Widely-Tunable Quantum Cascade-Based Sources for the Development of Optical Gas Sensors

sensors

Article Widely-Tunable Quantum Cascade-Based Sources for the Development of Optical Gas Sensors

Virginie Zéninari 1,* , Raphaël Vallon 1, Laurent Bizet 1, Clément Jacquemin 1, Guillaume Aoust 2, Grégory Maisons 2, Mathieu Carras 2 and Bertrand Parvitte 1

1 Unité Mixte de Recherche 7331, Groupe de Spectrométrie Moléculaire et Atmosphérique, Centre National de la Recherche Scientifique, Université de Reims Champagne Ardenne, 51097 Reims, France; [email protected] (R.V.); [email protected] (L.B.); [email protected] (C.J.); [email protected] (B.P.) 2 mirSense, Centre d’intégration NanoINNOV, 91120 Palaiseau, France; [email protected] (G.A.); [email protected] (G.M.); [email protected] (M.C.) * Correspondence: [email protected]; Tel.: +33-326-918-788



Received: 30 September 2020; Accepted: 16 November 2020; Published: 20 November 2020


Abstract: Spectroscopic techniques based on Distributed FeedBack (DFB) Quantum Cascade Lasers (QCL) provide good results for gas detection in the mid-infrared region in terms of sensibility and 1 selectivity. The main limitation is the QCL relatively low tuning range (~10 cm− ) that prevents from monitoring complex species with broad absorption spectra in the infrared region or performing multi-gas sensing. To obtain a wider tuning range, the first solution presented in this paper consists of 1 1 the use of a DFB QCL array. Tuning ranges from 1335 to 1387 cm− and from 2190 to 2220 cm− have been demonstrated. A more common technique that will be presented in a second part is to implement a Fabry–Perot QCL chip in an external-cavity (EC) system so that the laser could be tuned on its whole gain curve. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy, where the gas sample is placed within the laser resonator. Moreover, a technique only using the QCL compliance voltage technique can be used to retrieve the spectrum of the gas inside the cavity, thus no detector outside the cavity is needed. Finally, a specific scheme using an EC coherent QCL array can be developed. All these widely-tunable Quantum Cascade-based sources can be used to demonstrate the development of optical gas sensors.

Keywords: quantum cascade laser; widely tunable laser sources; mid-infrared laser sources; laser spectroscopy; QCL array; external-cavity systems; intra-cavity laser absorption spectroscopy; compliance voltage technique; gas sensors

1. Introduction Since the first realization of a Quantum Cascade Laser (QCL) [1], there has been a growing interest in its application to the development of optical gas sensors [2,3]. The main reason is that these semi-conductor lasers emit in the mid-infrared, where the strongest absorption lines lie for most of the absorbing gases, which is to the contrary of standard telecom diode lasers that are limited to the near-infrared, where low absorption harmonic bands lie. Various applications have emerged such as in air monitoring [4], chemical physics [5,6] or in the biomedical domain [7,8]. In this last field alone, we can cite for example a number of works dealing with cancer diagnosis [9], detection of glucose [10,11] and analysis of human respiration [12,13]. For most applications and particularly spectroscopic applications for gas detection, it is mandatory to have single frequency QCL operating devices. This tunable single-mode operation is usually reached in two ways. The first one consists in processing QCLs as Distributed FeedBack (DFB) lasers [14]

Sensors 2020, 20, 6650; doi:10.3390/s20226650 www.mdpi.com/journal/sensors Sensors 2020, 20, 6650 2 of 15 and the second one by incorporating QCLs within an external cavity including a rotating diffraction grating [15]. The laser team in GSMA (Groupe de Spectrométrie Moléculaire et Atmosphérique) in Reims has a long experience in the use of DFB QCLs in spectroscopy for applications to gas detection. This team has historically developed heterodyne detection [16] in the mid-infrared range using CO2 lasers. The first mid-infrared heterodyne detection using QCL as local oscillator was realized in 2004 [17]. Atmospheric absorption spectra of ozone (O3) were recorded at 9 µm. This work was continued by other teams for Earth atmospheric studies [18,19] or astronomical ones [20,21]. DFB QCLs were also used in Reims with multipass cell in standard Tunable Diode Laser Absorption Spectroscopy (TDLAS) 18 16 16 to demonstrate the water-vapor isotope ratio measurements in air: H2O ⁄H2O and HDO⁄H2O at 6.7 µm [22]; the simultaneous measurement of nitrous oxide (N2O) and methane (CH4) concentrations at ground level at 7.9 µm [23]; and the development of a field-deployable spectrometer using QCL technology at 4.5 µm to measure in situ N2O concentrations at ground level even in bad weather conditions [24]. In addition, the QCL power that is intrinsically higher than those of standard semi-conductor diode lasers is a key for photoacoustic gas detection [25]. Indeed the photoacoustic signal is directly proportional to the laser power. The first realization of photoacoustic spectroscopy with QCLs was demonstrated in 1999 [26]. This technique has been widely used for trace gas detection [27,28]. A recent extensive review can be found in [29]. Current works are dedicated to the development of miniaturized photoacoustic cells as the photoacoustic signal is inversely proportional to the cell volume. Two main ways are currently being explored: the Quartz-Enhanced PhotoAcoustic Spectroscopy (QEPAS) technique as for example in [30], and size-reduction of the photoacoustic cavity as for example in [31]. The advantage of QCL power for photoacoustic spectroscopy was successfully demonstrated in GSMA in [32] where the detection of methane was realized with a standard diode laser and compared with the QCL result. The high power of the QCL combined with the higher absorption coefficient in the mid-infrared had permitted to improve the detection limit of methane from the ppm to the ppb range. Then, the simultaneous detection of methane (CH4) and nitrous oxide (N2O) in atmospheric flux measurements [33] was demonstrated where a detection limit of 3 ppb of methane in air was obtained using a QCL emitting at 8 µm. Nitric oxide (NO) detection has also been performed with a photoacoustic sensor coupled with a continuous wave DFB QCL at 5.4 µm, working at cryogenic temperature [34]. All these GSMA realizations were obtained with continuous wave (cw) laser sources. However, still nowadays, some wavelengths can only be obtained with pulsed sources. Pulsed QCL spectrometers can be used to detect atmospheric gases in two ways [35]. The first one is the so-called interpulse technique using typically short (5–20 ns) pulses (see [36] for example). The second one called intrapulse technique uses quite long pulses (typically 500–800 ns) (see [37,38] for example). However, each of these techniques has drawbacks. Indeed, the gas absorption spectra are generally distorted due to the large influence of the pulse shape. That is why an alternative method for gas detection using pulsed QCL spectrometers was proposed in GSMA [39] and applied to the detection of ammonia (NH3) at 10 µm [40]. Even if one can demonstrate gas detection with pulsed QCL spectrometers, from a strictly spectroscopic point of view, spectra recorded with cw laser sources are of better quality. The main limitation of all these experiments is the DFB QCLs relatively low tuning range 1 (~10 cm− ). This drawback prevents from performing multi-gas sensing or monitoring more complex gaseous species that are characterized by broad absorption spectra in the mid-infrared region. To obtain a wider tuning range, two solutions are usually developed. The first solution consists in the use of a DFB QCL array [41]. This type of source initially developed in the Capasso group [42,43] is still under development [44,45] for the realization of optical sensors. Results for methane (CH4) detection using a DFB QCL array emitting from 7.2 µm to 7.5 µm are presented in Section 2.1. Another DFB QCL array associated to an arrayed waveguide grating [46] directly fixed on the chip has been developed to Sensors 2020, 20, 6650 3 of 15

