Utilizing Spectroscopy Techniques in Gas Analysis and Sensing

Spectroscopy

Various spectroscopic techniques are in use for gas analysis and sensing – fields of increasing importance – vital for atmospheric monitoring, emission control and industrial processes efficiency. They are applied on a mass scale on a more frequent basis today to ensure the wellbeing of humanity and the environment.

A comprehensive understanding of such techniques facilitates the development of state-of-the-art, efficient and cost-effective gas analysis solutions — an understanding well-suited to particular applications while squeezing the most out of the laser spectroscopy potential.

VIGO System has extensive experience in supplying components and solutions for gas analysis systems. The intention of this paper is to improve and spread the knowledge concerning the most prominent laser spectroscopic techniques and how to select the best detector for your laser gas analysis system.

Basics

Essentially, a laser gas analysis system is comprised of two main parts: a transmitter and a receiver. The transmitter is a laser system in combination with all the appropriate control electronics. The transmitter’s role is to emit the light into the gaseous sample prepared for analysis.

The receiver assembly is made up of the photodetector, along with all the suitable optical components and an appropriate readout system. Its role is to chronicle fluctuations in the interaction between the radiation introduced and the gaseous sample and then display them for an operator to decipher in a clear, easy-to-interpret way.

Typical layout of an infrared gas spectroscopy system.

Figure 1. Typical layout of an infrared gas spectroscopy system. Image Credit: VIGO System 

Selection of appropriate laser light source as well as suitable photodetectors and accompanying electronics always depends on the analysis technique used. They must all be matched carefully not only to the application but, most importantly, to the molecular species prepared for sensing and analysis.

Absorption Principles

The principal mechanism of light-matter interaction in laser spectroscopy is absorption – a portion of light is absorbed by the gaseous molecule, which results in the molecule’s energy level changing.

The wavelength of absorbed light must be fitted precisely to the energy gap between singular rovibrational levels. An individual absorbed wavelength signifies an absorption line on the radiation spectrum, and a mass of absorption lines makes for an absorption spectrum.

Normalised absorption spectra of most popular gaseous species.

Figure 2. Normalised absorption spectra of most popular gaseous species. Image Credit: VIGO System 

This is often called a molecule’s fingerprint, as it is unique for every molecular species. Each absorption line has contrasting intensity – the greater it is, the more light will be absorbed, enhancing the method’s sensitivity and enabling the detection of low concentration gases.

Quantitative description of absorption is aided by the Beer-Lambert-Bouguer law, which couples the intensity of absorbed light with type and concentration of the molecules:

I(λ,x) = I0 (λ) exp(-σ(λ)Nx)

Starting with shining the light of intensity (I0) onto a sample containing the absorbing species (which is typically dependent on the emitted wavelength λ, and requires positioning on to the absorption line selected). There, a portion of the light is absorbed, leading to the attenuation of the outcoming light intensity.

The absorption level is dependent on: optical path through the absorbing species x, concentration of the species N, and absorption cross-section σ, which has a particular value for each absorption line and, thus, for every single molecule and wavelength.

Based on this, identifying the presence and concentration of various types of molecules is made possible. Naturally, it is crucial to have knowledge about the precise values of the optical path and absorption cross-section in any given ambient conditions. The latter is readily available utilizing databases such as HITRAN.

Infrared wavelengths primarily correspond to energy gaps between varying rotational and vibrational states.

Particularly, the mid-infrared region (~3-14 µm) shows potential for carrying out absorption spectroscopy due to the greater intensity of particular absorption lines when compared to near-infrared (1-3 µm) wavelengths.

An increase of up to 100x in line intensity can be witnessed, e.g., for CO2 molecules (Fig. 3.) – this means the concentrations of the species detected can be much lower to produce a noticeable signal difference.

Comparison of absorption lines’ strength for different wavelength regimes.

Figure 3. Comparison of absorption lines’ strength for different wavelength regimes. Image Credit: VIGO System 

Another benefit of mid-IR is a plethora of absorbing species in this region and high separation of lines of various molecules, which helps facilitate the selectivity of detection. Greater absorption cross-sections mean a shorter optical path and, therefore, a setup that is more compact.

Presently, there are a great number of spectroscopic methods being used in the mid-infrared. Here, the principles of the key techniques are described, thus, indicating the appropriate detector choice for each given application.

Tunable Diode Laser Spectroscopy - Overview

TDLS is one of the most extensively used spectroscopic techniques in gas analysis and detection. It is also referred to as TDLAS or TLS. A simple TDLS setup is exhibited below. Here, the light source is typically a laser of sorts.

