Ph.D. Thesis - Physics

Figure 7-21: The two internal lenses mounted on the 40 K shield, above an ion trap. The
top of the 40 K shield is mounted in the octagon, and may rotate by small amounts. The
lenses are also mounted on a separate copper plate that can move freely, for centering the
lenses. The lenses are mounted on a cage system, so that the focus to the trap can be
adjusted.
The detection and PI beams were made to counter-propagate across the trap. This
setup has certain advantages and disadvantages. Counter-aligning was more practical given
the geometry of the experimental setup and the space available on the optical table. Also,
aligning was fairly easy. Normally, neutral strontium atoms would be observed using the
460 nm light. The beam of neutrals was centered on the trap, and then the height of
the beam was adjusted to the height computed by CPO (about 1.3 mm). The detection
lasers were then counter-aligned to this beam. Typically only small adjustments in the
laser positions were then required to detect trapped ions. The disadvantage is mainly that
back-reflections from the PI optics caused additional 422 nm scatter, which was reduced
when the PI lasers were blocked after loading.
Detection of ions was done on an Santa Barbara Instruments Group (SBIG) ST-3200ME
CCD camera with a 422 nm interference filter in place (as in Ch. 5). The imaging optics
are as follows. Inside the cryostat, two lenses were mounted on the 40 K shield, a 20 mm
aspheric lens and a 150 mm achromat. The focusing light emerging from the cryostat
was reflected off a silver mirror into another 150 mm achromat, which collimated the light
and served as the focusing lens. A final 150 mm achromat focused the image onto the
CCD. Fig. 7-21 shows the lens assembly, including the internal 20 mm asphere and 150 mm
achromat, mounted above an ion trap.
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