Simulation of mid-infrared transmission losses in semiconductor-core optical fibers
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The mid-infrared (IR) region of electromagnetic spectrum (2-15 ) has numerous applications, ranging from chemical & thermal sensing to medical surgery. Current IR fiber technology, that uses chalcogenides, halides, among other materials, has its shortcomings. Semiconductor materials like germanium (Ge) and silicon (Si) have exceptional properties like tunable band-gap, high refractive index & low extinction coefficients and low Rayleigh scattering due to highly crystalline structure, making them excellent materials for IR optical fibers and other IR based devices. Optical fibers fabricated with Ge-Si alloy cores are still in their nascent phase and this research aims to provide a detailed theoretical study of transmission losses in such fibers. Using Electromagnetic Finite Element Analysis (FEA) through COMSOL software, IR transmission through semiconductor core optical fibers was successfully modeled and simulated. The study focused on identifying the optimal geometry and operating wavelengths for pure semiconductor (Ge and Si) core fibers, Ge-Si alloy Step Index (SI) core fibers, as well as Graded Refractive Index (GRIN) Ge-Si alloy core fibers. Using optical waveguide theory, and available wavelength dependent attenuation values in the literature, properties like transmission losses and EM modal parameters were simulated for Ge core fibers and Ge core covered with ZnSe (thin ring) fibers. These predictions were in excellent agreement with published literature values, thereby validating the simulation methodology. This methodology was then applied to Ge-Si alloy core fibers with SI and GRIN structures, with properties such as refractive index and extinction coefficients calculated using techniques like the Kramers-Kronig relationships. Fiber properties such as attenuation, electric & magnetic fields, optical power flow and other important parameters, were investigated as function of wavelength of the IR light, fiber geometry, and composition profile of the Ge-Si alloy fiber core, to assess the optimum fiber design for each type of core. The simulation results showed that GRIN Ge-Si alloy fibers were the best solution, due to their low losses, low dispersion of EM signals and better optical power confinement over a wide spectral range.