Sharp vertices in asymmetric Y-junctions by double masking

A novel fabrication procedure is presented for the realization of sharp vertices in Y-junctions on InGaAsP/InP. Overlapping masking layers of silica and resist respectively define the two branches in the junction. A nearly perfect branching point is made by a reactive ion etch with two overlapping mask strips. An asymmetric Y-junction designed to function as a mode splitter is realized with this technique. It shows an improvement of 4-8 dB in mode splitting over a version made with a single masking layer. This agrees with BPM-simulations on the effect of blunting in these Y-junctions.>