2x2 Ultra-Broadband Multimode Interference Coupler with Subwavelength Gratings Fabricated by Immersion Lithography

We demonstrate an ultra-broadband 2x2 multimode interference (MMI) coupler leveraging subwavelength grating (SWG) nanostructures fabricated by immersion lithography. Excess loss of $\lt2$ dB, power imbalance $\lt1.5$ dB and phase error $\lt 5^{\circ}$ have been measured for a wavelength range of 1300 – 1680 nm. The small feature sizes, down to 75 nm, were well-defined using immersion lithography and optimized optical proximity correction.