crossovers are more prominent than the actual peaks because two velocity classes contribute to each crossover resonance, compared to one (zero-velocity) class for each hyperfine peak. Probe transparency is primarily caused by two effects: (i) saturation of absorption caused by the strong pump beam, and (ii) optical pumping into the $F=1$ ground hyperfine level for open transitions (i.e. those involving the $F'=1$ and 2 excited levels). In addition, there will be population redistribution among the magnetic sublevels, which can cause increased absorption or transparency depending on the $F$ values of the levels. The linewidth of the peaks in the figure is about 12 MHz, compared to the natural linewidth of 6 MHz. This increase is typical in SAS and arises due to a misalignment angle between the beams and power broadening.

(a)\
(b)\
(c)

Now let us consider the spectrum shown in (b) taken with the co-propagating configuration. The probe is locked to the $F=2 \rightarrow 3$ transition and the pump is scanned across the set of $F=2 \rightarrow F'=1,2,3$ transitions. Since the probe is locked, its transmitted signal primarily corresponds to absorption by zero-velocity atoms (i.e.atoms moving perpendicular to the laser beam) making transitions to the $F'=3$ level. The 