Hydrogen Sensor Based on Tunable Diode Laser Absorption Spectroscopy

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Article Hydrogen Sensor Based on Tunable Diode Laser Absorption Spectroscopy

Viacheslav Avetisov 1,*, Ove Bjoroey 1, Junyang Wang 1,2, Peter Geiser 1 and Ketil Gorm Paulsen 1

1 NEO Monitors AS, Prost Stabels vei 22, N-2019 Skedsmokorset, Norway; [email protected] (O.B.); [email protected] (P.G.); [email protected] (K.G.P.) 2 Yinian Sensors Technology Co., Ltd., Shenzhen 518126, China; [email protected] * Correspondence: [email protected]

Received: 1 November 2019; Accepted: 30 November 2019; Published: 3 December 2019

Abstract: A laser-based hydrogen (H2) sensor using wavelength modulation spectroscopy (WMS) was developed for the contactless measurement of molecular hydrogen. The sensor uses a distributed feedback (DFB) laser to target the H2 quadrupole absorption line at 2121.8 nm. The H2 absorption line exhibited weak collisional broadening and strong collisional narrowing effects. Both effects were investigated by comparing measurements of the absorption linewidth with detailed models using different line profiles including collisional narrowing effects. The collisional broadening and narrowing parameters were determined for pure hydrogen as well as for hydrogen in nitrogen and air. The performance of the sensor was evaluated and the sensor applicability for H2 measurement in a range of 0–10 %v of H2 was demonstrated. A precision of 0.02 %v was achieved with 1 m of absorption pathlength (0.02 %v m) and 1 s of integration time. For the optimum averaging time of · 20 s, precision of 0.005 %v m was achieved. A good linear relationship between H concentration and · 2 sensor response was observed. A simple and robust transmitter–receiver configuration of the sensor allows in situ installation in harsh industrial environments.

Keywords: gas sensor; hydrogen sensor; diode laser; TDLAS; WMS; absorption spectroscopy; laser spectroscopy; hydrogen

1. Introduction An increased demand for hydrogen gas sensors is strongly coupled with the expanded use of hydrogen gas (H2) in industry [1,2]. Hydrogen is an important feedstock in many industrial processes and applications, including the oil and gas industry, chemical plants, and the steel industry, among others. Refineries use hydrogen in many operations (e.g., hydrotreating of various refinery process streams and hydrocracking of heavy hydrocarbons). Analyzing hydrogen in complex and varying gas mixtures is challenging, and measurements are normally performed using gas chromatographs that entail slow response times and high operational costs. Since hydrogen is highly flammable, strict regulations for using H2 safety sensors apply. Many different types of hydrogen safety sensors are commercially available [3], and the common principle is a sensing element that is altered (e.g., by resistance) when in contact with hydrogen. This mode of operation precludes the use of these point-type hydrogen sensors in reactive, corrosive, and/or dusty gas streams. For this reason, contactless hydrogen sensing is highly desired. Diode lasers are extremely attractive for this purpose due to their inherently low-intensity noise and narrow linewidth. These properties enable the highly selective and sensitive probing of narrow and extremely weak H2 absorption lines. As a diatomic homonuclear molecule, H2 has no dipole moment that could create strong optical absorption in the infrared region. The absorption spectrum of H2 is therefore limited to vibrational bands of very weak

Sensors 2019, 19, 5313; doi:10.3390/s19235313 www.mdpi.com/journal/sensors Sensors 2019, 19, 5313 2 of 13

electric quadrupole transitions [4], which is why laser-based detection of H2 so far has been limited to extractive cavity-enhanced sensors based on cavity ring-down spectroscopy (CRDS) [5,6], intra-cavity output spectroscopy (ICOS) [7,8], and optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) [9]. Common in these techniques is confinement of the laser light in a high-finesse optical cavity by using a set of highly reflective mirrors. Up to several kilometers of effective absorption pathlength can be obtained, which allows for detection of very weak hydrogen quadrupole transitions. However, for use in industrial applications, where extremely high sample purity can be difficult to achieve, contamination of mirrors is an issue. If not properly handled, this will lead to degradation of the sensor sensitivity and can ultimately damage the coatings of the high-reflectivity mirrors. For this reason, cavity-based H2 sensors often impose rigid requirements on gas sampling and conditioning systems and typically require periodic maintenance. Tunable diode laser absorption spectroscopy (TDLAS) [10,11] is a sensitive and selective method that directly probes the process (in situ) without having to extract gas samples. Gas sensors based on TDLAS are frequently used for many industrial process-control, emission-monitoring, and safety applications, and are well accepted throughout many industries [12–16]. The underlying measurement principle is inherently contactless, and the instrumentation is consequently not exposed to potentially corrosive process gases. Concentration readings are made available in real time, which is ideal for fast and efficient process control and safety-related measurements. Furthermore, in situ measurements have low maintenance requirements and thus reduce the operational costs. In general, compared to cavity-enhanced techniques, TDLAS has less complexity and is more robust, which has made TDLAS-based sensors the preferred platform for many industrial process-control and safety applications. In this paper, we present the first laser-based infrared hydrogen absorption sensor for in situ hydrogen measurements. The sensor can be reconfigured for extractive measurements for applications where in situ installation is not feasible due to, for example, high pressure and/or high temperature. Optical windows isolate the process gas from the sensor so that the advantage of contactless measurement is maintained. The presented hydrogen sensor is based on the commercial LaserGas II platform [17] manufactured by NEO Monitors AS. The instrument was used to study a selected hydrogen absorption line in terms of line broadening and narrowing effects. To validate the measurements, lineshape modelling using different spectral line profiles was performed. The sensor was capable of measuring hydrogen with a precision of 0.02 %v m for 1 s of integration time, which is · better than most intended safety applications require (assuming a measurement range of 0–10 %v). Using a response time of 1 s, an estimated limit of detection (LOD) for H2 of 0.1 %v for 1 m of absorption pathlength was achieved.

