TDLAS
Encyclopedia
Tunable diode laser absorption spectroscopy (TDLAS) is a technique for measuring the concentration of certain species such as methane
, water vapor
and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry
. The advantage of TDLAS over other techniques for concentration measurement is its ability to achieve very low detection limits (of the order of ppb
). Apart from concentration, it is also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation. TDLAS is by far the most common laser based absorption technique
for quantitative assessments of species in gas phase.
, and then used to determine the gas concentration and other properties as described later.
Different diode lasers are used based on the application and the range over which tuning is to be performed. Typical examples are InGaAsP/InP (tunable over 900 nm to 1.6 µm), InGaAsP/InAsP (tunable over 1.6 µm to 2.2 µm), etc. These lasers can be tuned by either adjusting their temperature or by changing injection current density into the gain medium. While temperature changes allow tuning over 100 cm−1, it is limited by slow tuning rates (a few hertz), due to the thermal inertia of the system. On the other hand, adjusting the injection current can provide tuning at rates as high as ~10 GHz, but it is restricted to a smaller range (about 1 to 2 cm−1) over which the tuning can be performed. The typical laser linewidth is of the order of 10−3 cm−1 or smaller. Additional tuning, and linewidth narrowing, methods include the use of extracavity dispersive optics.
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,
where,
is the transmitted intensity of the radiation after it has traversed a distance 
through the medium,
is the initial intensity of the radiation,
is the absorbance of the medium,
is the absorption cross-section of the absorbing species,
is the number density
of the absorbing species,
is the line strength (i.e. the total absorption per molecule) of the absorbing species at temperature 
,
is the lineshape function for the particular absorption line. Sometimes also represented by 
,
is the center frequency of the spectrum.
of the absorbing species is known. However, it is possible to overcome this difficulty and measure the temperature simultaneously. There are number of ways to measure the temperature, a widely applied method, which can measure the temperature simultaneously, uses the fact that the line strength 
is a function of temperature alone. Here two different absorption lines for the same species are probed while sweeping the laser across the absorption spectrum, the ratio of the integrated absorbance, is then a function of temperature alone.
where,
is some reference temperature at which the line strengths are known,
is the difference in the lower energy levels involved in the transitions for the lines begin probed.
Another way to measure the temperature is by relating the FWHM
of the probed absorption line to the Doppler line width
of the species at that temperature. This is given by,
where,
is the weight of one molecule of the species, and
is the molecular weight of the species.
Note: In the last expression,
is in kelvins and 
is in g/mol.
However, this method can be used, only when the gas pressure is low (of the order of few mbar). At higher pressures (tens of millibars or more), pressure or collisional broadening becomes important and the lineshape is no longer a function of temperature alone.
. The shift in the frequency spectrum is related to the mean flow velocity by,
where,
is the angle between the flow direction and the laser beam direction.
Note :
is not same as the one mentioned before where it refers to the width of the spectrum. The shift is usually very small (3×10−5 cm−1 ms−1 for near-IR diode laser) and the shift-to-width ratio is of the order of 10−4.
(LAS) in general is that it relies on a measurement of a small change of a signal on top of a large background. Any noise introduced by the light source or the optical system will deteriorate the detectability of the technique. The sensitivity of direct absorption techniques is therefore often limited to an absorbance of ~10−3, far away from the shot noise level, which for single pass direct AS (DAS) is in the 10−7 – 10−8 range. Since this is insufficient for many types of applications, AS is seldom used in its simplest mode of operation.
There are basically two ways to improve on the situation; one is to reduce the noise in the signal, the other is to increase the absorption. 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. If the technique is applied to trace species detection, it is also possible to enhance the signal by performing detection at wavelengths where the transitions have larger line strengths, e.g. using fundamental vibrational bands or electronic transitions.
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.
In FMS, the light is modulated at a much higher frequency but with a lower modulation index. As a result, a pair of sidebands separated from the carrier by the modulation frequency appears, giving rise to a so called FM-triplet. The signal at the modulation frequency is a sum of the beat signals of the carrier with each of the two sidebands. Since these two sidebands are fully out of phase with each other, the two beat signals cancel in the absence of absorbers. 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.
