Design of tapered, uniform, and compact monolithically integrated semiconductor spot-size converters for efficient laser-fiber coupling

When a photonic device is directly butt-coupled to a single modefiber, often there is a 90% or more optical power loss, due to their highly unequal spot-sizes. Often, by using complex microlenses, the coupling efficiency has been increased, but however at the expense of a sub-micron alignment tolerance requirement, and consequently higher associated cost for a reliable operation. Recently, monolithically integrated devices, mostly using tapered structures, have been reported as useful to modify the spot-size of semiconductor photonic devices, which could assist in the rapid implementation of fiber-to-the-home'. In this work, spot-size converters with a tapered core, also using novel uniform directional couplers and MMI structures are discussed. A rigorous numerical approach, based on the full vectorial finite element method, is used to design various types of monolithically integrated spot-size converters for efficient coupling to an optical fiber and results of their performance reported.