Systems and methods for aligning beams in a coherent beam array

Abstract

The CBC system includes an array of beam sources generating coherent beams directed at a target. The beam sources are associated with adjustable phase modulators and beam steering devices. For each beam in a subset of beams, a corresponding beam steering device is operated to steer the beam and a corresponding phase modulator is operated to modulate a current phase of the beam between at least three phase states. The detector monitors an intensity parameter that varies with the intensity of radiation incident on the target. The controller calculates, for each beam in the subset, a current value representing a relative intensity of the beam based at least in part on the monitored intensity parameter in each of the at least three phase states. The calculated value is indicative of a current position of the beam relative to the target.

Description

[Technical Field]

[0001] The present invention relates to a coherent beam combining (CBC) system. [Background technology]

[0002] To achieve power scaling of laser sources, it is known to utilize CBC systems in which multiple coherent laser beams are combined. A typical implementation employs an array of fiber lasers, each seeded by a common "seeder" oscillator, to generate beams that are directed to combine at or before reaching the target.

[0003] To achieve effective coherent combining and a small combined beam spot size on the target, the phases of the different beams arriving at the target must be synchronized very precisely to minimize the phase difference between the beams. Furthermore, the effectiveness of a CBC system requires precise alignment of different beams on the same target spot (e.g., on the scale of a few microradians) to maximize overlap between the beams. Some conventional techniques for achieving precise beam alignment have relied on switching on individual beams and aligning them using a combination of detectors and imaging systems. However, such techniques require continuous on / off switching of the beams, thereby reducing the operational effectiveness of the CBC system. Furthermore, because alignment requires switching the beams off, alignment errors that occur during the engagement time on the target cannot be corrected using such techniques. Other conventional techniques have relied on iterative optimization over many degrees of freedom, such as stochastic parallel gradient descent. However, these techniques have very long convergence times, making them impractical for use in dynamically changing operating conditions. Summary of the Invention

[0004] The present invention is a system and method for aligning beams in a coherent beam array.

[0005] According to the teachings of an embodiment of the present invention, a method for aligning beams of a coherent beam combining (CBC) device directed toward a target is provided, the beams having associated thereto adjustable phase modulators and beam steering devices, the method including the steps of: (a) for each beam in a subset of the beams, actuating a corresponding beam steering device to steer the beam; (b) for each beam in the subset, actuating a corresponding phase modulator to modulate the current phase of the beam between at least three phase states; (c) monitoring an intensity parameter that varies as a function of the intensity of radiation incident on the target; and (d) for each beam in the subset, calculating a current value representing the relative intensity of the beam based at least in part on the monitored intensity parameter in each of the at least three phase states, the calculated value indicating the current position of the beam relative to the target.

[0006] Optionally, the method further comprises: (e) for each of the beams in the subset, adjusting a beam pointing direction of the beam according to the calculated current value.

[0007] Optionally, the method further includes (e) forming a comparison metric for each beam in the subset based on the calculated current value and at least one previous value representing a relative intensity associated with the beam, and (f) if the comparison metric meets a threshold criterion, (i) storing the calculated current value, and (ii) leaving the beam steered to the current position.

[0008] Optionally, the method further comprises (g) activating a beam steering device of the beam to steer the beam to a previous steering position if the comparison metric does not meet the threshold criterion.

[0009] Optionally, the method further comprises (g) selecting a next subset of beams; and (h) repeating (a) through (g).

[0010] Optionally, the step of calculating the current value is performed by evaluating an objective function having inputs based on the monitored intensity parameters in each of the at least three phase states.

[0011] Optionally, for each beam, the objective function has a single maximum that is achieved when the beam is positioned to be centered on the target.

[0012] Optionally, the intensity parameter is obtained from a beam-sensitive sensor located at the target.

[0013] Optionally, the intensity parameter is obtained from a sensor positioned to sense radiation reflected from the target.

[0014] Optionally, for each beam in the subset, the current value represents the relative intensity of that beam to the sum of all of the beams in the subset.

[0015] Optionally, the beams include at least 10 beams.

[0016] Optionally, the current phase of each beam is modulated in steps between at least three phase states.

[0017] Optionally, the current phase of each of the beams is substantially continuously modulated over a range of modulation frequencies encompassing at least three phase states.

[0018] Optionally, the current phase of each beam is sinusoidally modulated over a range of modulation frequencies encompassing at least three phase states.

[0019] Optionally, the current phases of multiple beams are varied sequentially.

[0020] Optionally, the subset is a majority subset.

[0021] Optionally, the subset includes all of the beams.

[0022] Optionally, the subset is a minority subset.

[0023] Optionally, the beams within the subset are steered in a random or pseudo-random manner.

[0024] In accordance with the teachings of embodiments of the present invention, there is also provided a system comprising: (a) an array of beam sources configured to generate a plurality of coherent beams for directing towards a target; (b) a plurality of adjustable phase modulators associated with the beam sources to enable adjustment of a relative phase offset of the beams; (c) a plurality of beam steering devices associated with the array of beam sources configured to steer the coherent beams; (d) a detector positioned to monitor an intensity parameter that varies as a function of the intensity of radiation incident on the target; and (e) a phase modulator associated with the detector to receive the intensity parameter and configured to adjust the phase modulator and the beam steering device. and a further associated control subsystem configured to: (i) operate beam steering devices associated with the subset of beam sources to steer beams in the subset generated by the beam sources; (ii) for each beam in the subset, operate a corresponding phase modulator to modulate the current phase of the beam between at least three phase states; and (iii) for each beam in the subset, calculate a current value representing the relative intensity of the beam based at least in part on the intensity parameters monitored in each of the at least three phase states, the calculated value indicating the current position of the beam relative to the target.

[0025] Optionally, the control subsystem is further configured to (iv) for each of the beams in the subset, actuate a corresponding beam steering device to adjust a beam pointing direction of the beam according to the calculated current value.

[0026] Optionally, the control subsystem is further configured to (iv) form, for each beam in the subset, a comparison metric based on the calculated current value representing the relative intensity of the beam and at least one prior value representing the relative intensity value associated with the beam, and (v) if the comparison metric meets a threshold criterion, (1) store the calculated current value, and (2) keep the beam steered to that position.

[0027] Optionally, the control subsystem is further configured to (vi) actuate a beam steering device for that beam to steer the beam to a previous steering position if the comparison metric does not meet the threshold criterion.

[0028] Optionally, the control subsystem is further configured to: (vi) select a next subset of beams; and (vii) repeat (i) through (vi).

[0029] Optionally, the control subsystem is configured to calculate the current value by evaluating an objective function having inputs based on the monitored intensity parameters in each of the at least three phase states.

[0030] Optionally, for each beam, the objective function has a single maximum that is achieved when the beam is positioned to be centered on the target.

[0031] Optionally, the detector is located at the target.

[0032] Optionally, the detector is positioned to sense radiation reflected from the target.

[0033] Optionally, for each beam in the subset, the current value represents the relative intensity of the beam to the sum of all of the beams in the subset.

[0034] Optionally, the array of beam sources comprises at least 10 beam sources.

[0035] Optionally, the current phase of each beam is modulated in steps between at least three phase states.

[0036] Optionally, each current phase of the beam is substantially continuously modulated over a range of modulation frequencies encompassing at least three phase states.

[0037] Optionally, each current phase of the beam is sinusoidally modulated over a range of modulation frequencies encompassing at least three phase states.

[0038] Optionally, the current phases of multiple beams are varied sequentially.

[0039] Optionally, the subset is a majority subset.

[0040] Optionally, the subset includes all of the beam sources.

[0041] Optionally, the subset is a small subset.

[0042] Optionally, the control subsystem is configured to operate the beam steering device to steer the beams in the subset in a random or pseudo-random manner.

