1 Department of Photonics Engineering, Technical University of Denmark2 Metro-Access and Short Range Systems, Department of Photonics Engineering, Technical University of Denmark
This thesis expands the state-of-the-art in coherent detection for optical fiber access networks employing vertical-cavity surface-emitting lasers (VCSELs) as light sources. Bit rates up to 10 Gb/s over 25 km single-mode fibre (SMF) transmission distance have been achieved supporting a passive optical splitting ratio for 199 users. Extended transmission reach over 40 km SMF and a splitting ratio supporting up to 1024 users have been experimentally demonstrated for a bidirectional bit rate of 5 Gb/s. These novel proposed VCSEL-based transmission systems satisfy the requirements for next generation optical fiber access networks regarding long reach, high splitting ratio, no optical amplification, no external modulation, and use of a single fiber for upstream and downstream transmission. An important contribution of this thesis is the novel concept of chirpassisted coherent detection for direct current modulated VCSELs. A coherent receiver approach that exploits adiabatic frequency chirping of direct modulated VCSELs to improve the extinction ratio of received signals is introduced. This concept enables coherent detection systems to be fully based on VCSELs in contrast to conventional coherent detection approaches that require narrow linewidth light sources. Moreover, the proposed receiver configuration is based on envelope detection that simplifies its implementation. The results presented in this thesis include the first reported experimental demonstration of all-VCSEL-based coherent transmission link with real-time demodulation. Furthermore, advanced modulation formats are considered in this thesis to expand the state-of-the-art in high-speed short-range data transmission system based on VCSELs. First, directly modulation of a VCSEL with a 4-level pulse amplitude modulation (PAM-4) signal at 50 Gb/s is achieved. This is the highest data rate ever transmitted with a single VCSEL at the time of this thesis work. The capacity of this system is increased to 100 Gb/s by using polarization multiplexing emulation and forward error correction techniques. Compared to a non return-to-zero on-off keying (NRZ-OOK) system with the same bandwidth, this approach reduces by 2 the number of transceivers and by 4 the number of parallel lines needed to provide future standard capacity links. Secondly, carrierless amplitude phase (CAP) modulation and half-cycle quadrature amplitude modulation (HC-QAM) are experimentally demonstrated to increase the capacity for a given bandwidth and reduce the impact of optical fiber chromatic dispersion for a given capacity. Finally, 2 Gb/s bipolar impulse-radio ultra-wide band (IR-UWB) data communication over a combined distance of 25 km SMF optical fiber and 4 m air. These results show the suitability of VCSELs light sources for applications in hybrid optical fiber-wireless transmission links. In conclusion, the results presented in this thesis have significantly extended the state-of-the-art in techniques for signal generation, transmission and detection for next-generation access networks and high- speed shortrange systems employing vertical-cavity surface-emitting lasers as light sources.