Holey Fibers with a Limited Number of Air Hole Rings in the Cladding: Design, Fabrication and Application for Some Interferometric Sensing Devices

Abstract

Large‐mode‐area all‐silica holey fibers (HFs), which are specially designed with a limited number of air hole rings in cladding, are presented. Fibers with 2, 3, and 4 air hole rings were inspected, and the transmission losses and the demands for the single‐mode waveguide operation in a series of the HF samples with different relative hole diameters d/Λ were analyzed. It has been found, in particular, that for the fibers there is an optimal ratio, d/Λ = 0.52±0.05, that provides a single‐mode output for shorter HFs (a few meters long), and low transmission and bending losses for longer HFs (hundreds of meters long). The HFs developed were used for the fabrication of compact modal interferometers, which represent special tapers with gradually collapsed air holes in the taper waist. The gradual collapsing of the holes is achieved by tapering the fibers with a “slow‐and‐hot” method (nonadiabatic process). As a result, at the excitation of the fundamental mode in the HF core, it couples to two high‐order modes of the solid taper waist. Due to the beating between the modes, the transmission spectra of the taper exhibit several interference peaks that are shifted to a shorter or longer wavelength under the influence of various external parameters. The use of the HF tapers in different sensing devices for the measuring of refractive indexes of liquids, high temperature, strain, and hydrogen sensing is demonstrated.