Laser-written 3D waveguide beam splitters in LBO crystals for second-harmonic generation with tailored modes

We have fabricated two categories of three-dimensional (3D) 1 × 3 waveguide beam splitters by femtosecond laser in lithium triborate (LBO) crystals. Both splitters are composed of laser-written depressed-cladding waveguides with circular cross-sections, and can realize efficient second-harmonic generation (SHG) when excited with a 1031-nm fundamental wavelength. With our beam splitter, a record SHG conversion efficiency of 9.22% has been achieved. By using laser-writing technique to tailor the cross-section of each output port of the splitter, which has three identical output ports, we have achieved the mode evolution of both fundamental and SHG waves from multi-mode to single-mode at three engineered output ports. This work successfully integrates 3D beam splitting with frequency conversion in a nonlinear crystal for the first time, to the best of our knowledge, paving a way to fabricate high-efficiency, and energy-saving photonic integrated circuits (PICs).