  We report on a simple and robust technique to generate a dispersive signal
which serves as an error signal to electronically stabilize a monomode cw laser
emitting around an atomic resonance. We explore nonlinear effects in the laser
beam propagation through a resonant vapor by way of spatial filtering. The
performance of this technique is validated by locking semiconductor lasers to
the cesium and rubidiumD2 line and observing long-term reduction of the
emission frequency drifts, making the laser well adapted for many atomic
physics applications.
