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Water is the most important molecule on earth. Without water, there would be no life possible on this planet. However, in some 'applications' the amount of water vapor needs to be exactly measured. In the Hitran database, many transitions can be found, and there are several strong absorption lines the 1390nm regime, for example. Some of these strong line at 1388nm are shown in the figure, detected with a simple water vapor absorption experiment.

In this figure, the detected optical density at the absorption maximum at a line at 1367.86nm is plotted versus the absorption coefficient calculated according to the Hitran database.

In another experiment, wavelength modulation spectroscopy was accomplished by mixing a low and a high tuning frequency. The 2f absorption spectra recorded with a Lock-In amplifier are shown in the figure.

These experiments have been performed with our TEC100 Littrow Laser System, as well as with our TEC500 Littman Laser System. We gratefully acknowledge finanical support by the BMBF within contract 13N7157. For more details, please check our publications.