2. Sensor Design The developed sensor follows the classical in situ TDLAS design and consists of transmitter and receiver units. The transmitter unit contains a diode laser, collimating optics, a microprocessor board, and all input–output electronics. The transmitter unit also has a built-in cell for H2 validation. The receiver unit incorporates a photodetector, focusing optics, and signal detection electronics (ampliﬁer, mixer, etc.). The sensor is based on the wavelength modulation spectroscopy (WMS) technique, which is well described in the literature [18–20]. This technique has been proven to be very useful in trace gas sensing due to its ability to perform very sensitive interference-free measurements directly in the process or across stacks without sample extraction and preconditioning. Since WMS provides nominally baseline-free absorption signals, it is especially suited for measuring weak absorbance. Recently published comparisons of WMS and direct absorption spectroscopy (DAS) techniques revealed that WMS is approximately one order of magnitude more sensitive [21–23]. Figure1a shows a photograph of the LaserGas II sensor mounted on the demo pipe using DN50 ﬂanges, and Figure1b depicts a schematic diagram and the basic principle of the sensor operation. Sensors 2019, 19, 5313 3 of 13 Sensors 2019, 19, x 3 of 14

Figure 1. (a) Tunable diode laser absorption spectroscopy (TDLAS) H2 sensor mounted on a demo pipe. FigureTransmitter 1. (a) unit Tunable is on thediode left andlaser receiver absorption unit onspectroscopy the right. Gas (TDLAS) inlet and H2 outlet sensor of themounted built-in on validation a demo pipe.gas cell Transmitter are indicated. unit is (b on) Schematic the left and overview receiver of un theit on principles the right. of Gas sensor inlet operation. and outlet A of sinusoidally the built-in validationmodulated gas current cell are ramp indicated. is applied (b to) Schematic the laser, whichoverview is swept of the in principles frequency of across sensor the operation. transition ofA sinusoidallyinterest. After modulated interacting current with the ramp sample, is applied the absorption to the laser, information which is swept is encoded in frequency in the transmitted across the transitionintensity, whichof interest. is measured After interacting using a photodetector.with the sample, The the photodetector absorption inform signalation is amplified, is encoded filtered, in the transmittedmixed, anddigitized. intensity, Finally, which digital is measured signal processing using a is photodetector. used to retrieve theThe concentration photodetector (and signal possibly is amplified,other relevant filtered, parameters). mixed, and digitized. Finally, digital signal processing is used to retrieve the concentration (and possibly other relevant parameters). A distributed feedback (DFB) diode laser from Nanoplus (Nanosystems and Technologies GmbH) emitting near 2122 nm was used in the sensor. The laser had an output power of about 5 mW at the A distributed feedback (DFB) diode laser from Nanoplus (Nanosystems and Technologies driving conditions specified herein. The temperature of the diode laser was stabilized at around 30 C GmbH) emitting near 2122 nm was used in the sensor. The laser had an output power of about 5 mW◦ with high accuracy (typically in the mK range) to set the average emission wavelength of the laser. at the driving conditions specified herein. The temperature of the diode laser was stabilized at around A DC current of 70 mA was applied to operate the laser above its threshold, and a current ramp of 30 °C with high accuracy (typically in the mK range) to set the average emission wavelength of the 10 mA with a duration of about 2 ms was used to tune the laser about 0.4 cm 1 across the H2 absorption laser. A DC current of 70 mA was applied to operate the laser above its threshold,− and a current ramp line of interest. The repetition rate of the current ramps was 150 Hz. After each ramp, the laser current of 10 mA with a duration of about 2 ms was used to tune the laser about 0.4 cm–1 across the H2 was switched off for signal normalization purposes. A sinusoidal current of 2 mA amplitude and absorption line of interest. The repetition rate of the current ramps was 150 Hz. After each ramp, the frequency f of 100 kHz was added to the current ramp in order to modulate the laser wavelength laser current was switched off for signal normalization purposes. A sinusoidal current of 2 mA for WMS implementation. The collimated laser beam was directed through the target gas, captured amplitude and frequency f of 100 kHz was added to the current ramp in order to modulate the laser by the receiver optics, and focused onto an InGaAs pin photodetector (Hamamatsu G12183-020K). wavelength for WMS implementation. The collimated laser beam was directed through the target The signal detected by the photodetector was bandpass-filtered to select the 2f component, and the gas, captured by the receiver optics, and focused onto an InGaAs pin photodetector (Hamamatsu filtered signal was detected using an analog mixer, a low-pass filter, and an amplifier. The bandwidth G12183-020K). The signal detected by the photodetector was bandpass-filtered to select the 2f of the detected signal was 10 kHz. Further, the 2f WMS signal was digitized using an AD converter component, and the filtered signal was detected using an analog mixer, a low-pass filter, and an and normalized to the measured direct signal. Before calculating the gas concentration, additional amplifier. The bandwidth of the detected signal was 10 kHz. Further, the 2f WMS signal was digitized digital signal processing was applied to improve the signal-to-noise ratio (SNR), which optionally using an AD converter and normalized to the measured direct signal. Before calculating the gas could be digital filtering, wavelet denoising, or baseline fitting. concentration, additional digital signal processing was applied to improve the signal-to-noise ratio (SNR), which optionally could be digital filtering, wavelet denoising, or baseline fitting.

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3.3. Line Line Selection Selection

TheThe absorption absorption spectrum spectrum of of H H2 2inin the the infrared infrared region region was was very very sparse sparse and and extremely extremely weak. weak. FigureFigure 22 shows shows a a simulation simulation of of H H2 2absorptionabsorption using using default default air air broadening broadening parameters parameters listed listed in in the the high-resolutionhigh-resolution transmission transmission molecular molecular absorption absorption database database (HITRAN) (HITRAN) [4]. [4 ].The The fundamental fundamental (1–0) (1–0) vibrationalvibrational electric–quadrupole electric–quadrupole transitions transitions of H of2 are H2 betweenare between 1900 1900and 3000 and nm. 3000 There nm. are There only are a few only linesa few visible lines due visible to the due large to therotational large rotational constant that constant follows that from follows the low from mass the of low the mass H2 molecule. of the H 2 Themolecule. first overtone The ﬁrst band overtone (2–0) band of (2–0)H2 is of located H2 is located between between 1100 1100and and1500 1500 nm. nm. Compared Compared to tothe the fundamentalfundamental band, band, the the overtones overtones ha haveve been been studied studied much much more more extensively extensively using using cavity-enhanced cavity-enhanced spectrometersspectrometers [6,24]. [6,24].

