In this thesis a Fully-Photonic DRoF (FP-DRoF) system is proposed for deploying of future super-broadband access networks. Digital Radio over Fibre (DRoF) is more independent of the fibre network impairments and the length of fibre than the ARoF link. In order for fully optical deployment of the signal conversion techniques in the FP-DRoF architecture, two key components an Analogue-to-Digital Converter (ADC) and a Digital-to-Analogue Converter (DAC)) for data conversion are designed and their performance are investigated whereas the physical functionality is evaluated. The system simulation results of the proposed pipelined Photonic ADC (PADC) show that the PADC has 10 GHz bandwidth around 60 GHz of sampling rate. Furthermore, by changing the bandwidth of the optical bandpass filter, switching to another band of sampling frequency provides optimised performance condition of the PADC. The PADC has low changes on the Effective Number of Bit (ENOB) response versus analogue RF input from 1 GHz up to 22 GHz for 60 GHz sampling frequency. The proposed 8-Bit pipelined PADC performance in terms of ENOB is evaluated at 60 Gigasample/s which is about 4.1. Recently, different methods have been reported by researchers to implement Photonic DACs (PDACs), but their aim was to convert digital electrical signals to the corresponding analogue signal by assisting the optical techniques. In this thesis, a Binary Weighted PDAC (BW-PDAC) is proposed. In this BW-PDAC, optical digital signals are fully optically converted to an analogue signal. The spurious free dynamic range at the output of the PDAC in a back-to-back deployment of the PADC and the PDAC was 26.6 dBc. For further improvement in the system performance, a 3R (Retiming, Reshaping and Reamplifying) regeneration system is proposed in this thesis. Simulation results show that for an ultrashort RZ pulse with a 5% duty cycle at 65 Gbit/s using the proposed 3R regeneration system on a link reduces rms timing jitter by 90% while the regenerated pulse eye opening height is improved by 65%. Finally, in this thesis the proposed FP-DRoF functionality is evaluated whereas its performance is investigated through a dedicated and shared fibre links. The simulation results show (in the case of low level signal to noise ratio, in comparison with ARoF through a dedicated fibre link) that the FP-DRoF has better BER performance than the ARoF in the order of 10-20. Furthermore, in order to realize a BER about 10-25 for the ARoF, the power penalty is about 4 dBm higher than the FP-DRoF link. The simulation results demonstrate that by considering 0.2 dB/km attenuation of a standard single mode fibre, the dedicated fibre length for the FP-DRoF link can be increased to about 20 km more than the ARoF link. Moreover, for performance assessment of the proposed FP-DRoF in a shared fibre link, the BER of the FP-DRoF link is about 10-10 magnitude less than the ARoF link for -19 dBm launched power into the fibre and the power penalty of the ARoF system is 10 dBm more than the FP-DRoF link. It is significant to increase the fibre link’s length of the FP-DRoF access network using common infrastructure. In addition, the simulation results are demonstrated that the FP-DRoF with non-uniform Wavelength Division Multiplexing (WDM) is more robust against four wave mixing impairment than the conventional WDM technique with uniform wavelength allocation and has better performance in terms of BER. It is clearly verified that the lunched power penalty at CS for DRoF link with uniform WDM techniques is about 2 dB higher than non-uniform WDM technique. Furthermore, uniform WDM method requires more bandwidth than non-uniform scheme which depends on the total number of channels and channels spacing.

published_or_final_version / abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

We optimize secret key rates (SKRs) of weak coherent pulse (WCP)-based quantum key distribution (QKD) over time-varying free-space optical channels affected by atmospheric turbulence. The random irradiance fluctuation due to scintillation degrades the SKR performance of WCP-based QKD, and to improve the SKR performance, we propose an adaptive scheme in which transmit power is changed in accordance with the channel state information. We first optimize BB84 and decoy state-based QKD protocols for different channel transmittances. We then present our adaptation method, to overcome scintillation effects, of changing the source intensity based on channel state predictions from a linear autoregressive model while ensuring the security against the eavesdropper. By simulation, we demonstrate that by making the source adaptive to the time-varying channel conditions, SKRs of WCP-based QKD can be improved up to over 20%.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title from title frame. Also available in printed format.

