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Sensitive multi-species trace gas sensor basedon a high repetition rate mid-infraredsupercontinuum source

KHALIL ESLAMI JAHROMI,* MOHAMMADREZA NEMATOLLAHI,QING PAN, MUHAMMAD ALI ABBAS, SIMONA M. CRISTESCU,FRANS J. M. HARREN, AND AMIR KHODABAKHSH

Trace Gas Research Group, Department of Molecular and Laser Physics, Institute of Molecules andMaterials, Radboud University, 6525 AJ Nijmegen, The Netherlands*Kh.Eslami@science.ru.nl

Abstract: We present a multi-species trace gas sensor based on a high-repetition-rate mid-infrared supercontinuum source, in combination with a 30 m multipass absorption cell, and ascanning grating spectrometer. The output of the spectrometer is demodulated by a digital lock-inamplifier, referenced to the repetition rate of the supercontinuum source. This improved thedetection sensitivity of the system by a factor 5, as compared to direct baseband operation. Thespectrometer provides a spectral coverage of 950 cm−1 (between 2.85-3.90 µm) with a resolutionof 2.5 cm−1 in 100 ms. It can achieve noise equivalent detection limits in the order of 100 ppbvHz−1/2 for various hydrocarbons, alcohols, and aldehydes.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Mid-infrared (MIR) laser absorption spectroscopy is an effective method for selective andnon-invasive detection of gas-phase molecular species at trace levels. In comparison to thenear-infrared (NIR) region, the MIR window (2–20 µm) is essentially advantageous in terms ofdetection sensitivity, as the fundamental rotational-vibrational bands in the MIR are typicallyorders of magnitude stronger in absorption than the NIR bands. Recent advancements inMIR laser technology, e.g. interband cascade and quantum cascade lasers, have improved therobustness, sensitivity, and spectral coverage of laser-based MIR spectrometers [1–4]. However,the majority of these approaches target single absorption lines of specific molecules, achievingsuperior selectivity and sensitivity in a limited spectral range [5–7].For molecules, the number of normal vibrational modes goes proportional with the number

of atoms (3N-6 for nonlinear molecules and 3N-5 for linear molecules, with N the number ofatoms) [8,9]. In addition, the moment of inertia of the molecule increases, thereby reducingthe rotational constants, and reducing the distance between the rotational absorption lines,substantially. Therefore, for large molecules in gas-phase observed close to atmospheric pressure,the rotational absorption lines within a vibrational transition band overlap and only a contour ofthe vibrational band can be detected. Even more challenging is the simultaneous detection ofmultiple large molecular species, especially when their broad absorption features overlap [10].As the vibrational bands are mostly quite wide (tens of cm−1), the light source should cover abroad spectral range. In addition, for sensitive absorption detection, high spectral brightness andspatial coherence are highly desirable.Traditionally, compact thermal sources (such as low-cost Globar) are used to provide a wide

spectral range. However, their omnidirectional output with low brightness restrains the detectionsensitivity of the spectroscopy, due to the limited achievable absorption path length. Thesesources are often used in combination with a Fourier Transform Spectrometer (FTS), the mostwidely used broadband spectroscopic method in the MIR region [11]. Although FTS systems

#396884 https://doi.org/10.1364/OE.396884Journal © 2020 Received 13 May 2020; revised 20 Jul 2020; accepted 10 Aug 2020; published 20 Aug 2020

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based on thermal sources can provide a high spectral resolution, they need a long averaging timeto achieve a spectrum with a high signal-to-noise ratio (SNR), especially in a high-resolutionmeasurement.

