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

We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey-Borde interferometer with spatially separated oscillatory fields on a thermal atomic beam. The 1S0↔3P1 intercombination line in neutral 40Ca is used as a frequency discriminator, with an extended cavity diode laser at 423 nm probing the ground state population after a Ramsey-Borde sequence of 657 nm light-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 9.2x10-14 at 1 s and 2.0x10-14 at 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 7x10-14/√(τ), highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 10-14 range frequency instability.