Integrated microlens-assisted perfectly vertical grating coupler for enhanced coupling efficiency

Perfectly vertical grating couplers (VGCs) are essential for the development of high-density, low-cost, and scalable photonic devices. This paper introduces an integrated microlens-assisted grating coupler (GC) designed to enhance coupling efficiency under vertical incidence conditions. The microlens, fabricated through a thermal reflow process, is integrated onto a standard 220 nm silicon-on-insulator (SOI) grating coupler. This hybrid integration approach provides greater flexibility in manipulating the transmission angle of vertically incident light, aligning it with the optimal coupling angle of the underlying grating, thereby effectively improving the overall coupling efficiency (CE) of the device. Our proposed microlens-assisted grating coupler achieves 2 dB higher coupling efficiency under the TE mode in the C-band compared to a bare grating coupler, significantly reducing alignment complexity and enabling wavelength division multiplexing applications. Furthermore, this methodology is applicable to polarization-insensitive vertical grating couplers, providing an increase of 1.3 dB and 1.4 dB for TE and TM modes, respectively. Compared to traditional nanostructures, our method offers simpler fabrication processes and more flexible optical characteristics, highlighting its significant potential for applications in ultra-high-capacity optical communications, high-performance optical detection, real-time biochemical sensing, and other related fields.