Saturated-absorption spectroscopy: eliminating crossover resonances by use of copropagating beams

We demonstrate a new technique for saturated-absorption spectroscopy by use of copropagating beams that does not have the problem of crossover resonances. The pump beam is locked to a transition, and its absorption signal is monitored while the probe beam is scanned. As the probe comes into resonance with another transition, the pump absorption is reduced and the signal shows a Doppler-free dip. We use this technique to measure hyperfine intervals in the D-2 line of Rb-85 with a precision of 70 kHz and to resolve hyperfine levels in the D-2 line of K-39 that are less than 10 MHz apart. (C) 2003 Optical Society of America.