demonstrate the detection of carbon monoxide (CO), acetylene (C2H2) and carbon dioxide (CO2) with a source emitting from 4.49 µm to 4.57 µm [47]. A more common technique to obtain a wider tuning range that will be presented in Section3 is to implement a Fabry–Perot QCL chip in an external-cavity (EC) system [48] so that the laser could be tuned on its whole gain curve. This technique has been demonstrated at room temperature [49] with high-power [50] for high resolution spectroscopy and chemical sensing [51]. In GSMA, the implementation of a commercial EC QCL emitting at 10.5 µm in a photoacoustic spectrometer was 1 realized [52] and enabled measurements on a broad spectral range up to 60 cm− . The tuning range of the source emitting from 10 µm to 10.5 µm demonstrates the possibility to detect small and complex molecules such as carbon dioxide (CO2) and butane (C4H10). Based on the same principle, an EC QCL spectrometer was designed in the lab at 7.5 µm. The QCL and its anti-reflection coating were specially 1 developed for this application [53]. A continuous wave emission from 1293 up to 1350 cm− was demonstrated, and measurements on acetone (C3H6O) and phosphoryl chloride (POCl3) were realized. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy (ICLAS). This technique consists of placing the gas sample within the laser resonator. In this case, absorption lines of the sample influence the spectrum during many round trips. The cavity output light can be detected to perform spectroscopy. This method, already well known in the visible [54–56] and the near infrared [57–60], is developing in the mid-infrared [61,62]. With a carefully optimized system, the effective pathlength can reach hundreds of kilometers, thus making it an appropriate technique for low absorption gases detection. Moreover, a technique using the QCL compliance voltage may be developed to retrieve the spectrum of the gas inside the cavity. In this way, no detector outside the cavity is needed as the cavity is the light source and the detector at the same time [63]. Preliminary results using this technique in GSMA and demonstrating the detection of methane (CH4) and water vapor (H2O) at 7.6 µm will be presented. Finally, another specific scheme combining EC systems and QCL array will be anew briefly presented in Section4. This new design is based on coherent quantum cascade laser micro-stripe arrays [64]. Phase-locking is provided by evanescent coupling between adjacent stripes, thus the far-field pattern shows a dual-lobe emission. This a priori drawback has been converted to a force by developing an EC system with this source, and acetone spectra have been recorded at 8.2 µm [65]. Another team has used the same design in [66].

2. Distributed FeedBack (DFB) Quantum Cascade Laser (QCL) Array To obtain a wide tuning range, the first solution consists in the use of a DFB QCL array [41]. A first system of DFB QCL array emitting from 7.2 µm to 7.5 µm has been developed in GSMA with the source from mirSense and will be presented in Section 2.1. This source has been used to demonstrate methane (CH4) detection. However, the main drawback of this monolithic source is the lateral shift of the beam with operating laser switching. A moveable lens and a parabolic mirror for the detection system can be used to compensate the shift. Moreover, it is a major drawback when using systems that are sensitive to alignment such as multipass cells. The beam lateral shift due to the numerous outputs of the array may be compensated by using a diffraction grating to group the wavelengths [67] or by the fabrication of integrated waveguides collecting beams in a single output [68]. In Section 2.2, an intermediate solution between these two methods demonstrated in GSMA will be anew presented. The DFB QCL array is combined with an Arrayed Waveguide Grating (AWG), usually called multiplexer, directly fixed on the chip. Thus, the system (QCLA+AWG) is not strictly monolithic but it is no more mandatory to realign the system when switching between lasers. The developed system is used to demonstrate the detection of carbon monoxide (CO), acetylene (C2H2) and carbon dioxide (CO2) with a source emitting at 4.5 µm [47]. Sensors 2020, 20, 6650 4 of 15

2.1. DFB QCL Array without Multiplexer

SensorsThe 2020 principle, 20, x FOR of PEER the REVIEW DFB QCL array without multiplexer and its application to gas detection4 of is 16 presentedSensors 2020, in 20 Figure, x FOR PEER1. REVIEW 4 of 16

Figure 1. Schematic of a DFB QCL array source for gas detection. Grey lines in the yellow square FigureFigure 1.1. SchematicSchematic ofof aa DFBDFB QCLQCL arrayarray sourcesource forfor gasgas detection.detection. GreyGrey lineslines inin thethe yellowyellow squaresquare indicate the lasers in the array. A mobile lens following the dashed arrows must be moved to adapt indicateindicate thethe lasers lasers in in the the array. array. A A mobile mobile lens lens following following the the dashed dashed arrows arrows must must be be moved moved to adaptto adapt to to the output of the chosen laser. theto the output output of theof the chosen chosen laser. laser. A room temperature DFB QCL array from mirSense is mounted onto a homemade water cooling AA roomroom temperaturetemperature DFBDFB QCLQCL arrayarray fromfrom mirSensemirSense isis mountedmounted ontoonto aa homemadehomemade waterwater coolingcooling with single stage Peltier element. Each laser has an approximate 20µ µm2 2 emission surface and the withwith singlesingle stagestage PeltierPeltier element.element. EachEach laserlaser hashas anan approximateapproximate 2020 µmm 2 emissionemission surfacesurface andand thethe lasers are periodically spaced by about 100µ µm. The DFB grating of each QCL is slightly different in laserslasers areare periodicallyperiodically spacedspaced byby aboutabout 100100 µm.m. TheThe DFBDFB gratinggrating ofof eacheach QCLQCL isis slightlyslightly didifferentfferent inin order to have different wavenumbers. The electrical contact is given by a gold common terminal and orderorder toto havehave didifferentfferent wavenumbers.wavenumbers. TheThe electricalelectrical contactcontact isis givengiven byby aa goldgold commoncommon terminalterminal andand by gold individual tracks under the lasers (see Figure 2a). A QCL pulser switching unit (LDD100, byby goldgold individualindividual trackstracks underunder thethe laserslasers (see(see FigureFigure2 a).2a). AA QCL QCL pulser pulser switching switching unit unit (LDD100, (LDD100, Alpes Lasers SA, St-Blaise, Switzerland) converts the voltage control into current pulses. The latest AlpesAlpes LasersLasers SA,SA, St-Blaise,St-Blaise, Switzerland)Switzerland) convertsconverts thethe voltagevoltage controlcontrol intointo currentcurrent pulses.pulses. TheThe latestlatest are transported to the array by a low impedance cable to avoid any signal deformation. The duration areare transportedtransported toto thethe arrayarray byby aa lowlow impedanceimpedance cablecable toto avoidavoid anyany signalsignal deformation.deformation. TheThe durationduration and the frequency of the pulses are determined by a pulse generator (BNC565, Berkeley Nucleonics andand thethe frequencyfrequency ofof thethe pulsespulses areare determineddetermined byby aa pulsepulse generatorgenerator (BNC565,(BNC565, BerkeleyBerkeley NucleonicsNucleonics Corporation, San Rafael, CA, USA). The array is composed of 34 DFB QCLs that are working in quasi- Corporation,Corporation, San Rafael, Rafael, CA, CA, USA). USA). The The array array is composed is composed of 34 of DFB 34 DFB QCLs QCLs that are that working are working in quasi- in continuous regime with an optical power of about 1 mW. Figure 2b shows the laser output power quasi-continuouscontinuous regime regime with withan opti an opticalcal power power of about of about 1 mW. 1 mW. Figure Figure 2b2b shows shows the laser outputoutput powerpower evolution of each QCL from the array with the voltage command. They all have the same tendency evolutionevolution ofof each each QCL QCL from from the the array array with with the the voltage voltage command. command. They They all haveall have the samethe same tendency tendency but but with different thresholds and maximum powers. No correlation was observed between the withbut with different different thresholds thresholds and maximum and maximum powers. po Nowers. correlation No correlation was observed was observed between between the emitted the emitted wavenumber and the threshold value. These disparities are due to imperfections during the wavenumberemitted wavenumber and the thresholdand the threshold value. These value. disparities These disparities are due are to due imperfections to imperfections during during the array the array fabrication. For this particular source, each DFB QCL is addressed by a single three-axis fabrication.array fabrication. For this For particular this particular source, eachsource, DFB each QCL DFB is addressed QCL is addressed by a single by three-axis a single motorizedthree-axis motorized tungsten probe with 0.3 micrometer accuracy and a metallic stripe on the common gold tungstenmotorized probe tungsten with probe 0.3 micrometer with 0.3 micrometer accuracy and accu aracy metallic and stripea metallic on thestripe common on the goldcommon terminal. gold terminal. The probe movement is automated and controlled by a Labview program. Theterminal. probe The movement probe movement is automated is automat and controlleded and controlled by a Labview by a program. Labview program.

