High-power, high-radiance lasers are becoming increasingly important for a multitude of applications. Yet it is difficult to scale up the power of a single laser beam due to operational problems like the avoidance of damage to optics and removal of waste heat. One promising solution is to combine multiple co-phased lasers, a technique referred to as Coherent Beam Combination (CBC). However, combining beams coherently with good beam quality and efficiency for high-power lasers has been an enduring challenge in the field of laser engineering. One reason for this is the need to actively monitor and correct the piston and tilt phase of each of the input beams of a CBC system.

X. Wang et al. now present a solution for co-phasing segmented laser arrays using the traditional tools of adaptive optical systems: The Shack-Hartmann wavefront sensor and a conventional segmented deformable mirror. The authors build on techniques developed in the 1980’s to co-phase the segmented primary mirrors of large, ground-based telescopes, including the 10-meter Kecks (Chanan et al. 1986). Such methods have become increasingly important as segmented telescopes have grown in size.

Wang et al.’s extension of this technique exploits diffractive effects caused by piston misalignments within individual Shack-Hartmann subapertures that span the gaps between adjacent laser beams. Large phase jumps split the Shack-Hartmann spot, moving the position of peak intensity in a predictable and easily measurable way. Combined with tilt information from subapertures centered on individual beams, the piston/tilt of each beam may be reconstructed completely from a Shack-Hartmann wavefront sensor alone.

The authors present a closed-loop control framework and demonstrate rapid convergence in both simulation and in experiment. The method converges in 2-3 iterations in closed-loop on a testbed with three laser beams, six Shack-Hartmann subapertures, and a segmented deformable mirror, producing a diffraction-limited spot with > 70% Strehl at red optical wavelengths (the Strehl is the ratio of observed peak intensity to the theoretical peak with a perfect imaging system). The detection and correction of piston misalignment in the laser array results in significant gain: The peak intensity is increased ~50% by correcting piston errors in addition to segment tilt errors.

These experiments demonstrate that laser array co-phasing can be rapid and robust with traditional, inexpensive tools such as Shack-Hartmann wavefront sensors and segmented deformable mirrors, and in particular, extend established techniques used to phase the segments of large telescopes to the correction of piston errors in laser arrays.

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