Editing Tunable diode laser absorption spectroscopy (section)

=== Modulation techniques ===
Modulation techniques make use of the fact that [[pink noise|technical noise]] usually decreases with increasing frequency (which is why it is often referred to as 1/f noise) and improve the signal to noise ratio by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques are wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS).

In WMS the wavelength of the light is continuously scanned across the absorption profile, and the signal is detected at a harmonic of the modulation frequency.

In FMS, the light is modulated at a much higher frequency but with a lower modulation index. As a result, a pair of sidebands separated from the carrier by the modulation frequency appears, giving rise to a so-called FM-triplet. The signal at the modulation frequency is a sum of the beat signals of the carrier with each of the two sidebands. Since these two sidebands are fully out of phase with each other, the two beat signals cancel in the absence of absorbers. However, an alteration of any of the sidebands, either by absorption or dispersion, or a phase shift of the carrier, will give rise to an unbalance between the two beat signals, and therefore a net-signal.

Although in theory baseline-free, both modulation techniques are usually limited by residual amplitude modulation (RAM), either from the laser or from multiple reflections in the optical system (etalon effects). If these noise contributions are held low, the sensitivity can be brought into the 10<sup>−5</sup> – 10<sup>−6</sup> range or even better.

In general the absorption imprints are generated by a straight line light propagation through a volume with the specific gas. To further enhance the signal, the pathway of the light travel can be increased with [[Multipass spectroscopic absorption cells|multi-pass cells]]. There is however a variety of the WMS-technique that utilizes the narrow line absorption from gases for sensing even when the gases are situated in closed compartments (e.g. pores) inside solid materia. The technique is referred to as [[gas in scattering media absorption spectroscopy]] (GASMAS).