Ultra-compact silicon multimode waveguide bends based on special curves for dual polarizations

Abstract. Multimode waveguide bends (MWBs) with very compact sizes are the key building blocks in the applications of different mode-division multiplexing (MDM) systems. To further increase the transmission capacity, the silicon MWBs for dual polarizations are of particular interest considering the very distinct mode behaviors under different polarizations in the silicon waveguides. Few silicon MWBs suitable for both polarizations have been studied. In this paper, we analyze several dual-polarization-MWBs based on different bending curve functions. These special curve-based silicon MWBs have the advantages of easy fabrication and low loss compared with other structures based on subwavelength structures, such as gratings. A comparison is made between the free-form curve (FFC), Bezier curve, and Euler curve, which are used in the bending region instead of a conventional arc. The transmission spectra of the first three TE and TM modes in the silicon multimode waveguide with a core thickness of 340 nm are investigated. The simulation results indicate that, with the premise of having the same effective radius, which is only 10  μm in this paper, the six-mode MWB based on the FFC has the optimal performances, including an extremely low loss <0.052  dB and low crosstalk below −25.97  dB for all six modes in the wide band of 1500 to 1600 nm. The MWBs based on the Bezier and Euler curve have degraded performances in terms of the loss and crosstalk. The results of this paper provide an efficient design method of the polarization insensitive silicon MWBs, which may leverage research for establishing complicated optical transmission systems that incorporate both the MDM and polarization-DM technology.