Application of Antimonide Lasers for Gas Sensing in the 3-4-μm Range

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Application of antimonidelasers for gas sensing in the 3–4-mm range

Peter Werle and Andrei Popov

Antimonide semiconductor laser devices designed for continuous-wave emission in the 3–4-mm spectralrange have been investigated with respect to spectroscopic applications. Representative data on themode structure, output power, noise characteristics, far-field pattern, and modulation response arepresented. Selected laser devices have been applied for methane ~CH4! and formaldehyde ~HCHO!measurements by use of a high-frequency modulated diode laser spectrometer. From an Allan varianceanalysis of experimental data a detection limit for HCHO of 120 pptv ~where 1 pptv 5 10212 volumemixing ratio! with a 40-s integration time and for CH4 of 2 ppbv ~where 1 ppbv 5 1029 volume mixingratio! with 20-s integration time were determined. The results show that, for selected gases, InAsSblasers can be an alternative to lead-salt diode lasers. © 1999 Optical Society of America

OCIS codes: 300.6260, 300.6340, 300.6380, 010.1120, 140.2020, 140.3070.

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1. Introduction

The reasons for limiting a widespread application oftunable diode laser absorption spectroscopy ~TDLAS!n industrial process control are found in the relativeomplexity of current instrumentation and from theack of high-quality, high-power diode lasers for

any spectral regions of interest.1 GaAs and InPlasers are widely commercially available. These di-ode lasers, made from the III–V group of semiconduc-tor materials, emit at red and near-infrared ~NIR!

avelengths from ;0.63 to 1.55 mm, including theGaAsyAlGaAs 0.78- and 0.83-mm lasers used, for ex-ample, in compact-disk players. The technology ofNIR 1.3- and 1.55-mm InGaAsPyInP diode lasers de-veloped for fiber-optic communication can be ex-tended to the fabrication of lasers that emit anywherein the wavelength interval of ;1.2–2 mm. A draw-back of NIR diodes is that only a limited number ofmolecular species have absorption features in thespectral region covered by these lasers. Further-

When this work was performed, P. Werle was with the Fraun-hofer Institut, Kreuzeckbahnstrasse 19, 82467 Garmisch-Partenkirchen, Germany, and A. Popov was with the Ioffe PhysicoTechnical Institute, 194021 St. Petersburg, Russia. A. Popov isnow with the Fachbereich Physik, AG Integrierte Optoelektronik,Universitat Kaiserslautern, 67653 Kaiserslautern, Germany. Thee-mail address for P. Werle is [email protected].

Received 17 June 1998; revised manuscript received 27 August1998.

0003-6935y99y091494-08$15.00y0© 1999 Optical Society of America

1494 APPLIED OPTICS y Vol. 38, No. 9 y 20 March 1999

more, the NIR absorptions are overtone or combina-tion bands that are typically one to several orders ofmagnitude weaker than the IR fundamental band,but many molecules of interest have NIR absorptionbands that are strong enough for detection at parts in106 and, in some cases, even parts in 109 levels.2,3

Lead-salt diode lasers, made from IV–VI semicon-ductor materials, operate in the 3–30-mm spectralregion, and therefore they cover the IR fundamentalband with strong absorptions for most atmospherictrace gases. They are used almost exclusively forspectroscopic applications. Because IV–VI lasersand their associated detectors operate at cryogenictemperatures, they are more expensive and morecumbersome to use than III–V devices. In trace-gas-monitoring applications, lead-salt laser instrumentshave routinely achieved ppbv levels ~1 ppbv 5 1029

volume mixing ratio! and in some cases even pptv ~1ptv 5 10212 volume mixing ratio! detection levels ofnumber of important molecular species.4 When

comparing detection sensitivities, one has to keep inmind that sensitivities in real field applications willlikely be less than those generated in the laboratorybecause of vibration and temperature and humidityexcursions, among other factors.

