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

In the derivation for the optical density, it was assumed that the probe beam
intensity I satisfies I ≪ Isat. Unfortunately this is not the regime with the best signal-
to-noise ratio. The best signal-to-noise ratio is found for intensities I ≈ Isat. However,
for a large enough number of atoms it is possible to achieve adequate signal-to-noise
ratios even in the low-intensity regime.
7.5.2 Fluorescence Imaging and Calibration
Fluorescence imaging of lithium atoms is very straight forward, and no special
considerations have to be made. Even though fluorescence imaging is simple it is very
useful for finding the image plane and calibrating the magnification of the imaging
system.
The image plane, and thus the position of the camera, can be found using a
split-imaging technique. A mask with two or more holes is placed directly behind the
imaging lens, as shown in figure 7.45. The fluorescence signal is then collected only
through those two holes. If the camera is not placed in the image plane, the two images
formed by the two spots will not overlap; they coincide only in the image plane. This
method is a quick and easy way to locate the image plane.
MOT
object
distance
image
distance
Figure 7.45: Schematic setup for finding the imaging plane using a split-imaging technique.
A mask with two or more holes is placed behind the imaging lens. The two
images of the MOT coincide only in the imaging plane.
CCD
The magnification of an imaging system is typically determined by taking a series
of images showing atoms in free fall. The center of mass position can then be used to
calibrate the magnification. This method, however, has a couple of problems. Any
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