Abstract
The 3-D beam propagation method (BPM) and a complementary mode solver for the passive fiber were applied for
modeling fiber amplifiers with a hexagonal structure of evanescently coupled cores that have been recently
experimentally realized. The modes and modal gains were calculated for 7- and 19-core systems. Diminishing the core
index step from Δn = 2.57 · 10<sup>-3</sup> to
Δn = 1.27 · 10<sup>-3</sup> leads to a reduction of the
amount of the guided modes from 7 to 3 and from 19 to 10 for the 7- and 19-core structures, respectively. The
in-phase mode that has the lowest small-signal gain for the larger index step turns to have the highest small-signal
gain at the lower index step. The mechanism lying behind the observed convergence of the wave field in the laser to
the in-phase-like mode was analyzed by a study of propagation of a linear combination of two multicore modes. It was
found that evolution of the amplified wave field in gain saturation regime can change from dominance of one to
another multicore mode at a small variation of the input wave field. The 3-D BPM modeling shows the shortage of
modal approach for analyzing the multicore fiber amplifier and indicates the importance of interference between the
competing modes, leading to the beatings in saturated gain.
© 2007 IEEE
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