Orientador: Newton Cesário Frateschi / Dissertação (mestrado) - Universidade Estaual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T07:11:07Z (GMT). No. of bitstreams: 1 Vallini_Felipe_M.pdf: 6816809 bytes, checksum: 7d9629364087ccf092b857c9650d464d (MD5) Previous issue date: 2009 / Resumo: A crescente demanda do uso de sistemas de comunicação óptica, seja pelo aumento do número de usuários ou pela quantidade de informação enviada, requer um aumento substancial na necessidade de desenvolvimento de novos sistemas e componentes. Em termos de componentes aplicados à comunicação óptica, aqueles utilizados para a amplificação óptica são de grande importância. Entre eles estão presentes os amplificadores ópticos de semicondutores. Para aplicações após a modulação óptica, pré-detecção e analógicas, é importante alta linearidade dos amplificadores, para que não haja distorção dos sinais e informações enviadas. Já os amplificadores em saturação (de baixa linearidade) permitem grande robustez de desempenho e a redução da complexidade (e do custo) de componentes de micro-óptica em integrações híbridas, neste caso utilizados para compensar perdas ópticas devido ao sistema e ao envelhecimento dos lasers e corrigir efeitos da polarização. Sendo assim, nos dois extremos de condição de operação, não saturado e em saturação profunda, temos a possibilidade de utilizar o dispositivo em aplicações lineares ou não-lineares, respectivamente. Como a saturação do ganho de um amplificador depende da densidade de portadores injetadas no mesmo, propomos o desenvolvimento de amplificadores com multicontatos para a injeção não homogênea de portadores. Com base na idéia proposta acima, nossa dissertação de mestrado trata do projeto de amplificadores multi-contatos partindo do estudo das propriedades de saturação do ganho. Fabricamos um laser de semicondutor, do qual parâmetros empíricos foram extraídos. Desenvolvemos uma simulação de amplificadores de contatos únicos e de multi-contatos, cujos dados de entrada foram os parâmetros empíricos obtidos. Com a mesma estrutura epitaxial utilizada na fabricação do laser, fabricamos amplificadores de contato único e de multi-contatos. Caracterizamos os dispositivos e obtemos boa concordância qualitativa com nossas simulações. Obtemos um controle da linearidade em amplificação de sinais contínuos de -7 a 7 dBm para uma baixa corrente de bombeio das cavidades dos dispositivos / Abstract: The dramatic increase in information density required for nowadays telecom systems demands constant improvements in optical communication technology. In this technology the components used for optical amplification are of great importance, particularly the semiconductor optical amplifiers for they provide high level of miniaturization and reduction in power consumption. Considering these components, after optical modulation, before detection and analog applications requires high linearity of the amplifiers. On the other hand, amplifiers in deep saturation (highly non-linear) yield big robustness and complexity reduction (and cost reductions) for hybrid integration. Also, non-linearity may be of interest for wavelength conversion, switching and pulse compression or dilatation. Therefore, the two extreme operation conditions, unsaturated or in deep saturation are of interest. Since the gain saturation of an amplifier depends on their carrier density, we propose the development of amplifiers with multi-contacts for non-homogeneous injection of carriers along the waveguide of the device. Based on this idea we present the development of multi-contact amplifiers in this dissertation. We first present the modeling of the device using semi-empirical parameters obtained from the fabrication of semiconductor lasers that uses the same epitaxial structures. Subsequently, we fabricated single-contact amplifiers and multi-contacts semiconductor amplifiers, the main subject of this work. The characterization of the devices was realized and good qualitative concordance with our simulations is shown. A linearity control of the amplification for DC signals was achieved for relatively low injection current in both cavity in a -7 to 7 dBm input power range / Mestrado / Mestre em Física

