4.3 Optimisation
29
provides noise analysis up to a Fourier frequency of 96 kHz with
140 dB dynamic range.
Ideally the spectrum analyser would be used with an independent
frequency discriminator that is insensitive to laser power fluctua-
tions [11]. Good results can be achieved by monitoring the in-loop
error signal but an out-of-loop measurement is preferable, such as
measuring the cavity transmission in a PDH application. To anal-
yse the error signal, connect the spectrum analyser to one of the
MONITOR
outputs set to
FAST ERR
.
High-bandwidth locking typically involves first achieving a stable
lock using only the fast servo, and then using the slow servo to
improve the long-term lock stability. The slow servo is required
to compensate for thermal drift and acoustic perturbations, which
would result in a mode-hop if compensated with current alone. In
contrast, simple locking techniques such as saturated absorption
spectroscopy are typically achieved via first achieving a stable lock
with the slow servo, and then using the fast servo to compensate
for higher-frequency fluctuations only. It may be beneficial to con-
sult the Bode plot (figure 4.3) when interpreting the error signal
spectrum.
When optimising the
FSC
, it is recommended to first optimise the fast
servo through analysis of the error signal (or transmission through
the cavity), 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 param-
eters 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
