Operating a near-concentric cavity at the last stable resonance

TL;DR

This paper demonstrates the operation of a near-concentric optical cavity at its last stable resonance, enabling strong atom-light interactions by maintaining a highly unstable cavity mode.

Contribution

The authors experimentally operate a near-concentric cavity at its last resonant length, overcoming stability challenges for enhanced light concentration.

Findings

Achieved stable operation at g=-0.999962(2)

Cavity beam waist of 2.4 μm at last resonance

Maintained resonance 207(13) nm shorter than critical point

Abstract

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.