PLANAR FABRICATION PROCESS OF A HIGH-COUPLING-EFFICIENCY INTERFACE BETWEEN OPTICAL WAVE-GUIDES OF LARGE INDEX DIFFERENCE

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 achi eved by matching the mode profiles in two waveguides as well as connec ting the waveguides with an antireflection (AR) section. Analysis show s a coupling efficiency as high as 98% between glass SiO2 and Gao(0.72 )Alo(0.23)As-Ga0.59Al0.1As waveguides with an AR section of TiO2. An i ndex difference of 1.8 for the TE(0) mode between these two waveguides is obtained. To implement the glass -SiO2 waveguide with an AR sectio n in the host AlGaAs waveguide, a new quadrilevel photomasking and lif t-off process is developed. A coupling efficiency of 83% was measured for the TE(0) mode at the 1.15-mu m wavelength, a very high value cons idering the large index difference (1.75) between the two waveguides t hat were connected. The design concept and the fabrication process dev eloped should facilitate efficient integration of low- and high-index waveguides on the same substrate to produce a greater variety of photo nic devices and modules.