Abstract
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 intercombination line in neutral is used as a frequency discriminator, with an extended cavity diode laser at probing the ground state population after a Ramsey–Bordé sequence of 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 at and at . 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 , highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with range frequency instability.
© 2010 Optical Society of America
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