Dual-frequency Doppler-free spectroscopy for simultaneous laser stabilisation in compact atomic physics experiments

Vapour cell spectroscopy is an essential technique in many fields; in particular, nearly all atom and ion trapping experiments rely on simultaneous spectroscopy of two atomic transitions, traditionally employing separate apparatus for each transition. Here, we demonstrate simultaneous spectroscopy on two atomic transitions using spatially-overlapped beams from two independent lasers, within a single spectroscopic apparatus. We show that, in addition to aiding compactness, this approach offers superior performance, leading to sharper spectroscopy peaks and stronger absorption signals. Doppler-free locking features become visible over a frequency range several hundred MHz wider than for standard saturated absorption spectroscopy. By exploring the full, 2D parameter space associated with dual-frequency spectroscopy, we reveal a lattice-like structure of sharp resonance features in 2D frequency space, which enhances experimental versatility by allowing laser frequency stabilisation anywhere within a wide manifold of locations in 2D frequency space. The process of simultaneous frequency stabilisation of two lasers is analysed in detail, revealing that it can be expected to produce significant improvements in laser stability when compared to conventional, single-frequency spectroscopy.