Polarization spectroscopy of a closed atomic transition: applications to laser frequency locking

We study polarization spectroscopy of Rb vapour. A weak probe beam analyses the birefringence induced in a room temperature vapour by a strong counterpropagating circularly polarized pump beam. In contrast to most other work on polarization spectroscopy, we use a polarization beam splitting cube and two detectors (rather than a polarizer and one detector) to analyse the probe beam. The signal is in the form Of a derivative of a Lorentzian. For theoretical analysis we study the closed atomic transition 5 (2)S(1/2) (F = 3) --> 5 (2)p(3/2) (F' = 4) in the D2 line of (85)Rb. We study the time needed to redistribute population among the m(F) states, derive an expression for the expected lineshape and present experimental data in excellent agreement with theory. The polarization spectrum provides an ideal error signal for frequency stabilization of a laser. We describe the geometry and parameters for optimizing the error signal.