Spectroscopic Gas Sensing Systems

Abstract

Spectroscopic gas sensing is becoming popular in wide range of areas including urban and industrial emission, environmental monitoring, chemical and industrial process control, medical diagnostics, homeland security and scientific research. Spectroscopy in the mid-infrared region (MIR, 2.5–25 μm) is attractive due to the strong fundamental ro-vibrational bands and the highly specific molecular signature, which allows both identification and quantification of the molecular species. In this chapter, we introduce the spectroscopic gas sensing systems based on a tunable laser. After a brief overview of the technical background (Sect. 13.1) and the principle of spectroscopy (Sect. 13.2), we discuss the system configurations, including pump suction system, diffusion gas sensors, open path sensing system and spectroscopic imaging systems. Two types of especially distinctive gas sensing, that is, miniaturized hollow waveguide gas sensor and standoff sensing with non-cooperative targets, are reviewed in this section. Recent developments of spectroscopic applications with II–VI lasers are in Sect. 13.4. Section 13.5 gives briefly the conclusion and prospects.

1 Introduction

Spectroscopic gas sensing, namely gas detection by analysis of the characteristic spectra of molecules, is becoming more popular in wide variety of areas including urban and industrial emission [1], environmental monitoring [2], chemical and industrial process control [3], medical diagnostics [4], homeland security [5] and scientific research [6]. Traditional approaches of spectroscopic gas sensing based on incoherent light sources are non-dispersive infrared (NDIR), differential optical absorption spectroscopy (DOAS), Fourier transform infrared spectroscopy (FTIR), photometry, etc.

NDIR is a technique to determine gas concentration by detecting a few absorption lines across a restricted wavelength range with fixed narrow-band filters in the infrared spectral region. A typical NDIR instrument uses a double-beam methodology and consists of a light source (e.g. halogen lamp, LED, etc.), filters, gas cells and infrared detectors. The sensing wavelength is carefully selected by the existing output range of infrared sources and within the ‘atmospheric windows’ to reduce spectral interferences. NDIR gas analysers have the merits of real-time response for easy algorithms, low-cost, compact, robust, in situ and stable and long-term operation needs minimum recalibration, which allow for measurement of a variety of components including CO₂, CO, CH₄, NO, SO_2, etc. However, NDIR gas analysers are suffered from low sensitivity and selectivity, and susceptible to ambient humidity and fluctuation of temperature and pressure.

DOAS, pioneered by Noxon [7], is a method to determine concentrations of trace gases by measuring their specific narrow-band absorption structures in the UV and visible spectral regions. DOAS can simultaneously detect multiple gases in an open optical path or in sampling mode. A typical DOAS instrument consists of a continuous light source (e.g. a Xe-arc lamp, a deuterium lamp, sunlight, etc.) and an optical setup to send and receive the light through the objective gases. The received light is sent to a grating spectrometer and converted to spectrum data. The narrow part of the absorption spectrum is separated and used to calculate the concentrations of the multiple species. By means of a typical length of the light path in the atmosphere ranges from several hundred metres to many kilometres, DOAS can measure concentrations of many different trace gases, including photochemical smog formation; O₃, NO₂, HCHO, HONO, H₂O, NO₃, SO₂, BrO, IO, OIO; and several aromatic hydrocarbons.

FTIR is an analytical technique used to measure the absorption of infrared radiation by the samples versus wavelength, in which both open optical path measurement and sampling sensing are available. The infrared absorption bands identify molecular components and structures. A typical FTIR spectrometer uses an interferometer to modulate the wavelength from a broadband infrared source. The signal obtained from the detector is an interferogram and is analysed with a computer using Fourier transforms to obtain a single-beam infrared spectrum. FTIR-based gas analyser is capable of low detection limits of ppb to ppm for multiple gas species in many gas analysis applications, such as toxic gas detection, monitoring industrial facilities, accidental releases and hazardous waste site emissions.

Practically, spectroscopic gas sensing with coherent light sources, for example, tunable lasers, ushered in a new era for their advantages of non-contact, fast response time, high-sensitivity and -selectivity, potential calibration-free, low maintenance requirements and long-life cycle. While, laser absorption spectroscopy (LAS) in the mid-infrared region (MIR, 2.5–25 μm) is attractive due to the strong fundamental ro-vibrational bands and the highly specific molecular signature, which allows both identification and quantification of the molecular species [2, 8]. Thanks to newly developed MIR devices, including quantum cascade lasers (QCLs) [9], interband cascade lasers (ICLs) [10] and II–VI semiconductor lasers [11], LAS technology progressed rapidly and developed high-performance, compact and rugged gas sensors.

In this chapter, we introduce primarily on LAS for trace gas detection. After a brief overview of the principle in Sect. 13.2, we discuss the spectroscopic gas sensing system configurations, including pump suction system, open path sensing system, gas diffusion sensors and spectroscopic imaging system. We discuss spectroscopic applications with II–VI lasers in Sect. 13.4. Section 13.5 is the conclusion and prospects.

