lower level. Such differences between the two levels have also been seen in EIT, arising from the fact that the closed transition is $F \rightarrow (F+1)$ for the upper hyperfine set and $F
\rightarrow (F-1)$ for the lower hyperfine set.

For a second set of experiments, we have used a single laser along with an acousto-optic modulator (AOM) to produce the pump-probe beams with a precisely-controlled frequency offset. We then obtain the entire spectrum by scanning the frequency of the AOM. The scan axis is guaranteed to be linear because it is determined by the frequency of the rf oscillator driving the AOM. A curve fit to the observed spectrum yields the hyperfine interval. The measurements have an accuracy of 20 kHz, which is comparable to the accuracy of other techniques.

In recent work from our laboratory, we have reported high-accuracy values for the hyperfine constants in the $D$ lines of all alkali atoms [@DAN08]. The hyperfine intervals in that work were obtained (with an accuracy of 6 kHz) by [*locking*]{} the AOM to the neighboring transition. One of the uncertainties when locking the AOM is whether the lock point is exactly at the center of the peak, since any 