Lasing spectrum narrowing in a seven-core fiber laser with an array of Bragg gratings inscribed by femtosecond pulses

The paper examines lasing regimes of a 7-core ytterbium fiber laser with a cavity based on an array of fiber Bragg gratings (FBGs) inscribed in each core by femtosecond pulses. Under pumping by a 50 W laser diode, the maximum output power is 33 W, and the differential lasing efficiency is ∼70% in both the presence and absence of optical coupling between the cores of an active multicore fiber (MCF). At the same time, a significant difference in the generated optical and radio frequency spectra is observed when use is made of two types of MCFs. It is shown that in an active MCF without coupling, the lasing wavelength in each core is determined by a specific FBG, and the total spectrum is broadened to ∼0.2 nm already at the threshold due to a relative shift of the spectral maxima of the FBG in different cores. However, in a fiber with coupling, despite the presence of a similar FBG shift, lasing occurs at a wavelength corresponding to a maximum of the geometric mean reflection coefficient of individual FBGs in the cores. In this case, the total optical spectrum is narrowed down to ∼0.04 nm and broadens with power. In the radio frequency spectrum, narrow peaks of intermode beats are observed against the background of a noise substrate, the amplitude of which also increases with power. A model is given to explain the experiments.