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

distribution of the MOT master laser is shown in figure 4.8.
error signal
from
lock-box
MOT master
laser
λ/2
λ/2
optical
isolator
anamorphic
prism pair
AOM
@
103 MHz
to saturation
absorption
spectrometer
+1
λ/4
λ/2
+1
to slave
lasers
to Fabry-Perot
cavity
PBSC
AOM
@
80 MHz
Figure 4.8: Distribution fo the MOT master laser. Figure courtesy of Gabriel Price.
Figure 4.9 shows the layout of the saturated absorption spectroscopy setup used
to lock the MOT master laser. The beam is split into a pump and a probe beam using
an uncoated glass plate. This ensures that the probe beam is significantly weaker than
the pump beam. The pump beam is deflected by a polarizing beam splitter cube and
double passes a 44 MHz AOM. This AOM is modulated with a frequency of 7 kHz
and a modulation depth of 4 MHz. In the double pass the polarization of the beam
is rotated by 90 ◦ , passing a quarter waveplate twice. On the second incidence on the
polarizing beam splitter cube the pump beam thus passes through the cube and into the
rubidium vapor cell. The pump beam is therefore counter-propagating the probe beam.
At the same cube the probe beam is deflected and the signal is recorded with a fast
photodiode. The signal is mixed with the FM frequency at a lock-in amplifier (Stanford
Research Systems, SR510). The error signal that results from sweeping the laser across
the |F = 2〉 → |F ′ = 3〉 transition is shown in figure 4.10.
The laser is locked to the |F = 2〉 → |F ′ = 2/3〉 crossover line, as this is the
most prominent feature. The error signal is fed into a PID circuit and amplified by a
high-voltage amplifier (Trek 601B-2) to feed back onto the grating angle.
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