Optical Amplifiers and their Applications in Nonlinear Fiber-Optic Communication Systems

The work presented in this dissertation deals with the characterization and modeling of optical amplifiers as well as their applications in nonlinear transmission systems with the main motivation to enhance system capacity. In the first part of the dissertation the work is focused on two different types of amplifiers: the semiconductor laser amplifier (SLA) and the erbium-doped fiber amplifier (EDFA). The noise characteristics of EDFAs are studied in detail, since EDFAs will be employed as in-line amplifiers and will thus determine a great deal of the system performance. The multi-functional performance of an SLA and the trade-offs between different functions of the SLA were investigated, since multi-functionality may facilitate monolithic integration. Utilizing a new and accurate pulsed source technique, the gain compression dependence of the noise figure (NF) of an EDFA was examined. An algorithm to map out all possible EDFA parameter configurations, including the NF, given constraints on the gain, the gain compression, and the output signal was developed. When no solutions existed, an additional degree of freedom in form of a post amplifier loss was introduced. The inclusion of amplified spontaneous emission (ASE) was shown to be crucial to accurately predict the EDFA performance. During the course of investigation a method to extract parameters in the effective ASE banddwidth model from experimental data was devised. In the second part, schemes to compress solitons and to overcome some of the obstacles hampering soliton transmission systems are studied: suppression of instabilites due to too short amplifier distances by means of distributed amplification, and reduction of interaction between adjacent solitons by alternating-amplitude (AA) modulation. Soliton compression by propagation through fiber junctions has been investigated experimentally and theoretically. The compressed solitons contained most of the original energy with minor oscillations in the pulse widths. A numerical investigation, showing the potential to double the system length with AA solitons, was conducted using a 100 Gbit/s, 400 km long AA soliton system with realistic fiber parameters and long pseudo random word. Lumped amplification perturb soliton transmission if the amplifier spacing is smaller than the soliton period, which scales as the pulse width squared. For high bit rate soliton systems the amplifier spacing becomes impractically small. One remedy is to use distributed amplification. Soliton transmission of 10 ps solitons over more than 90 km using distributed-EDFA transmission fiber was demonstrated. Finally, the impact of cross phase modulation induced spectral and temporal distortion in a fiber-based mid- span spectral inverter, intended for chromatic dispersion compensation, has been studied

Simulation and Optimization of SiC Field Effect Transistors

Silicon Carbide (SiC) is a wide band-gap semiconductor material with excel-lent material properties for high frequency, high power and high temperature elec-tronics. In this work different SiC field-effect transistors have been studied using theoretical methods, with the focus on both the devices and the methods used. The rapid miniaturization of commercial devices demands better physical models than the drift-diffusion and hydrodynamic models most commonly used at present. The Monte Carlo method is the most accurate physical methods available and has been used in this work to study the performance in short-channel SiC field-effect devices. The drawback of the Monte-Carlo method is the computational power required and it is thus not well suited for device design where the layout requires to be optimized for best device performance. One approach to reduce the simulation time in the Monte Carlo method is to use a time-domain drift-diffusion model in contact and bulk regions of the device. In this work, a time-domain drift-diffusion model is implemented and verified against commercial tools and would be suitable for inclusion in the Monte-Carlo device simulator framework. Device optimization is traditionally performed by hand, changing device pa-rameters until sufficient performance is achieved. This is very time consuming work without any guarantee of achieving an optimal layout. In this work a tool is developed, which automatically changes device layout until optimal device per-formance is achieved. Device optimization requires hundreds of device simulations and thus it is essential that computationally efficient methods are used. One impor-tant physical process for RF power devices is self heating. Self heating can be fairly accurately modeled in two dimensions but this will greatly reduce the computa-tional speed. For realistic influence self heating must be studied in three dimensions and a method is developed using a combination of 2D electrical and 3D thermal simulations. The accuracy is much improved by using the proposed method in comparison to a 2D coupled electro/thermal simulation and at the same time offers greater efficiency. Linearity is another very important issue for RF power devices for telecommunication applications. A method to predict the linearity is imple-mented using nonlinear circuit simulation of the active device and neighboring passive elements

Optical Amplifiers and their Applications in Nonlinear Fiber-Optic Communication Systems

