The frequency stability of current state-of-the-art stabilized clock lasers are limited by thermal fluctuations of the ultra-stable optical reference cavities used for their frequency stabilization. In this work, we study the possibilities for surpassing this thermal limit by exploiting the nonlinear effects from coupling of an optical cavity to laser cooled atoms having a narrow transition linewidth.

Here, we have realized such a system where a thermal sample of laser cooled strontium-88 atoms are coupled to an optical cavity.

The strontium-88 atoms were probed on the narrow 1S0-3P1 inter-combination line at 689 nm in a strongly saturated regime. The dynamics of the atomic induced phase shift and absorption of the probe light were experimentally studied in details with the purpose of applications to laser stabilization.

The atomic sample temperature was in the mK range which brought this system out of the domains described by previous theoretical works. A new theoretical cavity-QED model, which took into account the effects of finite atomic velocities, was developed and the theoretical predictions showed great agreement with the experimental observations. It was predicted by the experimental and theoretical studies, that an increase of the cavity finesse would bring the studied system into an interesting domain, where laser stabilization performance may compete with the current state-of-the-art stable laser systems. A new vacuum cavity system with increased finesse was hence realized.

This vacuum cavity also brought the system into a new domain where collective emission processes were observed. Collective emission processes can potentially be exploited as active light sources with unprecedented spectral purity and preliminary investigations of this collective emission process were carried out. The studies presented in this work open novel possibilities for alternative and simple strategies for surpassing the state-of-the-art laser stabilization and for realizing active light sources involving collective emission from narrow-line atoms.