4. Application example:
Pound-Drever Hall
locking
A typical application of the
FSC
is to frequency-lock a laser to an
optical cavity using the
PDH
technique (fig. 4.1). The cavity acts as
a frequency discriminator, and the
FSC
keeps the laser on resonance
with the cavity by controlling the laser piezo and current through its
SLOW
and
FAST
outputs respectively, reducing the laser linewidth. A
separate application note (
AN002
) is available that provides detailed
practical advice on implementing a
PDH
apparatus.
TRIG
CH1
Serial:
TRIG
FAST OUT SLOW OUT
MOD IN
POWER B
POWER A
MONITOR 1
MONITOR 2
SWEEP IN
GAIN IN
B IN
LOCK IN
Laser
EOM
Cavity
PBS
LPF
PD
Piezo
Current mod
Oscilloscope
AC
CH2
PZT
MOD
DLC
controller
SMA
A IN
Figure 4.1:
Simplified schematic for
PDH
-cavity locking using the
FSC
.
An electro-optic modulator (
EOM
) generates sidebands, which interact with
the cavity, generating reflections that are measured on the photodetector
(PD). Demodulating the photodetector signal produces a
PDH
error signal.
A variety of other methods can be used to generate error signals,
which will not be discussed here. The rest of this chapter describes
how to achieve a lock once an error signal has been generated.
23
