1 Department of Photonics Engineering, Technical University of Denmark2 Metro-Access and Short Range Systems, Department of Photonics Engineering, Technical University of Denmark3 Networks Technology and Service Platforms, Department of Photonics Engineering, Technical University of Denmark
The research work described in this thesis is devoted to experimental investigation of techniques for cost-effective high-speed optical communications supporting both wired and wireless services. The main contributions of this thesis have expanded the state-of-the-art in two main areas: high-speed optical/wireless integration and advanced modulation formats for intensity modulation with direct detection (IM/DD) optical systems. Regarding optical/wireless integration, this thesis focuses on integration of broadband ultra-wide band (UWB) and 60-GHz band wireless systems into optical fiber access networks to distribute wireless services in personal area networks (PANs). Photonic technologies to generate and distribute gigabit UWB and 60-GHz-band signals are proposed and demonstrated. Two novel methods are proposed and demonstrated to optically generate Federal Communications Commission (FCC)-compliant gigabit UWB signals and integrate them into baseband wavelength division multiplexingpassive optical networks (WDM-PONs). Performance of UWB signals and other wired/wireless signals in different scenarios including heterogeneous wired and wireless access networks, converged communication and sensing networks are experimentally assessed. For 60-GHz radio-over-fiber (RoF) systems, possibilities to simplify the system architecture and to use multimode fiber (MMF) together with single-mode fiber (SMF) to transport gigabit data at 60-GHz band are experimentally investigated. Additionally, the quality of video services distributed in a simplified 60-GHz RoF system is both experimentally and theoretically studied by analyzing the tradeoff between the distortion introduced by the video coding and distortion introduced by the optical/wireless transmission channel. Next to optical/wireless integration, this thesis explores techniques for delivery of high speed baseband signals in IM/DD optical links. To improve spectral efficiency (SE), a novel approach is introduced to implement advanced modulation formats, using subcarrier frequencies below the baud rate including half and quarter the baudrate. Compared to conventional single subcarrier modulation, quarter-cycle and half-cycle quadrature amplitude modulation (QAM) achieve a improvement of the SE by 37.5% and 25%, respectively. The feasibility of half-cycle QAM signals for IM/DD optical links is demonstrated by transmission of a 10-Gbps half-cycle 4-QAM signal over 20-km SMF and a 9-Gbps 4-QAM signal over 1 km MMF both using VCSELs and simple electrical transceiver structures. The proposed approach results in the 3 dB improvement of chromatic dispersion tolerance and 1.5 dB improvement of modal dispersion tolerance at the bit-error-ratio (BER) of 10􀀀5 compared to similar OOK systems operating at the same bitrate. In conclusion, techniques proposed in this thesis demonstrate the feasibility of employing photonic technologies for the generation and distribution of gigabit wireless signals. They also demonstrate seamless integrability of wired and wireless signals into a unified optical fiber platform. Additionally, half-cycle QAM modulation has prospects to increase SE and bit rates in high-speed short-range optical communication systems.