Although mathematical cryptography has been widely used, its security has only been proven under certain assumptions such as the computational power of opponents. As an alternative, quantum communication, in particular quantum key distribution (QKD) can get around unproven assumptions and achieve unconditional security. However, the key generation rate of practical QKD systems is limited by device imperfections, excess noise from the quantum channel, limited rate of true random-number generation, quantum entanglement preparation, and/or post-processing efficiency. This dissertation contributes to improving the performance of quantum communication systems. First, it proposes a new continuous-variable QKD (CVQKD) protocol that loosens the efficiency requirement on post-processing, a bottleneck for long-distance CVQKD systems. It also demonstrates an experimental implementation of the proposed protocol. To achieve high rates, the CVQKD experiment uses a continuous-wave local oscillator (CWLO). The excess noise caused by guided acoustic-wave Brillioun scattering (GAWBS) is avoided by a frequency-shift scheme, resulting in a 32 dB noise reduction. The statistical distribution of the GAWBS noise is characterized by quantum tomography. Measurements show Gaussian statistics upto 55 dB of dynamical range, which validates the security calculations in the proposed CVQKD protocol. True random numbers are required in quantum and classical cryptography. A second contribution of this thesis is that it experimentally demonstrates an ultrafast quantum random-number generator (QRNG) based on amplified spontaneous emission (ASE). Random numbers are produced by a multi-mode photon counting measurement on ASE light. The performance of the QRNG is analyzed with quantum information theory and verified with NIST standard random-number test. The QRNG experiment demonstrates a random-number generation rate at 20 Gbits/s. Theoretical studies show fundamental limits for such QRNGs. Quantum entanglement produced in nonlinear optical processes can help to increase quantum communication distance. A third contribution is the research on nonlinear optics of graphene, a novel 2D material with unconventional physical properties. Based on a quantum-dynamical model, optical responses of graphene are derived, showing for the first time a link between the complex linear optical conductivity and the quantum decoherence. Nonlinear optical responses, in particular four-wave mixing, is studied for the first time. The theory predicts saturation effects in graphene and relates the saturation threshold to the ultrafast quantum decoherence and carrier relaxation in graphene. For the experimental part, four-wave mixing in graphene is demonstrated. Twin-photon production in graphene is under investigation.

Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by Mark G. Allen. / Vita. Includes bibliographical references (leaves 155-166).

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 77-79). Also available in electronic version. Access restricted to campus users.

Thesis (M.S. in Computer Science)--Naval Postgraduate School, December 2003. / Thesis advisor(s): Bert Lundy, Su Wen. Includes bibliographical references (p. 87-90). Also available online.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Not available / text

The constantly increasing volume of information in modern society demands a better understanding of the physics and modeling of optical phenomena, and in particular, optical waveguides which are the central component of information systems. Two ways of advancing this physics are to push current technologies into new regimes of operation, and to study novel materials which offer superior properties for practical applications. This dissertation considers two problems, each addressing the above-mentioned demands. The first relates to the influence of high-order nonlinear effects on pulse collisions in existing high-speed communication systems. The second part is a study of pulse dynamics in a novel nanocomposite medium which offers great potential for both optical waveguide physics and applications. The nanocomposite consists of metallic nanoparticles embedded in a host medium. Under resonance conditions, the optical field excites plasmonic oscillations in the nanoparticles, which induce a strong nonlinear response.Analytical and computational tools are used to study these problems. In the first case, a double perturbation method, in which the small parameters are the reciprocal of the relative frequency of the colliding solitons and the coefficient of quintic nonlinearity, reveals that the leading order effects on collisions are radiation emission and phase shift of the colliding solitons. The analytical results are shown to agree with numerics. For the case of pulse dynamics in nanocomposite waveguides, the resonant interaction of the optical field and material excitation is studied in a slowly-varying envelope approximation, resulting in a system of partial differential equations. A family of solitary wave solutions representing the phenomenon of self-induced transparency are derived. Stability analysis reveals the solitary waves are conditionally stable, depending on the sign of the perturbation parameter. A characterization of two-pulse interaction indicates high sensitivity to relative phase, and collision dynamics vary from highly elastic to the extreme case where one wave is immediately destroyed by the collision, depositing its energy into a localized hotspot of material excitation. This last scenario represents a novel mechanism for "stopping light".

