Tunable Diode Laser Absorption Spectroscopy (TDLAS)

TDLAS exploits the rotational vibrational absorption features of gases for laser-based trace gas detection. Sometimes, it is referred to as TDLS, TLS, TLAS or (with a reference) even as TDLARS.

Key Features of TDLAS

TDLAS is a very strong tool for highly selective and sensitive measurements. It enables:

  • sensitive detection of ppm to ppb (or even ppt!) level concentrations
  • in situ measurements
  • contactless techniques
  • operation at or above room temperature
  • measurement of sticky gases
  • portable gas detectors

Compared to other highly sensitive technologies, such as gas chromatography TDLAS instruments show

  • high selectivity
  • low cost of ownership
  • fail-safe operation

Read more about application samples in our applications section.

Concept of TDLAS

TDLAS is one of the most sensitive, selective and robust technologies for trace gas monitoring. It is based on the Lambert-Beer law which states a logarithmic relation between the

  • transmission of light through a gas
  • product of the attenuation coefficient of the gas
  • distance the light travels through the gas

When a gas has an absorption feature at a specific wavelength, the transmitted intensity declines exponentially with:

I(ν,t) = I0(ν) e-S(T) g(ν,ν0) n L

With n being the number density of the molecular absorbers, I0(ν) the initial laser intensity and I(ν,t) the intensity detected after the probe with an absorption length L.

The absorption line profile is characterized by the temperature-dependent, spectrally integrated line strength S(T), and the normalized (area=1) shape function g(ν,ν0), which is centered at the wavelength ν0 [47].

Standard TDLAS setup

A standard TDLAS setup is illustrated in Figure 1. It consists of:

  • a wavelength tuning DFB laser; emitting monochromatic light at the absorption line of the trace gas
  • an optical lens to collimate the laser light
  • a gas sample cell; in this case filled with H2O
  • a photo detector on which the laser light is focused; measuring the transmission
Figure 1: Standard TDLAS set up
Standard TDLAS set up

Have a look at a nice animated model of tunable diode laser absorption spectroscopy. The link will refer you to the website of the project SensHy in which we collaborated with several European partners from industry and research.

Please click on the graphic to start the demonstration.

Figure 2: Animated model of tunable diode laser absorption spectroscopy
Animated model of tunable diode laser absorption spectroscopy

Selection of absorption line

Selecting a suitable absorption line for the TDLAS application is the principal challenge before designing the measurement set up. The outcome of the measurements is highly influenced by the strength of the absorption line as well as by interferences from other gases and the setup itself.

The HITRAN database outlines the absorption features of the most important gases. Please also visit our Applications by Gas section for our nanoplus selection.

Figure 3: Example of absorption features of water vapour in the 760 nm to 6000 nm range
Absorption features of water vapour in 760 nm to 6000 nm range

Technologies of TDLAS

TDLAS is carried out in form of e. g.:

  • direct absorption spectroscopy
  • 2f spectroscopy
  • cavity enhanced spectroscopy (CRDS)
  • light detection and ranging techniques (LIDAR)
  • photo acoustic spectroscopy (PAS)

Various application samples are provided in our applications section and in the cited papers below.

Related papers for TDLAS

In depth information about TDLAS is available in the selected papers below:

#3 Gas monitoring in the process industry using diode laser spectroscopy;
I. Linnerud, P.Kaspersen, T. Jaeger, Appl. Phys. B 67, 1998, pp. 297-305.

#10 Continuous wave, distributed feedback diode laser based sensor for trace-gas detection of ethanenanoplus Tittel ethan sensor;
K. Krzempek, R. Lewicki, L. Naehle, M. Fischer, J. Koeth, S. Belahsene, Y. Rouillard, L. Worschech, F.K. Tittel, Appl. Phys. B 106, 2, 2012, pp 251-255.

#15 Scanned-wavelength-modulation spectroscopy near 2.5 µm for H2O and temperature in a hydrocarbon-fueled scramjet combustor;
C. S. Goldenstein, I. A. Schultz, R. M. Spearrin, J. B. Jeffries, R.K. Hanson, Appl. Phys. B, 116, 3, Sept 2014, pp 717-727.

#16 Diode laser measurements of linestrength and temperature-dependent lineshape parameters of H2O-, CO2-, and N2-perturbed H2O transitions near 2474 and 2482 nm;
C.S. Goldenstein, J.B. Jeffries, R.K. Hanson, J. of Quantitative Spectr. & Radiative Transfer 130, 2013, pp. 100–111.

#17 Wavelength-modulation spectroscopy near 2.5 µm for H2O and temperature in high-pressure and -temperature gases;
C.S. Goldenstein, R.M. Spearrin, J.B. Jeffries, R.K. Hanson, Appl. Phys. B, 116, 3, Sept 2014, pp 705-716.

#28 In situ combustion measurements of H2O and temperature near 2.5 µm using tunable diode laser absorption;
A. Farooq, J.B Jeffries, R.K Hanson, Meas. Sci. Technol., 19, 2008, 075604, pp. 11.

