Ph.D. Thesis - Physics

C.4 Laser alignment and imaging
As discussed in the main text of the thesis, four lasers are used in the cryostat experiment.
Here, I will focus on the alignment procedure.
The PI lasers are aligned first, since they are brighter to the eye than the 422 laser. The
PI lasers are grazed across the center of the trap, then raised by a translation stage to the
correct height above the trap surface. For Uraniborg, this height is 1.3 mm. The detection
lasers are then counter-propagated against the PI lasers. This offers the advantage of very
straightforward alignment, but one disadvantage is that since the optics for the PI lasers are
not anti-reflection (AR) coated for 422 nm, back-reflections from the PI optics can increase
the scatter on the trap. It is possible to slightly adjust the angle at which the 422 and 1092
propagate to reduce this scatter.
The detection lasers are extended cavity diode lasers, but currently lack any fast ac-
tive stabilization. They may be locked to the Toptica wavemeter, which corrects drifts of
O(10 MHz) on a timescale of 2 s. This provides enough stability for the observation of ion
crystals, but would not be sufficient for experiments in which a well-defined laser detuning
is required (such as Doppler recooling).
Neutral strontium ions may be detected by removing the 422 nm interference filter in
front of the CCD camera, and shuttering all lasers other than the 460 nm laser. While
shuttering other lasers is not strictly necessary, it helps to reduce scatter and improve the
signal to noise ratio, since only 460 nm photons will carry the signal from the atomic beam.
A larger current is required to observe the beam of neutrals than for loading the trap. This
current may be as much as 3.5 A, compared to a loading current of 2.5 A or less. Therefore,
it is important to do a good alignment beforehand, to minimize the amount of time at which
the oven must be run at that high current.
C.5 What to do if you can’t trap ions
This section contains the author’s advice for debugging, in the event that ions cannot be
quickly trapped. The assumption is that the experimentalist is moving the position of the
detection lasers after each step, but not necessarily in a comprehensive fashion (covering
every spot on a 2-D grid). An ion cloud in Uraniborg is of a sufficient size (estimated to
be at least 200 µm) that an overlap of the laser with only part of the cloud will result in a
signal. Blocking and un-blocking the IR is a good way to tell ion signal apart from other
light, and switching off and back on the rf is the surest way to tell, especially if you have a
high loading rate.
So here are the debugging steps:
1. Double-check basic stuff: rf resonance is where you expect, temperature and pressure
are good, dc feedthrough is properly connected, etc.
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