Vapour-cell spectroscopy is widely used for the frequency stabilisation of
diode lasers relative to specific atomic transitions - a technique essential in
cold atom and ion trapping experiments. Two laser beams, tuned to different
frequencies, can be overlapped on the same spatial path as an aid to
compactness; this method also enhances the resulting spectroscopic signal via
optical pumping effects, yielding an increase in the sensitivity of
spectroscopically-generated laser stabilisation signals. Doppler-free locking
features become visible over a frequency range several hundred MHz wider than
for standard saturated absorption spectroscopy. Herein we present the measured
Doppler-free spectroscopy signals from an atomic vapour cell as a function of
both laser frequencies, showing experimental data that covers the full, 2D
parameter space associated with dual-frequency spectroscopy. We consider how
dual-frequency spectroscopy could be used for enhanced frequency-stabilisation
of one laser, or alternatively to frequency-stabilise two lasers
simultaneously, and analyse the likely performance of such stabilisation
methods based on our experimental results. We discuss the underlying physical
mechanism of the technique and show that a simple rate-equation model
successfully predicts the key qualitative features of our resultsComment: 12 pages, 5 figure