Nonlinear optical phenomena in multimode fibers

Thesis (Ph. D.)--University of Rochester. The Institute of Optics, 2018.Nonlinear optical effects are very well studied in optical fibers. However, most of these studies are restricted to fibers that support one spatial mode. Multimode fibers can provide several new nonlinear pathways due to intermodal effects, which makes light propagation through them difficult to understand. But several proposed applications have led to a resurgence in efforts to determine how nonlinearity manifests in multimode fibers. This thesis deals with many different studies undertaken to understand a subset of this large variety of nonlinear effects that occur inside a multimode fiber. The first part of the thesis looks at two different manifestations of intermodal nonlinearity, spatiotemporal solitons and nonlinear spectral effects. The existence and interaction of solitons in multimode fibers is considered. Solitons, or a solitary wave packet that maintains its shape during propagation, is a commonly observed nonlinear artifact that has several applications in fiber optics. Intermodal interaction is added to the picture and the stability and formation of solitary wave packets in a multimode fiber are studied using numerical simulations. Simulations and theory are also used to understand how solitons propagating in different modes of a multimode fiber interact with each other owing to nonlinearity. Nonlinear optics has been successfully exploited to generate sources with novel spectral profiles, including a broad supercontinuum, frequency-combs etc. A combination of several nonlinear effects in the medium is usually responsible for tailoring the source spectrum. Numerical results are presented that show a novel spectral suppression phenomenon in multimode fibers. Nonlinear spectral broadening is demonstrated through experimental results, using a special photonic crystal fiber. The second part of this thesis explores the impact of random linear coupling between modes in a multimode or multicore fiber and how it affects nonlinear coupling. Random linear coupling can occur due to several imperfections in a real optical fiber and is important to consider when modeling systems that employ multimode fibers, like in the case of space division multiplexing communication systems. A comprehensive theoretical model is proposed that can be used to simulate the average nonlinear behavior with any amount of random linear coupling present in the system. Specific regimes of linear coupling are found to be more detrimental to optical communication systems. An experimental study to quantify linear coupling in a multicore fiber, and future work pertaining to it, are also described