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 ${}^1\mathrm{S}_0\leftrightarrow {}^3\mathrm{P}_1$ intercombination line in neutral ${}^{40}\mathrm{Ca}$ is used as a frequency discriminator, with an extended cavity diode laser at $423\mathrm{nm}$ probing the ground state population after a Ramsey–Bordé sequence of $657\mathrm{nm}$ 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 $9.2\times {10}^{-14}$ at $1\mathrm{s}$ and $2.0\times {10}^{-14}$ at $64\mathrm{s}$ . 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 $7\times {10}^{-14}∕\sqrt{\tau }$ , 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.