Experiments to Control Atom Number and Phase-Space Density in ...
Experiments to Control Atom Number and Phase-Space Density in ...
Experiments to Control Atom Number and Phase-Space Density in ...
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In equilibrium, the temperature of the MOT is determ<strong>in</strong>ed by the comb<strong>in</strong>ation of<br />
beam <strong>in</strong>tensities <strong>and</strong> beam detun<strong>in</strong>g. At any given po<strong>in</strong>t dur<strong>in</strong>g the compression ramp<br />
<strong>to</strong>o large an <strong>in</strong>tensity leads <strong>to</strong> heat<strong>in</strong>g of the a<strong>to</strong>ms, while <strong>to</strong>o low an <strong>in</strong>tensity leads <strong>to</strong><br />
traploss. The ideal ramp shape therefore balances those two compet<strong>in</strong>g effects, <strong>and</strong> the<br />
ideal ramp shape will m<strong>in</strong>imize the temperature <strong>and</strong> maximize the number of a<strong>to</strong>ms <strong>in</strong><br />
the MOT.<br />
8.5 Lifetime of the Optical Dipole Trap<br />
After the successful load<strong>in</strong>g of a<strong>to</strong>ms <strong>in</strong><strong>to</strong> the optical dipole trap, the lifetime of<br />
a<strong>to</strong>ms <strong>in</strong> the trap is measured. Unfortunately, the determ<strong>in</strong>ed lifetime was only on the<br />
order of 15 seconds, much lower than the expected value. The ma<strong>in</strong> reason for chos<strong>in</strong>g<br />
the CO2 laser <strong>to</strong> create the optical dipole trap is its low <strong>in</strong>tensity noise. The low <strong>in</strong>tensity<br />
noise of the laser should, <strong>in</strong> pr<strong>in</strong>ciple, translate <strong>in</strong><strong>to</strong> a long lifetime <strong>in</strong> the optical dipole<br />
trap.<br />
With the assumption that the laser itself has low <strong>in</strong>tensity noise, the most likely<br />
source of noise is the RF source driv<strong>in</strong>g the CO2 amplifier. The orig<strong>in</strong>al RF source is<br />
a homebuilt DDS board. Noise near the trapp<strong>in</strong>g frequencies of the optical dipole trap<br />
is the most detrimental, so frequency spectra around the 40 MHz driv<strong>in</strong>g frequency are<br />
taken. However, the noise spectra are <strong>in</strong>conclusive.<br />
The next step is <strong>to</strong> directly measure the noise <strong>in</strong> the laser beam itself. To measure<br />
noise <strong>in</strong> the kHz range of the <strong>in</strong>frared laser beam a Vigo system pho<strong>to</strong>diode (PVM-<br />
10.6) is used. The pho<strong>to</strong>current is then amplified with a low-noise amplifier (Stanford<br />
Research Systems SR560). Special care is taken <strong>to</strong> not exceed the saturation value of<br />
this pho<strong>to</strong>detec<strong>to</strong>r. A schematic of the optical setup is shown <strong>in</strong> figure 8.12.<br />
99% of the beam power are reflected out of the beam us<strong>in</strong>g a ZnSe beam splitter<br />
(not shown figure 8.12). The experimental setup is then simulated us<strong>in</strong>g an identical lens<br />
(f = 5 <strong>in</strong>) <strong>to</strong> the lens <strong>in</strong> the experimental setup <strong>to</strong> focus the laser beam. A ZnSe w<strong>in</strong>dow<br />
with a thickness of 3 mm imitates the ZnSe w<strong>in</strong>dow <strong>in</strong> the viewport. The light then<br />
passes through an uncoated CaF2 w<strong>in</strong>dow with a thickness of 5 mm. This w<strong>in</strong>dow is<br />
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