Planar fabrication process of a high-coupling-efficiency interface between optical waveguides of large index...

A high-coupling-efficiency interface connecting two optical waveguides of large index difference is reported. Both theoretical analysis and an experimental study were performed. High coupling efficiency is achieved by matching the mode profiles in two waveguides as well as connecting the waveguides with an antireflection (AR) section. Analysis shows a coupling efficiency as high as 98% between glass-SiO(2) and Ga(0.72)Al(0.28)As-Ga(0.59)Al(0.41)As waveguides with an AR section of TiO(2). An index difference of 1.8 for the TE(0) mode between these two waveguides is obtained. To implement the glass-SiO(2) waveguide with an AR section in the host AlGaAs waveguide, a new quadrilevel photomasking and lift-off process is developed. A coupling efficiency of 83% was measured for the TE(0) mode at the 1.15-µm wavelength, a very high value considering the large index difference (1.75) between the two waveguides that were connected. The design concept and the fabrication process developed should facilitate efficient integration of low- and high-index waveguides on the same substrate to produce a greater variety of photonic devices and modules.