On-chip silicon shallowly-etched TM0-to-TM1 mode-order converter with high conversion efficiency and low modal crosstalk

Ever-increasing capacity requirements of optical interconnects drive the emergence and fast development of mode-division-multiplexing (MDM) transmission on-chip, where efficient mode control and conversion components become indispensable. Here, we propose an on-chip silicon ${\text{TM}_0} \mbox{-} \text{to}\mbox {-} {\text{TM}_1}$TM0-to-TM1 mode-order converter by leveraging shallowly etched rectangular slots on the top surface of silicon nanowire. The mode conversion region consists of two rectangular slots on the same side of a silicon nanowire and a smaller one between them to realize the efficient mode-order conversion from input ${\text{TM}_0}$TM0 to output ${\text{TM}_1}$TM1 mode with the help of multimode interference and accumulated phase difference. By studying the etching pattern on the silicon nanowire in detail, we have achieved an on-chip ${\text{TM}_0} \mbox{-} \text{to}\mbox {-} {\text{TM}_1}$TM0-to-TM1 mode-order converter with a high conversion efficiency of 97.5% and low modal crosstalk ${ \lt }{-} {23}\;\text{dB}$<−23dB in a conversion length of 11.8 µm; further, the insertion loss is only 0.29 dB at the wavelength of 1.55 µm. Moreover, the device working bandwidth and fabrication tolerance are also analyzed. Note that the proposed shallowly etched slots on the silicon nanowire can also be further developed to achieve the on-chip ${\text{TM}_0} \mbox{-} \text{to} \mbox{-} {\text{TM}_2}$TM0-to-TM2 mode-order conversion. With these characteristics, such a device could boost the development of MDM transmission on-chip with more TM-polarized mode channels.