SUPERCONTINUUM GENERATION IN SPACIALITY OPTICAL FIBERS : DESIGN AND ANALYSIS

Abstract

Optic fiber communication is the transmission of light as signal through an optically transparent medium which acts as channel by the principle of total internal reflection. The channel is a dielectric cylindrical waveguide of silica. The area in which the light travels is called core. Initially LED’s were used as light source which were having low intensity and low coherence. After invention of laser light for optical transmission through optical fiber, the phenomenon of nonlinear effects were discovered. The input-output power relation was no more linear and certain losses were observed and the losses increased with increase in input power. Investigation revealed that the high intensity optical light will give rise to certain nonlinear effects. This was due to response of the dielectric material to high intensity electromagnetic field. The polarization induced by electric dipoles is not linear to electric field. The nonlinear effect give rise to two important effects called Kerr effect which is second order nonlinearity and third order nonlinearity called Pockel effect. These effects have resulted in many phenomenon such as Cross-phase modulation, Self-phase modulation, Stimulated Raman Scattering, Four wave mixing etc. The effect is dependent on material and geometry of the fiber. Super Continuum Generation or SCG is a process that is a result due to interplay between linear and nonlinear effects. It is a process where a continuous spectrum of light is generated when a high intensity short duration laser pulse is propagated through a highly non-linear fiber. The resultant spectrum will have low temporal coherence (high bandwidth) and high spatial coherence. The project involves design and analysis of supercontinuum generation of speciality optical fibers. SCG have been simulated in co-axial multicore silica and tellurite fibers using RP Fiber power software. SCG has also been simulated in a Ge11.5As24Se64.5 Chalcogenide glass graded index photonic crystal fiber using vectorial finite element based COMSOL Multiphysics and MATLAB. ii Silica has a very low nonlinear refractive index. Silica based optical fibers display very low nonlinear coefficient. Higher optical power and longer length fiber are required for SCG. Tellurite has ten times higher nonlinear refractive index compared to silica. SCG can be achieved at low pulse power with smaller length cable. The step index fibers have very large effective area which is an undesirable feature for nonlinear application. Dispersion engineering in these fibers are difficult. PCF’s offer very small effective mode area. Dispersion characteristics can be modified as per requirement by arranging the air holes. Chalcogenide fibers have very high nonlinear coefficient and is transparent in near-IR and mid-IR region. Graded index assists in achieving a better dispersion profile. Hence a chalcogenide glass graded index PCF has been designed for generation an ultra broadband continuum at near-IR and mid-IR region with a low peak power. Such ultra-broadband supercontinuum spectrum is expected to have profound applications in various fields. The specific applications of mid-IR supercontinuum generation include spectroscopy, optical coherence tomography, frequency comb generation, early cancer detection, food quality control, security and sensing.