Development of an Automated Diode-Laser-Based Multicomponent Gas Sensor

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Development of an automated diode-laser-basedmulticomponent gas sensor

Dirk Richter, David G. Lancaster, and Frank K. Tittel

The implementation and application of a portable fiber-coupled trace-gas sensor for the detection ofseveral trace gases, including CO2, CH4, and H2CO, are reported. This particular sensor is based on acw fiber-amplified near-infrared ~distributed Bragg reflector! diode laser and an external cavity diodelaser that are frequency converted in a periodically poled lithium niobate crystal to the mid-IR spectro-scopic fingerprint region ~3.3–4.4 mm!. A continuous absorption spectrum of CH4 and H2CO from 3.37to 3.70 mm with a spectral resolution of 40 MHz ~;0.0013 cm21! demonstrated the spectral performancethat can be achieved by means of automated wavelength tuning and phase matching with stepper motorcontrol. Autonomous long-term detection of ambient CO2 and CH4 over a 3- and 7-day period was alsodemonstrated. © 2000 Optical Society of America

OCIS codes: 190.2620, 300.6340.

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1. Introduction

The spectroscopic detection and quantification of nu-merous trace-gas species has become an importantaspect in many industrial, urban, and environmentalapplications. More importantly, there is a need toselectively detect several different molecular gas spe-cies simultaneously by means of a single, tunable,and portable sensing device capable of high sensitiv-ity and rapid response time.

Characteristic absorption bands of most moleculescan be found in the near-infrared ~overtone! and mid-nfrared ~fundamental rovibrational! spectral region.he latter range has the advantage that the spectrum

s less congested and exhibits ;20–200 times stron-er absorption cross sections. Established tech-iques for multicomponent gas analysis includeonlaser-based Fourier-transform infrared spectrom-ters that can rapidly cover a broad spectrum ~;1 s!ith a typical resolution of 15 GHz.1 For highly

elective ~;50-MHz! absorption spectroscopy of traceases, various narrow-linewidth laser sources aresed in the mid-IR, including cw cyrogenically cooled

ead–salt diode lasers2–4 and, more recently, cw and

The authors are with the Rice Quantum Institute, Rice Univer-sity, Houston, Texas 77251-1892. The e-mail address for F. K.Tittel is [email protected].

Received 4 January 2000; revised manuscript received 24 May2000.

0003-6935y00y244444-07$15.00y0© 2000 Optical Society of America

4444 APPLIED OPTICS y Vol. 39, No. 24 y 20 August 2000

pulsed quantum cascade lasers. In the near-IRspectral region, room-temperature operation of cwsingle or multisection distributed feedback InGaAsPdiode lasers can be applied to overtone absorptionspectroscopy.6 An alternative, narrow-linewidthmid-IR spectroscopic source that has been appliedsuccessfully to trace-gas detection is based ondifference-frequency generation ~DFG! of two near-IRdiode lasers in a nonlinear optical material such asperiodically poled lithium niobate ~PPLN!.7–9

In this paper we describe the development andapplication of an automated mid-IR diode-laser-pumped DFG-based trace-gas sensor source that of-fers wide tunability from 3.3 to 4.4 mm and a spectralselectivity of ;40 MHz. A fiber-coupled tunablenear-IR external cavity diode laser ~ECDL! ~814–870

m! and a Yb-fiber-amplified distributed Bragg re-ector ~DBR! diode-laser ~1083 nm! are used as DFGump sources. The two pump lasers are difference-requency mixed in a fan-out-type PPLN crystal thatffers high optical conversion efficiency, broad spec-ral transmission ~0.3–5.3 mm!, and continuousuasi-phase matching. This sensor uses extractive,irect absorption spectroscopy at a reduced pressure;80 Torr! in a multiple-pass absorption cell fittedith astigmatic mirrors ~18 or 36 m!.10 Several

gases including CH4, CO2, H35,37Cl, H2CO, N2O,NO2, CH3OH, and C6H6 have been detected to datewithin the tuning range of this DFG-based gas sen-sor. Spectral tuning and quasi-phase matching bymeans of stepper motors are controlled by LabViewsoftware operating on a laptop PC and interfaced

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with serial ports. Furthermore, the fast measure-ment response time of typically 5–10 s, including av-eraging and automated data processing, allows thereal-time monitoring of in situ and remote chemicaland atmospheric trace-gas species.

