MOGlabs DLC202, Manual, Page: 37 / 82

3.5 Locking to an atomic transition:
AC
27
7. Find an appropriate spectral feature.
8. Adjust front-panel
INPUT OFFSET
and
LOCK OFFSET
to obtain
a zero-crossing
ERROR
signal at the desired frequency. The
slope should normally be positive (depending on
DIP
switches
10, 11
); it can be inverted by coarsely adjusting the
PHASE
control.
9. Set
SLOW
and
FAST
gains to minimum (fully anti-clockwise).
10. Switch
SCAN
/
LOCK
to
LOCK
.
11. Switch
OFF
/
LOCK
to
LOCK
.
12. Increase
SLOW
and
FAST
gains to minimise the error signal,
ideally using an external audio spectrum analyser. The gains
should be increased until the onset of oscillation, and then
reduced. See chapter 4 for additional discussion of feedback
optimisation.
Note that it is not necessary to “zoom in” on the desired lock point.
The controller will automatically lock to the zero-crossing closest to
the trigger point, i.e. to the centre of the oscilloscope trace.
When the laser is locked (step 10 above), the photodetector (
INPUT
)
signal should be fixed at the value corresponding to the lock fre-
quency – in this case zero since for
DC
locking, the controller locks
to the zero-crossing.
3.5 Locking to an atomic transition:
AC
Figures 3.5 and 3.6 show two alternate saturated absorption spec-
troscopy arrangements, useful for
AC
(“top of fringe”) locking. The
laser frequency can be directly modulated via the diode current (see
§ 2.4,
DIP
switch
3
), or using an external modulator. The controller
includes a modulator driver with sufficient power to drive a coil di-
rectly for Zeeman modulation, or an external modulator such as an
acousto-optic modulator can be used; see appendix C.
Sample oscilloscope traces obtained in
AC
locking mode are shown