Microlens-Integrated Grating Couplers for Efficient Free-Space Coupling in Silicon Photonics

Abstract

Grating-coupler arrays are used to couple free-space light into photonic chips, but the space required for tapers and routing waveguides limits their optical fill factor. Microlens arrays can increase the effective fill factor by collecting light over a larger area and focusing it onto each grating. Separately fabricated lens arrays, however, require assembly and precise alignment to the chip. Here we integrate refractive microlenses directly with high-efficiency, normal-incidence grating couplers using two-photon polymerization. Each lens has a 100 x 100 um2 aperture and focuses incident light onto the grating. Writing the lens body at coarse resolution and the optical surface at fine resolution reduces the print time from 9 h to 19 min per lens. Under illumination by a Gaussian beam with a 276 um mode-field diameter, a printed lens increases the measured coupled power by a factor of 33 (15.2 dB) relative to the same grating before printing. Loss normalization gives an estimated lens-to-mode efficiency of 68%. The lenses show no measurable change in coupling after three 3 h thermal cycles at 150C. These results support direct printing as an approach to increasing free-space collection while simplifying lens integration in photonic systems.