Martin Teichmann Atomes de lithium-6 ultra froids dans la ... - TEL

3.3 The laser system
3.3. THE LASER SYSTEM
Our laser system is divided into several distinct parts, shown in figure
3.4. We placed our master lasers on a separate table, not disturbed by
the vibrations that the experiment creates, especially while switching
the magnetic fields. The lasers are extended cavity diode lasers, locked
using saturated absorption spectroscopy onto three lines: the 6 Li D1 and
D2 lines and the 7 Li D2 line. The lithium level structure is shown in
figure 3.5. Using two double-pass acousto-optic modulators (AOM) we
can generate the two hyperfine lines in 7 Li used as principal and repump
beams. Due to the coincidence of the 7 Li D1 and the 6 Li D2 lines, we
repump 6 Li on the D1 line, otherwise the 6 Li light would disturb the 7 Li
MOT. Therefore we need an additional master laser for this line.
The mentioned double-pass AOMs together with those installed after
the 6 Li lasers allows us to tune the lasers in the needed range. The
resulting beams are injected into optical fibers and sent to the experiment,
where they are used to inject slave lasers. As the power of the 7 Li master
laser was not sufficient to stably inject the slave lasers on the experimental
table, we injected diodes with each beam. These acted as amplifiers.
Since we exchanged the 7 Li master laser with a more powerful diode,
the need for this amplification stage has disappeared, so we removed
the amplifier for the 7 Li principal beam. The other amplifier might be
uninstalled as well if the master laser turns out to be stable and powerful
enough over a long time. In the meantime, the 6 Li principal laser has
degraded to a point that we had to add an amplification stage to it.
After many years of successful operation, the tapered amplifier,
used to amplify the light for the MOT, had died. The spontaneous
emission background had raised to a level such that almost a third of
the emitted power was not actually in the desired mode but formed
a pedestal that was unfortunately too broad to be seen on a Fabry-
Pérot interferometer. This resulted in an unstable MOT which became
unusable. The replacement did not have a good fate: it died after
only two weeks of operation. This left us with no tapered amplifier,
since our model had gone out of production and no other model was
available on the market (nowadays, Toptica produces tapered amplifiers
for 670 nm). Fortunately, at the same time two models of high-power
laser diodes became available. Both models (Mitsubishi ML101J27 and
Hitachi HL6545MG) are specified for a wavelength of 660 nm and a
power of 120 mW. For the Mitsubishi diodes, we managed to convince
the distributor (Municom) to sell us diodes selected for a high wavelength,
typically specified as 663 nm at room temperature. According to the
47