Design optimization of monolithically integrated semiconductor spot-size converters for efficient laser-fiber coupling

When a photonic device is directly butt-coupled to a single mode fibre, often there is a power loss of 10 dB or greater, due to the highly unequal spot-sizes associated with each device. By using complex microlenses, the coupling efficiency may be increased at the expense of a resultant sub-micron alignment tolerance, which is very hard to achieve in reliable operation. Recently, monolithically integrated devices mostly using tapered structures, have been reported as useful to modify the spot-size of semiconductor photonic devices. In this work, spot-size converters with a tapered core, and also using novel uniform directional couplers and MMI structures are discussed and results reported. 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.