Thesis (M.Phil.) - University of Queensland, 2006. / Includes bibliography.

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Orientador: Newton Cesario Frateschi / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-10T18:33:04Z (GMT). No. of bitstreams: 1 Mialichi_JoseRoberto_M.pdf: 1964796 bytes, checksum: 5579bc0b247e2f473578a1a155424ebf (MD5) Previous issue date: 2006 / Resumo: Esta dissertação de mestrado apresenta estudos teóricos sobre o cálculo de ganho óptico em nanoestruturas baseadas no sistema InGaAs/InGaAsP. A partir da teoria de perturbação dependente do tempo, cuja perturbação é a interação de uma onda eletromagnética-matéria, e da teoria de Kane para a estrutura de bandas de semicondutores que relaciona a matriz de transição envolvida nos processos radiativos com parâmetros experimentais desta mesma banda, desenvolvemos todo o formalismo teórico deste cálculo do ganho óptico, além de criarmos uma ferramenta computacional para a aplicação do mesmo às nanoestruturas semicondutoras, tais como poço, fio e ponto quântico. Curvas espectrais de ganho são geradas para cada caso e as análises comparativas destas curvas demonstram que nanofios e nanopontos propiciam condições de alto ganho óptico devido ao confinamento de suas dimensões e, ainda melhor, controle espectral. O efeito da não-homogeneidade das dimensões dos pontos também é considerado. Subseqüentemente, aplicamos os resultados do cálculo de ganho óptico para calcular a corrente de limiar de lasers de semicondutores de faces clivadas a partir das equações de taxa de fótons e portadores na cavidade com a injeção de corrente elétrica. Um estudo é realizado para a otimização do comprimento da cavidade para cada um dos meios de amplificação de luz: bulk, poços, fios e pontos quânticos. Por fim, descrevemos nossos primeiros resultados para o crescimento epitaxial, através do método de crescimento epitaxial de feixe químico (CBE), dos componentes básicos de uma estrutura de laser de semicondutor. Fabricamos e caracterizamos um diodo de junção de InP e poços quânticos de InGaAs/InGaAsP. Amostras epitaxiais de lasers de poços quânticos obtidas externamente foram processadas em lasers de diversos comprimentos de cavidade e caracterizadas. Especialmente, apresentamos curvas de corrente de limiar em função do comprimento da cavidade do laser em concordância qualitativa com nossas simulações, estabelecendo o primeiro passo para o refinamento de nossa ferramenta computacional / Abstract: This master¿s dissertation presents the theoretical studies on the calculation of optical gain in nanostructures based on the InGaAs/InGaAsP/InP system. From the time dependent perturbation theory, where perturbation is the interaction of an electromagnetic wave with matter, and the Kane¿s theory for semiconductors band structure, relating the transition matrix involved in the radioactive processes with experimental parameters of the same band structure, we have developed a theoretical formalism for the optical gain calculation. We then developed a simulation tool to apply to semiconductors structures, such as well, wire, and quantum dot. Spectral gain curves are generated for each case and comparative analyses of these curves demonstrates that quantum wires and quantum dots provide higher optical gain due to its confined dimensions and, even better, much spectral control. The effects of non-homogeneous dot size distribution are also considered. Subsequently, we have applied the simulation tool to calculate the threshold current in semiconductor lasers with cleaved faces (edge emitting lasers), based on the rate equations for photons and carriers in the optical resonant cavity with the injection of electrical current. A study is performed for the optimization of the length of the cavity for each of active region medium: bulk, wells, wires, and quantum dots. Subsequently, we have described our first results for the epitaxial growth by chemical beam epitaxy (CBE) of the basic components of a semiconductor laser: InP junction diode and quantum wells of InGaAs/InGaAsP. Finally, epitaxial samples of quantum well lasers obtained externally were processed in lasers of several resonant cavity lengths and characterized. We have especially presented curves of threshold current as a function of the length of the resonant cavity of the laser in qualitative agreement with our simulations, establishing the first step for refining of our simulation tool / Mestrado / Física da Matéria Condensada / Mestre em Física