[0043] Unless otherwise defined herein, all technical and / or scientific terms used herein have the same meaning as understood by those skilled in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be practiced or used to test embodiments of the present invention, exemplary methods and / or materials are described below. In case of conflict, the present patent specification, including definitions, will control. Furthermore, the materials, methods, and examples are merely illustrative and are not intended to be necessarily limiting. [Brief explanation of the drawings]

[0044] Some embodiments of the present invention are described herein, by way of example only, with reference to the accompanying drawings. With detailed reference to certain drawings, it is emphasized that the details are shown by way of example and for purposes of illustrative description of embodiments of the invention. In this regard, the description given with respect to the drawings will make apparent to those skilled in the art how embodiments of the invention may be practiced.

[0045] Attention is now directed to the drawings, where like reference numbers or characters indicate corresponding or similar elements.

[0046] [Figure 1] FIG. 1 is a diagrammatic representation of a coherent beam combining system constructed and operable in accordance with the teachings of an embodiment of the present invention for aligning a coherent beam on a target using a beam director and based on radiation sensed at the target. [Figure 2] FIG. 2 is a diagrammatic representation of a coherent beam combining system constructed and operable in accordance with the teachings of an implementation of the present invention for aligning a coherent beam on a target using a beam director and based on radiation reflected from the target. [Figure 3] 2 is a diagrammatic representation of a coherent beam combining system similar to the system of FIG. 1, but with a beam director associated with each coherent beam. [Figure 4]3 is a diagrammatic representation of a coherent beam combining system similar to the system of FIG. 2, but in which each coherent beam has an associated beam director. [Figure 5] 5 is a diagrammatic representation of a beam steering apparatus of the beam director of FIGS. 1-4 in accordance with the teachings of an implementation of the present invention. [Figure 6] 5 is a diagrammatic representation of a beam steering device of the beam director of FIGS. 1-4 according to the teachings of a variant implementation of the present invention. [Figure 7] 5 is a diagrammatic representation of a beam steering configuration of the beam director of FIGS. 1-4 in accordance with the teachings of another variant implementation of the present invention. [Figure 8] FIG. 1 is a flow diagram illustrating a process for aligning a coherent beam on a target according to the teachings of an implementation of the present invention. [Figure 9] 1 is a diagrammatic representation of a photodetector for detecting radiation at a target and operating in conjunction with an imaging system in accordance with the teachings of an implementation of the present invention. [Figure 10] 1 is a diagrammatic representation of a photodetector for detecting radiation reflected from a target and operating in conjunction with an imaging system in accordance with the teachings of an alternative implementation of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0047] The present invention is a system and method for aligning beams in a coherent beam array.

[0048] The principles and operation of the systems and methods according to the present invention may be better understood with reference to the drawings accompanying this specification.

[0049] Systems and methods according to the present invention are particularly valuable when applied in the context of high energy laser or directed energy weapon systems.

[0050] Before describing at least one embodiment of the invention in detail, it is to be understood that the invention, in its application, is not necessarily limited to the details of construction and arrangement of components and / or methods set forth in the following description and / or illustrated in the drawings and / or examples. The invention may be practiced or carried out in other embodiments or in various ways.

[0051] Referring now to the drawings, Figures 1-4 illustrate various implementations of a coherent beam combining (CBC) system / device (generally designated 10 and hereinafter referred to as the "system") constructed and operable in accordance with certain non-limiting embodiments of the present disclosure. In overview, the system 10 includes an array of beam sources 12a, 12b, and 12c configured to generate multiple coherent beams 14a, 14b, and 14c (also referred to as "sub-beams") for direction toward a target 34. Multiple adjustable phase modulators 16a, 16b, and 16c are associated with each beam source 12a, 12b, and 12c to enable adjustment of the relative phase offset of the beams. While only three beam sources and phase modulators are illustrated here for simplicity, embodiments of the present disclosure are most preferably implemented with arrays containing anywhere from two to several hundred beams, and most typically with between 20 and 100 beam sources, with at least 10 beam sources being preferred. While a phase modulator is typically provided per beam source, it will be understood that one of the beam sources may be implemented as a constant reference phase with no adjustment, and the remaining beams and phase offsets may be measured and / or adjusted relative to that reference phase. Multiple beam steering devices (20a, 20b, and 20c), represented schematically in FIG. 1 as "black boxes," are associated with each beam source (12a, 12b, and 12c) to enable beam steering by adjusting at least one beam pointing parameter associated with the beam's pointing vector. A detector (28) (e.g., a photodetector) is positioned to monitor an intensity parameter that varies as a function of the intensity of radiation incident on the target (34) region. Optionally, detector 28 may be integrated with or cooperate with an imaging system used to aim system 10. Although the imaging system is not shown here, non-limiting examples of detector configurations with imaging systems will be described with reference to Figures 9 and 10.

[0052] A control subsystem (also referred to as a "controller") (30), including logic circuitry, is associated with the detector (28) to receive the monitored (measured) intensity parameters, and is further associated with each of the adjustable phase modulators (16a, 16b, and 16c) and the beam steering devices (20a, 20b, and 20c) to control the phasing and beam pointing parameters of the beam. Arrowheads pointing from the control subsystem (30) to the phase modulators (16c) and the beam steering devices (20c) represent the functional connection between all of the adjustable phase modulators (16a, 16b, and 16c) and the control subsystem (30), as well as the functional connection between all of the beam steering devices (20a, 20b, and 20c) and the control subsystem (30).

[0053] In certain preferred embodiments, the communication between the control subsystem 30 and the detector 28 is provided by a low-latency link (i.e., a link capable of providing a latency of preferably 1 millisecond or less), which may be embodied, for example, as a low-latency data link or a low-latency communication link in a communication network, including, for example, a wired network, a fiber optic connection, or a free-space optical communication network.

[0054] It will be understood that various implementation details will vary significantly depending on the intended use of the device. Applications can range from low-energy research tools for measuring rapid fluctuations in the optical properties of turbulent media to various communications applications (medium energy) to high-energy directed energy weapon systems. In each case, the array of beam sources (12a, 12b, and 12c) is most preferably an array of fiber lasers seeded by a common seeder oscillator (32). In high-energy applications, each fiber laser is preferably rated for a power output of at least 100 W, and in some cases at least 1 kW. The principles of the present disclosure can be used in devices operating at a variety of different wavelengths, but are typically implemented in the near-infrared (NIR) range. Seeder oscillators and fiber lasers suitable for operation in these ranges are well known in the art and readily commercially available.

[0055] The phase modulators (16a, 16b, and 16c) may be any type of phase modulator with suitable response time and low attenuation. To operate with relatively low-power signals, the phase modulators are placed after splitting the seeder (32), but preferably before the fiber laser amplifier. Suitable phase modulators are commercially available. One suitable, non-limiting example is a 10 GHz titanium-diffused Z-cut LiNbO3 phase modulator, identified by model number LN53S-FC, available from Thorlabs, NJ (USA).

[0056] A beam directing device (interchangeably referred to as a "beam director") (18) directs the beams (14a, 14b, and 14c) toward the target (34), or, for large targets, toward a specific region of the target, so that the beams (14a, 14b, and 14c) coherently combine (constructively interfere) upon or before reaching the target (34). In the context of this specification, coherent beams that are coherently combined upon or before reaching the target are either beams that are coherently combined at the target or beams that are coherently combined prior to the target and remain coherently combined for the remaining distance to the target.

[0057] The term "target" is used broadly herein to refer to an object against which a beam is incident, which may be a receiver (such as detector 28) for communications applications or a target for weapons applications in the military sense. The structure and placement of the detector 28 will depend on the specifics of the application. When the target forms part of the system design (e.g., for communications applications, research applications, etc.), the detector 28 is positioned to receive and sense radiation (coherent beams 14a, 14b, and 14c) emitted by the beam sources 12a, 12b, and 12c and configured to generate a signal (intensity parameter) indicative of the intensity of the radiation incident on the detector 28. FIG. 1 illustrates a non-limiting example configuration in which the target 34 is a "cooperative target," i.e., the "target" is part of the system 10 and includes a target-mounted photodetector 28. In such a cooperative configuration, the detector 28 itself may be considered the "target."

