Operating a near-concentric cavity at the last stable resonance

Near-concentric optical cavities of spherical mirrors can provide technical advantages over the conventional near-planar cavities in applications requiring strong atom-light interaction, as they concentrate light in a very small region of space. However, such cavities barely support stable optical modes, and thus impose practical challenges. Here, we present an experiment where we maintain a near-concentric cavity at its last resonant length for laser light at 780 nm resonant with an atomic transition. At this point, the spacing of two spherical mirror surfaces is 207(13) nm shorter than the critical concentric point, corresponding to a stability parameter g=-0.999962(2) and a cavity beam waist of 2.4 $\mu$m.