Modelação e simulação de fenómenos não-lineares em fibras ópticas microestruturadas

This Ph.D. embraced the study of the nonlinear optical phenomena occurring in microstructured optical _bres (MOFs), also called photonic crystal fibres (PCFs). In these fibres, there are the solid-core MOFs (SC-MOFs) and the hollow-core MOFs (HC-MOFs), which allow guided propagation of light in a diversity of conditions that are not possible with conventional fibres. The mechanisms of guidance of these fibres were studied, which for SC-MOFs and HC-MOFs included the computation of the guided modes, and for the latter included also the calculation of photonic bands, bandgaps, and density of states. In addition, the dispersion curves and the nonlinearity coefficients of SC-MOFs and of gas-filled HC-MOFs were calculated. The nonlinear propagation of ultra-short pulses of light in those fibres was modeled and simulated. With the simulations of this work, the supercontinuum generation, and the UV-light generation were predicted in certain conditions. In the last part of this work the electromagnetically induced transparency effect in hollow core fibres was investigated in detail.Este trabalho de doutoramento englobou o estudo de fenómenos ópticos não-lineares que ocorrem em fibras ópticas microestruturadas (MOFs), também chamadas fibras de cristais fotónicos (PCFs). Nestas fibras, temos as fibras microestruturadas de núcleo solido (SC-MOFs) e as fibras microestruturadas de núcleo oco (HC-MOFs), que permitem a guiagem de luz numa diversidade de condições não atingidas com as fibras convencionais. Estudou-se os mecanismos de guiagem de luz nestas fibras, o que quer para as SC-MOFs quer para as HC-MOFs incluiu a obtenção dos modos de propagacão, e para estas últimas incluiu ainda o calculo das bandas fotónicas, bandas proíbidas, e densidade de estados. Por outro lado, as curvas de dispersão e os coeficientes não-lineares de SC-MOFs e HC-MOFs preenchidas com gases foram calculados. Também se efectuou a modelação e a simulação da propagação não-linear de impulsos ultra-curtos nestas fibras. Com as simulações deste trabalho, foi prevista a geração do supercontínuo e de luz ultravioleta em certas condições. Na ultima parte deste trabalho o efeito da transparência electromagneticamente induzida em fibras de núcleo oco foi investigado em detalhe.Programa Doutoral em Físic

Finite element characterisation of photonic crystal fibres

Rigorous numerical simulations have been carried out by using the Finite Element Method (FEM) in order to calculate bending and leakage losses of Photonic Crystal Fibres (PCF). A modal solution approach including the implementations of the conformal transformation and the Perfectly Matched Layer (PML) were undertaken to determine the bending and leakage losses of several designs of Photonic Crystal Fibres. This was carried out by varying key parameters such as the pitch (Λ), diameter (d) and air-filling fraction (d/Λ). Output modal parameters including the effective indices, spot sizes, leakage and bending losses as well as the mode field profiles were obtained. These output parameters were obtained by varying the bending radius (R) from very large values to very low values for different dimensions of the PCF, with results being obtained for Transverse Magnetic and Transverse Electric (quasi-TM and quasi-TE) polarizations. These parameters were calculated by solving the Maxwell’s equations using the H-field vector formulation and with the inclusion of PML to solve complex eigenvalue equations. Generally, it was observed that for all Λ, d/Λ and the polarization considered, as R is reduced from a very high value to lower values, the bending losses increase and there is a sharp increase at some lower values of R. At some very low values of R, some oscillatory behaviour was observed in the curves obtained for the fibre losses, where further investigations were carried out. These oscillations appeared due to degeneration of the fundamental mode with the cladding modes. In most of the cases investigated, there was a correlation in the variation of effective indices the loss values and also in the variation of spot sizes. PCFs with non-identical air-holes were also investigated in which case the d ≠ d2 (diameter of 4 larger air-holes in the first ring) and knowing the values for TM and TE polarizations, it was possible to determine the birefringence, which is the difference between the effective indices for the TM and TE modes and also the loss ratio, which is the ratio of TM loss to that of the TE loss. All the input and output parameters that were considered with the symmetric air-holes were also considered in the case with fibre with asymmetric air-holes study. The results obtained are very important in the design of Single Mode Single Polarization PCF. Results have also been obtained from the studies done of asymmetric arrangement of air-holes which lead to the design of Single Mode Single Polarization PCF. Work was carried out on the design of a tapered PCF that could be efficiently coupled to a single mode fibre, SMF. This was achieved by increasing the number rings up to 10 rings of air-holes in the cladding and having the outermost ring with larger air-holes, the inner rings were near cutoff. This fibre was coupled to a conventional SMF to allow for better tolerance to fabrication errors. There has also been work carried out in polymer fibre namely Teflon and TOPAS in the terahertz regime. The conventional hexagonal arrangement of PCF was simulated and compared to spiral PCF in THz. An improved PCF design having a porous core with hexagonal arrangement and cladding was designed and analysed and low-loss guidance in THz was achieved. Thus overall a number of different PCF designs were considered and their properties evaluated and detailed knowledge has been obtained on potential performance of such fibres

