Modulation-Free Frequency Stabilization Based on Polarization-Split Sagnac Loop

A novel frequency stabilization method for external cavity diode lasers by using an all-pass polarization-split Sagnac loop is proposed. This scheme uses a quarter waveplate to convert the incident beam to a circularly polarized beam, and a polarization beam splitter for the input and output of the Sagnac loop. The two beams with perpendicular polarizations counter-propagate in the loop and experience different absorption of the Rb cell; therefore, the Sagnac loop output beam is converted into elliptical polarization. The dispersion phase shift induced by Rb saturation absorption can be obtained by balanced polarization-divided detection of the Sagnac loop output beam. The experimental results showed the feasibility and merits of the new method. This scheme is simple, robust, low-cost, and has potential applications in atomic physics experiments.