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30 Chapter 3. Operation
C1
C2
Ch1 100mV Ch2 100mV 20.0ms
C1
C2
Ch1 100mV Ch2 100mV 20.0ms
Figure 3.7: Examples of spectra for
AC
locking, for wide and narrow spans
(lower traces), with error signals (upper traces).
13. Increase
SLOW
and
FAST
gains to minimise the error signal,
ideally using an external audio spectrum analyser (see chapter
4). The gains should be increased until the onset of oscillation,
and then reduced. See chapter 4 for additional discussion of
optimisation.
Note again that it is not necessary to “zoom in” on the desired lock
point. The controller will automatically lock to the zero-crossing of
the error signal (in this case the peak of a spectral feature) closest
to the trigger point, at the centre of the oscilloscope trace.
When the laser is locked (step 11 above), the photodetector (
INPUT
)
signal should be fixed at the value corresponding to the lock fre-
quency. In contrast to the
DC
locking case, this should be the
INPUT
signal at the peak of the spectral feature, not zero.
3.6 Locking using an external signal
The MOG labs
DLC
can be used for Pound-Drever-Hall [6] or offset
locking [7, 8], or indeed using a wide variety of externally generated
dispersive signals. See appendix D for examples.
To operate with externally generated locking signal:
1. Connect the external error signal to the rear-panel
ERROR
ex-