Simultaneous modal phase- and group velocity matching in multiple step-index highly GeO2-doped optical fibers

Optical fibers are often used as a medium for nonlinear optical processes, especially for frequency doubling or second harmonic generation (SHG). Obtaining efficient SHG requires phase matching, which is challenging to achieve with ultrashort laser pulses. For that purpose we study how specific optical fiber designs can facilitate said phase matching. In this report we show that multi-step index fibers, which can be considered as an approximation of graded index fibers, can provide for simultaneous modal phase matching (MPM) and group velocity matching (GVM). That leads to an efficient pulsed second harmonic generation in optical fiber, which will open new opportunities for fiber sensing domain as well, where bio-sensing, medical sensing and strain sensing can be in the target.