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Experimental demonstration of using divergence cost-function in SPGD algorithm for coherent beam combining with

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    A new tip/tilt control method using a divergence cost function enhances coherent beam combining (CBC) stability. This novel approach improves correction range and performance in fiber amplifier arrays.

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    Area of Science:

    • Optics
    • Laser Physics
    • Optical Engineering

    Background:

    • Coherent beam combining (CBC) is crucial for high-power laser systems.
    • Effective phase and tip/tilt control are essential for maintaining beam quality in CBC.
    • Existing methods like power-in-the-bucket (PIB) have limitations in correction range and operational flexibility.

    Purpose of the Study:

    • To propose and experimentally demonstrate a novel tip/tilt control approach for CBC using a divergence cost function within the stochastic parallel gradient descent (SPGD) algorithm.
    • To compare the performance of the divergence cost function with the conventional PIB cost function for SPGD optimization.
    • To integrate phase-locking and tip/tilt control for enhanced performance in a multi-channel fiber amplifier array.

    Main Methods:

    • Development of a novel tip/tilt control strategy employing a divergence cost function for SPGD optimization.
    • Experimental setup utilizing a seven-channel 2-W fiber amplifier array.
    • Implementation of homemade piezoelectric-ring phase-modulators (PZT PM) for piston aberration correction and adaptive fiber-optics collimators (AFOC) for tip/tilt aberration correction.
    • Utilizing PIB for phase-locking (maximization) and divergence cost function for tip/tilt control (minimization).

    Main Results:

    • Demonstrated successful integration of phase-locking and tip/tilt control in a seven-channel fiber amplifier array.
    • Achieved a significant reduction in divergence metrics from an average of 432 μrad (open loop) to 89 μrad with tip/tilt control.
    • Observed a 32-fold increase in the power within the full width at half maximum (FWHM) of the main lobe during CBC.
    • Maintained a phase residual error below λ/15.

    Conclusions:

    • The proposed divergence cost function offers a superior alternative for tip/tilt control in SPGD-based CBC systems compared to the PIB method.
    • This approach provides a wider correction range, automatic program switching, and immunity to camera intensity saturation.
    • The experimental validation confirms the effectiveness of the novel control strategy for high-performance coherent beam combining.