Generation and optimization of picosecond optical pulses for use in hybrid WDM/OTDM networks

The burgeoning demand for broadband services such as database queries, home shopping, video-on-demand, remote education, telemedicine and videoconferencing will push the existing networks to their limits. This demand was mainly fueled by the brisk proliferation of Personal Computers (PC) together with the exceptional increases in their storage capacity and processing capabilities and the widespread availability of the internet. Hence the necessity, to develop high-speed optical technologies in order to construct large capacity networks, arises. Two of the most popular multiplexing techniques available in the optical domain that are used in the building of such high capacity networks, are Wavelength Division Multiplexing (WDM) and Optical Time Division Multiplexing (OTDM). However merging these two techniques to form very high-speed hybrid WDM/OTDM networks brings about the merits of both multiplexing technologies. This thesis examines the development of one of the key components (picosecond optical pulses) associated to such high-speed systems. Recent analysis has shown that RZ format is superior to conventional NRZ systems as it is easier to compensate for dispersion and nonlinear effects in the fibre by employing soliton-like propagation. In addition to this development, the use of wavelength tunability for dynamic provisioning is another area that is actively researched on. Self-seeding of a gain switched Fabry Perot laser is shown to one of the simplest and cost effective methods of generating, transform limited optical pulses that are wavelength tunable over very wide ranges. One of the vital characteristics of the above mentioned pulse sources, is their Side Mode Suppression Ratio (SMSR). This thesis examines in detail how the pulse SMSR affects the performance of high-speed WDM/OTDM systems that employ self-seeded gain-switched pulse sources

Multiwavelength Modelocked Semiconductor Diode Laser

Single-stripe GaAs/AlGaAs semiconductor optical amplifiers which simultaneously generates from four to more than twenty tunable WDM channels. A four channel version transmits approximately 12 picosecond pulses at approximately 2.5 GHz for an aggregate pulse rate of 100 GHz. Wavelength tuning over 18 nm has been demonstrated with channel spacing ranging from approximately 0.8 nm to approximately 2 nm. A second version uses approximately 20 wavelength channels, each transmitting approximately 12 picosecond pulses at a rate of approximately 600 MHz. A spectral correlation across the multiwavelength spectrum which can be for utilizing single stripe laser diodes as multiwavelength sources in WDM-TDM networks. A third version of multiple wavelength generation uses a fiber-array and grating. And a fourth version of wavelength generation uses a Fabry-Perot Spectral filter. Also solid state laser sources and optical fiber laser sources can be used

Multichannel grating cavity laser for optically multiplexed communication systems

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN008419 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Investigation of in-situ parameter control in novel semiconductor optical amplifiers

Fibre optic networks form the backbone of modern communications systems. As demand for ever increasing bandwidth continues to grow, technologies that enable the expansion of optical networks will be the key to future development. The semiconductor optical amplifier (SOA) is a technology that may be crucial in future optical networks, as a low cost in-line amplifier or as a functional element. As fibre networks extend closer to the end user, economical ways of improving the reach of these networks are important. SOAs are small, relatively inexpensive and can be readily integrated in photonic circuits. Problems persist with the development of SOAs, however, in the form of a relatively high noise figure and low saturation output power, which limits their use in many circumstances. The aim of this thesis is to outline a concept for control of these parameters such that the SOA can achieve the performance required. The concept relies on the control of the carrier density distribution in the SOA. The basic characteristics of the SOA and how they are affected by changes in the carrier density are studied. The performance of the SOA in linear and high power transmission of CW and pulsed signals is determined. Finally, the wavelength conversion characteristics of the SOA are outlined. The role of the carrier density control in shaping all of these characteristics will be explained

