  A ~545 mW single-frequency tunable 520 nm green laser has been demonstrated
using a periodically-poled potassium titanyl phosphate (PPKTP) bulk crystal
based on single-pass third-harmonic generation (THG) of a 1560 nm laser via
single-pass second-harmonic generation (SHG) followed by single-pass
sum-frequency generation (SFG). In single-pass SHG, two cascaded
periodically-poled magnesium-oxide-doped lithium niobate (PPMgO:LN) crystals
were used, and ~3.5 W 780.25 nm doubled laser output is produced, corresponding
to maximum doubling efficiency of 26.8%. The system can provide a pump source
(520 nm) for an optical parametric oscillator for two-color entangled
continuous-variable optical field generation at 1560 and 780 nm and two-color
local oscillators for homodyne detection.