The need to exploit the large fibre bandwidth and increase the reach has seen the application of the Raman fibre amplifier (RFA) become indispensable in modern light wave systems. The success and resilience of RFAs in optical communication is deeply rooted in their unique optical properties and new technologies which have allowed the amplifier to come of age. However, the full potential of RFAs in optical communication and other applications are yet to be realized. More so are its polarization properties which still remain largely unexploited and have not been fully understood. In this work, fundamental issues regarding distributed RFA have been investigated with the aim of acquiring a better understanding of the amplifier polarization characteristics which have potential applications. In particular the effects of polarization mode dispersion (PMD) and polarization dependent loss (PDL) have been demonstrated both by simulation and experiment. The possibility of Raman polarization pulling in single mode fibres (SMFs) has also been addressed. Polarization sensitivity of RFA has been known for a long time but the clear manifestation of it has become evident in the advent of modern low PMD fibre. Unlike EDFAs which make use of special doped fibre, RFAs require no special fibre for operation. Besides, RFA uses a very long length of fibre and as such the fibre polarization characteristics come into play during amplification. In the demonstrations presented in this thesis a fibre of PMD coefficient < 0.05 pskm-1/2 was regarded as low PMD fibre while one having coefficient ≥ 0.05 pskm-1/2 was categorized to have high PMD unless otherwise stated. Several experiments were performed to evaluate the RFA gain characteristics with respect to fibre PMD and the system performance in the presence of noise emanating from amplified spontaneous emission (ASE). Analysis of Raman gain statistics was done for fibres of low and high PMD coefficients. The statistics of PDG and on-off gain were eventually used to demonstrate the extraction of PMD coefficients of fibres between 0.01- 0.1 pskm-1/2 using a forward pumping configuration. It was found that, at increasing pump power a linear relationship exists between forward and backward signal gain on a dB scale. The interaction of PDL and Raman PDG in the presence of PMD were observed at very fundamental level. It was found the presence of PDL serves to reduce the available on-off gain. It was also established that the presence of PMD mediates the interaction between PDG/PDL. When PMD is high it reduces PDG but the presence of PDL introduces a wavelength dependent gain tilting for WDM channels. Further analysis revealed that signal polarization is influenced by the pump SOP due to the pulling effect which is present even at moderate pump power.

This dissertation investigates the design and performance issues of a recently demonstrated technique, termed as spectrum-slicing, for implementing wavelength-division-multiplexing (WDM) in optical fiber systems. Conventional WDM systems employ laser diodes operating at discrete wavelengths as carriers for the different data channels that are to be multiplexed. Spectrum-slicing provides an attractive low-cost alternative to the use of multiple coherent lasers for such WDM applications by utilizing spectral slices of a broadband noise source for the different data channels. The principal broadband noise source considered is the amplified spontaneous emission (ASE) noise from an optical amplifier. Each slice of the spectrum is actually a burst of noise that is modulated individually for a high capacity WDM system. The stochastic nature of the broadband source gives rise to excess intensity noise which results in a power penalty at the receiver. One way to minimize this penalty, as proposed and analyzed for the first time in this work, is to use an optical preamplifier receiver. It is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which optimizes the detection sensitivity (minimizes the average number of photons/bit) for a given error probability. Moreover the evaluated detection sensitivity represents an order of magnitude ( > 10 dB) improvement over conventional PIN receiver-based detection techniques for such spectrum-sliced communication systems. The optimum is a consequence of signal energy fluctuations dominating at low values of the signal time bandwidth product (m), and the preamplifier ASE noise dominating at high values of m. Operation at the optimum bandwidth renders the channel error probability to be a strong function of the optical bandwidth, thus providing motivation for the use of forward error correction coding (FEC). System capacity (for BER = ) is shown to be 23 Gb/s without coding, and 75 Gb/s with a (255,239) Reed Solomon code. The effect of non-rectangular spectra on receiver sensitivity is investigated for both OOK and FSK transmission, assuming the system (de)multiplexer filters to be N'th order Butterworth bandpass. Although narrower filters are recommended for improving power budget, it is shown that system penalty due to filter shape may be kept < 1 dB by employing filters with N > 2. Moreover spectrum-sliced FSK systems using optical preamplifier receivers are shown, for the first time, to perform better in a peak optical power limited environment. Performance-optimized spectrum-sliced WDM systems have potential use in both local loop and long-distance fiber communication systems which require low-cost WDM equipment for high data rate applications. / Ph. D.

