Design of a compact silicon-based TM-polarized mode-order converter based on shallowly etched structures

A mode-order converter is an indispensable component in some typical on-chip mode-division-multiplexing (MDM) systems. Here, we propose a compact and high-performance TM-polarized mode-order converter using shallowly etched structures on the silicon waveguide. This device consists of a rhombus etching part at the waveguide center and two same triangle etching parts on both sides of the waveguide along the propagation direction to achieve the mode conversion from the input ${{\rm TM}_0}$TM0 to the output ${{\rm TM}_2}$TM2 mode, where both the effective waveguide structure and the corresponding effective index distribution can be changed. By optimizing the dimensions of the rhombus and triangle etching parts as well as the relative position between them, we have realized the efficient ${{\rm TM}_0}$TM0 to ${{\rm TM}_2}$TM2 mode conversion in a conversion length of only 6 µm, which distinguishes from mainly reported TE-polarized mode-order converters, and the obtained mode conversion efficiency (CE), cross talk (CT), and insertion loss (IL) are $\sim{94}\% $∼94%, $ \lt - {15}\;{\rm dB}$<−15dB, and $\sim{0.5}\;{\rm dB}$∼0.5dB, respectively, at the wavelength of 1.55 µm. Meanwhile, the allowable bandwidth can be extended to 128 nm by keeping ${\rm CE} \gt {94}\% $CE>94%, where the mode CT and IL are $ \lt - {15}\;{\rm dB}$<−15dB and ${\rm IL} \lt {0.7}\;{\rm dB}$IL<0.7dB, respectively, in the same wavelength range. Also, the device fabrication processes and tolerance analyses have been done. We hope this device could be beneficial for the capacity improvement of the on-chip MDM systems.