68
4 Applications
4.3
Detection at Sum and Difference Frequencies
Certain applications require detection at the sum or difference frequency
of two chopped beams. Examples include pump‑probe experiments, or
two‑wave photomixing. In these experiments, the sample under study
responds non‑linearly to optical excitation. If two beams, having been
chopped at different frequencies
𝑓
INNER
and
𝑓
OUTER
, are combined on the
sample, the sample may produce a response at the sum and difference
frequencies
(𝑓
OUTER
+ 𝑓
INNER
)
and
(𝑓
OUTER
− 𝑓
INNER
)
.
The SR542 provides synthesized reference outputs at the sum and dif‑
ference frequencies, which can be provided to the reference input of a
lock‑in amplifier.
Lock-in Amplifier
SR542 Controller
f
OUTER
+ f
INNER
Ref Out
Ref In
Source #2
Nonlinear Optical
Media
Detector
f
OUTER
f
INNER
Source #1
f
OUTER
- f
INNER
Ref Out
Figure 4.3:
Two Beam Mixing Experiment. Source #1 and #2 need not be dif‑
ferent wavelengths.
SR542 Precision Optical Chopper
