Design and Fabrication of Single Transverse Mode Passive Large-Pitch Fibers

The fabrication processes of single transverse mode passive large-pitch optical fiber (LPF) have been proposed and investigated. The LPF design, combined with the fundamental mode operating principle of delocalizing higher-order modes, has led to impressive performance. In this study, two LPF preform designs were proposed based on the stacking of one type of inner glass tube, two (design of LPF1) or three (design of LPF2) different filler rods, and a core rod placed within an outer tube. The first passive LPF1 is fabricated from a single-step preform drawing process. This fiber exhibits single transverse mode propagation, featuring a core size of 45.5 µm and a normalized hole diameter of 0.454. For the first time, a single transverse mode of light propagation from an LPF with an elliptical-like hole shape was achieved. The second LPF2 design has been proposed and fabricated by using a two-step preform drawing process. Successful production of an LPF with a circular hole shape has been obtained and exhibits single transverse mode propagation, featuring a core size of 42.8 µm and a normalized hole diameter of 0.322. Furthermore, numerical analysis was also performed to study mode propagation for the LPF.