HOW THE TDLAS INSTRUMENT WORKS – INTERESTING FEATURES

TDLAS (Tunable diode laser absorption spectroscopy) is a method of measuring the levels of gases such as water vapor and methane in gaseous mixtures. The technology applied is absorption spectrometry where diode lasers are used. It has numerous advantages over other methods for measuring gaseous concentrations. It has the ability to measure very low concentrations in order of parts per billion (ppb). It can also measure other aspects apart from concentration. These include temperature, mass flux, velocity and pressure of the specific gas under study. This makes it a multipurpose instrument that can be used for quantitative tests of gaseous mixtures.

The working of TDLAS is basically explained by its name. The instrument is made up of a light source that produces laser light, transmitting optics, an absorbing medium, and reception detectors. The wavelength of laser produced by the tunable laser diode is adjusted according to the specific gas under study. Absorption characteristics of the gas are used in tuning the instrument. The gas under study absorbs some of the energy from the laser hence recording a lower signal intensity on the receiving optics. The extent by which the energy is absorbed is used to determine the concentration of the gas under study.

Different gases absorb light in different spectral regions and wavelengths. For this reason, the TDLAS has a variety of diode lasers used to measure gases of different wavelengths. Each of these diode lasers can be tuned over a certain wavelength range specific to its application. Some of the examples include InAsP, which can be tuned between 1.6-2.2 micrometer wavelength, and the InP which is tunable between 0.9-1.6 micrometers. The adjustment can be done by either increasing the temperature or altering the injection density of the light source. However, tuning using temperature is limited to slow rates but over wider wavelength ranges. Tuning through injection density on the other hand has faster rates but smaller wavelength ranges.

There are numerous methods used to increase the accuracy and sensitivity of the TDLAS instrument. One of the common methods is through introduction of a cavity in the region where the gases are to be measured. The laser beam bounces against the walls of the cavity back and forth hence increasing its interaction time with the gas under study. Accurate results are produced by the end of the exercise. The noise levels of the instrument are also reduced by using higher frequencies of laser light for measuring concentrations. The only main disadvantage of this instrument is that it is bulky and cannot be moved about freely.