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.
Description:
Supervised by
Dr. Mohammad Rakibul Islam
Professor,
Department of Electrical and Electronic Engineering,
Islamic University of Technology (IUT), Boardbazar, Gazipur-1704.