Single-Photon Atomic Cooling - Raizen Lab - The University of ...

ack into the AOM in a standard double pass configuration. The 1 st order spot
from the double passed beam passes through the PBSC once again, but because
it has passed through a λ/4 waveplate twice its polarization has rotated 90 ◦
and so is transmitted by the cube. The pump beam then passes though the
Rb vapor cell, overlapping the probe beam.
Because the probe beam has double passed a 44 MHz AOM (each time
the 1 st order beam was used) its frequency has been shifted up by 88 MHz. This
results in the pump and probe beam interacting with atoms which have velocity
such that they are Doppler shifted by 44 MHz and not with the stationary
atoms as discussed in Sec. 2.7.2. The result is that the signal produced by the
fast photodiode reflects the Doppler-broadened transition with peaks at each of
the real transitions (F = 2 → F ′ = 1, 2, 3) as well as the cross-over resonances
(F = 2 → 1/2, 1/3, 2/3) shifted by 44 MHz from their zero velocity value. This
signal is mixed in a lock-in amplifier (SRS SR510) with the f.m. modulation
frequency, resulting in a Doppler-free dispersion signal (see Sec. 2.7.2).
Any of the dispersive curves, shown in Fig. 3.9, could be used as an
error signal to lock the master laser frequency, but we used the F = 2 → 2/3
cross-over transition because it is the most prominent. This signal serves as
an error signal and is sent to a home-built PID [89] lockbox, which outputs
a control signal which we amplify with a Trek 601B-2 high voltage amplifier.
The amplified signal drives the piezo stack, controlling the lasing frequency
and closing the loop.
While this setup allows us to lock the MOT master laser, the laser out-
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