Noise and distortion limit the channel capacity and degrade the system performance in AM-VSB SCM CATV Distribution Networks. The use of star couplers and fiber amplifiers is an approach to overcome these problems and meet the increasing demand for large capacity and high quality. The capacity and performance analyses of three different network configurations are compared based on the results of computer modeling. In practice, the best configuration depends on the optical amplifiers and the pump lasers used in the system as well as the requirement on the number of channels and the number of subscribers. It is found that in a two-stage star network the largest improvement in the capacity and performance may be achieved when the optical amplifiers between the stages are individually pumped. For example, a capacity of 200 channels and 572 output nodes may be obtained. There is an optimal length of the erbium-doped fiber amplifier (EDFA) and there is also an optimal position where the EDFA is located between the two star couplers. The modeling is based on an extensive analysis of noise and distortion in the system. A discrepancy in the power behavior of laser relative intensity noise (RIN) is pointed out. A previous analysis on reflection-induced noise is extended to include the effect of quadruply-reflected light and optical amplifiers. Since chirp-dispersion distortion and gain tilt distortion are found to be negligible, clipping distortion is the main limitation on the system capacity. Optical amplifier noise provides an additional limitation to the system. Because the gain and amplified spontaneous noise depend on the input signal power and pump power, the gain and spontaneous emission factor are modeled for the comparison of the different systems. / Master of Science

Conjugated polymers are organic materials which are attractive as optoelectronic devices because they have a combination of broad band emission, high gain, versatility in processing, are ductile and can be electrically pumped. This thesis describes work conducted on such conjugated polymers as amplifier devices. The conjugated polymers used in this thesis were MEH-PPV, F8BT, GP1302 and ADS233YE. The amplifier devices used were grating coupled and end coupling waveguides. Amplification of light was demonstrated and characterised on single and multiple pulses using the grating coupled structure. Single pulse measurements obtained gains of 21 and 17 dB in a 1 mm long waveguide using the conjugated polymers MEHPPV and F8BT. Annihilation rate was also analysed in the single pulse method with MEH-PPV, giving a value of γ ≈ (3 ± 0.1) x 10⁻⁹ cm³/s. Amplification of a single pulse led to demonstrate amplification and the capability of the amplifier to function with multiple pulses, which resulted in F8BT being used as the gain medium. An average gain of 18 dB was obtained with F8BT in a 1 mm waveguide channel. Amplification was also investigated with end coupled waveguides. This led into investigating a suitable material or suitable combination of material for amplification with the waveguides. Switching of an amplified pulse was attempted on F8BT and GP1302 in the amplifier device at 5 kHz. Switching of F8BT was problematic which lead to attempt switching in GP1302 which was a co-polymer of PFO and F8BT. A 70 % switching effect was obtained with GP1302. Gain recovery dynamics of F8BT, GP1302 and ADS233YE was also investigated. These measurements established a switching rate of 500 GHz for GP1302 and ADS233YE, and F8BT showed partial gain recovery indicating the presence of long lived species. Switching was also attempted on a polymer laser. This resulted in a 100 % switched pulse with a combination of weak pump and strong switch pulse of 40 nJ and 2 μJ respectively. And a strong pump and weak switch pulse of 200 and 50 nJ respectively. Temporal delay of the switch pulse relative to the pump pulse resulted in re-timing of the laser output. Amplification and switching of light pulses were also attempted at a higher repetition rate of 50 kHz with F8BT, GP1302, ADS233YE and MEH-PPV. This resulted in strong amplification of light in MEH-PPV and F8BT with gains of 21 and 13 dB respectively in a waveguide length of 422 μm. Weak amplification of light in ADS233YE and GP1302 was also obtained with a maximum gain of 8 and 3 dB respectively. Switching was attempted on MEH-PPV and ADS233YE.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

