Polymeric optical mode converter for hybrid photonic integrated circuits

High optical loss due to mode mismatch at the interfaces of different components in a hybrid photonic integrated circuit (PIC) poses a major challenge in the implementation of such devices. Increased coupling efficiency can be achieved by incorporating an optical mode converter at the interface. This converter basically consists of a tapered waveguide section adapting different modal spot-sizes. Optimized coupling requires total control of the transverse optical field. This can be achieved by the ability to shape both the vertical as well as the lateral waveguide dimension. We present design, simulations, and fabrication considerations for a three-dimensional tapered waveguide structure for low loss mode conversion. Our mode converter concept is based on polymeric optical waveguides on silicon substrate. A gradually deeper trench is formed on the silicon substrate, using diffusion limited wet etching with a laterally tapered mask pattern. The structure is then planarized with a polymer and patterned laterally. Our method thus allows control of both the lateral and vertical waveguide dimensions. The concept is consistent with low-loss coupling to singlemode fibers as well as between laser and amplifier arrays and singlemode waveguides in a low-cost hybrid PIC solution.