Fiber laser coherent array for power scaling, bandwidth narrowing, and coherent beam direction control

Coherent beam combining of many fiber lasers realizes power and brightness scaling. We have proposed and demonstrated highly efficient coherent beam combining of N fiber lasers by Y-shaped array coupled with fused fiber couplers. Power-summed single beam output is obtained from one of the N fiber ports with the lowest loss by selective excitation of one of the N constructive-interference supermodes. The basic characteristics of the coherent array, scalability of power summation, and bandwidth narrowing property will be presented. A 2.65-W fiber laser with only a 12-MHz bandwidth has been obtained by arraying eight 10-nm-bandwidth erbium-doped fiber lasers with an 85% addition efficiency. The characteristic bandwidth narrowing by the Vernier effect can be applied to high-power, narrowband fiber laser beyond nonlinearity limitations. By threshold control of the supermodes, high-speed (> 1 kHz), high-contrast-ratio (> 100) coherent control of beam direction has been demonstrated in the N-8 coherent array. An electro-optically-driven, 8-channel variable optical attenuator array was implemented in the output ports, and the thresholds were controlled electrically. No requirements of complicated phase-control optics/electronics and perfect single beam formation without side lobes are unique advantages. Mass-free and reaction-free properties will be quite useful in material processing and space communications.