Page 117 - Photodetection and Measurement - Maximizing Performance in Optical Systems

System Noise and Synchronous Detection

110 Chapter Five

Modulated
probe-beam
Drive to experiment
motor

PD
V b
LED Reference
Reference output
optocoupler to lock-in
Figure 5.18 Chopper-disk used to modulate a con-
tinuous light source. By shifting the reference
optocoupler azimuthally, the phase difference
between reference and signal can be adjusted.

Modulated
Drive probe-beam
motor
to experiment
PD
LED Reference
output
to lock-in
Figure 5.19 “Thin chopper” provides weak modu-
lation at both even and odd harmonics
of the fundamental. A separate reference channel
allows use with a conventional lock-in amplifier.

the phase-alignment being carried out electronically in the lock-in amplifier.
Problems with many motorized choppers are their poor frequency stability,
leading to significant amplitude drifts when narrow bandwidths are used, and
mechanical vibration. Even if this is quite weak, it can cause strong optical
intensity modulation if laboratory hardware is insufficiently rigid. Precision
mirror mounts and fiber-alignment systems are particularly open to this. The
key characteristic is that the spurious intensity modulation is synchronous with
the lock-in reference or its harmonics. Hence synchronous demodulation may
not suppress these effects. Other types of mechanical beam modulators such as
resonant tuning fork systems may perform better. Best of all are the electrooptic
modulators without any moving parts whatsoever.
Usually we aim for 50/50 perfection in the chopper wheel, but this is not
always the best approach. In cases where the beam power is important, such as
when exposing photoresist or heating a substrate, the 50 percent power loss
may be unacceptable. Here a thin chopper may be useful (Fig. 5.19), still impart-

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