1. Radiation Trapping in O-RAFS
Ben Quinby Space Vehicles Scholar RVBY Dr. Nathan Lemke
Radiation Trapping
 420 nm fluorescence radiation is resonant with the
background Rubidium vapor
 Higher cell temperature makes a more efficient clock
 Higher temperature leads to more capture events
 Captured fluorescence is rarely released back as
420 nm light, leading to diminished fluorescence
readings
AF Relevance
Quantifying radiation trapping in O-RAFS will provide a
better understanding of how this may affect the quality of
the atomic clock. A higher stability clock will improve the
performance and resiliency of satellite-based navigation
systems.
Optical Rubidium Atomic Frequency Standard
(O-RAFS)
 A simple optical clock built from off-the-shelf components
 Short-term stability exceeds that of current clocks by 10x
 No laser cooling
 Minimal magnetic shielding
 Small size, weight, and power
Summary
We can conclusively say that we have seen radiation
trapping in O-RAFS. With this data we should be able to
calculate the optimal temperature and size of the
rubidium vapor cell to maximize the clock stability per unit
of laser power.
The detection of red fluoresce for the servo feedback will
will be pursued further, as it does not suffer from the
effects of radiation trapping, allowing the vapor cell to be
operated at higher temperatures.
Figure 2: The error signal arises from modulating the
laser frequency across the atomic spectrum. The plots
are all scaled to match their ending values in order to
make meaningful comparisons.
Figure 3: Red and blue fluorescence plotted and scaled to
match values at 80°C. We can clearly see radiation
trapping of the blue fluoresce really start to dominate the
signal at around 100°C and above.
Figure 1: Energy levels and important transitions of
Rubidium in the O-RAFS experiment. The dotted line is
the virtual state which allows for the two photon
absorption. The blue 420 nm transition is the
fluorescence we typically detect. The 776 nm transition
is the red fluorescence investigated here.
Figure 4: O-RAFS experimental schematic. Retro-
reflecting the laser light yields a Doppler free spectrum.
Figure 5: O-RAFS experimental setup. The outer box is a
magnetic shield
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