Soliton stability and trapping in multimode fibers.

We investigate the stability of optical solitons in few-mode fibers by solving numerically coupled nonlinear Schrödinger equations that include intermodal nonlinear coupling. While a single fundamental soliton propagating in any fiber mode is stable, simultaneous propagation of two or more solitons in different fiber modes is not always stable and leads to interesting effects resulting from intermodal nonlinear coupling. Under some conditions, soliton trapping is observed such that two solitons in different modes shift their spectra and travel at the same speed in spite of considerable intermodal differential group delay between them.