Dual-frequency operation of a vertical external cavity semiconductor laser for coherent population trapping cesium atomic clocks

Cs atomic clocks based on coherent population trapping require two phase-locked laser lines with output power in the 10-mW range and a frequency difference of about 9 GHz to provide the microwave interrogation. Stabilization of one laser frequency to the reference laser with a phase-locked loop and high-frequency modulation of a diode laser are the two most frequent solutions. Alternatively, dual-frequency operation of a single laser source might provide the simplest architecture. It is based on the simultaneous emission of two orthogonally-polarized laser beams sharing the same laser cavity, but with a slight anisotropy resulting in the frequency difference. The major advantage of this configuration lies in the fact that the frequency fluctuations of the two beams are strongly correlated. Such dual frequency oscillation has already been observed with rare-earth doped material lasers (Alouini et al., 1998); recently it has also been demonstrated with a 1-μm vertical-external cavity semiconductor laser (VECSEL), with the benefit of low phase and intensity noise thanks to the class-A regime dynamics of VECSEL (Cocquelin et al., 2009). In this work, we describe the first dual-frequency operation of an optically-pumped VECSEL emitting around the Cs D₂ line at 852 nm.