High-Performance Multimode Interference Coupler in Silicon Waveguides With Subwavelength Structures

The performance of multimode interference (MMI) couplers in silicon waveguides is limited by the high lateral refractive index contrast. Here we propose the use of subwavelength gratings (SWGs) in the lateral cladding regions of the MMI to reduce the index contrast. Our approach significantly reduces the mode phase error while at the same time allowing a single etch step process. Using a z-periodic lateral SWG, we design a 2 x 4 MMI that operates as a 90 degrees hybrid for a coherent optical receiver. This complex device exhibits a common mode rejection ratio (CMRR) and a phase error of less than -24 dBe and 2 degrees, respectively, over the full C-band. Compared to MMI with a homogenous lateral cladding, using subwavelength refractive index engineering effectively extends the receiver bandwidth from 36 to 60 nm.