EP-4735952-A1 - SYSTEMS AND METHODS FOR COHERENT BEAM COMBINING

EP4735952A1EP 4735952 A1EP4735952 A1EP 4735952A1EP-4735952-A1

Abstract

Multi-Channels coherent beam combining (CBC) using a mechanism for phase and/or polarization locking that uses a reference optical beam and an array of optical detectors each detector being configured and located to detect overall intensity of an optical interference signal caused by interfering of the reference beam and a beam of the respective channel, where the fast intensity per-channel detection allows simultaneous and quick phase/polarization locking of all channels for improving beam combining system performances.

Inventors

SCHIFFER, ZEEV
NAZAROV, Andrey
LEVY, DANIEL

Assignees

Elbit Systems Electro-Optics - Elop Ltd.

Dates

Publication Date: 20260506
Application Date: 20240716

Claims (20)

1. A method for locking phases of an array of channel optical beams propagated through channels of a coherent beam combining (CBC) system, the method comprising at least: (a) generating a reference optical beam; (b) generating an array of sample optical beams, sampled from the array of channel optical beams, and causing optical interference between each part of the reference optical beam and a corresponding sample optical beam, generating thereby an array of corresponding interference optical signals; (c) for each channel "I" and channel optical beam's phase testing iteration "i" , for testing channel beam phase cpu: sequentially changing a phase difference Atpii between the phase of the channel optical beam or part thereof and the corresponding part of the reference optical beam, by a discrete set of phase shifts 8<pk, where k=l...m, "m" being an integer number larger than 1, defining a discrete sequence of "m" shifting-modes; measuring updated power parameter value Puk of the interference optical signal for each phase shift 6<pk, resulting in a corresponding set 1 to m power parameter values, each associated with a different phase shift 8<pk, for the respective channel "I" and respective "i" phase testing iteration, where the power parameter is associated with an intensity of the corresponding interference signal of the corresponding channel "I" and phase testing iteration "i"; determining an updated value of a modes-parameter Ru, mathematically related to all "m" detected power parameter values Puk for the respective channel "I" and the respective phase testing iteration "i"; and (d) controlling phase locking of phase of each channel optical beam according to the phase of the corresponding channel optical beam that is associated with the value of the modes-parameter of the corresponding channel that complies with at least one locking criterion.
2. The method of claim 1, wherein the changing of the phase difference between each channel optical beam and the reference optical beam is carried out by one or more of: (i) phase-modulating the reference optical beam by using a phase modulation device configured to modulate the entire reference optical beam, according to a discrete sequence of phase shifts 6cpi<; (ii) phase shifting the phase of each channel optical beam and/or its corresponding sample optical beam, using a phase shifter (PS) of the respective channel, by a discrete sequence of phase shifts 6cpi<; and/or (iii) phase shifting each portion of the reference optical beam separately, by a discrete sequence of phase shifts 6<pk, using an array of phase modulators (PMs) located such that each PM receives therethrough and phase-modulates a different portion of the reference optical beam directed to interfere with a corresponding sample optical beam.
3. The method of claim 1, wherein the at least one criterion according to which the phase of each channel optical beam is controllably locked is adjustable based on a currently desired spatial phase distribution of a combined optical beam outputted from the CBC system.
4. The method of claim 3 further comprising controlling desired phase distribution of the combined optical beam by setting a different locking criterion to each channel, for causing intentional specific phase-differences between the channel optical beams, for enabling a desired phased-array beam steering of the combined optical beam.
5. The method of claim 1, wherein the at least one locking criterion comprises at least one of: an extremum value of the modes-parameter, such that the phase of the channel optical beam is locked to the phase <pn that yielded the minimum or maximum modes- parameter Rti value or an absolute value thereof | Ru |, between all modes-parameter values determined for all phases of the channel optical beam being tested in each phase locking session; and/or a minimum value of an absolute distance D between the modes-parameter Rti and a predefined desired modes-parameter Rd: minimum of: Du=| Rd - Ru| | , between all modes-parameter values determined for all phases of the channel optical beam being tested in each phase-locking session.
6. The method of claim 1, wherein, for each channel "I", the overall number of testing iterations in each phase-locking session is limited to a testing timeframe or to a predefined limiting number of testing iterations.
7. The method of claim 6, wherein the phase-locking method steps are performable in an ongoing and/or repeated manner such that phase-locking sessions for each channel are repeatedly performed during operation of the CBC system.
8. The method of claim 1, wherein the overall timeframe for each phase testing iteration is limited to a predefined testing timeframe ATt that corresponds to an estimated timespan AT e that is associated with an estimated timespan of a phase shifting sequence At m for measuring all "m" different phase shifting-modes for shifting the phase difference between the reference optical beam and the corresponding sample optical beam for the particular channel "I" , to enable detecting all power parameter values of all "m" shifting-modes.
9. The method of claim 1, wherein the number of shifting-modes is m=2 such that in a first shifting-mode, k=l, the phase difference between the phase of the reference and channel optical beam is shifted by 6<pi=0 and in a second shifting-mode, k=2, the phase difference is shifted by 6 2=71/2.
10. The method of claim 9, wherein the modes-parameter Ru of channel "I" and testing iteration "1" is mathematically proportional or equal to 7(loi-li) 2 + (Io 2 -I 2 ) 2 , wherein: I01 and I02 are predefined constants of desired intensity values of the interference optical signal, respectively, for the first and second shifting-modes: k=l and k=2; 11 is an updated intensity value of the specific channel "I" and specific iteration "1" for the first shifting-mode k=l, which is proportional to its corresponding power parameter value PM; and 12 is an updated intensity value of the specific channel "I" and specific and specific iteration "1", for the second shifting-mode k=2, which is proportional to its corresponding power parameter value PM.
