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 Δn (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 Δn-controlled waveguide where the core diameter is 3.7 μm and Δn changes from 0.0027 to 0.0009 toward one end in the 5 mm-gradient Δn, region, is about twice as large as that in a normal waveguide where Δn 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)