Figure 2. High-resolution transmission molecular absorption database (HITRAN) simulation of H2 Figureabsorption 2. High-resolution (1 %v m) using transmission default air broadening molecular absorp parameterstion database listed in the(HITRAN) HITRAN2016 simulation database. of H2 absorption (1 %v∙m)· using default air broadening parameters listed in the HITRAN2016 database. From the HITRAN simulations, three transitions in the fundamental vibrational band were 1 1 identifiedFrom the as potentiallyHITRAN simulations, suitable for Hthree2 gas transitions sensing: 2407 in the nm fundamental (4155.3 cm− ),vibrational 2223 nm (4497.8 band were cm− ), 1 –1 –1 identifiedand 2122 as nm potentially (4712.9 cm suitable− ). The for best H2available gas sensing: transition 2407 nm for (4155.3 industrial cm applications), 2223 nm (4497.8 is not cm necessarily), and –1 2122the nm strongest, (4712.9 butcm the). The one best with available least interference transition for from industrial gases such applications as water is vapor not necessarily (H2O), carbon the strongest,dioxide (CObut 2the), methane one with (CH least4), interference ammonia (NH from3), gases and carbonsuch as monoxidewater vapor (CO). (H2O), Figure carbon3 shows dioxide the (COsimulated2), methane transmission (CH4), ammonia spectra (NH of these3), and gases carbon for monoxide the wavelength (CO). Figure region 3 betweenshows the 2000 simulated nm and transmission2500 nm. The spectra hydrogen of these spectrum gases (line for positions)the wavelength is depicted region on between top of the 2000 plotted nm transmissionand 2500 nm. spectra. The hydrogenFor demonstration spectrum purposes,(line positions) H2O and is COdepicted2 concentrations on top of were the setplotted to 1 %v, transmission while CH4 ,spectra. NH3, and For CO demonstrationwere set to 0.1 purposes, %v. The hydrogen H2O and transition CO2 concentrations at 2122 nm were can be set identified to 1 %v, as while potentially CH4, NH best3, suitedand CO for werethe sensor,set to 0.1 since %v. itThe is nothydrogen only the transition strongest at H21222 line nm but can also be identified is located as at potentially the transmission best suited window for thefor sensor, the plotted since spectra.it is not only However, the strongest all these H2 gasesline but except also is CO located absorb at inthe the transmission close vicinity window of this for H 2 theline, plotted which spectra. is demonstrated However, by all the these inset gases of Figure except3 ,CO showing absorb a 1000in the times close magnified vicinity of portion this H2 ofline, the whichspectra is demonstrated around the 2122 by nmthe line.inset Eachof Figure of the 3, identifiedshowing a H 10002 lines times was magnified investigated portion in more of the detail spectra by a aroundcloser lookthe 2122 into nm high-resolution line. Each of simulatedthe identified spectra. H2 lines The Hwas2 line investigated at 2407 nm in is more close detail to the by strong a closer H2O lookabsorption into high-resolution band at 2700 nmsimulated and several spectra. strong The H 2HO2 linesline at overlap 2407 nm with is the close weak to Hthe2 line. strong Likewise, H2O absorptionthe line is withinband at the 2700 spectral nm and region several of the strong strong H CH2O lines4 absorption overlap band.with the Also, weak CO andH2 line. NH 3Likewise,should be theconsidered line is within to be the potentially spectral region interfering of the gases.strong ItCH should4 absorption be mentioned band. Also, that CO the and 2407 NH nm3 should line is freebe consideredof CO2 interference; to be potentially nevertheless, interfering using gases. this It line should for Hbe2 sensingmentioned is problematic that the 2407 due nm toline interference is free of COwith2 interference; the other mentioned nevertheless, gases. using The this H 2lineline for at H 22232 sensing nm is is free problematic of interference due to with interference CO2 and with H2O the(at other least mentioned around ambient gases. temperature),The H2 line at which2223 nm makes is free it attractiveof interference for some with applications.CO2 and H2O However,(at least aroundthe line ambient suffers fromtemperature), strong NH whic3 interferenceh makes it and, attractive to a somewhat for some lesser applications. extent, from However, CH4 interference the line suffersas well. from In addition,strong NH the3 interference strength of and, this lineto a issomewhat about one-third lesser extent, the strength from CH of the4 interference H2 line at as 2122 well. nm. InSince addition, the line the strength strength is of a crucialthis line factor is about for sensing one-third hydrogen, the strength the line of atthe 2122 H2 nmline (S(1) at 2122 (1–0) nm. transition, Since 1 the2121.8 line nm,strength 4712.9 is cma crucial− ) was factor chosen. for As sensing mentioned, hydrogen, this line the isline not at free 2122 of CHnm4 (S(1)and NH(1–0)3 interference,transition, –1 2121.8although nm, the4712.9 interference cm ) was is chosen. much weakerAs mentioned, than for this the line other is twonot free H2 lines.of CH In4 and addition, NH3 interference, a weak CO 2 1 althoughabsorption the lineinterference relatively is close much to weaker the H2 linethan (0.13 for the cm −otherapart) two must H2 lines. be considered In addition, for a applications weak CO2 –1 absorptionwith percentage line relatively levels of close CO2. to the H2 line (0.13 cm apart) must be considered for applications with percentage levels of CO2.