Optical pulse processing towards Tb/s high-speed photonic systems

Due to the continued growth of high-bandwidth services provided by the internet, there is a requirement to operate individual line rates in excess of 100 Gb/s in next generation optical communications systems. Thus, to implement these high-speed optical networks all-optical processing techniques are necessary for pulse shaping and pulse routing. Two sub-systems (pulse generation and wavelength conversion), which exploit optical processing techniques are explored within this thesis. Future systems will require high-quality pulse sources and this thesis develops the pulse generation technique of gain switching to provide simple and cost efficient pulse sources. The poor pulse quality typically associated with gain switching is enhanced by developing all-optical methods. The main attribute of the first pulse generation scheme presented is its wavelength tunability over 50 nm. The novelty of the second scheme lies in the ability to design a grating which has a nonlinear chirp profile exactly opposite to the gain-switched pulses. This grating used in conjunction with the gain-switched laser generates transform limited pulses suitable for 80 Gb/s systems. Furthermore the use of a vertical microcavity-based saturable absorber to suppress detrimental temporal pulse pedestals of a pulse source is investigated. Next generation networks will require routing of data in the optical domain, which can be accomplished by high-speed all-optical wavelength converters. A semiconductor optical amplifier (SOA) is an ideal device to carry out wavelength conversion. In this thesis pulses following propagation through an SOA are experimentally characterised to examine the temporal and spectral dynamics due to the nonlinear response of the SOA. High-speed wavelength conversion is presented using SOA-based shifted filtering. For the first time 80 Gb/s error-free performance was obtained using cross phase modulation in conjunction with blue spectral shifted filtering. In addition an important attribute of this work experimentally examines the temporal profile and phase of the SOA-based shifted filtering wavelength converted signals. Thus the contribution and effect of ultrafast carrier dynamics associated with SOAs is presented

Multiwavelength Modelocked Semiconductor Diode Laser

Single-stripe GaAs/AlGaAs semiconductor optical amplifiers which simultaneously generates from four to more than twenty tunable WDM channels. A four channel version trsnsmits approximately 12 picosecond pulses at approximately 2.5 GHz for an aggregate pulse rate of 100 GHz. Wavelength tuning over 18 nm has been demonstrated with channel spacing ranging from approximately 0.8 nm to approximately 2 nm. A second version uses approximately 20 wavelength channels, each transmitting approximately 12 picosecond pulses at a rate of approximately 600 MHz. A spectral correlation across the multiwavelength spectrum which can be for utilizing single stripe laser diodes as multiwavelength sources in WDM-TDM networks. A third version of multiple wavelength generation uses a fiber-array and grating. And a fourth version of wavelength generation uses a Fabry-Perot Spectral filter. Also solid state laser sources and optical fiber laser sources can be used

Monolithic Colliding Pulse Mode-Locked Quantum Well Lasers

The design, fabrication and characterisation of monolithic passive colliding pulse mode-locked (CPM) quantum well lasers are described. Firstly, a standard configuration of CPM laser is realised and mode-locking is obtained at repetition rates in the range of 73 to 129 GHz. These are the first published results on monolithic CPM quantum well laser at short wavelength, i.e. GaAs/AlGaAs based material. This device presented a previously unseen output laser polarisation dependence with the applied reverse bias to the absorber section of this laser. The generation of TE and TM polarised light from the CPM laser is analysed and discussed. A novel configuration of CPM laser, the multiple colliding-pulse mode-locked (MCPM) laser is realised. This multi-section device can have 1, 2, or 3 monolithically integrated saturable absorber sections in the cavity, inducing the laser to operate at the first up to the fourth harmonic of the repetition rate. This is the first observation of geometry dependent switcheable change of harmonics in mode-locked lasers. The different regimes of operation of the MCPM laser are investigated, comprising single mode, multimode, Q-switched and first to fourth harmonic mode-locking, generating pulses of around 1 to 3 ps width at up to 375 GHz repetition rate. Studies of the range of mode-locking at 240 GHz show that mode-locking occurs at two distinct regions of current and reverse bias. This previously unseen feature may represent an indication of the contribution of excitonic nonlinearities to the ultra-fast operation of the device. A monolithic CPM ring laser with two saturable absorbers in the cavity is described. Frequency domain measurements indicate mode-locking operation at 28 GHz repetition rate. The use of two saturable absorbers in colliding pulse configuration in the ring cavity showed improvements on the device operation. The design and fabrication of a CPM laser based device which is intended to perform clock recovery at high repetition rates, above 100 GHz, is described. This device consists of a standard CPM laser which has an extra waveguide in side-injection configuration. This extra waveguide amplifies and guides the optical signal from which the clock is to be recovered to the absorber section of the laser. To achieve clock recovery this signal should contribute to the saturation of the absorber section of the CPM laser, synchronising its saturation with the injected data signal. Preliminary characterisation and tests showed that the CPM laser part of the device works as a standard CPM laser, as expected

A regeneratively modelocked, fiber ring laser

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1997.Includes bibliographical references (leaves 65-70).by Matthew Edward Grein.M.S