Birefringence and Reflectivity of All-Polymer Tunable Bragg Grating Filters With Microheaters

We study the impact of the two heating schemes, i.e., top heating and buried heating, on the modal birefringence and reflectivity of all-polymer tunable grating filters. Numerical simulations show that with top microheaters, birefringence is thermally induced by a temperature gradient between the microheater and the waveguide. In the case of a buried microheater placed beneath the waveguide core, such thermally induced birefringence is effectively eliminated because of the almost uniform temperature distribution around the core region. Simulation results also indicate that the reflectivity of the polymer waveguide Bragg grating filter is reduced for the top heating scheme as the heating power increases whereas it nearly remains unchanged for the buried heating scheme. Experimentally, the thermally-induced part of the waveguide birefringence has been found to increase to 1 x 10(-3) when raising the specific electrical heating power to 70 mW/mm in the top microheater case. With the buried microheater structures, virtually no thermally induced birefringence was found, in consistency with the simulation results. The reflectivity changes of all-polymer tunable grating filters by heating are also studied. The results are considered helpful for designing polymer-based photonic devices that require birefringence control.