[0058] If the target is a remote object that does not form part of the system design, detector 28 is positioned to sense radiation reflected from target 34. Figure 2 illustrates a non-limiting example configuration in which target 34 is a "non-cooperative" target, i.e., the "target" is not part of system 10 and is not required to carry any sensors or play any active role in the system's operation. Here, a portion of the radiation incident on target 34 is reflected toward optical device 42 (shown as reflected radiation 15) and delivered to detector 28, which generates a signal (intensity parameter) indicative of the intensity of the radiation incident on an area of ​​target 34. Detector 28 and optical device 42 together form a "receiver" that receives target reflected radiation 15. To ensure that photodetector 28 measures the correct radiation, optical device 42, typically in the form of a receiver telescope, is positioned to define a field of view corresponding to the area from which the reflected radiation will arrive. The optical device is represented diagrammatically in Figure 2 by lens (42), but is typically an assembly of lenses, which may be refractive or reflective, or any combination thereof, to form a suitable telescope, as is known in the art. A narrow passband filter (not shown) may be advantageously disposed in the optical path to selectively pass the reflected laser illumination to the detector (28) while rejecting ambient background radiation, thereby improving the signal-to-noise ratio at the detector.

[0059] It is noted that in some scenarios, particularly when the distance to the target (either cooperative or non-cooperative) is long, the beams (14a, 14b, and 14c) may be transmitted along an optical path that passes through an optically inhomogeneous and / or transiently changing medium, such as the atmosphere, toward the target. Such a case is illustrated in FIG. 2, where the medium (generally designated 36) is represented diagrammatically by a cloud-like object. It is noted that in other scenarios, for example, when the distance to the target is shorter, such medium 36 does not exist. Furthermore, it is noted that while the beams (14a, 14b, and 14c) in the configuration illustrated in FIG. 1 are shown as being transmitted in the absence of an optically inhomogeneous and / or transiently changing medium, transmission conditions similar to those illustrated in FIG. 2 may also be applicable to the configuration illustrated in FIG. 1.

[0060] The beam director (18) includes beam steering devices (20a, 20b, and 20c) and optical devices (26) (schematically represented by lenses in FIGS. 1 and 2) for collimating and focusing the beams (14a, 14b, and 14c). While shown only diagrammatically here, the optical devices (26) typically include separate collimators for each beam and common, large-aperture focusing optics for collectively directing the beams toward the target. Alternatively, separate focusing optics can be provided for each beam. Figures 3 and 4 show systems generally similar to those of FIGS. 1 and 2, respectively, but in which separate collimating and focusing optics (labeled 26a, 26b, and 26c) are provided for each beam. Here, each beam source effectively has its own beam director (designated as 18a, 18b, and 18c), providing a modular design of the beam source-beam director combination, leading to increased flexibility and adaptability of the system. However, it should be noted that other beam source-beam director implementation combinations are possible, such as implementations in which a subset of the beam sources share common beam directing optics, or implementations in which some of the optical paths are shared and some are separated.

[0061] In a preferred embodiment, each of the beam steering devices (20a, 20b, and 20c), which in one exemplary implementation is implemented as an optical-mechanical device, includes beam steering optics having at least one optical element and at least one actuator that cooperates to adjust beam pointing parameters (azimuth and elevation) associated with the pointing vectors of the sub-beams (14a, 14b, and 14c) to steer the beam in azimuth and / or elevation.

[0062] 5-7 schematically illustrate details of the beam steering device (20), each representing a beam steering device (20a, 20b, and 20c) according to various non-limiting implementations. As described above, the beam steering device (20) includes beam steering optics (22) having at least one optical element coupled to at least one actuator (24) coupled to a control subsystem (30). Generally, the beam steering optics (22) and actuator (24) can be implemented using any suitable optical beam steering technology, including, but not limited to, MEMS-actuated mirrors, magnetically actuated mirrors, and the like. FIG. 5 illustrates one non-limiting implementation in which the actuator (24) is a mechanical actuator and the beam steering optics (22) includes an optical element (23) implemented as an adjustable steering mirror coupled to the mechanical actuator. The mechanical actuators are configured to selectively, and in some cases independently, adjust the orientation and / or position of the steering mirrors (e.g., via rotational and / or linear movement about one or more rotational axes) in response to control inputs received from the control subsystem (30) to adjust the elevation and azimuth angles (beam pointing parameters) of the exit beam (14O) in response to illumination by the incident beam (14I), where the incident beam (14I) is a beam from the output of a corresponding beam source (e.g., fiber laser 12a) and the exit beam (14O) is a deflected beam (e.g., beam (14a)) that is directed to the optics (26) for directing it toward the target (34). For example, rotation of the steering mirror about a first axis of rotation (arbitrarily shown here to correspond to the "x-axis") may be used to adjust the elevation angle of the output beam, while rotation of the steering mirror about a second axis of rotation orthogonal to the first axis (arbitrarily shown here to correspond to the "z-axis") may be used to adjust the azimuth angle of the output beam. The steering mirror is preferably fully reflective to light in the spectrum of the beam (e.g., the NIR range).

[0063] 6 shows another non-limiting implementation in which the beam steering optics (22) includes a pair of optical elements (23, 25) coupled to respective actuators (24), where the optical elements (23, 25) are implemented as a pair of independently controllable and adjustable steering mirrors (linked to separate mechanical actuators), one of which (in response to control input received from the control subsystem (30)) adjusts the orientation and / or position of the steering mirror (23), e.g., via rotation about a first axis of rotation (e.g., arbitrarily labeled the "x-axis" or "z-axis"), thereby controlling the elevation or azimuth angle of the beam to deflect the incident beam (14I) toward the other steering mirror (25) as a deflected beam (14D). Another one of the actuators (24) (in response to control input received from the control subsystem (30)) adjusts the orientation and / or position of the steering mirror (25), e.g., via rotation about a second axis of rotation (e.g., arbitrarily labeled "z-axis" or "x-axis"), thereby controlling the azimuth or elevation angle of the outgoing beam (14O) in response to illumination by the deflected beam (14D). Figure 7 shows yet another non-limiting implementation, similar to that of Figure 6, except that one of the optical elements (23) is implemented as a fixed mirror that deflects incoming radiation to the other optical element (25), which is implemented as an adjustable steering mirror coupled to the actuator (24). Here, the adjustable steering mirror (25) is independently rotatable about two orthogonal axes of rotation (here arbitrarily shown to correspond to the "x-axis" and "z-axis") to enable independent adjustment of the elevation and azimuth angles of the beam pointing vector.

[0064] While Figures 5-7 schematically illustrate the beam steering optics (22) implemented as one or more steering mirrors, other implementations are contemplated herein, including, for example, implementations in which steering prisms are used instead of steering mirrors. The steering prism can control the elevation and azimuth angles of the output beam via rotation of the prism about one or more axes of rotation. In such implementations, the reflective surfaces of the prism that control the output beam angle are preferably fully reflective to light within the spectrum of the beam. When implemented as a rectangular prism or any other type of prism with an outer surface from which the reflective surfaces extend, the outer surface is preferably formed from an optically transparent material that is transparent to light within the spectrum of the beam. Wedge prisms, such as those available from Thorlabs of Newton, New Jersey, are particularly useful for beam steering applications. Other beam steering implementations are also contemplated herein, including implementations that employ fiber motion along independent, orthogonal axes to independently control the azimuth and elevation angles. Non-mechanical beam steering implementations may also be used, for example, acousto-optic deflectors such as those available from Ilminster Somerset, UK. However, while acousto-optic deflectors have the advantage of providing high speed beam steering capabilities, they often introduce significant optical losses in the optical path to the target, making them less suitable for high energy applications.