2. Gas Sensor Configuration

The gas sensor developed for field operation utilizesboth a fiber-pigtailed, widely tunable 25-mW ECDLoperating at a center wavelength of 842 6 28 nm anda fiber-pigtailed 50-mW DBR diode laser operating ata 1083-nm wavelength ~see Fig. 1!. Continuous,

ode-hop-free tuning of the Littman-type ECDL iselected by simultaneous rotation and translationalovement of the internal feedback mirror about a

ivot point with respect to the grating by means of ainear translation stage.11 This linear translation

stage was modified to implement a stepper motor andwas operated by use of a compact computer-controlled driver. The fiber-coupled output power of;15 mW from the DBR diode laser was amplified to600 mW by a side-pumped Yb fiber amplifier using a2-W diode laser at 975 nm.12,13 Both ECDL and am-plified DBR diode-laser fiber-coupled beams passthrough polarization controllers before being com-bined by use of a four-port fiber coupler. Two per-cent of the combined beam power was connected to a

Fig. 1. Schematic of an automated DFG multicomponent gas senstage, and reference gas assembly, respectively. WDM, waveleng

second fiber coupler, separating the pump and signalbeams. Either optical fiber arm can be connected toa wavemeter for absolute frequency calibration. Forconvenience, the ECDL beam was monitored contin-uously with the wavemeter ~precision: 60.01 cm21

at l 5 1 mm! to provide absolute frequency tuningnformation by a general-purpose interface busGPIB! Personal Computer Memory Card Interna-ional Association ~PCMCIA! interface. Alterna-ively, a solid etalon in combination with a siliconhotodiode could be used as a relative wavelengthuning reference.14 The other 98% are terminated

in an 8-deg angle-polished fiber connector andmounted onto a ruggedized DFG conversion stage.This stage contains the optical elements and optome-chanical mounts that are pertinent to achieve widelytunable DFG, including a fiber port, imaging lens,PPLN crystal, and a CaF2 lens. We imaged the col-linear pump beams from the fiber tip into thetemperature-controlled antireflection-coated PPLNcrystal using an f 5 10-mm achromatic lens. A ki-nematic optical mount with vertical translation andtilt is used to align the crystal to the beam propaga-tion axis. This mount is also controlled by a stepper-motor-driven linear translation stage to provide theoptimal quasi-phase-matching grating period. Thedialing position is computed from an experimentally

M1, M2, and M3 are stepper motors for the ECDL, PPLN crystalivision multiplexer; DL, diode laser; CTRL, control.

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20 August 2000 y Vol. 39, No. 24 y APPLIED OPTICS 4445

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recorded phase-matching function. Both the single-mode fiber-launched pump and the signal beams pos-sess Gaussian beam profiles, and when mixed in a19-mm-long PPLN crystal they produce a diffraction-limited mid-IR DFG beam of ;2.9-mW power at;3.5-mm wavelength.8 The DFG beam is then col-limated by use of an f 5 50-mm CaF2 lens, passingthrough a Ge filter to block the unconverted pumpbeams and directed to a multipass cell ~physicalength, ;30 cm; actual path length, 18 or 36 m!.10

The sensor operated with a typical sensitivity of 2 31024 limited by the occurrence of etalon effects intro-duced by the optical elements. For convenient iden-tification and tracking specific absorption line centerpositions within the scan width, up to five individualreference gas cells ~L 5 5 cm! can be rotated into the

FG beam path by a stepper-motor-driven assembly.he sensor is also operated and controlled with aabView software code running on the laptop PC.igure 2 shows the electronic diagram of the gasensor. Both stepper motor controllers are inter-aced to the laptop PC by a universal serial bus port.

16-bit PCMCIA data-acquisition card is used tocquire the absorption signal synchronized to a trig-er signal, the gas sampling pressure, and tempera-ure. The card also controls a shutter through aigital transistor–transistor logic line for backgroundight subtraction that is necessary for an absolute

Fig. 2. Diagram of the electronic control and data-acquisitionacquisition; TEC, thermoelectric cooler.


spectroscopic absorption measurement. The entiresensor, including both optical and electronic ele-ments, is packaged into a suitcase with outside di-mensions of 61 cm 3 53 cm 3 20 cm. An additionalmall suitcase ~30 cm 3 25 cm 3 14 cm! contains aiaphragm pump and a mass-flow controller that aresed for extractive gas sampling at reduced pressureor optimum spectral selectivity and sensitivity.

