Distributed-feedback ~DFB! InGaAs laser diodesemitting in the near infrared are promising tools foratmospheric sounding.1–6 Indeed, from 1 to 2 mm aarge number of molecular species of interest for at-
ospheric science, for example, methane at 1.65 mm,ater vapor at 1.39 mm, carbon dioxide at 1.58 mm,ydrogen chloride at 1.74 mm, and hydrogen fluoridet 1.27 mm, exhibit rotation–vibration absorptionines that are suitable for in situ monitoring. Devel-pments in the field of near-infrared semiconductorasers have been driven mostly by the need for tele-ommunications. Nowadays, InGaAs laser diodesn pigtailed packages, with emission wavelengthshat range from 1 to 2 mm as specified by the pur-
chaser, are available from various suppliers. Thespectral-emission properties of DFB diodes make
The authors are with the Institut Pierre-Simon-Laplace, Serviced’Aeronomie, Centre National de la Recherche Scientifique, B.P. 3,91371 Verrieres-le-Buisson Cedex, France. The e-mail addressfor G. Durry is [email protected].
Received 28 May 1999; revised manuscript received 27 Septem-ber 1999.
7342 APPLIED OPTICS y Vol. 38, No. 36 y 20 December 1999
them particularly well suited for gas monitoring byinfrared absorption spectroscopy. In DFB technol-ogy, a grating is holographically printed in the activearea of the semiconductor to increase the cavity se-lectivity. Consequently, DFB devices exhibit reli-able single-mode emission and can be tuned withoutmode hops over a one-wave-number spectral rangesimply by ramping of the driving current at constanttemperature. The absence of mode hops over thefull spectral range is of particular interest for spec-troscopic measurements, as it permits continuousscanning over the molecular line shape with no poweror frequency artifacts. The linewidth is usually lessthan 10 MHz. The power output is of the order of afew milliwatts. Unlike cryogenically cooled mid-infrared lead-salt laser diodes ~which are usuallyused for atmospheric sensing7–12!, InGaAs devicesan be operated at room temperature. A Peltierooler–heater is sufficient to yield stable laser emis-ion. These properties are promising for field oper-tion, and such systems are much less cumbersome tose because a highly compact spectrometer can bebtained by the use of optical fibers for light connec-ion.
Among the various species that can be detected,H4 and H2O are both of great interest for facilitating
an understanding of the radiative, chemical, and dy-
tstf
r
s
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~
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namic processes in the atmosphere. Methane andwater vapor are greenhouse gases. Water vaporplays an important role in the radiative balance of theEarth, and there is a need for high-quality data onwater-vapor concentrations ~inaccuracy in the 5–10%range, high temporal resolution! to address issues inclimatology.13 The constant increase of atmosphericmethane over the past decades and its effect on cli-mate have drawn the attention of the atmosphericoptics scientific community. CH4 and H2O are alsoinvolved in ozone chemistry. Methane takes part inthe reactive chlorine cycle by reducting Cl to the res-ervoir species HCl ~CH4 1 Cl3HCl 1 CH3!. Watervapor is involved in the polar stratospheric cloudsthat are believed responsible for the Arctic–Antarcticozone holes and is involved in the species removal ofchlorine and nitrogen through hydrolysis reactions.Methane is a tracer: CH4 mixing ratio profiles canbe used to study air-mass transport among latituderanges or to constrain the chemical models.
One can reach all these scientific objectives by com-bining the high sensitivity and high spatial resolu-tion offered by diode laser probing techniques. Hightemporal and spatial resolution and a good accuracyin mixing-ratio retrievals are necessary for treatmentof issues in dynamics. Measurements based on la-ser diode absorption spectroscopy can meet these re-quirements. A 1-s measurement time and aprecision error of less than ;5% have been purposelytargeted for the spectrometre a Diodes Laser Accord-ables @Tunable Diode Laser Spectrometer ~SDLA!#balloonborne spectrometer described in this paper.Assuming an ascent–descent speed of a few metersper second for a stratospheric balloon, measurementsat 1-s intervals should give an ;10-m spatial resolu-tion in the vertical mixing-ratio profile. For ad-dressing chemical issues, lower spatial resolution~.100 m! in the concentration profile is often suffi-cient; successive mixing ratios obtained by laser di-ode probing with a 1-s period can be coadded, whichimproves the precision of the measurements. Thusa lightweight balloonborne ~or airborne! laser diodespectrometer performing CH4 and H2O simultaneousin situ monitoring at an ;1-s period with inaccuracyin the 5–10% range could address a large number ofcurrent atmospheric issues related to radiative,chemical, or dynamic phenomena by providing con-centration profiles with high spatial resolution.
