Sol-gel-derived 4 × 4 multimode interference coupler: Optimization and fabrication

The design and fabrication of a 4 × 4 multimode interference (MMI) coupler is reported. A photopatternable organic-inorganic sol-gel on silica-on-silicon is adopted for the device fabrication. Optimization of waveguide parameters led to the coupler excess loss below 1 dB and power imbalance below 0.2 dB. An optical 90-deg hybrid based on the coupler is realized and tested. The experimental data are in very good agreement with predicted simulation results, demonstrating the simplicity and exactness of the sol-gel method as an advantageous fabrication route in integrated optics.