(a) (b) (a) (b) FigureFigure 2. 2.( a(a)) SchematicsSchematics ofof thethe DFBDFB QCL array. The The electric electricalal contact contact (in (in yellow) yellow) of of the the DFB DFB QCLs QCLs (in Figure 2. (a) Schematics of the DFB QCL array. The electrical contact (in yellow) of the DFB QCLs (in (ingreen) green) is isgiven given by by a agold gold common common terminal terminal and and by gold individual trackstracks underunder the the lasers; lasers; ( b(b)) Power Power green) is given by a gold common terminal and by gold individual tracks under the lasers; (b) Power ofof the the 34 QCLs34 QCLs of the of array the vs.array voltage vs. voltage command. command. Supply conditions Supply conditions of all pulsed of lasers all pulsed are: temperature lasers are: of the 34 QCLs of the array vs. voltage command. Supply conditions of all pulsed lasers are: 20temperature◦C, pulse duration 20 °C, pulse 100 ns, duration repetition 100 ratens, repe 100 kHz,tition peakrate 100 current kHz, 1.3 peak A. current 1.3 A. temperature 20 °C, pulse duration 100 ns, repetition rate 100 kHz, peak current 1.3 A. AA spectral spectral variation variation is obtained is obtained by varying by varying the array the temperature,array temperature, which is which controlled is controlled by a TEC2000 by a A spectral variation is obtained by varying the array temperature, which is controlled by a fromTEC2000 Thorlabs from (Newton, Thorlabs NJ, (Newton, USA) associated NJ, USA) with associated a Peltier element.with a Peltier The wavenumber element. The measurement wavenumber is TEC2000 from Thorlabs (Newton, NJ, USA) associated with a Peltier element. The wavenumber measurement is realized with a Fourier Transform InfraRed spectrometer (FT/IR-6300 from Jasco, measurement is realized with a Fourier Transform InfraRed spectrometer (FT/IR-6300 from Jasco, Easton, MD, USA) with a minimal accuracy of 0.015 cm−1. Figure 3 presents the emission spectra in Easton, MD, USA) with a minimal accuracy of 0.015 cm−1. Figure 3 presents the emission spectra in pulsed mode from three DFB lasers of the array recorded for three different temperatures. Each laser pulsed mode from three DFB lasers of the array recorded for three different temperatures. Each laser covers a spectral range of about 2–3 cm−1 by tuning the temperature from 10 to 30 °C. A wide covers a spectral range of about 2–3 cm−1 by tuning the temperature from 10 to 30 °C. A wide

Sensors 2020, 20, 6650 5 of 15 realized with a Fourier Transform InfraRed spectrometer (FT/IR-6300 from Jasco, Easton, MD, USA) Sensors 2020, 20, x FOR PEER REVIEW 1 5 of 16 withSensors a 2020 minimal, 20, x FOR accuracy PEER REVIEW of 0.015 cm− . Figure3 presents the emission spectra in pulsed mode from5 of 16 three DFB lasers of the array recorded for three different temperatures. Each laser covers a spectral continuous spectral range1 can be obtained as each spectral range overlaps other ones. This result is rangecontinuous of about spectral 2–3 cm range− by can tuning be obtained the temperature as each from spectral 10 to range 30 ◦C. overlaps A wide continuousother ones. spectral This result range is canpresentedpresented be obtained in in Figure Figure as each 4. 4. spectral range overlaps other ones. This result is presented in Figure4.

Figure 3. Emission spectra of the QCLs #7, #9, and #14 from the DFB QCL array for three temperatures. Figure 3. Emission spectra of the QCLs #7, #9, and #14 from from the the DFB DFB QCL QCL array array for three three temperatures.