The link between the NIR lasers and the mid-IRlead-salt diode lasers are antimonide-containing com-pounds such as AlGaAsSb, InGaAsSb, and InAsSbP,which allow the extension of NIR lasers to wave-lengths longer than 1.8 mm for the III–V group ofsemiconductor materials.3 Room-temperature las-ing from 2 to 2.4 mm has been reported from simple

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double-heterostructure antimonide diode lasers.5As the wavelength increases up to 3.7 mm, the max-imum operating temperature decreases as a result ofincreasing optical and electrical losses. RecentlyIII–V devices based on InAsSbyInAsSbP manufac-tured at the loffe Physico Technical Institute ~St. Pe-ersburg, Russia! have been investigated with respecto spectroscopic applications.6–9 These double-

heterostructure devices were grown by liquid-phaseepitaxy on an InAs substrate and covered the spectralrange from 3 to 4 mm at liquid-nitrogen ~LN2! tem-peratures. In this paper a detailed characterizationshows that these devices are well suited for the de-tection of HCHO at 3.57 mm and CH4 at 3.25 mm andthat the single-mode power of more than 1 mW isespecially attractive for spectroscopic trace-gas mon-itoring with optical multipass cells.

Figure 1 gives an overview of different types ofsemiconductor laser, from the visible to the IR re-gions, together with some absorption bands of at-mospheric HCHO and CH4.10 Various systems

ave been developed for the detection of HCHO.hey are based mainly on fluorescence techniquesnd diode laser absorption spectroscopy.11,12 An-

other promising technique for compact, room-temperature solid-state sensors for HCHO in the3–5-mm spectroscopic region is based on difference-requency generation.13 Fast chemical sensors for

atmospheric-greenhouse-gas monitoring are impor-tant for applications in global warming control.14

CH4 absorbs thermal radiation approximately 30times more efficiently ~per unit mass! than CO2does. Its concentration increases ;20 ppbvyyear,produced by natural-gas releases, fuel combustion,

Fig. 1. Coverage of the spectral range from the visible to themid-IR by tunable diode lasers of different material systems. Forthe antimonide lasers in the 1.8–4-mm range, absorption bands for

CHO and CH4 are plotted together with the absorption band forwater vapor.

biomass burning, and rice paddy cultivation. TD-LAS monitoring of CH4 has been done with a lead-salt diode laser by use of the two strong fundamentalabsorption bands located near 7.7 ~n4! by Verma etal.15 and near 3.3 mm ~n3! by Edwards et al.,16 as wellas with weaker overtone bands at 1.3 mm ~n2 1 2n3!by Rooth and Kema17 and at 1.65 mm ~2n3! by Hovdeet al.18 So far, all mid-IR TDLAS instruments arebased on lead-salt lasers as the source of radiation,but it is still a problem to fabricate reliable lasers for3.3 mm. Antimonide diode lasers, which can operatet up to 4-mm wavelengths, are expected to obtainigher cw output powers since their thermal conduc-ivity and material stability under a strong dc injec-ion current are superior compared with those of leadalts.

2. Laser Diodes and Experimental Setup

The antimonide lasers under investigation are basedon an InAsSbyInAsSbP double heterostructuregrown on an InAs ~100! substrate. A 0.5–1.0-mm-hick active layer is enclosed between two 3-mm-thicknAsSbP claddings. The composition of the activeayer corresponds to a lasing wavelength of 3–4 mmt 77 K. For a more detailed description of the an-imonide lasers under investigation, the reader is re-erred to Refs. 5–9. The double-channel mesa laserith a 300-mm-length Fabry–Perot cavity and un-

oated facets is mounted on a copper heat sink @Fig.~a!# and installed in a Dewar @Fig. 2~b!# for cw oper-tion above LN2 temperatures. For high-FM appli-

cations and investigations of the wideband noisecharacteristics, a bias T was mounted close to thelaser for impedance matching. In general, we foundthat thermal cycling was not critical and if the laserwas cycled several times the operating points re-mained almost the same. This was different if thelaser had been dismounted and remounted in thesame or in another Dewar. Then thermal couplingmight have changed and previously measured modemaps would no longer be valid. Therefore the laserstayed in the same Dewar for a detailed spectralcharacterization in the laser test setup and for thesubsequent application measurements in the spec-trometer described below.