Naturally, the most common are distributed feedback (DFB) diode lasers, namely quantum cascade (QCL) or interband cascade lasers (ICL). With the onset of more affordable and widely available technology, VCSELs are now being incorporated into those setups as well.

As the entire principle of TDLS is predicated on the tunability of the light source, a steering console with suitable electronic devices is necessary to control and modulate the light coming to the sample.

The emitted light passes through the sample on a known, set optical path where it is absorbed partly by the analyzed gases.

Collection of the light signal is achieved using a photodetector, carefully selected to best fit the adapted light source and readout method. After experiencing some electronic processing, the collected signal is then ready for display and interpretation.

TDLS has some considerable advantages which account for its popularity. It is a relatively simple technique that does not necessitate the assembly of too many components in the setup.

Layout of a typical TDLS setup. System console used for controlling light source and detector readout is clearly visible.

Figure 4. Layout of a typical TDLS setup. System console used for controlling light source and detector readout is clearly visible. Image Credit: VIGO System 

Use of fast tunable lasers and fast-response photodetectors facilitate real-time operation. The system's well-structured design facilitates non-contact, remote measurements and enhancements in sensitivity and efficiency.

Of these, the two most common are enhancing the optical path through the absorbing sample and introducing the light source to modulation.

The light modulation techniques detailed below are: Direct Absorption Spectroscopy (DAS), Wavelength Modulation Spectroscopy (WMS) and Frequency Modulation Spectroscopy (FMS).

They may also be referred to using other names or terms. The elimination of random noises is made possible by demodulation of the received signal by suitable electronics and, therefore, enhances the lowest detectable fluctuation in the light intensity and also the lowest detectable concentration.

Optical path increase is most frequently achieved by introducing an external cavity. The most straightforward technique is Multipass Spectroscopy (MUPASS). A more intricate cavity is used in Integrated Cavity Output Spectroscopy (ICOS).

Light Modulation Techniques In TDLS

In the most common TDLS setup, the laser driving current is modulated, which leads to a change in both emitted wavelength and intensity. A wide range of modulation shapes can be used, as well as alternative types of modulation (e.g., by chopper).

Direct Absorption Spectroscopy is the most straightforward technique, where the modulation period (in terms of emitted wavelength) is much wider than the selected absorption profile for measurement. As a result, changes of intensity recorded in time display two symmetrical dips (Fig. 5.).

Exemplary detector signal intensity vs time in DAS. Note dips in intensity corresponding to passing through an absorption wavelength.

Figure 5. Exemplary detector signal intensity vs time in DAS. Note dips in intensity corresponding to passing through an absorption wavelength. Image Credit: VIGO System 

These represent passing through an absorption profile and are doubled because of the modulation symmetry. Naturally, the amplitude of the intensity dip can be utilized to quantify the concentration of molecular species. Therefore, DAS is one of the easiest modulation techniques to perform.

The technique primarily known as Wavelength Modulation Spectroscopy differs from DAS mostly in the range of wavelength modulation. Here, the modulation period is much narrower than the absorption profile. Thus, the modulation is thrust onto the absorption profile and can be in a number of relations to it (Fig. 6.).

Principle of WMS. Absorption profile is marked in blue. Wavelength modulation is imposed in relation to the profile – marked by vertical sine signals in gray. Modulation results in 1f or 2f signals depending on the modulation placements.

Figure 6. Principle of WMS. Absorption profile is marked in blue. Wavelength modulation is imposed in relation to the profile – marked by vertical sine signals in gray. Modulation results in 1f or 2f signals depending on the modulation placements. Image Credit: VIGO System 

According to the placement, the signal can be registered on the same frequency as the modulation’s (1f signal) or double the frequency (2f signal) when placing the modulation in the peak of the absorption profile.

After the demodulation, the absorption peak registered is shown to be different between 1f to 2f signals.

As can be seen from the prime signal curves (Fig. 7.), it is of greater advantage to use the 2f signal, as the curve that is generated has a clear shape of a peak, where interpretation of height is clearly relative to absorption intensity – similarly to DAS.

Differences between exemplary 1f and 2f signals registered in WMS.

Figure 7. Differences between exemplary 1f and 2f signals registered in WMS. Image Credit: VIGO System 

A more advanced technique is the Frequency Modulation Spectroscopy (FMS). Here, the frequency of the laser light is rapidly modulated, which generates additional sidebands showing in the laser’s emission spectrum.

