Implementation of nearly single-mode second harmonic generation by using a femtosecond laser written waveguiding structure in KTiOPO4 nonlinear crystal

We propose a hybrid photonic structure to realize guided-wave second harmonic generation (SHG) from near-infrared (NIR at 1064 nm) to visible (green light at 532 nm) wavelength with nearly single-mode output. The periodically arrayed tracks have been produced in KTiOPO4 nonlinear crystal by femtosecond laser writing, which enable efficient light field confinement with cladding-like refractive index distributions. Particularly, the track-cladding surrounded central region is free of any tracks, which serves as waveguiding cores. With designed core diameters, the structures could enable single-mode propagation at selected wavelength regime on purpose. In this work, the hybrid structure contains a larger-input core section and a connected smaller-output core section, which in principle supports nearly single mode for either fundamental pump beam or second harmonic beam in the input and output channel, respectively. Based on this hybrid structure we implement nearly single-mode SHG at 532 nm, and comparable normalized conversion efficiency (1.1%/W/cm) in the continuous-wave (CW) regime is obtained with respect to that (1.2%/W/cm) of multimode SHG from a single large-core channel structure. This work paves the way to realize mode profile controlling for selected wavelength by using laser-written arrayed tracks.