2 Principle

At its most basic, the interaction of light with matter, for example, gaseous molecule, two physical processes happen, absorption and scattering. Absorption is a basis of spectral analysis, quantitatively based on the Beer–Lambert law, which gives the relationship between the incident and the transmitted radiation through a gas cell or an open pathway filled with molecular gas sample:

$$ I\left(\nu \right)={I}_0\left(\nu \right)\times \exp \left\{-\sigma \left(\nu \right)\times L\times C\right\} $$

(13.1)

where I₀ and I are the incident and transmitted radiant powers, respectively; σ is the absorption cross-section of the molecule in cm²/molecule; L is the absorption path length in cm; C is the density of the molecule in molecule/cm³. Usually, the absorption cross-section σ is also used to describe the absorption intensity. The line strength is retrieved by spectrally integrating the absorption line shape and applying the ideal gas law,

$$ S(T)=\frac{K_{\textrm{B}} TA}{X_i LP\kern0.5em {r}_{iso}} $$

(13.2)

where K_B, T (K) and P (Pa) are the Boltzmann constant, gas temperature and total pressure of the gas sample, respectively; X_i is the amount fraction of i species; A (cm⁻¹) is integral absorbance; r_iso is a correction factor for isotopic fractionation of the gas sample.

2.1 Direct Absorption Spectroscopy

Direct absorption spectroscopy (DAS) is that a tunable narrow linewidth laser is tuned across the objective spectral line, and the light absorption in a sample is measured as a function of the wavelength. With the measured incident and transmitted laser powers, one could deduce the absorption intensity or gas concentration, and further the state parameters of the gas, for example, temperature or pressure.

DAS is the most common technique for simple-optical configuration, −signal processing and potential absolute measurement. However, DAS often suffers from low sensitivity (absorbance ~10⁻³–10⁻⁴) for the interference from low-frequency noise (i.e. 1/f noise) in the system and laser power fluctuation.

There are basically two ways to improve the spectroscopic sensitivity: (1) to reduce the noise in the signal, (2) to increase the absorption. The former can be achieved by using modulation techniques, for example, wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS), with a typical sensitivity of absorbance ~10⁻⁵–10⁻⁶. Whereas the latter can be obtained by placing the gas inside a cavity in which the light passes through multiple times to increase the interaction length, for example, multiple-pass and cavity-enhanced absorption spectroscopy (CEAS) [12]. Both ways of reducing noise and increasing absorption can be further applied in the same system, for example, cavity-enhanced wavelength modulation spectrometry and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) [13].

The absorbed energy during interaction of light with matter may possibly be converted to acoustic waves, that is, photoacoustic (PA) effect. By recording a photoacoustic spectrum, the amplitude of the acoustic wave is measured with the aid of a microphone as a function of the wavelength of the incident radiation, that is, photoacoustic spectroscopy. Photoacoustic spectroscopy is one of the most powerful techniques for gas sensing that covers a broad range of applications. The detection scheme photoacoustic spectroscopy can be excited by the absorption of modulated or pulsed radiation with very high sensitivity and selectivity.

2.2 Wavelength Modulation Spectroscopy

WMS is that a periodic sawtooth ramp ridden by a high-frequency sinusoidal is applied to the laser injection current, thus the laser wavenumber, v(t) = v_c + v_a cos ωt, is scanned across the transition of gas to be detected, where ν_c and ν_α, are the laser centre wavenumber and modulation depth, respectively; ω is the radian frequency. In case of ideal conditions, ignoring all kinds of interferences, the modulated absorption signal is detected by a photodiode and then processed using a lock-in amplifier (LIA) to demodulate the signal at the harmonics (1f, 2f, 3f, etc.). The second harmonic component (WMS-2f) is commonly used for calculating the concentration of target gas. In case of optically thin, that is, (v)L · C ≤ 0.05, the ideal 2f signal is modelled as:

$$ {A}_{\textrm{ideal}\kern0.5em 2f}=\frac{2{I}_0 CL}{\pi }{\int}_0^{\pi }-\alpha \left({\nu}_{\textrm{c}}+{\nu}_{\textrm{a}}\cos \theta \right)\cos 2\theta d\theta \propto {I}_0 CL $$

(13.3)

where α is the absorption coefficient and θ = ωt is the phase angle. When incident laser intensity I₀ and optical path L are constant, the amplitude of WMS-2f signal is proportional to the gas concentration.

Traditionally, tunable diode laser absorption spectroscopy (TDLAS) analyses a discrete narrow absorption line of small molecule for the detection of a single gas. The modulation index of the laser plays a pivotal role in WMS. A modulation index of 2.2 is recognized as the optimum to achieve the maximum SNR for WMS with an isolated spectrum line with Lorentzian, Gaussian or Voigt profile.

However, for larger molecules, for example, volatile organic compounds (VOCs), there are so many lines overlapping each other that results in the spectral features being broad and smooth except for occasional spikes [8, 14]. These spectral features are distinct from that of the discrete narrow absorption lines. Since the WMS-2f signal profile of broadband absorption with a modulation index of 2.2 is absolutely improper and may lead to broadening and overlapping by the adjacent spectrum, interference and optical fringes as well, detection of trace gas with broadband absorption is much more difficult. The overlapping may deteriorate and even disable the WMS measurement, especially for broadband spectra. So, the modulation index determination should balance the spectral discrimination and the SNR in WMS for a broadband spectrum.

Reference [14] provides a parameter of spectral discrimination (SD) and a criterion for optimizing the modulation index for broadband spectrum. Thanks to the recently developed broadband modulation absorption spectroscopy, one could detect some larger molecules (e.g. VOCs) or multiple gases with a simple system configuration by using a single semiconductor laser [14]. Further requirements of multi-gas sensing could be benefited from the broad tunable coverage of laser with II–VI semiconductors.

