Photonic integrated circuits for optical communications and microwave photonics XIEYIWEI 2018 This thesis explores and develops optical high spectral efficiency optical systems and wideband microwave photonic (MWP) subsystems using photonic integrated circuits (PICs). The thesis not only expands the application of PICs in optical communications and MWPs, but also proposes the solutions to some issues in PICs, and improve the merits to the design performance. The work promotes the development of PICs, opening a possibility for creating fully integrated, and multi-purpose optical processing ‘engine’ using a single device.