Although in theory baseline-free, both modulation techniques are usually limited by residual amplitude modulation (RAM), either from the laser or from multiple reflections in the optical system (etalon effects). If these noise contributions are held low, the sensitivity can be brought into the 10−5 – 10−6 range or even better.
In general the absorption imprints are generated by a straight line light propagation through a volume with the specific gas. To further enhance the signal, the pathway of the light travel can be increased with multi-pass cells. There is however a variety of the WMS-technique that utilizes the narrow line absorption from gases for sensing even when the gases are situated in closed compartments (e.g. pores) inside solid materia. The technique is referred to as gas in scattering media absorption spectroscopy
(GASMAS).
External cavities can either be of multi-pass type, i.e. Herriott or White cells
, of non- resonant type (off-axis alignment), or of resonant type, most often working as a Fabry–Pérot (FP) etalon. Multi-pass cells, which typically can provide an enhanced interaction length of up to ~2 orders of magnitude, are nowaday common together with TDLAS.
Resonant cavities can provide a much larger path length enhancement, in the order of the finesse of the cavity, F, which for a balanced cavity with high reflecting mirrors with reflectivities of ~99.99–99.999% can be ~ 104 to 105. It should be clear that if all this increase in interaction length can be utilized efficiently, this vouches for a significant increase in detectability! A problem with resonant cavities is though that a high finesse cavity has very narrow cavity modes, often in the low kHz range (the width of the cavity modes is given by FSR/F, where FSR is the free-spectral range of the cavity, which is given by c/2L, where c is the speed of light and L is the cavity length). Since cw lasers often have free-running linewidths in the MHz range, and pulsed even larger, it is non-trivial to couple laser light effectively into a high finesse cavity.
The most important resonant CEAS techniques are cavity ring-down spectrometry (CRDS), integrated cavity output spectroscopy (ICOS) or cavity enhanced absorption spectroscopy (CEAS), phase-shift cavity ring-down spectroscopy (PS-CRDS) and Continuous wave Cavity Enhanced Absorption Spectrometry (cw-CEAS), either with optical locking, referred to as (OF-CEAS), as has been demonstrated Romanini et al. or by electronic locking., as for example is done in the Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy
(NICE-OHMS) technique.
The most important non-resonant CEAS techniques are off-axis ICOS (OA-ICOS) or off-axis CEAS (OA-CEAS), wavelength modulation off-axis CEAS (WM-OA-CEAS), off-axis phase-shift cavity enhanced absorption spectroscopy (off-axis PS-CEAS).
These resonant and non-resonant cavity enhanced absorption technqies have so far not been used that frequently with TDLAS. However, since the field is developing fast, they will presumably be more used with TDLAS in the future.
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry
Laser absorption spectrometry
Laser absorption spectrometry refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry ....
. The advantage of TDLAS over other techniques for concentration measurement is its ability to achieve very low detection limits (of the order of ppb
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
). Apart from concentration, it is also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation. TDLAS is by far the most common laser based absorption technique
Laser absorption spectrometry
Laser absorption spectrometry refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry ....
for quantitative assessments of species in gas phase.
Working
A basic TDLAS setup consists of tunable diode laser light source, transmitting (i.e. beam shaping) optics, optically accessible absorbing medium, receiving optics and detector/s. The emission wavelength of the tunable diode laser, viz. 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, which can be detected by a photodiodePhotodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
, and then used to determine the gas concentration and other properties as described later.
Different diode lasers are used based on the application and the range over which tuning is to be performed. Typical examples are InGaAsP/InP (tunable over 900 nm to 1.6 µm), InGaAsP/InAsP (tunable over 1.6 µm to 2.2 µm), etc. These lasers can be tuned by either adjusting their temperature or by changing injection current density into the gain medium. While temperature changes allow tuning over 100 cm−1, it is limited by slow tuning rates (a few hertz), due to the thermal inertia of the system. On the other hand, adjusting the injection current can provide tuning at rates as high as ~10 GHz, but it is restricted to a smaller range (about 1 to 2 cm−1) over which the tuning can be performed. The typical laser linewidth is of the order of 10−3 cm−1 or smaller. Additional tuning, and linewidth narrowing, methods include the use of extracavity dispersive optics.
