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Glossary:Direct absorption spectroscopy
A frequently used method involves that a tunable narrow-linewidth laser (frequently a single-frequency laser) is tuned through some wavelength range, and the light absorption in some sample is measured as a function of that wavelength. The absorption is often obtained by measuring (a) the optical power of a laser beam which is transmitted through the investigated medium and (b) the optical power of a reference beam (obtained with a beam splitter between the laser and the investigated medium), which is not affected by the medium. That way, one can largely avoid that power fluctuations of the laser (intensity noise) affect the results. In many cases, one uses a balanced photodetector, essentially measuring the difference between two optical powers (rather than their ratio).[1][2]