Finite element characterisation of photonic crystal fibres

Rigorous numerical simulations have been carried out by using the Finite Element Method (FEM) in order to calculate bending and leakage losses of Photonic Crystal Fibres (PCF). A modal solution approach including the implementations of the conformal transformation and the Perfectly Matched Layer (PML) were undertaken to determine the bending and leakage losses of several designs of Photonic Crystal Fibres. This was carried out by varying key parameters such as the pitch (Λ), diameter (d) and air-filling fraction (d/Λ). Output modal parameters including the effective indices, spot sizes, leakage and bending losses as well as the mode field profiles were obtained. These output parameters were obtained by varying the bending radius (R) from very large values to very low values for different dimensions of the PCF, with results being obtained for Transverse Magnetic and Transverse Electric (quasi-TM and quasi-TE) polarizations. These parameters were calculated by solving the Maxwell’s equations using the H-field vector formulation and with the inclusion of PML to solve complex eigenvalue equations. Generally, it was observed that for all Λ, d/Λ and the polarization considered, as R is reduced from a very high value to lower values, the bending losses increase and there is a sharp increase at some lower values of R. At some very low values of R, some oscillatory behaviour was observed in the curves obtained for the fibre losses, where further investigations were carried out. These oscillations appeared due to degeneration of the fundamental mode with the cladding modes. In most of the cases investigated, there was a correlation in the variation of effective indices the loss values and also in the variation of spot sizes. PCFs with non-identical air-holes were also investigated in which case the d ≠ d2 (diameter of 4 larger air-holes in the first ring) and knowing the values for TM and TE polarizations, it was possible to determine the birefringence, which is the difference between the effective indices for the TM and TE modes and also the loss ratio, which is the ratio of TM loss to that of the TE loss. All the input and output parameters that were considered with the symmetric air-holes were also considered in the case with fibre with asymmetric air-holes study. The results obtained are very important in the design of Single Mode Single Polarization PCF. Results have also been obtained from the studies done of asymmetric arrangement of air-holes which lead to the design of Single Mode Single Polarization PCF. Work was carried out on the design of a tapered PCF that could be efficiently coupled to a single mode fibre, SMF. This was achieved by increasing the number rings up to 10 rings of air-holes in the cladding and having the outermost ring with larger air-holes, the inner rings were near cutoff. This fibre was coupled to a conventional SMF to allow for better tolerance to fabrication errors. There has also been work carried out in polymer fibre namely Teflon and TOPAS in the terahertz regime. The conventional hexagonal arrangement of PCF was simulated and compared to spiral PCF in THz. An improved PCF design having a porous core with hexagonal arrangement and cladding was designed and analysed and low-loss guidance in THz was achieved. Thus overall a number of different PCF designs were considered and their properties evaluated and detailed knowledge has been obtained on potential performance of such fibres.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Fiber amplifiers at 1.5 [my]m for gravitational wave detectors : power scaling, gain dynamics, and pump sources

[no abstract