An alternative approach is to utilize scanning tunable lasers such as optical parametric oscillator(OPO) [12], as well as external-cavity interband cascade lasers [13,14] and quantum cascadelasers [15] . Although they offer a high power, coherent beam, and can cover a few hundreds ofcm−1, their slow wavelength tuning process restricts the detection speed over broad wavelengthranges [16]. New detection methods, based on MIR dual-comb spectroscopy, have demonstratedsuperior performance in broad spectral coverage, high spectral resolution, short measurementtime, and high detection sensitivity. However, even with simplified implementations, their systemsetup is highly complex and costly [17,18].For years, due to lack of transparent MIR materials, supercontinuum (SC) sources had been

used for spectroscopy only in a limited spectral range up to the long-wavelength NIR [19–24]. Bythe advent of efficient and tailored highly nonlinear MIR optical fibers, SC generation range hasbeen extended to the longer wavelengths in MIR [25–29]. Recently, compact MIR SC sourceshave emerged as new candidates in broadband MIR spectroscopy [30–34]. The SC sourcesgenerally lack shot-to-shot (temporal) coherence. However, a precise control over the phasestability is not a prerequisite for direct absorption spectroscopy. The relief of this technicalconstraint makes the SC sources competitive in terms of low system complexity and costs.Recently, the superior brightness of MIR SC sources (exceeding the brightness of synchrotrons)[35], as well as an ultra-broadband spectral coverage (1.4-13.3 µm) [36] have been demonstrated.Robust MIR SC sources providing a high power spectral density (> 200 µW/nm) have evenbecome commercially available. These favorable characteristics of MIR SC sources open up thepossibility of multi-gas sensing of larger molecules at short time scales.Conventionally, grating-based spectrometers are very popular due to the low price and ease

of use. They can generally provide enough spectral resolution to be captured by a fixed linearcamera or alternatively a single-point detector while scanning the grating [33]. In addition,higher spectral resolution can be obtained by combining a virtually imaged phase array (VIPA)with a grating and a 2D camera [37]; although this is quite a complex and costly setup, especiallyin the MIR range. We have recently demonstrated a scanning grating-based spectrometer usinga MIR SC source [33]. Within this system, we reduced the relative intensity noise (RIN) ofthe SC source by balance detection scheme, thereby minimizing optical power fluctuations anddrift of the spectrometer. However, this method required a rather complex optical setup andthe noise reduction performance was, still, very sensitive to optical alignment. A well-knownnoise reduction technique in spectroscopy is to modulate the light (by intensity or wavelengthmodulation) and utilize a lock-in amplifier to demodulate the signal after detection. For single-frequency continuous-wave lasers, intensity modulation can be achieved using a mechanicalchopper in the path of the beam. For pulsed sources, however, one can directly use the repetitionrate of the source as the modulation frequency and demodulate the light after the detection system.This method works best for high repetition rate pulsed sources, for which the short integrationtime after demodulation will not limit the measurement speed. Here, we implement a lock-indetection scheme, referenced to the repetition rate of the supercontinuum source. The aim isto keep a simple optical setup with a broad spectral coverage and a short measurement time,while improving the detection sensitivity. We characterize the spectroscopic sensor in termsof detection precision, linearity and long-term stability, as well as spectral resolution, spectralcoverage, and multi-species detection.

2. Experimental setup and method

A schematic overview of the experimental setup is shown in Fig. 1. We used a broadbandMIR SC source (NKT Photonics) with a pulse repetition rate of 2.5 MHz, a total power of

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∼0.5 W, and a beam divergence of less than 2 mrad. The SC source has an averaged powerspectral density of ∼200 µW/nm in the spectral range between 2 to 4 µm. For our experimentthe MIR SC light is split into two beams, the first beam is directed into a multipass absorptioncell (HC30L/M-M02, ∼0.85 L volume, Thorlabs) with a nominal optical path length of 31.2 m,containing the gas sample. The pressure and gas flow in the absorption cell are under controlusing pressure meter/controller (EL-PRESS, Bronkhorst) and flow meter/controller (EL-FLOWPrestige, Bronkhorst), respectively. A 50 cm plano-convex lens (LA5464-E, Thorlabs) is usedto focus the beam at the center of the multipass cell. The output beam of the cell is sentto a diffraction grating (450 l/mm, GR1325-45031, Thorlabs) mounted on a galvo scanner(GVS011/M, Thorlabs). The galvo scanner is driven by a sinusoidal wave at 20 Hz generated by aLabVIEW-based program. To avoid the deterioration of the spectral resolution, which generallyhappens by focusing with spherical mirrors, the diffracted beam is only vertically focused on ahorizontal line by a concave cylindrical mirror (CCM254-100-P01, Thorlabs). During every scan,the spectrum is recorded in the time domain by a MIR thermo-electrically cooled single-pointphotodetector (PVI-4TE, 5 MHz, VIGO System). The output signal of the detector is used asthe modulated input signal for a fast dual-phase field-programmable gate array (FPGA)-basedlock-in amplifier (Moku:Lab, Liquid Instruments).

