1. Introduction
The
MOGL
abs
FSC
provides the critical elements of a high-bandwidth
low-latency servo controller, primarily intended for laser frequency
stabilisation and linewidth narrowing. The
FSC
can also be used for
amplitude control, for example to create a “noise-eater” that sta-
bilises the optical power of a laser, but in this manual we assume
the more common application of frequency stabilisation.
1.1
Basic feedback control theory
Feedback frequency stabilisation of lasers can be complicated. We
encourage readers to review control theory textbooks [1, 2] and lit-
erature on laser frequency stabilisation [3].
The concept of feedback control is shown schematically in figure 1.1.
The frequency of the laser is measured with a
frequency discrimi-
nator
which generates an
error signal
that is proportional to the
difference between the instantaneous laser frequency and the de-
sired or
setpoint
frequency. Common discriminators include optical
cavities and Pound-Drever-Hall (PDH) [4] or H¨ansch-Couillaud [5]
detection; offset locking [6]; or many variations of atomic absorption
spectroscopy [7–10].
+–
0
Servo
Laser
Frequency
discriminator
Error
signal
Control
signal
dV/df
Figure 1.1:
Simplified block diagram of a feedback control loop.
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