High performance silicon-on-sapphire subwavelength grating coupler for 2.7µm wavelength

Couplers are important parts in integrated optical circuit and high efficiency compact couplers are in great demand. We report on high-efficiency silicon-on-sapphire (SOS) grating couplers at wavelength of 2.7μm. A 75% coupling efficiency and 50nm etching depth bandwidth (the tolerance of etching height) from a standard single-mode fiber to an SOS waveguide is obtained. A basic design principle of the grating coupler is presented, and some improved structures to enhance the coupling efficiency are proposed and estimated. The FDTD method is utilized to simulate and design the grating operated under TE polarization. With our optimization design, the coupling efficiency can be largely increased and beyond 80%. The gratings open the path to silicon photonic chips for the mid-infrared enabling new nonlinear optical functions as well as new spectroscopic lab on-a chip approaches.