The Nyquist WDM system realizes a terabit high spectral efficiency transmission system by allocating several subcarriers close to or equal to the baud rate. This system achieves optimal performance by maintaining both temporal and spectral orthogonality. However, ISI and ICI effects are inevitable in practical Nyquist WDM implementations due to the imperfect channel response and tight channel spacing and may cause significant performance degradations. Our primary research goals are to combat the ISI effects via the transmitter digital pre-shaping and to remove the ICI impairments at the receiver using MIMO signal processing. First we propose two novel blind channel estimation techniques that enable the transmitter pre-shaping design for the ISI effects mitigation. Both numerical and experimental results demonstrate that the two methods are very effective in compensating the narrow band filtering and are very robust to channel estimation noise. Besides pre-shaping, the DAC-enabled transmitter chromatic dispersion compensation is also demonstrated in a system with high LO laser linewidth. Next a novel “super-receiver” structure is proposed, where different subchannels are synchronously sampled, and the baseband signals from three adjacent subchannels are processed jointly to remove ICI penalty. Three different ICI compensation methods are introduced and their performances compared. The important pre-processes that enable a successful ICI compensation are also elaborated. Despite ICI compensation, the joint carrier phase recovery based on the Viterbi-Viterbi algorithm is also studied in the carrier phase locked systems. In-band crosstalk arises from the imperfect switch elements in the add-drop process of ROADM-enabled DWDM systems and may cause significant performance degradation. Our third research topic is to demonstrate a systematic way to analyze and predict the in-band crosstalk-induced penalty. In this work, we propose a novel crosstalk-to-ASE noise weighting factor that can be combined with the weighted crosstalk weighting metric to incorporate the in-band crosstalk noise into the Gaussian noise model for performance prediction and analysis. With the aid of the Gaussian noise model, the in-band crosstalk-induced nonlinear noise is also studied. Both simulations and experiments are used to validate the proposed methods.

The Internet, with cloud computing, offers amazing services that require a fast, intelligent, reliable network connection. Current networks, electrical or optical, need to work together to provide the user with a high-quality connection. The IP protocol as Layer 3 and an optical network as Layer 2 need to talk to each other and help each other instead of working separately. Therefore, this thesis proposes using software-defined network (SDN) technology for integrating the IP protocol into an optical network to fill the gap between the two layers and to give the network more intelligence and flexibility for new connection requests, choosing the best route, and monitoring the network. A two-layer SDN centralized controller design has been used. The Layer 1 SDN controller is the centralized controller that connects and updates all Layer 2 SDN controllers which control traffic in each domain. New connection requests are processed in the SDN controller and the traffic is forwarded by the optical network. SDN technology and the integration of IP into the optical network promise to enhance network connectivity.

Underwater communication systems are in high demanded for subaquatic environment activities as the sea is an enormous and mostly unexplored place. The ten-meter long and few giga-bit per second range optical communication technique is feasible and has bright future compared to the mature but low data rate (few kilobits per second) acoustic technology and short distance (several meters) radio-frequency signaling schemes. The underwater wireless optical communication (UWOC) technique takes advantage of wide bandwidth, low attenuation effect in the visible range for multiple applications such as seafloor and offshore exploration, oil pipe control and maintenance, and pipeline leak detection. Nowadays, visible light-emitting diode (LED)-based and laser diode (LD)-based UWOC system are attractive and much related research is being conducted in the field. However, the major challenges of developing UWOC systems are the attenuation, scattering and turbulence effects of the underwater environment. The temperature gradient, salinity gradient, and bubbles make underwater optical channel predictable challenging and degrade the optical beam propagating distance and quality. Most studies focus on the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations as well as the presence of air bubbles. In this thesis, we experimentally investigate the reciprocity nature of underwater turbulence caused by the turbidity, air bubbles, temperature variations, and salinity. Bit error rate measurement and statistical data analysis reveal the high reciprocal nature of turbulence that can be induced by the presence of bubbles, temperature, and salinity. The mitigation strategies for the different turbulence scenarios are discussed.

The Internet of Underwater Things (IoUT) paradigm is expected to enable various practical applications such as environmental monitoring, underwater exploration, and disaster prevention. Supporting the concept of IoUT requires robust underwater wireless communication infrastructure. Optical wireless communication has the superiority of wide bandwidth, low latency, and high data capacity over its counterparts, namely, acoustic and radio-frequency. However, the transmission of the optical beam has inherent drawbacks in a harsh environment. Obstructions such as geometrical underwater terrains and underwater turbulence can pose a serious challenge to the alignment of the transmitter and the receiver. Non-line-of-sight (NLOS) configuration is a generalized alignment scheme between the transmitter and the receiver such that the strict requirement of precise alignment (point-to-point) is no longer needed. In this dissertation, the effectiveness of NLOS to withstand challenging underwater turbulence is examined. Thermal gradients with a maximum temperature difference of 10 ◦C had a negligible effect on the received power. The presence of air bubble clouds caused an increase of 38% of the received power when the bubble area increased from 5.2 to 80 mm$^2$. Additionally, various salinity concentrations ranging between 30-40‰ are emulated. A gain of 32.5% in the signal-to-noise ratio is observed when the salinity gradient increased from 0.08 to 0.4‰·cm$^{−1}$. Moreover, a reduction of 2.35 dB/m of the pathloss is noticed. The bit-error ratio is used to examine the communication quality using on-off-keying modulation scheme. In addition, this dissertation shows a practical wavelength-division multiplexing method based on large-area detection and wide field-of-view (FoV) photonic receiver. The dual-antenna is made of scintillating fibers with distinctive characteristics. An aggregated data rate of 1 Gbps is achieved. Two methods of wavelengths separation are demonstrated. Additionally a field deployment verification in an outdoor water pool is conducted at a maximum separation distance of 10m. The presented promising results pave the way for a robust underwater wireless optical sensor network that serves as a building block for achieving the goal of establishing IoUT.

