  An experimental method is developed for the robust frequency stabilization
using a high-finesse cavity when the laser exhibits large intermittent
frequency jumps. This is accomplished by applying an additional slow feedback
signal from Doppler-free fluorescence spectroscopy in an atomic beam with
increased frequency locking range. As a result, a stable and narrow-linewidth
556 nm laser maintains the frequency lock status for more than a week, and
contributes to more accurate evaluation of the Yb optical lattice clock. In
addition, the reference optical cavity is supported at vibration-insensitive
points without any vibration isolation table, making the laser setup more
simple and compact.
