Alpes Lasers . TC-3 Datasheet download pdf (Page 37)

The advantage of the TILDAS technique is mainly its sensitivity. First of all, under good

modulation condition, an a.c. 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, requiring also a monomode emission from the laser itself. This technique has

already been successfully applied with Distributed Feedback Quantum Cascade Laser (DFB-

QCL). Some examples in the literature include:

- E. Whittaker et al, Optics Letters 1998 (PDF 229kB)

- F. Tittel et al., accepted for publication in Optics Letters.

Photoacoustic detection

In the photoacoustic technique, the optical beam is periodically modulated in amplitude before

illuminating 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 photoacoustic detection are

i) a signal is detected only in the presence of absorption from the molecule

ii) no mid-ir detectors are needed.

For these reasons, photoacoustic detection has the potential of being both cheap and very

sensitive. However, ultimate sensitivity is usually limited by the optical power of the source.

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Alpes Lasers . TC-3 Datasheet Page 37