Therefore, the phase-locked unit 30 returns to step 105 and performs one or more new iterations (steps 105 to 127) until the convergence condition is met.

[0148] In fact, the phase change performed by the phase modulator 42 enables the phase locking unit 30 to change the position of the recombined beam 75, in particular to move the main lobe of the recombined beam 75, for example until the beam spot 130 is again located completely within the active area 131.

[0149] In fact, by moving the photodiode 23 it is possible to steer the recombined beam 75 and therefore the recombined output beam 2 .

[0150] Applicant has determined that method 100 can achieve very fast and precise beam steering, e.g., precisely control the position of recombined output beam 2 at target T. For example, by moving photodiode 23 with motor 25, CBC system 1 can adjust the position of the beam at the location of target T by even a few micrometers, even when target T is located at a distance of about 100 m from CBC system 1.

[0151] According to one embodiment, the optical delay line 44 may be a variable delay line. Figure 6 shows a flow chart of a method 150 performed by the delay optimization unit 31 to equalize the optical paths of the secondary beams 12A-12D in the channels 20A-20D, according to one embodiment.

[0152] Method 150 begins at step 151 when delay optimization unit 31 determines that the optical paths of channels 20A-20D need to be equalized.

[0153] For example, method 150 may be performed during calibration of CBC system 1, such as prior to first use of CBC system 1, or may be performed periodically during use, such as upon confirmation of certain conditions.

[0154] Delay optimization unit 31 selects one of channels 20A-20D to be equalized and sets one of channels 20A-20D as a reference channel, step 153.

[0155] For example, in the following, the first channel 20A is the reference channel and the second channel 20B is the channel to be equalized.

[0156] However, any of the channels 20A-20D may be the reference channel. For example, if the channels 20A-20D are arranged in a honeycomb structure of the aperture combiner 45, the channel whose aperture is located at the center of the honeycomb structure may be the reference channel.

[0157] In step 153, delay optimization unit 31 selects first channel 20A and second channel 20B by turning off third channel 20C and fourth channel 20D.

[0158] For example, the third channel 20C and the fourth channel 20D are turned off by ceasing emission of the corresponding amplifier 40. For example, the delay optimization unit 31 may provide signals s3 and s4 such that the respective amplifiers 40 block the propagation of the third and fourth intermediate beams 21C and 21D.

[0159] Then, in step 155, the delay optimization unit 31 determines a coarse estimate of the optical path difference between the second channel 20B and the reference channel 20A. For example, the coarse estimate may have an accuracy of between a few centimeters and a few meters of optical path difference.

[0160] In this embodiment, the delay optimization unit 31 performs a frequency modulated continuous wave (FMCW) technique to find a coarse estimate of the optical path difference between the second channel 20B and the reference channel 20A.

[0161] In particular, the delay optimization unit 31 provides a chirp signal CHIRP to the laser source 3 and obtains an intensity signal INT from the photodiode 23 .

[0162] The chirp signal CHIRP has a chirp frequency α which causes a time modulation of the wavelength of the primary beam 8 and in particular causes the wavelength of the primary beam 8 to describe a triangular slope.

[0163] When there is a delay OPD2 between the reference channel 20A and the second channel 20B, the intensity signal INT has a beat frequency f b Has a humming note.

[0164] Delay OPD2 and beat frequency f b The relationship between

number

[0165] Then, in step 157, delay optimization unit 31 provides a delay control signal d2 to optical delay line 44 of the second channel 20B to compensate for delay OPD2.

[0166] In effect, the delay control signal d2 shortens or lengthens the optical path of the optical delay line 44 of the second channel 20B to compensate for the delay OPD2.

[0167] For example, the optical delay line 44 of the second channel 20B may have a movable opto-mechanical element configured to change the optical path length of the optical delay line 44, and the delay control signal d2 may control an actuator, such as a rolling actuator, configured to move the optical delay line 44 of the second channel 20B.

[0168] The delay optimization unit 31 then finds a fine estimate of the optical path difference between the second channel 20B and the reference channel 20A.

