Low Loss Porous Core Photonic Crystal Fiber Design in THz Regime

Abstract

Photon, the massless particle responsible for electromagnetic radiations travels at the speed of light which is the key element in terahertz (THz) based fiber communications. The efficient transmission of THz wave using waveguides has been a major challenge since early days of this technology. Loss and dispersion play vital part in THz wave guidance. They should be reduced to obtain accurate THz wave propagation. Metallic waveguide tolerates Ohmic losses and dielectric waveguides experience huge material absorption losses. Dry air is the best transparent medium for THz radiations. Considering above losses, the fiber needs to develop. This thesis reports the development of fibers to guide THz radiations. The objective of this research work is to design optical waveguides using more advanced form optical fiber called porous core photonic crystal fiber (PCPCF) in the terahertz regime. The porous core fiber has an arrangement of subwavelength featured air-holes in the cross section. The designed PC-PCF would be a possible candidate for THz wave guidance. A considerable effort has been put into designing advanced porous core PCF structures with increased flexibility and the ability to minimize losses in terms of geometric structure. In this thesis, different structure is designed to obtain porous core photonic crystal fibers with low effective material loss (EML), low confinement loss (CL), high core power fraction and flatten dispersion for THz wave propagation. The preparation of proposed structure air holes, both in periodic cladding and porous core, made it possible to guide most of the light through low loss air, which is confirmed by numerical analysis of optical properties of the fiber while preserving the single mode condition. Numerical analysis of the proposed geometric structure of the fiber is rigorously performed using finite element method (FEM) with perfectly match layer (PML) boundary conditions to characterize the wave guiding properties. This thesis examines crucial design parameters such as effective material loss, core power fraction, birefringence, dispersion, and confinement loss of the proposed porous core fiber. Topas is used as background material due to its constant refractive index behavior in terahertz regime and its lower bulk absorption loss. In this thesis work, four PC-PCF structures have been proposed, designed and numerically investigated. The proposed PC-PCF contains different shape and size of air hole dimensions in the cross sections. Among all the proposed PC-PCFs, the circular XVI cladding and circular core PC-PCF structure exhibits comparatively better results. This circular PC-PCF shows 0.04 𝑐𝑚�������� and 1.96 × 10�������� 𝑐𝑚�������� of EML and confinement loss respectively. So, the total loss is 0.04 𝑐𝑚�������� as the confinement loss is negligible. Also, at 84% porosity the proposed circular PC-PCF exhibits 55.8% of core power fraction which can be considered enough for THz wave propagation. The sol-gel fabrication technique offers more freedom to high porosity geometric structures. Therefore, sol-gel can be considered as the fabrication technique for this proposed circular PC-PCF. Finally, the results of the analysis are further compared with those of previous reported contributions and found comparative. The proposed designs can be taken into consideration for THz communications.