[0065] Regardless of implementation, the actuator(s) (24) are coupled (e.g., via an electronic link) to a control subsystem (30) and configured to receive control inputs (control signals) from the control subsystem (30). The control subsystem (30) provides the control inputs (control signals) to the actuator(s) to control the actuator(s) to adjust the orientation and / or position of the optical element(s) of the beam steering optics (22) to adjust the azimuth and / or elevation angles of the coherent beams, thereby adjusting the beam pointing vector of each beam.

[0066] The logic circuitry of the control subsystem (30) may be implemented as appropriately configured hardware using digital and / or analog processing, including, but not limited to, one or more application-specific integrated circuits (ASICs), one or more field-programmable gate arrays (FPGAs), a general-purpose computer system configured by software running under a suitable operating system, or any hardware / software / firmware combination configured to perform the functions described herein at an appropriate speed. The control subsystem (30) will also typically include a data storage device and appropriate input / output interfaces to control the phase modulators and accept inputs from the detector(s), all of which will be apparent to those of ordinary skill in the art. The data processing requirements of the system are not unusual, even for high-speed measurements, and can be handled by standard processing equipment. Due to the relatively large number of outputs (phase modulators for each beam) that must be rapidly adjusted in parallel, specialized hardware adaptations beyond the interfacing capabilities of most off-the-shelf hardware may be required, but such adaptations are readily accomplished by those skilled in the art using standard components.

[0067] It has been found that using intensity parameter measurements with blind optimization approaches (e.g., stochastic parallel gradient descent, or SPGD) to reduce beam misalignment (pointing vector adjustment) often has significant drawbacks that make such approaches impractical under various operating conditions. For example, intensity fluctuations caused by phase noise due to reflections from diffuse objects, turbulence, and scintillation are generally on time scales comparable to or greater than the steering components (24), which can result in signal-to-noise ratios (e.g., SNR<1 dB) that are too small to be useful for pointing vector adjustment. Furthermore, blind optimization methods (e.g., SPGD) require numerous iterative steps and are therefore often too slow to converge effectively. Instead, a system (10) according to an embodiment of the present disclosure performs deterministic (i.e., quantitative) measurements of intensity parameters and uses those measurements to extract information about the beam pointing error / misalignment due to modulation of the current phase by the phase modulators (16 a, 16 b, and 16 c), embedded in the phase information. In a particularly preferred implementation, the deterministic measurements, in combination with a beam model-based analysis, are used to calculate a value for each beam representing the beam's relative intensity, whereby the calculated value indicates the position of the beam's center relative to the target. In a preferred implementation, the calculated value represents the beam's relative intensity relative to the sum of all beams. The model-based analysis includes identifying or generating an objective function that represents the relative intensity and has a single maximum value that is achieved when the beam's center is centered on the target (or, if the target is a cooperative target, when centered on the detector). The objective function is derived based on a beam model analysis, and the details of the analysis depend on the particular beam model and the form of phase modulation introduced into the transmitted beam.As a result, the objective function takes as input the deterministic measurements resulting from the modulation of the current phase by the phase modulators (16a, 16b, and 16c) and is repeatedly evaluated (by the control subsystem (30)) while iteratively adjusting the beam pointing vector throughout the engagement of the system (10) with the target or until a stopping condition is reached, such as, for example, until the beam pointing error is within an acceptable tolerance range or until the representative relative intensity calculated (via evaluation of the objective function) is at a sufficient intensity level.

[0068] In one non-limiting example implementation, the control subsystem (30) may employ a Gaussian beam model, where the intensity, I(r), of the coherently combined beam on the target (34) (or detector (28) as in the case of FIGS. 1 and 3) is expressed by the following equation:

[0069]

number

[0070] For simplicity, if the target 34 (or detector 28, as in the examples of Figures 1 and 3) is assumed to be sufficiently small and located at the origin, i.e., r = 0, then the dependence on r in Equation 1 is eliminated. Using these assumptions, the formula in Equation 1 can be reduced to:

[0071]

number

[0072] The intensity I is determined when the phase difference between the beams is minimized (preferably zero) and each of the beams (e.g., sub-beams 14a, 14b, and 14c) is centered on the target 34, i.e., r for all i=1...N. i = 0, achieving a maximum value. It has been shown that modulation resulting from modulating the current phase by the phase modulators (16a, 16b, and 16c) can be used to correct phase offsets between multiple beams in the CBC system (10). In one particularly relevant example, the aforementioned intensity parameters, which vary as a function of the intensity of radiation incident on the target (34), are monitored (measured) by the detector (28) while each phase modulator is operated to modulate the current phase of its corresponding transmit beam between at least three phase states, typically an initial "unaltered" phase state and two altered phase states. Monitoring the intensity parameters typically involves generating, by the detector (28), intensity signals indicative of the intensity of radiation incident on the target (34). The control subsystem (30) receives the monitored intensity parameters (i.e., intensity signals) from the detector (28) and identifies variations in the monitored intensity parameters due to modulation of the current phase of each transmit beam relative to the sum of all other beams. The control subsystem (30) then calculates, for each beam, a phase offset of the current phase (based on the identified variations) relative to the phase representing the sum of all other beams.

[0073] This process of modulating the phase of the sub-beams, identifying corresponding variations in the measured intensity parameters, calculating the phase offset of the sub-beams, and correcting that phase offset is preferably repeated in a rapid cycle, thereby correcting in real time for dynamic variations in operating conditions that may result from fluctuations in the beam-generating hardware or fluctuations in atmospheric conditions caused by turbulence. Further details of this process of phase correction are fully described in WO2020 / 016824 A1, which is incorporated herein by reference in its entirety.

[0074] According to certain aspects of the present disclosure, the beam pointing error and / or beam center position information embedded in the intensity parameter measurements is ascertained by calculating a value representing the relative intensity of the beam for each beam using the intensity parameter measurements (and in some cases phase offset information) resulting from modulation of the current phase by the phase modulators (16a, 16b, and 16c).

[0075] In some implementations, the current phase of each transmit beam is modulated at a modulation frequency. In some particularly preferred implementations, the current phase of each transmit beam is modulated in steps between a base phase value (unaltered phase state) and at least two discrete modulation values ​​(altered phase states). In other implementations, the current phase of each transmit beam is modulated substantially continuously over a range of modulation frequencies, encompassing the aforementioned at least three phase states.

[0076] According to one preferred, but non-limiting example, the control subsystem (30) varies the phase of each beam (14a, 14b, and 14c) in a stepped manner, where discrete steps equivalent to a step function variation of the phase state may be used, for example, by employing a rectangular wave or square wave phase modulation, such as commonly used in square wave dithering techniques, where the modulation of the different beams is preferably performed sequentially to isolate the effect on the intensity measurement resulting from each modulation.

[0077] In one particularly preferred example, the phase of each beam is sequentially varied according to a discrete three-step modulation, in which a phase modulator associated with each beam is configured to modulate the beam's phase between its current base value (initial, unaltered phase state) and two discrete modulation values: a first altered phase state (i.e., first discrete modulation value) and a second altered phase state (i.e., second discrete modulation value). The phase values ​​represent a phase offset of each beam's current phase relative to the phase representing the sum of all other beams. In the initial, unaltered phase state, the phase offset is effectively zero (i.e., there is no phase offset of the beam's current phase relative to the phase representing the sum of all other beams). In the first altered phase state, i.e., when the phase assumes the first discrete modulation value, the beam's current phase has a positive phase offset or shift (say, +δ) relative to the phase representing the sum of all other beams. In the second, altered phase state, i.e., when the phase assumes a second discrete modulation value, the current phase of the beam has a negative phase offset or shift (say, −δ) relative to the phase representing the sum of all other beams. In some cases, the phase offset value + / −δ is the actual discrete modulation value. However, in other cases, + / −δ can be calculated using, for example, the techniques described in WO2020 / 016824 A1.