Oceans cover 71% Oceans cover 71% surface of the earth and are rich in oil and gas resources, marine living resources, renewable energy, mineral resources. The depths of the oceans are often thought of as a silent world, but that was never the case, and oceans have become noisier as human technologies have advanced. Humans have not only added noise to the ocean; they have also eliminated natural sounds. One of the primary noise sources is sonar. Sonar technology is widely used in fish detection, ocean floor mapping, and vehicle navigation. The noise in the ocean dramatically affects the animal’s survival and breaks marine ecological balance. Herein, underwater wireless optical communication (UWOC) and fiber communication and sensing (FC&S) technologies are proposed to minimize the acoustic noise in the ocean. Compared to noisy, powerful acoustic communication technology, UWOC has the merit of silence and takes advantage of high bandwidth, high transmission speed, and power efficiency. Multi-functions FC&S system turns the submarine telecommunication cable network into sensor network. UWOC and underwater FC&S technology can boost the development of Underwater Internet of things (UIoT) by establish large-scale underwater sensor networks. This dissertation aims to investigate and address noisy ocean issues and build large-scale underwater sensor networks by optical communication and sensing technology. The dissertation proposes using UWOC and FC&S technology to replace the conventional acoustic communication technology and reduce the noise in the ocean. UWOC helps achieve high-speed wireless communications between sensors, vehicles, and even humans for UIoT. The significant challenges of developing UWOC systems are the complex underwater environment's attenuation, scattering, and turbulence effects. This dissertation studied the turbulence effects on the UWOC system’s performance and addressed the pointing-acquisition-and-tracking issues. The diffuse-line-of-sight configuration and scintillating-fiber-based detector help the mobile UWOC systems relieve the strict requirements on PAT. FC&S technology is proposed to build underwater communication and sensor networks. Studies pave the way for UIoT and keep ocean silent. Such modality is much sought-after for implementing robust UWOC links in a complex oceanic environment, building large-scale sensor networks across the oceans, and minimizing noise pollution in the ocean.

With the rapid development of fiber access networks, data centers, 5th generation cellular networks, and many more, there is an increasing demand for cost effective light sources possessing specification including high frequency modulation efficiency, low noise figure, and high data rate up to 40 Gb/s or even 100 Gb/s. Semiconductor lasers are considered the most attractive candidate in such applications for their low cost, high energy efficiency, and compact size. The focus of this thesis is the development of novel designs of semiconductor DFB lasers for device performance improvement with the help of numerical simulation tools. The governing equations used in the simulation of DFB lasers are briefly explained, which covers the calculation of optical field, carrier transport, material gain, and thermal diffusion. The TWM based on these governing equations are adopted for the numerical laser solver used in the following chapters for device performance simulation. Three novel DFB structures are then proposed in the thesis to achieve different specifications. The first proposed structure is a three-electrode DFB laser which can be directly frequency modulated. Numerical simulation shows that a high frequency modulation efficiency of 26GHz/mA from 0 to 100GHz and 17GHz/mA from 100GHZ to 200GHz can be achieved, respectively. Large-signal simulation of the waveform and eye-diagram of a frequency shift-keying (FSK) signal generated by the laser is also performed by converting it to an amplitude shift-keying (ASK) signal through an optical slope filter. The second proposed structure is a DFB laser with asymmetric λ/8 phase-shifted grating designed to flatten the relaxation oscillation peak through longitudinal spatial hole burning (LSHB) effect. Optimization of the phase-shift position to be 25% (in terms of the total length of the cavity) away from the high reflective (HR)-coated facet leads to reduced power leakage thus a higher quality factor of the cavity. The combined effect provides an improved RIN figure for the proposed DFB laser. The third proposed structure is a DFB laser with periodic current blocking grating. This novel grating is designed to improve the modulation bandwidth of DFB lasers by exploiting the enhancement of net differential gain. The effectiveness of the design is verified numerically, and excellent 3dB bandwidth enhancement are found for both uniform grating and λ/4 phase-shifted grating structures. / Thesis / Doctor of Philosophy (PhD) / Semiconductor lasers are by far the most ubiquitous of all lasers, with their applications ranging from communication to manufacturing and from cooling of atoms to sensing of minor movement. And as the fabrication technique of semiconductor laser mature, numerical simulation tools now play the critical role in laser development. This thesis focuses on the design and simulation of novel structures for distributed-feedback (DFB) lasers to improve the performance of such devices, including the frequency tuning efficiency, relative intensity noise (RIN), and modulation bandwidth. The proposed new structures and the underlying ideas led to them are thoroughly explained in the thesis. The device performances are also investigated numerically by applying traveling wave method (TWM). Simulation results are presented and discussed to provide design guidelines for the proposed structures.