FigureFigure 4. 4.Emission Emission spectraspectra ofof allall thethe QCLsQCLs fromfrom the the DFB DFB QCL QCL array array for for one one temperature temperature (20 (20 C).°C). Figure 4. Emission spectra of all the QCLs from the DFB QCL array for one temperature (20 ◦°C). 1 1 AA total total range range of of more more than than 50 50 cm cm− −1from from 1335 1335 to to 1387 1387 cm cm−−1 cancan bebe achieved.achieved. NoteNote thatthat thetheFTIR FTIR A total range of more than 50 cm−1 from 1335 to 1387 cm−1 can be achieved. Note that the FTIR used to record those spectra is not time-resolved and cannot be synchronized with the laser emission. usedused to to record record those those spectra spectra is is not not time-resolved time-resolved and and cannot cannot be be sync synchronizedhronized with with the the laser laser emission. emission. Thus, the recorded spectra may be distorted and cannot be used to demonstrate single-mode emission Thus,Thus, thethe recordedrecorded spectraspectra maymay bebe distorteddistorted anandd cannotcannot bebe usedused toto demonstratedemonstrate single-modesingle-mode but only to evaluate the emission wavenumber. emissionemission but but only only to to evaluate evaluate the the emission emission wavenumber. wavenumber. As presented in Figure1, the DFB QCL array source developed for gas detection has to be AsAs presented presented in in Figure Figure 1, 1, the the DFB DFB QCL QCL array array so sourceurce developed developed for for gas gas detection detection has has to to be be used used used with a 4 mm focal length lens mounted on a motorized translation (M-112.1DG from Physik withwith aa 44 mmmm focalfocal lengthlength lenslens mountedmounted onon aa motorizedmotorized translationtranslation (M-112.1DG(M-112.1DG fromfrom PhysikPhysik Instrumente, Karlsruhe, Germany) in order to collimate the beam of each laser of the array. The gas of Instrumente,Instrumente, Karlsruhe, Karlsruhe, Germany) Germany) in in order order to to collimate collimate the the beam beam of of each each laser laser of of the the array. array. The The gas gas interest is methane filled in a 15-cm-long cell at ambient temperature and at 0.1 bar pressure. As the ofof interest interest is is methane methane filled filled in in a a 15-cm-long 15-cm-long cell cell at at ambient ambient temperature temperature and and at at 0.1 0.1 bar bar pressure. pressure. As As beam moves with each laser change, the detection system (a HgCdTe detector at liquid nitrogen thethe beam beam moves moves with with each each laser laser change, change, the the detection detection system system (a (a HgCdTe HgCdTe detector detector at at liquid liquid nitrogen nitrogen temperature) is placed after a parabolic mirror that compensates the beam shift and focalizes the beam temperature)temperature) isis placedplaced afterafter aa parabolicparabolic mirrormirror thatthat compensatescompensates thethe beambeam shiftshift andand focalizesfocalizes thethe on the detector. beambeam on on the the detector. detector. Each laser operates in quasi-continuous regime with 100 ns pulses and a period of 10 µs. Signals are EachEach laser laser operates operates in in quasi-continuous quasi-continuous regime regime with with 100 100 ns ns pulses pulses and and a a period period of of 10 10 µs. µs. Signals Signals recorded during a 90 s long temperature ramp between 10 ◦C and 30 ◦C. The power transmitted through areare recorded recorded during during a a 90 90 s s long long temperature temperature ramp ramp between between 10 10 °C °C and and 30 30 °C. °C. The The power power transmitted transmitted the gas cell and the chip temperature are synchronously recorded by an oscilloscope (Waverunner throughthrough thethe gasgas cellcell andand thethe chipchip temperaturetemperature areare synchronouslysynchronously recordedrecorded byby anan oscilloscopeoscilloscope 104Xi from Teledyne Lecroy, Chestnut Ridge, NY, USA). The gas transmission spectrum is obtained (Waverunner(Waverunner 104Xi104Xi fromfrom TeledyneTeledyne Lecroy,Lecroy, ChesChestnuttnut Ridge,Ridge, NY,NY, USA).USA). TheThe gasgas transmissiontransmission from two successive records with and without gas. The conversion from temperature to wavenumber spectrumspectrum isis obtainedobtained fromfrom twotwo successivesuccessive recordsrecords withwith andand withoutwithout gas.gas. TheThe conversionconversion fromfrom temperaturetemperature toto wavenumberwavenumber isis obtainedobtained assumingassuming thatthat itsits variationvariation isis linearlinear forfor aa DFBDFB laser.laser. TheThe resultresult of of the the measurement measurement on on a a part part of of the the whole whole wa wavenumbervenumber range range is is presented presented in in Figure Figure 5a. 5a. Figure Figure 5b5b showsshows thethe calculatedcalculated transmissiontransmission fromfrom HITRANHITRAN data data [69][69] inin thethe samesame experimentalexperimental conditionsconditions when adding an apparatus function of 0.25 cm−1 to be close to the one of the spectroscopic system. when adding an apparatus function of 0.25 cm−1 to be close to the one of the spectroscopic system.

Sensors 2020, 20, 6650 6 of 15 is obtained assuming that its variation is linear for a DFB laser. The result of the measurement on a part of the whole wavenumber range is presented in Figure5a. Figure5b shows the calculated transmission Sensorsfrom 2020 HITRAN, 20, x FOR data PEER [69 REVIEW] in the same experimental conditions when adding an apparatus function6 of 16 of 1 0.25 cm− to be close to the one of the spectroscopic system. One can remark a good agreement on the Onewavenumber can remark data a good and theagreement quite good on the agreement wavenumb wither the data absorption and the quite depths. good agreement with the absorption depths.

(a) (b)

FigureFigure 5. 5. (a()a )Transmission Transmission spectra spectra of of the the DFB DFB QCL QCL array array through through the the methane methane gas gas cell cell and and (b (b) ) transmittancetransmittance calculated calculated from from HITRAN HITRAN data data in in the the same same cond conditionsitions than than the the experience. experience.

ThisThis first first system system of of DFB DFB QCL QCL array array emitting emitting from from 7.2 7.2 µmµm to to 7.5 7.5 µmµm has has been been used used to to demonstrate demonstrate thethe possible possible detection detection of of methane. methane. This This laser laser source source is is a afirst first prototype prototype and and emits emits only only in in the the pulsed pulsed modemode with with a a low low power. power. Thus, Thus, at at the the moment, moment, this this kind kind of of source source cannot cannot be be competitive competitive with with current current existingexisting optical optical sensors sensors using using high-power high-power standard standard DFB DFB QCLs. QCLs. However, However, the the next next prototypes prototypes that that wouldwould emit emit in in cw cw mode mode with with a a higher higher power power should should be be competitive competitive either either for for multi-species multi-species gaseous gaseous moleculesmolecules such such as as methane methane and/or and/or for for the the detection detection of of complex complex species species with with broad broad absorption absorption spectra spectra inin the the mid-infraredmid-infrared regionregion thanks thanks to to its its wide wide emission emission range. range. For example,For example, in the in demonstrated the demonstrated spectral spectralrange and range using and the using photoacoustic the photoacoustic spectrometer spectrom demonstratedeter demonstrated in [33] in in combination [33] in combination with this with type thisofsource type of emitting source 20emitting mW in 20 continuous mW in contin mode,uous one couldmode, beone able could to demonstrate be able to demonstrate the simultaneous the simultaneousdetection of methane detection (CH of4 methane), sulphur (CH dioxide4), sulphur (SO2), anddioxide water (SO vapor2), and (H 2waterO) with vapor 5 ppb, (H 22O) ppb with and 5 20 ppb, ppb 2respective ppb and 20 detection ppb respective limits. Moreover, detection thislimits. emission Moreover, range this istotally emission suitable range for is thetotally detection suitable of for acetone the detection(C3H6O) of and acetone propyne (C3H (C63O)H 4and). The propyne detection (C3H limits4). The that detection could belimits achieved that could with be the achieved photoacoustic with thespectrometer photoacoustic [33] spectrometer are 5 ppb and [33] 120 are ppb, 5 ppb respectively. and 120 Finally,ppb, respectively. this source Finall shouldy, alsothis source be used should for the alsodetection be used of otherfor the complex detection species of other with complex very good species detection with limits:very good 60 ppb detection of acetonitrile limits: 60 (CH ppb3CN), of acetonitrile70 ppb of acrolein (CH3CN), (C3 H704O), ppb 40 ppbof acrolein of propane (C3 (CH43O),H6 ),40 and ppb 1 ppb of ofpropane nitrobenzene (C3H6), (C 6andH5NO 1 2ppb). of nitrobenzene (C6H5NO2). 2.2. DFB QCL Array with Multiplexer 2.2. DFBAs QCL previously Array with mentioned, Multiplexer the beam lateral shift with laser changing is the main drawback of the monolithicAs previously source mentioned, presented the in beam Section lateral 2.1. shift It imposes with laser the usechanging of a moveable is the main lens drawback and a parabolic of the monolithicmirror for thesource detection presented system. in Section Moreover, 2.1. its It useimposes with systemsthe use thatof a aremoveable sensitive lens to alignmentand a parabolic such as mirrormultipass for the cells detection becomes system. far more Moreover, complicated. its use This wi drawbackth systems is avoidedthat are sensitive with the systemto alignment presented such in asthis multipass section. Thecells principle becomes of thefar DFBmore QCL complicated. array with This multiplexer drawback and is its avoided application with to gasthe detectionsystem presentedrealized in in GSMA this section. with a The source principle from mirSenseof the DFB is QCL presented array in with Figure multiplexer6. and its application to gas detection realized in GSMA with a source from mirSense is presented in Figure 6.

