Relaxed-tolerance subwavelength grating coupler

Short-wavelength mid-infrared (MIR) silicon photonics at 2–2.5 μm wavelengths has a wide range of applications in optical communications, chemical analysis, and environmental monitoring. In this spectral region, subwavelength grating (SWG) couplers can be fabricated by using commercial multi-project wafer (MPW) services, due to their larger device feature sizes compared with those in the telecommunication band. However, conventional SWG couplers are still sensitive to variations in dimensions produced by errors in fabrication processes, restricting the fabrication reproducibility of the devices. Here, we demonstrated a relaxed-tolerance SWG coupler that has relaxed fabrication tolerance to overcome this limitation. The approach is based on the design of the SWG coupler with dual-hole structures to reduce the influence of random fabrication variations on the grating effective refractive index. Theoretically, the slope of the center wavelength shift of the relaxed-tolerance SWG coupler to the variation of the subwavelength hole size is 0.58, while that is 0.63 for the conventional SWG coupler. We demonstrated the relaxed-tolerance SWG coupler with a peak coupling efficiency of −6.2 dB at a wavelength of 2.038 μm with a 1-dB bandwidth of about 30 nm. Additionally, fabrication reproducibility and optical fiber coupling tolerance are also presented. The study paves a promising way toward developing short-wavelength MIR grating couplers with high performance, excellent repeatability, and cost-efficiency.