[0169] In particular, in step 159, the delay optimization unit 31 measures the fringe visibility V2 from the image IMG received from the CCD camera 24.

[0170] In step 159, only the first channel 20A and the second channel 20B are activated, so that the recombined beam 75 is formed by interference between the first and second intermediate beams 21A, 21B.

[0171] Therefore, the image IMG captured by the CCD camera 24 represents an interference pattern between the first and second intermediate beams 21A, 21B.

[0172] Fringe Visibility V2

number

[0173] The delay optimization unit 31 checks in step 161 whether the fringe visibility V2 satisfies the interference quality condition. In this embodiment, the fringe visibility V2 is set to a fringe visibility threshold V, which may be chosen, for example, by a user during calibration of the CBC system 1. th The delay optimization unit 31 checks whether the delay is equal to or higher than .

[0174] In the negative case where the output from step 161 is branch N, the delay optimization unit 31 updates, in step 163, the delay control signal d2 provided to the optical delay line 44 of the second channel 20B to increase the fringe visibility V2.

[0175] The delay control signal d2 is updated using a well-known search or optimization algorithm, for example a bisection algorithm.

[0176] If the optical delay line 44 of the second channel 20B has a movable opto-mechanical element, the delay control signal d2 is updated so that the optical delay line 44 of the second channel 20B is updated by its respective actuator to appropriately shorten or lengthen the optical path of the second beam 12B accordingly.

[0177] The delay optimization unit 31 then repeats step 159 of measuring the updated fringe visibility V2 and step 161 of checking whether the interference quality condition is met.

[0178] The interference quality threshold is reached, i.e., V2 ≥ V th If so, the delay optimization unit 131 returns to step 153 by selecting the j+1-th channel on branch Y output from step 161. In the example considered, the delay optimization unit 31 selects the third channel 20C.

[0179] Therefore, the delay optimization unit 31 activates the third channel 30C and deactivates the second channel 20B.

[0180] According to this embodiment, the first channel 20A is again used as the reference channel.

[0181] Delay optimization unit 31 then repeats steps 155 to 163 for the third channel 20C.

[0182] After the optical path of the third channel 20C has also been equalized, steps 153 through 163 are repeated for the fourth channel 20D.

[0183] 7 shows an experimental example of a 3D representation of the spatial distribution of the intensity of a portion of the sample beam 63 acquired in use by the CCD camera 24. By taking into account the splitting properties of the first and second beam splitters 60, 65, said intensity distribution represents the intensity distribution of the recombined beam 75 and hence of the recombined output beam 2 directed towards the target T.

[0184] By measuring the peak intensity of the main lobe from the image IMG acquired by the CCD camera 24 or by the intensity signal INT provided by the photodiode 23, the ratio of the peak intensity of the recombined beam 75 to the peak intensity of the single intermediate beam 21A-21D, e.g., η=I max,CBC / I max,SB It is possible to find the efficiency of the CBC system 1 by calculating

[0185] Theoretically, the efficiency of the CBC system 1 is N 2 where N is the number of channels.

[0186] The applicant has confirmed that the efficiency of the CBC system 1 can achieve high values ​​close to the theoretical value.

[0187] In particular, for a CBC system with channel number N=7, Applicant assumes that N 2 We found an experimental efficiency of 38.5 against the theoretical value of = 49. Therefore, the CBC system may have an overall efficiency of about 79% against the theoretical value.

[0188] According to one embodiment, as shown in FIG. 8, the CBC system 1 may also include a temperature analysis unit or module 199 configured to perform a method shown in FIG. 9 and labeled 200 for simulating the effects of temperature changes in the CBC system 1, in particular those induced by the high optical power of the recombined beam 75.

[0189] In particular, the method 200 may be performed on a particular component of the CBC system 1 to optimize the parameters of that component.

[0190] In particular, method 200 can be performed on any of the optical elements of focusing optics 17. In fact, recombined beam 75 can reach high optical power values, for example on the order of tens of kW, that can cause high temperature changes in the optical elements of focusing optics 17.