Sensors 2020, 20, x FOR PEER REVIEW 7 of 16

Sensors 2020, 20, 6650 7 of 15 Sensors 2020, 20, x FOR PEER REVIEW 7 of 16

Figure 6. Schematic of a DFB QCL array source with multiplexer for gas detection.

−1 A 2190–2220Figure cm 6. Schematicspectral range of a DFB covered QCL arrayby the source chip with has multiplexerbeen reached. for gasCarbon detection. monoxide (CO) spectra were recorded with a single-path 51-cm-long cell. Flame gas mixture spectra were also A 2190–2220 cm 1 spectral range covered by the chip has been reached. Carbon monoxide (CO) recordedA 2190–2220 where CO cm and−−1 spectralacetylene range (C2H covered2) lines may by the be identified.chip has been Finally, reached. in order Carbon to demonstrate monoxide (CO)the spectra were recorded with a single-path 51-cm-long cell. Flame gas mixture spectra were also recorded efficiencyspectra were of the recorded multiplexer, with thea single-path developed 51-cm-longsystem was cell. combined Flame gaswith mixture a multi-pass spectra 5.1-m-long were also where CO and acetylene (C2H2) lines may be identified. Finally, in order to demonstrate the efficiency Whiterecorded cell towhere demonstrate CO and acetylene detection (Cof2 Hcarbon2) lines dioxide may be (CO identified.2). More Finally, details inon order this experiment to demonstrate can bethe of the multiplexer, the developed system was combined with a multi-pass 5.1-m-long White cell to foundefficiency in [47]. of the multiplexer, the developed system was combined with a multi-pass 5.1-m-long demonstrate detection of carbon dioxide (CO2). More details on this experiment can be found in [47]. White cell to demonstrate detection of carbon dioxide (CO2). More details on this experiment can be 3. External-Cavity Quantum Cascade Laser (EC QCL) 3.found External-Cavity in [47]. Quantum Cascade Laser (EC QCL) 3.1. Standard EC QCL Systems and Intra-Cavity Laser Absorption Spectroscopy 3.1.3. External-Cavity Standard EC QCL Quantum Systems Cascade and Intra-Cavity Laser (EC Laser QCL) Absorption Spectroscopy A more common technique to obtain a wide tuning range is to implement a Fabry–Perot QCL A more common technique to obtain a wide tuning range is to implement a Fabry–Perot QCL chip3.1. inStandard an external-cavity EC QCL Systems (EC) andsystem Intra- soCavity that thLasere laser Absorption could be Spectroscopy tuned on its whole gain curve [48]. chip in an external-cavity (EC) system so that the laser could be tuned on its whole gain curve [48]. The most standard solutions for EC-systems are presented in Figure 7. Figure 8a presents the front The mostA more standard common solutions technique for EC-systemsto obtain a wide are presented tuning range in Figure is to 7implement. Figure8a a presents Fabry–Perot the front QCL facet Littrow configuration (Figure 7a) applied to gas detection. facetchip in Littrow an external-cavity configuration (EC) (Figure system7a) applied so that tothe gas laser detection. could be tuned on its whole gain curve [48]. The most standard solutions for EC-systems are presented in Figure 7. Figure 8a presents the front facet Littrow configuration (Figure 7a) applied to gas detection.

(b)

(a) (b)

(a)

(c)

FigureFigure 7. 7.SchematicSchematic of Fabry–Pérot of Fabry–P éQuantumrot Quantum Cascade Cascade Laser chip Laser source chip in source an external-cavity in an external-cavity system insystem (a) front in facet (a) front Littrow facet (configuration,c Littrow) configuration, (b) back facet (b) back Littrow facet configuration Littrow configuration and (c) front and facet (c) front Littman facet configuration.LittmanFigure 7. configuration. Schematic of Fabry–Pérot Quantum Cascade Laser chip source in an external-cavity system in (a) front facet Littrow configuration, (b) back facet Littrow configuration and (c) front facet Littman Inconfiguration. GSMA, the implementation of a commercial EC QCL emitting at 10.5 µm in a photoacoustic spectrometer was realized and enables measurements on broad spectral range up to 60 cm−1. The tuningIn range GSMA, of the implementationsource emitting fromof a commercial 10 µm to 10.5 EC QCLµm demonstrates emitting at 10.5 the µm possibility in a photoacoustic to detect spectrometer was realized and enables measurements on broad spectral range up to 60 cm−1. The tuning range of the source emitting from 10 µm to 10.5 µm demonstrates the possibility to detect

Sensors 2020, 20, x FOR PEER REVIEW 8 of 16 small and complex molecules such as carbon dioxide (CO2) and butane (C4H10) [52]. Based on the same principle, an EC QCL spectrometer was designed in the lab at 7.5 µm. Two methods were used for the rotation of the grating: step by step and continuous rotation. The continuous rotation method provided good results for a shorter acquisition time. A continuous wave emission from 1293 up to 1350 cm−1 was demonstrated, and measurements on acetone (C3H6O) and phosphoryl chloride (POCl3) were realized [53]. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy (ICLAS) where the gas sample is placed within the laser resonator (see Figure 8b). The ICLAS was developed at the beginning of the 70s [70]. This method, already well known in the visible and the near infrared, is developing in the mid-infrared [61,62]. Absorption lines influence the spectrum during many round trips. The cavity output light can be detected to perform spectroscopy. With a carefully optimized system, the effective pathlength can reach hundreds of kilometers,Sensors 2020, 20 thus, 6650 making it an appropriate technique for low absorption gases detection. 8 of 15

(a) (b)

Figure 8. 8. (a)( aSchematic) Schematic of a ofstandard a standard Fabry–Pérot Fabry–P Quantumérot Quantum Cascade Cascade Laser chip Laser source chip in source an external- in an cavityexternal-cavity system for system gas detection for gas detection in front facet in front Littrow facet configuration Littrow configuration and (b) the and same (b) the set-up same used set-up to performused to performIntra-Cavity Intra-Cavity Laser Absorption Laser Absorption Spectroscopy. Spectroscopy.

3.2. ECIn QCL—Voltage GSMA, the implementation Intracavity Sensing of a commercial EC QCL emitting at 10.5 µm in a photoacoustic 1 spectrometer was realized and enables measurements on broad spectral range up to 60 cm− . The tuning In addition to the ICLAS technique, a technique using the QCL compliance voltage can be used range of the source emitting from 10 µm to 10.5 µm demonstrates the possibility to detect small and to retrieve the spectrum of the gas inside the cavity, thus no detector outside the cavity is needed [63]. complex molecules such as carbon dioxide (CO2) and butane (C4H10)[52]. Based on the same This technique is based on the fact that the compliance voltage of a semi-conductor laser is sensitive principle, an EC QCL spectrometer was designed in the lab at 7.5 µm. Two methods were used to optical feedback. For EC QCLs, the cavity losses can be changed by introducing absorbing for the rotation of the grating: step by step and continuous rotation. The continuous rotation materials such as gases into the laser cavity. The principle of the EC QCL—Voltage Intracavity method provided good results for a shorter acquisition time. A continuous wave emission from Sensing and its application1 to gas detection is presented in Figure 9. 1293 up to 1350 cm− was demonstrated, and measurements on acetone (C3H6O) and phosphoryl chloride (POCl3) were realized [53]. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy (ICLAS) where the gas sample is placed within the laser resonator (see Figure8b). The ICLAS was developed at the beginning of the 70s [ 70]. This method, already well known in the visible and the near infrared, is developing in the mid-infrared [61,62]. Absorption lines influence the spectrum during many round trips. The cavity output light can be detected to perform spectroscopy. With a carefully optimized system, the effective pathlength can reach hundreds of kilometers, thus making it an appropriate technique for low absorption gases detection.