To investigate the suitability of the antimonide la-sers for trace-gas measurements, a laser test setupwas used, that allowed recording of laser opticalpower, mode structure, and wideband noise charac-teristics over a broad range of injection currents andjunction temperatures.19 The schematic layout ofthe setup is shown in Fig. 2~c!. The laser was

ounted in a LN2-cooled Dewar ~Laser Photonics,L5739!, was temperature and current stabilized overthe range from 77 to 120 K and between 0.300 mAwith an ILX Lightwave LDC 3742 power supply, andwas additionally mounted on an xyz stage for focuslignment. An off-axis parabolic mirror with a5-mm diameter was used to collimate the laser out-ut beam. Electrically actuated mirrors were usedo pass the beam through the various elements. Theaser beam could be directed on a calibrated pyroelec-

20 March 1999 y Vol. 38, No. 9 y APPLIED OPTICS 1495

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tric powermeter ~Laser Precision, RS 5900! or steeredthrough a 0.5-m monochromator ~Digikrom 480!. Abypass was implemented to allow the measurementof the unfiltered beam. Another mirror was used toswitch between two modes of operation: either a200-MHz-bandwidth HgCdTe detector ~SAT!, fol-lowed by a low-noise preamplifier ~SAT IR 500! and ahigh-frequency spectrum analyzer ~Marconi SA 2383!were used for noise measurements, or a chopper ~SR540! and a HgCdTe detector ~Polytec HCT 70! com-bined with a lock-in amplifier ~SR 530! were selectedfor relative power measurements while the mono-chromator was scanning the mode structure. AHe–Ne laser was coupled into the system by a remov-able beam splitter to prealign the whole system. Aproblem did arise from the fact that the InAsSb lasersunder investigation had a much larger half-widthemittance angle as the previously used lead-salt la-sers. Because of the limited size of the off-axis par-abolic mirror ~25-mm diameter at 32-mm focallength!, a separate external optical output-powermeasurement with a 25-mm BaF2 lens was made toget the full emitted power on the meter. The laserfield patterns were recorded by a pyroelectric cameraplaced directly in front of the laser.

The mid-IR TDLAS system,20 which has been usedfor the spectroscopic detection of CH4 and HCHObased on antimonide lasers is shown in Fig. 2~d!.

he optical setup is mounted on a 1 m 3 0.6 m optical

Fig. 2. ~a! InAsSb laser in a standard housing, mounted close towideband noise characteristics, and far-field pattern in a testspectrometer. OAP’s, off-axis parabolic mirrors.


readboard. The antimonide diode laser is housedn the same LN2-cooled Dewar, which was used for

the spectral characterization in the laser test setupabove. To accommodate a possible deviation anglebetween the cone of laser emission and the lasermount axis, the LN2 Dewar is mounted on an xyzstage alignable to within 630°. The beam from theTDL is first collimated by an off-axis paraboloid andthen directed by a sequence of mirrors through thesample cell and onto a LN2-cooled HgCdTe photovol-taic detector. A visible 670-nm laser beam is com-bined by means of a pellicle beam splitter with theinvisible IR beam to assist in alignment. Because oflow transmission in the IR, the pellicle beam splitteris kinematically mounted and can be removed duringthe trace-gas measurements. Typical line strengthsare such that an atmospheric concentration of 1 ppbvproduces an absorption of only 1 part in 107 over a10-cm path length. Conventional absorption spec-troscopy techniques will clearly not be able to mea-sure such small absorptions. With TDLAS thisproblem is overcome by use of a multipass cell withpath lengths of 100 m or even more. Such cellsachieve the long path by using mirrors to fold theoptical path, giving typically 100 passes of a 1-mbase-length cell. A commercial White cell module~Mutek MDS 1600! with a base length of 62.5 cm andan adjustable path length of up to 100 m is showntogether with the focusing optics in Fig. 2~d!.

! bias T in the Dewar for recording ~c! mode maps, output power,; ~d! for analytical measurements the Dewar is moved to the

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3. Experimental Results

Different InAsSb lasers were investigated in detailand preselected for ambient measurements by use ofthe test setup described in Section 2. Figure 3 sum-marizes some typical results. For the laser shownhere, we observed a mode hop at 90 mA immediatelyabove threshold of ;80 mA at 95 K. Then a domi-nant single-mode emission, starting from 90 up to200 mA was observed @Fig. 3~b!# together with anoutput power of up to 700 mW. The noise measure-ments in Figs. 3~c! and 3~d! illustrate the superiornoise characteristics in the single-mode regime ~B!