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Figure 3. HITRAN simulation of transmission spectra of H O, CO (1 %v m), CH , NH , and CO 2 2 · 4 3 (0.1Figure %v m)3. HITRAN and positions simulation of the Hof transmissionlines. Inset: enlarged spectra viewof H2 ofO, absorptionCO2 (1 %v∙ spectram), CH around4, NH3, theand 2122-nm CO (0.1 · 2 H%v2 line.∙m) and positions of the H2 lines. Inset: enlarged view of absorption spectra around the 2122-nm H2 line. 4. Experimental Results and Modelling of the H2 Lineshape 26 4. ExperimentalThe line strength Results of theand S(1) Modelling (1–0) H2 oftransition the H2 Lineshape is 3.2 10− cm/molecule [4]. Simulations using 1 × an air broadening coefficient of 0.05 cm− /atm (default value listed in HITRAN2016) and the Voigt The line strength of the S(1)6 (1–0) H2 transition is 3.2 × 10–26 cm/molecule [4]. Simulations using profile indicated about 5 10− relative peak absorbance for 1 %v H2 over 1 m of the absorption × –1 pathlengthan air broadening (1 %v m) coefficient under ambient of 0.05 conditionscm /atm (default (Figure 2value). Although listed in such HITRAN2016) weak absorbance and the can Voigt be profile indicated ·about 5 × 10–6 relative peak absorbance for 1 %v H2 over 1 meter of the absorption detected by an in situ TDLAS sensor, the line appears to be too weak for the sensor to detect H2 at sub-percentagepathlength (1 %v levels∙m) under (i.e., well ambient below conditions 1 %v m). (Figure However, 2). itAlthough was found such that weak simulations absorbance using can the be · Voigtdetected profile by an resulted in situ in TDLAS incorrect sensor, predictions the line in appears this particular to be too case, weak because for the the sensor Voigt to profile detect isH not2 at adequatesub-percentage for modelling levels (i.e., hydrogen well below absorption 1 %v∙m). lines. However, it was found that simulations using the Voigt profile resulted in incorrect predictions in this particular case, because the Voigt profile is not It is important to note that, due to the low mass of the H2 molecule, Doppler broadening is significantlyadequate for largermodelling than hydrogen what is typical absorption for other lines. molecules at this wavelength. The Doppler half It is important to note that, due to the low mass of the H2 molecule,1 Doppler broadening is width at half maximum (HWHM) for this transition is 0.0204 cm− at 296 K. The small collisional cross-sectionsignificantly andlarger absence than ofwhat a dipole is typical moment for resultother inmolecules very weak at collisional this wavelength. broadening, The while Doppler the highhalf –1 ratewidth of velocity-changingat half maximum collisions(HWHM) contributesfor this transition to unusually is 0.0204 strong cm narrowing at 296 K. (theThe Dickesmall narrowingcollisional ecross-sectionffect). The combinationand absence of a these dipole eff ectsmoment results result in ain line very profile weak thatcollisional deviated broadening, significantly while from the thehigh Voigt rate profile.of velocity-changing Hence, more advanced collisions profiles contributes must beto used,unusually such asstrong the Hartmann–Tran narrowing (the profileDicke (HTP)narrowing [25] oreffect). other The simpler combination profiles of that these include effects collisional results in narrowing,a line profile such that asdeviated the Rautian significantly profile (RP)from [26the] andVoigt Galatry profile. profile Hence, (GP) more [27 advanced]. Recently profiles non-Voigt must linebe used, profiles such have as the been Hartmann–Tran implemented profile (HTP) [25] or other simpler profiles that include collisional narrowing, such as the Rautian in the HITRAN database [28], where HTP was explicitly recommended for modelling the H2 profile (RP) [26] and Galatry profile (GP) [27]. Recently non-Voigt line profiles have been lines. However, by the time of writing, the non-Voigt line parameters for H2 in air were not availableimplemented in the in HITRANthe HITRAN database. database The [28], line where parameters HTP was used explicitly here for recommended the S(1) (1-0) for line modelling for pure the H2 lines. However, by the time of writing, the non-Voigt line parameters for H2 in air were not H2 (self-broadening) were reported by Wcisło et al. [28]. The reported self-broadening coefficient for available in the HITRAN database.1 The 1line parameters used here for the S(1) (1-0) line for pure H2 this H2 line was γsel f = 0.0019 cm− atm− , and the velocity-changing collision (narrowing) coefficient (self-broadening)vc were1 reported1 by Wcisło et al. [28]. The reported self-broadening coefficient for this was ν = 0.0448 cm− atm− . The combination of very weak self-broadening and strong narrowing sel f –1 –1 H2 line was 𝛾 = 0.0019 cm atm , and the velocity-changing collision (narrowing) coefficient was should result in sub-Doppler widths of the H2 line for pressures around ambient air pressure. Indeed, –1 –1 𝜈 = 0.0448 cm atm . The combination of very weak self-broadening and strong narrowing significant reduction of the H2 linewidth with pressure and a corresponding increase in peak amplitude should result in sub-Doppler widths of the H2 line for pressures around ambient air pressure. Indeed, have been reported for all studied H2 lines in the overtone (2-0) vibrational band [6,24] for pure H2 gas. However,significant nitrogen reduction and of air the broadening H2 linewidth and narrowing with pressure parameters and ahave corresponding not yet been increase reported. in peak amplitude have been reported for all studied H2 lines in the overtone (2-0) vibrational band [6,24] for

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In this paper, the results of measuring the self-broadening and narrowing parameters and the corresponding parameters for nitrogen and air for the S(1) (1–0) line are presented. Since the laser intensity modulation amplitude, modulation phase, and phase shift between the laser intensity and wavelength modulations all influence the shape of the WMS absorption signals, performing WMS signal fitting using advanced line profiles with many fitting parameters can be ambiguous. Such an approach requires an exceptional signal-to-noise ratio and precision of the recorded WMS signals (better than 1% rel.), which is a very challenging task, especially in cases of weak H2 transition. Since such precision could not be achieved using the given sensor, no WMS signal fittings were performed in this study. Another approach was chosen based on findings by Reid and Labrie [29], who investigated the behavior of the 2f WMS lineshape as a function of the ratio between modulation amplitude a and absorption HWHM ∆: m = a/∆. The WMS lineshapes for the Voigt profile and the limiting cases of the Voigt, Doppler, and Lorentzian profiles were modelled. It was found that the peak amplitude of the WMS lineshape (WMS-PA) was maximized at m = 2.2 independent of the ratio of collisional broadening to Doppler broadening (i.e., regardless of whether Voigt was in the Doppler or Lorentzian regime). To find out if the same value of m = 2.2 was also valid for an absorption lineshape exhibiting strong collisional narrowing, numerical simulations were performed for HTP, RP, and GP using the collisional broadening and narrowing parameters within the range of values reported for different H2 lines. In particular, the gas pressure region where collisional narrowing resulted in the most prominent effect on the H2 lineshape, between 0.1 and 1.0 atm, was of interest. The details of the RP and GP calculations can be found in the paper by Wang et al. [30]. HTP was calculated according to Forthomme et al. [31], who provide the Matlab implementation as a supplement to the paper. The instantaneous laser frequency of the laser modulated at a frequency f around its center frequency v can be written as v(t) = v + a cos(2π f t) (1)

