MOGlabs DLC202, Manual, Page: 35 / 82

3.4 Locking to an atomic transition:
DC
25
Rb
87
F=2
Rb
85
F=3
Rb
85
F=2
Rb
87
F=1
Frequency (GHz)
0 −2 −4 2 4
In
tensi
ty
−0.2
0
0.2
0.4
0.6
0.8
1.0
Saturated absorption spectrum for natural Rb
Figure 3.2: A saturated absorption spectrum of rubidium using a standard
uncoated laser diode and low diffraction efficiency grating in Littrow con-
figuration (upper trace). The entire 780 nm rubidium hyperfine structure
can be scanned, for both naturally occurring isotopes (nearly 10 GHz), by
ramping both the external cavity length and simultaneously the injection
current, with appropriate adjustment of the feed-forward
BIAS
. The figure
also shows (lower trace) the
AC
-modulation error signal (see § 3.5).
an atomic vapour absorption cell. A Fabry-Perot optical cavity or
other frequency reference could also be used.
The photodetector can be used in single channel mode (default) or
with balanced differential inputs, for example to subtract a Doppler
background from a saturated absorption spectrum.
Sample oscilloscope traces obtained in
DC
locking (“side of fringe”)
mode are shown below, for wide and narrow spans. These traces
were obtained with an 8 cm long Rb vapour cell at room temperature.
To operate in
DC
locking configuration:
1. Select
DC
locking by setting internal
DIP
switch
7
to
ON
.
2. If using differential inputs, set internal
DIP
switch
8
to
ON
.
3. Connect the diode,
TEC
and temperature sensor as above. Also
connect a two-channel oscilloscope to the
CHANNEL A, B
and
TRIG
outputs.