We describe the dual-frequency operation of an optically-pumped vertical external cavity semiconductor laser (VECSEL) stabilized onto an Cs atomic transition. It is based on the simultaneous emission of two cross-polarized adjacent longitudinal modes inside the same laser cavity, which provides a strong correlation between the two laser lines. The frequency difference, in the GHz range, is fixed by the intracavity phase anisotropy, and precisely tuned with an electro-optic modulator (EOM). For this work, we additionally take benefit of the class-A dynamical behaviour of VECSEL which results in a shot-noise limited relative-intensity-noise on a wide spectral range.

The GaAs/AlGaAs active structure is pumped with a 1W-fiber coupled laser diode at 670 nm. The laser cavity has been carefully designed for improved thermal and mechanical stability, and compactness. It consists in a 15-mm concave output coupler, a glass Fabry-Perot etalon, a YVO₄ birefringent plate and a MgO:SLT EOM. The output power at each frequency reaches 20 mW. The frequency difference is phase-locked to a microwave reference source through the EOM voltage with a MHz bandwidth, resulting in a high-purity optically-carried microwave signal. Simultaneously, one laser line is locked on a Cs atomic hyperfine transition at 852 nm through a low-bandwidth servo-loop on the cavity length. The performance of our laser source is thus fully compatible with the excitation of Cs atoms in coherent population trapping atomic clocks.