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Chapter 9
Application notes
9.1 Detection techniques
9.1.1 Direct absorption
In a direct abs orption measurement, the change in
intensity of a beam is recorded as the latter crosses
a sampling cell where the chemica l to be detected
is contained.
This measurement technique has the advantage
of simplicity. In a version of this technique, the light
interacts with the chemical through the evanescent
field of a waveguide or an optical fiber.
9.1.2 Frequency mo dulation technique
(TILDAS)
In this technique, the frequency of the laser is mod-
ulated sinusoidally so as to b e perio dically in and
out of the absorption peak of the chemical to be de-
tected. The absorption in the cell will convert this
FM modulation into an AM modulation which is
then detected usually by a lock-in technique.
The advantage of the TILDAS technique is mainly
its sensitivity. First of all, under good modula-
tion condition, an AC signal on the detector is only
present when there is absorption in the chemical
cell. Secondly, this signal discriminates efficiently
against slowly varying absorption backgrounds. For
this reason, this technique will usually work well for
narrow absorption lines, requir ing also a monomode
emission from the laser itself.
QCLaser Chemical Cell Detector
Fig.51: Direct absorption technique
QCLaser ChemicalCell Detector
a.c.modulation of
thelaser frequency
A.M.modulation of
thebeamdetected
w1 w2
w
w1 w2
w
1
Fig.52: Frequency modulation technique (TILDAS)
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