Ultra-compact and low-loss silicon nitride optical mode-size converter

We designed and implemented a compact silicon nitride mode-spot converter that is non-linear and stepwise segmented. The mode-spot converter with a length of 20 μm and a simulated transmission efficiency of 95.7% was obtained using the three-dimensional finite-difference time-domain method and particle swarm optimization algorithm. This device can connect a 10-μm wide waveguide with a 1-μm wide single-mode waveguide for O-band operation. Compared with a linear taper with equivalent transmission efficiency, this device reduces the footprint by 80%. The mode-spot converter was fabricated on a silicon nitride platform using electron-beam lithography and inductively coupled plasma etching processes. The fabricated device exhibits a measured average insertion loss of 0.30 dB in the O-band and 1 dB bandwidth of more than 100 nm.