[0191] According to method 200, at step 202, temperature analysis unit 199 receives data representative of a property of a part under test (hereinafter cut data), which may be, for example, convex lens 55 or concave lens 56, or any other optical component of focusing optics 17 shown in and discussed with reference to FIG.

[0192] This cut data may include, for example, the geometry of the inspected component, the optical properties, particularly the absorbance, and the thermal properties of the substrate material of the inspected component.

[0193] In step 204, the temperature analysis unit 199 receives laser data describing properties of the laser beam being analyzed, here recombined beam 75, such as optical power, spot size, wavelength, etc.

[0194] Then, in step 206, the temperature analysis unit 199 solves the 3D partial differential heat equations of the inspected part using the cut data and the laser data as inputs and provides as output a temperature map indicative of the temperature changes induced in the inspected part by the propagation of the laser beam.

[0195] In step 208, a temperature analysis unit 199 uses the temperature map as input to calculate the local change in refractive index of the component under test induced by temperature changes. In particular, unit 199 provides as output a map of updated refractive index n(x,y,z,ΔT), e.g., given by n(x,y,z,ΔT)=n0(x,y,z)+Δn(x,y,z,ΔT), where the change in refractive index Δn as a function of temperature change ΔT depends on the substrate material of the component under test.

[0196] Then, in step 210, unit 199 uses the updated refractive index n(x, y, z, ΔT) to find the phase change Δφ induced by the change in refractive index. Specifically, in this embodiment, unit 199 calculates the phase change Δφ by the equation

number

[0197] Unit 199 in step 212 transforms the phase change Δφ(x,y,ΔT) in polar coordinates Δφ(r,θ,ΔT).

[0198] In detail, Unit 199 is:

number

[0199] Zernike polynomials are single exponent

number

[0200] Then, in step 214, the unit 199 performs a simulation algorithm for the optical propagation of the laser beam in the CBD system 1. The algorithm for optical propagation is known per se in the art and is, for example, based on a model of the CBD system 1 prestored in the unit 199 and provided by a user, and the Zernike polynomials Z i and are taken as input.

[0201] The simulation algorithm provides output data of the optical properties of the recombined beam 75, such as the wavefront properties of the recombined beam 75, after propagation through the CBC system, and in particular after propagation through the part under test.

[0202] Based on this output data, unit 199 in step 216 determines cut data, e.g., the material of the inspected part that optimizes the optical properties of the recombined beam to ensure minimal distortion of the wavefront of the recombined beam 75 and minimal absorption of the recombined beam 75 for a particular application.

[0203] In particular, methods 90, 100, 150, and 200 all contribute to improving the performance of the CBC system 1 and achieving efficiency values ​​closer to theoretical values.

[0204] Finally, it will be apparent that modifications and variations may be made to what has been described and illustrated herein without departing from the scope of the present invention as defined in the appended claims.

[0205] For example, the number of channels N may differ from that discussed with reference to FIG. 1, and in particular the CBC system 1 may have up to 19 channels.

[0206] For example, the phase lock unit 30 may implement a different algorithm, such as the LOCSET algorithm.

[0207] For example, the phase locking unit 30 may be implemented using an FPGA, a multi-channel DAC unit, or different hardware. For example, the phase locking unit 30 may comprise a multi-channel DAC unit, be coupled to a CCD camera, and be configured to extract peak intensities from the image IMG.

[0208] The sample beam 63 may be split directly from the intermediate beams 21A-21D before recombination. In this case, the focusing optics would comprise one or more beam splitters disposed between the forward coupling section 45B and the convex lens 55.

[0209] Alternatively, each opto-mechanical element also includes a respective fiber splitter that extracts a small portion of the laser power, e.g., less than 0.1%, In this case, each intermediate beam includes a first portion that propagates through the aperture and is focused by focusing optics onto a target, and a second portion that is extracted by the fiber splitter and focused by focusing optics of an image sensor and / or intensity sensor.

