Ultracompact Low-Loss Grating Couplers

Abstract

Fiber-to-chip couplers play a crucial role in interfacing on-chip photonic circuits with other optical systems or off-chip devices. Downsizing the couplers via topology optimization addresses the demand for high-density integration and improves the scalability of photonic integrated systems. However, these optimized couplers have yet to reach the performance level demonstrated by their conventional counterparts, leaving room for further improvement. In this work, we apply topology optimization to design single-polarization 1D and dual-polarization 2D grating couplers incorporating bottom reflectors and achieve sub-decibel coupling efficiency. Both types of couplers are fabricated on the silicon-on-insulator platform with dimensions of mere 14 $μ$m $\times$ 14 $μ$m and are compatible with standard single-mode fibers at normal incidence. From our experimental characterization, the measured peak coupling efficiency of the topology-optimized 1D and 2D couplers is -0.92(1) dB and -0.86(13) dB, respectively, within the telecom C-band. Our demonstration provides a coupling solution for photonic applications requiring high efficiency and high-density integration, such as spatial division multiplexing and photonic quantum technologies.

Figures (3)

• Figure 1: Schematic of the designed ultracompact topology-optimized 1D (left) and 2D grating couplers (right) incorporating a bottom reflector on the SOI platform. The topology-optimized 1D grating coupler couples only one specific linear polarization state (LP$^{y}_{01}$). In contrast, the topology-optimized 2D grating coupler couples light with arbitrary polarization (i.e. the superposition state of LP$^{x}_{01}$ and LP$^{y}_{01}$) into two waveguides with a proportion determined by the polarization state.
• Figure 2: Simulation and optimization results of the topology-optimized grating couplers. (a) Evolution of the topology-optimized 1D grating coupler's coupling efficiency during the optimization. The material binarization starts after iteration 61. (b) Rendered image of the topology-optimized 1D grating coupler. (c) Simulated coupling efficiency of the topology-optimized 1D grating coupler within the range from 1530 nm to 1560 nm. The coupling efficiency at 1550 nm is $-0.37$ dB. (d) Evolution of the topology-optimized 2D grating coupler's coupling efficiency during the optimization. The material binarization starts after iteration 97. (e) Rendered image of the topology-optimized 2D grating coupler. The two waveguides connecting to the 2D grating coupler are labeled as X and Y arms as the light with polarization along $x$ and $y$-axes propagates to the arms, respectively. (f) Simulated coupling efficiency of the original (blue solid line) and modified (black dashed line) topology-optimized 2D grating coupler within the range from 1530 nm to 1560 nm. The coupling efficiency at 1550 nm is $-0.46$ dB and $-0.47$ dB for the original design and the modified one, respectively.
• Figure 3: Fabricated topology-optimized 1D and 2D grating couplers and the experimental characterization. (a) Scanning electron microscope (SEM) image of the topology-optimized 1D grating coupler. (b) SEM image of the topology-optimized 2D grating coupler. (c) Simulated (black line) and measured (blue line) coupling efficiency of the topology-optimized 1D grating coupler versus wavelength. The blue shaded region indicates the deviations of the measurements. The measured peak coupling efficiency is $-0.92(1)$ dB at 1548.0 nm and the measured coupling efficiency at 1550.0 nm is $-1.21(2)$ dB. (d) Simulated (black line) and measured coupling efficiency of the topology-optimized 2D grating coupler versus wavelength when the input light is at X (orange line) and Y (blue line) arms. The shaded regions indicate the deviations of the measurements. The measured peak coupling efficiency is $-0.86(13)$ dB at 1548.4 nm for the light from the X arm and $-1.03(16)$ dB at 1544.0 nm for the light from the Y arm. At the target wavelength of 1550 nm, the measured coupling efficiency is $-1.09(10)$ dB and $-1.25(11)$ dB for the light from the X and Y arms, respectively. Note that the simulated coupling efficiency shown here corresponds to the post-processed structure. (e) Measured polarization extinction ratio of the topology-optimized 2D grating coupler between two arms versus wavelength. At the wavelength of 1550 nm, the measured polarization extinction ratio is $57.31(50)$ dB and $54.41(22)$ dB for maximizing the power from X and Y arms, respectively. (f) Measured power dependency of the X (orange) and Y arms (blue), along with the sum of their powers (black), on the HWP angle $\theta$ at the wavelength of 1550 nm.