Helical versus fundamental solitons in optical fibers

We consider solitons in a nonlinear optical fiber with a single polarizatio n in a region of parameters where it carries exactly two distinct modes, vi z., the fundamental one and the first-order helical mode. From the viewpoin t of applications to dense-WDM communication systems, this opens a way to d ouble the number of channels carried by the fiber. Aside from that, experim ental observation of helical (spinning) solitons (that can be launched and detected, using helicity-generating phase masks) and collisions between the m and with fundamental solitons in (ordinary or hollow) optical fibers is a n issue of fundamental interest, especially because it has been very recent ly found that spatiotemporal spinning solitons in bulk optical media with v arious nonlinearities are unstable. We introduce a system of coupled nonlin ear Schrodinger equations for fundamental and helical modes, computing nons tandard values of the cross-phase-modulation coupling constants in it, and investigate, analytically and numerically, results of "complete" and "incom plete" collisions between solitons carried by the two modes. We conclude th at the collision-induced crosstalk is partly attenuated in comparison with the usual WDM system, which sometimes may be crucially important, preventin g merger of the colliding solitons into a breather. The interaction between the two modes is found to be additionally strongly suppressed in compariso n with that in the WDM system in the case when a dispersion-shifted or disp ersion-compensated fiber is used.