CW frequency doubling of 1029 nm radiation using single pass bulk and waveguide PPLN crystals

Abstract Following various works on second harmonic process using periodically poled Lithium Niobate crystals (PPLN), we report on the performances comparison between commercial bulk and proton-exchanged waveguide crystals operating at 1029 nm. We use a continuous wave (CW) amplified Yb doped single fiber laser delivering up to 500 mW in single mode regime. In case of bulk crystal we generate 4 mW using 400 mW IR power. The use of waveguide crystal leads to an increase of the harmonic power up to 33 mW with input IR power limited to 200 mW. Nevertheless, this impressive efficiency was affected by the long term degradation of the non-linear waveguide crystal. A decrease of about 40% of the conversion efficiency was observed after 2 years of use, neither associated to the degradation of the coupling efficiency nor to that of the AR coating. This behavior could be attributed to a slow diffusion of protons in the guide.