Continuously apodized fiber-to-chip surface grating coupler with refractive index engineered subwavelength structure

Abstract

We demonstrate a fully etched, continuously apodized fiber-to-chip surface grating coupler for the first time (to our knowledge). The device is fabricated in a single-etch step and operates with TM-polarized light, achieving a coupling efficiency of $3.7\mathrm{dB}$ and a $3\mathrm{dB}$ bandwidth of $60\mathrm{nm}$. A subwavelength microstructure is employed to generate an effective medium engineered to vary the strength of the grating and thereby maximize coupling efficiency, while mitigating backreflections at the same time. Minimum feature size is $100\mathrm{nm}$ for compatibility with deep-UV $193\mathrm{nm}$ lithography.