11. A phase-locking subsystem, for locking temporary phase of each optical beam of a plurality of channel optical beams propagated through channels of a coherent beam combining (CBC) system, wherein the phase-locking subsystem is configured to temporarily lock a phase of each channel optical beam, based on: changing a phase-difference between a reference optical beam and each channel optical beam of each channel "I" of the CBC system, by shifting the phase difference Acpi by a set of "m" predetermined phase shifts 8<pk, wherein "k" is an integer number from 1 to m and "m" is an integer number larger than 1, defining thereby an "m" number of shifting-modes; and controlling phase-locking of phase of each channel optical beam according to the phase of the corresponding channel optical beam that is associated with a value of a modes-parameter of the corresponding channel that complied with at least one locking criterion, wherein the modes-parameter value is determined based on values of an intensity related power parameter measured for all shifting-modes.
12. A phase-locking subsystem for locking phases of an array of channel optical beams propagated through channels of a coherent beam combining (CBC) system, the phaselocking subsystem comprising at least: a referencing unit configured to generate a reference optical beam; a sampling unit configured to generate an array of sample optical beams, sampled from an array of channel optical beams, propagated via channels of the CBC system and cause optical interference between each part of the reference optical beam and a corresponding sample optical beam, generating thereby an array of corresponding interference optical signals; and a processing and control unit, configured to control phase-locking of phase of each channel optical beam of each channel at least by performing the following steps for each channel "I" and channel optical beam's phase testing-iteration "i" , for testing channel beam phase cpu: (i) sequentially changing a phase difference Atpii between the phase of the channel optical beam and the reference optical beam, by a discrete set of phase shifts 6<pk, where k=l...m, "m" being an integer number larger than 1, defining a discrete sequence of "m" shifting-modes; (ii) measuring updated power parameter value Puk of the interference optical signal for each phase shift 6<pk, resulting in a corresponding set of 1 to m power parameter values, each associated with a different phase shift 6<pk, for the respective channel "I" and respective "i" phase testing iteration, where the power parameter is associated with an intensity of the corresponding interference signal of the corresponding channel "I" and phase testing iteration "i"; and (iii) determining an updated value of a modes-parameter Ru, mathematically related to all "m" detected power parameter values Puk for the respective channel "I" and the respective phase testing iteration "i"; and (iv) controlling phase-locking of phase of each channel optical beam according to the phase of the corresponding channel optical beam that is associated with the value of the modes-parameter of the corresponding channel that complied with at least one locking criterion.
13. The phase-locking subsystem of claim 12, wherein the changing of the phase difference between each sample optical beam and the reference optical beam is carried out by one or more of: (i) phase-modulating the reference optical beam by using a phase modulation device configured to modulate the entire reference optical beam, by a discrete sequence of phase shifts 6< k; (ii) phase shifting the phase of each channel optical beam and/or its corresponding sample optical beam, using a phase shifter (PS) of the respective channel, by a discrete sequence of phase shifts 6< k; and/or (iii) phase shifting each portion of the reference optical beam separately, by a discrete sequence of phase shifts 6<pk, using an array of phase modulators (PMs) located such that each PM receives therethrough and phase-modulates a different portion of the reference optical beam directed to interfere with a corresponding sample optical beam, wherein the sample optical beams are sampled from the output optical beams of the CBC system.
14. The phase-locking subsystem of claim 12, wherein the at least one criterion according to which the phase of each channel optical beam is controllably locked depends on a currently desired phase distribution of a combined optical beam outputted from the CBC system.
15. The phase-locking subsystem of claim 14, wherein the processing and control unit is further configured to control desired phase distribution of the combined optical beam by setting a different locking criterion to each channel, for resulting with intentional specific phase-differences between the reference optical beam and each of the channel optical beam for each channel, such as to enable a desired phased-array beam steering of the combined optical beam.
16. The phase-locking subsystem of claim 12, wherein the at least one locking criterion comprises at least one of: an extremum value of the modes-parameter, such that the phase of the channel optical beam is locked to the phase <pn that yielded the minimum or maximum modes- parameter Rti value or an absolute value thereof | Rn | ; and/or a minimum value of an absolute distance between the modes-parameter Rti and a predefined desired modes-parameter Rd: minimum of: | Rd - Rn|.
17. The phase-locking subsystem of claim 12, wherein, for each channel "I", the overall number of phase testing iterations in each phase-locking session is limited to a testing timeframe or to a predefined limiting number of testing iterations.
18. The phase-locking subsystem of claim 12, wherein the phase-locking steps are performable in an ongoing and/or repeated manner such that phase-locking sessions for each channel are repeatedly performed during operation of the CBC system.
19. The phase-locking subsystem of claim 12, wherein the overall timeframe for each testing iteration is limited to a predefined testing timeframe ATt that corresponds to an estimated timespan of a modulation-sequence At m timeframe for measuring all "m" different phase shifting-modes for shifting difference between the reference optical RECTIFIED SHEET (RULE 91) beam and the corresponding sample optical beam for the particular channel "I" , to enable detecting all power parameter values of all "m" shifting-modes.
20. The phase-locking subsystem of claim 12, wherein the number of shifting-modes is m=2 such that in a first shifting-mode, k=l, the phase difference between the phase of the reference and sample optical beam is shifted by 6<pi=0 and in a second shifting-mode, k=2, the phase difference is shifted by 6 2=71/2.

