We report a new type of polymer waveguide for two-dimensional (2-D) mode size transformation in which \Delta n (refractive index difference between core and cladding) is properly controlled along the waveguide axis while the core cross section is kept uniform. The beam diameter in the \Delta n-controlled waveguide where the core diameter is 3.7 \mum and \Delta n changes from 0.0027 to 0.0009 toward one end in the 5 mm-gradient \Delta n region, is about twice as large as that in a normal waveguide where \Delta n is 0.0027. The optical loss accompanied by mode size transformation is calculated to be less than 2.5% based on beam propagation methods (BPM's).
Ryoji Inaba, Miwa Kato, Masakazu Sagawa, and Haruo Akahoshi, "Two-Dimensional Mode Size Transformation by \Delta n-Controlled Polymer Waveguides," J. Lightwave Technol. 16, 620- (1998)