Laser frequency stabilization using Doppler-free bi-polarization spectroscopy

We present a new sub-Doppler polarization spectroscopy based technique for locking the frequency of a laser system to an atomic transition. A near symmetric, background-free and dispersion-like reference signal for locking at the transition center without frequency modulation is obtained by subtracting polarization spectra of two probe beams which overlap with two oppositely circularly polarized pump beams. Using this technique, we demonstrate frequency locking of a diode laser to the 5 2S1/2 (F = 2) → 52P3/2 (F′ = 3) transition of 87Rb. The frequency fluctuations and the drift were measured to be less than 0.3 MHz and 0.05 MHz, respectively, for observation period of more than 3 h. A simple model description of this technique is also presented.