Ultra-Compact Broadband In-Line Mode Converter Based on a Width-Modulated Silicon Waveguide

Mode division multiplexing (MDM) technology is becoming increasingly important for modern optical communication systems. Here, an ultra-compact broadband in-line mode converter for quasi-TE
₀₀
and quasi-TE
₁₀
on the silicon-on-insulator platform is proposed and demonstrated experimentally. In our device, the mode-conversion region consists of a continuously width-modulated waveguide with a footprint size as small as 1.32 × 4.52 μm
²
. Its modulation profile is designed by using the particle swarm optimization algorithm. This device has a simulated conversion efficiency of about −0.174 dB and an insertion loss less than 0.153 dB within 100-nm wavelength bandwidth from 1500 nm to 1600 nm. Our design exhibits a favorable fabrication error tolerance and the fabricated device has achieved nearly the same conversion efficiency as the simulated one. Our concept can also be applied to design other high-performance mode converters, i.e., converting modes between quasi-TE
₂₀
and quasi-TE
₀₀
. Our work suggests a very promising path for realizing compact integrated MDM systems.