Diffraction-limited output from multi-core fibers using coherent beam combination and a diffractive optical element

The introduction of large core fibers [1] led to great advances in high power near-diffraction limited fiber lasers and amplifiers. However, any power-scaling is ultimately limited by the achievable index uniformity across a large core as well as self-focussing. The index uniformity limits the maximum possible mode size and self-focussing limits the achievable peak power. Here we suggest a solution to both problems by implementing a novel mode-coupling free multi-core design based on large core fibers, which enables the application of slow adaptive optics schemes to coherent beam-combination for the generation of diffraction-limited beams. The great advantage compared to other beam combination techniques [2] is that the whole signal is contained in only one fiber structure. The approach is based on propagation in a passive seven-core leackage channel fiber (LCF) with a core to core spacing 3 Λ of 110 µm and an overall diameter of 500 µm (side to side). Each pure silica glass core region was surrounded by two hexagonally arranged rings of low index fluorine glass rods with a diameter of d = 0.6 Λ and an effective refractive index 0.0012 smaller than that of pure silica. The individual core diameters were 51 µm (NA ≈ 0.03) resulting in an overall usable mode area of 9700 µm
²
and high quality fundamental mode propagation. The cross section of the fiber is shown in Fig. 1 (a). It has already been demonstrated that each core behaves like an individual isolated waveguide with respect to optical properties but all external perturbations have a similar effect on all cores [3]. This drastically reduces the required phase control bandwidth and piston phase, respectively.