The 2f WMS lineshape WMS2 f (ν, a) in the limit of low absorbance, which is true in this case, is proportional to the second Fourier component of the absorbance [32,33]. Since the absorbance in turn is proportional to the line profile function g(νd), where νd is detuning from the absorption line center ν (ν = v ν ), the 2f WMS lineshape can be calculated from the following integral: 0 d − 0 Z 1 τ WMS2 f (νd, a) g(vd + a cos(2π f t)) cos(2π2 f t)dt (2) ∝ τ 0 where τ is an integration time, τ f 1. − In the modelling of the WMS lineshapes using different line profiles, the line profile function (e.g., HTP) was substituted into Equation (2), and the integral was calculated numerically. The modelling shows that the peak value for WMS-PA appears to be approximately at the same value of m = 2.2 0.1 in each case of HTP, RP, and GP used in the WMS lineshape. Figure4 shows ± the results of modelling the S(1) (1–0) H2 line using HTP and the line parameters for 100% H2 gas by Wcisło et al. The results for the Doppler profile were obtained by setting the H2 pressure to P = 0.01 atm, which ensures that both collisional broadening and narrowing effects are negligible. The results for the Lorentzian profile were obtained by setting P = 10 atm, which ensures that both Doppler broadening and collisional narrowing are negligible. The plots for P = 0.25 atm and P = 0.5 atm represent the region where collisional narrowing is significant. No significant differences between the WMS-PA m-dependence for HTP, RP, and GP were found. This means that it is possible to indirectly measure the HWHM of an absorption line by varying the modulation amplitude of the laser while measuring the WMS-PA. Finally, the HWHM pressure dependence obtained from the measurements can be compared with the corresponding theoretical pressure dependence from the model. Such comparison should provide a good estimation of the collisional broadening and narrowing coefficients (with accuracy of the parameters determined by this method of about 10% rel.). Sensors 2019, 19, 5313 7 of 13 Sensors 2019, 19, x 7 of 14

Figure 4. CalculatedCalculated wavelength wavelength modulation modulation spectros spectroscopycopy (WMS) (WMS) peak peak amplitude amplitude (Hartmann–Tran (Hartmann–Tran proﬁle (HTP)) as a function of normalized modulation amplitude m for H at and 0.25 and 0.5 atm profile (HTP)) as a function of normalized modulation amplitude m for H2 at and 0.25 and 0.5 atm (100% H ) together with the Lorentzian limit at 10 atm and the Doppler limit at 0.01 atm. HTP line (100% H2) together with the Lorentzian limit at 10 atm and the Doppler limit at 0.01 atm. HTP line parameters for S(1) (1–0) H transition given in [28] were used in the modelling. parameters for S(1) (1–0) H2 transition given in [28] were used in the modelling.

To improve the SNR of the WMS-PA measurement, the sensor was connected to a Herriott-type To improve the SNR of the WMS-PA measurement, the sensor was connected to a Herriott-type multipass cell with 12 m pathlength, which was filled with pure H2 gas at different pressures up to multipass cell with 12 m pathlength, which was filled with pure H2 gas at different pressures up to 4 atm. For measurement in nitrogen and air gas mixtures, pure hydrogen was diluted to 10 %v in the 4 atm. For measurement in nitrogen and air gas mixtures, pure hydrogen was diluted to 10 %v in the corresponding balance gas using a HovaGAS G6 (IAS GmbH) gas mixer. The Doppler HWHM at corresponding balance gas using a HovaGAS G6 (IAS GmbH) gas mixer. The Doppler HWHM at 23 23 C of 0.204 cm 1 was used to calibrate the laser modulation amplitude a to the unit cm 1 such that °C ◦of 0.204 cm–1 was− used to calibrate the laser modulation amplitude a to the unit cm–1 such− that the the experimentally obtained HWHM converged to the theoretical Doppler HWHM at zero pressure. experimentally obtained HWHM converged to the theoretical Doppler HWHM at zero pressure. The The theoretical HWHM was obtained from the calculated HTP for different pressures. The results theoretical HWHM was obtained from the calculated HTP for different pressures. The results from from the calculations were compared with the measured values, and the broadening and narrowing the calculations were compared with the measured values, and the broadening and narrowing coefficients used in the calculations were adjusted until the best fit with the measured results was coefficients used in the calculations were adjusted until the best fit with the measured results was achieved (ignoring line shift effects). It should be noted that accuracy of the experimental data achieved (ignoring line shift effects). It should be noted that accuracy of the experimental data (measured WMS-PA as a function of m) did not allow us to distinguish between HTP, RP, and GP. (measured WMS-PA as a function of m) did not allow us to distinguish between HTP, RP, and GP. The calculated HWHM using any of these three profiles showed very similar pressure dependence, The calculated HWHM using any of these three profiles showed very similar pressure dependence, and the obtained broadening and narrowing parameters were essentially the same, while HTP was and the obtained broadening and narrowing parameters were essentially the same, while HTP was used solely to comply with the new recommendation for the H2 absorption lines adopted in HITRAN. used solely to comply with the new recommendation for the H2 absorption lines adopted in HITRAN. The parameter for broadening speed dependence had a negligible influence on the resulting pressure The parameter for broadening speed dependence had a negligible influence on the resulting pressure dependency of the HWHM. In the modelling, this parameter was set to one-tenth of the broadening dependency of the HWHM. In the modelling, this parameter was set to one-tenth of the broadening parameter, and the correlation parameter was set to zero. parameter, and the correlation parameter was set to zero. Figure5a shows the measured HWHM using the WMS-PA approach of the H 2 line for pure Figure 5a shows the measured HWHM using the WMS-PA approach of the H2 line for pure (100 (100 %v) H2 gas and the calculated HTP HWHM that resulted in the best fit. The coefficients obtained %v) H2 gas and the calculated HTP HWHM that resulted in the best fit. The coefficients obtained 1 1 vc 1 1 were γsel f = 0.0024(3) cm−–1 atm–1− and ν = 0.037(5) cm–1− atm–1− (T = 296 K). These coefficients are were 𝛾 = 0.0024(3) cm atm and 𝜈sel f = 0.037(5) cm atm (T = 296 K). These coefficients are in in reasonable agreement with the values reported by Wcisło et al.: 0.0019(1) and 0.0448, respectively reasonable agreement with the values reported by Wcisło et al.: 0.0019(1) and 0.0448, respectively (reported for T = 315 K). As seen in Figure5a, the absorption line of pure H 2 is extremely narrow (reported for T = 315 K). As seen in Figure 5a, the absorption line of pure H2 is extremely narrow (more than twice as narrow as the Doppler HWHM) in a broad pressure range from around ambient (more than twice as narrow as the Doppler HWHM) in a broad pressure range from around ambient up to several atm. For demonstration purposes, Figure5b compares HTP calculated for 1 atm using up to several atm. For demonstration purposes, Figure 5b compares HTP calculated for 1 atm using the obtained parameters with the Doppler profile of the same integral. The peak amplitude of the H2 the obtained parameters with the Doppler profile of the same integral. The peak amplitude of the H2 line (100%) at atmospheric pressure appears to be more than a factor of 1.6 larger than if the H2 line line (100%) at atmospheric pressure appears to be more than a factor of 1.6 larger than if the H2 line were pure Doppler (i.e., without any collisional broadening at all). were pure Doppler (i.e., without any collisional broadening at all).

