MOGlabs DLC202, Manual, Page: 43 / 82

4. Optimisation
Laser frequency stabilisation is a complex and ongoing research
topic. A thorough treatment would require extensive discussion of
control theory, actuator response, mechanical design, laser-atom in-
teractions and electronics. Here we consider the problem from a
pragmatic perspective.
The laser is assumed to be moderately stable, operating close to the
desired frequency, with a linewidth of a few MHz averaged over a
typical measurement time of about one second. The very short-term
linewidth is determined by the Schawlow-Townes (S-T) limit, which
is typically of order 100 kHz. The
MOG
labs
DLC
will stabilise the
laser frequency to an external reference, usually an atomic absorp-
tion feature, and reduce the effective linewidth as close as possible
to the S-T limit.
Achieving the best frequency locking stability requires careful op-
timisation of the signal-to-noise ratio (SNR) of the frequency dis-
crimination signal obtained from the saturated absorption or other
reference. Then the phase and gain settings must be optimised,
preferably by measuring the feedback error signal spectrum.
4.1 Frequency reference
The frequency reference is critical to the performance of the
MOG
labs
DLC
: the controller cannot reduce the laser frequency noise without
an appropriate frequency-dependent reference signal.
The
DLC
has been designed to work with a saturated absorption
reference, as shown in figures 3.5 and 3.7. Users should familiarise
themselves with saturated absorption spectroscopy, for example as
described in Demtr¨
oder [5].
The frequency discriminator (“
ERROR
”) SNR should be optimised to
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