Practically, apart from the WMS-2f signal descript in Eq. (13.3), the detected signal consists of random noises and the derivation of optical fringes. The optical fringes appear as unpleasant spectral features which are usually mixed with the target absorption and constitute one of the major obstacles in trace gas detection. In a well-designed and well-fabricated spectroscopic system, the optical fringes should be well reduced, and only small residual fringes remain with sinusoidal waveforms [15]. While, random noise is little time-varying wiggles superimposed on the true underlying signal, with a small standard deviation. Thus, the detected signal could be described as:

$$ {A}_{\textrm{detected}\ 2f}={\textrm{e}}_n+\sum {a}_j(t)\times \cos \left({\omega}_j(t)\times t\right)+{A}_{\textrm{ideal}\ 2f} $$

(13.4)

where α_j(t) and ω_j(t) are the instantaneous amplitude and frequency of j^th fringe component, respectively; A_ideal 2f is the WMS-2f signal modelled by Eq. (13.3).

The profiles of second harmonic of absorption, fringes and noise will inherit the feathers of their origination. These profile differences among WMS-2f, harmonic of optical fringes and noise will be novel breakthrough point to distinguish and eliminate the interference from the signal [14].

2.3 Frequency Modulation Spectroscopy

FMS is a method of optical heterodyne spectroscopy capable of rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FMS optical spectrum of the probe wave is distorted by the spectral feature of interest. Recently, dispersion spectroscopy, namely chirped laser dispersion spectroscopy or heterodyne phase-sensitive dispersion spectroscopy attracts the attention of both immunities to optical intensity changes and superb linearity in the measurement of concentration.

3 System Configurations

A typical LAS consists of a laser, a photodetector and an optical configuration for light interaction with gas. For modulation-based LAS, there are additionally a modulator and a demodulator, the latter usually by a lock-in amplifier (LIA).

The laser is the key component of LAS, which usually needs to be continuously tunable, mode-hop-free, reliable, low-intensity noise and single-frequency with narrow linewidth (typically <1 MHz). The spectral resolution of a TDLAS system is largely depended on the dynamic spectral characteristics during the laser tuning [16], and then determines the selectivity of gas sensing for a gas mixture. Recently developed single-frequency tunable lasers, for example, distributed feedback (DFB) QCL, DFB ICL and single-frequency II–VI semiconductor lasers are all excellent candidates for MIR spectroscopic gas sensing systems.

High-sensitive and low-noise detectors are essential for trace gas detection. Mercury–cadmium–telluride (MCT, or HgCdTe) semiconductor-based detector is popular for its higher speed and wide MIR spectral response. The limitation of the MCT detector is the need for cooling to reduce noise due to dark current. Alternatively, newly developed quantum heterostructure detectors could take a vital part in future infrared detection.

The optical configuration provides interaction between light and gas samples, and the interaction length directly relates to the detection sensitivity. Thus, a long interaction length is desired to achieve high sensitivity. Long path absorption cell and open long path are commonly used in LAS to measure low-concentration components or to observe weak spectra in gas. Traditional multipass cells (MPCs), such as White or Herriott cell, are still widely used, but the requirements of compact, small sample volume and fast response time have stimulated the development of new type of gas cells. Recently, modified MPCs, circular multi-reflection (CMR) cells and hollow waveguides (HWGs)-based gas cells hint at the glorious perspective of compact integrated sensors. On the other hand, the need for open-path gas detection, for example, leak detection, aroused the development of standoff remote sensing with or without a retroreflector. Here in this section, the laser spectroscopic gas sensing systems are classified and reviewed according to the optical configuration that they utilize.

3.1 Pump Suction System (Sampling Sensing System)

Gas cells have been introduced for path length enhancement to achieve high sensitivity in spectroscopic analysis systems. In actual applications, a gas cell based-system usually requires a pump for sampling gas, namely pump suction sampling system.

3.1.1 Multipass Cell-Based System

MPCs are commonly used in laboratories or in industrial process applications for more than 70 years, where the simplest designs are the two-mirror Herriott design or the three-mirror White cell. Conventional MPCs commonly have disadvantages in high requirement of alignment stability, long gas exchange time, high cost and large physical size. Recent publications present also more sophisticated designs, such as modified MPC, circular multi-reflection (CMR) cell and quasi-chaotic cell designs. MPC technology has evolved rapidly in the aspects of compactness, small sample volume and fast response time. Herein modelling on C. Robert’s classification [17], the MPC optical systems are summarized and classified into six categories as shown in Table 13.1.

Table 13.1 Summary of the various types of multipass cells up to now

3.1.2 Hollow Waveguide-Based Sensor

HWGs, capillary tubes with metal/dielectric internal coatings, are promising for transmitting laser beams owing to the advantages of flexibility, high power threshold, low transmission loss, absence of end reflections and broadband ranging from X-ray to ultraviolet, visible, infrared and terahertz. They are ideal candidates for gas cells in spectroscopic system, which could be classified to three categories: ① Ag/AgI-coated HWGs (Ag/AgI-HWG); ② photonic bandgaps HWGs (PBG-HWG); ③ substrate-integrated HWGs (iHWG), respectively [2, 30, 31], as shown in Fig. 13.1. Compared with modified MPCs and CMR cells, HWGs have advantages of easy configurations, compact structure and higher path-to-volume ratio. Hence, HWGs can easily be integrated into the spectroscopic gas sensing system, as shown in Fig. 13.2, for environmental monitoring [32, 33] and breath analysis [4, 34].

