Novel optical transmitters for high speed optical networks

The objective of this thesis is to investigate the performance of novel optical transmitter lasers for use in high speed optical networks. The laser technology considered is the discrete mode laser diode (DMLD) which is designed to achieve single wavelength operation by etching features on the surface of the ridge waveguide. This leads to a simplified manufacturing process by eliminating the regrowth step used in conventional approaches, presenting an economic approach to high volume manufacture of semiconductor lasers. Two application areas are investigated in this work. The bit rate in next generation access networks is moving to 10 Gbit/s. This work characterises the performance of DMLDs designed for high speed operation with the objective of identifying the limitations and improving performance to meet the specifications for uncooled operation at 10 Gbit/s. With the deployment of advanced modulation formats the phase noise of the laser source has become an important parameter, particularly for higher order formats. DMLDs were developed for narrow linewidth operation. The linewidth of these devices was characterised and a value as low as 70 kHz was demonstrated. Transmission experiments were also carried out using a coherent transmission test bed and the performance achieve is compared with that of an external cavity laser

Controlo de subsistemas de processamento integralmente óptico

Mestrado em Engenharia Electrónica e TelecomunicaçõesO presente trabalho tem por objectivos o desenvolvimento e optimização de dois controladores para subsistemas ópticos: um comutador integralmente óptico e um gerador de banda lateral única. Inicialmente, foram abordados os componente ópticos envolvidos e as suas principais características. Foram desenvolvidos, implementados e, posteriormente, testados de forma individual os circuitos de controlo de cada componente óptico. Foram, também, implementados os respectivos controladores principais de cada subsistema. Seguidamente, foram construídos os protótipos dos dois subsistemas. Por fim, os protótipos foram caracterizados e foi possível verificar as suas funcionalidades através dos resultados obtidos.This thesis purpose is the design and implementation of two controllers for optical subsystems: one for an all-optical switching system and other for an all-optical sideband suppressor. Initially, it was introduced the optical components involved and its main characteristics. It was developed, implemented and finally tested independently the control circuits of each optical component. Their main controllers for each subsystem were also implemented. Then, the prototypes of the two subsystems were built. Finally, the prototypes were characterized and their functionality could be checked through the overall results

Componentes optoelectrónicos para redes fotónicas de alto débito

O presente trabalho tem por objectivo o estudo dos diversos componentes optoelectrónicos que constituem uma rede fotónica, com vista ao aumento da flexibilidade, débito, conectividade e desempenho. Para tal, são analisados os blocos e funções essenciais que permitem a implementação de uma rede óptica transparente e multiplexada no comprimento de onda. Inicialmente, caracterizaram-se os diversos componentes optoelectrónicos envolvidos em redes ópticas multicomprimento de onda, tendo-se dado uma ênfase especial ao meio de transporte de informação, nomeadamente, às propriedades não lineares das fibras ópticas. Foi implementado laboratorialmente um demonstrador de um nó óptico de extracção e inserção de canais sintonizável, baseado numa rede de Bragg gravada em fibra óptica, tendo esta configuração de nó evoluído para um estágio de protótipo comercial. Foi, também, implementado um demonstrador de um nó óptico de cruzamento de canais, completamente reconfigurável baseado em componentes discretos disponíveis comercialmente, onde se demonstra uma arquitectura estritamente não bloqueante. Foram estudadas as diversas técnicas de conversão de comprimento de onda e implementado um módulo capaz de realizar conversão de comprimentos de onda, no domínio óptico e independente do ritmo e formato de modulação, que suporta ritmos de transmissão de 2.5 Gbit/s, baseado na modulação cruzada do ganho num amplificador óptico semicondutor reflectivo. Por fim, foi desenvolvida uma metodologia transparente de análise do desempenho em sistemas de comunicações ópticas utilizando amostragem assíncrona. Foi derivado um modelo para os histogramas assíncronos em função das características dos impulsos, tendo-se implementado um módulo de análise de qualidade de serviço de uma rede óptica.The present work intends to study several optoelectronic components used in a high speed photonic network, in order to increase flexibility, debit, connectivity and performance. The basic essentials blocks and functions to the implementation of a transparent and wavelength multiplexed optical network are analysed. Initially, the optolectronic components typically used in a multiwavelength network are characterised. A specific emphasis is focused on the transport medium, mainly the non-linear proprieties of the optical fibres. A tunable optical add-drop multiplexer node demonstrator based in a fibre Bragg grating was experimentally implemented. A completely reconfigurable optical cross connect based on commercially available discrete components was also implemented where a strictly non blocking architecture was demonstrated. Several optical wavelength conversion techniques were studied and a wavelength conversion module working on the optical domain, independent of the bitrate and signal format which support a 2.5 Gbit/s bitrate was implemented. This module is based on the cross gain modulation in a semiconductor optical reflective amplifier. An optical performance monitoring transparent methodology using asynchronous sampling was developed. A numerical model for the asynchronous histograms based on the optical pulses characteristics was derived. An optical network quality of service analysing module was implemented

Computer modelling of electro-optic modulators

An electro-optic modulator is an essential part of an optical communications system. An ideal modulator would have low drive voltage, large bandwidth, small size, low insertion loss, a very high extinction ratio, very low chirp and low power consumption. However, there is a trade-off between some of these characteristics, and consequently the modulator is designed with a compromise. It is due to these complex devices that computer modelling has become a necessary and integral part of device design, which is made easier and more economically acceptable by the larger capacities of today’s computers. In this work, a finite element method-based approach is used to design and develop computer programs in order to simulate the major characteristics of electro-optic modulators (i.e. low drive voltage, a large bandwidth, small size, low insertion loss) and fabrication variables and combine these separate computer programs into one complete suite to model electro-optic waveguide modulators for a good and complete design process for these devices. The devices examined in this work are the TkLiNbCfi directional coupler-based modulator, the Ti:LiNb03 Mach-Zehnder interferometric modulator and a GaAs/AlGaAs interferometric-based electro-optic modulator. These computer programs give the optical mode for each of the devices examined in this work both before and after application of an electric field, giving exact values for the refractive and any change in refractive index with applied electric field. In this way it is the drive voltage and size of the device that can be adjusted and examined in order to obtain an optimum design. The electrode design is important to modulators particularly bandwidth but is also essential for decreasing optical loss without increasing the drive voltage. A program was designed and developed using a perturbational approach in order to model this optical loss. For the TiiLiNbCfi directional coupler based modulator, the light coupling process between the two adjacent waveguides as well as the power transfer efficiency were also modelled using a FEM LSBR-based approach. However, the directional coupler based devices are slower than their MZI-based counterparts. A TkLiNbCfi MZI-based modulator was also studied with both channel and ridge waveguides. It was found that a ridge waveguide is optimally beneficial only if the FWHM (width and height) of the mode fit into the ridge. The bending waveguides at the input and output of the Ti:TiNb03 MZI-based modulator were modelled using a FEM BPM-based approach. This approach yields accurate results for structures where the cross-section of the device is changing. A computer program was also developed to incorporate the RF characteristics of the device in order to find the bandwidth of the device for perfect velocity and impedance matching conditions. Furthermore, smaller studies were undertaken for a ThLiNbCfi MZI-based modulator for electrical sensing applications as well as for a GaAs/AlGaAs interferomtric-based modulator. The main work of this thesis was to improve these device structures in order to provide optimal results. The suite of computer programs yielded results very close and similar to that examined in the literature. Most importantly, the computer modelling approach to device design yields accurate results for both simple and more complex devices structures and is a cheaper long-term approach to device design