High-power lasers for directed-energy applications: comment.

Sprangle et al. [Appl. Opt.

Comparative efficiency analysis of fiber-array and conventional beam director systems in volume turbulence.

The performance of two prominent laser beam projection system types is analyzed through wave-optics numerical simulations for various atmospheric turbulence conditions, propagation distances, and adaptive optics (AO) mitigation techniques. Comparisons are made between different configurations of both a conventional beam director (BD) using a monolithic-optics-based Cassegrain telescope and a fiber-array BD that uses an array of densely packed fiber collimators. The BD systems considered have equal input power and aperture diameters.

Target-in-the-loop remote sensing of laser beam and atmospheric turbulence characteristics.

A new target-in-the-loop (TIL) atmospheric sensing concept for in situ remote measurements of major laser beam characteristics and atmospheric turbulence parameters is proposed and analyzed numerically. The technique is based on utilization of an integral relationship between complex amplitudes of the counterpropagating optical waves known as overlapping integral or interference metric, whose value is preserved along the propagation path. It is shown that the interference metric can be directly measured using the proposed TIL sensing system composed of a single-mode fiber-based optical transceiver and a remotely located retro-target.

Deep turbulence effects mitigation with coherent combining of 21 laser beams over 7 km.

We demonstrate coherent beam combining and adaptive mitigation of atmospheric turbulence effects over 7 km under strong scintillation conditions using a coherent fiber array laser transmitter operating in a target-in-the-loop setting. The transmitter system is composed of a densely packed array of 21 fiber collimators with integrated capabilities for piston, tip, and tilt control of the outgoing beams wavefront phases. A small cat's-eye retro reflector was used for evaluation of beam combining and turbulence compensation performance at the target plane, and to provide the feedback signal for control of piston and tip/tilt phases of the transmitted beams using the stochastic parallel gradient descent maximization of the power-in-the-bucket metric.

Scintillation resistant wavefront sensing based on multi-aperture phase reconstruction technique.

A scintillation resistant sensor that allows retrieval of an input optical wave phase using a multi-aperture phase reconstruction (MAPR) technique is introduced and analyzed. The MAPR sensor is based on a low-resolution lenslet array in the classical Shack-Hartmann arrangement and two high-resolution photo-arrays for simultaneous measurements of pupil- and focal-plane intensity distributions, which are used for retrieval of the wavefront phase in a two stage process: (a) phase reconstruction inside the sensor pupil subregions corresponding to lenslet subapertures and (b) recovery of subaperture averaged phase components (piston phases). Numerical simulations demonstrate the efficiency of the MAPR technique in conditions of strong intensity scintillations and the presence of wavefront branch points.