Fig. 1. simplified schematic representation of the sensor setup (BS: beam splitter, L: lens,CM: cylindrical mirror).

The second beam is detected by another MIR single-point detector (PVI-4TE-5 MHz, VIGOSystem), providing the repetition rate of the SC source. Since the reference trigger signal was notaccessible in this version of the SC source, the second detector output signal is employed as anexternal reference signal for the lock-in amplifier. This second detector is not necessary using anewer version of the SC source, in which a reference trigger signal output is directly providedby the manufacturer. The galvo-scanner position signal and the demodulated lock-in amplifieroutput signal are transferred to a computer via a data acquisition card (USB-6211, NationalInstruments). These signals are synchronized for every grating scan via a LabVIEW-basedprogram to acquire reproducible absorbance spectra. To calibrate the frequency axis of thespectrometer, we separately measured the broadband spectra of different gas species coveringthe entire bandwidth of the spectrometer. We compared the measured spectra to correspondingsimulated spectra calculated using HITRAN [38] or PNNL [39] database (depending on theavailability of the species) and a Gaussian instrumental line-shape. Using a 9th order polynomialequation, we fit the linear point numbers of the measured spectra to the wavenumber of thesimulated spectra. The retrieved coefficients of the fit were used for frequency calibration of theother measurements. The calibration stays the same, as long as the driving parameters of thegalvo scanner is not changed.Generally, choosing the time constant (i.e., filter bandwidth) of the lock-in amplifier is very

crucial. A short time constant yields a low signal-to-noise ratio (SNR), while a long time constantlimits the time resolution and the measurement speed. Since we obtain the spectra in the time

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domain, a long time constant limits the spectral resolution of the spectrometer to a lower spectralresolution than the resolving power of the diffraction grating. To prevent degradation of thespectral resolution at higher scan frequency of the grating, a shorter time constant of the lock-inamplifier should be considered. Therefore, finding an optimized lock-in time constant with aproper grating scan frequency is essential.

3. Evaluation of system characteristics

We investigated the effect of the time constant of the lock-in amplifier on the spectral resolutionand signal-to-noise ratio (SNR). Figure 2 represents the measured absorbance for 47 ppmvmethane in nitrogen (at 900 mbar and room temperature) obtained at different time constantsin 1 s averaging time. Evidently, increasing the time constant enhances the SNR, but it limitsthe spectral resolution. The optimized time constant for 20 Hz scanning rate of the grating wasfound to be ∼4 µs. Any higher time constant value results in a reduced spectral resolution. Forlonger time constants (>4 µs), the cut-off frequency of the low-pass filter of the lock-in amplifierattenuates the higher frequency components of the narrow absorption lines, reducing the spectralresolution. Therefore, the time constant is the limiting factor for the spectral resolution. Notethat the sampling rate of the spectrum does not need to match the time constant of the lock-inamplifier. Although we used a data acquisition card with a maximum sampling rate of 250 kS/s,we employed a sampling rate of 50 kS/s to record the spectrum. This value is sufficient to record3-4 data points in the full-width at half-maximum of the narrowest absorption lines, which isneeded for the fitting routine to operate properly. A higher sampling rate provides more datapoints in the measured spectrum; however, it also adds more noise to the measurement whichfinally reduces the effectiveness of the fitting process.

Fig. 2. Methane absorbance spectra (∼47 ppmv in nitrogen, 900 mbar, 1 s averaging)measured at different lock-in amplification time constants.

