peaks remain quite prominent with good signal-to-noise ratio. The additional peak at $-157$ MHz increases in height, but not significantly. By comparison, a good saturated-absorption spectrum requires the pump-probe power ratio to be accurately controlled to a value of $3:1$, with loss in signal at lower pump powers and power broadening at higher powers. In Fig. \[coprop\](b), we show a multipeak Lorentzian fit to the spectrum measured with a pump power of 15 $\mu$W. The residuals show that the line shape of all the peaks is Lorentzian.

(a)\
(b)

Spectrum in $^{87}$Rb, $F=1 \rightarrow F'=0,1,2$
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The same advantages are seen in the spectrum of transitions starting from the lower ground hyperfine level ($F=1
\rightarrow F'=0,1,2$) shown in Fig. \[rb10\]. The Doppler-subtracted saturated-absorption spectrum on top has 6 peaks including the 3 crossover resonances. The spectrum with the co-propagating beams shown below is taken with the probe locked to the $F=1 \rightarrow 0$ transition. The beam powers are 9 $\mu$W (probe) and 15 $\mu$W (pump). It appears on a flat background and shows the 3 hyperfine peaks without any crossovers in between. The 3 hyperfine peaks are located at 0, $+72.223$ MHz, and $+229.166$ MHz. Two of the additional peaks 