Abstract

Mode-division multiplexing (MDM) is a promising approach to increase data transmission capacity by using different modes as independent data channels. Intermodal crosstalk and coherent mode interference are detrimental to the performance of an MDM system and should be avoided in the mode converters and mode expanders for MDM. The Mikaelian lens is well known to be free from spherical aberrations and is therefore an excellent candidate for focusing high order waveguide modes which have main mode field distributions located off-axis. Here we proposed an ultracompact mode-independent mode-size converter using an integrated Mikaelian lens formed by refractive index engineering using a subwavelength grating. The Mikaelian lens is fabricated on the silicon-on-insulator (SOI) platform and can be used to convert the size of high order modes at the interface between the silicon waveguide and a multimode fiber in MDM systems. We present a proof-of-concept demonstration of a low loss spot size converter for three transverse electric (TE) modes between a 14 μm wide multimode waveguide grating coupler and a 1.56 μm wide multimode silicon waveguide. The measured insertion loss of the Mikaelian lens mode size converter is 0.21 dB for TE₀ mode, 0.26 dB for TE₁ mode and 0.28 dB for TE₂ mode. 1-dB bandwidth of all the three modes are about 60 nm. The 15 μm long Mikaelian lens has comparable experimental performance as a 450 μm long adiabatic taper.

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