We demonstrate a technique to lock the frequency of a laser to a transitionbetween two excited states in Rb vapor in the presence of a weak magneticfield. We use a ladder configuration from specific hyperfine sublevels of the5S 1/2, 5P 3/2, and 5D 5/2 levels. This atomic configuration can showElectromagnetically Induced Transparency and Absorption processes. The errorsignal comes from the difference in the transparency or absorption felt by thetwo orthogonal polarizations of the probe beam. A simplified model is in goodquantitative agreement with the observed signals for the experimentalparameters. We have used this technique to lock the frequency of the laser upto 1.5 GHz off atomic resonance.
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