3.2. EC QCL—Voltage Intracavity Sensing In addition to the ICLAS technique, a technique using the QCL compliance voltage can be used to retrieve the spectrum of the gas inside the cavity, thus no detector outside the cavity is needed [63]. This technique is based on the fact that the compliance voltage of a semi-conductor laser is sensitive to optical feedback. For EC QCLs, the cavity losses can be changed by introducing absorbing materials suchFigure as gases 9. Schematic into the laser of ancavity. External The Cavity principle Quantum of the Cascade EC QCL—Voltage Laser—Voltage Intracavity Intracavity Sensing for and its applicationgas detection to gas in detection front facet isLittrow presented configuration. in Figure Note9. that, in this case, no detection system is needed asIn the order cavity to is test the this light technique source and in the GSMA, detector a at previously the same time. existing EC QCL mounted in back facet Littrow configuration (Figure7b) was used with a QCL device designed for operation at a center wavelength of 4.5 µm. The external cavity length of 5 cm used a diffraction grating blazed for 4.5 µm with an angle of 44.8◦. Although it was possible to demonstrate a wide emission range from 2050 to 1 1 2190 cm− , a lot of QCL mod-hops appear every 0.31 cm− , which is logical for a QCL length of 0.5 cm and a refractive index of 3.27. Moreover, as there was some problems of mechanical instabilities with the rotation platform of the grating, it was never possible to record gas spectra with this system. Five QCL chips were tested with a front facet Littrow configuration (Figure7a) in order to choose the more powerful chip in cw mode with the wider wavelength range. The best chip has a cw max 1 power of 16 mW and emits in pulsed operating mode from 1283 to 1384 cm− . A second EC QCL mounted in Littman configuration (Figure7c) was assembled using the best QCL device designed for operation at a center wavelength of 7.5 µm. The Littman configuration was chosen in order to have a fixed beam at the output of the cavity and had an external cavity length of 38 cm. This quite big value was chosen in order to have sufficient place to insert a 20-cm-long gas cell in the set-up. The EC Sensors 2020, 20, x FOR PEER REVIEW 8 of 16

small and complex molecules such as carbon dioxide (CO2) and butane (C4H10) [52]. Based on the same principle, an EC QCL spectrometer was designed in the lab at 7.5 µm. Two methods were used for the rotation of the grating: step by step and continuous rotation. The continuous rotation method provided good results for a shorter acquisition time. A continuous wave emission from 1293 up to 1350 cm−1 was demonstrated, and measurements on acetone (C3H6O) and phosphoryl chloride (POCl3) were realized [53]. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy (ICLAS) where the gas sample is placed within the laser resonator (see Figure 8b). The ICLAS was developed at the beginning of the 70s [70]. This method, already well known in the visible and the near infrared, is developing in the mid-infrared [61,62]. Absorption lines influence the spectrum during many round trips. The cavity output light can be detected to perform spectroscopy. With a carefully optimized system, the effective pathlength can reach hundreds of kilometers, thus making it an appropriate technique for low absorption gases detection.

(a) (b)

Figure 8. (a) Schematic of a standard Fabry–Pérot Quantum Cascade Laser chip source in an external- cavity system for gas detection in front facet Littrow configuration and (b) the same set-up used to perform Intra-Cavity Laser Absorption Spectroscopy.

3.2. EC QCL—Voltage Intracavity Sensing In addition to the ICLAS technique, a technique using the QCL compliance voltage can be used Sensorsto retrieve2020, 20, the 6650 spectrum of the gas inside the cavity, thus no detector outside the cavity is needed9 of [63]. 15 This technique is based on the fact that the compliance voltage of a semi-conductor laser is sensitive to optical feedback. For EC QCLs, the cavity losses can be changed by introducing absorbing QCL used a diffraction grating blazed for 8 µm with an angle of 36.5◦. The first-order diffraction from thematerials grating wassuch directed as gases to into a galvanometer-mounted the laser cavity. The tuningprinciple mirror of the (model EC QCL—Voltage 6220H from Cambridge Intracavity Technology,Sensing and Bedford, its application MA, USA), to gas which detection selected is presented the operation in Figure wavelength 9. of the EC QCL.

FigureFigure 9. Schematic9. Schematic of of an an External External Cavity Cavity Quantum Quantum Cascade Cascade Laser—Voltage Laser—Voltage Intracavity Intracavity Sensing Sensing for for gasgas detection detection in frontin front facet facet Littrow Littrow configuration. configuration. Note Note that, that, in thisin this case, case, no no detection detection system system is neededis needed asas the the cavity cavity is theis the light light source source and and the the detector detector at theat the same same time. time.

The QCL chip is placed inside a modified Laser Laboratory Housing (LLH) adapted for mirSense chips. The EC QCL can be operated in both pulsed and continuous-wave configurations. For pulsed operation, a Digital Delay/Pulse generator from Berkeley Nucleonics Corporation commands a pulser switching unit (LDD100 from Alpes Lasers SA, St-Blaise, Switzerland) that generates a current pulse transported to the array via a low-impedance flat cable. For cw operation, a stabilized current supplier LDX-3232 from ILX Lightwave (Bozeman, MT, USA) with a 4 A maximal current is used. The QCL device temperature is controlled by a LDT-5980 from Newport (Irvine, CA, USA) associated with a PT100 and a Peltier element. The compliance voltage of the QCL device was measured via a FPGA PXI-7841 card from National Instruments (Austin, TX, USA) that can record up to 2 105 samples/s. The compliance voltage × variation of the EC QCL across a mirror movement of 3◦ corresponding to a wavenumber variation 1 ± of about 100 cm− is presented in Figure 10a. Such a figure with a U form is called mark. Figure 10b presents a detail of the mark of Figure 10a. The magnitude of the compliance voltage is negative due to the polarity of the QCL device. Figure 10a shows two different types of peaks inside the mark. The first one that is zoomed in Figure 10b corresponds to QCL mod-hops. The interval between two peaks 1 equals 0.5 cm− , which corresponds to the free spectral range (FSR) of the QCL chip. Notwithstanding 1 1 these peaks, one can remark other peaks around 1320 cm− and 1340 cm− . These peaks correspond to water vapor absorption lines because of the presence of water vapor in the air of the cavity. In order to suppress mod-hops on the gas spectra, a new acquisition method has been developed [71]. It is based on a simultaneous wavenumber double scanning. A broad scanning realized by the mirror covers the whole spectral range of the laser. Another finer scanning is operated by a current ramp so as to cover the FSR of the QCL. The current ramp is much slower than the mirror ramp. Thus, each recorded mark is spectrally shifted to another one. Several processing methods have been tested [71] on two records, with and without gas inside the cell. The best one allows to demonstrate preliminary results on the detection of methane (CH4) and water vapor (H2O) at 7.6 µm. Figure 11a presents the voltage difference between the two records with and without gas. The gas inside the cavity corresponds to 0.3% of methane in ambient air at atmospheric pressure. One can also estimate that ambient air contains approximately 3% of water vapor. Figure 11b shows the corresponding absorption coefficient in the experimental conditions. Once again, one can observe the good agreement between experimental and calculated spectra in terms of absorption positions and depths. Sensors 2020, 20, x FOR PEER REVIEW 9 of 16

