Wireless communications has gone through tremendous growth in the past decades. There has been a shift in wireless network research from spectral efficiency and quality of service (QoS) constraints to energy efficiency and green communications to reduce power consumption. Green energy resources such as solar, wind, thermal and mechan- ical vibrations can be employed to increase the energy efficiency of energy-constrained networks such as wireless sensor networks. Converting the available energy in the sur- rounding area into electricity, energy harvesting (EH), has been the subject of recent research. EH from radio frequency (RF) signals can be utilized to prolong the lifetime of devices in energy-constrained systems. Wireless power transmission (WPT) for EH is a promising solution to provide a reliable source of energy for devices which are di cult to service due to mobility and/or hard to reach locations. The integration of relaying into conventional wireless networks is promising to increase the coverage area and reduce power consumption. However, the extra power consumed to relay signals may be a problem that can be mitigated by WPT. WPT has made it possible for relays to power themselves by capturing ambient energy wirelessly. The received signal at the relay can be utilized to both forward information and harvest energy. This dissertation focuses on practical energy harvesting schemes in wireless com- munication networks. Further, the broadcast nature of wireless systems makes wire- less transmissions more vulnerable to eavesdropping compared to wired signals. The goal of this work is to develop EH schemes that are capable of supplying sustainable energy to the relays and overcoming the secrecy hazards from potential eavesdroppers. Power splitting (PS) and time switching (TS) are studied in communication networks to prolong the lifetime of an energy-constrained relay. First, a dual hop system with an amplify and forward (AF) relay employing wireless information and power transfer (WIPT) via power splitting is studied. Optimal transmit antenna selection that max- imizes the end-to-end signal to noise ratio (SNR) at the destination is considered and the outage probability is derived. It is shown that the outage probability increases with the number of transmit antennas but this also increases the system complexity. Since the spectral efficiency with two-way relaying is higher than with one-way relaying, a two-way EH-based relay network with an eavesdropper is investigated. The secrecy capacity at the users is derived for two diversity combining cases at the eavesdropper, selection combining (SC) and maximal ratio combining (MRC). A friendly jammer is introduced to increase the secrecy capacity of the users by reducing the received signal to noise ratio at the eavesdropper since the signal of the jammer is considered as noise at the eavesdropper. The corresponding optimization problem is reformulated using the single condensation method (SCM) and geometric programming (GP) into a convex optimization problem. Then, GP is used to jointly optimize the power splitting factor of the relay and transmit powers of the two users and jammer to maximize the secrecy capacity of the system. Imperfect cancellation of the jamming signal at the relay is assumed. It is shown that increasing the power allocated to the jammer decreases the secrecy capacity at the users. However, when perfect jamming signal cancellation is assumed, increasing the power allocated to the jammer increases the secrecy capacity at the users. The secrecy capacity is also shown to be greater with a jammer than without a jammer. Channel state information uncertainty at the eavesdropper is also considered as an extra noise source. TS at the relay of a two-way EH-based relay network was also considered. GP is used to jointly optimize the time switching ratio of the relay and transmit powers of the two users and jammer to maximize the secrecy capacity of the system. It is shown that PS two-way relaying achieves a better secrecy capacity than TS two-way relaying. / Graduate

In this work, we have reported the design, MBE-growth and fabrication of strain-balanced n-type modulation-doped (MD) InGaAlAs/InGaAs multiple quantum wells laser/SOAs on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Two kinds of samples having similar structure but different fundamental transition wavelength of 1.55 £gm and 1.48 £gm are separately prepared for investigating their characteristics in optical amplification under forward bias and electro-absorption under reversed bias. Also, the technique of growing high-quality InGaAlAs with solid-source molecular beam epitaxy has been established and the resulting InGaAlAs bulk and QWs samples are extensively characterized by double-crystal X-ray diffraction, transmission electron microscopy, electroluminescence, and photoluminescence measurements. For £f = 1.55 £gm samples, ridge-waveguide lasers of Fabry-Perot (FP) type and tilted-end-facet (TEF) type were fabricated by a new developed multi-step wet-etching process. When injection current density > 20A/cm^2, electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at £f = 1460 nm than the e1-hh1 transition at £f = 1550 nm. The FP laser shows a lasing peak of £f = 1514 nm at threshold. Additional lasing wavelength at £f =1528 nm and 1545 nm were observed sequentially as the injection current increased. However, for the TEF laser, only the emission at £f = 1511 nm was observed. These TE-polarized lasing wavelengths are consistent with the £_-like absorption peaks in photocurrent spectra. The lasing performance is possible attributed to optical transitions within quantum dots/wires which are formed by the strain-field profile and alloy segregation/migration. For £f = 1.48 £gm samples, the differential absorption spectroscopy, which measures the change of transmission (£GT/T) in the presence of electric field, is used to study the electro-absorption modulation behavior of MD-SOA¡¦s. A sample with n-type modulation-doping amounting to a sheet density of 3.5 ¡Ñ 10^11 cm^-2 per QW and combining with a hole-stopping barrier represents the largest chirp parameter (£Gn/£Gk) under reversed bias, which offers an excellent platform to realize electro-refractive devices with larger refractive index changes (£Gn) but lower differential absorption (£G£\) near £f = 1.55 £gm, which is also our interested region of operation. In addition, we have succeeded in reducing the length of conventional constant-width multimode interference (MMI) coupler of K = 0.15 and 0.28 more than 32% by a novel stepped-width design concept. By extending the stepped-with idea, we show that it is possible to obtain 2x2 waveguide couplers with new power splitting ratios of 7%, 64%, 80% and 93% for cross coupling by cascading two short MMI sections. We further realize freely chosen power splitting ratio by interconnecting a pair of unequal-width waveguides as the phase-tuning section into the middle of two short MMI sections. These compact and low loss MMI-based devices use only rectangular geometry without any bent, curved, and tapered waveguides. They offer valuable new possibilities for designing waveguide-based photonic integrated circuits.