  Avoiding laser frequency drifts is a key issue in many atomic physics
experiments. Several techniques have been developed to lock the laser frequency
using sub-Doppler dispersive atomic lineshapes as error signals in a feedback
loop. We propose here a two-beam technique that uses non-linear properties of
an atomic vapor around sharp resonances to produce sub-Doppler dispersive-like
lineshapes that can be used as error signals. Our simple and robust technique
has the advantage of not needing either modulation or magnetic fields.
