Intra-Cavity Absorption Measurements

In the absorption measurements described above an external laser beam was used. A great increase in sensitivity can be achieved if the sample is placed inside the laser cavity [9.49,50]. This is due to the mode competition in a multi-mode laser. The principle of intra-cavity absorption measurements is given in Fig. 9.4. 

A multi-mode dye laser is run in the wavelength region where the species under investigation absorbs. As an example we will consider the case of molecular iodine (I2). If the intra-cavity cell is first taken out and the laser is adjusted to the desired wavelength region, laser-induced fluorescence will be observed in an external iodine cell. Normally, several molecular transitions are induced. If the intra-cavity iodine cell is now introduced the 

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The CRDS technique uses the same principle as intra-cavity spectroscopy, namely increasing the effective absorption path length. The difference is that in CRDS the absorption coefficient is determined from a time measurement, i.e., the decay time of the ringing cavity , while in intra-cavity spectroscopy the gain competition between different resonator modes is used as the enhancement factor. 

Figure 29.9 Measurement of combustion radicals in a low-pressure, stoichiometric (CH4 = N2 = 02) flame , temperature data (measured by CRDS on OH rotational lines) k, OH concentration data (measured by CRDS) o, HCO concentration data (measured by intra-cavity laser absorption spectroscopy). The solid lines are from the model calculations using experimental temperature profiles. Reproduced from Cheskis et al, Appi. Phys. B, 1998, 66 377, with permission of Springer Science and Business Media...

Fluctuations of mode frequencies due to density fluctuations of the dye liquid or caused by external perturbations, prevent stationary conditions in a multimode laser. We can define a mean "mode lifetime t " which represents the average time a specific mode exists in a multimode laser. If the measuring time for intracavity absorption exceeds the mode lifetime t, no quantitatively reliable information on the magnitude of the absorption coefficient a(w) of the intra-cavity sample can be obtained. 

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