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Tunable Diode Laser Absorption Spectroscopy


Tunable Diode Laser Absorption Spectroscopy

What is 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.

Basics of Tunable Diode Laser Absorption Spectroscopy

Key Features & Technology

Key Features

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

TDLAS Technology

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.

(Source: #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.)

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

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

Types 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)
Standard TDLAS setup

A standard TDLAS setup 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
Find the perfect wavelength

Your setup

Select your target wavelength

Selecting a suitable absorption line for the TDLAS application is the principal challenge before designing the measurement setup. 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.

In our Applications by Gas section, we present the most commonly used target wavelengths for major industrial gases.


Papers & Articles

Further reading related to TDLAS

#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 ethane;
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.

#70 In situ H2O and temperature detection close to burning biomass pellets using calibration-free wavelength modulation spectroscopy;
Z. Qu, F.M. Schmidt, App. Phys. B, 2015, 119, pp. 45-53.

#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.

#72 TDLAS-based NH3 mole fraction measurement for exhaust diagnostics during selective catalytic reduction using a fiber-coupled 2.2-µm DFB Diode laser;
F. Stritzke, O. Diemel, S. Wagner, App. Phys. B, 2015, 119, pp. 143-152.

#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.

#93 Interband cascade laser-based optical transfer standard for atmospheric carbon monoxide measurements;
J. A. Nwaboh, Z. Qu, O. Werhahn and V. Ebert, App. Optics, Vol. 56, No. 11, April 10, 2017, pp. E84-E93.

#94 Compact optical probe for flame temperature and carbon dioxide using interband cascade laser absorption near 4.2 μm;
J. J. Girard, R. M. Spearrin, C. S. Goldenstein, R. K. Hanson, Elsevier, Combustion and Flame, Vol. 178, April 2017, pp. 158 – 167.

#96 Fiber-coupled 2.7 μm laser absorption sensor for CO2 in harsh combustion environments;
R. M. Spearrin, C. S. Goldenstein, J. B. Jeffries and R. K. Hanson, Meas. Sci. Technol. 24, April 2013, 055107.

#100 Multiheterodyne spectroscopy using interband cascade lasers;
L. A. Sterczewski, J. Westberg, C. L. Patrick, C. S. Kim, M. Kim, C. L. Canedy, W. W. Bewley, C. D. Merritt, I. Vurgaftman, J. R. Meyer and G. Wysocki, Opt. Eng. 57(1), 011014, Jan. 2018.

# 157 MHz laser absorption spectroscopy via diplexed RF modulation for pressure, temperature, and species in rotating detonation rocket flows;
A. Nair, D. Lee, D. I. Pineda, J. Kriesel, W. A. Hargus Jr., J. W. Bennewitz, S. A. Danczyk, R. M. Spearrin, Appl. Physics B, Lasers and Optics, 126, 2020.

# 160 Narrow linewidth characteristics of interband cascade lasers;
Y. Deng, B.-B. Zhao, X.-G. Wang, C. Wang, Appl. Phys. Lett., 116, 2020, 201101-5.

Further technical advice

Need help?


The high-resolution transmission molecular absorption database from the Harvard-Smithsonian Center for Astrophysics provides an excellent compilation of spectroscopic parameters.

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