[pt] Os amplificadores a fibra dopada com érbio (EDFAs) têm se mostrado como uma alternativa bastante atraente para aumentar a capacidade de sistemas de comunicações por fibras ópticas operando na terceira janela de transmissão de fibras convencionais (lambda = 1.55 mi m). Consideráveis esforços têm sido direcionados ao aprimoramento do desempenho desses dispositivos. O potencial de aplicação em telecomunicações é grande e motiva o estudo do desempenho dos enlaces com o uso de EDFAs. A maior parte das contribuições teóricas sobre influência de amplificadores ópticos no desempenho dos receptores consiste de métodos que são fáceis de usar mas muito aproximado - o que pode conduzir a estimativas bastantes imprecisas -, ou muito precisos mas de utilização extremamente difícil - devido a uma complexidade inerente. São apresentados modelos para a simulação de sistemas de comunicações ópticos digitais que consideram as perdas no sinal e as distorções da forma de onda devidas ao efeito combinado do chirping do laser, da dispersão da fibra e da largura de banda finita do receptor. É proposta uma nova abordagem para o cálculo da taxa de erro do sistema e da sensibilidade, que possui a precisão e a simplicidade desejadas. O método desenvolvido considera a interferência intersimbólica (causada pelas distorções) e das contribuições de ruídos dos amplificadores, utilizando-se o método semi-analítico. Esses modelos e o método desenvolvido foram utilizados para introduzir a simulação de sistemas ópticos com EDFAs em um programa já existente de análise de sistemas. Foram realizadas simulações para avaliar as contribuições dos diversos parâmetros dos EDFAs e as implicações de algumas propriedades do sistema no desempenho total do sistema óptico. A comparação com resultados de análises rigorosas, encontrados na literatura, validou a abordagem proposta. / [en] Erbium-doped fiber amplifiers (EDFAs) have shown to be a very attractive alternative to improve the capacity of optical-fiber communications systems operating in the third transmission window of conventional fibers (lambda = 1.55 mi m). Considered efforts have been made towards the performance enhancement of these devices. The possible application potential in telecommunications is indeed large, and has simulated the study of system performance using EDFAs. Most of the previous theoretic contribuitions on the influence of optical amplifiers in the receiver performance consist of methods that are either easy to use but over-approximated - what can lead to quite inaccurate evaluations - or very precise but extremely difficult to use - due to some inherent complexity. Models for the simulation of digital optical communication systems are presented, which account for signal loss and distortion - due to laser chirping, fiber dispersion and detector finite bandwidth. A new semi-analytical method for the evaluation of bit-error rate (BER) and receiver sensitivity is introduced, which possesses the desired accuracy and simplicity. It takes into account the presence of inter-symbol interference (due to signal distortion) and the optical amplifier noises. These modesl and the developed method jhave been the used for introducing simulation of optical systems with EDFAs into a previously existing system evaluation software package. A numbe of simulations was carried out in order to estimate the contribution of EDFA parameters and implications of some system properties in the final performance of the optical system. The comparison with results from rigorous analysis, found in the literature, has validated the proposed approach.

[pt] Neste trabalho apresenta-se um estudo do comportamento dinâmico de laser semicondutor modulado diretamente em intensidade sob fracos e fortes sinais. Através da utilização de dispositivo passivo (filtro supressor) e ativo (FET de dupla porta) de microondas aplaina-se a resposta em freqüência numa faixa em torno de 1,5 GHz. Resultados medidos e modelados são comparados mostrando boa concordância. / [en] This work presents a study of the dynamic behaviour of a semiconductor laser under small and large signal direct intensity modulation. A flat frequency response can be obtained in a bandwidth of around 1,5 GHz by using passive (suppressor filter) and an active (dual-gate FET) microwave device. The measured and modeled results are compared showing good agreement.

