Phasing Concept For An Array Of Mutually Coherent Laser Transmitters

Abstract

A concept is described for phasing the outputs of a multiple telescope array used as a laser transmitter. The technique uses samples of the transmitted beams to control optical path lengths through the separate telescopes so that the beams add constructively at the receiver. The phasing concept is applicable both to systems that provide inputs to the multiple telescopes by dividing a single laser beam and to systems in which the inputs to the telescopes are multiple, phase-locked laser beams. The approach is also compatible with single line and multiline lasers and does not entail stringent alignment requirements. The concept uses a three-step procedure to find the zero optical path difference condition and to effect fine control of the optical path lengths through the different telescopes. Algorithms are described for estimating the phase mismatch from focal plane measurements. The technique is susceptible to errors induced by local aberrations within the individual telescopes. The errors and one possible solution, the use of redundant measurements, are discussed. Performance requirements for a phased-array laser transmitter are described.