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We demonstrate a direct inscription of a fiber Bragg grating (FBG) in the active cores of an Yb-doped large mode area multicore fiber (MCF). An ultrashort pulsed laser is used to inscribe the FBG simultaneously in all six cores. In order to validate the FBG reflection and uniformity, the FBG is incorporated as a rear mirror in a fiber laser oscillator setup. The MCF, which has been fabricated in-house, has six cores located in a hexagonal-ring shape, each with a 19 µm diameter and an NA of ∼0.067. A reflection of ∼96% was measured at a center Bragg wavelength of ∼1062nm for the inscribed FBG. The laser performance of the MCF with the femtosecond inscribed FBG at its end shows a similar performance to lasing with a free-space commercial volume Bragg grating as the rear-reflector. A slope efficiency of ∼72.4% and a maximum (pump limited) output power of 51.8 W have been obtained for the FBG setup. An …