High-Efficiency Fiber-Chip Edge Coupler for Near-Ultraviolet Integrated Photonics

Abstract

Integrated photonics is demanded in applications operating at ultraviolet to visible wavelengths, such as atomic/quantum systems, on-chip broadband receivers, and far-field structured illumination autofluorescence microscopy. A fundamental challenge in these applications is efficient edge coupling from a single-mode fiber (SMF) to on-chip photonic components, which is critical for on-chip integration. In this paper, a high-efficiency edge coupler based on an alumina-on-insulator platform is introduced and experimentally validated. The coupler employs a symmetric double-tip taper and a multimode interference (MMI)-based optical combiner. The double-tip taper effectively expands the mode field diameter at the chip facet to match that of the SMF at the initial stage. Then the MMI-based combiner efficiently combines the two channels in the taper into a highly confined strip waveguide. A coupling loss of 2.85 dB/facet has been achieved for the transverse magnetic mode at the wavelength of 407 nm, which is the lowest insertion loss for fiber-chip coupling on this platform to the best of the knowledge. The design can significantly reduce the insertion loss associated with fiber-chip coupling, offering a key component for diverse areas ranging from atomic/quantum photonic integrated circuits and ultra-high capacity communications to optical microscopes.