Power Control In Optical CDMA

Optical CDMA (OCDMA) is the multiplexing technique over the fiber optics medium to increase the number of users and this is a step towards all optical Passive Optical Networks (PON). Optical OFDM, WDM and Optical TDM have also been studied in this thesis which are also candidates to all optical passive optical networks. One of the main features of Optical CDMA over other multiplexing techniques is that it has smooth capacity. The capacity of OCDMA is constrained by the interference level. Hence, when some users are offline or requesting less data rates, then the capacity will be increased in the network. Same feature could be obtained in other multiplexing techniques, but they will need much more complicated online organizers. However, in OCDMA it is critical to adjust the transmission power to the right value; otherwise, near-far problem may greatly reduce the network capacity and performance. In this thesis Power control concepts are analyzed in optical CDMA star networks. It is applied so that the QoS of the network get enhanced and all users after the power control have their desired signal to interference (SIR) value. Moreover, larger number of users can be accommodated in the network. Centralized power control algorithm is considered for this thesis. In centralized algorithm noiseless case and noisy case have been studied. In this thesis several simulations have been performed which shows the QoS difference before and after power control. The simulation results are validated also by the theoretical computation.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format

Integrated Inp Photonic Switches

Photonic switches are becoming key components in advanced optical networks because of the large variety of applications that they can perform. One of the key advantages of photonic switches is that they redirect or convert light without having to make any optical to electronic conversions and vice versa, thus allowing networking functions to be lowered into the optical layer. InP-based switches are particularly attractive because of their small size, low electrical power consumption, and compatibility with integration of laser sources, photo-detectors, and electronic components. In this dissertation the development of integrated InP photonic switches using an area-selective zinc diffusion process has been investigated. The zinc diffusion process is implemented using a semi-sealed open-tube diffusion technique. The process has proven to be highly controllable and reproducible by carefully monitoring of the diffusion parameters. Using this technique, isolated p-n junctions exhibiting good I-V characteristics and breakdown voltages greater than 10 V can be selectively defined across a semiconductor wafer. A series of Mach-Zehnder interferometric (MZI) switches/modulators have been designed and fabricated. Monolithic integration of 1x2 and 2x2 MZI switches has been demonstrated. The diffusion process circumvents the need for isolation trenches, and hence optical losses can be significantly reduced. An efficient optical beam steering device based on InGaAsP multiple quantum wells is also demonstrated. The degree of lateral current spreading is easily regulated by controlling the zinc depth, allowing optimization of the injected currents. Beam steering over a 21 microns lateral distance with electrical current values as low as 12.5 mA are demonstrated. Using this principle, a reconfigurable 1x3 switch has been implemented with crosstalk levels better than -17 dB over a 50 nm wavelength range. At these low electrical current levels, uncooled and d.c. bias operation is made feasible. The use of multimode interference (MMI) structures as active devices have also been investigated. These devices operate by selective refractive index perturbation on very specific areas within the MMI structure, and this is again realized using zinc diffusion. Several variants such as a compact MMI modulator that is as short as 350 µm, a robust 2x2 photonic switch and a tunable MMI coupler have been demonstrated

Design and characterization of InP based Mach-Zehnder modulators at 2μm wavelength

The Mach-Zehnder modulators (MZMs) based on InP are the key building blocks of photonic integrated circuits (PICs) due to low drive voltage and higher electro-optic (EO) bandwidth. They are the most suitable candidates to replace the widely deployed large footprint Lithium Niobate (LiNbO3) based MZMs. This thesis is focused on the design and development of travelling wave InP MZMs operating in the conventional optical C-band and also at 2000 nm which is one of the newly proposed possible alternatives for optical transmission to avoid highly anticipated ‘Capacity Crunch‘in the currently deployed standard single mode fiber (SSMF) in the next decade. InP MZMs working around the 1550 nm wavelength range were developed and characterised under DC and high frequency in order to validate the optimal electrode design. The highlight of presented work is the development of the first InP MZMs for operation around 2000 nm wavelengths for used in future optical transmission systems. To make the operation feasible around 2000 nm wavelength, compressively strained InGaAs QWs are used in the optical waveguide. The developed modulators exhibit a 3-dB EO bandwidth of 9 GHz with switching voltage as low as 3.2 V for a 3 mm long electrode. It is also shown that maximizing the electro-optical overlap by increasing the number of quantum wells can significantly reduce the Vπ, hence the modulator driving conditions for higher order modulation formats, without sacrificing the modulation bandwidth and device dimensions. Further, the devices are packaged using specially designed RF interposer to be used in an efficient, high-capacity WDM transmitter for communication over 1.15 km hollow-core photonic bandgap fiber (HC-PBGF) at 2 μm wavelength. A WDM capacity of 40 Gb/s is accomplished by using four 10 Gb/s NRZ-OOK externally modulated channels for the first time and transmission performance is evaluated using a direct detection receiver

