Modulation-free frequency stabilization system of external cavity diode laser based on Sagnac loop

A novel frequency stabilization method of ECDL using a phase difference biased Sagnac loop was proposed. This scheme uses polarization beam splitter (PBS) for input and output of Sagnac loop instead of the beam splitter, and a quarter waveplate to introduce a bias phase difference of /2 between the counterpropagating pump and probe beams with perpendicular polarization. A polarizer is inserted after the output of the Sagnac loop, so that the dispersion phase shift induced by Rb resonance can be obtained. By this method, an error signal with larger amplitude can be acquired, which provides an ideal error signal to stabilize the frequency of the laser. Compared with the previously proposed method, whose bias phase difference is introduced by polarization dependent phase shift in total-internal-reflection(sin=0.64), the amplification factors of dispersive signal sin can be achieved to theoretical maximum. This scheme is simple, robust, and has potential applications in atomic physics experiment and so on.