We also evaluated the effect of using the lock-in amplifier, which is synchronized with therepetition rate of the SC source (at 2.5 MHz) and compared it with the direct signal from the MIRdetector. Figure 3(A) displays the absorbance spectrum of 100 ppmv ethane in nitrogen (froma calibrated bottle, Linde gas) measured with (in red) and without (in blue) lock-in amplifier(average time 1 s). As expected, employing the lock-in amplifier enhances the SNR. Figure 3(B)depicts the 1 s averaged ethane absorbance spectrum (in red) obtained with lock-in and with a30 s averaging time (in blue) without lock-in. Without the lock-in amplifier, a 30 s averagedspectrum has an improved SNR by a factor of

√30 ≈ 5.5, compared to a 1 s averaged spectrum.

This 30 s averaged spectrum without lock-in amplifier also has roughly the same SNR as a 1s averaged spectrum using the lock-in amplifier. Therefore, assuming that the measurements

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are white-noise-dominant, it implies that the lock-in amplifier enhances the SNR by a factor of√30 ≈ 5.5.

Fig. 3. Absorbance spectra of 100 ppmv ethane diluted in nitrogen measured at 900 mbar(A) with using the lock-in amplifier (in red) and without using the lock-in amplifier (in blue),both averaged for 1 s, (B) with using the lock-in amplifier (in red) averaged for 1 s, andwithout using the lock-in amplifier (in blue) averaged for 30 s.

To evaluate the effect of using the lock-in amplifier on the long-term stability and minimumdetectable concentration, we obtained the background spectra every 100 ms for 6000 s, whilethe cell was evacuated (pressure below 1 mbar). We normalized each consecutive spectrumto the first spectrum taken, and acquired the Noise Equivalent Absorbance (NEA). Followingthis, we fitted a methane reference spectrum (taken experimentally), as well as a 4th orderpolynomial (to fit the baseline) to the obtained absorbance spectra. The retrieved noise equivalentmethane concentrations for both of the cases (with and without lock-in amplifier) are shown inFig. 4(A). Due to a high white noise level at low frequencies in baseband detection, the calculatedconcentration is more scattered, as compared to when the lock-in is applied with a referencefrequency of 2.5 MHz. Figure 4(B) shows the corresponding Allan-Werle plots fitted by time

Fig. 4. (A) Noise equivalent methane concentrations (obtained every 100ms for 6000 s)for the measurements with (red dots) and without (blue dots) the lock-in amplifier. (b) Thecorresponding Allan-Werle plots of the retrieved methane concentrations when the lock-inamplifier was used (red curve) and was not used (blue curve). The fitted time dependency ofτ−1/2 (green dash lines) represents the white noise contribution.

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dependency lines of τ−1/2, representative of the white noise contribution. They demonstrate amethane detection sensitivity of 500 ppbv Hz−1/2 and 100 ppbv Hz−1/2 without and with lock-inamplification, i.e. a factor 5 improvement. For both cases, averaging over a longer time helpsto reduce the detection limit in the white-noise-dominant region, over 3 orders of magnitude intime. With lock-in, the methane detection limit is 4 ppbv for ∼8.5 minutes averaging time, whilewithout lock-in, it is 40 ppbv after ∼3.5 minutes of averaging.

Next, we assessed the system linearity response to various ethane concentrations in a dynamicrange of 1 to 100 ppmv by dilution of 100 ppmv ethane from a calibrated gas bottle in nitrogen.We used the 100 ppmv ethane concentration as a reference spectrum for a linear fitting routine.Figure 5 shows the retrieved ethane concentrations from the fit versus the applied concentrations.A linear fit to this dataset has a Pearson correlation coefficient (Pearson’s r) close to one,demonstrating a high agreement between the applied and the calculated concentration values.

Fig. 5. System linearity response to different applied ethane concentrations. The uncertaintyis based on ±σ values calculated from 1 min measurement of each gas concentration.

