| Summary: | The need to satisfy bandwidth-hungry applications with a growing number of internet users has raised the requirements for capacity and reach in optical communication systems. Recent advances in field modulation and digital coherent detection enabled applications in long- and medium-reach systems. For short-reach systems, such as intra- and inter-data center connections, solutions which consider their sensitivity to both cost and system complexity are required. Systems based on intensity modulation and direct detection (IM/DD) are good candidates to fulfill the requirements of many short-reach applications. In an IM/DD channel, chromatic dispersion and the dispersion-induced frequency-selective power fading are major issues which limit the system performance. The primary objective of this research is to enhance the transmission performance in IM/DD systems, with a focus on using a transmitter based on a directly-modulated laser (DML). The capacity and reach of an IM/DD system are improved by employing higher-order modulation formats and mitigating chromatic dispersion and the resulting frequency-selective power fading. For this purpose, two main approaches are investigated by simulation. In both approaches, higher-order modulation formats are enabled by employing subcarrier modulation (SCM) with a transmitter based on the direct modulation of a 10 Gb/s commercial packaged laser. For modeling the laser, a rate-equations model is adopted. The values of the rate equations parameters and the elements in an equivalent circuit model for the electrical interface are extracted based on a series of measurements using a comprehensive procedure. In the first approach, several radio frequency (RF) channels using the same modulation format are multiplexed before driving the DML. The frequency-selective power fading is mitigated by optimizing the modulation conditions of the DML and employing power loading of the RF channels. Using a practical two-tone test and the received signal-to-noise-and-distortion ratios, an iterative procedure is presented to calculate the required power loading which equalizes the performance of the channels. In the second approach, vestigial sideband transmission is employed to mitigate the frequency-selective power fading of the channel. The use of the Kramers-Kronig (KK) receiver with a DML is validated, and the KK receiver is utilized to enhance the system performance with the compensation of chromatic dispersion in single-channel transmission. PhD
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