Local phase control for a planar array of fiber laser amplifiers

Abstract

Arrays of phase-locked lasers have been developed for numerous directed-energy applications. Phased-array designs are capable of producing higher beam intensity than similar sized multi-beam emitters, and also allow beam steering and beam profile manipulation. In phased-array designs, individual emitter phases must be controllable, based on suitable feedback. Most current control schemes sample individual emitter phases, such as with an array-wide beam splitter, and compare to a master phase reference. Reliance on a global beam splitter limits scalability to larger array sizes due to lack of design modularity. This paper describes a conceptual design and control scheme that relies only on feedback from the array structure itself. A modular and scalable geometry is based on individual hexagonal frames for each emitter; each frame cell consists of a conventional lens mounted in front of the fiber tip. A rigid phase tap structure physically connects two adjacent emitter frame cells. A target sensor is mounted on top of the phase tap, representing the local alignment datum. Optical sensors measure the relative position of the phase tap and target sensor. The tap senses the exit phase of both emitters relative to the target normal plane, providing information to the phase controller for each emitter. As elements are added to the array, relative local position data between adjacent phase taps allows accurate prediction of the relative global position of emitters across the array, providing additional constraints to the phase controllers. The approach is scalable for target distance and number of emitters without loss of control.

A fast high-precision six-degree-of-freedom relative position sensor

“…3, based on a design for an extensible phased array laser system. 28,29 Target acquisition algorithms determine the target centroid x c , y c  in the IRFPA pixel array coordinate system. Using camera calibration information, the target centroid x c , y c  is then converted to a target axis θ T , φ T  as polar (θ) and azimuthal (φ) directions in the infrared camera mount mechanical datum coordinate system, using transformations (a LI (b) (e) deduced from innovative calibration algorithms.…”

“…Using camera calibration information, the target centroid x c , y c  is then converted to a target axis θ T , φ T  as polar (θ) and azimuthal (φ) directions in the infrared camera mount mechanical datum coordinate system, using transformations (a LI (b) (e) deduced from innovative calibration algorithms. [30][31][32][33] Conversion from image coordinates to the target axis is accomplished using a composite inverse transformation: 29 ° ° (25) where F is the transformation from the target axis to ideal pixel coordinates, D is a lens distortion mapping, and A is the ideal affine transformation from scene coordinates to pixel coordinates, all determined during camera calibration. The relationship between pixel coordinates and the mechanical mounting is also characterized during fabrication.…”

“…3 shows a phased array laser system with multiple target acquisition sensors. 28,29 The relative position of adjacent target sensors is determined using 6-DOF relative position sensors. The emitter plane maintains a rigid spatial relationship within the mechanical datum coordinate system, and the phase tap maintains a rigid spatial relationship to the reflective plane.…”