Large bandwidth lasers coherent beam combining in a 7-element YDFL array using a tailored SPGD algorithm

Abstract

Coherent beam combining techniques aim to obtain high-power laser beams with good quality and thus enabling several specific applications such as long-range communications, remote power delivering or L-DEW applications. They have received a renewed interest given the very good performances obtainable with high power fiber lasers adopting MOPA architectures and phase locking feedback loops. In the case of fiber lasers, the specific power of a single amplifier is limited by the deleterious effect of SBS. One can enhance the threshold of SBS widening the bandwidth of the field in the amplifier but this contrasts with the need of a spectrally pure field for the adoption of CBC architectures based on phase detection. Arrays with a large number of elements have been demonstrated for narrow-band systems, while wideband phase locking has been reported only in the case of a limited number of elements. The adoption of multi-KW fiber laser amplifiers requires bandwidths of the order 30 GHz. In this paper, we report on the experimental demonstration of a 7-element array coherently combined up to a spectral bandwidth of 47 GHz. The architecture is based on a narrow line single mode master oscillator whose emission is phase modulated to widen the bandwidth. Amplified fields are summed in a tiled aperture geometry and far field PIB is adopted as a metrics for a perturbation hill-climbing algorithm. Phase-locking results and convergence dynamics are analyzed in relation to the bandwidth properties of the oscillator modulation.