The work presented in this dissertation deals with the characterization and modeling of optical amplifiers as well as their applications in nonlinear transmission systems with the main motivation to enhance system capacity. In the first part of the dissertation the work is focused on two different types of amplifiers: the semiconductor laser amplifier (SLA) and the erbium-doped fiber amplifier (EDFA). The noise characteristics of EDFAs are studied in detail, since EDFAs will be employed as in-line amplifiers and will thus determine a great deal of the system performance. The multi-functional performance of an SLA and the trade-offs between different functions of the SLA were investigated, since multi-functionality may facilitate monolithic integration. Utilizing a new and accurate pulsed source technique, the gain compression dependence of the noise figure (NF) of an EDFA was examined. An algorithm to map out all possible EDFA parameter configurations, including the NF, given constraints on the gain, the gain compression, and the output signal was developed. When no solutions existed, an additional degree of freedom in form of a post amplifier loss was introduced. The inclusion of amplified spontaneous emission (ASE) was shown to be crucial to accurately predict the EDFA performance. During the course of investigation a method to extract parameters in the effective ASE banddwidth model from experimental data was devised. In the second part, schemes to compress solitons and to overcome some of the obstacles hampering soliton transmission systems are studied: suppression of instabilites due to too short amplifier distances by means of distributed amplification, and reduction of interaction between adjacent solitons by alternating-amplitude (AA) modulation. Soliton compression by propagation through fiber junctions has been investigated experimentally and theoretically. The compressed solitons contained most of the original energy with minor oscillations in the pulse widths. A numerical investigation, showing the potential to double the system length with AA solitons, was conducted using a 100 Gbit/s, 400 km long AA soliton system with realistic fiber parameters and long pseudo random word. Lumped amplification perturb soliton transmission if the amplifier spacing is smaller than the soliton period, which scales as the pulse width squared. For high bit rate soliton systems the amplifier spacing becomes impractically small. One remedy is to use distributed amplification. Soliton transmission of 10 ps solitons over more than 90 km using distributed-EDFA transmission fiber was demonstrated. Finally, the impact of cross phase modulation induced spectral and temporal distortion in a fiber-based mid- span spectral inverter, intended for chromatic dispersion compensation, has been studied

Multi‑objective Pareto and GAs nonlinear optimization approach for fyback transformer

Design and optimization of high-frequency inductive components is a complex task because of the huge number of variables to manipulate, the strong interdependence and the interaction between variables, the nonlinear variation of some design variables as well as the problem nonlinearity. This paper proposes a multi-objective design methodology of a 200-W flyback transformer in continuous conduction mode using genetic algorithms and Pareto optimality concept. The objective is to minimize loss, volume and cost of the transformer. Design variables such as the duty cycle, the winding configuration and the core shape, which have great effects on the former objectives but were neglected in previous works, are considered in this paper. The optimization is performed in discrete research space at different switching frequencies. In total, 24 magnetic materials, 6 core shapes and 2 winding configurations are considered in the database. Accurate volume and cost models are also developed to deal with the optimization in the discrete research space. The bi-objective (loss–volume) and tri-objective (loss–volume–cost) optimization results are presented, and the variations of the design variables are analyzed for the case of 60 kHz. An example of a design (30 kHz) is experimentally verified. The registered efficiency is 88% at full load.WIRI

Core Loss Calculation of SymmetricTrapezoidal Magnetic Flux Density Waveform

Existing empirical core loss models for symmetric trapezoidal flux waveform (TzFW) stillsuffer some issues such as the inaccuracy and the complexity. These issues are mainly due to the lack ofan accurate model of the relaxation loss generated during the off-time. This paper aims to understand therelaxation loss and develop an accurate model using the superposition technique. The developed model givesan accurate prediction of the on-time loss and the relaxation loss and shows the dependency of each on theduty cycle. The research shows that the core loss at low duty cycle is several times the core loss at full dutycycle. The developed model is verified with experimental results and compared to the Improved Steinmetzequation (ISE). The model error is reduced to lower than 15% compared to 50% of the ISE. Finally, an easymethod using multiplication factors with the ISE model is given to simplify the developed model

A Modified Higher Operational Duty Phase Shifted Full Bridge Converter for Reduced Circulation Current

