ALL-OPTICAL MICROWAVE FREQUENCY STANDARD - A PROPOSAL

We describe a simple method to produce a clock transition with purely optical means by modulated pumping. We observe the field-independent g round state resonance of Rb-87 atoms using sinusoidal modulation of th e injection current of an AlGaAs laser diode emitting at 780 nm (FM mo dulation). We detect the 6.835 GHz resonance with a subharmonic modula tion frequency of 1.139 GHz. A high-contrast resonance peak is observe d and a condition for zero light shift is found. The linewidth is 3 kH z (at 6.835 GHz) in this preliminary experiment, due to the small size of the light beam (approximately 2 mm diameter) and the low buffer ga s pressure (680 Pa) that was used. In the first part of this article, we develop a theoretical model that explains the main features of the experiment which is described in the second part.