#32 Single-frequency Sb-based distributed-feedback lasers emitting at 2.3 µm above room temperature for application in tunable diode laser absorption spectroscopy;
A. Salhi, D. Barat, D. Romanini, Y. Rouillard, A. Ouvrard, R. Werner, J. Seufert, J. Koeth, A. Vicet, A. Garnache, Appl. Opt., 45, 20., pp. 4957-4965.

#35 TDLAS-based sensors for in situ measurement of syngas composition in a pressurized, oxygen-blown, entrained flow coal gasifier;
R. Sur, K. Sun, J.B. Jeffries, R.K. Hanson, R.J. Pummill, T. Waind, D.R. Wagner, K.J. Whitty, Appl. Phys. B, 116, 1, 2014, pp. 33-42.

#43 Chemical analysis of surgical smoke by infrared laser spectroscopy;
Michele Gianella, Markus W. Sigrist, Appl. Phys. B, 109, 3, Nov. 2012, pp. 485-496.

#46 TDLAS-based open-path laser hygrometer using simple reflective foils as scattering targets;
A. Seidel, S. Wagner, V. Ebert, Appl. Phys. B, 109, 3, Nov. 2012, pp. 497-504.

#47 High-speed tunable diode laser absorption spectroscopy for sampling-free in-cylinder water vapor concentration measurements in an optical IC engine;
O. Witzel, A. Klein, S. Wagner, C. Meffert, C. Schulz, V. Ebert, Appl. Phys. B, 109, 3, Nov. 2012, pp. 521-532.

#48 Absolute, spatially resolved, in situ CO profiles in atmospheric laminar counter-flow diffusion flames using 2.3 µm TDLAS;
S. Wagner, M. Klein, T. Kathrotia, U. Riedel, T. Kissel, A. Dreizler, V. Ebert, Appl. Phys. B, 109, 3, Nov. 2012, pp. 533-540.

#52 Antireflection-coated blue GaN laser diodes in an external cavity and Doppler-free indium absorption spectroscopy;
L. Hildebrandt, R. Knispel, S. Stry, J.R. Sacher, F. Schael, Appl. Opt., 42, No. 12, 2003, pp. 2110-2118.

#56 Widely tunable quantum cascade lasers with coupled cavities for gas detection;
P. Fuchs, J. Seufert, J. Koeth, J. Semmel, S. Hoefling, L. Worschech, A. Forchel, App. Phys. Lett., 97, 2010, 181111.

#62 High-sensitivity interference-free diagnostic for measurement of methane in shock tubes;
R. Sur, S. Wang, K. Sun, D. F. Davidson, J. B. Jeffries, R. K. Hanson, J. of Quant. Spectrosc. and Rad. Transfer, Vol. 156, May 2015, pp. 80–87.

#65 H2O temperature sensor for low-pressure flames using tunable Diode laser Absorption near 2.9 µm;
S. Li, A. Farooq, R.K. Hanson, Meas. Sci. Technol., 22, 2011, pp. 125301-125311.

#67 New Opportunities in Mid-Infrared Emission Control;
P. Geiser, Sensors, 2015, pp. 22724-22736.

#68 Field laser applications in industry and research;
F. D'Amato, A. Fried, App. Phys. B, 2015, 119, pp. 1-2.

#69 A quartz-enhanced photoacoustic sensor for H2S trace-gas detection at 2.6µm;
S. Viciani, M. Siciliani de Cumis, S. Borri, P. Patimisco, A. Sampaolo, G. Scamarcio, P. De Natale, F. D'Amato, V. Spagnolo, App. Phys. B, 2015, 119, pp. 21-27.

#71 Novel utilisation of a circular multi-reflection cell applied to materials ageing experiments;
D.A. Knox, A.K. King, E.D. McNaghten, S.J. Brooks, P.A. Martin, S.M. Pimblott, App. Phys. B, 2015, 119, pp. 55-64.

#73 Time-multiplexed open-path TDLAS spectrometer for dynamic, sampling-free, Interstitial H218O and H216O vapor detection in ice clouds;
B. Kuehnreich, S. Wagner, J.C. Habig, O. Moehler, H. Saathoff, V. Ebert, App. Phys. B, 2015, 119, pp. 177-187.

#78 Ppb-level formaldehyde detection using a CW room-temperature interband cascade laser and a miniature dense pattern multipass gas cell;
L. Dong,Y. Yu,.C. Li, S. So, F. Tittel, Optics Express Vol. 23, Issue 15, 2015, pp. 19821-19830.

# 84 Optical gas sensing: a review;
J. Hodgkinson, R. P. Tatam, Measurement Science and Technology, Vol. 24, No. 1, 2013

# 85 Frequency modulation characteristics for interband cascade lasers emitting at 3 µm;
J. Li, Z. Du, Y. An, Appl. Phys. B, 2015, 121:7–17.

#87 Optical‑feedback cavity‑enhanced absorption spectroscopy with an interband cascade laser: application to SO2 trace analysis;
L. Richard, I. Ventrillard, G. Chau, K. Jaulin, E. Kerstel, D. Romanini, Appl. Phys. B, 2016, 122:247.

#90 Optical feedback cavity-enhanced absorption spectroscopy with a 3.24 µm interband cascade laser;
K. M. Manfred, G. A. D. Ritchie, N. Lang, J. Roepcke, J. H. van Helden, Appl. Phys. Lett. 106, 2015, 221106.

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