Besides many advantages, the reduction in the operational duty of traditional phase shifted full converter limits its scope in applications where a wide range of input voltage is the main requirement. Operation with low duty cycle extends freewheeling interval, which results in degraded performance such as more circulation current, increased conduction loss in power devices, narrow range of zero voltage switching and increased EMI. To overcome these drawbacks, this work suggests a modified phase shifted full bridge converter that keeps the operational duty of the converter high for a wide range of input voltage. This cuts the freewheeling interval and improves performance. The proposed converter consists of four low profile transformers having reconfigurable interconnection structure. There are two distinct reconfigurable modes, a low gain mode and a high gain mode, which can be adopted in accordance with the variation in line voltage. The proposed work is validated in LTspice simulation and hardware characterization for a wide range of input voltage 100-400Vdc/12Vout and up to the load power of 1.2kW

High Performance Planar Magnetics Based on an Unbalanced-Flux Approach

This paper presents a new design concept to increase the efficiency and the power density of planar magnetics. In contrast to the existing magnetics, which are built using the balanced magnetic flux density design, the proposed design concept generates unbalanced magnetic flux density across the different parts of the magnetic core. The theoretical analysis shows that the core loss of the unbalanced-flux design can be reduced by more than 50% compared to the existing one. The core loss reduction brings several benefits to planar magnetics such as: high power capability, better thermal performance and wider safe operating area (SOA). The proposed design is experimentally evaluated and compared with the balanced-flux design. The experimental results are in good consistency with its theoretical counterparts. The measured core loss are decreased by more than 50% and the power density is increased by more than 250%

Battery powered inductive welding system for electrofusion joints in optical fiber microducts

Optical fiber microducts are joined together by mechanical joints. These mechanical joints are bulky, require more space per joint, and are prone to air pressure leakage and water seepage during service. A battery powered electrofusion welding system with a resistive-type joint has been recently developed to replace mechanical joints. These resistive-type electrofusion joints require physical connectors for power input. Due to a different installation environment, the power input connectors of resistive optical fiber microduct joints may corrode over time. This corrosion of connectors will eventually cause water seepage or air pressure leakage in the long run. Moreover, due to connector corrosion, resistive-type optical fiber microduct joints cannot be re-heated in future if the need arises. In this study, an inductively coupled electrofusion-type joint was proposed and investigated. This inductive-type electrofusion joint is not prone to long-term corrosion risk, due to the absence of power connectors. Inductive-type electrofusion joints can be re-heated again for resealing or removal in the long run, as no metal part is exposed to the environment. The battery powered inductive welding system can be easily powered with a 38 volts 160 watt-hour battery. The inductive-type electrofusion joint was welded within one second, and passed a 300-newton pull strength test and a 10-bar air pressure leakage test. It was demonstrated that the power input requirement for inductive electrofusion joints is 64% higher than that of resistive electrofusion joints. However, these inductive joints are relatively easy to manufacture, inexpensive, have no air leakage, and no water seepage risk in highly corrosive environments.

LTspice electro-thermal model of joule heating in high density polyethylene optical fiber microducts

At present, optical fiber microducts are joined together by mechanical type joints. Mechanical joints are bulky, require more space in multiple duct installations, and have poor water sealing capability. Optical fiber microducts are made of high-density polyethylene which is considered best for welding by remelting. Mechanical joints can be replaced with welded joints if the outer surface layer of the optical fiber microduct is remelted within one second and without thermal damage to the inner surface of the optical fiber duct. To fulfill these requirements, an electro-thermal model of Joule heat generation using a copper coil and heat propagation inside different layers of optical fiber microducts was developed and validated. The electro-thermal model is based on electro-thermal analogy that uses the electrical equivalent to thermal parameters. Depending upon the geometric shape and material properties of the high-density polyethylene, low-density polyethylene, and copper coil, the thermal resistance and thermal capacitance values were calculated and connected to the Cauer RC-ladder configuration. The power input to Joule heating coil and thermal convection resistance to surrounding air were also calculated and modelled. The calculated thermal model was then simulated in LTspice, and real measurements with 50 µm K-type thermocouples were conducted to check the validity of the model. Due to the non-linear transient thermal behavior of polyethylene and variations in the convection resistance values, the calculated thermal model was then optimized for best curve fitting. Optimizations were conducted for convection resistance and the power input model only. The calculated thermal parameters of the polyethylene layers were kept intact to preserve the thermal model to physical structure relationship. Simulation of the optimized electro-thermal model and actual measurements showed to be in good agreement.