We propose a modulation-free technique for frequency stabilization of an external-cavity diode laser (ECDL) by using a phase-difference biased Sagnac interferometer to produce dispersion spectroscopic error signals. A half-wave plate and a total internal reflection prism are inserted into the loop to provide a phase-difference bias between the clockwise and counterclockwise beams with perpendicular polarizations, instead of the previous method with misaligned optical paths. In the experiments, the frequency of the Littman–Metcalf configuration ECDL is locked at the transition of the Rb atomic vapor, and the frequency fluctuation is suppressed from 8 to less than peak to peak. It is shown that this scheme is simple, robust, low cost, and it shows promise for use in a variety of related applications.
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