Hollow core fibre for THz applications

Abstract

This work is focused on the analysis of Hollow Core Photonic Band Gap Fibres (HC-PBGFs) for THz applications. Waveguide design in this spectral region is a major challenge due to the high conductivity losses of metals and high absorption of the dielectrics. HC-PBGFs are dielectric waveguides in which the power fraction propagating in the dielectric is very low, and losses due to material absorption are dramatically reduced. HC-PBGFs have been proposed and developed from the end of 90's for applications in visible and Near InfraRed (NIR) spectrum [1], [2] and in few years propagation losses have been dramatically reduced from tens of dB/m to few dB/Km. These fibres suffers of high leakage and scattering loss [3], [4]. Leakage loss is due to the finite number of air hole composing the cladding lattice [9]. It can suppressed by incorporating enough air hole ring around the hollow core, though the higher is the number of hole rings, more difficult is the fabrication process. Scattering loss is due to fibre nonuniformity which couples guided mode with radiating modes. Fluctuation of technological origin can be reduced by more careful fibre fabrication. On the contrary surface roughness from surface capillarity waves cannot eliminated because of they thermodynamic origin [4]. However all these losses are inversely proportional to the wavelength, thus at THz frequencies their values do not significantly affect the overall loss mechanism. In particular, the purity of the aperture filed distribution at each PBGF section leads to a highly collimated output beam which can be eventually exploited for the design and development of feeds, even in array configuration, coupled to optical or electronic devices. The possible employment of such a structure as an aperture antenna has not been investigated, yet. The numerical analysis has been performed by using a complex modal solver based on the Finite element Method (FEM) [7], already applied in the past for the loss analysis of several kind of photonic band gap fibres [8], [9]