Coherent control of laser arrays by complex resonator

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We have proposed and demonstrated highly efficient coherent beam combining of fiber lasers and laser diodes by Y-shaped array coupled with fused fiber couplers. Near-field coupling of N fiber lasers by use of N×N fused fiber couplers enables a power-summed output from one of N fiber ports without any fiber-length adjustment. The wavelengths satisfying constructive interference have the highest effective reflectivities and are selectively excited as a coherently-added single beam output. High addition efficiencies of 94, 96, and 85% have been obtained for N=2, 4, and 8 erbium-doped fiber lasers, respectively. The measured frequency characteristics of the coupled array well matched with the theory. The measured array-number dependence of the addition efficiency is well matched with the simple-scaling theory we developed. The clarified limit of coherent addition (N=8〜10) is found to be an inherent problem in length-different laser arrays. 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. We also proposed that harmonic mode locking can be attained by coupling ultrafast lasers. In a preliminary experiment, 20-th harmonic sub-picosecond pulse train with >40-dB mode suppression ratio has been successfully obtained. In conclusion, it has been shown that coherent arraying of multiple lasers realizes not only power and brightness scaling but various new-functionality lasers.