Intermodal group-velocity engineering for broadband nonlinear optics

Interest in the nonlinear properties of multi-mode optical waveguides has seen a recent resurgence on account of the large dimensionality afforded by the platform. The large volume of modes in these waveguides provides a new spatial degree of freedom for phase matching nonlinear optical processes. However, this spatial dimension is quantized, which narrows the conversion bandwidths of intermodal processes and constrains spectral and temporal tailoring of the light. Here we show that by engineering the relative group velocity within the spatial dimension, we can tailor the phase-matching bandwidth of intermodal parametric nonlinearities. We demonstrate group-velocity-tailored parametric nonlinear mixing between higher-order modes in a multi-mode fiber with gain bandwidths that are more than an order of magnitude larger than that previously thought possible for intermodal four-wave mixing. As evidence of the technological utility of this methodology, we seed this process to generate the first high-peak-power wavelength-tunable all-fiber quasi-CW laser in the Ti:sapphire wavelength regime. More generally, with the combination of intermodal interactions, which dramatically expand the phase-matching degrees of freedom for nonlinear optics, and intermodal group-velocity engineering, which enables tailoring of the bandwidth of such interactions, we showcase a platform for nonlinear optics that can be broadband while being wavelength agnostic. (C) 2018 Chinese Laser Press