Quantum Instruments Cascade Laser Starter Kit Manuel d'utilisateur télécharger pdf (Page 55)

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 ac oustic 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 a re needed.

For these reasons, photoaco us tic detection has

the potential of being cheap and very sensitive. How-

ever, ultimate sensitivity is us ually limited by the

optical power of the source.

QC Laser Chemical Cell

Microphone

Detection of the

acoustic wave at w

a.c.modulation of

the laser amplitude

Fig.53: Photoacoustic technique

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Quantum Instruments Cascade Laser Starter Kit Manuel d'utilisateur Page 55