[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.

Dans cette thèse nous décrivons l'étude expérimentale et théorique d'un modulateur à électro-absorption (EAM) en vue de son intégration verticale sur un VCSEL (Laser à Cavité Verticale Emettant par la Surface) pour des communications optiques à très hautes fréquences. La modulation externe de la lumière émise par le VCSEL, alimenté en continu, permet de s'affranchir de la limite physique due à la dynamique des porteurs et devrait donc permettre d'augmenter la bande passante comparé à un VCSEL à modulation directe. Notre approche permet de considérablement diminuer la surface utile de par son intégration monolithique verticale et ainsi la consommation électrique. La première partie est consacrée au design du composant EAM-VCSEL. Tout d'abord nous expliquerons l'effet d'électro-absorption et comment le modéliser, sa combinaison avec la méthode de transfert matricielle, et son implémentation pour optimiser les paramètres physiques des puits quantiques tels que : l'épaisseur, la concentration d'aluminium dans les barrières et le champ électrique à appliquer. Ensuite, basé sur l'état de l'art des modulateurs verticaux, nous présenterons la conception d'une structure Fabry-Pérot asymétrique pour augmenter l'effet d'absorption dans la cavité. Cette structure optimisée du modulateur sera ensuite intégrée sur une structure standard de VCSEL tout en considérant le découplage optique entre ces cavités. Je présenterai ensuite la fabrication de ces composants complexes. Nous aborderons la croissance des structures EAM et EAM-VCSEL avec notamment l'optimisation de la calibration des cellules et les caractérisations après croissance. Ensuite nous présentons un procédé lift-off innovant visant à faciliter le procédé complet. Ce procédé sera utilisé pour fabriquer le modulateur afin de le caractériser en régime statique. Nous démontrons ainsi son fonctionnement en mesurant la réflectivité en fonction de la température et de la tension appliquée ce qui nous permet d'avoir une validation du modèle précédent. Nous présentons enfin la caractéristique LIV du modulateur-VCSEL ainsi que son spectre en longueur d'onde.[...] / In this PhD thesis, we describe experimental and theoretical studies on Electro-Absorption Modulator (EAM) for its vertical integration onto a Vertical-Cavity Surface-Emitting Laser (VCSEL) to reach high-speed modulation for optical communications. The external modulation of the emitted light by the VCSEL, biased in continuous-wave, avoids the physical limitation due to the carrier dynamics encountered in directly-modulated lasers and thus enables very high frequency bandwidth. Furthermore, this fully monolithic integrated approach decreases the footprint and thus the energy consumption. In the first part of the manuscript, we describe the design of the EAM-VCSEL device. First, we explain the electro-absorption effect and its modelling; its combination with the transfer matrix method, and the implementation of the combined model for optimization of the quantum well parameters: thickness, barrier Al-content and applied electric field. Then, based on the state-of-the-art of the vertical modulators, we design an Asymmetric Fabry-Perot modulator structure to improve the electro-absorption in the top cavity. This EAM structure is integrated onto a standard VCSEL device while considering the decoupling between the two cavities. I present afterwards the fabrication of these complex devices. The epitaxial growth of EAM and EAM-VCSEL structures is presented as an implementation of pre-growth calibration routine to achieve the expected characteristics and the post-growth characterization of the wafers. Then, we present an innovative technological solution used to facilitate the global process. We used a double resist stack for a self-aligned process for the modulator, useful for the mesa etch, the sidewalls passivation and the metallization, all realized with only one single photolithography step. This process is used for the static characterization of the EAM. We demonstrate the feasibility of these devices by carrying out static reflectivity measurements as a function of the temperature and the applied EAM voltage. This allows to validate the previous absorption model. We also present the static characterization of the EAM-VCSEL with LIV curves and spectrum measurements. [...]

