Fractional frequency instability in the 10−14 range with a thermal beam optical frequency reference

摘要

We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey–Bordé interferometer with spatially separated oscillatory fields on a thermal atomic beam. The S01↔P13intercombination line in neutral Ca40is used as a frequency discriminator, with an extended cavity diode laser at 423 nmprobing the ground state population after a Ramsey–Bordé sequence of 657 nmlight–field interactions with the atoms. Evaluation of the instability of the Ca frequency reference is carried out by comparison with (i) a hydrogen-maser and (ii) a cryogenic sapphire oscillator. In the latter case the Ca reference exhibits a square-root Λ variance of 9.2×10−14at 1 sand 2.0×10−14at 64 s. This is an order-of-magnitude improvement for optical beam frequency references, to our knowledge. The shot noise of the readout fluorescence produces a limiting square-root Λ variance of 7×10−14/τ√, highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 10−14range frequency instability.