In this paper we describe the laser diode balloon-borne SDLA spectrometer. The instrument is de-voted to the in situ monitoring of CH4 and H2O in theupper troposphere and lower stratosphere. TheSDLA project started in 1997, supported by the Cen-tre National d’Etudes Spatiales ~CNES!, the Frenchspace agency and the Centre National de la Recher-che Scientifique ~CNRS!. Its goal was to be part ofthe Third European Stratospheric Experiment onOzone ~THESEO! in early 1999. To prepare forhese scientific flights, the SDLA was test flown on atratospheric balloon in October 1998. Then, withinhe framework of the THESEO campaign, a flightrom a stratospheric balloon took place in late Feb-
2
uary 1999 from Kiruna, Sweden, to take CH4 andH2O measurements in the polar vortex. That flightwas followed by a second flight, in May 1999, at mid-latitudes ~in southern France! whose main purposewas to determine, through tracer profiles, whetherchemically activated air coming from high latitudescan contribute to ozone depletion at mid-latitudes.
In Section 2 we explain the principles of detectionand mixing-ratio retrieval. In Section 3 the balloon-borne SDLA spectrometer is described. In Section 4we describe the methane and water-vapor absorptionspectra obtained during the balloon flights.
2. Differential Absorption Spectroscopy
A. Spectroscopy and Gas Sensitivities
Atmospheric methane and water vapor can be mea-sured in the near infrared in the spectral regions near1.653 and 1.393 mm, respectively, where rotation–vibration absorption lines can be found without in-terference from other atmospheric species. Apigtailed DFB InGaAs laser diode mounted in a but-terfly package ~purchased from Sensors Unlimited,Inc.! with a central emission wavelength at 1.653 mmsweeps over the R~3! rotation–vibration line ~at6046.9 cm21! of the 2n3 overtone band14,15 of CH4. Asecond device from Sensors Unlimited, at 1.393 mm,cans mainly the P~3! rotation–vibration line ~at
7181.17 cm21! of the n1 1 n3 combination band16 ofH2O. We also have at our disposal fiber DFB In-GaAs diodes at 1.662 mm ~developed by Thomson-LCR, France!, which allow us to monitor the 2n3 R~0!line ~at 6015.6 cm21! of CH4 and some spectral fea-tures of the 2n3, Q branch ~near 6000 cm21!. The
homson laser diodes have larger output power ~;8mW! than the devices from Sensors Unlimited, Inc.;1 mW!.
The line strengths of the lines selected for bothpecies are of the order of from ;10220 to ;10221 cm2
molecule21 cm21. In the near infrared, transitionsare weaker than in the mid infrared, where funda-mental rovibronic transitions take place; the pre-dicted absorption depths according to altitude forCH4 and H2O obtained by simulation with the U.S.Standard Atmosphere 76 concentration profiles forboth species are listed in Table 1. For a flight in the10–30-km altitude range, the sensitivity required forthe instrument in terms of absorbance is ;1024 ~as-suming an L 5 50-m absorption path length!. Thefeasibility of detection down to 1025–1026 has al-ready been demonstrated by use of near-infrared la-ser diodes in conjunction with high-frequencymodulation techniques or ultrasensitive dual-beamspectroscopy.16,17
In selecting the detection technique, our main con-cern was the accuracy achievable in the mixing-ratioretrieval ~;5%! and the frequency of the measure-ment ~1 Hz! to comply with the scientific goal as-igned to the instrument for the THESEO campaign.ence, the SDLA is based on a differential spectros-
opy technique. This technique permits precise de-ermination of the zero-absorption baseline, which
0 December 1999 y Vol. 38, No. 36 y APPLIED OPTICS 7343
s
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o
td
Table 1. Simulated Fractional Absorption and Linewidth ~FWHM! According to Altitude for the 2n , R~3! Transition of CH ~at 6046.9 cm21! and for
7
remains one of the main sources of error in concen-tration measurements. This technique also givesaccess to the full molecular line shape and makes itpossible to check the consistency of each spectrumbefore starting the concentration retrieval ~See Sub-ection 2.D below!.To obtain an absorbance sensitivity of ;1024 the
detection strategy consists in working at a lower fre-quency ~to get low-noise electronics and a large dy-namic range for the measurements! and is based on abalanced detection setup. We use a 16-bit samplerto obtain a large dynamic range for the measure-ments. Figure 1 is a schematic of the detection prin-ciple of the SDLA instrument ~the detection for theCH4 channel is shown; the same setup is used forH2O!.
