Two-dimensional mode size transformation by Δn-controlled polymer waveguides

We report a new type of polymer waveguide for two-dimensional (2-D) mode size transformation in which /spl 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 /spl Delta/n-controlled waveguide where the core diameter is 3.7 /spl mu/m and /spl Delta/n changes from 0.0027 to 0.0009 toward one end in the 5 mm-gradient /spl Delta/n, region, is about twice as large as that in a normal waveguide where /spl 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).