Ultra-Broadband Mode Converter and Multiplexer Based on Sub-Wavelength Structures

Current bandwidth capacity provided by wavelength-division multiplexing and polarization-division multiplexing is insufficient to keep up with the increasing bandwidth demand required for new services. Mode-division multiplexing technology paves the way to further increase transmission and bandwidth capacities in photonic interconnects. In this work, we propose an ultra-broadband two-mode converter and de/multiplexer based on a sub-wavelength engineered multimode interference coupler, a 90<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> phase shifter, and a symmetric Y-junction for the silicon-on-insulator platform. Sub-wavelength grating waveguides enable dispersion engineering to further increase the bandwidth operation of conventional multimode interference coupler and, subsequently, of mode de/multiplexer based on them. Full three-dimensional simulations of the designed mode converter and de/multiplexer show insertion loss below than 0.84 dB and crosstalk lower than <inline-formula> <tex-math notation="LaTeX">$-$</tex-math></inline-formula>20 dB over an unprecedented bandwidth of 300 nm (1.4–1.7 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m). The overall footprint of the proposed device is only 36 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula>m <inline-formula><tex-math notation="LaTeX">$\times$</tex-math></inline-formula> 3.7  <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m.

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