This thesis presents an investigation of the lasing dynamics and optical amplification devices using conjugated polymers. Spectroscopic studies of conjugated polymers and dendrimers were also performed. Conjugated polymers and dendrimers are materials with great potential as display materials and tuneable lasers due to their broad spectra and high optical gains. The effect of conjugation is studied in MEH-PPV and an anisotropy measurement of two different cored dendrimers has been shown to verify a theoretical prediction on their depolarisation. Singlet emission from a highly efficient phosphorescent dendrimer is also observed and is the first known report of fluorescence from this class of dendrimers. Conjugated polymers exhibit optical gain over broad spectral ranges, which has led to much interest in their potential as novel laser gain media. Investigations into lasing from conjugated polymers has been confined mainly to studying the lasing properties and not the temporal dynamics of the laser pulses. In this work an investigation into the lasing dynamics of a 2D-DFB conjugated polymer laser is demonstrated with the first subpicosecond laser pulses observed for a polymer laser. A novel encapsulated laser fabricated via a soft lithography route was also studied and exhibited laser pulse of 6 ps duration. The high gain observed over broad spectral ranges also means that these materials are suitable for use as optical amplifiers. Broadband gain in a conjugated polymer solution was demonstrated with a gain of 30 dB accessible across a 60 nm wavelength range. In the solid state the limited thickness of films (~ 100 nm) and the uneven nature of the film edges had limited the ability to study the amplification of a probe signal. The first practical solid state conjugated polymer amplifier has been demonstrated. The device uses grating structures to couple a probe signal into and out of the gain region. The gain dynamics of different length amplifiers were studied and an 18 dB gain was observed in a 300 µm device length using a conjugated polymer blend of RedF and F8BT. Further work on a conjugated polymer MEH-PPV led to a 21dB gain in a 1 mm device.

Cladding pumped single-frequency Yb3+-doped fiber amplifiers at 976 nm were investigated. Over 4 W output power was obtained and further power scaling can be achieved by reducing the cladding diameter of the Yb3+-doped fiber.

Foi desenvolvida e construída uma cavidade bombeadora para um meio de ganho de Cr:LiSAF na forma de bastão, bombeado por duas lâmpadas, visando sua operação como amplificador de pulsos ultracurtos a taxas de repetição elevadas, e com redução da carga térmica sobre o meio de ganho. A cavidade e o meio de ganho foram caracterizados sob operação como laser e como amplificador de pulsos ultracurtos. Atuando como laser, obtivemos 30 Hz de taxa de repetição e 20 W de potência média, resultado inédito, e ganho máximo de 1,5 por passagem. Alterando as características do bombeamento, foi possível operar o laser a 8 Hz e 16 W de potência média, com ganho máximo de 3,6 por passagem. A cavidade bombeadora, na configuração de maior ganho, foi integrada e sincronizada a um sistema Chirped Pulse Amplification baseado em cristais de Ti:Safira, concebido e construído visando a sua conjugação com o amplificador de Cr:LiSAF. O amplficiador introduziu um fator de amplificação de 150 em pulsos de 20 ps, produzindo pulsos com potência de pico de 0,5 TW. Também foi apresentada uma nova técnica para medida do limiar de ablação por pulsos ultracurtos, e sua teoria analítica foi desenvolvida. Esta técnica foi utilizada na determinação do limiar de ablação de cristais de Cr:LiSAF no regime de 20 ps. / We have developed and built a pumping cavity for a Cr:LiSAF rod, pumped by two flashlamps, in order to operate it as an ultrashort laser pulse amplifier at the highest possible repetition rate, under reduced thermal load. The pumping cavity and the gain medium were characterized under laser and ultrashort pulse amplifier operation. Operating as a laser, 30 Hz repetition rate and 20 W average power were obtained for the first time at a maximum gain per pass of 1.5. Changing the pumping characteristics, the laser provided 16 W at 8 Hz repetition rate, at a maximum gain of 3.6. The cavity, in the highest gain configuration, was integrated and synchronized to a Ti:Sapphire Chirped Pulse Amplification system, conceived and built to comprise the Cr:LiSAF amplifier. The amplifier provided amplification by a factor 150 to the 20 ps stretched pulses, resulting in pulses with 0.5 TW of peak power. Also, a new technique to measure ultrashort pulses ablation threshold was presented, and its analytical theory was developed. This technique was used to determine the ablation threshold for Cr:LiSAF crystals in the 20 ps regime.