Ultracompact mode-order converting power splitter for mid-infrared wavelengths using an MMI coupler embedded with oblique subwavelength grating wires

Abstract

An ultracompact mode-order converting power splitter using a multimode interference (MMI) coupler with subwavelength gratings (SWGs) is proposed for mid-infrared (MIR) wavelength. Two symmetric oblique SWG-wires are embedded in the central input-region that is connected with the input waveguide. As a result, the input TE0 mode is evenly coupled from the input waveguide into the MMI coupler due to the adiabatical mode evolution, and then two desired TE1 modes are formed via self-imaging effect and further guided into the two output channels. Additionally, tapered structures and straight SWG wires are introduced into the two output channels to better obtain the required phase difference for the improvement of converting efficiency. Consequently, both TE0-to-TE1 mode-order converting and power splitting, for the first time, are attained simultaneously in the present single device. Moreover, TE0-to-TE2 power splitting can be readily achieved by flexibly changing the dimensions of the proposed device. Results show that a compact device length of 7μm with an insertion loss (IL) of 0.41 dB, a conversion efficiency (CE) of 99% and a crosstalk (CT) of -19.77 dB is obtained at the wavelength of 2μm; its bandwidth can be enlarged to 220 nm for IL < 0.79 dB and CE> 91%. Besides, fabrication tolerances are addressed and mode propagation profiles are illustrated.