[0078] A control subsystem (30) operates the phase modulator of the ith beam to vary the phase φ between three phase states. iWhen modulating i, the control subsystem (30) operates the remaining phase modulators to maintain the phase of all other beams (i.e., φ_k, for k=1···N, where k≠i) in an unchanged (i.e., unshifted) phase state. Detector (28) measures intensity parameters at the three phase states to generate three intensity signals and, in a preferred implementation, identifies variations in the intensity parameters due to modulation. When the phase of the ith beam assumes the unchanged phase state (i.e., when all phases of the beams are unchanged / unshifted), detector (28) measures the intensity to obtain a first intensity parameter measurement, designated I0. When the phase of the ith beam assumes the first changed phase state (i.e., when the current phase φ i is shifted by +δ relative to the phase representing the sum of all other beams), the detector (28) detects I(φ i +δ) when the phase of the ith beam assumes a second altered phase state (i.e., the current phase φ i is shifted by -δ relative to the phase representing the sum of all other beams), the detector (28) detects I(φ i The detector 28 preferably is synchronized with the phase modulator (via the control subsystem 30) so that the detector 28 performs (at least) three intensity measurements at time intervals that coincide with the modulation interval. Preferably, the time between the three intensity measurements is on the order of a few microseconds, so that the total intensity measurement period required to perform the three intensity parameter measurements is on the order of a few microseconds.

[0079] The intensity fluctuations resulting from the modulation (I + ) is the intensity measurement I(φ i +δ)(shifted by +δ, phase φ i The intensity fluctuations resulting from the modulation (I) are obtained (e.g., calculated by the control subsystem 30) as the ratio between the intensity of all beams except for I (unmodified phase) and the intensity measurement I (unmodified phase of all beams).- ) is the intensity measurement I(φ i -δ) (shifted by -δ, phase φ i The phase of all beams except for Θ is obtained as the ratio (e.g., calculated by the control subsystem (30)) between the intensity measurement I (the intensity when the phase of all beams is unchanged) and the intensity measurement I (the intensity when the phase of all beams is unchanged).

[0080] Using the Gaussian beam model, the relative intensity of the ith beam to the sum of all beams (with unchanged phase) can be expressed as:

[0081]

number

[0082] The formula in (3) can be derived by expanding equation (2) (which involves many intermodulation product terms) and isolating the terms that depend only on i (i.e., ignoring all intermodulation terms and all k≠i terms). The objective function F(φ i , δ) is the relative intensity of the beam in Equation (3), and the phase offset (i.e., modulation level) δ and the relative intensity of the intensity fluctuation due to modulation (I + and I - ) can be derived by expanding equation (2) and is given by

[0083]

number

[0084] Note that the phase terms in all of the intermodulation products are resolved down to the phase difference between the phases of the various beams. i +δ), the phase φ i +δ represents the current phase of the ith beam shifted by +δ (in other words, φ i+δ is the current phase shifted by an offset +δ of the current phase relative to the phase representing the sum of all other beams). Similarly, the term I(φ i -δ), the phase φ i -δ represents the current phase of the ith beam shifted by δ (in other words, φ i -δ is the current phase shifted by an offset -δ of the current phase relative to the phase representing the sum of all other beams).

[0085] The control subsystem (30) controls the intensity fluctuations (I + and I - ) and then,δ,I + and I - as input to calculate the objective function F(φ i , δ) is evaluated. i , δ) when the beam is centered on the target (34) (i.e., r i = 0). The objective function F therefore represents the relative intensity of the beam in equation (3) and indicates the position of the center of the beam relative to the target (34), and also indicates the beam pointing error (i.e., beam misalignment). The objective function F(φ i A non-maximum output value produced by evaluating the objective function F(φ, δ) indicates that the position of the beam center does not coincide with the center of the target (34) and therefore there is some amount of beam pointing error in some direction. i , δ) may not provide the direction in which the beam needs to be moved to correct the beam pointing error to achieve maximum beam intensity, but it can be used as an optimization or merit function that can be calculated / evaluated iteratively while adjusting the beam pointing vector up to a stopping criterion such as a pointing error performance threshold or a beam intensity threshold.

[0086] For example, if the evaluation of the objective function for a subset of beams by the control subsystem (30) produces a low output value, the control subsystem (30) may operate the beam steering devices associated with that subset (i.e., some or all) in parallel to adjust the beam pointing vector of the beam (i.e., adjust the azimuth and / or elevation angle of the beam). The detector (28) may then perform new measurements of the intensity parameters at the three phase states after the beam pointing vector has been adjusted, and the control subsystem (30) may then adjust the modulation (I) based on the new measurements performed by the detector (28). + and I - ) and obtain the new I + and I - For each sub-beam, the objective function F(φ i , δ) may be re-evaluated. This process of adjusting the beam pointing vector, measuring intensity parameters at three phase states, and evaluating the objective function can be repeated iteratively in rapid cycles, thereby correcting beam pointing errors in real time at high convergence rates. In various practical applications, these rapid iterations of cycles are performed at least 100 times per second. The beam alignment process according to the teachings of the present disclosure relies on optimizations performed separately for each beam. In this way, the degrees of freedom for optimization of various beams are uncorrelated, resulting in a reduction in the overall degrees of freedom in the optimization space, compared to traditional blind optimization approaches (e.g., SPGD), which have correlated degrees of freedom and result in a vast optimization space.

[0087] As should be understood, the above discrete 3-step modulation example can be extrapolated up to M-steps for any integer M>2.

[0088] Thus, while the embodiment described thus far relates to a particularly preferred, non-limiting implementation in which the modulation between the current base value and (at least) two discrete modulation values ​​is performed in a stepped manner, other embodiments are possible in which the control subsystem (30) simultaneously varies the current phase of the beams substantially continuously over a range of modulation frequencies, encompassing at least the three aforementioned phase states. Thus, for example, the current phase of the beams may be varied according to a sinusoidal modulation. Other continuous functions (such as variations of a sawtooth) may also be used. In one preferred, non-limiting example, the control subsystem (30) modulates the phase of each beam with a phase change modulation amplitude A at a modulation frequency ω. m The amplitude of the intensity fluctuations occurring at the target at frequencies corresponding to the first two harmonics (frequencies ω and 2ω) is defined as I ω and I 2ω is the phase offset Φ of the central phase state of a beam modulated at frequency ω relative to the average phase. ω is given by the following formula:

[0089]

number

[0090]

number

[0091] Attention is now directed to FIG. 8 , which illustrates a flow diagram detailing a process (method) (800) according to an embodiment of the disclosed subject matter. This process includes aligning a beam of the system (10) on a target by correcting / adjusting the beam pointing direction of each sub-beam according to the value of the sub-beam's objective function. Reference is also made to elements illustrated in FIGS. 1-7 . The process (800) and steps (sub-processes) of FIG. 8 are performed by the system (10) and its associated components, including, for example, the phase modulators (16 a, 16 b, and 16 c), the beam steering devices (20 a, 20 b, and 20 c), the detector (28), and the control subsystem (30). The sub-processes of the process (800) are preferably performed automatically and preferably in real time. The process (800) is an iterative process, whereby the steps of the process (800) may be repeated periodically or continuously, for example, while the system (10) is engaged with its target, for a set number of iterations, or until a stopping criterion is met.

[0092] The process (800) begins at step (802), where the beam pointing vector of a selected current subset (i.e., group) S of beams is adjusted. The beams in subset S can be randomly selected (by the control subsystem (30)) or preselected by preprogramming the beams into the memory of the control subsystem (30). The adjustment is performed by the control subsystem (30) operating beam steering devices associated with the beams in subset S to steer the beams by adjusting the beam pointing parameters of the beams. As described above, the actuation to adjust the beam pointing parameters (which define the beam pointing vector) can be accomplished by sending control inputs to actuators of the beam steering devices to adjust the position and / or orientation (via rotation about one or more rotational axes) of the beam steering optics to change the azimuth and / or elevation of the beam. The beam pointing parameters (i.e., the defined beam pointing vectors) of the beams are preferably adjusted in a random, pseudo-random, or predetermined manner. For example, for each beam, the associated beam steering device may adjust the azimuth and / or elevation by a random or pseudo-random amount, preferably selected from predefined azimuth and elevation ranges. Preferably, the azimuth / elevation adjustments of the steered beams are varied within a subset of the beams to ensure that the center of the combined beam remains in more or less the same position.

