Abstract

It was shown that cesium double transitions could be used to obtain very high contrast Ramsey fringes [1]. Such a scheme needs to uses lin⊥lin configuration, where two linear and orthogonal laser beams couple the atomic transitions, with a frequency detuning equal to the level splitting. The frequency difference should be very stable, and one solution is to use a dual–frequency vertical external cavity surface emitting lasers (DF–VECSEL) such as the one shown in Fig.1. A birefringent crystal in the cavity allows to emit slightly detuned orthogonal polarisations with a very stable frequency difference, which is targeted to be limited by the Dick effect [2]. However, in this reference, the laser noise contribution to the Allan standard deviation is dominant : the relative intensity noise (RIN) level of -115 dB/Hz technically limits the clock stability to σ_y (τ) = 1.6×10⁻¹² τ^−1/2 while state–of–the– art atomic clocks based on CPT give σ_y (τ) = 3.2×10⁻¹³ τ^−1/2 [3].

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