3. Wavelength Tuning and Quasi-Phase-MatchingCharacteristics

The wavelength tuning curve for the Littman-typeexternal cavity pump laser as a function of the step-per motor rotation ~dial position! is shown in Fig. 3.A miniature coupler from the stepper motor shaft tothe ECDL tuning screw was used. This coupler per-mitted angular, radial, and lateral misalignment ofthe stepper motor shaft and therefore suppressedstress to the ECDL tuning mechanism. The steppermotor resolution was 1y3600 steps per revolution andprovided an experimentally measured linear ECDLfrequency tuning rate of 0.018 cm21ystep. The fre-quency tuning was superimposed by a sinusoidallikemodulation, corresponding to a peak deviation of 0.15cm21 as can be seen from the inset of Fig. 3. Thisinset also shows a residual backlash of 225 6 15 stepsinherent to the ECDL device used in this research.

Continuous quasi-phase matching over the entire

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tuning range of the sensor is achieved by use of aPPLN crystal with a fan-out grating structure.15 Inthis configuration the grating period continuously in-creases from 22.4 to 23.3 mm over a 1-cm-wide crys-tal. Plotted in Fig. 4 is the relative crystal positionversus the DFG wavelength for two temperatures,26 °C and 44 °C. The PPLN crystal temperaturecan be set to match the available grating periods withthe ECDL wavelength tuning range, which for thissensor is 44 °C. For comparison, a multichannelPPLN crystal ~common channel period spacing of 0.1mm! requires a temperature adjustment to optimallyphase match intermediate wavelengths. For exam-ple, for a pump–signal–idler wavelength combina-tion, which requires a grating period that liesbetween two nominal PPLN crystal channel gratingperiods of 0.1-mm difference, a temperature adjust-ment of 615 °C is necessary.8,16 The measuredPPLN phase-matching bandwidth is 28 cm21 ~Fig. 4!.

Fig. 3. ECDL frequency tuning characteristic with stepper motorcontrol.

Fig. 4. Continuous quasi-phase matchin

his value can be compared with a theoretically com-uted phase-matching bandwidth of 12.5 cm21 at 3.6

mm for normal-incidence nondiverging pump beams.Similar to the ECDL stepper motor interface, a min-iature joint coupling was used to connect to the PPLNcrystal linear translation stage ~Vernier set screw,0.5-mm linear translation per revolution!. In addi-tion, a 48-pitch, 1:1 Miter gear was implemented torealize a compact stepper motor arrangement with nonoticeable loss of precision.

The simultaneous computer control of wavelengthtuning and quasi-phase matching allows multispe-cies gas measurements within the instrument tuningrange from 3.3 to 4.4 mm. For example, Fig. 5 shows

continuous absorption spectra of methane ~CH4!nd formaldehyde ~H2CO! acquired over a 270-cm21-

th a fan-out grating-type PPLN crystal.

Fig. 5. Measured absorption spectra of formaldehyde and meth-ane over 270 cm21 centered at 2850 cm21. Estimated total pres-sure was 50 Torr ~8% CH4 and 0.65% H2CO!; absorption pathlength was 5 cm.

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wide tuning range from 2700 cm ~3.70 mm! to 2970cm21 ~3.37 mm! with a DFG linewidth resolution of 40MHz. We performed the measurement using a5-cm-long reference gas cell containing 8% of CH4and 0.65% of H2CO with an estimated total pressuref 50 Torr. A 200-Hz current modulation of the DBRiode laser provided an effective scanning width of.33 cm21. The ECDL was tuned consecutively by

an eight-step ~;0.15-cm21! increment to take intoccount the above-mentioned sinusoidal offset. Ab-olute wavelength accuracy was provided by aavemeter. Each spectrum was averaged over five

cans ~25 ms! and subtracted by a five-scan averageark voltage measurement to obtain absolute trans-ission. The recorded spectroscopic data files that