B. Laser Emission
A low-frequency ramp at 100 Hz is used to scan theDFB InGaAs laser diodes over the selected atmo-spheric absorption lines by modulation of the drivingcurrent at constant temperature. The choice of alow-frequency ramp was driven mainly by the use ofa high-resolution ~16-bits! digitizer that consequentlyhas a limited available sampling frequency. Thepigtailed laser diode mounted in an emission module~Fig. 1! is junctioned to a 20y80 fused coupler. Theweaker beam is passed through an 8-cm reference cellthat contains methane and is focused onto a InGaAsphotodetector. The reference cell is filled with 40Torr of methane, which provides a 55% absorptiondepth and a 0.04-cm21 ~;1.2-GHz! linewidth for the2n3, R~J 5 3! multiplet at 6046.9 cm21. We use thereference spectrum to perform the wavelength cali-bration of the atmospheric spectra. The spectralrange Ds scanned by the laser diode during rampingf the driving current can be retrieved from the ref-rence spectrum and then used to frequency scale thetmospheric spectra. Nonlinearities in the laser fre-uency tuning are modeled by a third-order polyno-ial and taken into account when the wavelength
alibration is made. With a low 100-Hz rampingrequency, the methane device exhibits an ;2%inear-frequency variation over a full spectral range
aThe mixing ratios and the atmospheric P and T profiles weparameters, from the HITRAN database. The optical path lengt
344 APPLIED OPTICS y Vol. 38, No. 36 y 20 December 1999
f 0.5 cm21. We check the values of the polynomialcoefficients after each flight by fitting laboratorymethane Doppler-broadened spectra. The linear-ization of the wavelength scale accounts for less thana 0.5% variation in the absorption. After threestratospheric flights, no significant drifts of the laser
Fig. 1. Schematic of the detection principle of the SDLA. Everysecond, four spectra are recorded simultaneously, with 512 samplepoints on each. The reference spectrum is used for wavelengthcalibration. Channels A and B are used for the intensity calibra-ion process. The atmospheric absorption is extracted from theifferential analogical signal A 2 B. Taking the difference allows
the sloping background to be removed and the complete dynamicrange of the measurements ~16-digit sampler! to be used for theweak atmospheric absorption signal. See text for more details.
tained from the U.S. Standard Atmosphere 76; the molecular5 50 m.
on of
n!
re obh is L
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id
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emission properties were observed. The referencepressure was purposely chosen such that the refer-ence linewidth equals the linewidth of the atmo-spheric methane absorption at mid-flight profile ~;15km!. This choice permits good precision in thewavelength calibration, as the atmospheric lineshape tends to narrow with altitude ~see Table 1!.For water vapor, a 5-Torr reference pressure waschosen, which provides a 56% absorption depth and a0.028-cm21 linewidth for the n1 1 n3, P~J 5 3! tran-sition at 7181.17 cm21. The reference absorptionspectrum is also used to stabilize the laser emissionthrough a temperature control, which is a proportion-ally integrated servo system that is capable of asmuch as ;1 mK stability by driving the laser Peltiercooler–heater.
