Abstract

In the past decade, CW fiber lasers have revolutionized industrial material processing because of their high efficiency, robustness and diffraction-limited beams obtained in kW regimes. However, the output power from a single fiber laser is limited by phenomena such as non-linear effects or excessive heating. To overcome these issues, incoherent beam combining (IBC) of multiple independent fiber lasers has become a common approach for further power scaling, which however comes at the expense of the resulting beam quality. Although the brightness conservation law states that IBC cannot be done without deterioration of the beam quality, minimizing this degradation is of paramount importance to achieve the best performances in material processing applications. The beam quality of a fiber laser is usually characterized by the well-known M² parameter, which has a value of 1 for a Gaussian beam and of 1.10 for typical kW single-core fiber lasers. The all-fiber signal combiner (SC), obtained by connecting a tapered fiber bundle (TFB) to a multimode delivery fiber (Fig. 1 (a)), has proven to be a robust approach to realize IBC and reach multi-kW powers [1]. In this work, we investigate the beam quality obtained at the output of a 7×1 SC. The best reported results with such a configuration is a M² of 4.6 when the lasers are combined into a 50-µm core multimode fiber and a M² of 6.5 when combined into a 100-µm core multimode fiber [2,3]. In this paper, we experimentally demonstrate the fabrication of a 7×1 SC exhibiting a record low M² of 3.75 into a 50-µm core multimode fiber.

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