  We report a simple method to suppress the light shift in optical pumping
systems. This method uses only frequency modulation of a radio frequency or
microwave source, which is used to excite an atomic resonance, to
simultaneously lock the source frequency to the atomic resonance and lock the
pumping light frequency to suppress the light shift. We experimentally validate
the method in a vapor-cell atomic clock and verify the results through
numerical simulation. This technique can be applied to many optical pumping
systems that experience light shifts. It is especially useful for atomic
frequency standards because it improves long-term performance, reduces the
influence of the laser, and requires less equipment than previous methods.