The main part of the laser emission ~80%! is di-rected to the multipass Herriott cell through a mono-mode silica optical fiber. Ramping the drivingcurrent at constant temperature results in scanningthe laser frequency over a spectral range Ds but,
nfortunately, also produces an amplitude modula-ion, which results in a strongly sloping background.ence the signal obtained at the output of the mul-
ipass cell consists of this sloping background with aeak atmospheric absorption signal lying on top of it
;0.025% of the light is absorbed by CH4 at 30 km,assuming a 56-m optical path!. To remove the slop-ing baseline and use the complete dynamic range ofthe measurements on the atmospheric signal we useda differential absorption setup. At the output of theoptical fiber, the collimated beam is split with a beamsplitter. Ten percent of the laser beam is directed toan InGaAs photodetector and gives reference channelB ~Fig. 1!. Reference channel signal B is taken atthe output of the optical fiber, just before injectioninto the optical cell, to take into account possiblefluctuations of the optical fiber losses when it is sub-jected to the atmospheric surroundings ~severe tem-perature gradients!. The remainder of the laserbeam is passed through the multipass cell and, afterpropagating in the atmosphere, it falls onto a secondInGaAs detector to give atmospheric channel signal A~Fig. 1!. The analogical difference between A and Bs then taken and sampled over 16 digits to give theifference signal, SD ~see Fig. 1!.
C. Detection and Acquisition
We use 1-mm-diameter InGaAs photodetectors fromEpitaxx, Inc. ~ETX 1000T! in the 1.393 mm spectralregion to measure water vapor. Methane is moni-tored at 1.653 mm by long-wavelength-enhanced In-GaAs photodetectors from Sensors Unlimited ~SU000-2.2!. Dark currents at the working tempera-ure are in the 10–50-nA range. The detectors areperated in a photoconductive mode with an ;100-nsesponse time. The bandwidth of the complete an-logical electronics is 300 kHz. The differentialetup that yields the A 2 B analogical difference
signal is based on the use of low-noise Burr–BrownModel 627 operational amplifiers.
We simultaneously recorded 512 sample points on
2
each of the three channels A, B, and SD within 10 ms.Simultaneously, 512 samples were also taken within10 ms from the 55% methane absorption referencespectrum Sref. The sampling of the four signals, A,B, SD, and Sref, is triggered simultaneously to ensurecontinuation of the wavelength-calibration process.Forty elementary A, B, SD, and Sref spectra are co-added to improve the signal-to-noise ratio. The totalmeasurement time to obtain four averaged A, B, SD,and Sref is thus 400 ms. We used the remainingtime within the 1-s sampling time to perform thecoadditions and to add to the data frame, local pres-sure, temperature, and Global Positioning System~GPS! information. This 1-s measurement timematches the scientific requirements described above.It is also well adapted to avoid laser-frequency driftduring coaddition of successive elementary spectra.Indeed, we have taken great care in verifying in thelaboratory that, when coadditions of 40 successive10-ms spectra are made, there is no laser-frequencydrift to produce strong distortions in the averagedspectrum. By superimposing successive 10-ms spec-tra, we verified that, on a 1-s time scale, frequencydrift is less than 0.1% of the linewidth.
At 1-s intervals, four averaged spectra, A, B, SD,and Sref, are recorded, with 512 samples on each. Atthe same time, four other averaged spectra, A, B, SD,nd Sref, are recorded on the water-vapor channel.
Thus, every second, 8 3 512 samples are recordedonboard and sent to the ground through a high-data-rate telemetry link. Figure 2 shows signals A, B,and SD and reference spectrum Sref for a 1-s methanemeasurement performed at 6.9 km during the testflight of the SDLA. Figure 3 shows the recordedsignals for a 1-s water-vapor measurement per-formed at 22.6 km during the second flight of theSDLA within the framework of the THESEO cam-paign.
D. Differential Detection Setup
The molecular transmission can be expressed as ~seeig. 1!