Fig. 13.1

Schematic structure (a) Ag/AgI-coated hollow waveguide; (b) photonic bandgap hollow waveguide (PBG -HWG); (c) substrate-integrated hollow waveguide (iHWG). (Reprinted with permission from Ref. [18]. Copyright 2017: Peking University Press)

Fig. 13.2

Overview of the most prevalent hollow waveguide-based laser gas sensing principles. The laser source includes the following: NIR-TDL near-infrared tunable diode laser, ICL interband cascade laser, QCL quantum cascade laser, EC-QCL external cavity coupled QCL, OPO optical parametric oscillator. The detector includes the following: pyroel pyroelectric detector, DTGS deuterated triglycine sulphate detector, thermo thermopile detector, MCT mercury cadmium telluride semiconductor detector, QCD quantum cascade detector. (Reprinted with permission from Ref. [18]. Copyright 2017: Peking University Press)

In spectroscopic gas sensor, an HWG is exploited as both light waveguide and gas transmission cell. However, the capillary structure of HWG makes it necessary to pay extra attention to the following issues when used for accurate gas measurement, including transmission characteristics of laser beam propagating in HWG, effective optical path [35], influence of adsorption effect of polar molecules on gas measurement [36, 37] and non-uniform distribution of pressure during gas flowing in HWG [38]. Additionally, it is worth noting that filling PBG-HWG with analyte gas for sensing is difficult owing to considerable back-pressure building up in the hollow structure. However, techniques to improve the sample filling time have been proposed, which include increasing the pressure difference across the fibre to drive the gas through and introducing holes to allow gas flow or diffusion along the waveguide’s length.

3.2 Diffusion Sensors

Different from pump suction sampling system, diffusion-type gas sensor can avoid the need for a gas valve and pump. A diffusion-type gas sensor can be obtained by reforming the system via removing the sealing cover of the gas cell or replacing it with an air-permeable shell. These kinds of sensors are widely used to monitor the target gas in the atmospheric environment, or loaded on an unmanned aerial vehicle or a mobile vehicle to perform highly sensitive detection of the target gas in the area of interest. However, the open-path-cell optical system is very easy to cause contamination of the multi-reflective lens and requires frequent maintenance.

On the other hand, an alternative diffusion-type laser gas sensor has been developed for point measurement, which is similar to a semiconducting metal oxide gas sensor or catalytic sensor in shape and size but better in selectivity and sensitivity. Most recently, a novel compact intrinsic safety full range methane (CH₄) microprobe sensor based on the TDLAS technique has been proposed for leaking detection in explosion risk environment, such as natural gas industries, petrochemical enterprises and coal mines. A minimized diffused laser CH₄ probe sensor with sensitivity of ppm is shown in Fig. 13.3 [39].

Fig. 13.3

Photograph of CH₄ probe sensor. Length (a), (b) diameter and (c) structure of the probe. (Reprinted with permission from Ref. [39]. Copyright 2021: Elsevier)

3.3 Open Path Sensing

3.3.1 Open Path Detection with Retroreflectors

A simple standoff open-path TDLAS can be realized by means of a transceiver coupled with a retroreflector, which retrieves path-averaged gas concentrations by a long-path transmission measurement. A schematic diagram is shown in Fig. 13.4 to describe typical TDLAS systems with three different modes of operation, that is, DAS, WMS and FMS. The optical signal is directly collected through the photodetector and then processed by the computer, as shown in Fig. 13.4a. DAS 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. Since DAS is vulnerable to the effects of background noise, it is seldom used for standoff detection applications.

Fig. 13.4

Schemes of open-path spectroscopic gas systems with an optical reflector and direct absorption spectroscopy (DAS) (a), wavelength modulation spectroscopy (WMS) (b) and frequency modulation spectroscopy (FMS) (c). RF radio frequency, LIA lock-in amplifier, OAPM off-axis parabolic mirror. (Reprinted from Ref. [42]. Published 2020 by MDPI as open access)

Alternatively, modulation spectroscopy is widely used to improve SNR. As shown in Fig. 13.4b, WMS is characterized by the use of a large modulation depth generating a large number of sidebands, and a modulation frequency is much smaller than the linewidth of the target gas (f < 1 MHz). The modulated laser is then collimated and passes through a gaseous medium through an off-axis parabolic mirror (OAPM) with a small hole and then illuminates onto an opposite optical reflector. The reflected laser propagates back to the gaseous medium and is concentrated by the OAPM onto a photodetector. The detector signal is demodulated in two LIAs, respectively, with reference-in signals of one- and two-times modulation frequency. The first (1f) and second harmonic (2f) signals are obtained simultaneously and then processed in a computer or an embedded processor for WMS-2f/1f calibration-free measurement to avoid the influence of light intensity fluctuations in open-path detection. While in the case of FMS, the modulation depth is small, but the frequency is very high (f > 100 MHz), which is the same magnitude as the line width of the target gas. Therefore, a radio frequency (RF) signal generator is usually utilized to generate such a high modulation signal, and a bias tee is used to superimpose the RF signal with the scanning signal to drive the laser, as shown in Fig. 13.4c. The frequency-modulated laser reflected by the mirror at a far end carries both absorption and dispersion information of the target gas, which is detected by a high-speed detector. The detector signal is sent to a radio frequency lock-in amplifier (RF-LIA). The absorption component and dispersion component are obtained by in-phase and quadrature-phase demodulation in the RF-LIA, respectively, to retrieve gas concentration [40]. Although FMS has lower 1/f noise, it has a higher cost of LIA and other optoelectronic devices than WMS as a result of the requirement for higher modulation frequency. Therefore, WMS techniques are most widely used in TDLAS for standoff gas detection [41, 42].

