Photonic lattice-like waveguides in glass directly written by femtosecond laser for on-chip mode conversion

We report on a conceptually new type of waveguide in glass by femtosecond laser direct writing, namely, photonic lattice-like waveguide (PLLW). The PLLW's core consists of well-distributed and densified tracks with a sub-micron size of 0.62 µm in width. Specifically, a PLLW inscribed as hexagonal-shape input with a ring-shape output side was implemented to converse Gaussian mode to doughnut-like mode, and high conversion efficiency was obtained with a low insertion loss of 1.65 dB at 976 nm. This work provides a new freedom for design and fabrication of the refractive index profile of waveguides with sub-micron resolution and broadens the functionalities and application scenarios of femtosecond laser direct-writing waveguides in future 3D integrated photonic systems.