Fig. 3. Characteristic results obtained from the laser test setup:aser excess noise at a mode-hop position, ~e! far-field pattern, ~f !

relative to the simultaneous emission of two modesnear the mode-hop position ~A!. These results arethe same as those for lead-salt diode lasers.1,19 Thefar-field distribution @Fig. 3~e!# is especially impor-tant for high-FM application because maximumbeam intensity has to be focused on a small-areadetector element. The modulation response dis-played in Fig. 3~f ! illustrates that relaxation oscilla-tions occur well beyond the modulation frequenciesused for high-FM techniques.8 The results summa-rized in these plots are representative for most anti-monide lasers we have investigated so far. In

ode map, ~b! power measurement, ~c! relative intensity noise, ~d!odulation response.

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general we found that there are no significant dis-crepancies in performance compared with that oflead-salt diode lasers, besides the fact that the ob-served single-mode emission power is up to 1 order ofmagnitude higher for the antimonide lasers underinvestigation.

For trace-gas-sensing applications, two lasers havebeen selected: one for HCHO emitting at 3.57 mm2800.2 cm21! and one for CH4 operating at 3.25 mm

~3076.5 cm21!. Characteristic results obtained fromhe test setup and the spectrometer are displayed inig. 4 for HCHO ~left-hand side! and CH4 ~right-hand

side!; Fig. 4 shows the laser mode map @Fig. 4~a!#, thedirect absorption signals during the injection currentscan @Fig. 4~b!#, time-series concentration data ob-ained from FM signals @Fig. 4~c!#, and the corre-

Fig. 4. Characteristic results obtained from the spectrometer fordirect absorption signals during scan, ~c! time-series concentratioines indicate the expected noise reduction for white noise.


ponding Allan Plots to characterize the systemtability @Fig. 4~d!#.First we briefly discuss the HCHO measurement.uring the investigations of Section 2 with the test

etup the mode structure was recorded for differentemperatures. During this experiment the temper-ture of the laser was set to 95 K, which resulted inasing at 3.57 mm with 400-mW optical power @Fig.~a!#. The laser tuning rates at 95 K were typicallyelow 300 MHzymA. In this spectral region a seriesf strong absorption lines suitable for the detection ofCHO at ambient concentrations was found when a

eference absorption cell with a high HCHO concen-ration was placed in the laser beam @Fig. 4~b!#. The

hite cell in the spectrometer was set to a 30-mbsorption path, and the pressure inside the cell was

O ~left-hand side! and CH4 ~right-hand side!: ~a! mode map, ~b!a, ~d! Allan plots19 to characterize system stability. The dashed

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set to 30 mbars. The laser was repetitively tunedover an absorption line at 2800.28 cm21 ~linestrength S 5 4.4 3 10220 cmymolec! and modulatedat 120 MHz. Because of the application of thesingle-tone high-FM technique, the absolute value ofthe direct absorption signal was lost and therefore acalibration system, which provided zero air ~air de-void of the target gas! and a constant amount ofHCHO calibration gas, was added to the setup. Forthe determination of the ambient-gas concentration,calibration gas, zero air ~background!, and ambient-air spectra were recorded in the long-path absorptioncell. After the subtraction of the zero-air spectrumfrom the calibration and the ambient spectra, theconcentration was calculated by a multilinear regres-sion analysis.21 This baseline cancellation reducesthe influence of baseline features caused by the non-linearity in the emitted laser power versus injectioncurrent or because of the presence of fringes in theoptical system. However, baseline cancellationworks only if the baseline distortion is stable overtime. Therefore we carefully analyzed the systemstability in terms of the Allan variance.20 Figure4~c! shows time-series data of calibration gas re-corded over a period of 10 min. From the corre-sponding Allan plot @Fig. 4~d!# the optimumaveraging time can be derived to be of the order of40 s. This time defines the maximum interval for acomplete measurement cycle: sample measure-ment, background measurement, and gas-exchangetime. If a measurement cycle exceeds this experi-mentally determined time, the data are no longerreliable. The measured standard deviation of theHCHO measurement was 500 pptv at a 1.5-s integra-tion time and a 6.25-Hz bandwidth. In terms ofnoise-equivalent optical absorption, this translatesinto 1 3 1026 at a 1-Hz bandwidth. For the ultimatedetection limit for the given experimental conditionswe obtained 120 pptv at a 40-s integration time. Itshould be noted that this precision has been actuallycalculated from the measured concentrations and isnot an extrapolation.20 For integration times longerthan 40 s the system stability is insufficient, and a1yf-type noise dominates the measurements. Thisnoise contribution has been attributed to turbulentfluctuations in the multipass cell. The fluctuationsare due to the expansion of the ambient air into thecell, where the pressure is regulated to 30 mbars.We have recently shown that the spectra of the den-sity fluctuations are of the 1yf-type.22 Theseefractive-index fluctuations translate into phaseuctuations, and averaging of 1yf-type noise will no

