Intermodally phase-matched nonlinear effects in optical nanofibers and their applications

Abstract

In nonlinear optics, tightly confined modes in subwavelength photonic waveguides have been expected to increase the efficiency of nonlinear interaction between lights with large wavelength difference because they possess various spatial structures and intriguing optical features. In this thesis, third-harmonic generation processes are experimentally implemented as an example of intermodally phase matched nonlinear effects in silica nanofibers, and their applicability was analyzed and presented. Firstly, tapered optical fiber supporting the adiabatic propagation of higher-order modes is demonstrated by adding the wet-etching technique on conventional flame brushing method. Also, widely tunable nearly transform-limited narrow-linewidth mode-locked fiber laser is implemented, which can be employed as a promising pump source for nonlinear frequency conversion with narrow phase-matching bandwidth and high group-velocity mismatch. Based on these two achievements, complete characterization of intermodally phase-matched third-harmonic generation in optical nanofiber is firstly realized. Finally, it is verified both in theory and experiment that optical vortex mode can be excited with single pump in fundamental mode via spin-orbit-interaction assisted third-harmonic generation. We believe that our work could contribute to the development of various optical technologies based on intermodal interaction between guided modes confined within photonic waveguides.