[0210] 4, e.g., mirrors 58 and / or 59 are absent, so optical path 53 is not folded, and lenses 55 and 56 may have different focal lengths and / or different shapes.

[0211] According to one embodiment, the present coherent beam recombination system is configured to provide a recombined output beam, a laser source providing a source beam having a linewidth; a beam expander coupled to the laser source and configured to provide an expanded beam having a linewidth greater than the source beam; a splitter configured to split the expanded beam into a plurality of secondary beams; a plurality of channels coupled to the splitter, each configured to receive a respective secondary beam and provide a respective intermediate beam, each channel comprising an optical amplifier, a phase modulator, an optical delay line, and an optomechanical element to provide a respective intermediate beam; an optical sensor configured to provide a detection signal representative of an intensity of a received optical beam; focusing optics configured to receive the intermediate beams, provide a recombined output beam from a first portion of each intermediate beam, and provide a sampled recombined beam from a second portion of each intermediate beam to an optical sensor; a control unit coupled to the optical sensor and to the plurality of channels; The control unit may include: providing each of a plurality of phase control signals to a phase modulator of a respective channel; receiving a detection signal from the optical sensor, the detection signal representing an intensity of the sampled recombined beam; calculating a cost function from the detected signals, the cost function being a function of the intensity of the sampled recombined beam; performing an optimization algorithm of the cost function, the optimization algorithm being configured to maximize the intensity of the sample recombined beam; providing a plurality of updated phase control signals based on results of the optimization algorithm; The phase locking module is configured to:

[0212] Additionally, according to one embodiment, the method is configured to provide a recombined output beam, comprising: a laser source providing a source beam having a linewidth; a beam expander coupled to the laser source and configured to provide an expanded beam having a linewidth greater than the source beam; a splitter configured to split the expanded beam into a plurality of secondary beams; a plurality of channels coupled to the splitter, each configured to receive a respective secondary beam and provide a respective intermediate beam, each channel comprising an optical amplifier, a phase modulator, an optical delay line, and an optomechanical element to provide a respective intermediate beam; an optical sensor configured to provide a detection signal representative of an intensity of a received optical beam; focusing optics configured to receive the intermediate beams, provide a recombined output beam from a first portion of each intermediate beam, and provide a sampled recombined beam from the second portion of each intermediate beam to an optical sensor; a control unit coupled to the optical sensor and to the plurality of channels; The method may be for controlling a coherent beam recombination system comprising: The phase lock module in the control unit providing each of a plurality of phase control signals to a phase modulator of a respective channel; receiving a detection signal from the optical sensor, the detection signal representing an intensity of the sampled recombined beam; calculating a cost function from the detection signals, the cost function being a function of the intensity of the sampled recombined beam; performing an optimization algorithm of the cost function, the optimization algorithm configured to maximize the intensity of the sample recombined beam; providing a plurality of updated phase control signals based on results of the optimization algorithm; Includes:

