Valerio Setti

Waveguiding mechanism in tube lattice fibers

Waveguiding mechanism and modal characteristics of hollow core fibers consisting of a single or a regular arrangement of dielectric tubes are investigated. These fibers have been recently proposed as low loss, broadband THz waveguides. By starting from a description in terms of coupling between air and dielectric modes in a single tube waveguide, a simple and useful model is proposed and numerically validated. It is able to predict dispersion curves, high and low loss spectral regions, and the conditions to ensure the existence of low loss regions. In addition, it allows a better understanding of the role of the geometrical parameters and of the dielectric refractive index. The model is then applied to improve the tradeoff between low loss and effectively single mode propagation, showing that the best results are obtained with a heptagonal arrangement of the tubes.

Flexible tube lattice fibers for terahertz applications.

In this paper a flexible hollow core waveguide for the terahertz spectral range is demonstrated. Its cladding is composed of a circular arrangement of dielectric tubes surrounded by a heat-shrink jacket that allows the fiber to be flexible. Characterization of straight samples shows that the hollow core allows the absorption caused by the polymethylmethacrylate tubes of the cladding to be reduced by 31 times at 0.375 THz and 272 times at 0.828 THz with respect to the bulk material, achieving losses of 0.3 and 0.16 dB/cm respectively. Bending loss is also experimentally measured and compared to numerical results. For large bending radii bending loss scales as Rb-2, whereas for small bending radii additional resonances between core and cladding appear. The transmission window bandwidth is also shown to shrink as the bending radius is reduced. An analytical model is proposed to predict and quantify both of these bending effects.

Extra loss due to Fano resonances in inhibited coupling fibers based on a lattice of tubes

Confinement loss of inhibited coupling fibers with a cladding composed of a lattice of tubes of various shapes is theoretically and numerically investigated. Both solid core and hollow core are taken into account. It is shown that in case of polygonal shaped tubes, confinement loss is affected by extra loss due to Fano resonances between core modes and cladding modes with high spatial dependence. This explains why hollow core Kagome fibers exhibit much higher confinement loss with respect to tube lattice fibers and why hypocycloid core cladding interfaces significantly reduce fiber loss. Moreover it is shown that tube deformations, due for example to fabrication process, affect fiber performances. A relationship between the number of polygon sides and the spectral position of the extra loss is found. This suggests general guide lines for the design and fabrication of fibers free of Fano resonance in the spectral range of interest.

Confinement Loss in Kagome and Tube Lattice Fibers: Comparison and Analysis

In this paper, a thorough numerical analysis of the confinement loss in kagome and tube lattice fibers is presented. The results show that the confinement loss strongly depends on the shape of the struts composing the core boundary and the cladding. This explains why confinement loss in kagome fibers is much higher than in tube lattice ones. In fact, the closer to a perfectly circular arc the struts, the lower the confinement loss. For this reason, struts shape must be carefully controlled during the fabrication process.

Terahertz Tube Lattice Fibers With Octagonal Symmetry

A new hollow core fiber for terahertz applications consisting of dielectric tubes arranged with an octagonal symmetry is presented. Simulation results have shown that this fiber exhibits an effectively single-mode propagation with lower propagation loss with respect to hollow core fibers with a hexagonal symmetry. The suppression of higher order modes is due to a better phase-matching condition with cladding modes propagating into the hollow core of tubes. In particular, by using tubes with 44-μm thickness and outer diameters as low as 0.5 mm, a rejection of high order modes higher than 20 dB/m and fundamental mode propagation loss lower than 5 dB/m from 2.30 to 2.85 THz can be obtained.

Fano Resonances in Polygonal Tube Fibers

In this paper, the dispersion and the confinement loss properties of polygonal tube fibers are investigated. Fano resonances between the hollow core modes and the high order dielectric modes are found inside transmission bands. A theoretical model based on the approximation of the polygonal tube as a perturbed circular one is proposed. The model is able to predict number and spectral position of the Fano resonances. It is shown that there always is a lower bound condition on the number of sides of the polygonal fiber which guarantees the absence of Fano resonances in a given frequency range. Numerical results are given in order to support the model.

Confinement Loss of Tube Lattice and Kagome Fibers

Confinement loss of two kinds of broad band hollow core fibers, the tube lattice fibers and the kagome fibers, are numerically investigated and compared.

Elliptical hollow tube waveguides

Dispersion and loss properties of hollow core tube fibers with elliptical cross section are numerically investigated. Results show unusual characteristics. For example, the birefringence always goes to zero in the middle of the low loss region irrespectively of the ellipticity and it always assume opposite sign approaching the two edges of the low loss region.

Fano resonances in kagome fibers

Confinement Loss of microstructured fibers whose cladding is composed by a triangular arrangement of tubes of various shapes is theoretically and numerically investigated. Kagome Fibers belong from this family of fibers with cladding tubes with hexagonal shape. The shape of the cladding tubes is proved to strongly affect the performance of the microstructured fiber. In order to understand the reasons for this behavior, the spectral properties of the tubes that constitute the cladding are investigated first. It is proved that also these tubes suffer from additional Fano Resonances when they are given a polygonal shape. It is proved that, by using the analytical model developed for the stand alone polygonal tubes, it is possible to predict the spectral position of Fano Resonances also in microstructured fibers. This is extremely important since it suggest new ways to reduce confinement loss in kagome fibers and microstructured fibers in general.