Dual-frequency spectroscopy for compact systems and enhanced laser stabilisation

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; we show that this method also enhances the resulting spectroscopic signal via optical pumping effects, yielding an estimated increase in the sensitivity of spectroscopically-generated laser stabilisation signals by a factor of 3. 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 and develop a simple theoretical model that explains the key features of the technique. We also discuss how the method can be used to frequency-stabilise two lasers simultaneously, or to provide enhanced stabilisation for one laser.