Obviously, the scheme is like the TDLAS with a gas cell system, except that the open path is used to replace the multipath gas absorption cell. Moreover, there is almost no extra loss in detection capability compared with the latter, due to the use of optical reflectors. A broadly tunable laser, for example, ECQCL or II–IV laser, could be used for standoff gas-phase chemical detection over hundred-metre distances using a corner-cube retroreflector, from which multiple gases could be monitored in real time.

With open-path spectroscopic gas sensing, a safeguard concept of an optical fence is developed for the protection of outdoor facilities, such as for leakage detection and warning in the oil depot [43], as shown in Fig. 13.5. The transceiver of the system is directed in sequence to inexpensive retroreflectors placed at opposite ends of the facility and measures the transmittance along each of the optical paths, as shown in Fig. 13.5. If a threat cloud crosses the ‘optical fence’, the sensing system will detect its presence. In addition to sequentially interrogating multiple-beam paths, it is also practicable to configure a system of mirrors such that the laser beam crosses the facility in such a way that the transceiver does not require to move. Moreover, the system is suitable for the protection of large indoor areas, such as airports. Advantages of this method include the capability of detecting a diversity of gases with high sensitivity over large distances (several kilometres). Furthermore, the system needs no consumables and leads to low maintenance compared to a network of point sensors.

Fig. 13.5

Optical fence system for threat cloud detection and facilities protection by constructing an ‘optical fence’ with (a) laser transceiver scanning mode and (b) laser transceiver fixed mode. (Reprinted from Ref. [42]. Published 2020 by MDPI as open access)

Note that the WMS signal is intensity-dependent and requires normalization for the received light power, which might be a major source of significant measurement errors in particular at low-light conditions. In order to mitigate this limitation of WMS, Wysocki and Weidman introduced a novel technique, named chirped laser dispersion spectroscopy (CLaDS) [44], for quantitative trace gas detection based on molecular dispersion measurements. Rather than measuring absorption, ClaDS detects refractive index changes that occur in the vicinity of a molecular transition. The essence of phase detection makes it highly immune to amplitude noise and transmission fluctuations, which means ClaDS is particularly suitable for open-path remote or standoff sensing.

3.3.2 Standoff Sensing Without Retroreflectors

Although the TDLAS systems can achieve highly sensitive detection in open-path with the help of retroreflectors or other cooperative targets, they are not feasible for mobile or multidirectional measurements. This is because the direction of the beam is defined by the system layout and has to be realigned when the system is moved. In the last decade, special attention has been devoted to the research and development of a real standoff sensing with backscattered light with non-co-operators, called backscatter-TDLAS. Here in this section, we focus on this kind of standoff open-path system that retrieves path-averaged gas concentrations by collecting the backscattered light from a distant non-cooperative topographic target.

In backscatter-TDLAS, a transceiver projects the laser beam onto a remote non-cooperative surface instead of an optical reflector and only receives a small fraction of the passively scattered laser light returned from the surface. The biggest challenge of this technique is the collection of weak backscattered light to ensure adequate detection of SNR. The commonly used optical structure for backscatter-TDLAS can be generally divided into two categories. The first approach uses a large-diameter Fresnel lens [45], as shown in Fig. 13.6a. The simple structure is conducive to integrating a compact, lightweight and low-cost handheld system for short-path standoff detection. A 5 cm diameter receiver is adequate to collect sufficient scattered light for ~10 m distance standoff detection [45]. The second approach, which utilizes a large-aperture telescope [46], as shown in Fig. 13.6b, can be used for long-range standoff detection. On the other hand, the overall performance of the instrument greatly depends on the optical properties of the backscattering target. Most recently, backscattering properties and hemispherical reflectance of some common topographic targets have been measured in the visible, NIR and MIR spectral ranges [47], which is useful for optimizing active standoff TDLAS detection and DIAL with hard-target, as well as for increasing their overall efficiency.

Fig. 13.6

Typical transceiver units of the TDLAS system with the non-cooperative target (a) with a single Fresnel lens and (b) with a large-aperture telescope. (Reprinted from Ref. [42]. Published 2020 by MDPI as open access)

The typical application of backscatter-TDLAS is to detect hazardous chemicals leakage in the field of pipelines, for example, CH₄, H₂S, etc. The characteristic absorption of CH₄ at 1.65 μm band is a superior choice for commercial sensors, because of the rugged and easy-to-align fibre-coupled DFB laser diode, the low-cost optical components commercially available in optical communication and the field-tested durability. A handheld remote methane leak detector employs a 10 mW scale laser and is capable of detecting a few ppm-m methane at about 30 m range [48]. By inserting an erbium-doped fibre amplifier that boosts the laser output power ~500× to the 5 W scale, and increasing the telescope diameter, they have extended the standoff distance to 3000 m (increased the standoff range by approximately two orders of magnitude) [49]. Then, they provided relatively low-cost lightweight and battery-powered aerial leak sensors. A miniaturized ultra-lightweight TDLAS sensor flies aboard a small quad-rotor uncrewed aerial vehicle (UAV) for landfill methane monitoring less than 1 W of electrical power [49]. Additionally, they presented measured range limits from a variety of common tomographic targets (as many as 22 different surfaces). The latest progress is that a miniaturized, downward-facing remote methane leak detector mounted on a small UAV has been developed to investigate natural gas fugitive leaks and further to localize the leakage, as well as quantify the emission rate. Recently, benefit from the broad tuning range and high power, DFB QCL has become a reliable MIR light source for a variety of trace gas sensing applications either in pulsed or continuous mode. DFB QCLs can work at room temperature with a high output optical power, are relatively compact and can address mid- to long-wave infrared spectral range applications where many trace gases exhibit significant absorption features. Consequently, DFB QCLs have been successfully used for many standoff detection applications with sub-ppm levels, for example, CH₄, N₂O, CO, CO₂, peroxide-based explosives, etc.

