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Starter Kit Instruction Manual Maintenance 54
9.1.3 Photoacoustic detection
In the photoaco us tic detection, the optical beam is
periodically modulated in amplitude before illumi-
nating the cell containing the absorbing chemical.
The expansion generated by the periodic heating
of the chemical creates an acoustic wave which is
detected by a microphone.
The two very important advantages of photoa-
coustic detection are:
• a signal is detected only in the presence of
absorption from the molecule;
• no mid-IR detectors are needed.
For these reasons, photoacoustic detection has
the potential of being cheap and very sensitive. How-
ever, ultimate sensitivity is usually limited by the
optical power of the source.
QCLaser Chemical Cell
Microphone
w
Detection of the
acoustic waveat w
a.c.modulationof
thelaser amplitude
Fig.53: Photoacoustic technique