Abstract

Achieving low-loss coupling between single-mode fibers and photonic integrated circuits plays a crucial role in emerging enabling technologies including quantum information, advanced imaging, and precision metrology. High coupling efficiency in the UV/visible spectrum remains challenging due to the small waveguide dimensions at shorter wavelengths. Here, we present an ultra-low loss edge coupler made of a SiOx/AlOx mode converter. The coupling loss between a standard single-mode fiber and an AlOx waveguide below 0.31 dB and 0.62 dB is experimentally demonstrated at a working wavelength of 375 nm and 405 nm, respectively. The design exhibits a large bandwidth of 110 nm below 2.6-dB loss over the entire operating wavelength range of the single mode fiber, as obtained from the simulation. This coupler not only facilitates low-loss fiber-to-chip coupling but also significantly reduces spurious light scattering, making it particularly valuable for applications in PIC-based far-field advanced microscopy and trapped-ion quantum information systems.