TDLAS standoff detection, based on MIR DFB QCLs with a non-cooperative target, is not only employed to detect chemicals in the gas phase but is also used as a powerful method to identify bulk materials and trace contaminants on surfaces. Most recently, a TDLAS system with three MIR QCLs has been reported for detection and quantification of explosives in soils at a distance of tens of centimetres. Using multivariate analysis and artificial intelligence techniques, the system is capable of distinguishing between soils contaminated with DNT, TNT or RDX and uncontaminated soils with high accuracy.

The concept, based on TDLAS standoff detection with non-cooperative targets, has also been employed for in situ measurement of combustion diagnosis. A single-ended laser-absorption-spectroscopy (SE-LAS) sensor has been developed to the anal size of the return scattering light from native surfaces, such as the piston of an automotive engine, which benefits of ease of installation and mitigates the invasive drawbacks [50]. The SE-LAS sensor can collect 10 μW backscatters through a 2-mm-diameter aperture in the case of a 20 mW DFB laser as the light source. Afterwards, the authors demonstrated the feasibility of spatially resolved measurements of gas properties using a SE-LAS sensor in conjunction with two-line thermometry. A 1-D distribution of H₂O mole fraction and temperature with a spatial resolution of 5 mm were obtained. The method can be extended to measurements for other species’ distribution and 2-D scanning. SE-LAS MIR sensors based on QCLs have also been developed for simultaneous in situ measurements of H₂O, CO₂, CO and temperature in combustion flows [50]. Most recently, C. S. Goldenstein has designed and demonstrated a compact SE-LAS sensor for measuring temperature and H₂O in high-temperature combustion gases by collecting laser light backscattered off native surfaces [51]. The SE-LAS sensor achieved an optical collection efficiency and provided a measurement accuracy and precision that is similar to or better than the conventional line-of-sight-based LAS sensor.

In practical remote detection by standoff WMS with a non-cooperative target, the received light energy varies, due to the variation of the scattering surface characteristics, as well as the change of the distance between the sensor and the tomographic surface. Therefore, measures should be taken to eliminate the fluctuation of the light intensity detected, among which the most commonly used effective method is using the first harmonic WMS-1f to normalize the second harmonic WMS-2f, namely WMS-2f/1f technique. Indeed, there are several methods to realize the calibration-free measurement [52]. Ding et al. [53] put forward a scheme called ‘Baselineoffset’ WMS, which means the zero response of the detector has been offset by a reference cell inserted into the measuring optical path. This scheme inherits the merits of WMS and can achieve high SNR, especially in a low-concentration environment.

This type of standoff detection has great potential in applications of atmospheric environmental monitoring, leak detection and security early warning, benefiting from great robustness and flexibility. The current performance of this technique is summarized in. However, as we can see, the detection distance of TDLAS with non-cooperative target ranges only from a few metres to tens of metres. Although it can be extended by approximately two orders of magnitude via utilizing a fibre-amplified source and increasing the telescope diameter, standoff range improvement needs to be compromised with human eye safety in practical applications. Another research direction is to develop chip-scale low-power integrated-optic gas-phase chemical sensors based on TDLAS, which are beneficial towards robot-assisted gas remote sensing and leak rate quantification and localization with small UAVs.

3.4 Spectroscopic Imaging

Two-dimensional (2D) or even three-dimensional (3D) measurement of gas flow field has attracted more and more attention, since the actual flow field is usually non-uniform distribution. For example, in the combustion flow field, there will be obvious gradients in gas concentration and temperature due to factors such as gas flow, mixing, chemical reaction and heat conduction. The two-dimensional spatial imaging of gas flow field is helpful to analyse the reaction mechanism of combustion, so as to verify the theoretical model and numerical model of the combustion flow field. Moreover, in the application of leakage detection, high spatial resolution imaging of leakage gas is helpful to locate the leakage point accurately. Imaging the gas flow field can make people more intuitive to observe and study the flow field state. Therefore, various laser imaging methods for gas flow fields have been developed in recent years.

By means of the interaction of emission, scattering, refraction and absorption, several non-invasive optical measurement techniques have been used to image the gas flow field. Some representative active laser gas imaging techniques mainly include laser-induced fluorescence (LIF), Rayleigh Scattering, Raman Scattering, LAS, etc. LIF is a promising optical measurement technique to characterize concentration fields with high spatial and temporal resolution; however, the high optical complexity and high cost hinder its practical applications. Rayleigh scattering is a simple way to detect the molecular number density and temperature of the flow field, but it is not selective. Raman scattering provides a detection method for gases with no inherent dipole moment. The system is simple in structure, but the scattering signal is very weak, usually requiring a very high-power light source. LAS employs laser as the light source and measures the flow field information by detecting the change in the intensity of the laser after transmission through the gas flow field. LAS is the most widely used technology for 2D measurement in the actual flow field due to its stability and robustness.

