Spectrum collapse in a 7-core fiber laser with wavelength-shifted FBGs: modeling and comparison with experiment

Multicore fiber is a prospective medium for high-power lasers.All-fiber cavities based on FBGs inscribed by femtosecond pulses in uncoupled cores generate independently at wavelengths defined by the individual FBG reflection.However, separate core generation spectra collapse into a single narrow and stable line at strong optical coupling between the cores.The developed model shows that the collapsed spectrum linewidth depends on the number of cores, their coupling strength, geometric-mean reflection width of individual FBGs.It is revealed that physical mechanisms and quantitative features of linewidth broadening defined by spatial-hole-burning and Kerr effects are sufficiently modified in comparison with single-core lasers.A performed comparison of the theory with experiment demonstrates a fairly good correspondence of key characteristics, namely generation wavelength, spectrum shape and its linewidth value depending on power.An extreme linewidth narrowing is predicted for a high-power multicore fiber laser with greatly increased number of cores and their coupling strength.