[0093] It is noted that process (800) may include an initialization step (not shown) prior to the first iteration (i.e., the first execution of step (802)) in which the control subsystem (30) selects a selected subset S of beams. In some non-limiting implementations, the subset is the entire set of beams (i.e., the subset includes all of the beams). However, it is noted that utilizing the entire set of beams as the subset may result in a situation in which all of the beams are steered such that the combined beam deviates from the target. Therefore, in a more preferred non-limiting implementation, the subset is a majority subset (i.e., including at least half of the beams, but not all of the beams), which in some cases includes at least 70% of the beams, in other cases includes at least 80% of the beams, and in still other cases includes at least 90% of the beams. In other non-limiting implementations, the subset is a minority subset (i.e., including less than half of the total number of beams). In such a minority subset implementation, the beams are preferably divided into equal groups such that each subset includes the same number of beams. For example, if there are 100 beams, the beams may be divided into 5 subsets (groups) of 20 beams each.

[0094] Process (800) then moves to step (804), where, for each beam in S, control subsystem (30) actuates the beam's corresponding phase modulator to modulate the beam's current phase between at least three phase states. In some preferred, but non-limiting, implementations, the modulation of different beams is discrete step modulation (comprising modulation levels resulting in phase shifts of +δ and −δ), which is performed sequentially to isolate the contribution to intensity measurements from each modulation. Thus, when using discrete step modulation, step (804) is divided into m substeps, where m is the number of beams in subset S (i.e., m is the size of S). When continuous (e.g., sinusoidal) modulation is used, all of the beams in S can be modulated simultaneously, and frequency analysis (e.g., Fourier analysis) can be used to isolate the contribution to intensity measurements from each modulation.

[0095] In step 806, detector 28 measures the intensity (in response to illumination by the beam) to obtain intensity parameter measurements for the beam in S as the current phase of the beam is modulated between (at least) three phase states. If sequential discrete step modulation is used, an intensity measurement is made for each discrete step (for each beam). In such a case, detector 28 preferably measures the intensity parameter measurements I(φ), I(φ), and I(φ). i +δ) and I(φ i The detector 28 is synchronized (via the control subsystem 30) with the phase modulator to perform (at least) three intensity measurements for each beam of S at time intervals that coincide with the modulation interval of the beam to obtain I(φ). Specifically, for a given beam of S, the detector 28 measures I(φ i The detector 28 measures the intensity parameter during a modulation interval in which the current phase of a given beam is modulated to assume a first changed phase state, e.g., a phase offset of +δ, so as to measure I(φ iThe detector (28) measures the intensity parameter during a modulation interval in which the current phase of the given beam is modulated to a second altered phase state, e.g., a phase offset of -δ, to measure I, and the detector (28) measures the intensity parameter during a modulation interval in which the current phase of the given beam is in an unaltered phase state, e.g., no phase offset, to measure I. If continuous (e.g., sinusoidal) modulation is used, intensity measurements are made for each modulation frequency.

[0096] In step 808, the intensity fluctuation resulting from the modulation is obtained. In an implementation using sequential discrete step modulation, the intensity fluctuation resulting from the modulation (I + and I - ) are the intensity parameter measurements I0, I(φ i +δ), and I(φ i -δ). In implementations using continuous (e.g., sinusoidal) modulation where the phase is continuously modulated at the modulation frequency, the intensity fluctuations resulting from the modulation are obtained by the control subsystem (30) identifying the fluctuations in the measured intensity resulting from the modulation occurring at multiple harmonic frequencies of the modulation frequency. This can be the modulation frequency itself and a second harmonic (twice the modulation frequency). As mentioned above, frequency analysis (e.g., Fourier analysis) can be used to separate the intensity fluctuations resulting from each modulation.

[0097] In step 810, the control subsystem 30 calculates, for each beam in S, a current value representing the beam's relative intensity based at least in part on the intensity parameters monitored in each of the at least three phase states. In particular, the control subsystem 30 determines the current value of a beam in S by evaluating an objective function for that beam using the intensity fluctuations resulting from the modulation (obtained in step 808). By representing a relative intensity, the calculated current value also indicates the current position of the beam's center with respect to the target (where the current position is defined by the beam's beam steering vector). As previously mentioned, in a preferred embodiment, the calculated value represents the beam's relative intensity with respect to the sum of all beams in S.

[0098] In a non-limiting implementation using sequential discrete step modulation and assuming a Gaussian beam model, the objective function F takes as inputs δ, I as described above with reference to equation (4): + , and I - In a non-limiting implementation using continuous (e.g., sinusoidal) modulation and assuming a Gaussian beam model, the objective function F can take A as input, as described above with reference to equation (5). m , I ω , and I 2ω It is possible to take.

[0099] If there are m beams in the subset, then m objective functions are evaluated to generate m values ​​in step 810. The m objective functions may be evaluated, for example, in parallel, serially, or in groups. Typically, the control subsystem 30 can complete the evaluation of the objective functions for all beams in the subset within a few hundred microseconds (or less if the subset is a small subset).

[0100] Referring now to steps (812)-(816), it should first be noted that these steps (812)-(816) are performed for each beam in subset S. However, for clarity and brevity, the details of steps (812)-(816) are only described for a particular beam in subset S. The performance of steps (812)-(816) can be performed for all beams in S, for example, in parallel, serially, or in groups.

[0101] In step (812), the control subsystem (30) performs a comparison based on the calculated current value (generated in step (810)) and the previous value (and optionally one or more additional prior values) to determine whether to accept the beam steering adjustment (performed in step (802)) for the beam or reject the beam steering adjustment (performed in step (802)) for the beam. The previous value is the value calculated by the previous evaluation of the objective function for that beam (from the previous iteration of process (800)). The previous value may be a value retrieved from a stored memory of the control subsystem (30) or a stored memory electronically linked to the control system (30). Generally, in step (812), the control subsystem (30) performs a comparison based on the calculated current value and the previous value (and optionally one or more additional prior values) to form a comparison metric, and then determines whether the comparison metric meets a threshold criterion.

[0102] In one non-limiting implementation, the control subsystem (30) applies proportional control in step (812) by comparing the calculated current value (produced in step (810)) with the previous value. In this somewhat simplified implementation, the comparison metric is defined as the change between the current value and the previous value. If the current value is greater than the previous value (i.e., a positive comparison metric), this indicates that the beam steering adjustments made to the beam (performed in step (802)) have reduced the alignment error of that beam (i.e., the beam steering adjustments made in step (802) have moved the center of the beam closer to the center of the target).

[0103] In another non-limiting example, the control subsystem 30 applies derivative-based control in step 812 by comparing the rate of change of the calculated relative intensities. In such an implementation, the comparison metric is the rate of change of the calculated relative intensities, formed based on multiple prior values. For example, at a given iteration i, the control subsystem 30 calculates the change between the relative intensities calculated in iterations i and i-1 and compares that change to the change between the relative intensities calculated in iterations i-1 and i-2 to determine the rate of change of the calculated relative intensities. If the rate of change meets a threshold criterion, e.g., if the rate of change is greater than a certain value (which may be a fixed value or a value that varies from iteration to iteration), it indicates that the beam steering adjustments for that beam (performed in step 802) have reduced the alignment error of that beam.

[0104] It is noted that if the previous value for a particular beam is not available (e.g., if the steering adjustment performed in step (802) was the first steering adjustment for a beam in any subset), the current value can be compared to a default initial value, e.g., 0 or a value between the maximum and minimum achievable values ​​of the objective function.