There are mainly two ways for LAS to realize 2D gas flow measurement, that is, laser absorption tomography (LAT) and laser absorption imaging (LAI). LAT technology is a combination of TDLAS and computer tomography (CT) technology. In order to cover the entire gas flow field to be measured, a typical LAT system is by using motion machinery to control a single transceiver to scan and measure from different angles and positions (Fig. 13.7a) or arrange multiple laser transceivers at different angles (Fig. 13.7b). The absorbance of multiple light beams in different directions, that is, the projections, is used to reconstruct the two-dimensional distribution of the gas concentration or temperature field by means of a tomographic algorithm. LAT with mobile scanning method can obtain many projections, but its time resolution is low and cannot meet the needs of combustion diagnosis due to the use of scanning components. Therefore, LAT with a fixed optical path method is more preferable in practical applications. Reasonable beam arrangements and reconstruction algorithms are helpful to improve the quality of reconstructed images, which makes LAT competitive for online and in situ combustion diagnosis.

Fig. 13.7

Comparison of three imaging methods. (a) Moving Line-of-Sight LAS; (b) Multi-projection TDLAT technology; (c) LAI method

LAI technology is developed based on LAT technology, which utilizes high-speed infrared cameras instead of traditional photodetectors to achieve projection acquisition as shown in Fig. 13.7c. The system is relatively simple and has an extremely high spatial resolution. The optical arrangement for the LAI system involves one or more high-speed infrared cameras that image a flow-field backlit with tunable infrared laser radiation. Using MIR LAI technology, Wei et al. [54] characterize the thermochemical structure of a laboratory Bunsen style C₂H₆-air flame by 2D measuring C₂H₆ (@3.34 μm), CO (@4.97 μm) and CO₂ (@4.19 μm), respectively, in the axisymmetric laminar flame, with an effective spatial resolution for LOS absorbance of ∼50 μm in the horizontal direction and ∼125 μm in the vertical direction. Subsequently, the research group used Tikhonov regularized linear tomography method to extend the mid-infrared LAI to 3D measurement of flame temperature and species concentration. Most recently, the team developed a deep learning method for laser absorption tomography to effectively integrate physical prior information related to flow field thermochemistry and transmission. Compared with LAT technology, the advantage of LAI technology is that it increases the spatial resolution to an order of micrometres. This is of great significance to the combustion field with steep gradients. However, limited by the frame rate of the camera, LAI is usually based on DAS technology, which is impeded to further improve the measurement sensitivity with the help of the anti-noise advantage of WMS technology.

In addition, mid-infrared LAI is a powerful tool for a sensitive and quantitative visualization of gas leaks [55]. A standoff methane leak detection system has been developed recently within 2 m by a 3270 nm ICL and an infrared camera, which is demonstrated visualization of methane leakage rates down to 2 ml/min by images and sequences at frame rates up to 125 Hz. The gas plume and leak can be localized and quantified within a single image by DAS with pixel-wise sensitivities around 1 ppm·m. This method improves the efficiency of leakage detection and localization and provides a new solution for safety monitoring of the petroleum industry and other industrial applications (Fig. 13.8).

Fig. 13.8

Diagram of the standoff MIR LAI experiment system built by the Fraunhofer Institute of Physical Measurement Technology in Germany. (Reprinted from Ref. [55]. Published 2021 by the Optical Society as open access)

4 II–VI Laser Application in Spectroscopic Gas Sensing

II–VI semiconductors, comprising elements of Groups II and VI of the Periodic Table, include materials ME (M = Cd, Zn, Hg; E S, Se and Te), such as cadmium selenide (CdSe), cadmium sulphide (CdS), cadmium telluride (CdTe), zinc selenide (ZnSe), zinc sulphide (ZnS) and zinc telluride (ZnTe). II–VI semiconductors feature broad infrared transparency, low phonon frequency and low optical losses. When doped with transition metal ions, these media exhibit a four-level energy structure, the absence of excited state absorption and broad absorption and emission bands. Significant results have been achieved in the development of II–VI semiconductors-based MIR tunable lasers, for example, Cr:ZnSe, Cr:ZnS and Fe:ZnSe. These results include access to a broad spectral range of 1.8–8.4 μm with high quantum efficiency, tunability exceeding thousands of nanometres, output powers exceeding hundred-Watt in continuous-wave operation, multi-Joule output energies in free-running and gain-switched regimes [56, 57].

The II–VI semiconductors-based lasers are still in developing stages and rarely commercialized. Applications with the state of art of lasers are still not very common. Even though, a few pioneer reports show exciting and glorious prospects. Cr:ZnSe lasers are the pioneers in the applications of gas sensing for their broad tuning ranges and high output power in the 2–3.5 μm wavelength region. Fischer et al. [58] presented the first photoacoustic spectroscopic measurements of trace gas using Cr²+:ZnSe laser emitting at wavelengths between 2.2 and 2.8 μm. Their experimental setup, shown in Fig. 13.9, consists of an continuous wave (cw) Cr²+:ZnSe laser source, a chopper, a PA cell and a detector employed as power reference in order to power-normalize the detected PA signal. An FTIR spectrometer is used to monitor and to coarse-tune the wavelength.

Fig. 13.9

Experimental set-up consisting of the Er³⁺:fibre laser pumped Cr²⁺:ZnSe laser source, a chopper operated at 5.7 kHz, the PA gas cell and a detector for power normalizing the measured PA signal. OC output coupler, HR high reflectivity mirror. (Reprinted with permission from Ref. [58]. Copyright 2005: Elsevier)

The average output power of the Cr²+:ZnSe laser is up to 500 mW across the entire tuning range of 2.2–2.8 μm. The wavelength selection of the laser is provided by a tandem of fused silica prisms. Wavelength tuning is performed by rotating the end mirror. The coarse wavelength tuning was obtained by a micrometre screw, while fine-tuning and spectrum scanning was achieved additionally via a piezo actuator. The gas measurement cell is a single-pass in-house built PA cell equipped with four microphones. The measured data is compared with the calculated data taken from the HITRAN database which results in a conversion coefficient from the measured normalized PA signal. They presented measurements on methane, nitrous oxide, and ambient air and deduced detection limits of 0.2 ppm for carbon dioxide, 0.8 ppm for methane and 2.7 ppm for carbon monoxide.

