The development of integrated photonic technologies has enabled the miniaturisation and mass production of optical components for fibre-based optical transmission systems. This resulted in an exponential growth in bandwidth availability over the last few decades due to drastic cost reductions and increases in energy efficiency. However, as the world transitions to a carbon-neutral economy, the requirements imposed on the optical network get more and more challenging to meet. The use of intensity modulation with direct detection (IMDD) has historically been the preferred solution for short-reach applications, but coherent technologies have proven in recent years that they can outperform IMDD even after a few tens of km. Some of the main technologies competing in the short-reach market, ordered in increasing complexity and performance, are directly modulated lasers (DML), externally modulated lasers, and coherent transmission. DMLs provide high power and energy efficiency, but the frequency chirp induced by the modulation makes it challenging to achieve high data rates and transmission distances. External modulation allows for improved performance at the expense of additional insertion losses, while coherent transmission provides great improvements in reach and spectral efficiency at the expense of increased complexity, component cost and energy consumption. This thesis explores the use of photonic integrated circuits and low-complexity digital signal processing to enable cost-effective and energy-efficient increases in transmission capacity/reach in short-reach optical transmission systems. We study the use of the three technologies mentioned above for different use cases relevant for current and future optical links ranging from intra-data centre point-to-point links to point-to-multipoint systems for optical access networks.