1 TDLS Technology
1.1 What does TDLS mean? How does it work?
Tunable Diode Laser Spectroscopy (TDLS) is an infrared absorption measurement.
Many gases have characteristic absorption bands in the infrared wavelength region. In a typical set-up IR light from a light source traverses a cavity and falls onto a photo detector. Interaction of infrared light (absorption) with gas molecules in the cavity (or measurement cell) leads to a decrease of IR radiation intensity on the detector, depending on the gas concentration.
The absorption bands of gases with relatively small molecules show a well resolved fine structure, consisting of many individual absorption lines. In TDLS, a single-mode tunable diode laser is the light source for the gas measurement. Its wavelength is chosen to be centered onto one of the fine absorption lines of the target gas. The laser is then tuned by temperature or current to scan this absorption line within a very narrow range (~2 nm) to obtain the gas concentration.
Typically one laser is used to measure one gas. Due to the sharpness of the lines (~ 0.1 nm) there is practically no cross-sensitivity with other gases. In the case where the absorption bands of two different gases are interlaced, it is sometimes possible to monitor the concentration of two different gases with one laser.
Due to the fact that telecom-type laser diodes are only available at wavelengths up to 2.7 µm, TDLS is rarely done in the wavelength range of the fundamental gas absorption bands (3 to 9 µm). Instead, the first overtones of the absorptions bands are used. These bands are situated conveniently in the range of 0.76 to 2.3 µm and enable the use of affordable near-infrared telecom lasers and detectors.
|Gas absorption band as seen in low resolution (e.g. NDIR). High-resolution observation reveals a fine structure of individual rotation & vibration bands.
|TDLS scans individual rotation or vibration bands of the molecules in a high-resolution measurement.
1.2 Which gases can be measured with the TDLS?
TDLS in general can measure gases that have relatively simple molecular structures where the absorption bands have a distinct and resolved fine structure.
Currently the following gases are available: NH3, CH4, CO2, C2H6, C2H2, H2O and HCl
Further gases that can be measured with TDLS, but that are not currently available by Axetris include: HF, O2, N2O, H2S, CO, NO, NO2, PH3, HI, HBr, HCN, H2CO, H2CO and C2H4 (further information on request).
1.3 Which gases cannot be measured with the TDLS?
Gases that do not have near-infrared absorption bands cannot be measured. These includes H2, N2, O3, Cl2, He, Ne, Ar, etc. Also, more complex or ring-shaped molecules cannot be measured because they do not show narrow and well separated rotation and vibration bands, e.g. propane, butane, pentane, ethanol, propanol, acetone, ethyl ether, benzene, toluene, etc.
1.4 What kind of laser diodes are used in the TDLS laser gas detection (LGD)?
The measurement technique is based on telecom-type single-mode lasers. Appropriate laser diodes are Distributed Feedback (DFB) lasers, whose laser structure is arranged parallel to the surface and emit the laser light from the edge of the laser chip, and Vertical Cavity Surface Emitting Lasers (VCSEL's), in which the layers are stacked vertically and the laser light is also emitted vertically to the surface.
Laser diodes in the near IR are used for all currently available LGD modules. Further developments are planned for the LGD Compact in which quantum cascade lasers are to be used for mid IR range.
1.5 How is the TDLS influenced by external pressure variations?
Pressure broadening affects absorption peak-height only slightly. In first order approximation the collision broadening effect increases with increasing pressure. This enlarges the absorption band and reduces it in strength, however this effect is compensated by the higher number of absorbing molecules/volume, which in turn increases the absorption strength again.
In NDIR measurements the pressure effect is much bigger and directly proportional to the increase in surface of the absorption line (i.e. the complete envelope with dozens of vibration and rotation bands due to the low resolution of the technique).
It is always recommended to do a new span and offset correction when the environmental conditions are changing a lot.
1.6 How is the TDLS influenced by external temperature variations?
Different temperatures have more influence on the peak intensity, the peak width is hardly affected.
All LGD products are calibrated at an ambient temperature of 20°C or hot system to an expected operating temperature.
If the gas to be measured has a different temperature than the gas cell itself, incorrect measurement results may be obtained.
1.7. How is the target gas measurement influenced by other gases in the gas matrix?
The background gas matrix of the target gas to be measured influences also the shape of the beam. In normal ambient air the peak of the target gas is broadened and reduces its intensity compared to a pure N2 background. This effect is only a % of reading and negligible in the majority of cases where concentration changes are just in the range of ppm level.
It is always recommended to do a new span and offset correction when the environmental conditions like gas matrix are changing a lot.