Saturated-absorption spectroscopy: Eliminating crossover resonances using co-propagating beams

We demonstrate a new technique for saturated-absorption spectroscopy using co-propagating 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 $^{85}$Rb with a precision of 70 kHz, and to resolve hyperfine levels in the $D_2$ line of $^{39}$K that are less than 10 MHz apart.