SD 5 G@AT~s! 2 B#, (1)
where T~s! is the molecular transmission and G is again applied to the analogical difference A 2 B beforedigitization. As can be inferred from Table 1, ab-sorption decreases with altitude, and a variable gainG is needed to adapt the signal to the full dynamicrange of the measurements. In our setup, G takesthe value 2n ~n 5 0–6! and can be changed from theround station through a telecommand link. Wean rewrite Eq. ~1! in a more convenient form, intro-ucing misbalancing term derr and molecular absorp-
tion !~s!:
SD 5 Gderr 2 GA!~s!, (2)
derr 5 A 2 B. (3)
ain G1 applied to channel B ~Fig. 1! can be adjustedo maintain a good balance between A and B. This
0 December 1999 y Vol. 38, No. 36 y APPLIED OPTICS 7345
u
tttcdGnttmro
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7
gain can be modified from the ground station~through the telecommand link! to maintain the bal-ance throughout the complete flight. G1 can takethe value ky256, with k 5 1, 2, 3, . . . , 256. A causeof this balance degradation could be a modification ofthe reflectivity coefficient of the Herriott-cell mirrors,which have been subjected to the atmospheric envi-ronment ~the signal in channel A then decreases!.
Knowing Gderr, A, B, and G, we can retrieve mo-lecular absorption !~s! by using
!~s! 5Gderr 2 SD
GA. (4)
An expression similar to Eq. ~4! can be obtained byse of B instead of A, as A 5 B 1 derr.Misbalancing term Gderr can be retrieved from
channel SD. From Eq. ~2!, when there is no absorp-tion, SD equals Gderr. The idea is then to scan the
Fig. 2. Atmospheric methane spectrum recorded at 6.9 km duringthe SDLA balloon test flight on 23 October 1998. As explained forFig. 1, a methane measurement consists of four spectra registeredsimultaneously. ~a! Reference spectrum recorded at the output ofa reference cell filled with 40 Torr of methane; it features the 2n3,R~3! transition of CH4 at 1.653 mm ~6046.9 cm21!. ~b! Signals Aand B recorded at the input ~B! and the output ~A! of the opticalmultipass cell open to the atmosphere ~Fig. 1!. The sloping back-ground is the response in amplitude of the laser diode when thedriving current is ramped to tune the frequency. Channel A hasa very weak absorption information ~;0.05% of the laser beam isabsorbed in the cell at 25 km!. ~c! Analogical difference A 2 Btaken to extract the absorption information from channel A. Thevalue of gain G applied before digitization is 16 3 10. The atmo-spheric line shape of the CH4 multiplet is collisionally broadenedcompared with the reference spectrum. 1 mbar 5 1 hPa 5 100 Pa.
346 APPLIED OPTICS y Vol. 38, No. 36 y 20 December 1999
molecular absorption line over a spectral range thatis large enough to yield a zero-absorption signal atthe beginning and at the end of scanning interval Ds.These regions of zero absorption signal are used toreconstruct, through a polynomial interpolation, theGderr misbalancing term. To match the ramping ofthe laser diode tuning current, we use a full spectralrange Ds such that
Ds < 10dmol, (5)
where dmol is the linewidth ~HWHM! of the atmo-spheric species. The value of the ramp can be mod-ified from the ground station ~through theelecommand link! to adapt the spectral window Dso the narrowing of the molecular linewidth as a func-ion of altitude ~see Table 1!. The operation of re-onstructing the misbalancing term is equivalent toetermining the baseline in direct absorption.reat care should be taken in the choice of the poly-omial degree to avoid distortion in the baseline. Ahird-order polynomial is used here. Precision in de-ermining the baseline is within 1%, with an esti-ated contribution to the overall error in mixing-
atio retrieval within 1–2%. We take 512 samplesver the full spectral range Ds; they provide enough
Fig. 3. Atmospheric water-vapor spectrum recorded at 22.6 kmduring the SDLA balloon flight at mid-latitudes ~in southernFrance! on 10 May 1999. ~a! The reference spectrum, ~b! channelsA and B, and ~c! differential signal SD ~see Fig. 2 for more details!.
he spectra feature the n1 1 n3, P~3! transition of water vapor at7181.17 cm21. At 22 km, ;2% of the light is absorbed in the cell~20 times as much as for methane absorption at the same altitude!.
r
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points to permit us to carry out the polynomial re-construction over the zero-absorption regions, takinginto account relation ~5!.
Practically, at the output of the optical cell, A T~s!is sampled @T~s! is the molecular transmission# andsaved as channel A. To make the intensity calibra-tion by using Eq. ~2!, one can extract A from therecorded signal A T~s! by using the same interpola-tion technique as described above on full-transmission regions ~the technique used in theresearch reported in this paper!. Another solutionwould consist of using B instead of A in the same way.