longer increase precision.20,22 The main limitationtherefore comes from the turbulent gas flow in thecell as well as from the presence of weak fringes in thesystem. However, the results of these measure-ments are well in line with NO2 measurements madewith a lead-salt diode laser in the same system near6 mm.20 This application of antimonide lasers forHCHO measurements at ambient concentration lev-els clearly demonstrated that they can compete withlead-salt lasers.

In a second series of experiments we investigatedthe performance of the antimonide laser selected forCH4 measurements. The mode map obtained at 82K is displayed in Fig. 4~a! ~right-hand side!. Thehreshold current was ;80 mA dc and the laser out-

put power was in the range 0.2–0.6 mWyfacet. Thelaser showed single-mode operation over a broad tem-perature and current range. Lasing at 3.25 mm~3076 cm21! was observed close to the threshold.The laser tuning rates at 82 K were typically ;300

HzymA ~at lower temperatures they are higher byfactor of 2–3!. This behavior was attractive for

eld measurements because the noise of the currentontroller ~;10 mA! will not limit the required fre-uency stability. To set the optimal condition foraser driving we made tests with a single-pass ab-orption cell filled with CH4 at a high concentration

@Fig. 4~b!#. The measured absorption spectra werecompared with those given in the HITRAN databaseto identify the strongest absorption lines that wouldnot interfere with other gases. Only one group ofstrong CH4 lines could be detected at 3076.5 cm21

~3.25 mm!, slightly above threshhold at low currentalues, where the emitted laser power unfortunatelyas below 200 mW. With the mode at 3105 cm21

~3.22 mm! providing higher laser power, only absorp-ion lines that were weaker by 2 orders of magnitudeould be accessed, and therefore we selected an ab-orption line with a line strength S 5 1.2 3 10219

cmymolec at 3076.56 cm21 for the CH4 measure-ments.

The FM TDLAS instrument was set to high-speedmode, which allows fast measurements with a repe-tition rate of 10 Hz, an integration time of 0.06 s, anda bandwidth of 83 Hz. This spectrometer mode isused to determine the exchange of trace gases be-tween biosphere and atmosphere with the eddy cor-relation method for emission and depositionmeasurements.15,16 This technique correlates thevertical wind and the trace-gas concentration fluctu-ations. For this type of measurement the concentra-tion fluctuation must be recorded at ;10 Hz todetermine the trace-gas flux correctly. During themodification of the system for eddy correlation mea-surements, the White cell was replaced by a Herriottcell with a fixed path length of 100 m ~AerodyneResearch!. For reliable measurements of CH4fluxes, the precision of the measured concentrationfluctuation should be better than 5 3 1023. For typ-ical ambient CH4 concentrations of 1800 ppbv, thiscorresponds to a detection limit of ;10 ppbv in 0.1-saveraging time or 3.3 ppbv in 1 s. This value wasobtained previously with the same spectrometer witha lead-salt diode laser at 7.8 mm and a CH4 absorp-tion line that was weaker by a factor of ;2–4, com-pared with the strongest one in the n3 band near 3.3mm. The reference channel consisted of a cell filledwith a high CH4 concentration and produced a strongabsorption signal for line locking. The high-resolution time-series data from the measurementcell ~flushed with CH4 calibration gas of 1.8 ppmv at30 mbar! are displayed in Fig. 4~c!. Based on these


