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

needed when forming a MOT or optical molasses because 87 Rb atoms can decay
into the |F = 1〉 ground state manifold where they are no longer resonant with
the MOT beams, removing them from the cooling cycle. The repump beam is
resonant with the |F = 1〉 → |F ′ = 2〉 transition and reintroduces atoms into
the |F = 2〉 → |F ′ = 3〉 cycling transition.
The construction of the repump master laser is identical to that of
the MOT master laser and so the details will not be repeated here. The
temperature and injection current are also controlled in a manner identical to
that used for the MOT master laser.
As with the MOT master laser, feedback on a piezo stack positioned
behind the grating allows us to lock the frequency of the repump master laser.
In this case we lock the laser to the F = 1 → 1/2 cross-over resonance fre-
quency. This locking scheme is accomplished by picking off a small portion
(∼ 1 mW) of the output of the repump master laser with a polarizing beam
splitting cube (PBSC) and sending it to a saturation absorption spectroscopy
setup. This setup yields the Doppler-free spectrum of the |F = 1〉 → |F ′ 〉
transitions as well as the cross-over resonances.
The saturation absorption spectroscopy setup used to lock the repump
master laser differs from that used to lock the MOT master laser. The layout of
this scheme is shown in Fig. 3.16. The portion of the repump master laser sent
to the saturation absorption spectroscopy setup encounters a thick glass plate
which splits it into three beams: a strong pump beam, a weak probe beam and
a weak reference beam. The probe and reference beam are displaced from one
113