Equation ~4! shows clearly that one must knowelectronic gain G accurately to retrieve the molecularabsorption; assuming perfect balancing ~derr 5 0! anddifferentiating Eq. ~4!, we can relate the uncertaintyin !~s! directly to the uncertainty in G. Conse-quently @see formula ~7! below#, to keep the contribu-tion to the total uncertainty in the mixing ratio below0.1% implies knowing gain G with a precision errorbetter than 0.1%. The value of G was calibrated inthe laboratory, and great care was taken concerningthe thermal drift of the electronics components: G isknown with a precision error better than 0.1%.Each time an atmospheric spectrum is recorded, ap-plied gain G is also recorded and added to the data-base. From Eq. ~2!, in the zero-absorption @!~s! 5 0#egion over the scanning ranges Ds, the following
relation holds:
G <SD
A 2 Bif !~s! 5 0. (6)
o the recording of A, B, and SD over a matchedspectral range Ds allows us to check out the gain Gapplied before we start the process of mixing-ratioretrieval. Figure 4 is an example of molecular ab-sorption retrieval with the SD spectrum and misbal-ancing term derr.
During the 3-h balloon flight the laser diodes areswitched off ~approximately ten times during thecomplete flight! and signals A and B are recorded to
easure the photodetector dark currents and ~solar!mbient radiation that introduce offset and can causeiscrepancies in the mixing ratio retrievals. Assum-ng perfect balancing ~derr 5 0! and differentiating
formula ~4!, we can relate the uncertainty in !~s!irectly to the uncertainty in A ~or B!. Conse-uently, keeping the contribution to the total uncer-ainty in the mixing ratio below 1% impliesetermining the signal in channel A with a precisionrror better than 1%. The averaged signal in chan-el A is ;2 V; the measured contribution of the meth-ne detector dark current and of the ambientadiation is ;50 mV. Therefore channel A ~or B! is
accordingly corrected before the formula in Eq. ~4! isapplied. Spontaneous emission of the laser diodeswas quantified in the laboratory but was revealed tobe negligible because of the selectivity of the DFBlaser cavity.
The signal recorded in channel A contains absorp-tion information, but, even with a 16-bit sampler, the
2
sensitivity is not high enough that the stratosphericmethane concentration can be retrieved from the sig-nal in channel A ~direct absorption!. In Fig. 5 wecompare the difference signal ~Adigitized 2 Bdigitized!with the analogical difference signal measured inchannel SD ~differential absorption! at two altitudes toshow the effects of numeric noise on methane absorp-tion spectra. Therefore, measuring the concentrationof methane in the stratosphere ~the corresponding ab-sorption depth can be found in Table 1! absolutelyrequires a differential detection setup above 20 km.
E. Mixing-Ratio Retrieval
The molecular absorption is related to the mixingratio through the Beer–Lambert18 relation for weakabsorption:
!~s! < rmolN~T, P!L (lines
kvN~T !F~T, P, s!pdapp~s!, (7)
Fig. 4. Retrieval of molecular absorption !~s!. ~a! A methanebsorption spectrum recorded at 17.5 km during the balloon testight of SDLA on 23 October 1998. The SD differential channel is
plotted against the misbalancing term G derr ~an offset has beenintroduced for clarity!. The misbalancing term was extractedfrom the SD channel by a 3-deg polynomial interpolation on theero-absorption regions on both parts of the spectral interval. ~b!nowing the G derr term, channel A, and gain G, one can retrieveolecular absorption !~s! by using Eq. ~2!. The wavelength scal-
ing was carried out by use of the reference spectrum. The meth-ane mixing ratio can then be retrieved from the molecularabsorption by the Beer–Lambert law. At 17.5 km, the molecularabsorption for the 2n3, R~3! multiplet of CH4 is ;0.25% and theinewidth is ;0.037 cm21 ~FWHM!. The R~3! multiplet consists
of three individual lines that are not resolved at atmospheric pres-sure, one of which has a stronger line strength that gives a typicalform to the line shape.