The sidebands are displayed on the left and right of the main emission profile and are equally distanced from the center.

The beating signal of the laser frequency side is measured in this methodology. In standard conditions, i.e., without absorbing species in the light path, the beating is neutralized, which results in a plain DC signal on the detector.

However, when the intensity of one of the sidebands is reduced due to absorption, the beating signal is non-zero and results in the introduction of another AC signal in addition to the DC background. Amplitude of the AC signal can, therefore, be interpreted as reliant on the absorption intensity.

For this reason, it is of key importance in FMS is to take care when positioning the modulated laser’s emission spectrum in respect to the absorption profile – so that one of the sidebands stands for the wavelength of much greater absorption than the other. This technique is rarely utilized but is, nevertheless, extremely sensitive.

Principle of FMS. Left: modulated signals show the light sidebands, however, without absorption, the resulting signal is DC. Right: with one of the sidebands aligned with an absorption line, the detected signal shows an additional AC modulation.

Figure 8. Principle of FMS. Left: modulated signals show the light sidebands, however, without absorption, the resulting signal is DC. Right: with one of the sidebands aligned with an absorption line, the detected signal shows an additional AC modulation. Image Credit: VIGO System 

Extending the Light Path in TDLS

The second way of increasing the sensitivity of TDLS is to enhance the optical path. Most commonly, external cavities are used – due to the desire to preserve the compactness of the gas sensing devices.

A collection of such techniques falls under the umbrella term Cavity-Enhanced Absorption Spectroscopy (CEAS). We will provide insight into two of the most popular cavity techniques: MUPASS and ICOS.

Image Credit: VIGO System 

Multipass absorption spectroscopy (MUPASS) relies on a simple increase in the optical path via a carefully designed cavity, where – as the name implies – the light beam travels multiple times before hitting the detector. The gas sample is enclosed by the cavity.

There are numerous layouts of the multipass cavity available. The most common are White or Herriott cells, which are tubular in shape and have mirrors on the ends to reflect the laser beam in such a way that facilitates the highest possible optical path within the cavity.

Layout of a typical MUPASS system. Here a Herriott cell is used; however, other cell designs are possible.

Figure 9. Layout of a typical MUPASS system. Here a Herriott cell is used; however, other cell designs are possible. Image Credit: VIGO System

Currently, such cells are being made commercially available and make fast prototyping and cost-effective manufacturing possible. There are also other more specific cavity designs, such as circular, vertical and so on.

Those more sophisticated designs allow an optical path through the analyzed sample of hundreds of meters – all within a tabletop instrument. Another widely used cavity technique is the Integrated Cavity Output Spectroscopy.

The first variant of this method incorporates a resonant cavity with the length between the high reflectivity mirrors carefully tuned to the laser’s emitted frequency. However, this proved to be impractical outside the laboratories.

The more popular and robust variant of this method is the ICOS-OA – and off-axis variation, which utilizes the same cavity, but in a non-resonant way.

Layout of a typical ICOS-OA system. Note the input of the light at an angle (“off-axis”) and collection of light signal from the whole surface of the back mirror.

Figure 10. Layout of a typical ICOS-OA system. Note the input of the light at an angle (“off-axis”) and collection of light signal from the whole surface of the back mirror. Image Credit: VIGO System 

Here, the detected light signal behind the cavity is very low because of the high reflectivity of the mirrors, but output from the whole back mirror surface is collected and, because of the design of the cavity, the light path is virtually infinite hence why the ICOS-OA is a very interesting and sensitive technique.

Choosing the Right Detector for TDLS

TDLS is a reasonably simple and highly efficient technique for gas analysis. In order not to waste its potential, all the elements of the system need to be suited to the job. High performance light detectors are a crucial part.

The spectral and frequency response need to be carefully matched to the laser source and modulation technique in order to provide the best SNR.

Another vital parameter to be taken into account is the linearity of the detector’s response. High linearity range allows easier readout and interpretation of signal and, therefore, simplifies the design of the electronic part of the setup.

All of these requirements are met by VIGO’s photovoltaic (PV) detectors – available in a broad range of spectral variants and with lowtime constants, ensuring proper frequency response.

In PV detectors, biasing is available and recommended especially for high frequency systems – biasing of photodiodes greatly enhances the speed of response and linearity range, thus making it the perfect solution for demanding TDLS applications.

VIGO PVs are available as MCT (mercury-cadmium-telluride) detectors and also RoHS-compliant III-V materials detectors. For custom applications, we are happy to provide tailored spectral responses, bandpass filters, design-in electronics, etc.