The work described in this thesis investigates the effects of bandstructure modifications, brought about by Landau confinement, hydrostatic pressure and uniaxial stress, on recombination processes in narrow-gap materials and laser diodes. The effects of Landau confinement on the characteristics of InSb-based emission devices operating at a wavelength of ~5mum at 77K were studied. The change in performance due to the magnetic field applied along both the cavity and the growth direction and thereby simulating quasi-quantum wire and quasi-quantum dot structures clearly demonstrated the benefits, such as reduced threshold and temperature sensitivity, gained by the reduced dimensionality. On the other hand, suppression of LO-phonon emission due to the discrete nature of the density of states was observed, for the first time, in an interband laser device. Interband recombination dynamics were studied in In1-xGaxSb and PbSe over a range of excited carrier densities and temperatures down to 30K. Detailed analysis of the results found that the Auger-1 mechanism is reduced in In1-xGaxSb as a function of Ga-fraction due to the increased bandgap energy, in good agreement with theoretical predictions. In PbSe, the Auger-1 rate was observed to dominate at low excited carrier concentrations in spite of near-mirror bands, and was found to be approximately constant between 300K and 70K and was seen to be quenched in the low temperature regime. Stimulated emission was seen to be the most efficient recombination mechanism at high excited carrier densities at low temperatures. The Auger coefficient in PbSe was found to be one to two orders of magnitude lower than for materials with a Kane band structure (Hg1-xCdxTe) with comparable bandgap. An experimental technique was developed which enables measurements at high hydrostatic pressures and high magnetic fields at low temperatures. Hydrostatic pressures were applied to a 1.5mum laser diode at different temperatures revealing the effects of pressure on the band structure and hence the laser characteristics. A visible laser diode was measured under the simultaneous application of hydrostatic pressure and uniaxial stress. The change in performance was satisfactorily explained in terms of leakage of carriers into the X-minimum in the cladding region, the process that has been suspected of being one of the major loss mechanisms in visible laser diodes. This copy of the thesis has been supplied on the condition that anyone who consults it is understood to recognise that the copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without the prior written consent of the author or the University (as may be appropriate).

Femtosecond (fs) laser micromachining has been a useful technique either to modify and remove materials or to change the properties of a material, and can be applied to transparent and absorptive substances. Recently high-power fs laser radiation has drawn intensive attention for the induction of refractive index change to fabricate micro-structures in dielectric materials. This thesis studies the optical properties of a novel fiber random grating fabricated by fs laser micromachining technique and extends its applications from optical sensing to random fiber lasers and optical random bit generations. The thesis mainly consists of three parts. In the first part, the physical mechanism behind the fs laser micromachining technique and the fabrication of the fiber random grating are introduced. By employing a wavelength-division spectral cross-correlation algorithm, a novel multi-parameter fiber-optic sensor based on the fiber random grating is proposed and demonstrated to realize simultaneous measurements of temperature, axial strain, and surrounding refractive index. In the second part, Brillouin random fiber laser (BRFL) and Erbium-doped fiber random laser (EDFRL) are introduced, respectively. Firstly, we propose a novel Brillouin random fiber laser with a narrow linewidth of ~860 Hz based on the bi-directionally pumped stimulated Brillouin scattering (SBS) in a 10-km-long optical fiber. A random fiber Fabry-Perot (FP) resonator is built up through the pump depletion effects of SBS at both ends of the fiber. The novel laser is successfully applied for linewidth characterization beyond 860 Hz of light source under test. Secondly, the random grating-based FP resonator is introduced to build up a novel BRFL with narrow-linewidth of ~45.8Hz and reduced lasing threshold. The intensity and frequency noises of the proposed random laser are effectively suppressed due to the reduced resonating modes and mode competition. Finally, the fiber random grating is used as random distributed feedback in an EDFRL to achieve both static (temperature, strain) and dynamic (ultrasound) parameter sensing. Multiple lasing lines with high signal-to-noise ratio (SNR) up to 40dB are achieved, which gives an access for a high-fidelity multiple-static-parameter sensing application. By monitoring the wavelength shifts of each peak, temperature and strain have been simultaneously measured with small errors. The fiber random grating in the EDFRL is also able to sense the ultrasound waves. By achieving single mode lasing with the EDFRL, ultrasound waves with frequencies from 20kHz to 0.8MHz could be detected with higher sensitivity and SNR improvement up to 20dB compared with conventional piezoelectric acoustic sensors. In the third part, we demonstrate that a semiconductor laser perturbed by the distributed feedback from a fiber random grating can emit light chaotically without the time delay signature (TDS). A theoretical model is developed by modifying the Lang-Kobayashi model to numerically explore the chaotic dynamics of the laser diode subjected to the random distributed feedback. It is predicted that the random distributed feedback is superior to the single reflection feedback in suppressing the TDS. In experiments, The TDS with the maximum suppression is achieved with a value of 0.0088, which is the smallest to date.