To date, group-III-nitride has undergone continuous improvements to provide a broader range of industrial applications, such as solid-state lighting (SSL), visible light communications (VLC), and light projection. Recently, VLC has attained substantial attention in the field of wireless communication because it offers ~ 370 THz of bandwidth of unregulated visible spectrum, which makes it a critical factor in the evolution of the 5G networks and beyond. GaN-based light-emitting diode (LED) and laser diode (LD) have become increasingly appealing in energy-sufficient SSL replacing conventional light sources. However, III- nitride LEDs suffer from efficiency-droop in their external quantum efficiency associated with high current densities, and their modulation bandwidth is limited to 10 ~ 100 MHz. Although LDs have shown gigabit-modulation bandwidth, unfavorable artifacts, such as speckles are observed, which may raise a concern about eye safety. This dissertation is devoted to the fabrication and electrical and optical characterization of a new class of III-nitride light-emitter known as superluminescent diode (SLD). SLD works in an amplified spontaneous emission (ASE) regime, and it combines several advantages from both LD and LED, such as droop-free, speckle-free, low-spatial coherence, broader emission, high-optical power, and directional beam. Here, SLDs were fabricated by a focused ion beam by tilting the front facet of the waveguide to suppress the lasing mode. They showed a high-power of 474 mW on c-plane GaN-substrate with a large spectral bandwidth of 6.5 nm at an optical power of 105 mW. To generate SLD- based white light, a YAG-phosphor-plate was integrated, and a CRI of 85.1 and CCT of 3392 K were measured. For the VLC link, SLD showed record high-data rates of 1.45 Gbps and 3.4 Gbps by OOK and DMT modulation schemes, respectively. Additionally, a widely single- and dual-wavelength tunability were designed using SLD-based external cavity (SLD-EC) configuration for a tunable blue laser source. These results underscore the practicality of c-plane SLDs in realizing high-power, high data rate, speckle-free, and droop-free SSL-VLC apparatus. Additionally, the SLD-EC configuration allows a wide range of applications, including biomedical applications, optical communication, and high-resolution spectroscopy.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20090号 / 工博第4257号 / 新制||工||1659(附属図書館) / 33206 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 野田 進, 教授 藤田 静雄, 教授 川上 養一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

This dissertation investigates the monolithic integration of dual optical elements on high power semiconductor lasers for emission around 980nm wavelength. In the proposed configuration, light is coupled out of the AlGaAs/GaAs waveguide by a low reflectivity grating coupler towards the substrate where a second monolithic optical element is integrated to improve the device performance or functionality. A fabrication process based on electron beam lithography and plasma etching was developed to control the grating coupler duty cycle and shape. The near-field intensity profile outcoupled by the grating is modeled using a combination of finite-difference time domain (FDTD) analysis of the nonuniform grating and a self-consistent model of the broad area active region. Improvement of the near-field intensity profile in good agreement with the FDTD model is demonstrated by varying the duty cycle from 20% to 55% and including the aspect ratio dependent etching (ARDE) for sub-micron features. The grating diffraction efficiency is estimated to be higher than 95% using a detailed analysis of the losses mechanisms of the device. The grating reflectivity is estimated to be as low as 2.10-4. The low reflectivity of the light extraction process is shown to increase the device efficiency and efficiently suppress lasing oscillations if both cleaved facets are replaced by grating couplers to produce 1.5W QCW with 11nm bandwidth into a single spot a few mm above the device. Peak power in excess of 30W without visible COMD is achieved in this case. Having optimized, the light extraction process, we demonstrate the integration of three different optical functions on the substrate of the surface-emitting laser. First, a 40 level refractive microlens milled using focused ion beam shows a twofold reduction of the full-width half maximum 1mm above the device, showing potential for monolithic integration of coupling optics on the wafer. We then show that differential quantum efficiency of 65%, the highest reported for a grating-coupled device, can be achieved by lowering the substrate reflectivity using a 200nm period tapered subwavelength grating that has a grating wavevector oriented parallel to the electric field polarization. The low reflectivity structure shows trapezoidal sidewall profiles obtained using a soft mask erosion technique in a single etching step. Finally, we demonstrate that, unlike typical methods reported so far for in-plane beam-shaping of laser diodes, the integration of a beam-splitting element on the device substrate does not affect the device efficiency. The proposed device configuration can be tailored to satisfy a wide range of applications including high power pump lasers, superluminescent diodes, or optical amplifiers applications.

