The fast development of Photonic Integrated Circuits (PICs) has led to advances not only in the fundamental elements (such as ultra-low loss waveguide platforms and arrayed waveguide gratings (AWGs)) but also in the functionalities including large-scale optical interconnects, miniaturized imaging systems, low-loss and wide bandwidth communication transceivers, and precision metrology. A complete design/simulation, fabrication, and characterization routine is demonstrated on the ultra-low-loss multi-layer Si3N4 waveguide platforms in addition to the standard III-V and SOI PIC platforms, offering the superior performance of flexibility in the vertical integration and of minimizing the system throughput loss.To catch up with the needs and expectations from the modern wavelength division multiplexing (WDM) communication infrastructure, AWGs stand out of its alternatives (cascaded Mach-Zehnder Interferometer lattice filter and Echelle gratings) in the finite impulse response (FIR) multiplexers (MUXs) and demultiplexers (DEMUXs) categories with better performance on the device crosstalk level, insertion loss, and device footprint. A concise and comprehensive AWGs modelling and simulation routine based on Fourier optics is presented. I will introduce the modelling and characterization results of both standard Gaussian and flat-top AWGs on the developed Si3N4 waveguide platform with capabilities as MUXs and wavelength routers in the optical communication O and C band as the building blocks for complex PICs. I will present the new generation PICs based on our proposed Segmented Planar Imaging Detector for Electro-optical Reconnaissance (SPIDER) for a small-scale interferometric imaging system, relying on the sampling in the spatial Fourier domain via multiple baselines in PICs for an image reconstruction offline. The Si3N4 PIC contains path-length matching waveguides, large channel spacing AWGs as DEMUXs, multimode interferometers (MMI), and phase tuning sections. Thorough characterization and study of the PIC performance are carried out, and the sustainability of the PIC for Low Earth Orbit (LEO) mission is confirmed. In the end, I will propose and show novel and compact prism-waveguide couplers utilizing the evanescent wave coupling phenomenon together with mode size converter (negatively-tapered waveguide) for the efficient on-chip coupling towards the low-index (fluoride compounds) ultra-high-Q resonators for the first time. Thus, it paves the way towards the fully monolithic integrated optical frequency comb (OFC) sources for practical applications in optical communications, precision navigation, and optical sensing.