0 December 1999 y Vol. 38, No. 36 y APPLIED OPTICS 7347
tprl
twlmie
lfitrbrtsti
Wbfllpts
rDo
iwutflWVetl
msl
7
where rmol is the mixing ratio, N~T, P! is the totalmolecular density, L is the absorption path length,Kv~T! is the molecular line strength, f~T, P, s! is themolecular normalized profile, and dapp~s! is the ap-paratus function of the spectrometer. The molecu-lar parameters used for the data processing areextracted from the HITRAN database19 ~see alsoRefs. 14, 15, and 20 for complementary laboratorymeasurements!. All the molecular parameters, par-icularly the line strengths and the temperature de-endence of the air-broadened linewidths, have beeneconfirmed in the laboratory with a precision error ofess than a few percent.
The width of the apparatus function is much lesshan the molecular linewidth ~the DFB laser line-idth is less than 10 MHz, compared with a mean
inewidth of 1 GHz for the methane triplet at 1.653m!. In addition, with differential absorption there
s no troublesome electronic filtering, and the differ-ntial absorption gives access directly to the molecu-
Fig. 5. Differential signal SD compared with the Adigitized 2Bdigitized signal for two methane atmospheric spectra recorded at6.9 and 28.1 km during the SDLA test flight. The effect of thelimited dynamic range of the measurements can clearly be seen onthe spectrum at 28 km. Taking the analogical difference beforedigitizing ~SD channel! permits the full dynamic range to be usedfor the atmospheric information. The spectra are recorded in 10ms each, 40 successive elementary spectra are coadded, and thespectra are then sampled over 16 digits. At 6 km, ;0.4% of thelaser beam is absorbed by methane ~on a 56-m optical path length!;at 28 km, ;0.02% is absorbed.
348 APPLIED OPTICS y Vol. 38, No. 36 y 20 December 1999
ar line shape. Hence, convolution by the apparatusunction in formula ~7! can be neglected. Integrat-ng formula ~7! over the spectral range by summinghe 512 sample points then directly gives the mixingatio, removing the pressure and temperature contri-utions to the line shape. This is a practical way toetrieve the mixing ratio that is well adapted to fu-ure onboard real-time retrievals. At any rate, thepectral range swept by the laser diode must be re-rieved with a precision error of less than 1% @thentegration of formula ~7! is directly proportional to
Ds#. The zero-absorption regions ~the line basis!must be free of distortions to avoid introducing strongdiscrepancies when the integration is performed overthe full spectral range. Hence before implementingsuch a real-time procedure one must be sure thatthere are no systematic artifacts ~unexpected noise,Fabry–Perot fringes, temporal drifts in the laseremission properties etc.! in the atmospheric spectra.
e decided, in a first step, to save the spectra on-oard and to carry out the data processing after theight. The spectra are processed one by one by a full
ine-shape fitting technique. An automatic retrievalrocedure will be implemented for future flights afterhe first set of atmospheric data yielded by the in-truments has been thoroughly checked out.In a first step, a nonlinear fit is applied to the
eference spectrum to retrieve the spectral intervals swept by the laser. Residuals obtained by fittingf the reference line shape and reconstituted Ds are
compared in successive atmospheric spectra to en-sure that there is no change in the laser emissionproperties during the flight. In a second step, theatmospheric concentration is determined by applica-tion of a nonlinear fit to the molecular absorption!~s! in conjunction with the corresponding in situatmospheric P and T measurements. Before start-ng the nonlinear least-squares interative techniquee make sure that in the !~s! signal there are nonexpected artifacts ~Fabry–Perot fringes, distor-ions as a result of misalignment of the cell during theight, etc.! or unexpected noise in the line basis.ith respect to the molecular line shape, we use aoigt profile for methane and water vapor. How-ver, the methane molecular model was extended toake into account possible line mixing between theines of the 2n3, R~3! multiplet according to the model
proposed by Pine21 ~for the n3 band!. This line-ixing effect depends on the total pressure, and we
uspect that it could be of importance at tropopauseevels of ;200 to ;300 mbars. We can couple two
lines of the multiplet that have the same nuclear spinsymmetry such that the resultant profile for the mul-tiplet cannot be modeled precisely ~within 1% of re-siduals! just by summing independent Voigt lineshapes, introducing an estimated uncertainty in themixing ratio in the 3–4% range. We just mentionthe problem here, as it is now being subjected tofurther spectroscopic studies in the laboratory. Forthe first set of atmospheric data presented in thispaper, we used a Voigt representation. A retrievalprogram that uses Levenberg–Marquardt nonlinear
1
sFtlrPdmtripi
soppSscasm6ssfitm
csbi
least squares requires a few seconds to perform amixing-ratio retrieval. During a 3-h flight with con-tinual recording at 1-s intervals, ;10,000 spectra arerecorded for each CH4 and H2O channel. The pro-grams that process this large amount of data weredeveloped with Matlab software on a Pentium II PC.