nclude an absorption scan, dark voltage scan, andorresponding wavelength log were automaticallyrocessed with a LabView software-programmed al-orithm to connect the acquired spectra ~indicated byhe dashed circles in the inset of Fig. 5! and to removeedundant data points of consecutive data scans.urthermore, a polynomial-fit algorithm can be ap-lied to improve the baseline overlap of two consec-tive scans.Automated compensation of wavelength drift that

s due to temperature changes is a further advantagef our using stepper-motor-controlled wavelengthuning and quasi-phase matching. Critical compo-ents that are susceptible to temperature fluctua-ions include detector amplifier, diode-laser drivers,emperature controllers, and, most significantly, theavelength stability of the ECDL. Figure 6 showsn example of automated wavelength drift compen-ation by means of the two stepper motors. For thiseasurement the absorption peak location of a CO2

transition line at ;4.2 mm was monitored after theDFG-based gas sensor was moved from an outdoor toan indoor environment with an ambient temperaturedifference of ;5 °C. At the beginning of the mea-surement the absorption line was centered within theabsorption scan width of 0.32 cm21. We pro-rammed the LabView software to compensate for


the drift of the absorption by appropriately wave-length tuning the ECDL if the peak location wasoutside the programmed maximum and minimumboundary conditions. For comparison, the totalwavelength drift that can occur if the gas sensor sys-tem is not actively controlled is also shown in Fig. 6.

4. Spectroscopic Evaluation of theDifference-Frequency Generation-Based Gas Sensor

The DFG-based gas sensor was used in several long-term tests. In a first test, the sensor was tuned to aCO2 line at 2385.77 cm21 ~4.2 mm! and monitored the

O2 concentration in ambient laboratory air contin-uously at reduced pressure of 80 Torr for a 3-dayperiod as depicted in Fig. 7. The rovibrational CO2absorption lines at ;4.2 mm have a frequency spacingof ;0.7 cm21 ~21 GHz!. In this case, it is advanta-geous to perform spectroscopic absorption measure-ments of CO2 at a reduced sampling pressure to avoida zero baseline offset. The CO2 concentration in-creases and decreases rapidly, respectively, at timeswith and without human activity in the laboratory asshown in Fig. 7. The dynamic detection range atthis CO2 absorption line is 0.5–2200 parts per million~ppm! ~100% absorption! at a pressure of 80 Torr foran 18-m optical absorption path-length-configuredmultipass cell and a residual optical noise of 2 31024. After 2 h into the CO2 sampling test, humanbreath was briefly sampled and caused a sharp in-crease to CO2 levels of more than 2200 ppm, saturat-ing the absorption signal. However, it is possible totune the DFG wavelength to a weaker CO2 transitionto measure higher concentrations or to lower frequen-cies to access a stronger CO2 transition at 4.2 mmwhen low CO2 concentrations are measured. TheDFG-based gas sensor measured higher ambientbackground levels than the atmospheric concentra-tions measured by the National Oceanic and Atmo-spheric Administration.17 This may be due to thefact that the CO2 and other gas species concentrationevels tend to be higher in urban areas such as Hous-on, Texas.

Fig. 6. Comparison of the relative frequency drift for a stepper-motor-controlled and free-running ECDL operation as a function oftime.

Fig. 7. Ambient CO2 laboratory sampling over a 42-h time period.

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In another long-term test, the DFG sensor wastuned to a methane absorption line at 3028.751 cm21

~3.3 mm! to monitor ambient air continuously for a-day period. The DFG-based gas sensor was lo-ated in an air-conditioned laboratory environmentith the sensor being located close to an air-

onditioner ventilation inlet. For this test, the mul-ipass cell was aligned to a 36-m optical pathonfiguration for increased detection sensitivity.

Fig. 8. Continuous detection of ambient CH4 for a 7-day period.

Fig. 9. Intercomparison of long-term CH4 concentration measureconcentrations and deviation in parts per billion.