Concentration measurement
The basic principle behind the TDLAS technique is simple. The focus here is on a single absorption line in the absorption spectrum of the a particular species of interest. To start with the wavelength of a diode laserLaser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
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
Beer-Lambert law
In optics, the Beer–Lambert law, also known as Beer's law or the Lambert–Beer law or the Beer–Lambert–Bouguer law relates the absorption of light to the properties of the material through which the light is travelling.-Equations:The law states that there is a logarithmic dependence between the...
, 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,where,
is the transmitted intensity of the radiation after it has traversed a distance through the medium, is the initial intensity of the radiation, is the absorbance of the medium, is the absorption cross-section of the absorbing species, is the number densityNumber density
In physics, astronomy, and chemistry, number density is an intensive quantity used to describe the degree of concentration of countable objects in the three-dimensional physical space...
of the absorbing species,
is the line strength (i.e. the total absorption per molecule) of the absorbing species at temperature , is the lineshape function for the particular absorption line. Sometimes also represented by , is the center frequency of the spectrum.Temperature measurement
The above relation requires that the temperature of the absorbing species is known. However, it is possible to overcome this difficulty and measure the temperature simultaneously. There are number of ways to measure the temperature, a widely applied method, which can measure the temperature simultaneously, uses the fact that the line strength is a function of temperature alone. Here two different absorption lines for the same species are probed while sweeping the laser across the absorption spectrum, the ratio of the integrated absorbance, is then a function of temperature alone.where,
is some reference temperature at which the line strengths are known, is the difference in the lower energy levels involved in the transitions for the lines begin probed.Another way to measure the temperature is by relating the FWHM
Full width at half maximum
Full width at half maximum is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value....
of the probed absorption line to the Doppler line width
Doppler broadening
In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...
of the species at that temperature. This is given by,
where,
is the weight of one molecule of the species, and is the molecular weight of the species.Note: In the last expression,
is in kelvins and is in g/mol.However, this method can be used, only when the gas pressure is low (of the order of few mbar). At higher pressures (tens of millibars or more), pressure or collisional broadening becomes important and the lineshape is no longer a function of temperature alone.
Velocity measurement
The effect of a mean flow of the gas in the path of the laser beam can be seen as a shift in the absorption spectrum, also known as Doppler shiftDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. The shift in the frequency spectrum is related to the mean flow velocity by,
where,
is the angle between the flow direction and the laser beam direction.Note :
is not same as the one mentioned before where it refers to the width of the spectrum. The shift is usually very small (3×10−5 cm−1 ms−1 for near-IR diode laser) and the shift-to-width ratio is of the order of 10−4.Limitations and means of improvements
The main disadvantage of absorption spectrometry (AS) as well as laser absorption spectrometryLaser absorption spectrometry
Laser absorption spectrometry refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry ....
(LAS) in general is that it relies on a measurement of a small change of a signal on top of a large background. Any noise introduced by the light source or the optical system will deteriorate the detectability of the technique. The sensitivity of direct absorption techniques is therefore often limited to an absorbance of ~10−3, far away from the shot noise level, which for single pass direct AS (DAS) is in the 10−7 – 10−8 range. Since this is insufficient for many types of applications, AS is seldom used in its simplest mode of operation.
There are basically two ways to improve on the situation; one is to reduce the noise in the signal, the other is to increase the absorption. 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. If the technique is applied to trace species detection, it is also possible to enhance the signal by performing detection at wavelengths where the transitions have larger line strengths, e.g. using fundamental vibrational bands or electronic transitions.
Modulation techniques
Modulation techniques make use of the fact that technical noise usually decreases with increasing frequency (which is why it is often referred to as 1/f noise) and improve the signal to noise ratio by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques are wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS).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.
In FMS, the light is modulated at a much higher frequency but with a lower modulation index. As a result, a pair of sidebands separated from the carrier by the modulation frequency appears, giving rise to a so called FM-triplet. The signal at the modulation frequency is a sum of the beat signals of the carrier with each of the two sidebands. Since these two sidebands are fully out of phase with each other, the two beat signals cancel in the absence of absorbers. 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.
Although in theory baseline-free, both modulation techniques are usually limited by residual amplitude modulation (RAM), either from the laser or from multiple reflections in the optical system (etalon effects). If these noise contributions are held low, the sensitivity can be brought into the 10−5 – 10−6 range or even better.