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data, the performance of the spectrometer was againanalyzed in terms of the Allan variance. With theselected laser a maximum integration time of ;20 san be derived. The standard deviation of the mea-ured CH4 concentration was 37 ppbv for an integra-ion time of 0.06 s, which translates into 9 ppb at 1 s.he Allan plot of Fig. 4~d! ~right-hand side! gives evenlightly better results for a 1-s integration time, andor the ultimate detection limit for the given experi-ental conditions we obtain ;2 ppbv at a 20-s inte-

gration time. For longer integration times thesystem stability is insufficient and again a 1yf-typenoise dominates the measurements, as we discussedabove in the context of the HCHO measurements.22

In terms of optical absorption this performance isequivalent to 2.7 3 1024 at a 1-Hz bandwidth.

If we compare this result with the above-stated 3.3ppbv in 1 s for the lead-salt system, at least a factorof 3 is missing. Besides the already discussed tur-bulence and fringe problems, the measurementsshown here suffer from the lack of laser power avail-able on the detector. In principle, high-FM spec-troscopy is capable of quantum-noise-limited perfor-mance. The signal-to-noise ratio under quantum-limited conditions scales with the square root of thepower available on the detector, and to obtainquantum-limited conditions, the laser power has tobe high enough to make shot noise the dominatingnoise source.23 We should keep in mind that thepectrometer has originally been designed for appli-ations with lead-salt diode lasers and is optimizedor the 5–10-mm range. Therefore, for measurementpplications of antimonide lasers, the HgCdTe detec-ors with a peak D* at 10.6 mm should be replaced by

InSb ~77-K! detectors, which are more suited for the3–4-mm spectral range under investigation. In ad-dition, at 3.3 mm, the reflectivity of the mirrors in thelong path cell is reduced and only ;4 mW of the laserpower reaches the photodetector. Taking into ac-count ~1! the lower mirror reflectivity that causes areduced transmission of the optical system, ~2! thelow power of the laser, which has to be operatedslightly above threshold, and ~3! the lower spectralresponse of the HgCdTe detector in this spectralrange, we find that the detection limit is comparablewith that of lead-salt lasers.

4. Summary and Conclusion

In summary, a series of InAsyInAsSbP double-heterostructure lasers was investigated with respectto spectroscopic applications. Therefore mode mapswere recorded for different temperatures of the p–n-junction. These measurements were completed byrecording the wideband noise characteristics to de-termine the relative intensity noise at single-modepositions and in the vicinity of mode hops. As statedabove for lead-salt diode lasers,19 the presence of spu-rious side modes dramatically increased the noise inthe detection circuitry and therefore degraded thesystem performance. Additionally the far field wasinvestigated to check the focusing quality, which canbe obtained to optimize the power transmission of the


system. Investigations of the modulation responsecompleted the picture and showed that relaxationoscillations occur well beyond the modulation fre-quencies normally applied in FM spectrometers.

Measurements of HCHO and CH4 with selectedantimonide laser devices have been presented.From our first experiments we determined a detec-tion limit for HCHO of 120 pptv with a 40-s integra-tion time or ~aL!min 5 1026 at Df 5 1 Hz, and for CH4we obtained 2 ppb with a 20-s integration time~aL!min 5 2.7 3 1024 at Df 5 1 Hz. The achievabledetection limits scale with the integration time, asillustrated by the corresponding Allan plots. As thespectrometer is usually operated with lead-salt diodelasers, it has not been optimized for the spectralrange between 3 and 4 mm: The spectral response ofthe HgCdTe detectors degrades near 3 mm, and InSbdetectors should be used instead. With an opti-mized system with respect to optical power transmis-sion and an antimonide laser that emits at the rightwavelength at higher injection currents, the potentialof the increased line strength in the n3 band of CH4should be feasible with antimonide lasers. Further-more, we expect substantial improvement in systemperformance by applying double-modulation schemesfor the suppression of fluctuations and baselinedrifts, which currently limit, in most cases, the sen-sitivity.24

