Fiber-coupled laser-written PPKTP nonlinear optical waveguides for enhanced pure heralded single-photon and polarization-entangled photon pair generation

We report on fiber-coupled nonlinear optical waveguides in periodically poled potassium titanyl phosphate (PPKTP), fabricated via femtosecond laser direct writing for efficient quantum light generation through spontaneous parametric down-conversion. The platform achieves high nonlinear conversion efficiency with fiber integration for compact, alignment-free operation. We discuss applications including enhanced performance for low-efficiency parametric processes and high-visibility polarization-entangled photon pair generation. This work demonstrates laser-written PPKTP waveguides as promising building blocks for in-fiber integrated quantum photonic circuits and quantum technologies.