Inverse Design of Low-Loss Subwavelength Grating Couplers

A conventional single-polarization surface grating coupler (GC) consists of periodically-defined trenches in the longitudinal direction to wave propagation. By contrast, subwavelength grating couplers (SWGCs) leverage additional degrees-of-freedom in either the longitudinal or transverse directions to wave propagation to offer improved insertion loss, bandwidth, or other metrics [1]–[4]. In this paper, as part of our broader effort to integrate machine learning methods into fab-robust photonic design [5], [6], we have optimized 8° SWGCs using our custom AI-enhanced global photonic optimization protocol called PHORCED [6]. Our design was first optimized in 2D, then converted to 3D using a custom per-period effective medium conversion model. In optimization, we parameterized longitudinal grating variables (pitch and duty cycle) with a finite Fourier series expansion [5]. We parameterized transverse grating variables by representing each grating trench with a continuous homogenized refractive index value, n hom . The objective of each optimization was the average coupling efficiency at 11 wavelengths in the $\lambda=1540\text{nm}$ to $\lambda=1560\text{nm}$ band. We assume a mode field diameter of $10.4\mu\mathrm{m}$ for the resulting Gaussian beam. The result of our 2D SWGC optimization is depicted in Fig. 1(a), where the SWGC consists of a gradual transition in the homogeneous effective index along the length of the grating, providing directionality-limited loss of 0.56dB and 40nm 1dB-bandwidth in 2D simulation.