Single-Photon Atomic Cooling - Raizen Lab - The University of ...
Single-Photon Atomic Cooling - Raizen Lab - The University of ...
Single-Photon Atomic Cooling - Raizen Lab - The University of ...
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oadened background can be removed from this signal by adding a fast fre-<br />
quency dither to the pump beam and then mixing the saturation absorption<br />
spectrum with the dither frequency ωdither in a lock-in amplifier (see Fig. 2.17).<br />
<strong>The</strong> removal <strong>of</strong> the Doppler broadened background occurs because if the laser<br />
beam is <strong>of</strong>f resonance then the pump and probe beam do not interact and a<br />
frequency dither <strong>of</strong> the pump beam does not induce an intensity modulation<br />
in the probe beam at ωdither. <strong>The</strong> lock-in amplifier output is therefore zero<br />
for all such laser frequencies. If however the laser frequency is near resonance<br />
then a frequency dither <strong>of</strong> the pump beam does induce a modulation in the<br />
intensity <strong>of</strong> the probe beam at ωdither. Furthermore, the amplitude <strong>of</strong> the mod-<br />
ulation is proportional to the slope <strong>of</strong> the saturation absorption spectrum so<br />
the output <strong>of</strong> the lock in amplifier is the Doppler background free derivative<br />
<strong>of</strong> the spectrum. This curve is perfect for use as an “error signal” in a laser<br />
frequency feedback loop because it is zero on resonance and has an opposing<br />
sign on either side <strong>of</strong> resonance.<br />
This scheme is used to lock our near resonance laser during the exper-<br />
iment. <strong>The</strong> detailed layout <strong>of</strong> our locking scheme can be found in Ch. 3.<br />
2.8 Imaging<br />
During the course <strong>of</strong> the experiment the atomic cloud is imaged so that<br />
we can extract information about the ensemble, such as its density distribution,<br />
total atom number and temperature. We use two atomic imaging techniques<br />
to accomplish this. <strong>The</strong> first, absorption imaging, relies on the atomic sample<br />
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