3. Operation
A typical application of the FSC is to frequency-lock a laser to an
optical cavity using the PDH technique, as shown in figure 3.1. The
cavity acts as a frequency discriminator, and the FSC keeps the la-
ser on resonance with the cavity by controlling the laser piezo and
current through its SLOW and FAST outputs, reducing the laser line-
width. A separate application note (AN002) is available that provides
practical advice on implementing a PDH apparatus.
TRIG CH1
Serial:
TRIG FAST OUT SLOW OUT MOD IN POWER B POWER A
MONITOR 1MONITOR 2 SWEEP IN GAIN IN B IN A IN
LOCK IN
Laser
EOM
Cavity
PBS
LPF
PD
PiezoCurrent
Oscilloscope
AC
CH2
Figure 3.1: 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.
19
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