The topic of this dissertation is the development of low repetition rate and low noise semiconductor-based laser sources with a focus on linearly chirped pulse laser sources. In the past decade chirped optical pulses have found a plethora of applications such as photonic analogto-digital conversion, optical coherence tomography, laser ranging, etc. This dissertation analyzes the aforementioned applications of linearly chirped pulses and their technical requirements, as well as the performance of previously demonstrated chirped pulse laser sources. Moreover, the focus is shifted to a specific application of the linearly chirped pulses, timestretched photonic analog-to-digital conversion (TS ADC). The challenges of surpassing the speeds of current electronic converters are discussed, while the need for low noise linearly chirped pulse lasers becomes apparent for the realization of TS ADC. The experimental research addresses the topic of low noise chirped pulse generation in three distinct ways. First, a chirped pulse (Theta) laser with an intra-cavity Fabry-Pérot etalon and a long-term referencing mechanism is developed that results in the reduction of the pulse-topulse energy noise. Noise suppression of > 15 times is demonstrated. Moreover, an optical frequency comb with spacing equal to the repetition rate (≈100 MHz) is generated using the etalon, resulting in the first reported demonstration of a system operating in the sub-GHz regime based on semiconductor gain. The path for the development of the Theta laser was laid by the precise characterization of the etalon used in this laser cavity design. A narrow linewidth laser is used in conjunction with an acousto-optic modulator externally swept for measuring the etalon's iv free spectral range with a sub-Hz precision, or 10 parts per billion. Furthermore, the measurement of the etalon long-term drift and birefringence lead to the development of a modified intra-cavity Hänsch-Couillaud locking mechanism for the Theta laser. Moreover, an external feed-forward system was demonstrated that aimed at increasing the temporal/spectral uniformity of the optical pulses. A complete characterization of the system is demonstrated. On a different series of experiments, the pulses emitted by an ultra-low noise but high repetition rate mode-locked laser were demultiplexed resulting in a low repetition rate pulse train. Experimental investigation of the noise properties of the laser proved that they are preserved during the demultiplexing process. The noise of the electrical gate used in this experiment is also investigated which led into the development of a more profound understanding of the electrical noise of periodical pulses and a mechanism of measuring their noise. The appendices in this dissertation provide additional material used for the realization of the main research focus of the dissertation. Measurements of the group delay of the etalon used in the Theta laser are presented in order to demonstrate the limiting factors for the development of this cavity design. The description of a balancing routine is presented, that was used for expanding the dynamic range of intra-cavity active variable delay. At last, the appendix presents the calculations regarding the contribution of various parameters in the limitations of analog-todigital conversion.

This thesis presents a study of lasing properties and optical amplification in semiconducting conjugated polymers and dendrimers. Configured as surface-emitting distributed feedback lasers, the effect of incorporating wavelength-scale microstructure on the output of the devices is examined along with the ability to create such structures using simplified fabrication processes such as soft lithography. Conjugated materials have received a great deal of interest due to their broad spectral absorption, emission, ability to exhibit gain and ease of processing from solution. As a result, they show great potential for a variety of applications such as photovoltaics, displays, amplifiers and lasers. To date however, there has only been one demonstration of a polymer optical amplifier. A broadband, solution based polymer amplifier is presented where the gain overlaps with the transmission window of polymer optical fibres. The effect of transitions that reduce the availability of gain in conjugated polymers is also examined by studying saturation of absorption in thin films. Producing wavelength scale microstructure is traditionally a slow, expensive technique. Here, solvent assisted micromoulding is used to pattern polymer films in less than two minutes. The effect of the variations in the pattern transfer on the laser characteristics is examined. The micromoulding technique is then applied to fabricating novel device types such as circular gratings and flexible plastic lasers. Encapsulation of the micromoulded laser is then shown to improve the lifetime of the device by over three orders of magnitude. The degradation effects witnessed during this extended operation are characterised quantitatively, an area of study where little data exists in the literature. A novel class of branched dendrimer materials whose properties can be independently tuned due to their modular architecture are configured as blue-emitting distributed feedback lasers. The ability to tune the emission wavelength by varying the film thickness is demonstrated. By changing the chemical groups contained within the molecule, further tuning of the emission can be obtained along with the demonstration of a highly efficient blue-emitting dendrimer laser. Chemosensing using dendrimer lasers is presented by demonstrating the incredibly sensitive response of the laser device to trace vapours of nitro-benzene compounds. The future application of which could be highly beneficial in the detection of explosives.