From the limited number of antimonide lasers wehave investigated up to now, we can say that theobserved single-mode emission power was up to 1order of magnitude higher than that for lead-saltdiode lasers. This is attractive, especially forhigh-FM applications, and might lead to an improvedsystem performance in the future. Therefore anti-monide lasers can offer operational benefits com-pared with those of lead-salt diode lasers while stillmaintaining the high sensitivity that is the result ofprobing fundamental rovibrational absorption tran-sitions. The antimonide lasers still need a cryogenicDewar; these lasers operate in single longitudinalmodes over limited regions since they are Fabry–Perot devices, and they are not easily fiber coupled, sothe sensor still remains complex.

Since 1996 lead salts and antimonides have beenfacing competition from a relatively new direction.Until recently all semiconductor lasers, regardless oftheir operating wavelength, relied on direct band-to-band transitions in bulk material or analogous tran-sitions in type I quantum wells. Rapid progress hasbeen reported in mid-IR lasers based on new types oftransitions. The most notable examples of thesetypes of lasers are the quantum-cascade lasers andtype II quantum-well lasers. Quantum-cascade la-sers operate on transitions within the conductionsubbands of multiple quantum wells and have beenpioneered by Bell Laboratories. Operation aboveroom temperature was recently reported for the firsttime for quantum-cascade lasers emitting at 5 mm inpulsed mode and up to T 5 140 K in cw operation.25

The other new development is the type II quantum-well laser in GaInSbyInAs, which operates pulsed at

ground based measurements of formaldehyde,” J. Geophys.
250 K at 3.2 mm. While these new types of laser arestill in the experimental stage, they appear to offerthe prospect of more robust construction and higher-temperature operation than is possible with the ma-terials that have been used up to this point.26
This study has been funded by the European Com-munity under Copernicus contract CIPA-CT94-0158and by a DAAD-NATO grant A97y52215 for AndreiPopov. We thank Victor Sherstnev for the preselec-tion of lasers, Robert Mucke for his assistance inpreparing the manuscript, and Bernd Janker for hisengineering support. The valuable suggestions ofthe reviewers to improve the manuscript are grate-fully acknowledged.

References1. P. Werle, “A review of recent advances in semiconductor laser

based trace gas analyzers,” Spectrochim. Acta A 54, 197–236~1998! and references therein.

2. P. Werle, R. Mucke, F. D’Amato, and T. Lancia, “Near-infraredtrace gas sensors based on room temperature diode-lasers,”Appl. Phys. B 67, 307–315 ~1998!.

3. R. U. Martinelli, R. J. Menna, P. K. York, D. Z. Gabuzov, H.Lee, J. Abeles, N. A. Morris, J. C. Conolly, S. Y. Narayan, J. S.Vermaak, G. H. Olsen, D. E. Cooper, C. B. Carlisle, and H.Riris, “Tunable single-frequency III–V semiconductor diode la-sers with wavelengths from 0.76 to 2.7 mm,” in Application ofTunable Diode Lasers and Other Infrared Sources for Atmo-spheric Studies and Industrial Monitoring, A. Fried, ed., Proc.SPIE 2834, 2–16 ~1996!.

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7. A. Popov, A. Baranov, V. Sherstnev, Y. Yakovlev, B. Scheu-mann, R. Mucke, and P. Werle, “Investigation of the modestructure and noise characteristics of InAsSbyInAsSbP laserswith respect to spectroscopic applications,” Infrared Phys.Technol. 37, 117–121 ~1996!.

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9. A. Popov, V. Sherstnev, Y. Yakovlev, R. Mucke, and P. Werle,“Tuning and spectral performance of mid-infrared InAsSb la-sers,” in Application of Tunable Diode Lasers and Other Infra-red Sources for Atmospheric Studies and IndustrialMonitoring, A. Fried, ed., Proc. SPIE 2834, 46–56 ~1996!.

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Application of Antimonide Lasers for Gas Sensing in the 3-4-μm Range

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