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Adaptive Photonic Phase-Locked Elements (APPLE)

The Adaptive Photonic Phase-Locked Elements (APPLE) program will enable all-electronic combining of high power laser beams within an agile conformal, aperture. The APPLE vision is a coherently-phased array of optical apertures that directly addresses the long standing DoD need for flexible, multi-function laser systems for applications in laser radar, laser target designation, laser communications, and ultimately, high power laser systems. It is a practical approach to synthesizing high-power directed energy weapon lasers from lower power modules, and provides electro-optical systems with the same mission flexibility and performance enhancements that microwave phased arrays provide for RF systems. The APPLE vision is a modular, phase-locked, coherently combined laser system that can be electronically steered to large angles while automatically compensating for optical aberrations introduced by atmospheric turbulence and thermal aberrations within the beam director. The modular feature enables the coherent combining of multiple optical apertures to create a transmit/receive array of arbitrary size, similar to the scaling of RF phased array antennas through addition of multiple RF receive/transmit modules. This approach enables the bootstrapping of higher-power laser systems from multiple lower-power modules, thereby leapfrogging bottlenecks to creating ever higher-power monolithic laser systems. The program will initially focus on the development of a single 2.5 cm aperture at 100 W.

The system is modular, can be scaled to any number of apertures, and supports both transmit and receive functions.

In APPLE, multiple optical apertures are close-packed in an array. The beams emitted by each laser can be electronically steered over a wide FOR, either independently, or as a group. Each aperture is illuminated by a separate branch of a fanout from a fiber laser-based master-oscillator power-amplifier train. The amplitude, phase, and polarization of the energy sent to each aperture can be independently adjusted under control of a combination local/global control system. Detectors are provided at each aperture to measure, local to the aperture, the optical return from a target. These independent measurements are summed to provide a global signal, from which global metrics are computed and stochastic parallel gradient descent algorithms are used to provide feedback for control of the amplitude, phase shift, and polarization controllers for each aperture. Under guidance of this SPGD control system, the transmitted beamlets are altered in intensity, phase, and polarization until the combined far-field beam is coherent and any atmospheric or aero-atmospheric effects between transmitter and target are compensated. Adaptive compensation is inherent to the APPLE approach, and no separate wave-front sensors are required. The system is modular, can be scaled to any number of apertures, and supports both transmit and receive functions.