Soliton self-mode conversion: revisiting Raman scattering of ultrashort pulses

Coherent frequency conversion in compact integrated formats via guided-wave nonlinear optics has not been shown to be power scalable to date, because single-mode waveguides need dispersion control, achieved by shrinking mode size and hence reducing power-handling capacity, whereas power-tolerant multimode waveguides yield spatially incoherent, hence uncontrollable, nonlinear coupling. Here we report the discovery of a new manifestation of Raman scattering of ultrashort pulses that is power scalable while yielding pure spatially coherent beams. The phenomenon of soliton self-mode conversion (SSMC) described in this paper exploits the group-velocity diversity of multimode waveguides, enabling noise-initiated Raman scattering of an ultrashort pulse to occur exclusively between two distinct spatial eigenmodes and only those two modes. This exclusivity helps in naturally maintaining spatial coherence, which is usually the bane of multimode waveguide nonlinear optics. And, the fact that this phenomenon occurs in mode-size-scalable multimode waveguides yields the power scalability. SSMC is wavelength agnostic, since it can occur at virtually any wavelength in which a multimode fiber is transparent, and we demonstrate its versatility by frequency-converting a conventional 1-μm fiber laser into MW-peak power, ∼75-fs pulses at the biologically crucial 1300-nm spectral range. This represents an enhancement, by roughly two orders of magnitude, of nonlinear frequency-converted power levels of ultrashort pulses at 1300 nm directly out of any flexible fiber, to date.