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Figure 5. (a) Half width at half maximum (HWHM) of H2 100 %v as a function of pressure together

Figurewith a 5.fit (usinga) Half the width HTP; at half(b) calculated maximum HTP (HWHM) at P = of 1.0 H2 atm100 compared %v as a function to the of corresponding pressure together Doppler with aprofile. fit using the HTP; (b) calculated HTP at P = 1.0 atm compared to the corresponding Doppler profile. Figure 5. (a) Half width at half maximum (HWHM) of H2 100 %v as a function of pressure together with a fit using the HTP; (b) calculated HTP at P = 1.0 atm compared to the corresponding Doppler For gasgas sensing, sensing, it isit important is important to measure to measur the collisionale the collisional parameters parameters (broadening (broadening and narrowing) and profile. innarrowing) nitrogen andin nitrogen air balances. and air These balances. parameters These para aremeters not available are not in available the literature in the forliterature the given for the H2 transition. Figure6a shows the pressure dependence of measured H HWHM for 10% H in nitrogen givenFor H 2 transition.gas sensing, Figure it is 6a important shows the pressureto measur dependencee the collisional of measured2 parameters H2 HWHM (broadening for2 10% Hand2 in balance and the best fit of the calculated HTP HWHM. Despite significantly larger nitrogen broadening narrowing)nitrogen balance in nitrogen and the and best air fit balances. of the calculat Theseed para HTPmeters HWHM. are not Despite available significantly in the literature larger nitrogen for the comparedbroadening to compared the self-broadening, to the self-broadening, the H2 line in the nitrogen H2 line around in nitrogen ambient around pressures ambient is narrower pressures than is given H2 transition. Figure 6a shows the pressure dependence of measured H2 HWHM for 10% H2 in the Doppler broadened line. In Figure6b, the modelled HTP at 1.0 atmosphere is compared with nitrogennarrower balance than the and Doppler the best broadened fit of the calculat line. Ined Figu HTPre HWHM. 6b, the modelledDespite significantly HTP at 1.0 larger atmosphere nitrogen is thecompared Doppler with profile the Doppler of the same profile integral. of the sa Theme peakintegral. amplitude The peak of amplitude the H2 line of at the 1 atmH2 line in nitrogenat 1 atm broadening compared to the self-broadening, the H2 line in nitrogen around ambient pressures is is 1.2 times larger than if the line had only Doppler broadening. The collisional broadening and narrowerin nitrogen than is the1.2 Dopplertimes larger broadened than ifline. the Inline Figu hadre 6b,only the Doppler modelled broadening. HTP at 1.0 Theatmosphere collisional is 1 1 narrowingbroadening coe andffi cientsnarrowing for nitrogen coefficients balance for ni weretrogen evaluated balance towere be γevaluatedN2 = 0.0087(8) to be cm𝛾 −=atm 0.0087(8)− and compared with the Doppler profile of the same integral. The peak amplitude of the H2 line at 1 atm vc 1 1 ν –1= 0.071(7)–1 cm atm , respectively–1 –1 (T = 296 K). A few measurements of H in air balance at ambient2 incmN 2nitrogenatm and is 𝜈1.2− =times 0.071(7)− larger cm thanatm if, respectivelythe line had ( Tonly = 296 Doppler K). A few broadening. measurements2 The ofcollisional H in air pressurebalance at were ambient made pressure as well. were The made H2 line as appearedwell. The somewhatH2 line appeared narrower somewhat than in nitrogennarrower balance. than in broadening and narrowing coefficients for nitrogen balance were evaluated to be 𝛾 = 0.0087(8) Itnitrogen was assumed balance. that It the was collisional assumed narrowing that the parametercollisional wasnarrowing the same parameter as in nitrogen. was Thethe broadeningsame as in cm–1atm–1 and 𝜈 = 0.071(7) cm–1atm–1, respectively (T = 296 K). A few measurements of H2 in air 1 1 coenitrogen.fficient The in air broadening was then estimatedcoefficient to in be airγ wasAIR = then0.0081(8) estimated cm− toatm be −𝛾. Table= 0.0081(8)1 summarizes cm–1atm the–1 results. Table balance at ambient pressure were made as well. The H2 line appeared somewhat narrower than in nitrogenfor1 summarizes the 2122 balance. nm the H 2resultsItline was obtained forassumed the 2122 in thisthat nm study. theH2 linecollisional obtained narrowing in this study. parameter was the same as in –1 –1 nitrogen. The broadening coefficient in air was then estimated to be 𝛾 = 0.0081(8) cm atm . Table 1 summarizes the results for the 2122 nm H2 line obtained in this study.

Figure 6. (a) HWHM of 10 %v H2 in N2 balance gas as function of pressure and a ﬁt using the HTP;

(Figureb) calculated 6. (a) HWHM HTP at Pof= 101.0 %v atm H2 compared in N2 balance to the gas corresponding as function of Doppler pressure proﬁle. and a fit using the HTP; (b) calculated HTP at P = 1.0 atm compared to the corresponding Doppler profile.

Figure 6. (a) HWHM of 10 %v H2 in N2 balance gas as function of pressure and a fit using the HTP;

(b) calculated HTP at P = 1.0 atm compared to the corresponding Doppler profile.

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1 1 Table 1. H2 2122 nm line parameters obtained in this study (T = 296 K); unit is cm− atm− .