To evaluate the effective SNR and sensor precision in a wide spectral range, we obtained theaveraged absorbance spectrum of 100 ppmv ethane in nitrogen every 1 s for 1 min. Figure 6(A)shows the overlay of the first and the second measured absorbance spectra. Following that, thesecond absorbance spectrum (absorbance values at each measurement point) was plotted againstthe first one as shown in Fig. 6(B) with green dots. Then, we applied a linear fit to the data points(red line). The linear fit slope is close to one, implying an excellent reproducibility. We repeatedthe same procedure for all of the obtained absorbance spectra (with respect to the first one).An effective SNR of ∼615 was calculated from the average of the slopes, µ, and their standarddeviation, σ, using SNR = µ/σ. Therefore, the system precision in detecting 100 ppmv ethanein nitrogen was evaluated to be ∼160 ppbv (100 ppmv/SNR) corresponding to ∼0.16% precisionat 1 s averaging time.We also evaluated the sensor ability to quantify the concentration of gases in a complex

gas mixture. In general, the quantification of the trace gas concentrations in a multi-speciesgas mixture with overlapping absorbance features is more challenging than the detection ofsingle-species. The absorbance spectrum measured in a complex gas mixture is the summationof all of the absorbance of the contributing gases. To obtain the individual absorbance featureand quantify the concentration for each gas, a non-negative least square (NNLS) curve fittingtechnique was applied. This method has been explained in our previous publications [10,33].For this experiment, we made a diluted mixture containing ethyl acetate and ethylene bothwith a concentration of 48.3± 2.5 ppmv in nitrogen (both mixtures originated from calibrated

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Fig. 6. (A) Overlay of two consecutive ethane absorbance spectra measured within 1 saveraging at 900 mbar. (B) A linear fit (red line) to the data points obtained from thefirst-second measured absorbance scatter plot (green dots).

gas bottles). These gas species have overlapped spectral features in a wide spectral range.The measured absorbance of the mixture and each individual gas are shown in Fig. 7(A). Byperforming the NNLS global fitting technique using the measured individual spectra as thereferences for the fit, the concentration of each gas in the mixture was calculated as shown inFig. 7(B). The uncertainty with the 1.5 interquartile range was obtained from 60 independentmeasurements in 1 min. The results demonstrate that the retrieved concentrations are close to theapplied concentrations within the dilution error (∼ ±5%).

Fig. 7. (A) The measured absorbance profiles for 48.3 ppmv ethyl acetate (in red) and 48.3ppmv ethylene (in blue) both diluted in nitrogen, as well as their mixture (in green). (B)The retrieved concentrations for each gas in the gas mixture calculated from NNLS fittingapproach.

Spectral resolution and spectral range are important spectroscopic characteristics for multi-species detection. To determine the sensor spectral resolution, we experimentally measuredthe absorbance of 98 ppmv ethylene (diluted in nitrogen) at 900 mbar and compared it with asimulated absorbance model based on HITRAN database convoluted with a Gaussian profilewith a variable full width at half maximum (FWHM). The Gaussian profile was used to modelthe resolving power of our spectrometer. We fitted the measurement and the model with the

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line width of the Gaussian profile as the fitting parameter. The retrieved linewidth from the fitcan be used for convoluting the model spectra of the other species for future measurements,since the spectral resolution of the spectrometer is fixed and does not vary over time unless thealignment of the beam is changed. Figure 8(A) indicates that the measured absorbance (in red)matches nicely with the simulation (in blue) when the FWHM of the Gaussian profile is 2.5 cm−1.Therefore, we estimate the spectral resolution of the spectrometer to be ∼2.5 cm−1. In order tocheck the maximum spectral coverage, we increased the scan range of the grating by ∼2 times, byincreasing the amplitude of the galvo scanner; meanwhile, the scan frequency was reduced to 10Hz to avoid any possible damage to the galvo scanner. We applied a mixture of 50 ppmv ethyleneand 50 ppmv nitrous oxide (N2O), both diluted in nitrogen. The measured absorbance spectrumand the corresponding simulations are shown in Fig. 8(B) demonstrating that the sensor cansupport a broad spectral range more than 950 cm−1 (from 2.85 to 3.90 µm or ∼2560-3510 cm−1).