Approximately 20 min into the test, the DFG-basedgas sensor was set to acquire a dark voltage beforeand after each absorption scan and to compute anaverage absorption value. This effectively reducesthe noise present in a CH4 measurement evident inFig. 8. Furthermore, one can see that the methaneconcentration peaks around midnight and increasedto over 5000 parts per billion ~ppb! after heavy raintorms. These methane trends were confirmed to beeal by an intercomparison with another fiber-oupled DFG-based gas sensor.18 At the bottom of

Fig. 8 the corresponding DFG output power signal isplotted. A general drift of 0.03%yh with a 1.3% stan-dard deviation is measured, except of a few incidentswhen a cooling fan attached to the Yb fiber amplifierhousing stopped operation. This effectively changedthe temperature-dependent pump diode wavelengthwhich resulted in a shift to a weaker absorption cross-section region of the Yb-doped fiber amplifier and ledto a reduced amplifier gain.

A second CH4 sensor, described in Ref. 18, was setup in close vicinity and operated at the same DFGwavelength and measured ambient laboratory airwith a 36-m open path-length multipass cell at atmo-spheric pressure. Figure 9 shows the superimposedCH4 concentration measurements versus time plotsof the two independently operating DFG-based trace-gas sensors over a 4-day period. Both sensors fol-

s by two independent DFG-based gas sensors indicating both CH4
ment

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lowed the same profile and measured the CH4concentration with an average difference of 50 ppb~standard deviation!, which is well within the accu-acy of both instruments ~5%!. For calibration of theFG-based gas sensors, a calibrated gas mixture wasown through the multipass cells.8 Using LabView

software, we first processed the absorption scans toobtain a flat baseline, and then we fitted the absorp-tion signal to a Lorentzian line-shape function usinga nonlinear least-squares Levenberg–Marquardt fit.8The obtained spectroscopic parameters also agreewith the HITRAN19 database. Furthermore, thegood agreement between atmospheric and low sam-pling pressure ~88 Torr! measurements indicates ahigh spectral purity of the two DFG-based gas sen-sors and confirms the applicability of a Lorentzianline-shape function.

5. Summary

We reported the development and application of aportable, compact, widely tunable mid-IR DFG-basedgas sensor that can be operated automatically withstepper motor wavelength tuning and continuousPPLN quasi-phase matching. Fiber optics andfiber-coupled diode-laser pump sources make such adevice suitable for nonlaboratory use of gas sensingapplications. Long-term monitoring of CO2 and

H4 and the acquisition of a continuous CH4 andH2CO absorption spectra over 270 cm21 further dem-onstrates the key advantages of this DFG-based gassensor, including robust long-term maintenance-freeoperation and wide tunability for multicomponentgas detection with high sensitivity, selectivity, andreal-time response.

To achieve better sensitivity, more power is re-quired to apply advanced detection techniques suchas dual-beam detection to effectively cancel out theoccurrence of optical etalon effects.20 Potentially,the same sensor architecture also can be used to ac-cess the second atmospheric window from 6 to 16 mmwith orientation-patterned epitaxial-grown quasi-phase-matched GaAs crystal for optical frequencyconversion.21

The authors acknowledge D. P. Leleux ~Rice Uni-versity! for the support and help in implementing thetepper motor control hardware and software. Fi-ancial support was provided by NASA, the Texasdvanced Technology Program, the Welch Founda-

ion, the National Science Foundation, and the Gulfoast Hazardous Substance Research Center.

References and Notes1. Midac Corporation, 17911 Fitch Ave., Irvine, Calif. 92614;

http:yywww.midac.com.2. A. Fried, B. Henry, B. Wert, S. Sewell, and J. R. Drummond,

“Laboratory, ground-based, and airborne tunable diode lasersystems: performance characteristics and applications in at-mospheric studies,” Appl. Phys. B 67, 317–330 ~1998!.

3. D. D. Nelson, M. S. Zahniser, J. B. McManus, C. E. Kolb, andJ. L. Jimenez, “A tunable diode laser system for the remotesensing of on-road vehicle emissions,” Appl. Phys. B 67, 433–441 ~1998!.


based gas monitors,” Spectrochim. Acta Part A 54, 197–236~1998!.

5. F. Capasso, C. Gmachl, D. L. Sivco, and A. Y. Cho, “Quantumcascade lasers,” Phys. World 12, 27–33 ~1999!.

6. B. L. Upschulte, D. M. Sonnenfroh, and M. G. Allen, “Mea-surements of CO, CO2, OH, and H2O in room temperature andcombustion gases by use of a broadly current-tuned multisec-tion InGaAsP diode laser,” Appl. Opt. 38, 1506–1512 ~1999!.

