Active optical frequency standards provide interesting alternatives to their passive counterparts. Particularly, such a clock does not require any local oscillator to keep the optical phase during a dead time between interrogations, because it continuously generates high-stable narrow-line laser radiation. Here we investigate the spectral properties and the stability of active clocks, including homogeneous and inhomogeneous broadening effects. We find that for short averaging times the stability is limited by shot noise from the limited emitted laser power and at long averaging times by phase diffusion of the laser output. Operational parameters for best long-term stability were identified. Using realistic numbers for an active clock with 87 Sr we find that an optimized stability of σy(τ ) ≈ 4 × 10 −18 / τ [s] is achievable.

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