Claims

1. 1. A coherent beam recombination system (1) configured to provide a recombined output beam (2), comprising: a laser source (3) providing a source beam (8) having a linewidth; a beam expander (5) coupled to the laser source and configured to provide an expanded beam (10) having a linewidth larger than the source beam; a splitter (7) configured to split the expanded beam into a number of secondary beams (12A-12D); a plurality of channels (20A-20D) coupled to the splitter, each configured to receive a respective secondary beam (12A-12D) and provide a respective intermediate beam (21A-21D), each channel comprising an optical amplifier (40), a phase modulator (42), an optical delay line (44), and an opto-mechanical element (47) for providing the respective intermediate beam; an optical sensor (23, 24) configured to provide a detection signal (INT, IMG) representative of the intensity of the received optical beam; a focusing optic (17) configured to receive the intermediate beams (21A-21D), provide the recombined output beam (2) from a first portion of each intermediate beam, and provide a sampled recombined beam (63) from a second portion of each intermediate beam to the optical sensor; a control unit (30) coupled to the optical sensor and to the plurality of channels; Equipped with The control unit A plurality of phase control signals (u １ , u ２ , u ３ , u ４ ) to said phase modulator (42) of a respective channel (20A-20D); receiving the detection signal (INT) from the optical sensor, the detection signal representing an intensity of the sampled recombined beam; calculating a cost function (J) from the detected signal (INT), the cost function being a function of the intensity of the sampled recombined beam; performing an optimization algorithm on the cost function, the optimization algorithm configured to maximize the intensity of the sample recombined beam; providing a plurality of updated phase control signals based on results of the optimization algorithm; A phase locking module (26) configured as follows: The optical delay lines are variable delay lines, each configured to vary the optical path length of the respective channel, the control unit (30) further comprising a delay equalization module (31) coupled to the channels and to the optical sensor, the delay equalization module comprising: selecting (153) an optimized channel and a reference channel from said plurality of channels (20A-20D); determining (155) a coarse estimate of the optical delay between the optimized channel and the reference channel from the detected signal (INT); Based on the coarse estimate, a delay control signal (d ２ ) (157), determining (159) a fine estimate of the optical delay between the channel to be optimized and the reference channel from the detected signal (IMG); The delay control signal (d ２ ) (161, 163), It is configured as follows: Coherent beam recombination system.

2. 2. The coherent beam recombination system of claim 1, further comprising an actuator (25) coupled to the optical sensor (23) and configured to cause a displacement of the optical sensor (23).

3. The coherent beam recombination system according to claim 1 or 2, wherein the optimization algorithm is a stochastic parallel gradient descent algorithm.

4. 4. The coherent beam recombination system of claim 1, wherein the beam expander (5) comprises a noise generator (36, 37, 38) for providing an expanded signal, and an expanded phase modulator (35) for receiving the source beam (8) and providing the expanded beam (10) and controlled by the expanded signal.

5. 5. A coherent beam recombination system as claimed in any one of claims 1 to 4, wherein each optical mechanical element (47) has an aperture (46) providing the respective intermediate beam and comprises a respective intensity mask (50) configured to block transmission of a portion of each of the secondary beams (12A-12D).

6. 6. A coherent beam recombination system as claimed in any one of claims 1 to 5, wherein each optical-mechanical element (47) has an aperture (46) for providing the respective intermediate beam, the apertures (46) being arranged in a tiled aperture configuration.

7. 7. A coherent beam recombination system as claimed in any one of claims 1 to 6, wherein the sampled recombined beam (63) forms an interference pattern having a main lobe, and the optical sensor comprises a photodiode (23) coupled to a pinhole (71) having a size smaller than the size of the main lobe.

8. 8. The coherent beam recombination system of claim 1, wherein the delay equalization module is coupled to the laser light source (3) and configured to perform a frequency modulated continuous wave technique to determine the coarse estimate of the optical delay between the channel to be optimized and the reference channel.

9. The optical sensor comprises an image sensor (24) configured to provide an image (IMG) of the sampled recombined beam, and the delay equalization module further comprises: receiving the image (IMG) of the sampled recombined beam; measuring fringe visibility of the sampled recombined beam from the image; When the fringe visibility falls below a fringe visibility threshold, the delay control signal (d ２ ) to update The coherent beam recombination system according to claim 1 , configured as follows:

