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No description available.

No description available.

Initiated by studying the characteristics of our 1^st generation opto-electronic packet switch (VIVALDI), we identified the impacting factors for the performance of a generic system under a burst-like traffic environment. Analyzing the complex relationship between system resource (i.e. connectivity and throughput) and performance (i.e. packet delays), a novel architecture based on a revolutionary principle of group switching was designed that produced significant reduction in the hardware required and the difficulties in operations, and yet still achieved an acceptable level of performance. The centre of the architecture lays a novel solid state switching device with ultra fast response characteristics – in the order of microseconds. The deployment of this new device further reduces the required level of system complexity – making the architecture a truly practical solution. An experimental demonstrator was designed and built to verify the feasibility of the new device in the application of optical switching. This thesis is structured in five sections. The first section (chapter 1 & 2) states the motivations and goals of our research work and looks at the currently available methods of optical switching. The second section (chapter 3) discusses the analysis conducted on the 1^st generation VIVALDI architecture. The results and analytical methods used form a foundation for our more advanced architecture. The third section (chapters 4-7) presents the origination of the group switching principle and the architectural design of the novel system. Simulations are used to assess the performance of the system. In the fourth section (chapter 8), we discuss the results of our experimental work from the optical core demonstrator. Finally, in section five (chapter 9), we conclude our findings from the project and provide guidelines to future research works.

Free space optical communications has a number of advantages for the transmission of high bandwidth data. However, for many applications, its use is limited by the precise alignment tolerances required to maintain a reliable link. In this dissertation, the design and construction of a free space adaptive optical interconnect demonstrator system is reported, in which the transmitted beam was actively steered to compensate for misalignment. The target application considered was a board to board interconnect for use within computer systems. Beam steering was performed using binary phase gratings displayed on a Spatial Light Modulator (SLM), for which an ‘off the shelf’ ferroelectric Liquid Crystal on Silicon (LCOS) micro display was used here. The gratings were generated in hardware, as an integrated part of the SLM driver, using a novel and compact implementation for which the details are described. This was capable of generating gratings at high frame rates, and applied a scrolled addressing scheme to ensure DC balance of the pixels whilst maintaining an uninterrupted optical path. Data transmission through a bulk optical relay containing this SLM was successfully demonstrated at 2.5Gbps. The transmitter and receiver modules were custom built for these experiments using an 850nm multi mode VCSEL and PIN photodiode, driven by standard telecommunications components. Losses due to the optical components, SLM, grating diffraction efficiency and scrolled addressing scheme totalled between 15.1 and 17dB. These corresponded well with the values estimated for the components, and with further device optimisation it was expected that they could be substantially reduced. Beam steering at the receiver plane was achieved over a 6.4x6.4mm range with a resolution of 25μm. This was sufficient to track a detector with a coupling loss of no more than 0.05dB. The feasibility of adaptive alignment correction in a free space optical interconnect, using a liquid crystal SLM for beam steering, was demonstrated through a series of experiments. The considerations relevant to extending this system for multiple parallel channels are discussed.

In this work, an ultrashort optical pulse source, generated through gain-switching of a semiconductor distributed feedback (DFB) laser followed by nonlinear fibre compression scheme, is experimentally demonstrated. Incorporating this with a novel fibre loop mirror, known as the dispersion-imbalanced loop mirror (DILM), simultaneous pulse compression and pedestal suppression are achieved. This generates ultrashort pulses with 350fs full-width half maximum (FWHM) and high quality extinction ratio of 30dB (instrument limited). With such short pulses, timing jitter becomes critical in avoiding signal-to-noise ratio (SNR) degradation. By employing a novel self-seeding scheme with the gain-switched laser, an instrument-limited jitter of 150fs is achieved. This makes the experimentally demonstrated femtosecond pulse source as an ideal candidate for high speed OTDM applications. The performance of the DILM as a noise suppressor is also studied. Interferometric noise, also known as incoherent crosstalk, has been a limiting factor in high-speed multichannel optical communication systems. With the use of the DILM, interferometric noise suppression is achieved with a power penalty improvement of more than 9dB. This is observed to be consistent throughout a wide range of wavelength from 1531-1560nm. This shows that using the DILM as a noise suppressor out-performes any electronic techniques. The combined DILM and self-seeding scheme provides a high quality pulse source which successfully meets the requirements of high capacity data systems. Having achieved this goal, further investigations of next generation sources, such as solid sate lasers for ultrahigh performance system applications are undertaken. A solid state mode-locked laser (CR⁴⁺:YAG), which has a 3dB spectral width of 40nm, is demonstrated for spectral slicing using an AWG. A total capacity of 1.36Tb/s (40Gb/s x 32 channels) is achieved. This is the highest capacity ever reported using a single pulse source in the C-band region. The output Q-factor of 8 to 13 demonstrates the feasibility of such innovative approach.

In this thesis, spectral effects of QD lasers including injection locking of QD lasers and their linewidth enhancement factor measurement are first studied. Experimental studies show that the QD laser performance can be greatly improved by injection locking. Linewidth enhancement factor measurement reveals that chirp of QD lasers under moderate bias is inherently smaller than typical QW lasers. Large alpha factor values up to 6.3 induced by the excited-state carrier effect are observed under high bias current operation. In addition, a novel optical injection gain-tuning method is invented for easy implementation of linewidth enhancement factor measurement. In addition, applications of QD lasers for multi-mode fibre (MMF) transmission are investigated. Beam profiling is employed to achieve spot size reduction and single transverse mode operation of a 980 nm QD vertical-cavity surface-emitting laser (VCSEL). Furthermore, error-free operation of 10 Gb/s data transmission over 300 m of MMF is demonstrated using the post-processed QD VCSEL. Finally, applications of QD Febry-Perot (FP) lasers and modulators for single mode fibre (SMF) transmission are demonstrated. A directly modulated QD laser at 10 Gb/s is applied for both SMF and MMF data transmission. Error-free transmission is achieved over 4 km of SMF and 300 m of MMF. The potential application of 2.5 Gb/s QD electro-absorption modulator (EAM) is investigated. Substantial quantum-confined Stark shifts (QCSE) are observed in QD material subject to reverse bias. Based on the QCSE observed, a high speed QD EAM is mounted and demonstrated for 2.5 Gb/s error-free back-to-back modulation. This is the first time that the QD material has been shown to be applicable for semiconductor EAMs, and QD EAMs have the potential advantages of low insertion loss, high saturation power and short response time.

