Transmission Bandwidth Enlargement with Elliptical Cladding Elements in Hollow Core Inhibited Coupling Fibers

Hollow Core Inhibited Coupling Fibers (HCICFs) have been gaining more and more attention in the photonics community thanks to their unique features such as low loss and large bandwidth, up to breaking the Rayleigh scattering limit of solid core silica fibers [1], [2]. Recently works have shown loss and bandwidth are mainly set by Tube Tunneling Loss (TTL) and Core mode - Dielectric mode Coupling Loss (CDCL) respectively [3]. TTL is caused by the power tunneling of the Fundamental Core Mode (FCM) through the tubes composing the cladding and it can be reduced by inserting one or more nested tubes. However this technique does not affect CDCL, which is related to the coupling between FCM and Dielectric Modes (DMs), mainly confined in the tube membranes. Since CDCL dominates at long wavelengths [3] it sets the red edge of the transmission band and thus the fiber bandwidth. CDCL mainly depends on the azimuthal Fourier spectrum of the FCM computed on the tube inner boundaries and on the spectral distribution of the DM cut-off frequencies [4]. Both of them do not depend on the number of nested tubes, while they are affected by the tube shape. Acting on the shape is thus possible to modify CDCL, enlarging the bandwidth. The goal of this work is to share the first results emerging form this activity. For the investigation, we considered an “artificial” fiber as shown in Fig. 1(a). This kind of fiber is a useful tool for analyzing the CDCL, by removing the effects of the other mechanism contributing to the total CL. In such fibers, an “artificial” material, with refractive index lower than the one of the material filling core and tubes, has been set in the innermost part of the tube holes, in order to minimize the TTL, and in-between the tubes to minimize the tube-tube leakage. The plot of Fig. 1(a) compares the CL, numerically computed and only due to CDCL, for elliptical tubes with different aspect ratio <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$b/a$</tex>. The fiber parameters core size, tube thickness, number of tubes and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$a$</tex> are kept constant. The case of perfectly circular tubes <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(b/a=\mathrm 1)$</tex> shows the narrowest transmission band, which increases with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$b/a$</tex> up to getting the maximum extension for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$b/a=\mathrm 1.3$</tex>; after that, the trend reverses. The potential transmission band enlargement is about 200nm in the telecom band. Moving to real fibers, Fig. 1(b) compares CL of a eight circular double nested tubes with a single nested elliptical tube with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$b/a=\mathrm 1.3$</tex>. The two fibers show the same minimum loss, but the elliptical tube one exhibits a transmission band enlargement of more than 150nm if a loss of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10^{-1}dB/km$</tex> is kept as reference value.