As modern computing systems scale toward higher integration and bandwidth, electrical interconnects increasingly limit system efficiency, signal integrity, and packaging scalability. Optical interconnects offer superior bandwidth and power characteristics, yet their adoption is hindered by challenges in optical coupling, alignment precision, and assembly complexity—particularly in chiplet-based and 2.5D integration platforms. This dissertation presents a sub-micron passive self-aligned optical interconnect platform to address these bottlenecks. By combining silicon microfabrication and nano-scale 3D printing, the proposed structures—namely the Positive Self-Alignment Structure (PSAS) and the Tilted Silicon Chiplet (TSC)—enable high-precision fiber placement for surface coupling, eliminating the need for active alignment. Experimental characterization achieves sub-micron alignment accuracy and insertion losses comparable to active alignment approaches, while supporting scalable packaging. The platform advances previous generations of PSAS and transitions from a multi-element Fiber-Interconnect Silicon Chiplet Technology (FISCT) architecture to a simplified, tilted chiplet form (FISCT-T), improving mechanical robustness and integration density. Alignment tolerance is studied through simulation and experiment, providing practical insights into grating coupler coupling under misalignment. Demonstrations are performed on both passive photonic integrated circuits (PICs) and foundry-fabricated active photonic dice, demonstrating its viability as a post-fabrication strategy for scalable optical I/O in 2.5D and co-packaged optics (CPO) systems.