Fjodorow et al. [59] demonstrated an intracavity absorption spectroscopy (ICLAS) system by using a single-crystal pulsed Fe:ZnSe laser continuously tunable from 3.76 to 5.29 μm at room temperature. The experimental setup consists of a laser resonator formed by two aluminium mirrors M1 and M2, an Fe:ZnSe crystal placed close to M1 under an angle of θ = 51° to the normal of M1. A gas cell is installed in the resonator for ICAS measurements in human breath and in gas flows of N₂O (diluted in N₂), as shown in Fig. 13.10.

Fig. 13.10

Intracavity laser absorption spectroscopic sensing system. (Reprinted from Ref. [59]. Published 2021 by the Optical Society as open access)

The system was applied for measurements of ① CO₂ isotopes in the atmosphere and in human breath, ② CO in breath (after cigarette smoking) and in the smoke of a smouldering paper and ③ N₂O in a gas flow. They achieved detection limits: 0.1 ppm for ¹²CO₂ and ¹³CO₂, 3 ppm for CO and 1 ppm for N₂O. The authors believe that the low sensitivity is primarily limited by the short pump-pulse duration of 40 ns. Possibilities for sensitivity enhancement by up to a factor of 10⁷ are discussed.

Compared to conventional spectroscopy techniques, ICLAS offers several key advantages. In contrast to the conventional scheme of light source → sample → detector, with ICAS the sample is placed inside the laser resonator. The successive interaction of laser photons with the broadband gain and the narrow-band absorption determines the laser emission spectrum. The laser light passes through the sample many times, resulting in large effective absorption path lengths of up to 7 × 10⁷ m [59]. The most important advantage of ICAS compared to conventional absorption spectroscopy techniques is the ability to compensate broadband losses (e.g. light scattering and absorption by particles or by dirty windows of a technical apparatus, as well as beam steering) by the broadband gain medium. This mechanism makes ICAS unique and ideally suitable for measurements in challenging environments.

Wang et al. [60] reported a single-frequency continuous wave (cw) Cr:ZnSe laser with tuning range of 427 nm from 2164 to 2591 nm potentially competitive for trace gas sensing applications. Bernhardt et al. [61] presented a proof-of-principle experiment of frequency-comb Fourier-transform spectroscopy with two Cr²⁺:ZnSe femtosecond oscillators directly emitting in the 2.4 μm mid-infrared region. The acetylene absorption spectrum in the region of the ν₁ + ν₅₁ band, extending from 2370 to 2525 nm, could be recorded within a 10 μs acquisition time without averaging with 12 GHz resolution. Voronina et al. [62] presented a differential absorption LIDAR for atmospheric constituents and pollutants measurements using a Cr²⁺:ZnSe laser with spectral wavelength tunability range on a line-by-line basis. Frolov et al. [63] developed single-crystal Fe:CdTe lasers, which could operate in 77 K and room temperature pumped by 40-ns pulses from a Q-switched Er:YAG laser at 2.94 μm or a Fe:ZnSe laser at 4.1 μm. Furthermore, a record 2300-nm smooth and continuous wavelength tunability over 4.5–6.8 μm is achieved. They used the Fe:CdTe laser for intracavity absorption spectroscopy of atmospheric H₂O sensing.

5 Prospects for II–VI Laser in Spectroscopic Gas Sensing

MIR spectral trace gas sensing is particularly attractive for the unique and strong fingerprint absorption of the molecule. A higher sensitivity has been achieved by solving all the challenges of the spectral feature, that is, broad, serried crowded and even overlapped, in the MIR region. Benefiting from the progress of high-power and broad tuning of MIR lasers and also the spectroscopic technologies of multicomponent simultaneous detection is an expected achievement.

The development of high-performance MIR lasers in the aspects of compact, output power of hundreds of milliwatts, low emission linewidth of ~100 kHz and broad tuning range will promote the development and application of MIR tunable laser-based trace gas sensors, especially in multiple gas sensing, large molecules or organic molecules and remote sensing applications. Though more than one hundred types of gas have been detected by tunable laser-based sensors, there are huge demands for higher detection sensitivity, or in extreme conditions, or scientific exploration, or more other types of gas need to be detected. Thus, we believe greater progress will be performed in the next decade, which may include the following:

• 1. Multicomponent and VOCs Sensing System. Multicomponent sensors will achieve a more progress benefiting from the wider wavelength coverage by II–VI semiconductors-based laser, integrated laser arrays, OFC or EC-QCL. More species could be detected simultaneously by a particular devised broadband laser, which could expand their applications in scientific research, including combustion diagnosis, chemical reaction process dynamics, exhaled breath analysis and metabolomics.

• 2. Standoff Remote Sensing. The techniques of open-path standoff detection by backscatter MIR light provide a promising way for prompt and flexible assessment of atmospheric environmental, leaks, explosive and security by handheld devices or equipped with UAV. The detection sensitivity could be substantially improved by newly developed high-performance MIR detectors and the progress of high-power DFB-QCL.

• 3. Ultra-sensitive Sensing. With the development of a mid-infrared laser source and high-performance detector, combined with cavity enhancement technology and noise immunity technology, ultra-high detection sensitivity becomes possible.