In general the absorption imprints are generated by a straight line light propagation through a volume with the specific gas. To further enhance the signal, the pathway of the light travel can be increased with multi-pass cells. There is however a variety of the WMS-technique that utilizes the narrow line absorption from gases for sensing even when the gases are situated in closed compartments (e.g. pores) inside solid materia. The technique is referred to as gas in scattering media absorption spectroscopy
Gas in scattering media absorption spectroscopy
Gas in scattering media absorption spectroscopy is an optical technique for sensing and analysis of gas located within porous and highly scattering solids, e.g. powders, ceramics, wood, fruit, translucent packages, pharmaceutical tablets, foams, human paranasal sinuses etc. It was introduced in...
(GASMAS).
Cavity-enhanced absorption spectrometry (CEAS)
The second way of improving the detectability of TDLAS technique is to extend the interaction length. 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. This has led to a group of techniques denoted as cavity enhanced AS (CEAS). The cavity can either be placed inside the laser, giving rise to intracavity AS, or outside, when it is referred to as an external cavity. Although the former technique can provide a high sensitivity, its practical applicability is limited because of all the non-linear processes involved.External cavities can either be of multi-pass type, i.e. Herriott or White cells
White cell (spectroscopy)
A White cell is a type of long path gas phase spectroscopy cell that is commonly used to measure low-concentration components in gases or liquids. In 1942 John U...
, of non- resonant type (off-axis alignment), or of resonant type, most often working as a Fabry–Pérot (FP) etalon. Multi-pass cells, which typically can provide an enhanced interaction length of up to ~2 orders of magnitude, are nowaday common together with TDLAS.
Resonant cavities can provide a much larger path length enhancement, in the order of the finesse of the cavity, F, which for a balanced cavity with high reflecting mirrors with reflectivities of ~99.99–99.999% can be ~ 104 to 105. It should be clear that if all this increase in interaction length can be utilized efficiently, this vouches for a significant increase in detectability! A problem with resonant cavities is though that a high finesse cavity has very narrow cavity modes, often in the low kHz range (the width of the cavity modes is given by FSR/F, where FSR is the free-spectral range of the cavity, which is given by c/2L, where c is the speed of light and L is the cavity length). Since cw lasers often have free-running linewidths in the MHz range, and pulsed even larger, it is non-trivial to couple laser light effectively into a high finesse cavity.
The most important resonant CEAS techniques are cavity ring-down spectrometry (CRDS), integrated cavity output spectroscopy (ICOS) or cavity enhanced absorption spectroscopy (CEAS), phase-shift cavity ring-down spectroscopy (PS-CRDS) and Continuous wave Cavity Enhanced Absorption Spectrometry (cw-CEAS), either with optical locking, referred to as (OF-CEAS), as has been demonstrated Romanini et al. or by electronic locking., as for example is done in the Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy
Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy
Noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy is an ultra-sensitive laser-based absorption technique that utilizes laser light to assess the concentration or the amount of a species in gas phase by absorption spectrometry .- Principles :The NICE-OHMS technique combines...
(NICE-OHMS) technique.
The most important non-resonant CEAS techniques are off-axis ICOS (OA-ICOS) or off-axis CEAS (OA-CEAS), wavelength modulation off-axis CEAS (WM-OA-CEAS), off-axis phase-shift cavity enhanced absorption spectroscopy (off-axis PS-CEAS).
These resonant and non-resonant cavity enhanced absorption technqies have so far not been used that frequently with TDLAS. However, since the field is developing fast, they will presumably be more used with TDLAS in the future.
See also
- Absorption spectroscopyAbsorption spectroscopyAbsorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
- Absorption spectrometry
- Cavity Ring Down Spectroscopy (CRDS)Cavity ring down spectroscopyCavity ring-down spectroscopy is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light. It has been widely used to study gaseous samples which absorb light at specific wavelengths, and in turn to...
- Diode LasersLaser diodeThe laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
- Laser Absorption SpectrometryLaser absorption spectrometryLaser absorption spectrometry refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry ....
- Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy (NICE-OHMS)Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular SpectroscopyNoise-immune cavity-enhanced optical-heterodyne molecular spectroscopy is an ultra-sensitive laser-based absorption technique that utilizes laser light to assess the concentration or the amount of a species in gas phase by absorption spectrometry .- Principles :The NICE-OHMS technique combines...