In order to test this technique in GSMA, a previously existing EC QCL mounted in back facet Littrow configuration (Figure 7b) was used with a QCL device designed for operation at a center wavelength of 4.5 µm. The external cavity length of 5 cm used a diffraction grating blazed for 4.5 µm with an angle of 44.8°. Although it was possible to demonstrate a wide emission range from 2050 to 2190 cm−1, a lot of QCL mod-hops appear every 0.31 cm−1, which is logical for a QCL length of 0.5 cm and a refractive index of 3.27. Moreover, as there was some problems of mechanical instabilities with the rotation platform of the grating, it was never possible to record gas spectra with this system. Five QCL chips were tested with a front facet Littrow configuration (Figure 7a) in order to choose the more powerful chip in cw mode with the wider wavelength range. The best chip has a cw max power of 16 mW and emits in pulsed operating mode from 1283 to 1384 cm−1. A second EC QCL mounted in Littman configuration (Figure 7c) was assembled using the best QCL device designed for operation at a center wavelength of 7.5 µm. The Littman configuration was chosen in order to have a fixed beam at the output of the cavity and had an external cavity length of 38 cm. This quite big value was chosen in order to have sufficient place to insert a 20-cm-long gas cell in the set-up. The EC QCL used a diffraction grating blazed for 8 µm with an angle of 36.5°. The first-order diffraction from the grating was directed to a galvanometer-mounted tuning mirror (model 6220H from Cambridge Technology, Bedford, MA, USA), which selected the operation wavelength of the EC QCL. The QCL chip is placed inside a modified Laser Laboratory Housing (LLH) adapted for mirSense chips. The EC QCL can be operated in both pulsed and continuous-wave configurations. For pulsed operation, a Digital Delay/Pulse generator from Berkeley Nucleonics Corporation commands a pulser switching unit (LDD100 from Alpes Lasers SA, St-Blaise, Switzerland) that generates a current pulse transported to the array via a low-impedance flat cable. For cw operation, a stabilized current supplier LDX-3232 from ILX Lightwave (Bozeman, MT, USA) with a 4 A maximal current is used. The QCL device temperature is controlled by a LDT-5980 from Newport (Irvine, CA, USA) associated with a PT100 and a Peltier element. The compliance voltage of the QCL device was measured via a FPGA PXI-7841 card from National Instruments (Austin, TX, USA) that can record up to 2 × 105 samples/s. The compliance voltage variation of the EC QCL across a mirror movement of ±3° corresponding to a wavenumber variation of about 100 cm−1 is presented in Figure 10a. Such a figure with a U form is called mark. Figure 10b presents a detail of the mark of Figure 10a. The magnitude of the compliance voltage is negative due to the polarity of the QCL device. Figure 10a shows two different types of peaks inside the mark. The first one that is zoomed in Figure 10b corresponds to QCL mod-hops. The interval between two peaks equals 0.5 cm−1, which corresponds to the free spectral range (FSR) of the QCL chip. Notwithstanding these peaks, one can remark other peaks around 1320 cm−1 and 1340 cm−1. These peaks correspond to water vapor absorption lines because of the presence of water vapor in Sensors 2020, 20, 6650 10 of 15 the air of the cavity.

Sensors 2020, 20, x FOR PEER REVIEW 10 of 16

In order to suppress mod-hops on the gas spectra, a new acquisition method has been developed [71]. It is based on a simultaneous wavenumber double scanning. A broad scanning realized by the mirror covers the whole spectral range of the laser. Another finer scanning is operated by a current ramp so as to cover the FSR of the QCL. The current ramp is much slower than the mirror ramp. Thus, each recorded mark is spectrally shifted to another one. Several processing methods have been tested [71] on two records, with and without gas inside the cell. The best one allows to demonstrate preliminary results on the detection of methane (CH4) and water vapor (H2O) at 7.6 µm. Figure 11a presents the voltage difference between the two records with and without gas. The gas inside the cavity corresponds to 0.3%(a) of methane in ambient air at atmospheric pressure.(b) One can also estimate that ambient air contains approximately 3% of water vapor. Figure 11b shows the corresponding FigureFigure 10. 10. ((aa)) QCL QCL device device compliance compliance voltage voltage when when varying varying the the mirror mirror angle angle ±3°3 andand ( (bb)) a a zoom zoom on on absorption coefficient in the experimental conditions. Once again, one± can◦ observe the good FigureFigure 10a10a around around 1305 1305 cm cm−1.1 . agreement between experimental− and calculated spectra in terms of absorption positions and depths.

(a) (b)

Figure 11. ((aa)) Experimental Experimental spectra spectra and and ( (bb)) absorption absorption coefficient coefficient calculated from from HITRAN data assuming 0.3% of methane and 3% of water va vaporpor in ambient air at atmospheric pressure.

4. External External Cavity Cavity Coherent Coherent Quantum Quantum Cascade Cascade Laser Array Finally, another specificspecific scheme combining EC systems and QCL array is anew anew briefly briefly presented presented inin thisthis part.part. ItIt isis based based on on coherent coherent quantum quantum cascade cascade laser laser micro-stripes micro-stripes (µ-stripes) (µ-stripes) array array [64]. The[64]. main The maingoal isgoal to obtainis to obtain a very a very powerful powerful source. source. This This coherent coherentµ -stripesµ-stripes array array configuration configuration waswas proposed toto solve both the problems of beambeam qualityquality and thermalthermal dissipationdissipation of wide active region InP-based QCLs. Phase-locking Phase-locking is is provided provided by by evanescent evanescent co couplingupling between between adjacent adjacent stripes stripes and and leads leads to the creation of a so-called supermode. In In the the proposed proposed configuration, configuration, the the far-field far-field intensity pattern for thisthis supermode is aa dual-lobedual-lobe emission.emission. The spacingspacing of thethe µµ-stripes-stripes greatlygreatly increasesincreases the thermal dissipation of of the the device device and and thus thus reduces reduces the the laser laser heating heating when when operating operating at at high high power. power. The source emitting at 8.2 µmµm is formed of 4 µ stripes 2 µmµm width and 8 µmµm spacing. The a priori priori drawback of dual-lobe emissionemission (with(with anan angularangular separationseparation of of almost almost 45 45°)◦) has has been been converted converted into into a aforce force by by developing developing an an EC EC system system with with this this source. source. OneOne lobelobe isis usedused asas anan output beam while the optical feedback is is obtained obtained on on the the other other lobe lobe us usinging a a diffraction diffraction grating grating in in Littrow Littrow configuration. configuration. The schematic schematic of of this this configuration configuration and and its its application application to gas to gas detection detection is presented is presented in Figure in Figure 12. The 12. 1 possibilityThe possibility to tune to tuneup to up 40 to cm 40−1 cm of −theof emitted the emitted wavelength wavelength was wasdemonstrated demonstrated [65]. [This65]. Thissource source was implementedwas implemented with a with cell containing a cell containing acetone. acetone. The obta Theined obtainedexperimental experimental spectrum is spectrum in good agreement is in good withagreement a FTIR with spectrum a FTIR and spectrum a spectrum and a from spectrum PNNL from database PNNL database[72]. To our [72 ].knowledge, To our knowledge, this was thisthe first was realization of an external cavity with a coherent laser array in the mid-infrared. More details on this experiment can be found in [65].

