MODELING AND DESIGN OF SPECIALTY OPTICAL FIBERS AND WAVEGUIDES FOR SUPERCONTINUUM GENERATION

Abstract

This thesis focuses on the modeling and design of specialty optical fibers and waveguides for supercontinuum generation, targeting applications across diverse domains of nonlinear optics and photonic systems. By employing highly nonlinear materials such as silica, chalcogenides, and organic liquids, these designs aim to generate broad supercontinuum spectra spanning from the visible to the mid-infrared. The approach emphasizes short interaction lengths and low input peak powers, while maintaining high temporal coherence to maximize bandwidth and ensure spectral flatness. These specialty optical fibers and waveguides find applications in various fields like biomedical, military and sensing technologies. Supercontinum generation is the process where amalgamation of various nonlinear phenomena including stimulated Raman scattering, self-phase modulation, cross-phase modulation, and fourwave mixing takes place. They work together on an intense pump beam, leading to a significant amount of spectral broadening compared to the original pump beam in an optical fiber. The generated supercontinuum sources can be used to discern chemicals, inspect food quality, detect explosives and hazardous gases, ulcer and cancer diagnosis, frequency comb generation, optical imaging, and optical coherence tomography. In this thesis, we have numerically designed photonic crystal fibers and waveguides for supercontinuum generation across the visible to mid-infrared spectrum using the finite element method. By optimizing core and cladding geometries and adjusting core – cladding materials, we have minimized dispersion at the pump wavelength. The impact of input peak power, pulse width, fiber length and coherence on supercontinuum broadening has been numerically analyzed. The proposed waveguide structure with a parabolic core, implemented in a chalcogenide glass results in the generation of a mid-IR supercontinuum spectrum when pumped in the normal dispersion region. This design is crucial as the fabricated rectangular waveguide does not remain rectangular after laser post processing but becomes similar to the proposed parabolic design. Another chalcogenide based graded-index hybrid cladding photonic crystal fiber is suitable for the generation of an ultra-broadband supercontinuum spectrum from 1 µm to 11 µm in MIR domain when pumped with the peak power of 0.75 kW at 5µm with 50 fs pulse width. We have also proposed liquid-infiltrated photonic crystal fibers to enhance the nonlinearity of silica fibers, enabling the generation of a highly coherent broadband supercontinuum. Ethanol based photonic crystal fiber results in a flat-top dispersion profile with a low peak power of 0.55kW at a pump wavelength of 1.55 μm. Another photonic crystal fiber design infiltrated with nitrobenzene results ix in a spectrum ranging from 1.3 μm to 2.0 μm for the circular design (#C), from 0.9 μm to 2.3 μm for the elliptical x-polarised design (#EX) and from 1.0 μm to 2.4 μm for the elliptical y-polarised design (#EY). For dual infiltrated photonic crystal fibers we have a broadband supercontinuum spanning from 1.3 µm – 1.8 µm for design #A, 1.0 µm – 2.4 µm for design #B, 1.2 µm – 2.5 µm for design #C and 0.9 µm – 2.5 µm for design #D is obtained by using a fiber length of 4 mm to 8 mm with 50 fs secant laser pulse source and pump power of 600 W to 800 W. The suggested waveguide and photonic crystal fiber designs can be employed for various critical applications involving detection of explosives, various gases, cancers, ulcers and enhanced monitoring systems for food quality detection.