Low-loss and tight-bend waveguides via multiple scans of a femtosecond laser

In this paper, in situ multiple scans of a femtosecond laser are employed to optimize the refractive index distribution in a glass optical waveguide. The emergence of second bright core boundary (SBCB) and the increasing density of three- and four-membered rings in silica were thoroughly investigated. They can lead to a higher refractive index in a waveguide core and enhance the light confinement capability, which can increase the spatial integration density of bent waveguide structures. A 90° tightly bent single-mode waveguide is prepared at 808 nm, achieving a minimum bending loss of 1.028 dB/cm at a bending radius of 10 mm. This study will facilitate the preparation of on-chip large-scale, highly integrated photonic chips.