Spectrum collapse in a 7-core Yb-doped fiber laser with an array of fs-inscribed fiber Bragg gratings
Alexey G. Kuznetsov, Alexey A. Wolf, O. N. Egorova, S. L. Semjonov, А. В. Достовалов, E. V. Podivilov, Sergey A. Babin
Abstract
Femtosecond inscription of fiber Bragg gratings (FBGs) in each core of a cladding-pumped seven-core Yb-doped fiber enables efficient (≈70%) 1064-nm lasing in a robust all-fiber scheme with ≈33 W power, nearly the same for uncoupled and coupled cores. However, the output spectrum is quite different: without coupling, seven individual lines corresponding to the in-core FBG reflection spectra sum up into a broad (0.22 nm) total spectrum, whereas the multiline spectrum collapses into a single narrow line at strong coupling. The developed model shows that the coupled-core laser generates coherent superposition of supermodes at the wavelength corresponding to the geometric mean of the individual FBG spectra, whereas the generated laser line broadens, with a power (0.04-0.12 nm) like the single-core mode of a seven-times larger effective area.