VIGO preamplifiers are also available with low noise electronics and fitted spectral response to provide the highest performance of the detection module. In laser applications where the wavelength changes in small ranges – just like TDLS – fringing becomes a big problem.

Fringing is described as additional modulation of the system’s transmission coefficient, caused by different interferences of the light at various wavelengths. Fringing occurs in the detector itself as well due to the interferences on different wafer levels.

Solving the fringing problem can mean a huge boost in sensitivity and efficiency of the TDLS systems. VIGO provides standard wedged windows with AR coatings to prevent unwanted interferences from occurring in the detector’s window.

AR coating on the detector’s active area is also available upon demand. VIGO is developing new innovative products with anti-fringing solutions implemented on the wafer level, which are planned to enter serial production soon – so be sure to stay in touch and find out about our latest developments.

Table 1. Source: VIGO System 

Gaseous species Selected absorption line [µm] Recommended VIGO System detector/module
Water vapour H2O 2.9 PV-xTE-3
PVA-xTE-3 (RoHS compliant)
6.3 PV-xTE-6
UM-I-6
Carbon dioxide CO2 4.3 PV-xTE-5
PVA-xTE-5 (RoHS compliant)
Affordable Module AM03120 (RoHS compliant)
9.4 PV-xTE-10.6
PVM-xTE-10.6
Methane CH4 3.3 PV-xTE-3.4
PVA-xTE-3 (RoHS compliant)
Nitrous oxide N2O 4.5 PV-xTE-4
PVA-xTE-5 (RoHS compliant)
Affordable Module AM03120 (RoHS compliant)
Nitrous dioxide NO2 6.2 PV-xTE-6
UM-I-6
Carbon monoxide CO 4.6 PV-xTE-4
PVA-xTE-5 (RoHS compliant)
Affordable Module AM03120 (RoHS compliant)

 

Cavity Ring-Down Spectroscopy

This technique, known as CRDS, also relies on the external cavity filled with the gas sample. However, there are crucial differences in the method of operation of a CRDS system in comparison to TDLS. The light source is usually a mechanically modulated or pulsed laser.

The cavity has high reflectivity mirrors on both ends, aligned so that the light beams pass through the same path over and over. After each pass, a small portion of the light signal exits the cavity through the back mirror.

Therefore, the recorded response consists of a series of signals. An exemplary single ring-down response is described below

Typical CRDS system with a vertical cavity used. Below: principle of detecting a CRDS system response.

Figure 11. Typical CRDS system with a vertical cavity used. Below: principle of detecting a CRDS system response. Image Credit: VIGO System 

First, the laser pulse enters the cavity. During this phase, the first series of stronger and stronger signals is recorded – marking the build-up phase. Then, the laser input is shut down.

After that, the light is still in the cavity, but with each pass, the beam is weaker and weaker – naturally due to absorption by the sample. Therefore, the signals registered by the detector also get exponentially weaker – this is called the ring-down phase.

By fitting the curve to the registered series of signals and calculating the decay time, one can get information about the concentration of the gas in the sample can be obtained.

Figure 12. Comparison of ring-down signals with and without absorbing sample. A: response with absorbing sample in place. B: response without absorption. Formulas linking concentration of a sample with ring-down time are provided. Note constitution of a ring-down curve of numerous single signals. Image Credit: VIGO System 

CRDS is a highly accurate and sensitive technique, beating records in terms of lowest detectable concentrations – however, for proper operation, good calibration and stable cavity designs are required. The continuous development of CRDS allows it to be used even in such demanding conditions as airborne systems.

Similarly to TDLS, the most important parameters for a detector to fulfill its role in a CRDS system are: spectral and frequency response, which has to be carefully matched to cavity operation mode and expected ring-down times, as well as measured concentrations of gases and strength of absorption lines.

Again, the photovoltaic VIGO detectors both from MCT and III-V materials with our low noise electronics are well-suited for work in CRDS systems. Anti-fringing solutions are of lesser importance here. However, AR coatings can also help in achieving better results.

In CRDS, other issues arise. Stability of the detector readouts is even more important than in the other techniques, and due to the technique’s nature, high detectivity is crucial because of extremely low light signals reaching the detector (caused by the high reflectivity mirrors being used).

Therefore, the best pick for CRDS systems is thermoelectrically cooled PV detectors with immersion lenses. Those are a unique sales point for our detectors – monolithically integrated GaAs hyper hemispherical lenses enabling the increase of the detectivity of a detector element by as much as 11 times.

