Low-Noise Dual-Frequency Laser for Compact Cs Atomic Clocks

We report the dual-frequency and dual-polarization emission of an optically-pumped vertical external-cavity semiconductor laser. Our laser source provides a high-purity optically-carried RF signal tunable in the GHz range, and is specifically designed for the coherent population trapping (CPT) of Cs atoms in compact atomic clocks. The laser spectrum is stabilized onto a Cs atomic transition at 852.1 nm, and the frequency difference is locked to a local oscillator at 9.2 GHz. Special attention has been paid to the evaluation of the frequency, intensity and phase noise properties. A maximum phase noise of -90 dBrad²/Hz has been measured. Finally, we estimate the contribution of the laser noise on the short-term frequency stability of a CPT atomic clock, and predict that a value below 3 × 10^-13 over one second is a realistic target.