10. 1. A method for controlling a coherent beam recombination system (1) configured to provide a recombined output beam (2), comprising: The coherent beam recombination system comprises: a laser source (3) providing a source beam (8) having a linewidth; a beam expander (5) coupled to the laser source and configured to provide an expanded beam (10) having a linewidth larger than the source beam; a splitter (7) configured to split the expanded beam into a number of secondary beams (12A-12D); a plurality of channels (20A-20D) coupled to the splitter, each configured to receive a respective secondary beam (12A-12D) and provide a respective intermediate beam (21A-21D), each channel comprising an optical amplifier (40), a phase modulator (42), an optical delay line (44), and an opto-mechanical element (47) for providing the respective secondary beam; an optical sensor (23, 24) configured to generate detection signals (INT, IMG) representative of the intensity of the received optical beam; a focusing optic (17) configured to receive the intermediate beams (21A-21D), provide the recombined output beam (2) from a first portion of each intermediate beam, and provide a sampled recombined beam (63) from a second portion (63) of each intermediate beam to the optical sensor; A control unit (30); Equipped with The method includes, by a phase locking module (26) of the control unit, A plurality of phase control signals (u １ , u ２ , u ３ , u ４ ) to said phase modulator (42) of a respective channel (20A-20D); receiving the detection signal (INT) from the optical sensor, the detection signal representing an intensity of the sampled recombined beam; calculating a cost function (J) from said detected signal (INT), said cost function being a function of the intensity of said sampled recombined beam; performing an optimization algorithm on the cost function, the optimization algorithm being configured to optimize an intensity of the sampled recombined beam; providing a plurality of updated phase control signals based on results of the optimization algorithm; Inclusive of the optical delay lines are variable delay lines, each configured to vary the length of the optical path of a respective channel, the control unit (30) further comprising a delay equalization module (31) coupled to the channels and to the optical sensor; The method includes, by the delay equalization module: selecting (153) an optimized channel and a reference channel from said plurality of channels (20A-20D); determining (155) a coarse estimate of the optical delay between the channel to be optimized and the reference channel from the detected signal (INT); Based on the coarse estimate, a delay control signal (d ２ ) (157) determining (159) a fine estimate of the optical delay between the channel to be optimized and the reference channel from the detection signal (IMG); The delay control signal (d ２ ) (161, 163), and The method further comprises:

11. The method of claim 10, wherein the coherent beam recombination system further comprises an actuator (25) coupled to the optical sensor (23), the optical sensor having an active area (131) for receiving a spot of the sampled recombined beam, and the method further comprises a beam manipulation step including reducing the intensity of the sampled recombined beam detected by the optical sensor by controlling the displacement of the optical sensor by the actuator so that the spot of the sampled recombined beam is partially located outside the active area.

12. The phase lock unit (30) Obtaining 123 an updated detection signal; Calculating 125 an updated cost function; Checking 127 whether 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; performing a new iteration of the optimization algorithm if the convergence condition is not verified, and providing a number of new updated phase control signals based on results of the new iteration; The method according to claim 10 or 11, further comprising:

13. The optimization algorithm is an iterative stochastic parallel gradient descent algorithm, which, for each iteration, is performed by the phase-locking module: generating (105) a plurality of random dephasing signals (δu(k)), one for each channel; providing (107) each of a plurality of positive phase control signals to a respective phase modulator, each positive phase control signal being the sum of a respective real phase control signal (U(k-1)) determined in a previous iteration and a respective one of said random dephasing signals; Obtaining 109 a positive detection signal; Calculating 111 a positive cost function from the positive detection signals; providing (113) each of a plurality of opposite phase control signals to a respective phase modulator, each opposite phase control signal being a difference between the respective phase control real signal and the respective random phase relaxation signal; Obtaining 115 a negative detection signal; Calculating (117) a negative cost function from the negative detection signals; Inclusive of each of the updated phase control signals is a function of the respective most recent phase control signal, the respective random phase relaxation signal, and a difference between the positive cost function and the negative cost function; 13. The method according to any one of claims 10 to 12.

14. The control unit further comprises a temperature simulation module (199), the method further comprising: receiving first data representative of a geometry and material of the optical component; receiving second data representative of a property of the laser beam interacting with the optical component; and determining from the first data and the second data a phase change induced by the optical component due to interaction with the laser beam; and running an algorithm for simulating optical propagation within the coherent recombination system, the simulation algorithm configured to receive the phase changes as input and output optical properties of the recombined output beam; determining first optimization data for the optical component based on results of the simulation algorithm; 14. The method of claim 10, further comprising simulating optical components of the coherent recombination system, the optical components comprising:

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