The ever-increasing data traffic demand drives the evolution of telecommunication networks, including the last-mile access networks as well as the long-haul backbone networks. This Ph.D. dissertation focuses on system design and signal processing techniques for next-generation converged optical-wireless access systems and the high-speed long-haul coherent optical communication systems. The convergence of high-speed millimeter-wave wireless communications and high-capacity fiber-optic backhaul networks provides tremendous potential to meet the capacity requirements of future access networks. In this work, a cloud-radio-over-fiber access architecture is proposed. The proposed architecture enables a large-scale small-cell system to be deployed in a cost-effective, power-efficient, and flexible way. Based on the proposed architecture, a multi-service reconfigurable small-cell backhaul network is developed and demonstrated experimentally. Additionally, the combination of high-speed millimeter-wave radio and fiber-optic backhaul is investigated. Several novel methods that enable high-spectral-efficient vector signal transmission in millimeter-wave radio-over-fiber systems are proposed and demonstrated through both theoretical analysis and experimental verification. For long-haul core networks, ultra-high-speed optical communication systems which can support 1Terabit/s per channel transmission will soon be required to meet the increasing capacity demand in the core networks. Grouping a number of tightly spaced optical subcarriers to form a terabit superchannel has been considered as a promising solution to increases channel capacity while minimizing the need for high-level modulation formats and high baud rate. Conventionally, precise spectral control at transmitter side is required to avoid strong inter-channel interference (ICI) at tight channel spacing. In this work, a novel receiver-side approach based on “super receiver” architecture is proposed and demonstrated. By jointly detecting and demodulating multiple channels simultaneously, the penalties associated with the limitations of generating ideal spectra can be mitigated. Several joint DSP algorithms are developed for linear ICI cancellation and joint carrier-phase recovery. Performance analysis under different system configurations is conducted to demonstrate the feasibility and robustness of the proposed joint DSP algorithms, and improved system performance is observed with both experimental and simulation data.

Electroabsorption modulators (EAMs) are fundamental components in optical communication systems. Their response is governed by a set of parameters inherent to their internal structure and by the external electrical components used to drive them. The first part of this thesis discusses a new method for the extraction of values for these parameters. The use of EAMs as both optical modulators and photodetectors is exploited for the purpose of parameter extraction. The proposed method allows the estimation of the parameters which govern the internal frequency response of EAMs without any knowledge of the characteristics of the electrical interconnect used to drive them. The procedure also removes the need for an accurately calibrated optical transmitter and receiver pair normally used during the characterization of optoelectronic components. Analytic description and experimental verification are presented. In the second part of this thesis, direct demultiplexing of a 10 Gbit/s channel from a 160 Gbit/s optical time division multiplexing (OTDM) signal using a single lumped, electrically driven EAM is experimentally presented for the first time. Direct demultiplexing is made possible by using an electrical driving signal for the EAM consisting of a sum of in-phase harmonics of the base channel rate. The use of a single EAM as an optical gate is quasi-analytically compared with the common approach of cascading two EAMs when performing 160 Gbit/s to 10 Gbit/s demultiplexing. The analysis reveals that the use of a single EAM is beneficial in terms of lower penalty with respect to degradations in the extinction ratio and width of the pulses used in the OTDM signal. The last part of this thesis introduces an electrical distributed oscillator which works in a regime of oscillation similar to that of mode locked laser (MLL) systems. The oscillator offers the flexibility of producing other waveforms not found in MLLs and has the potential to generate the required electrical driving signal for a single EAM OTDM demultiplexer. Other possible applications of the oscillator include the generation of short pulses for use in radar systems and wireless personal area networks. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2011-06-30 14:41:42.625

This thesis is dedicated to the study of gain transients of discrete fiber Raman amplifiers and to the all-optical gain-clamping technique which is used to mitigate those transients. / First, we study the standing-wave and the traveling-wave gain-clamping techniques when applied to a single discrete fiber Raman amplifier in the context of WDM channel add and drop. We take into account the operational regime of the amplifier and the location of the surviving channel in the amplification band. We demonstrate that the gain-clamped amplifier has to be operated in a regime below the critical regime to ensure that gain-clamping will be in effect. The efficiency of gain-clamping also depends on the feedback level of the lasing signal and on the implementation. / Next, we investigate the dynamic behaviour of a single discrete fiber Raman amplifier fed by multi-channel packet traffic. Our study shows that the efficiency of the gain-clamping technique to reduce the gain transients is dependent upon the operational regime of the amplifier and the packet duration. However, we also demonstrate that gain-clamping is not required to control the gain transients as the gain variations of the unclamped amplifier are small enough to be neglected. / We then theoretically analyse the dynamic response of cascades of discrete fiber Raman amplifiers subject to WDM channel add and drop. We consider cascades of mixed unclamped and gain-clamped amplifiers, varying the number and the position of the gain-clamped amplifiers in the cascade and taking into account the location of the surviving channel and the operational regime of the amplifiers. Our results show that the location of the gain-clamped amplifiers in a mixed cascade affects the transient characteristics and that it is possible to control the transients within tolerable limits. / Finally, we investigate the gain transients that occur in hybrid amplifiers in the presence of channel add and drop. We demonstrate that the gain-clamping technique can be used to mitigate the gain transients in hybrid amplifiers and that the surviving channel location does not influence the transient characteristics, contrary to the case of single and cascaded fiber Raman amplifiers.