Multiple Quantum Well Structures As Optical Waveguides

This thesis is concerned with the design, fabrication and characterisation of semiconductor optical waveguides in which the high index guiding layer is a multiple quantum well structure (MQWS), consisting of alternate layers of high and low band gap semiconductors with the electrons and holes in the MQWS being confined to the low band gap material. This confinement in two dimensions alters greatly the electronic and optical properties of the MQWS in comparison to the bulk properties of the constituent layers. The basic concepts involved in MQW waveguides are introduced using an elementary quantum mechanical analysis of quantum wells together with a brief description of the properties of dielectric waveguides, A more detailed treatment of the electronic and optical properties of MQWS and a review of published experimental work is used to show that the fundamental absorption edge is much more abrupt than that in the corresponding bulk material with strong excitonic characteristics being evident even at room temperature. In addition, the absorption edge is seen to be anisotropic with the fundamental energy gap being larger for light polarised perpendicular to the MQW layers. This anisotropic absorption edge, together with the layered dielectric nature of MQWS, makes them birefringent with a smaller refractive index for light polarised perpendicular to the MQW layers. The quantum confinement of carriers in MQWS also enhances their electroabsorption and electro-optic properties through the quantum confined Stark effect. Standard techniques used in the design, fabrication and analysis of bulk semiconductor waveguides are developed for application to MQW waveguides. These include analytical and numerical techniques for the design of dielectric waveguides; dry etching and metallisation processes for the fabrication of devices; and a laser/optics system to analyse the waveguide devices. To verify these techniques they are first applied to the well-understood case of n/n+ GaAs waveguides and are used to successfully fabricate and analyse single-mode, passive, rib waveguides at l=1.15mum. The electro-optic coefficient is also measured in an active, planar n/n+ waveguide and found to be close to that reported by other workers. The design techniques are then applied to MOWS waveguides resulting in the design of a MQW double heterostructure (MQW-DH), p-i-n diode which was predicted to produce the required Quantum properties (strong, room temperature, excitonic behaviour), waveguide properties (single-mode propagation up to the fundamental absorption edge) and electronic properties (a high reverse bias breakdown voltage and uniform applied electric field). Most of the theoretical work and all the experimental work included is devoted to MQWS in the (Al,Ga)As, III-V semiconductor alloy system. Accordingly, the methods available for growing MQWS in this system are reviewed with Molecular Beam Epitaxy (MBE) being found the most likely method to satisfactorily reproduce the desired structure. MQW-DH were grown at two establishments and are initially studied by photoluminescence and scanning electron microscopy before their planar optical waveguide characteristics are checked using the laser system. Only one sample is found to satisfy all the design requirements, and then only partially. Detailed analysis of the properties of MQW waveguides is therefore limited to this structure. Passive MQW-DH waveguides are demonstrated to exhibit an anisotropic absorption edge as predicted, and it is shown that the design and fabrication techniques developed can be successfully used to obtain single, double and multi-mode strip loaded waveguides. Single-mode waveguides are also used to fabricate passive directional couplers with coupling lengths in good agreement with theoretically predicted values. A semi-empirical model is put forward to describe the band edge electro-absorption of MQWS. Although simple, the model is in qualitative and approximate quantitative agreement with published results. (Abstract shortened by ProQuest.)

Numerical analysis of light absorbing semiconducting devices beyond the conventional 3dB bandwidth

This thesis describes an investigation by computer simulation of the performance of two semiconductor device types at the heart of optical high speed data communications, namely the PIN photodiode and the electroabsorption modulator. Both device types operate by light absorption and are therefore likely to have similar factors that limit their performance at high speed. In order to have high speed detection, the PIN photodiode has been investigated through varying the materials, and used a photodiode structure to improve its bandwidth. If output signals beyond the 3dB frequency limit can be well detected by the photodiode, then significant improvements in the detection speed can be achieved. This possibility is a motivation of this thesis. In this study the InP /InGaAs/InP PIN photodiode is chosen because the light at 1.55 urn wavelength can be absorbed by InGaAs. At 1.55J.lm, the fibre is on low dispersion and low loss. A numerical model of a PIN photodiode has been written in C. Comprehensive modelling ofthe PIN photodiode requires a self-consistent solution of the Poisson's equation for calculating the electrostatic potential and the continuity equations for the electron and hole currents. The PIN photo diode model has included the therm ionic current over the hetero-junction, drift current and diffusion current that other models often ignore. After completing an extensive study of the large signal performance of PIN photodiodes at data rates much higher than the conventional 3dB bandwidth, the model was extended to investigate InP/InGaAsP/lnGaAs MQW -EAM under high speed applied bias pulses. The numerical modelling of the MQW-EAM requires a self-consistent solution of the Poisson's equation, the Schrcdinger 's equation and the current continuity equation. The Schrodinger 's equation is for the estimation of carrier concentration in the quantum wells. The MQW-EAM numerical model has applied a special technique for adding the carrier concentration in the quantum wells to the charge density. 11 Large performance has been successfully analysed on PIN photodiode to reveal that optical pulses at repetition frequencies are substantially higher than the conventional 3dB limit can detect (1 ps FWHM and up to 240Gb/s repetition rate) to give photocurrent pulses with an open eye diagram even in the presence of simulated noise, however, these output current pulses tend to spread and merge together sometimes. This tendency can be counteracted to a reasonable extent by using a suitable repetition time, and a large input average power. Similar Gaussian shaped applied bias pulses have also applied to the MQW-EAM, in order to generate fast Gaussian shaped light power, however; the output light pulses spread out nearly 3 times compared with its applied bias pulses under 5ps FWHM and 48Gb/s repetition rate. Thinner l-layer and less quantum wells in the MQW-EAM might be the solution. iiiEThOS - Electronic Theses Online ServiceGBUnited Kingdo

