The advantage of TDLAS over other techniques for concentration measurement is its ability to achieve very low detection limits (of the order of ppb).
[2][3] TDLAS is by far the most common laser based absorption technique for quantitative assessments of species in gas phase.
VCSEL, DFB, etc., is tuned over the characteristic absorption lines of a species in the gas in the path of the laser beam.
This causes a reduction of the measured signal intensity due to absorption, which can be detected by a photodiode, and then used to determine the gas concentration and other properties as described later.
These lasers can be tuned by either adjusting their temperature or by changing injection current density into the gain medium.
To start, the wavelength of a diode laser is tuned over a particular absorption line of interest and the intensity of the transmitted radiation is measured.
The transmitted intensity can be related to the concentration of the species present by the Beer-Lambert law, which states that when a radiation of wavenumber
passes through an absorbing medium, the intensity variation along the path of the beam is given by,[6] where, The above relation requires that the temperature
The former can be achieved by the use of a modulation technique, whereas the latter can be obtained by placing the gas inside a cavity in which the light passes through the sample several times, thus increasing the interaction length.
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.
As a result, a pair of sidebands separated from the carrier by the modulation frequency appears, giving rise to a so-called FM-triplet.
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.
In general the absorption imprints are generated by a straight line light propagation through a volume with the specific gas.
This can be obtained by placing the species inside a cavity in which the light bounces back and forth many times, whereby the interaction length can be increased considerably.
Although the former technique can provide a high sensitivity, its practical applicability is limited because of all the non-linear processes involved.
Multi-pass cells, which typically can provide an enhanced interaction length of up to ~2 orders of magnitude, are nowaday common together with TDLAS.
Oxygen tunable diode laser spectrometers play an important role in safety applications in a wide range of industrial processes, for this reason, TDLS are often an integral part of modern chemical plants.
This technology is employed on flares, in vessel headspace and in other locations where explosive atmospheres must be prevented from forming.