Radio over fibre (RoF) has long been proposed as a suitable alternative technology for distributed antenna systems (DAS). In this thesis the concept of optical DAS is extended and improved to the idea of an optical <i>distributed antenna network (DAN) </i>to meet future requirements. Active RoF link solutions are also demonstrated to improve link performance and simplify design. It is shown that modern communication signals can be multicast simultaneously from a number of low power antennas in a DAN to expand the area of coverage without causing “black-spots”. The required output power at each antenna is reduced by 15 dB by using 3 distributed antennas instead of 1. As a result of this finding, it becomes desirable to use many more antenna units in the DAN, and the cost of installation must be considered. Current systems generally use separate antennas for the uplink and downlink to provide isolation. A new active solution to provide isolation requiring only a single antenna when time division duplexed signals are used, based on switching of the downlink laser bias current, is proposed and demonstrated. It is shown to have no negative impact on the throughput of IEEE 802.11g signals while increasing the uplink dynamic range by 10 dB. To meet future inbuilding wireless service requirements, DAN will have to support sensing functions as well as communications. It is shown that RFID signals can be carried on RoF without effecting the RFID performance. A low cost electronic predistortion scheme is demonstrated to allow simultaneous delivery of IEEE 802.11g with an EVM of 3.1% at +15 dBm and RFID signals at +28 dBm over a single RoF link while meeting spurious emissions limits. More advanced network functions such as the ability to dynamically route RF signals will become a key requirement in more complex DANs. Previously semiconductor optical amplifiers (SOAs) have been demonstrated as switching elements in small RoF switches using a single SOA in each path, limiting the number of ports which could be supported. A new switch architecture is investigated using cascaded SOAs. A testbed is demonstrated using 3 cascaded SOAs to simulate part of a 32x32 switch. It is shown that the switch can carry IEEE 802.11g signals with an output EVM of 4% indicating that two such switches could be cascaded achieving a 1024x1024 switch while maintaining an EVM below 5.6 %.

In order to facilitate the increasing traffic demands of optical communications networks high-speed transmitters will be required in future systems. This thesis focuses on routes to achieving 40Gb/s data rates using advanced modulation formats and direct modulation of Distributed Feedback (DFB) semiconductor laser diodes. In particular four-level signalling is employed as a way of achieving 40Gb/s data rates from existing state-of-the-art DFB lasers which are typically limited to speeds of around 20GHz. The potential of using four-level signalling to extend the reach of future systems is demonstrated for the first time by 40Gb/s uncooled signal generation. Transmission tests confirm the feasibility of the technique used and spectral efficiency measurements further show the benefit that can be gained by using advanced modulation formats. A novel technique is presented and demonstrated experimentally for the first time. The new technique, which exploits multi-level signalling of multi-section lasers to achieve high-data rates, can also alleviate some of the complicated electronics usually required by multi-level signalling systems. In this work a twin-contact DFB laser is employed to demonstrate the principle of four-level signal generation by differential amplitude modulation. Results are presented which show the feasibility of the technique and observations are made on the limitations of the device used in experimental demonstration. This leads to the use of a time-resolved travelling wave laser model. The model is adapted to support simulation of multi-section devices with spatially varying injection currents. Extensive simulation is used to suggest an optimised device structure for the generation of four-level signals. A three-contact device geometry is suggested and further work is presented on fabricating such a structure. Focussed-ion-beam-etching of a ridge-waveguide index coupled DFB is carried out to fabricate, for the first time, the optimised device structure. Results are presented which confirm the benefits that can be gained by employing the novel technique of differential amplitude modulation of multi-contact devices for high-speed multi-level signal generation.

From the literature review of promising networking architectures, a MPLS-enabled networking architecture was selected for SAN applications over an optical network and the SAN data transmission protocol over such network architecture was simulated through a simulation platform constructed by Network Simulator (NS-2) software. The strategies in designing a MPLS-enabled router prototype were specified from the case study of currently commercial available SAN switches/routers. The architecture design of a MPLS-enabled router prototype was emulated by using OPNET Modeler. The network-wide simulation results from NS-2 showed that the utilization of iSCSI transmission protocol with appropriate setting of TCP parameters such as window size can increase the network throughput and the link bandwidth efficiency for SAN applications. A realistic system-level performance simulation of the proposed router prototype by OPNET Modeler software indicated that the labelled packet size of 1500 byte length under the proposed switching architecture with the SLM reconfiguration time of 1us is suitable to minimise the buffer size usage and maintain an acceptable switching latency less than 20ms no matter when the SANs traffic is intensive or not. The optical transmission performance of the implemented 2x2 shutter-based free-space multiwavelength optical switch prototype evaluated by CWDM transceiver based testbed has proved successfully that a link budget between -26.72dB and -28.88 dB can be achieved with a satisfying BER of less than 10^-12 in a single mode fibre to multimode fibre transmission architecture. From the analysis of optical components, the optical system link budget can be improved up to 4.3dB. The system crosstalk of the optical switching prototype was calculated to be between -21.6dB and -24.4dB.