  We demonstrate a technique to lock the frequency of a laser to a transition
between two excited states in Rb vapor in the presence of a weak magnetic
field. We use a ladder configuration from specific hyperfine sublevels of the
5S 1/2, 5P 3/2, and 5D 5/2 levels. This atomic configuration can show
Electromagnetically Induced Transparency and Absorption processes. The error
signal comes from the difference in the transparency or absorption felt by the
two orthogonal polarizations of the probe beam. A simplified model is in good
quantitative agreement with the observed signals for the experimental
parameters. We have used this technique to lock the frequency of the laser up
to 1.5 GHz off atomic resonance.
