Micro-cavity devices such as micro-ring resonators are frequency selective elements that can perform a variety of functions such as add/drop filtering, switching, and modulating in chip-scale wavelength-division-multiplexing (CS-WDM) systems. Their compact size and low-power-consuming property have enabled them to become a strong candidate as building blocks in the future photonic integrated circuits (PICs). In this dissertation, InP/InGaAsP-based micro-ring (or micro-disk) resonators vertically coupled with bus waveguides were investigated mainly as active bandpass filters serving as multiplexer/demultiplexers (MUX/DEMUXs) in the WDM communication system.