Quantum Instruments Cascade Laser Starter Kit - Manual (Page 55)

Starter Kit Instruction Manual Maintenance 54
9.1.3 Photoacoustic detection
In the photoacoustic 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.
QC Laser Chemical Cell
Microphone
w
Detection of the
acoustic wave at
w
a.c. modulation of
the laser amplitude
Fig.53: Photoacoustic technique