[0105] In step 812, if the comparison metric meets the threshold criterion, i.e., if the beam steering adjustment in step 802 reduces the alignment error, process 800 moves from step 812 to step 814, where control subsystem 30 accepts the beam steering adjustment for the beam (performed in step 802). For example, using a simplified proportional control example, process 800 moves from step 812 to step 814 if the current value is greater than the previous value (i.e., the comparison metric is positive). By accepting the beam steering adjustment, the beam continues to be steered to the current beam pointing / steering position. In addition, the current value for the beam is stored (e.g., in memory of control subsystem 30 or in a memory linked to control subsystem 30). The current value is stored so that in the next iteration in which the same beam is steered, the stored current value serves as the "previous value" for comparison in step 812.

[0106] If the comparison performed in step (812) does not meet (i.e., fails to meet) the threshold criterion, i.e., if the beam steering adjustment in step (802) does not reduce the alignment error, process (800) moves from step (812) to step (816), where control subsystem (30) overrules the beam steering adjustment for that beam (performed in step (802)). For example, using a simple proportional comparison example, process (800) moves from step (812) to step (816) if the current value is not greater than the previous value (i.e., the comparison metric is negative). In one embodiment, overruling in step (816) further includes control subsystem (30) actuating the beam steering device for the beam to steer the beam back to its previous beam pointing position (i.e., the beam pointing (i.e., steering) position the beam was in immediately prior to step (802)). For example, consider a particular beam having a beam pointing vector pointing to point P at the start of a given iteration. Using a simple proportional control example, if in step (802) a beam is steered so that its beam pointing vector points to point Q, and in step (812) it is determined that the current value is less than the previous value, then the control subsystem (30) (as part of step (816)) may actuate the beam steering device for that particular beam to steer the beam back to point at point P.

[0107] After steps 812-816 have been performed for all beams in subset S, process 800 proceeds to step 818, where the next beam subset S is selected (e.g., by control subsystem 30) for beam steering adjustment. The next subset may be identical to the preceding subset (i.e., the immediately preceding subset) or may be different from the preceding subset. The two subsets may contain common beams or may be disjoint subsets (if the subsets are minority subsets). If the subset is a majority subset, the next and previous subsets necessarily intersect (i.e., at least one of the beams is included in both the preceding and next subsets).

[0108] The process 800 then returns to step 802, where the beam steering devices for the next subset of beams are actuated (randomly, pseudo-randomly, or in a predetermined manner) by the control subsystem 30 to adjust the beam pointing vectors of the beams. Steps 804 through 816 are then performed as before, but for the beams in the next subset.

[0109] Generally, process 800 is repeated multiple times for each subset. For example, if the beam is divided into three subsets, process 800 may be performed such that steps 802-816 are performed for a first subset, and in step 818 a second subset is selected, steps 802-816 are performed for the second subset, and in step 818 a third subset is selected, steps 802-816 are performed for the first subset, and in step 818 a second subset is selected, and so on until process 800 terminates.

[0110] At each iteration of process 800, the control subsystem 30 forms a comparison metric and evaluates the comparison metric against a threshold criterion. For example, the comparison metric may be formed by comparing the current value of a beam in the selected subset with the previous value of that beam to check whether the current value has increased (indicating the beam is closer to alignment than previously) or decreased (indicating the beam is more out of alignment than previously). As explained, if a previous value is not available for a particular beam (i.e., if the beam has not previously been steered to perform intensity measurements and objective function evaluations), a default initial (i.e., dummy) value may be used.

[0111] The control subsystem (30) can complete the evaluation of the objective function for all beams in a single subset within a few hundred microseconds, so that the evaluation of the objective function for all beams can typically be completed in less than 1 millisecond.

[0112] It is noted that the subsets are selected such that each beam is steered and its objective function is evaluated multiple times (i.e., in multiple iterative steps), e.g., on the order of tens of thousands of times, while the system 10 engages the target. The assignment of beams to subsets is performed by the control subsystem 30 and is preferably static, i.e., the control subsystem 30 assigns beams to each subset prior to the first execution of the process 800. However, in certain non-limiting implementations, the assignment of beams to each subset can be dynamic.

[0113] In some non-limiting implementations, for each beam, the corresponding beam steering device adjusts (in step 802) the beam pointing vector of the beam by a relatively small amount, which in some cases may be proportional to a fraction of the previous value (i.e., the value calculated in step 810 in the previous iteration for the beam). It is also noted that while two exemplary control techniques are described above, control subsystem 30 may apply a variety of control algorithms, including, for example, proportional-integral-derivative (PID) control, as part of performing steps 812-816, as is well known in the art of control systems.

[0114] The process 800 may continue to iterate for the duration that the system 10 is engaged with the target, or until a stopping criterion is reached, such as, for example, the beam pointing error for each of the beams is within an acceptable tolerance, or the calculated values ​​(via evaluation of an objective function) representing the relative intensities for all of the beams are at a sufficient intensity level. For example, although not shown in the drawings, the control subsystem 30 may terminate the beam adjustment process after several iterations when it successfully reaches step 814 for all of the beams.

[0115] As mentioned above, in certain non-limiting implementations, process 800 may include an initialization step in which control subsystem 30 selects a selected subset S of beams. The initialization step may include performing steps similar to steps 804-810, in which phase modulators of beams in the subset are activated to modulate the current phases of the beams, intensity measurements are performed, intensity variations are obtained (based on the intensity measurements), and objective functions of all beams in the selected subset are evaluated (based on the intensity variations) to generate calculated current values ​​for each beam in the subset. In a first performance of step 802, beam steering devices of beams in the subset may be activated to steer the beams proportionally or according to the calculated current values.

[0116] In certain non-limiting implementations, the beam steering in step 802 may be performed in a non-random manner, for example, if the previous value is the result of performing step 814, then each beam in the subset may be steered according to the previous value associated with that beam.

[0117] It is noted that, in general, the beam alignment correction process (i.e., process (800)) can be incorporated as a subprocess of the process for phase correction described in WO 2020 / 016824 A1. Because both the phase correction process and the alignment correction process utilize intensity measurements, particularly intensity fluctuations resulting from modulation of the beam's current phase, the phase correction process and the alignment correction process can be implemented in a nested-loop process. For example, step (802), in which the beam's current phase is modulated between at least three phase states, can generally continue throughout the duration of engagement of system (10) with the target, since it is required to correct for phase differences between the beams that constantly evolve over the engagement time with the target. The remaining steps of process (800) can be repeated (looped) for each beam to perform alignment correction for each beam. In working conditions where beam misalignment is dynamic, e.g., due to varying atmospheric conditions caused by atmospheric turbulence, thermal stresses, focusing errors, etc., the alignment correction process may continue for the entire duration of the target engagement time, preferably with a high revisit rate. In working conditions where beam misalignment is static, e.g., due to instrumentation, the alignment correction process may continue periodically or intermittently.

[0118] As mentioned above, the detector (28) may be integrated with or cooperate with an imaging system used to aim the system (10). Figure 9 schematically illustrates an example arrangement of the detector (28) with an imaging system within the context of the systems illustrated in Figures 1 and 3. The image sensor (camera 44) is positioned to receive radiation (coherent beams (14a, 14b, and 14c)) emitted by the beam sources (12a, 12b, and 12c). The beam director (18) (or multiple directors (18a, 18b, and 18c)) is preferably configured to simultaneously direct and focus the beam onto the detector (28) and the camera (44). The control subsystem (30) receives (via a data connection) the image generated by the camera (44) and identifies image pixels that roughly correspond to the position of the detector (28).

[0119] FIG. 10 schematically illustrates an exemplary arrangement of a detector (28) with an imaging system within the context of the systems shown in FIGS. 2 and 4, in which radiation (beams (14a, 14b, and 14c) transmitted by a beam source, not shown) is reflected from a target (34). A beam splitter (46) is positioned to receive incident radiation (light) reflected from the target (34) and collected by a telescope arrangement (42). The beam splitter (46) directs a portion of the received radiation to the detector (28), which senses the intensity of the laser spot on the "target" for processing by the control subsystem (30), and directs a portion of the received radiation to the camera (46), which forms an image spot on the target emanating from the reflected radiation. The control subsystem (30) receives the image generated by the camera (46) (via a data connection) and identifies image pixels that roughly correspond to the location of the spot on the target.

