APPLICATION SPECIFIC DESIGN OF PHOTONIC CRYSTAL FIBERS, WAVEGUIDES AND DEVICES

Abstract

Photonic Crystals (PhC) are micro-structured materials with periodic variation of dielectric constant in one, two, or three dimensional length scale and periodicity comparable to the wavelength of light. Because of the periodicity, the PhC exhibit strong reflection for a range of wavelengths resulting in the formation of Photonic Band Gap (PBG). By selecting an appropriate crystal structure of certain materials, one can get a PBG or a frequency range in which propagation or existence of electromagnetic waves is forbidden. Light with frequency that falls within this forbidden frequency range or PBG can be guided through such PhC by creating a waveguide. Similarly, light can be confined or trapped inside such a PhC by creating a cavity. The confined modes of a photonic cavity could be used in optical resonators, laser cavities, filters, switches, demultiplexers, sensors etc. Waveguide and cavity can be created by introducing defects in the periodic structure of PhC on different platforms like Silicon Carbide (SiC), Gallium Arsenide (GaAs) etc predicted for use in all optical networks. The most significant application of PhC is the design of a novel waveguide known as Photonic Crystal Fiber (PCF) which is essentially a fused silica optical fiber with a periodic distribution of voids or air holes in the cladding that run parallel to its axis. PCF has the extraordinary ability to carry more light in the core, confinement characteristics not possible in conventional optical fiber and characterized by simpler and economic fabrication technique. Nowadays, PCF is finding applications in optical fiber communications, fiber lasers, nonlinear devices, high-power transmission, and highly sensitive gas sensors. Since its inception, PCF is mainly fabricated from silica glass material. But today, researchers have been profusely using materials like fluoride glass, polymer-based and chalcogenide glass in fabrication of PCF. Use of these materials in PCF offer advance characteristics like high non-linearity, high refractive index, high mode confinement, Raman amplification etc. In this thesis, first, introductions of PhC and PCF have been given and discussed regarding PhC and PCF based devices. This is followed by detailed discussion of theory of light guidance through PhC and PCF, Photonic band gap engineering in PhC, analysis of PhC and PCF structures and different PhC and PCF fabrication ix methods. Computational methods utilized in the analysis of PhC and PCF including an overview of the thesis work have been discussed in the introduction. Investigating photonic structures which are less sensitive to environmental fluctuations like temperature is a valuable area of research. Here, in this thesis, it has been proposed to design photonic crystals using temperature resilient material Silicon Carbide (SiC) and study the variation of width of photonic band gaps in SiC photonic crystals with change in temperature and a comparison with Silicon (Si) photonic crystals using Plane Wave Expansion (PWE) method. Further, SiC point defect cavity has been created in the PhC and analysis of the SiC photonic crystal cavity defect modes have been carried out using Finite Difference Time Domain (FDTD) method. The effect of temperature on different parameters of the proposed designs like band gap width, defect cavity mode dispersion, the resonance mode and quality factor of resonant mode have been studied. The SiC PhC devices can be used for high temperature and power transmission which is difficult to achieve with conventional Si or GaAs based photonic crystals and devices. Apart from applications in optical communication, various other SiC based devices such as optical filters, switches and lasers etc can be designed. The SiC based devices are stable at high power and high temperature, at which the silicon based devices, cannot sustain such high power and temperature. Apropos of the recent trend of using new fabrication material in PCF, in this thesis, the application specific design of PCF having solid core with regular and irregular cladding geometries, using different and new material and doping the cladding have been studied. First, the design of a PCF in Fluoropolymer material has been done and analyses of the proposed PCF has been carried out in terms of the parameters like effective refractive index of the guided mode and dispersion for wavelength range 10µm to 300µm using full vectorial Finite Element Method (FEM) and MATLAB computational tool. The transmission characteristics of Fluoropolymer PCF are found to be comparable with the earlier published result. The proposed Fluoropolymer PCF may find applications in long distance telecommunication and in mid-infrared region. Next, the design of a PCF with irregular cladding geometry has been proposed. The PCF has been named after its W letter shaped refractive index profile as W-type PCF. The analysis of the proposed W-type PCF have been done in terms of the parameters x like bend loss of the guided mode, effect of temperature on bend loss and nonlinearity using full vectorial FEM . The transmission characteristics of W-Type PCF obtained are found better compared with the earlier published results. The power coupling in W-type PCF is more compared to conventional PCF. The bend insensitive nature of the proposed W-type PCF structures makes them good candidate for large mode area fiber design and fit for fiber to home applications. Then, cladding doped large mode area W-type photonic crystal fibers are designed. The analyses of these proposed structures in terms of the parameters like confinement loss, effective refractive index of the guided mode, bend loss, birefringence, sensitivity, effective area and non-linear co-efficient using full vectorial FEM have been carried out. The proposed W-type PCF structures found to possess very low confinement loss and low macro bend loss. The W-type PCF is found to be birefringent and sensitive and can find application in telecommunication and sensing. In addition to the above proposed PCF structures, a PCF structure has been designed for nonlinear applications in novel Ga-Sb-S based chalcogenide glass which is a different and new PCF fabrication material. The propagation characteristics of the designed GaSb-S based chalcogenide glass PCF structure like effective refractive index of the guided mode, dispersion, effective area, non-linear co-efficient have been investigated by employing full vectorial FEM and MATLAB computational tool. The transmission characteristics of Ga-Sb-S-PCF have been obtained for wavelength range 0.8µm to 14µm. The nonlinear coefficient as high as 14.92 W-1m-1 with effective mode area of 3.37μm2 at the operating wavelength of 1.55μm for the proposed photonic crystal fiber structure has been found. The proposed PCF structure exhibit flat and low dispersion value between spectral spanning 2.4μm - 2.7μm with maximum dispersion variation of 20 ps/nm km. The PCF structure possesses zero dispersion wavelength value at 2.6μm. This novel Ga-Sb-S material based PCF structure has been studied for the first time since the inception of the novel material Ga-Sb-S and can be a promising candidate for nonlinear applications such as mid-infrared supercontinuum generation, slow light generation, and mid-infrared fiber lasers. In the last, the thesis includes summery and future scope of the research work reported.