Sensors 2020, 20, 6650 11 of 15 the first realization of an external cavity with a coherent laser array in the mid-infrared. More details Sensorson this 2020 experiment, 20, x FOR PEER can REVIEW be found in [65]. 11 of 16

Figure 12. Schematic of an External Cavity Coherent Quantum Cascade Laser Array for gas detection. Figure 12. Schematic of an External Cavity Coherent Quantum Cascade Laser Array for gas detection. 5. Conclusions 5. Conclusions The laser team in GSMA in Reims has a long experience in the use of Distributed FeedBack QuantumThe laser Cascade team Lasersin GSMA in spectroscopy in Reims has for a applicationslong experience to gas in detection.the use of TheDistributed main limitation FeedBack of Quantum Cascade Lasers in spectroscopy for applications to gas detection. The main1 limitation of all all these experiments is the DFB QCLs’ relatively low tuning range (~10 cm− ) that prevents from −1 thesemonitoring experiments complex is the species DFB with QCLs’ broad relatively absorption low spectratuning range in the (~10 infrared cm ) region that prevents or performing from monitoringmulti-gas sensing. complex species with broad absorption spectra in the infrared region or performing multi-gasTo obtain sensing. a wider tuning range, the first solution that has been presented in Section2 of this paper To obtain a wider tuning range, the first solution that has been presented in Section 2 of this consists in the use of a DFB QCL array. Methane (CH4) detection using a DFB QCL array emitting paperfrom 7.2consistsµm to in 7.5 theµm use has of been a DFB demonstrated. QCL array. In Methane a second time,(CH4) another detection DFB using QCL a array DFB associatedQCL array to emittinga multiplexer from 7.2 directly µm to fixed 7.5 µm on thehas chip been has demonst been presentedrated. In a to second demonstrate time, another the detection DFB QCL of carbon array associated to a multiplexer directly fixed on the chip has been presented to demonstrate the detection monoxide (CO), acetylene (C2H2) and carbon dioxide (CO2), with a source emitting around 4.5 µm. ofAs carbon a first monoxide proof of concept, (CO), acetylene to our knowledge, (C2H2) and the carbon prototypes dioxide demonstrated (CO2), with a in source this work emitting show around a lot of 4.5possibilities µm. As a first for multi-gas proof of concept, sensing to and our for knowledg monitoringe, the complex prototypes species demonstrated with broad absorptionin this work spectra show ain lot the of infraredpossibilities region. for multi-gas One of the sensing next step and for for these monitoring QC-based complex sources species will be with to obtainbroad absorption continuous spectrawave functioning, in the infrared because region. spectra One that of arethe recordednext step with for pulsedthese QC-based sources remain sources of lowerwill be quality to obtain than continuousthose recorded wave with functioning, cw sources. because Another spectra step that will are consist recorded in the with improvement pulsed sources of the remain covered of spectral lower quality than those recorded1 with cw sources. 1Another step will consist in the improvement of the range, first up to 100 cm− then up to 200 cm− . −1 −1 coveredAmore spectral common range,technique first up to to100 obtain cm athen wider up tuningto 200 cm range. that has been presented in Section3 is toA implement more common a Fabry–Perot technique QCL to obtain chip ina wider an external-cavity tuning range (EC) that systemhas been so presented that the laser in Section could be3 istuned to implement on its whole a Fabry–Perot gain curve. QCL The chip use in of an an extern EC systemal-cavity also (EC) allows system to perform so that Intra-Cavitythe laser could Laser be tunedAbsorption on its Spectroscopywhole gain curve. (ICLAS), The where use of the an gas EC sample system is also placed allows within to theperform laser resonator. Intra-Cavity Moreover, Laser Absorptiona technique usingSpectroscopy the QCL compliance(ICLAS), where voltage the can gas be usedsample to retrieveis placed the spectrumwithin the of thelaser gas resonator. inside the Moreover,cavity, thus a notechnique detector using outside the theQCL cavity compliance is needed. voltage Preliminary can be used results to retrieve demonstrating the spectrum the detection of the gas inside the cavity, thus no detector outside the cavity is needed. Preliminary results demonstrating of methane (CH4) and water vapor (H2O) at 7.5 µm have been presented. Finally, another specific thescheme detection combining of methane EC systems (CH4) andand QCLwater array vapor has (H been2O) anewat 7.5 presentedµm have inbeen Section presented.4. For thisFinally, last anothersystem, specific the main scheme next step combinin will beg to EC realize systems the sameand QCL type array of source has b buteen emitting anew presented in cw mode. in Section On the 4.opposite For this oflast pulsed system, emission the main presented next step will in Section be to realize 2.1, cw the emission same type is of more source simple but emitting to implement. in cw mode.Moreover, On the the opposite quality of of the pulsed spectra emission obtained presented is much better in Section and helps 2.1, cw in obtainingemission goodis more spectroscopic simple to implement.measurements. Moreover, In addition, the quality in this of case, the spectra the average obtained power is much increases, better which and helps makes in cwobtaining sources good ideal spectroscopicfor photoacoustic measurements. spectroscopy, In addition, where the in signal this case, is directly the average proportional power toincreases, the laser which power. makes cw sources ideal for photoacoustic spectroscopy, where the signal is directly proportional to the laser power. For EC systems, the overall tuning range is usually limited by two main parameters. The first one corresponds to the width of the gain curve of the FP QCL chip. This value is defined by the design of the chip. The second one is linked to the reflection at the output of the QCL FP chip. This parameter must be minimized in such a coupled cavities system in order to help the feedback from the

Sensors 2020, 20, 6650 12 of 15

For EC systems, the overall tuning range is usually limited by two main parameters. The first one corresponds to the width of the gain curve of the FP QCL chip. This value is defined by the design of the chip. The second one is linked to the reflection at the output of the QCL FP chip. This parameter must be minimized in such a coupled cavities system in order to help the feedback from the diffraction grating. This can be achieved through the deposition of an anti-reflection (AR) coating on the front face of the QCL chip. Thus, the development of new AR coatings and of a broadband gain middle QCL chip are key issues for external cavity systems and will consist in the next steps of these works. For the particular compliance voltage system, next improvements will consist in making the set-up stability better, to shorten the cavity length and to test the limits of the developed system by the measurement of voltage evolution with gas concentration. A more long-term objective consists in the development of real time sub-ppm detection.

Author Contributions: Conceptualization, V.Z.; software, R.V., L.B., C.J., G.A., G.M. and B.P.; validation, V.Z. and B.P.; formal analysis, V.Z., R.V., L.B., C.J. and B.P.; resources, G.A., G.M. and M.C.; data curation, V.Z., R.V., L.B., C.J. and B.P.; writing—original draft preparation, V.Z.; writing—review and editing, V.Z., R.V., L.B. and B.P.; visualization, V.Z., R.V., L.B. and B.P.; supervision, V.Z., R.V, M.C. and B.P.; project administration, V.Z., M.C. and B.P.; funding acquisition, V.Z., G.A., G.M., M.C. and B.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by French National Research Agency ANR ASTRID “COCASE”, grant number 11-ASTR-0027 (https://anr.fr/Project-ANR-11-ASTR-0027); by ANR ECOTECH “MIRIADE”, grant number 11-ECOT-004 (https://anr.fr/Project-ANR-11-ECOT-0004); by ANR ASTRID “QUIGARDE”, grant number 12-ASTR-0028 (https://anr.fr/Projet-ANR-12-ASTR-0028) and by French Army General Delegation DGA-REI “SELECTIF”, grant number 2009.34.0040. Acknowledgments: The authors thank Frédéric Polak for his valuable technical assistance. Conflicts of Interest: The authors declare no conflict of interest.

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