[0120] While the exemplary objective functions used to implement the beam alignment methods described herein are derived based on the assumption of a general Gaussian beam model, it should be noted that this beam model assumption is merely one non-limiting example of a beam model assumption that is particularly useful within the context of certain coherent beam combining systems. Other beam models, including but not limited to, Bessel beam models and higher-order (or transverse) Gaussian beam modes, may also be used. Objective functions based on beam phase modulation via discrete stepped modulation or continuous / sinusoidal modulation may also be derived using the same or similar principles used in deriving the objective functions of equations (4) and (5), as should be apparent to those skilled in the art.

[0121] The description of various embodiments of the present disclosure has been presented for illustrative purposes, but is not intended to be exhaustive and limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terms used herein have been selected to best explain the principles of the embodiments, practical applications or technical improvements over technologies found in the market, or to enable those skilled in the art to understand the embodiments disclosed herein.

[0122] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0123] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments and / or does not necessarily exclude the incorporation of features from other embodiments.

[0124] It will be appreciated that certain features of the invention that are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for clarity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination, or as appropriate in any other described embodiment of the invention. Certain features described in the context of various embodiments are not considered essential features of an embodiment, unless the embodiment cannot be practiced without those elements.

[0125] To the extent that the appended claims have been drafted without multiple dependencies, this has been done solely to accommodate formal requirements in jurisdictions that do not permit such multiple dependencies. It should be noted that all possible combinations of features implied by multiple dependency of the claims are expressly contemplated and should be considered part of the present invention.

[0126] While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims.

Claims

1. 1. A method of aligning a beam of a coherent beam combining (CBC) device directed towards a target, the beam having an associated adjustable phase modulator and a beam steering device, the method comprising: (a) for each of the beams in the subset of beams, actuating a corresponding beam steering device to steer the beam; (b) for each of the beams in the subset, actuating a corresponding phase modulator to modulate the current phase of the beam between at least three phase states; (c) monitoring an intensity parameter that varies as a function of the intensity of radiation incident on said target; (d) for each of the beams in the subset, calculating a current value representing the relative intensity of the beam based at least in part on the intensity parameters monitored in each of the at least three phase states, the calculated value indicating a current position of the beam relative to the target.

2. The method of claim 1 , further comprising: (e) for each of the beams in the subset, adjusting a beam pointing direction of the beam according to the calculated current value.

3. (e) for each of the beams in the subset, forming a comparison metric based on the calculated current value and at least one prior value representing the relative intensity associated with the beam; (f) if the comparison metric meets a threshold criterion; (i) storing the calculated current value; The method of claim 1 , further comprising: (ii) keeping the beam steered to the current position.

4. 4. The method of claim 3, further comprising: (g) activating the beam steering device of the beam to steer the beam to a previous steering position if the comparison metric does not meet the threshold criterion.

5. (g) selecting a next subset of the beams; The method of claim 3, further comprising: (h) repeating (a) through (g).

6. 2. The method of claim 1, wherein the step of calculating the current value is performed by evaluating an objective function having inputs based on the intensity parameters monitored in each of the at least three phase states.

7. The method of claim 6 , wherein for each beam, the objective function has a single maximum value that is achieved when the beam is positioned so that it is centered on the target.

8. The method of claim 1 , wherein the intensity parameter is obtained from a beam-sensitive sensor located on the target.

9. The method of claim 1 , wherein the intensity parameter is obtained from a sensor positioned to sense radiation reflected from the target.

10. The method of claim 1 , wherein for each of the beams in the subset, the current value represents the relative intensity of the beam to the sum of all of the beams in the subset.

11. The method of claim 1 , wherein the beams include at least 10 beams.

12. The method of claim 1 , wherein the current phase of each of the beams is modulated in a stepped manner between the at least three phase states.

13. The method of claim 1 , wherein the current phase of each of the beams is modulated substantially continuously over a range of modulation frequencies encompassing the at least three phase states.

14. The method of claim 1 , wherein the current phase of each of the beams is sinusoidally modulated over a range of modulation frequencies that encompasses the at least three phase states.

15. The method of claim 1 , wherein the current phases of multiple beams are varied sequentially.

16. The method of claim 1 , wherein the subset is a majority subset.

17. The method of claim 1 , wherein the subset includes all of the beams.

18. The method of claim 1 , wherein the subset is a minority subset.

19. The method of claim 1 , wherein the beams within the subset are steered in a random or pseudo-random manner.

20. (a) an array of beam sources configured to generate a plurality of coherent beams for directing toward a target; (b) a plurality of adjustable phase modulators associated with the beam sources to enable adjustment of the relative phase offset of the beams; (c) a plurality of beam steering devices associated with the array of beam sources configured to steer the coherent beams; (d) a detector positioned to monitor an intensity parameter that varies as a function of the intensity of radiation incident on said target; (e) a control subsystem associated with the detector to receive the intensity parameter, the control subsystem further associated with the phase modulator and the beam steering device; (i) operating the beam steering devices associated with a subset of the beam sources to steer the beams generated by the beam sources in the subset; (ii) for each of the beams in the subset, actuating a corresponding phase modulator to modulate the current phase of the beam between at least three phase states; (iii) for each of the beams in the subset, calculating a current value representing a relative intensity of the beam based at least in part on the intensity parameter monitored in each of the at least three phase states, such that the calculated value indicates a current position of the beam with respect to the target; a control subsystem.

21. The control subsystem includes:

21. The system of claim 20, further configured: (iv) for each of the beams in the subset, actuating a corresponding beam steering device to adjust a beam pointing direction of the beam according to the calculated current value.

22. The control subsystem includes: (iv) for each of the beams in the subset, forming a comparison metric based on the calculated current value representing the relative intensity of the beam and at least one prior value representing a relative intensity value associated with the beam; and (v) if the comparison metric meets a threshold criterion; (1) storing the calculated current value; and (2) so that the beam remains steered to that position; The system of claim 20 further comprising:

23. The control subsystem includes:

23. The system of claim 22, further configured to: (vi) actuate the beam steering device of the beam to steer the beam to a previous steering position if the comparison metric does not meet the threshold criterion.

24. The control subsystem includes: (vi) selecting a next subset of the beams; (vii) The system of claim 22, further configured to repeat (i) through (vi).

25. 21. The system of claim 20, wherein the control subsystem is configured to calculate the current value by evaluating an objective function having inputs based on the intensity parameters monitored in each of the at least three phase states.

26. 26. The system of claim 25, wherein for each beam, the objective function has a single maximum value that is achieved when the beam is positioned so that it is centered on the target.

27. The system of claim 20 , wherein the detector is located at the target.

28. 21. The system of claim 20, wherein the detector is positioned to sense radiation reflected from the target.

29. 21. The system of claim 20, wherein for each beam in the subset, the current value represents the relative intensity of the beam to the sum of all the beams in the subset.

30. 21. The system of claim 20, wherein the array of beam sources comprises at least 10 beam sources.

31. 21. The system of claim 20, wherein the current phase of each of the beams is modulated in steps between the at least three phase states.

32. 21. The system of claim 20, wherein the current phase of each of the beams is substantially continuously modulated over a range of modulation frequencies encompassing the at least three phase states.

33. 21. The system of claim 20, wherein the current phase of each of the beams is sinusoidally modulated over a range of modulation frequencies encompassing the at least three phase states.

34. 21. The system of claim 20, wherein the current phases of multiple beams are varied sequentially.

35. 21. The system of claim 20, wherein the subset is a majority subset.

36. The system of claim 20 , wherein the subset includes all of the beam sources.

37. 21. The system of claim 20, wherein the subset is a minority subset.

38. 21. The system of claim 20, wherein the control subsystem is configured to operate the beam steering device to steer the beams in the subset in a random or pseudo-random manner.

Images