MOGlabs DLC202, Manual, Page: 45 / 82

4.2 Noise spectra 35
10
1
10
2
10
3
10
4
10
5
Frequency (Hz)
-140
-120
-100
-80
-60
dBV rm
s Unlocked
Piezo locked
Piezo+current locked
Off resonance
Dark
Figure 4.1: Error signal spectra, with laser unlocked, locked with
SLOW
(piezo) feedback only, and with
SLOW
and
FAST
(piezo+current) feedback.
The off-resonance and dark noise spectra provide information on the effec-
tive noise floor.
4.2 Noise spectra
The master, slow and fast gains can be set as described in chapter 3,
increasing them until the onset of oscillation, and then reducing
slightly. If possible, an audio frequency spectrum analyser can be
used to provide better guidance. A generic computer sound card with
spectrum analysis software gives reasonable results up to 20 kHz. A
good sound card (24-bit 200 kHz, e.g. Lynx L22 or E-Mu 1212m)
provides noise analysis up to 100 kHz with 140 dB dynamic range,
surpassing most standalone audio spectrum analysers, at very low
cost. Connect the spectrum analyser to the
CHANNEL B
output, and
set the
CHAN B
selector to
ERROR
.
You should see curves similar to those shown in fig. 4.1. The noise
spectrum with laser unlocked was obtained in scan mode, but with
zero span, and the frequency carefully set to an atomic resonance
(the highest saturated absorption dip in fig. 3.7). Similarly for the
Off resonance curve, but with the laser tuned far away from all res-