26
Chapter 4. Application example: Pound-Drever Hall locking
steps are illustrative of the process required to achieve a stable lock.
Values listed are indicative, and will need to be adjusted for specific
applications. Further advice on optimising the lock is provided in
4.2.1 Locking with fast feedback
1. Connect the error signal to the
A IN
input on the back-panel.
2. Ensure that the error signal is of order 10 mVpp.
3. Set
INPUT
to ∆ (offset mode) and
CHB
to 0.
4. Set
MONITOR 1
to
FAST ERR
and observe on an oscilloscope.
Adjust the
ERR OFFSET
knob until the
DC
level shown is zero.
If there is no need to use the
ERROR OFFSET
knob to adjust
the
DC
level of the error signal, the
INPUT
switch can be set to
DC
and the
ERROR OFFSET
knob will have no effect, preventing
accidental adjustment.
5. Reduce the
FAST GAIN
to zero.
6. Set
FAST
to
SCAN+P
, set
SLOW
to
SCAN
, and locate the reso-
nance using the sweep controls.
7. Increase
FAST GAIN
until the error signal is seen to “stretch
out” as shown in figure 4.2. If this is not observed, invert the
FAST SIGN
switch and try again.
8. Set
FAST DIFF
to
OFF
and
GAIN LIMIT
to 40. Reduce
FAST INT
to 100 kHz.
9. Set
FAST
mode to
LOCK
and the controller will lock to the
zero-crossing of the error signal. It may be necessary to make
small adjustments to
FREQ OFFSET
to lock the laser.
10. Optimise the lock by adjusting the
FAST GAIN
and
FAST INT
while observing the error signal. It may be necessary to relock
the servo after adjusting the integrator.
