Effect of Nested Elements on Avoided Crossing between the Higher-Order Core Modes and the Air-Capillary Modes in Hollow-Core Antiresonant Optical Fibers

Laurent Provino

doi:10.3390/fib6020042

Abstract

Optimal suppression of higher-order modes (HOMs) in hollow-core antiresonant fibers comprising a single ring of thin-walled capillaries was previously studied, and can be achieved when the condition on the capillary-to-core diameter ratio is satisfied (d/D≈0.68). Here we report on the conditions for maximizing the leakage losses of HOMs in hollow-core nested antiresonant node-less fibers, while preserving low confinement loss for the fundamental mode. Using an analytical model based on coupled capillary waveguides, as well as full-vector finite element modeling, we show that optimal d/D value leading to high leakage losses of HOMs, is strongly correlated to the size of nested capillaries. We also show that extremely high value of degree of HOM suppression (&sim;1200) at the resonant coupling is almost unchanged on a wide range of nested capillary diameter dNested values. These results therefore suggest the possibility of designing antiresonant fibers with nested elements, which show optimal guiding performances in terms of the HOM loss compared to that of the fundamental mode, for clearly defined paired values of the ratios dNested/d and d/D. These can also tend towards a single-mode behavior only when the dimensionless parameter dNested/d is less than 0.30, with identical wall thicknesses for all of the capillaries.

Highlights

A new form of silica hollow-core fiber consisting of a single ring of touching or non-touching antiresonant elements (ARE) surrounding a central hollow-core has emerged in recent years
We investigated numerically and analytically the effect of nested thin-walled capillaries on the modal properties of a hollow-core fiber consisting of a single ring of six non-touching capillaries of the same wall thickness, mounted inside a thick-walled glass capillary
We observed a dependence with respect to the dimensionless parameter d Nested /d on the (d/D )max value at which the resonant coupling between the higher-order core modes and an air-capillary mode, characterized by an avoided crossing and an extremely high degree of higher-order modes (HOMs) suppression, is optimal

Summary

Introduction

A new form of silica hollow-core fiber consisting of a single ring of touching or non-touching antiresonant elements (ARE) surrounding a central hollow-core has emerged in recent years.This antiresonant fiber (ARF) has been investigated as a result of the discovery of the importance of core wall shape in the attenuation reduction in Kagome-structured hollow-core fiber in 2010 [1].it was possible to reduce the Kagome cladding to just one single glass layer of ARE without significantly increasing fiber attenuation [2]. It was proven that ARFs, with an inverted optical core boundary, possess large transmission bandwidth and low attenuations in the mid-infrared spectral region due to both low leakage losses and weak coupling of air-core modes with the cladding structure [3,4,5]. A modified form of the basic design with contactless capillaries has been proposed and fabricated in order to remove the additional optical resonances in the transmission bands related to nodes between cladding elements [8] In this way, the loss level can be further decreased in the mid-infrared wavelength range [8,9]. By adding one or more nested capillaries within the node-less cladding structure, Fibers 2018, 6, 42; doi:10.3390/fib6020042 www.mdpi.com/journal/fibers

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