Experiments to Control Atom Number and Phase-Space Density in ...

(a) (b)
Figure 2.21: Saturated absorption spectroscopy spectrum. (a) In the absence of the
pump beam, the spectrum shows the Doppler-broadened profile. (b) In the presence of
the pump beam, a narrow peak appears in the spectrum, known as Lamb dip or hole
burning. Figure courtesy of Gabriel Price.
Assume there exists more than one transition separated by less then the Doppler
width, e.g. two distinct excited states. The spectrum will then display three lines. Two
of these lines are associated with the transitions from the ground to the two distinct
excited states. The third line is located midway between these transitions and is known
as cross-over transition or cross-over resonance. The emergence of this line is connected
with a group of atoms moving at a velocity v such that one transition is resonant with
the probe while the other is resonant with the pump beam.
The Doppler-broadend background can be removed by adding an acousto-optic
modulator (AOM) to the setup, see figure 2.22. In addition to the slow frequency
sweep of the laser frequency, a fast dithering frequency (fdither) is added to the pump
beam. The photodiode signal and the dither frequency fdither are then mixed in a lock-in
amplifier. Away from a transition the dither will not induce a change in the power of
the probe beam, and the signal from the lock-in amplifier will be close to zero. Near the
resonance however, the small change in the pump laser frequency dramatically changes
the absorption profile. The generated error signal is proportional to the slope of the
saturation absorption spectrum and the error signal is thus the Doppler-free derivative
of the spectrum. This error signal provides a good signal for a frequency feedback loop.
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