Abstract
The background remaining in the first derivative signal of saturated absorption was removed by means of the dual lock-in amplifiers in series, which were synchronized with a modulation frequency of the laser frequency and a chopping frequency of the saturating beam, respectively. Using the background-free first derivative signal, the dye laser frequency was stabilized to an iodine hyperfine transition with a small modulation width of 500 kHz. The frequency stability of 2×10-11 was obtained.