Growth, characterization and design of INP-based strained-layer multiple quantum wells for optical modulator devices

Experimental methods and background -- Sample cleaning -- Growth of strained-layer, InP-based thin film materials by MOVPE -- Structural characterization by x-ray diffraction -- Wet chemical etching of InP in HCI-based solutions -- Au-based contact contact metallizations on InP -- Measurement of field-dependent absorption curves by photocurrent detection -- Detailed balance efficiency limit in quantum well solar cells -- Strain and relaxation in multiple quantum well stacks -- Band alignment engineering for the quantum-confined stark effect -- Band alignment engineering for high-speed, low drive field quantum-confined stark effect devices

Quantum well intermixing in 1.55 um InGaAs/AlInGaAs and InGaAs/InGaAsP structures and applications

The work described in this thesis is aimed at the exploring the possibility of optically integrating multi-bandgap energy devices on appropriate semiconductor substrates, using the technology of quantum well intermixing. A novel quantum well intermixing technique, based on sputtering process induced disordering (SID), has been developed for the first time, addressing multi-bandgap active device integration. Using this technique, blue shifts have been precisely tuned from 0 nm to over 160 nm for the InGaAs/AlInGaAs and from 0 nm to 100 nm for the InGaAs/InGaAsP MQW systems. Assessment of post-process material characteristics has shown that good electrical and optical qualities were maintained in the bandgap widened regions of both the InGaAs/AlInGaAs and InGaAs/InGaAsP material systems. This novel technique has been used to create multi-wavelength light sources that are of potential application in WDM systems and 2x2 crosspoint optical integrated switches. The expected performance has been achieved. A reactive ion etching process, using CH4/H2 etching gas, has been investigated, particularly for effective etching in the InGaAs/AlInGaAs MQW system. A 'standard' process for the InGaAs/AlInGaAs material system has been developed, based on experimental research. Modelling and design of 3-dB MMI couplers have been carried out. An improved Ti/Si02 mask for reactive ion etching has been successfully employed to ensure the waveguide profiles of fabricated MMI couplers meet the design specification, especially regarding the lateral profiles of the MMI section. Characterisation has shown the waveguide profile is close to the design requirement (side wall angle is of 81±2°). The principle operation of the so-called terahertz optical asymmetric demultiplexer has been qualitatively described and the design of a Mach-Zehnder interferometer (MZI) type demultiplexer has been carried out. Three kinds of MZI demultiplexers with different geometric structures have been fabricated using SID technique. Assessment of the devices has been carried out, including the operation of semiconductor amplifier, propagation loss of the device, etc

Heterogeneous Integrated Photonic Transceiver on Silicon

The demand for high-speed and low-cost short-distance data links, eventually for chip-level optical communication, has led to large efforts to develop high density photonics integrated circuits (PICs) to decrease the power consumption and unit price. Particularly, silicon based photonic integration promise future high-speed and cost-effective optical interconnects to enable exascale performance computers and datacenters. High-level integration of all photonics components on chip, including high speed modulators and photodetectors, and especially lasers, is required for scalable and energy efficient system topology designs. This is enabled by silicon-based heterogeneous integration approach, which transfers different material systems to the silicon substrate with a complementary metal–oxide–semiconductor (CMOS) compatible process. In this thesis, our work focuses on the development of silicon photonic integrated circuit in the applications of high speed chip level optical interconnects. A full library of functional devices is demonstrated on silicon, including low threshold distributed feedback (DFB) lasers as a low power laser source; high extinction ratio and high speed electroabsorption modulators (EAM) and ultra-linear Mach-Zehnder interferometer (MZI) modulators for signal modulation in the data transmitter; high speed photodetectors for the data receiver; and low loss silicon components, such as arrayed waveguide grating (AWG) routers and broadband MZI based switches. The design and characterization of those devices are discussed in this thesis. A highly integrated photonic circuit can be achieved with co-design and co-process of all types of functional photonic devices. Selective die bonding method is performed to integrate multiple III-V dies with different band-gap onto a single photonic die. A reconfigurable network-on-chip circuit was proposed and demonstrated, with state-of-the-art high-speed silicon transceiver chip. With over 400 active and passive components heterogeneously integrated on silicon, photonic circuit with multiple- wavelength-division multiplexing (WDM) transceiver nodes achieved a total capacity up to 8×8×40 Gbps. This high capacity and dense integrated heterogenous circuit shows its potential as a solution for future ultra-high speed inter- and intra-chip interconnects