γ νvc

H2–N2 0.0087(8) 0.071(7) H –air 0.0081(8) 1 2 − H2–H2 0.0024(3) 0.037(5) 2 H2–H2 0.0019(1) 0.0448 1 2 Assumed to be the same value as for N2. Results for T = 315 K reported by Wcisło et al. [28]

5. H2 Sensor Performance The simulation using HTP with the line strength listed in HITRAN and the collisional parameters obtained in this study for H2 in nitrogen and air indicates that 1 %v H2 over a 1-m optical pathlength (1 %v m) gives about 2.2 10 5 of relative peak absorbance. This is about four times stronger than · × − the absorbance calculated using the Voigt profile with default HITRAN parameters. To achieve the required LOD of 0.1–0.2 %v m (assuming a measurement range of 0–10 %v), the sensitivity of the · TDLAS sensor must be in the range of 2 to 4 10 6 of relative absorbance. The two main factors × − limiting the sensitivity of most TDLAS sensors are (i) optical fringe-noise, sometimes called etalon noise, which is interference of the laser light caused by partially reflective surfaces in the system, and (ii) coupling of stray light into the active laser area (laser feedback noise). Considerable effort was made in the optomechanical design of the H2 sensor to eliminate most sources of optical feedback and etalon noise. All optical components used were wedged and tilted and all optical surfaces AR coated. The optical system was designed in a way to avoid distances that could create etalon fringes with periods close to the H2 absorption width. In addition, the modulation amplitude was carefully optimized to improve sensitivity to and selectivity of the H2 absorption. The H2 line is approximately five times narrower than the potentially interfering lines of CO2, CH4, and NH3. If the modulation amplitude is optimized for the H2 line (1 atm, N2/air) using m = 2.2, the corresponding value of m for the interfering lines will be around 0.4–0.5, such that the WMS signals for these lines are efficiently suppressed by approximately four times (see Figure4). The modulation amplitude can be further adjusted down to m = 1.5 for the H2 line without significant reduction of H2 WMS-PA (0.9 of the maximum WMS-PA), while the m value for the interfering lines reduces to about 0.3 (about 0.15 of the maximum WMS-PA), or an additional two-times reduction of the corresponding WMS-PA. In total, such optimization of the modulation amplitude provides about seven times (4 0.9 2) suppression × × of the interference signal. Thus, the WMS technique is particularly useful for discrimination between absorption features of different widths, which is a great advantage over DAS, especially when it comes to sensing hydrogen. A notable byproduct of relatively weak laser wavelength modulation is that the residual amplitude modulation contributing to baseline variations in the WMS signal [34] becomes negligible, thus improving detection sensitivity. The sensor was calibrated with a known concentration of H2 in N2 balance and characterized for different pressures and temperatures by measuring pressure and temperature dependencies of the H2 WMS signal using a heated cell. For evaluation of the sensor performance, the transmitter and receiver were set 1 m apart on a test bench. A 0.7-m optical cell fitted with wedged windows was placed between the transmitter and the receiver. Gas mixtures were directed into the cell at flow rates from 1 to 5 L/min. The pressure and temperature of the cell were close to ambient. For long-term stability tests, the cell was filled with a known gas mixture and sealed. The results were converted to the gas concentration unit, %v m. For zero H measurement, the cell was removed from the test bench · 2 and ambient air was measured. Sensors 2019, 19, 5313 10 of 13

Figure7a shows the obtained 2f WMS signals for 0.5, 0.2, and 0.1 %v of H 2 in nitrogen for an optical pathlength of 1 m. The signal for zero hydrogen concentration was recorded when the sensor was measured in ambient air. Each signal was acquired during 1 s as a result of averaging 150 laser Sensorswavelength 2019, 19, x scans. To improve sensitivity, the signals can be further processed using, for10 example, of 14 wavelet denoising or band-pass filtering. Here, convolution with a Mexican hat wavelet function function was used. The width of the function was chosen to match the width of the H2 absorption was used. The width of the function was chosen to match the width of the H2 absorption feature. feature.Figure Figure7b demonstrates 7b demonstrate the improveds the improved SNR after SNR convolution. after convolution. The baseline The slope baseline and high-frequency slope and high-frequencynoise components noise were components removed. were It is seenremoved. that the It peakis seen from that 0.1 the %v peakm is clearlyfrom 0.1 distinguishable %v∙m is clearly from · distinguishablethe noise level, from which the indicates noise level, that thewhich obtained indicate sensitivitys that the of obtained the sensor sensitivity to fractional of the absorbance sensor to was fractionalbetter than absorbance 2 10 6. was The better SNR approach than 2 × 10 was–6. appliedThe SNR to approach estimate thewas LOD applied of the to sensor. estimate In the this LOD approach, of × − theLOD sensor. corresponds In this approach, to the concentration LOD corresponds level at which to the the concentration measured signal level (peak-to-peak) at which the reachesmeasured three signaltimes (peak-to-peak) the signal noise reaches (peak-to-peak) three times of the the signal baseline. noise By (peak-to-peak) using this criterion, of the thebaseline. LOD ofBy the using H2 sensorthis criterion,was estimated the LOD to of be the 0.1 H %v2 sensorm for was the 1-sestimated integration to betime. 0.1 %v Figure∙m for7 cthe demonstrates 1-s integration the time. selectivity Figure of 7c the · 2 demonstratessensor, showing the selectivity the convolved of the 2fsensor, WMS showin signalsg of the 0.5 convolved %v of H2 ,2f 10 WMS %v of signals CO2, andof 0.5 5 %v CHof H4 (all, 10 in %vnitrogen of CO2, and balance) 5 %v recordedCH4 (all in at nitrogen 1 atm pressure. balance) It recorded should beat mentioned1 atm pressure. that It as should the pressure be mentioned increases, thatthe as selectivity the pressure improves, increases, because the selectivity the m value improves, for the interfering because the lines m value decreases for the at ainterfering much higher lines rate 2 decreasesthan the atm avalue much of higher the H2 rateline than due tothe the m significantvalue of the di ffHerence line due in pressure-broadening to the significant difference parameters. in pressure-broadening parameters.

Figure 7. (a) 2f WMS sensor signals for three H2 concentrations and one zero signal. Corresponding

Figureconcentrations 7. (a) 2f WMS at 1 msensor of optical signals pathlength for three are H2 0.5,concentrations 0.2, and 0.1 %v.and Zero one signalzero signal. was recorded Corresponding in ambient concentrationsair. (b) Signals at after1 meter convolution of optical with pathlength matched are Mexican 0.5, 0.2, hat and wavelet 0.1 %v. function Zero signal demonstrating was recorded improved in ambientSNR. (air.c) Convolved (b) Signals 2fafter WMS convolution signals of wi 0.5th %v matched of H2, 10Mexican %v CO hat2, and wavelet 5 %v function CH4. demonstrating

