Saturated absorption spectroscopy of calcium in a hollow-cathode lamp

We report on a technique for obtaining dispersive saturated absorption features by two pump beams derived from an acousto-optic modulator. One of the beams is frequency shifted by the modulator allowing for a variable frequency offset of the resulting signal. They are used to saturate the atomic transition in the low-density vapour of the lamp. The sensitivity of the technique is sufficient to observe clear signals from isotopes of calcium unresolved by the inhomogeneous broadening and with natural abundances of less than 0.2%. The technique is used for offset locking the frequency of a Ti:Sapphire laser used for laser cooling and trapping of calcium