Design, Realization, and Characterization of 3-D Taper for Fiber/Micro-Waveguide Coupling

Three-dimensional (3-D)-tapered structures are designed to carry out efficient coupling from a single-mode optical fiber to a submicrometer square waveguide. The main advantages of such a structure are wavelength and polarization insensitivities and compactness. 3-D taper realization in silicon-on-insulator micro-waveguide is achieved using two electronic lithography steps, the first one being grayscale lithography. Experimental characterizations clearly evidence optical mode expansion fitted to the optical fiber mode and a coupling loss of about 7 dB for a 170-nm square cross section waveguide. Low polarization and wavelength sensitivities are also demonstrated using such a structure