Description

SYSTEMS AND METHODS FOR COHERENT BEAM COMBINING [0001] The present disclosure relates in general to systems and methods for coherent beam combining incorporating a phase locking and/or polarization locking mechanism. BACKGROUND [0002] Near diffraction-limit High power lasers, such as amplification fiber lasers (fiber amplifiers), have a variety of scientific and industrial implementations and enable achieving high power output optical signals having excellent beam quality. However, for a single fiber laser, maintaining its near diffraction limit beam quality may be limited, mainly due to three physical phenomena: Stimulated Brillouin Scattering, Stimulated Raman Scattering and modal thermal instability. To overcome these limitations, techniques for combining multiple optical beams are used, which combine multiple optical beams emanating from multiple fiber lasers into a single combined optical beam. [0003] The techniques and system layouts used for combining multiple optical beams depend, inter alia, on the spectral coherency of these optical beams, where combination of spectrally coherent optical beams, known as coherent beam combining (CBC), may be carried out by using a phased array (also known as "side-by-side CBC) such as an array of collimators, each collimating a separate optical beam. Other techniques for CBC involve using one or more diffraction grating elements (also known as "field aperture techniques"). SUMMARY [0004] Aspects of disclosed embodiments pertain to a method for locking phases of an array of channel optical beams propagated through channels of a coherent beam combining (CBC) system, the method comprising at least: [0005] (a) generating a reference optical beam; [0006] (b) generating an array of sample optical beams, sampled from the array of channel optical beams, and causing optical interference between each part of the reference optical beam and a corresponding sample optical beam, generating thereby an array of corresponding interference optical signals; [0007] (c) for each channel "I" and channel optical beam's phase testing iteration "i", for testing channel beam phase cpu: [0008] sequentially changing a phase difference Atpii between the phase of the channel optical beam or part thereof and the corresponding part of the reference optical beam, by a discrete set of phase shifts 8<pk, where k=l...m, "m" being an integer number larger than 1, defining a discrete sequence of "m" shifting-modes; [0009] measuring updated power parameter value Puk of the interference optical signal for each phase shift 6<pk, resulting in a corresponding set 1 to m power parameter values, each associated with a different phase shift 8<pk, for the respective channel "I" and respective "i" phase testing iteration, where the power parameter is associated with an intensity of the corresponding interference signal of the corresponding channel "I" and phase testing iteration "i"; [0010] determining an updated value of a modes-parameter Ru, mathematically related to all "m" detected power parameter values Puk for the respective channel "I" and the respective phase testing iteration "i"; and [0011] (d) controlling phase locking of phase of each channel optical beam according to the phase of the corresponding channel optical beam that is associated with the value of the modes-parameter of the corresponding channel that complies with at least one locking criterion. [0012] According to some embodiments, the changing of the phase difference between each channel optical beam and the reference optical beam may be carried out by one or more of: [0013] (i) phase-modulating the reference optical beam by using a phase modulation device configured to modulate the entire reference optical beam, according to a discrete sequence of phase shifts 6cpi<; [0014] (ii) phase shifting the phase of each channel optical beam and/or its corresponding sample optical beam, using a phase shifter (PS) of the respective channel, by a discrete sequence of phase shifts 6cpi<; and/or [0015] (iii) phase shifting each portion of the reference optical beam separately, by a discrete sequence of phase shifts 6<pk, using an array of phase modulators (PMs) located such that each PM receives therethrough and phase-modulates a different portion of the reference optical beam directed to interfere with a corresponding sample optical beam. [0016] According to some embodiments, the at least one criterion according to which the phase of each channel optical beam is controllably locked may be adjustable based on a currently desired spatial phase distribution of a combined optical beam outputted from the CBC system, for example, by controlling desired phase distribution of the combined optical beam by setting a different locking criterion to each channel, for causing intentional specific phase-differences between the channel optical beams, for enabling a desired phased-array beam steering of the combined optical beam. [0017] According to some embodiment