Design of microstructured fibers for hollow core guidance

Instead of the traditional index guidance, microstructured fibers can guide light in a core of refractive index lower than that of its cladding using mechanisms like photonic band gap guidance, inhibited coupling guidance and anti-resonant guidance. Their guidance is usually leaky and depends on the photonic properties of their structured cladding. Specifically, photonic band gap guidance is possible with photonic crystals, whose photonic band gaps appear below the refractive index index of the core. Guidance in a low-index core or hollow core guidance, is of interest for applications in the fields of bioanalytic, quantum gas, lasers and others that involve interacting of the light with confined matter of low refractive index. My work is aimed at investigating the possibility of hollow core guidance with an all-solid microstructured cladding. Ideally, such a hollow core waveguide is expected to have obvious guidance advantages over capillaries. Besides, it also surpasses the holey hollow core band gap fibers in the optofluidic applications by avoiding undesired penetration of the liquid into the cladding channels. To achieve the design of the ideal hollow core waveguide, I developed two models for all relevant modes in microstructured fibers: an analytical method with binary functions and a reflection-based planar model. While the binary functions for photonic band gap is more about ideal periodic structures extended into infinite, the reflection and transmission analysis with a reflection-based planar model is more practical to be used for waveguides with finite periodic structures and deliberately induced disorder

Identification of zero density of states domains in band gap fibers using a single binary function

Here we introduce a new calculation method to find the domains of zero density of states for photonic band gap guiding fibers consisting of arrays of high refractive index strands in a low refractive index cladding. We find an analytic expression that associates any combination of geometric parameter, effective index, material and wavelength with a single binary function which allows direct determination whether the density of cladding states is zero or not. The method neither requires the typically used root finding procedure for dispersion tracking nor simulation volume discretization. We verify the validity of our approach on well-established results and reveal as example that band gap regions are mainly determined by the two lowest order Bessel function orders. Our method allows for extensive parameter scans and evaluation of photonic band gap structures against structural and material inaccuracies with substantially reduced simulation effort

Evaluating resonances in PCSEL structures based on modal indices

The frequently sought after combination of characteristics in semiconductor lasers of high power together with narrow beam divergence and monochromatic output is usually difficult to attain. The photonic crystal surface emitting laser (PCSEL) is one category of device, however, which tends to provide the above-mentioned desirable output features. The PCSEL uses a large area optically active surface but with a two-dimensional periodic structure that enables it to generate high power in a narrow vertically emitted beam yet maintaining single wavelength operation. A primary requirement to model PCSELs is to obtain the optical field resonances that identify the lasing mode. This study presents an alternative method for evaluating the resonances, based essentially on the transfer-matrix technique and wave propagation in multilayer medium, which is relatively easy to formulate, and has quite modest demands on computing requirements

Multilayer waveguide modes based analysis of 2-D photonic crystals-pertinent to modelling PCSELs

Semiconductor lasers with the combination of characteristics such as large output power, single mode operation and good beam quality are very often desired. The photonic crystal surface emitting laser (PCSEL) has shown significant promise and has received much attention with the purpose of achieving devices with the desired characteristics. The evaluation of the resonant modes of the structure is a primary requirement in modelling PCSELs. However, conventional techniques such as PWE, CMT and FDTD are either computationally very time consuming or mathematically rather intensive. The aim of this thesis is to develop a new model for evaluating resonance of 2-D photonic crystal, pertinent to the lasing mode of PCSEL. Such aim is achieved by first studying wave characteristics of 1-D periodic structure and understanding the eigenmode and eigenfunction of both infinite and finite periodic structure. It is shown that the eigenmode of the infinite periodic structure is the Bloch mode while the eigenmode of the finite periodic structure is represented by optical tunnelling type of solution. The solutions correspond to the characteristic impedance of the periodic structure. The concept of eigenmode of finite periodic structure is then used to establish the 2-D model of photonic crystal. The essential underlying concept of the analysis procedure presented in this work is based on viewing the 2-D photonic crystal as a laterally periodic multilayer waveguide which is longitudinally segmented. Such model matches with conventional model favourably and proved to be versatile, efficient, fast (for 500×500 periods takes ~7min using laptop: 2 core at 1.70 GHz, negligible memory usage. (Compare to FDTD for 20×20 periods takes 5h using supercomputer system: 12 core, 24GB RAM). Thus, the model has the potential of generating more comprehensive models of photonic crystal based devices. Experimental work including fabrication, characterisation further proved the validity of the model. PCSEL with external reflection is experimental studied. It is shown that the lasing characteristics can be modified through introduced external reflection

Modal Index Analysis of Resonances of PCSEL

This paper presents a new modal index analysis method for evaluating the resonances of PCSEL structures which is versatile, efficient and fast. Hence it is envisaged that the implementation of this method will enhance the potential to generate more comprehensive models of photonic crystal based devices, say, PCSELs, that include, for example, aspects of inversion population distribution and also time dependence while still retaining relatively modest demands on computational resources

An efficient optimized independent component analysis method based on genetic algorithm

Three simulation experiments are designed to evaluate and compare the performance of three common independent component analysis implementation algorithms – FastICA, JADE, and extended-Infomax. Experiment results show that the above three algorithms can’t separate the mixtures of super-Gaussian and sub-Gaussian precisely, and FastICA fails in recovering weak source signals from mixed signals. In this case an independent component analysis algorithm, which applies genetic algorithm to minimize the difference between joint probability and product of marginal probabilities of separated signals, is proposed. The computation procedure, especially the fitness evaluation when signals are in discrete form, is discussed in detail. The validity of the proposed algorithm is proved by simulation tests. Moreover, the results indicate that the proposed algorithm outperforms the above three common algorithms significantly. Finally the proposed algorithm is applied to separate the mixture of rolling bearing sound signal and electromotor signal, and the results are satisfied

Theoretical study of small signal modulation behavior of Fabry-Perot Germanium-on-Silicon lasers

This work investigated the small signal performance of Fabry-Perot Ge-on-Si lasers by modeling and simulations. The 3dB bandwidth dependence on the structure parameters such as poly-Si cladding thickness, Ge cavity width and thickness, and minority carrier lifetime were studied. A 3dB bandwidth of 33.94 GHz at a biasing current of 270.5 mA is predicted after Ge laser structure optimization with a defect limited carrier lifetime of 1 ns