www.thalesgroup.com Thales R&T Contribution to ICAN Highly scalable collective techniques for coherent fiber beam locking and combining ICAN workshop Marie Antier 1, Jérôme Bourderionnet 1, Christian Larat 1, Eric Lallier 1, Eric Lenormand 1, Jérôme Primot 2, Arnaud Brignon 1 1 Thales Research & Technology, France 2 ONERA, France Research & Technology
2 / Introduction Advantages of fiber amplifiers Strong confinement of the optical fields Yb doping (3 level system) can be used effectively Small quantum defect è high efficieny Large equivalent surface è heat removal Very high average power Modularity Limitations in pulse regime Nonlinear effects Laser induced damage In order to combine both high average power and high peak power Coherent combining of multiple fiber amplifiers Each amplifier operates at a power level below the parasitic effect threshold The number of fibers is adjusted depending on the applications
3 / Coherent fiber beam combining Control loop Master Oscillator Random phase: open loop Phase modulators N amplified fibers khz perturbations Phase detection In phase: closed loop
4 / Methods for active phase errors measurement and control Hill climbing: Optimization of the peak energy in the far field (power in the bucket) Pros: one detector required, easy implementation Cons: trade-off correction loop bandwidth / number of fibers LOCSET: Heterodyne phase measurement with frequency tag of each fiber Pros: no trade-off bandwidth / nb of channels, one detector Cons: Requires RF components (modulators, mixers) for each channel => expensive Interferometric technique: Measurement of the fringe pattern with a camera Pros: collective measurementè Record of 64 fibers (TRT) Requirements : è Control loop sampling: khz è Scalability to high channel counts M. Vorontsov et. al. Optics Letters, 22, pp 907 (1997) T. Shay et al. Opt. Express 14, 25, (2006) C. Bellanger et al. Opt. Lett. 35, 3931 (2010)
5 / Interferometric technique principle Optical beamlet (collimated fiber output) u Relative phase shift given by fringe sets position u Phase shift for all fibers determined in a single camera aquisition Reference plane wave Phase-lock loop OFF Camera Far field
6 / Interferometric technique principle Optical beamlet (collimated fiber output) Nb of pixels per fiber u Low speed additional loop for combined pattern optimization (calibration) Reference plane wave Camera u High speed stabilization loop locks the fringes position Phase-lock loop ON Far field
7 / Interferometric experimental setup PM Fibers 4 x 4 fibers = 16 combined fibers khz control loop: l l l Hamamatsu camera Labview Real time 16 phase modulators
8 / Characterization of the control loop Loop bandwidth Sampling frequency Combining efficiency: 84% of the theoretical efficiency noise DAC limit Phase error: λ/60 rms, 450Hz bandwidth with 1kHz sampling frequency Shannon s limit è Minimum of 8 pixels per fiber Scalability M. Antier et al. JSTQE 20 (2014)
9 / Scalability of the system 4Mpixels CMOS camera already available à 2000x2000 pixels à High frame rate > 0.5 khz High speed data transfer frame grabber Processing : algorithm with 4.10 5 op/s per fiber è 4 Gop/s with 10,000 fibers è Available with a single GPU à Up to 40,000 combined fibers with one single camera fiber array of 200 x 200 4 Mpixels 12bits 500f/s 25 Gb/s (4 CoaxPress) up to 40 Gb/s (high speed PCI)
10 / Phase plate for optimization of the combining efficiency Near field Far field Normalized intensity 1 0.5 0 Gaussian beams +10% -λ/p 0 λ/p Far field intensity Top-hat beams Normalized intensity (db) 0-10 -20-30 7dB Uniform pupilla -λ/p 0 λ/p Far field intensity
11 / On going and future development Fiber array Hexagonal fused silica holder (on going development -19 fibers) Scalable to a larger number of fibers Massively integrated splitters and phase modulators Si photonics technology Fully packaged on chip 64 modulators (few cm 2 ) at 1.55µm under fabrication Transposable technology with 1µm
12 / Conclusion Conclusion Coherent beam combining of 64 fibers already demonstrated with single image acquisition Successful fiber phase locking with a minimum number of 8 pixels per fiber khz combining loop bandwidth demonstrated Scalability up to 10,000 fibers using off-the-shelf components Optimization of the efficiency of the combining up to 80%
13 / Thales R&T contribution to ICAN-B feedback Master oscillator ICAN P Or ICAN DPA... Transport fiber... Coherent beam combining output Coherent combining of a large number of fibers in the fs regime must be demonstrated within Fet-Open Project Phase measurement & Locking Phase & Delay (synchronization) control Interferometric technique Multiple interleaved loops Spatial recombination Optimization of the near field spatial distribution
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