Spatio-temporal behaviour of femtosecond solitons in graded-index multimode fibers

We present a new class of spatiotemporal walk-off solitons in graded-index multimode optical fibers, characterized by unique properties. Their energies and pulsewidths depend only on chromatic and modal dispersion of the fiber, and on the input laser beam coupling conditions. Whereas the soliton properties are independent of the input pulse duration. The generated multimode solitons are composed by non-degenerate modes. However, upon propagation over distances comprising several hundreds of meters, the multimode solitons naturally evolve into singlemode pulses. For this reason, walk-off solitons are metastable objects, which are irreversibly attracted by the fundamental mode of the fiber.