3.3 Optimisation 25
When optimising the FSC, it is recommended to first optimise the
fast servo through analysis of the error signal, and then the slow
servo to reduce sensitivity to external perturbations. In particular,
SCAN+P mode provides a convenient way to get the feedback sign
and gain approximately correct.
Note that achieving the most stable frequency lock requires careful
optimisation of many aspects of the apparatus, not just the parame-
ters of the FSC. For example, residual amplitude modulation (RAM)
in a PDH apparatus results in drift in the error signal, which the
servo is unable to compensate for. Similarly, poor signal-to-noise
ratio (SNR) will feed noise directly into the laser.
In particular, the high gain of the integrators means that the lock
can be sensitive to ground loops in the signal-processing chain, and
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.
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 to prevent coupling
this noise back into the control signal. A “filter” option is therefore
provided to cause the fast servo response to roll-off at a specific
frequency. This option is mutually-exclusive to the differentiator,
and should be tried if enabling the differentiator is seen to increase
the measured noise.
The slow servo can then be optimised to minimise the over-reaction
to external perturbations. The high gain limit means that the fast
servo will respond to external perturbations (e.g. acoustic coupling)
and the resulting change in current can induce mode-hops in the
To Next Page
Loading...