1 ×3 Silicon Oxynitride Tunable Optical Waveguide Attenuators Based on the Multimode Interference Effect

The silicon oxynitride (SiON) films of various refractive indices achieved through the careful adjustments of oxygen-to-nitrogen contrast ratios were deposited on silicon substrates using the plasma-enhanced chemical vapor deposition (PECVD) method. The refractive indices of SiON films spanning from 1.47 to 1.94 were realized by manipulating the flow rates of pertinent gaseous precursors. The Fourier transform infrared (FTIR) spectra were gathered in order to verify that a significant reduction in the infrared absorption of the N–H bond could in fact be achieved with the thermal annealing process. Afterward, the integrated 1 ×3 SiO2/SiON/SiO2 tunable multimode interference (MMI) optical waveguide attenuators were then designed and simulated using the beam propagation method (BPM), and the optimal structures were then proceeded for device fabrication. The positive thermooptic (TO) effect of SiON was employed to change the self-imaging light pattern as generated by the side-heated MMI region for purposes of optical steering and beam attenuation. The experiment results showed that a 22-dB attenuation could be realized with the heating power of 2.2 W. Finally, when the heating power of ~1.73 W was applied, the rise and fall times of 1 ×3 tunable MMI optical waveguide attenuators were obtained as 455 and 420 µs, respectively.