An external cavity 1540-nm diode laser was frequency doubled in a 3-cm-long periodically poled LiNbO3 waveguide doubler with 150% W-1 conversion efficiency, thereby generating more than 3 mu W at 770 nm. Second-harmonic light was used to detect and lock to sub-Doppler lines of the K-39 D-1 transition. (C) 1998 Optical Society of America.