  We report on double-resonant highly efficient sum-frequency generation in the
blue range. The system consists of a 10-mm-long periodically poled KTP crystal
placed in a double-resonant bow-tie cavity and pumped by a fiber laser at
1064.5 nm and a Ti:sapphire laser at 849.2 nm. An optical power of 375 mW at
472.4 nm in a TEM$_{00}$ mode was generated with pump powers of 250 mW at 849.2
nm and 200 mW at 1064.5 nm coupled into the double-resonant ring resonator with
88$\%$ mode-matching. The resulting internal conversion efficiency of 95($\pm
3$)$\%$ of the photons mode-matched to the cavity constitutes, to the best of
our knowledge, the highest overall achieved quantum conversion efficiency using
continuous-wave pumping. Very high conversion efficiency is rendered possible
due to very low intracavity loss on the level of 0.3$\%$ and high nonlinear
conversion coefficient up to 0.045(0.015) W$^{-1}$. Power stability
measurements performed over one hour show a stability of 0.8$\%$. The generated
blue light can be tuned within 5 nm around the center wavelength of 472.4 nm,
limited by the phase-matching of our nonlinear crystal. This can however be
expanded to cover the entire blue spectrum (420 nm to 510 nm) by proper choice
of nonlinear crystals and pump lasers. Our experimental results agree very well
with analytical and numerical simulations taking into account cavity impedance
matching and depletion of the pump fields.