7. D. Richter, D. G. Lancaster, R. F. Curl, W. Neu, and F. K.Tittel, “Compact mid-infrared trace gas sensor based on dif-ference frequency generation of two diode lasers in periodicallypoled LiNbO3,” Appl. Phys. B 67, 347–350 ~1998!.

8. D. G. Lancaster, D. Richter, and F. K. Tittel, “Portable fibercoupled diode laser based sensor for multiple trace gas detec-tion,” Appl. Phys. B 69, 459–465 ~1999!.

9. M. Seiter and M. W. Sigrist, “On-line multicomponent trace-gas analysis with a broadly tunable pulsed difference fre-quency laser spectrometer,” Appl. Opt. 38, 4691–4698 ~1999!.

10. J. B. McManus, P. L. Kebabian, and M. S. Zahniser, “Astig-matic mirror multipass absorption cells for long-path-lengthspectroscopy,” Appl. Opt. 34, 3336–3348 ~1995!.

11. R. J. Lang, D. G. Mehuys, and D. F. Welch, “External cavity,continuously tunable wavelength source,” U.S. patent,5,771,252 ~23 June 1998!.

12. J. P. Koplow, L. Goldberg, and D. A. V. Kliner, “Compact 1-WYb-doped double-cladding fiber amplifier using V-groove sidepumping,” IEEE Photon. Technol. Lett. 10, 793–795 ~1998!.

13. L. Goldberg, B. Cole, and E. Snitzer, “V-groove side pumped1.5 mm fibre amplifier,” Electron. Lett. 33, 2127–2129 ~1997!.

14. K. W. Aniolek, T. J. Kulp, B. A. Richman, S. E. Bisson, P. E.Powers, and R. L. Schmitt, “Trace gas detection in the mid-IRwith a compact PPLN-based cavity ring-down spectrometer,”in Application of Tunable Diode and Other Infrared Sources forAtmospheric Studies and Industrial Processing Monitoring II,A. Fried, ed., Proc. SPIE 3758, 62–73 ~1999!.

15. P. E. Powers, T. J. Kulp, and S. E. Bisson, “Continuous tuningof continuous-wave periodically poled lithium niobate opticalparametric oscillator by use of a fan-out grating design,” Opt.Lett. 23, 159–161 ~1998!.

16. D. H. Jundt, “Temperature-dependent Sellmeier equation forthe index of refraction, ne, in congruent lithium niobate,” Opt.Lett. 22, 1553–1555 ~1997!.

7. National Oceanic and Atmospheric Administration ~NOAA!,Climate Monitoring and Diagnostics Laboratory ~CMDL!, Car-bon Cycle Group, http:yywww.cmdl.noaa.govyccgg.

18. D. G. Lancaster, R. Weidner, D. Richter, F. K. Tittel, and J.Limpert, “Compact CH4 sensor based on difference frequencymixing of diode lasers in quasi-phase matched LiNbO3,” Opt.Commun. 175, 461–468 ~2000!.

19. L. S. Rothman, R. R. Gamache, A. Goldman, L. R. Brown, R. A.Toth, H. M. Pickett, R. L. Poynter, J.-M. Flaud, C. Camy-Peyret, A. Barbe, N. Husson, C. P. Rinsland, and M. A. H.Smith, “The HITRAN database: 1986 edition,” Appl. Opt. 26,4058–4097 ~1987!.

20. D. G. Lancaster, A. Fried, B. Wert, B. Henry, and F. K. Tittel,“Difference-frequency-based tunable absorption spectrometerfor detection of atmospheric formaldehyde,” Appl. Opt. 39,4436–4443 (2000).

21. L. A. Eyres, P. J. Tourreau, T. J. Pinguet, C. B. Ebert, J. S.Harris, M. M. Fejer, B. Gerard, and E. Lallier, “Quasi-phasematched frequency conversion in all-epitaxial, orientation pat-terned 200-mm-thick GaAs films,” Stanford University AnnualReport 1998–1999 ~Center for Nonlinear Optical Materials,Stanford University, Stanford, Calif., 1999!, pp. 37–41.

Documents

Development of an Automated Diode-Laser-Based Multicomponent Gas Sensor