Fig. 8. (A) The measured absorbance spectrum of 98 ppmv ethylene diluted in nitrogen (inred) averaged for 1 min at 900 mbar pressure along with the simulated absorbance spectrumusing HITRAN database with 2.5 cm−1 resolution (in blue, inverted). (B) The measuredabsorbance spectrum for a mixture of 50 ppmv ethylene and 50 ppmv nitrous oxide bothdiluted in nitrogen (in red) along with the corresponding simulated spectra of nitrous oxide(in blue, inverted) and ethylene (in green, inverted) using HITRAN database.

Since the digital lock-in amplifier also provides an internal programmable reference frequencybased on the FPGA clock, we compared the system operation with the external reference (fromthe reference MIR detector) with the internal reference of the lock-in (2,500,000.000 Hz); thelatter is equal to the nominal repetition rate of the SC source. Figure 9(A) demonstrates theobtained absorbance spectra of 47.5 ppmv methane diluted in nitrogen averaged for 60 s in bothcases. Interestingly, the absorbance spectrum obtained by external reference (in red) agrees verywell with the spectrum recorded using the internal lock-in reference (in blue, inverted). In otherwords, the mutual drifts and fluctuations of the two independent oscillators are negligible foran averaging time of 60 s. To evaluate this, we plot (point-by-point) the absorbance values ofthe spectrum measured with the internal reference (on the y-axis) in terms of the absorbancevalues of the spectrum measured with the external reference (on the x-axis) as shown by thegreen square markers in Fig. 9(B). The linear fit to the points (red line) has both Pearson’s r andslope close to 1, demonstrating an excellent agreement of the two spectra. Therefore, we can usethe internal reference of the lock-in amplifier instead of the external reference from the SC source,without any degradation of the measured spectrum up to 60 s measurement time, which furthersimplifies the experimental setup. Note that a more detailed study on the long term stability ofthe system is needed, to thoroughly compare the long-term performance of the two schemes.

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Fig. 9. (A) Measured absorption spectra of 47.5 ppmv methane diluted in nitrogen obtainedwhen the digital lock-in amplifier was referenced externally to the SC source (in red) andinternally to its FPGA clock (in blue, inverted). (B) Methane absorbance values of thespectrum measured with the internal reference in terms of the absorbance values of thespectrum measured with the external reference (green square markers) along with a linear fitto the data points (red line).

4. Conclusion

We have developed a multi-species trace gas sensor based on a high repetition rate mid-infraredsupercontinuum source and a scanning grating spectrometer in combination with a digital lock-inamplifier. We demonstrated that by employing the lock-in approach the detection sensitivitycan be improved by ∼5 times compared to the direct baseband operation, yielding a detectionlimit in the order of 100 ppbv Hz−1/2 for various hydrocarbons, alcohols, and aldehydes. Atthe optimized lock-in amplifier time constant, the spectrometer features more than 950 cm−1

(between 2.85 and 3.90 µm) spectral coverage with 2.5 cm−1 spectral resolution in a 100 msmeasurement time. For retrieving the concentration of gases in complex mixtures, we utilized anon-negative least square global fitting routine, which is able to retrieve the concentration ofeach species, despite their partial spectral overlap. The performance of the sensor in terms ofprecision, linearity, and long term stability was studied, as well. The results clearly demonstratethe potential of the developed sensor for various applications requiring multi-species trace gasdetection with a simple and cost-effective system at seconds timescale; such as environmentalmonitoring, quality control, and biomedical research.

Funding

Interreg (363); Nederlandse Organisatie voor Wetenschappelijk Onderzoek (14709); H2020Industrial Leadership (732968).

Acknowledgments

The authors acknowledge Interreg North-West Europe program (project number 363), theNetherlands Organisation for Scientific Research (NWO, project number 14709), and the H2020Industrial Leadership programme (project number 732968). The authors would also like to thankNKT Photonics for providing the mid-infrared supercontinuum light source.

Disclosures

The authors declare no conflicts of interest.

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