30
Chapter 4. Application example: Pound-Drever Hall locking
care should be taken to eliminate or mitigate these. The earth of
the
FSC
should be as close as possible to both the laser controller
and any electronics involved in generating the error signal.
One procedure for optimising the fast servo is to set
FAST DIFF
to
OFF
and adjust
FAST GAIN
,
FAST INT
and
GAIN LIMIT
to reduce the
noise level as far as possible. Then optimise the
FAST DIFF
and
DIFF
GAIN
to reduce the high-frequency noise components as observed
on a spectrum analyser. Note that changes to
FAST GAIN
and
FAST
INT
may be required to optimise the lock once the differentiator has
been introduced.
In some applications, the error signal is bandwidth-limited and only
contains uncorrelated noise at high frequencies. In such scenarios
it is desirable to limit the action of the servo at high frequencies
to prevent coupling this noise back into the control signal. A
filter
option is provided to reduce the fast servo response above a specific
frequency. This option is mutually-exclusive to the differentiator,
and should be tried if enabling the differentiator is seen to increase
G
ain (dB)
20
0
40
60
–20
10
2
10
1
10
3
10
4
10
5
10
6
10
7
Fourier frequency [Hz]
Inte
grato
r
Double in
teg
rat
or
Proportional
High fr
eq
. cut
off
Diff
eren
tia
tor
Inte
grato
r
10
8
FAST LF GAIN (limit)
FAST
GAIN
DIFF GAIN (limit)
FAST INT
FAST DIFF
SL
OW INT
Filt
er
Figure 4.3:
Conceptual Bode plot showing action of the fast (red) and slow
(blue) controllers. The corner frequencies and gain limits are adjusted with
the front-panel knobs as labelled.
