  We report a simple technique for stabilization of a laser frequency at the
wings of an atomic resonance. The reference signal used for stabilization
issues from interference effects obtained in a low-quality cavity filled with a
resonant atomic vapour. For a frequency detuned at 2.6 GHz from the $^{133}$Cs
D$_2$ 6S$_{1/2}$ F=4 to 6P$_{3/2}$ F'= 5 transition, the fractional frequency
Allan deviation is $10^{-8}$ for averaging times of 300 s, corresponding to a
frequency deviation of 4 MHz. Adequate choice of the atomic density and of the
cell thickness allows locking the laser at detunings larger than 10 GHz. Such a
simple technique does not require magnetic fields or signal modulation.
