Damage Evolution and Optical Degradation in Potassium Titanyl Phosphate During Long‐Term Nanosecond Second‐Harmonic Generation

This study presents an experimental investigation of second harmonic generation (SHG) in potassium titanyl phosphate (KTP) crystals subjected to long‐term nanosecond‐pulsed Nd:YAG irradiation (1064 nm, 6 ns, 1 Hz). The work evaluates the stability of SHG output alongside progressive optical degradation under a laboratory‐accessible configuration. Frequency‐doubled emission was characterized spectrally, confirming stable generation of a ~531 nm output, while the measured spectral widths are interpreted as instrument‐limited. Transmission measurements using He–Ne (632.8 nm) and violet diode (400 nm) probes revealed a pronounced time‐dependent loss of transparency, with a steeper reduction at 632.8 nm. Photoluminescence spectroscopy revealed irradiation‐induced modifications, including a ~18 nm blue shift of the principal emission peak, broadening of the emission profile, and the emergence of a defect‐related luminescence band centered near 525 nm. Complementary fluorescence imaging under ultraviolet, visible, and white‐light excitation visualized spatially localized defect formation and heterogeneous damage development. The findings highlight the trade‐off between short‐term SHG performance and long‐term reliability and provide a practical diagnostic workflow (transmission + PL + fluorescence imaging) for monitoring degradation and informing robust operation of ns‐pulsed SHG systems under similar conditions. The study is intended as a practical degradation analysis under fixed laboratory operating conditions rather than a formal laser‐damage‐threshold investigation.