improved SNR. (c) Convolved 2f WMS signals of 0.5 %v of H2, 10 %v CO2, and 5 %v CH4. To examine the capability of the sensor in terms of measurement precision and long-term stability, the sensor was installed on a cell to measure 1.0 %v m H in nitrogen, and the data were To examine the capability of the sensor in terms of measurement· 2 precision and long-term logged over 500 min with a 1-s resolution. Figure8a shows the plot of the measured concentration. stability, the sensor was installed on a cell to measure 1.0 %v∙m H2 in nitrogen, and the data were Along with stochastic noise, an oscillating pattern can be observed. This pattern was attributed to logged over 500 min with a 1-s resolution. Figure 8a shows the plot of the measured concentration. optical fringes from the cell windows, which were drifting due to the changing ambient temperature. Along with stochastic noise, an oscillating pattern can be observed. This pattern was attributed to The corresponding Allan deviation (1σ)[35] is plotted in Figure8b. The precision without averaging optical fringes from the cell windows, which were drifting due to the changing ambient temperature. (i.e., with a sensor update time of 1 s) was 0.02 %v m. The precision improved with averaging, and after The corresponding Allan deviation (1σ) [35] is plotted· in Figure 8b. The precision without averaging 20 s of averaging 0.005 %v m was achieved. Between 1 and 10 min of averaging time, the Allan (i.e., with a sensor update time· of 1 s) was 0.02 %v∙m. The precision improved with averaging, and deviation increased up to approximately 0.02 %v m. This behavior can be attributed to a combination of after 20 s of averaging 0.005 %v∙m was achieved. Between· 1 and 10 min of averaging time, the Allan etalon noise from the cell windows and the sensor optical system. Thus, long-term drift and short-term deviation increased up to approximately 0.02 %v∙m. This behavior can be attributed to a combination (1 s) precision were both at about the same value of 0.02 %v m, which demonstrates excellent overall of etalon noise from the cell windows and the sensor optical system.· Thus, long-term drift and short- performance of the sensor. term (1 s) precision were both at about the same value of 0.02 %v∙m, which demonstrates excellent overall performance of the sensor.

SensorsSensors 20192019, ,1919, ,x 5313 1111 of of 14 13 Sensors 2019, 19, x 11 of 14

Figure 8. (a) Long-term measurement of 1 %v H2 over 1 meter with a 1-s update time; (b) Allan Figure 8. (a) Long-term measurement of 1 %v H2 over 1 m with a 1-s update time; (b) Allan deviation deviationofFigure sensor 8. of measurements( asensor) Long-term measurements measurement (1σ). Demonstrated (1σ). Demonstrated of 1 %v precision H2 over precision is1 0.02meter is %v 0.02 withm %v at a∙ 1m1-s sat withupdate 1 s with no time; averaging no averaging (b) Allan and · and0.005deviation 0.005 %v %vm of with∙sensorm with 20 measurements s20 averaging. s averaging. (1 σ). Demonstrated precision is 0.02 %v∙m at 1 s with no averaging and 0.005· %v∙m with 20 s averaging.

ForFor linearity evaluation,evaluation, the the HovaGAS HovaGA gasS gas mixer mixer was usedwas toused generate to generate H2 concentrations H2 concentrations stepwise stepwisedownFor from downlinearity 10 to from 0.1 evaluation, %v10 byto diluting0.1 %vthe by pureHovaGA diluting hydrogenS pugasre in mixerhydrogen nitrogen wasbase. inused nitrogen Each to concentrationgenerate base. Each H2 concentrationconcentrations was measured wasforstepwise aboutmeasured 4down min. for Figurefrom about 109a 4 showsto min. 0.1 Figure%v the recordedby 9adiluting shows measurements. puthere recorded hydrogen The measurements. in measurements nitrogen base. The for Eachmeasurements each concentrationconcentration for eachstepwas concentrationmeasured were averaged for aboutstep and were plotted4 min. averaged Figure in Figure 9aand shows9b plotted against the in recorded the Figure nominal 9bmeasurements. against concentration the nominal The set measurements in concentration the gas mixer for setsoftware.each in theconcentration gas The mixer linear software.step ﬁt haswere a The slopeaveraged linear well fitand within has plotted a 0.5%slop ein ofwell Figure the within sensor 9b against0.5% range, of the andthe nominalsensor the R-square range, concentration and value the of set in the gas mixer software. The linear fit has a slope well within 0.5% of the sensor range, and the R0.9998-square conﬁrms value of the 0.9998 excellent confirms linearity the excellent of the H 2linearitysensor. of the H2 sensor. R-square value of 0.9998 confirms the excellent linearity of the H2 sensor.

Figure 9. (a) Sensor response to diﬀerent H2 concentrations (10.0, 8.0, 6.0, 4.0, 2.0, 1.0, 0.5, 0.2, 0.1 %v)

Figurein the 0.79. (a m) Sensor cell; (b )response plot of measured to different concentration H2 concentrations as a function (10.0, of8.0, concentration 6.0, 4.0, 2.0, 1.0, set in0.5, gas 0.2, mixer 0.1 %v) and inlinearFigure the 0.7 ﬁt. 9. m(a )cell; Sensor (b) responseplot of measured to different concentration H2 concentrations as a fu nction(10.0, 8.0, of concentration6.0, 4.0, 2.0, 1.0, set 0.5, in 0.2,gas 0.1mixer %v) andin thelinear 0.7 fit.m cell; (b) plot of measured concentration as a function of concentration set in gas mixer and linear fit.

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6. Conclusions A TDLAS sensor was developed for industrial in situ and noncontact measurement of hydrogen. The sensor uses a well-proven LaserGas II platform by NEO Monitors AS and a 2122-nm DFB laser. The selected quadrupole absorption line of H2 was characterized in terms of collisional broadening and narrowing effects. The line exhibited sub-Doppler width in wide pressure ranges not only for pure hydrogen but also for hydrogen in nitrogen and air balances. The broadening and narrowing coefficients for nitrogen and air were obtained by fitting the measured linewidth to the calculated linewidth from the Hartmann–Tran profile. The performance of the sensor was evaluated. Measurement precision of 0.02 %v H2 for just 1 m of the absorption pathlength was achieved for 1 s of integration time. Using the SNR approach, the corresponding LOD was estimated to be 0.1 %v of H2 in air, which is sufficient for safety applications. By increasing the absorption pathlength and/or integration time, the performance of the sensor can be further improved, which should widen its applicability to industrial processes with sub-% H2 levels.

Author Contributions: Conceptualization, V.A.; Methodology, V.A. and O.B.; Investigation, V.A. and O.B.; Software, J.W. and P.G.; Formal analysis, V.A. and J.W.; Writing—original draft preparation, V.A.; Writing—review and editing, P.G.; Project administration, V.A., P.G., and K.G.P.; Supervision, K.G.P. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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