Precise mode control of laser-written waveguides for broadband,low-dispersion 3D integrated optics

Three-dimensional(3D)glass chips are promising waveguide platforms for building hybrid 3D photonic circuits due to their 3D topological capabilities,large transparent windows,and low coupling dispersion.At present,the key challenge in scaling down a benchtop optical system to a glass chip is the lack of precise methods for controlling the mode field and optical coupling of 3D waveguide circuits.Here,we propose an overlap-controlled multi-scan(OCMS)method based on laser-direct lithography that allows customizing the refractive index profile of 3D waveguides with high spatial precision in a variety of glasses.On the basis of this method,we achieve variable mode-field distribution,robust and broadband coupling,and thereby demonstrate dispersionless LP21-mode conversion of supercontinuum pulses with the largest deviation of<0.1 dB in coupling ratios on 210 nm broadband.This approach provides a route to achieve ultra-broadband and low-dispersion coupling in 3D photonic circuits,with overwhelming advantages over conventional planar waveguide-optic platforms for on-chip transmission and manipulation of ultrashort laser pulses and broadband supercontinuum.