  The stabilization of lasers to absolute frequency references is a fundamental
requirement in several areas of atomic, molecular and optical physics. A range
of techniques are available to produce a suitable reference onto which one can
'lock' the laser, many of which depend on the specific internal structure of
the reference or are sensitive to laser intensity noise. We present a novel
method using the frequency modulation of an acousto-optic modulator's carrier
(drive) signal to generate two spatially separated beams, with a frequency
difference of only a few MHz. These beams are used to probe a narrow absorption
feature and the difference in their detected signals leads to a dispersion-like
feature suitable for wavelength stabilization of a diode laser. This simple and
versatile method only requires a narrow absorption line and is therefore
suitable for both atomic and cavity based stabilization schemes. To demonstrate
the suitability of this method we lock an external cavity diode laser near the
$^{85}\mathrm{Rb}\,5S_{1/2}\rightarrow5P_{3/2}, F=3\rightarrow F^{\prime}=4$
using sub-Doppler pump probe spectroscopy and also demonstrate excellent
agreement between the measured signal and a theoretical model.