When used with thermoelectrical cooling, these detectors provide unmatched parameters for CRDS applications.

Dual Frequency Comb Spectroscopy

One of the most sophisticated techniques in gas spectroscopy, the Dual Comb Spectroscopy (DCS), is a slowly emerging but highly promising method. At the heart of the system lie two frequency combs – light sources with extraordinary emission spectra.

A frequency comb emits light at numerous separate wavelengths at once, with equal spacing between the single emission peaks. The emission spectrum looks like a comb – hence the name. Spectroscopy using the frequency combs as light sources opens up unprecedented possibilities in multiple gas detection with a single setup.

The DCS principle is usually as follows. Two frequency combs with slightly different repetition frequencies are used. Light from one of the combs goes through the analyzed sample. Then both of the light signals are directed along the same axis, and a detector records an interferogram of the two signals.

Finally, the data acquisition system performs a Fourier transformation in order to extract an absorption spectrum of the sample.

Figure 13. Layout of a typical DCS system. Image Credit: VIGO System

DCS shows numerous advantages over the traditional spectroscopy techniques: not only wide wavelength range emitted at once but also solid-state operation with almost no moving mechanical parts – enabling robustness and ease of alignment.

The advent of more affordable frequency combs has resulted in increased popularity of DCS – and also stringent requirements for the detectors used in the technique. High linearity of response and low fringing levels are highly desired in DCS.

But, most importantly, the detector needs to have an extraordinarily fast response (matching the combs’ repetition rate) and a wide spectral range, not hindering the potential of the frequency combs.

Hence why our recommendations for the DCS are photovoltaic multijunction PVM detectors or photoelectromagnetic PEM detectors, combining wide spectral range (with even response over the whole range) with short time constants.

Even faster readout can be achieved using biased PV detectors, especially in our standard Ultra High Speed Modules (UHSM).

Table 2. Comparison of different detector types for DCS. Source: VIGO System

Detector type Spectral range [µm] Operation frequency
PEM 2-12 >150 MHz
PVM 2-13 >110 MHz
PV (biased) 3-12 >1 GHz (UHSM modules)

 

Application Overview

Constant development of spectroscopy systems, environmental pressure for gas control and desire for a cleaner and more efficient world are the key drivers for new applications of gas sensing systems. In this brief overview, we provide a few ideas for potential applications.

The most prominent field in which gas sensing is developing is environmental protection and research. Monitoring gas and aerosols levels in the atmosphere is increasingly important in the face of ongoing climate change and allows us to better understand climate and our influence on it.

For gases whose impact we already know – e.g., carbon dioxide, methane and others – emission control is vital and increasingly regulated by law. This creates the need for affordable and sensitive gas analysis systems.

Environmental measurements are taken not only by simple ground-based setups but also in advanced airborne systems or in extreme conditions, like monitoring volcanic processes. The compactness and robustness of TDLS or CRDS systems enable precise in-situ measurements in all conditions.

A huge portion of hazardous gases emitted into the atmosphere stems from the industry. This field is not only increasingly pressured to improve its environmental record but is also on the constant lookout for optimization of various processes.

Therefore, monitoring of various gaseous species, e.g., in furnaces, incinerators or during production, is very interesting, as it provides real-time insight into the efficiency of industry and also enables more exact control over toxic or hazardous gaseous substances which may leak into the atmosphere.

This is important not only for the benefit of the environment but also for workplace safety. The automotive is also becoming more interested in gas analysis – mainly for control of exhaust fumes during the design phase as well as periodical vehicle testing.

Increasingly robust systems are slowly facilitating real-time fumes monitoring – with devices mounted directly on the vehicle in question. Benefits for the environment as well as air pollution control in cities are some of the most significant goals in the development of such systems.

The medical is patiently awaiting the advent of affordable and sensitive gas spectroscopy systems as research in breath analysis progresses. Accurate detection of biomarkers – substances indicating a certain abnormal state in the human body – would help in the diagnosis of cancer, pulmonary diseases and stomach infections.

Cheap, fast and non-invasive diagnostic tools, like spectroscopic breath analyzers using RoHS compliant detectors, would greatly improve our healthcare possibilities. These are some of the most prominent and interesting gas analysis applications, but naturally, the list can go on.

No matter what your application is – VIGO System is ready to deliver cutting-edge detectors for your needs and share our broad know-how in their applications.

This information has been sourced, reviewed and adapted from materials provided by VIGO System.

For more information on this source, please visit VIGO System.

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