Spectral and noise characterization of a 852 nm dual-frequency VECSEL

Coherent population trapping (CPT) is a common technique used in compact atomic clocks which requires two-phase coherent laser modes with a frequency difference in the GHz range for alkali atoms [1]. To improve the performance vs size trade-off of Cs atomic clocks, we develop a laser source generating two crosspolarized coherent laser fields at 852 nm. It relies on the dual-frequency and dual-polarization operation of an optically-pumped vertical external-cavity semiconductor laser [2]. This emission is induced by intracavity birefringent components which induce a controllable phase anisotropy within the laser cavity and force emission on two cross-polarized longitudinal modes. The semiconductor (SC) chip is grown on a 350 μm thick GaAs substrate and includes 7 GaAs quantum wells (8 nm-thick) embedded in Al0.22Ga0.78As barriers, and a high reflectivity Bragg reflector composed of 32.5 pairs of AlAs/Al0.22Ga0.78As. The semiconductor micro-cavity, consisting in the Bragg mirror and the air/semiconductor interface, is resonant at the emission wavelength and thus enhances the modal gain. Under continuous pumping, the SC chip delivers up to 8% optical gain. The 10 mm long cavity includes a birefringent YVO4 plate which induces a 50 μm separation modes inside the semiconductor structure and an electro-optic crystal (MgO:SLT) for continuous tuning of the frequency difference. Additionally, a wedged Fabry-Perot etalon made of a 100 μm-thick YVO4 is inserted in the cavity. The frequency difference between the cross-polarized modes is tunable by rotating the etalon thanks to its intrinsic birefringence: a 2° angle corresponds to a frequency difference of 9 GHz. And the tuning of the laser central wavelength is achieved by translation of the etalon.