Formula ~7! shows that atmospheric pressure andtemperature have to be known within appropriateaccuracy. N~T, P!, the total number of molecules, isgiven by Mariotte’s law and is directly related to thetotal atmospheric pressure. To keep precision errorof better than a few percent in the mixing ratio im-plies a precision error of the order of 1% of the totalpressure. Such an accuracy in the measurements isnot obvious with existing pressure sensors operatedin an atmospheric environment @see Subsection 3.C!.The problem is still more acute at high altitudeswhen the molecular line shape is a pure Dopplerprofile and does not contain any information on thetotal pressure. The mixing-ratio retrieval then de-pends completely on determination of the total pres-sure. Taking into account the achievable precisionerrors in atmospheric temperature ~,0.1 °C!, in pres-sure ~,0.01 hPa!, in gain G ~,0.1%! applied to thedifferential signals, and in background determina-tion ~1–2%! and considering the average noise in therecorded atmospheric CH4 spectra over a completeflight ~a few times 1025 expressed in absorbanceunits; see Section 4 and Fig. 9 below!, we obtained aprecision error in the mixing ratio of ;5% up to 25 kmand in the 5–10% range at higher altitudes, accordingto formula ~7!.
3. SDLA Balloonborne Spectrometer
A. Optical Multipass Cell
The balloonborne SDLA is based on the use of a mul-tipass Herriott22–24 cell open to the atmosphere.Figure 6 shows the experimental arrangement of theSDLA gondola. The instrument performs in situmeasurements as it probes the atmospheric air sam-ple located between the two mirrors of the optical cell.We designed and fabricated the complete Herriottcell. It uses gold-coated spherical mirrors ~radius,080 mm; diameter, 100 mm! and provides a 56-m
optical path length. Spherical rather than toroidalmirrors were chosen to permit the use of highly pol-ished ~superpolished! substrates to limit the lightcattered by the mirrors, which is a cause of parasiteabry–Perot fringes. The surface smoothness is
hus less than 0.5 nm rms. We chose a line basisength of 1 m for the cell to avoid too many successiveeflections, which are also potential sources of Fabry–erot fringes, and to reduce the loss of power. In-eed, assuming an ;97% reflection coefficient for theirrors, ;80% of the injected power is lost. A slight
ilt misalignment was applied to the emission andeception antireflection-coated optics to avoid fring-ng. The optical module was revealed to be com-letely free of fringe patterns in both optical channelsn the 1024–1025 absorption range.
Two laser diodes are coupled to the optical cell with
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optical fibers to monitor CH4 and H2O simulta-neously. A small Cassegrain-type telescope locatedon the upper part of the Herriott cell permits easyinjection detection for both laser beams, according tothe principle illustrated in Fig. 7. The two laserspropagate over separate optical paths in the cell, andeach is focused on a dedicated detector; the methanelaser has no effect on the water-vapor detector ~andvice versa!. Silica monomode fibers are used to con-duct both laser emissions to the optical module ~Cas-egrain telescope plus Herriott cell!. At the fiberutput, the laser beam is split into two parts. Tenercent of the laser emission is directed to an InGaAshotodetector to construct reference signal B ~seeubsection 2.B!. After going through the atmo-pheric medium, the laser beam is collected and fo-used onto a second InGaAs photodetector to producetmospheric signal A. Reference and atmosphericignals are then transferred to a data-processingodule located on the upper part of the gondola ~Fig.
!, where the A 2 B difference is determined and theignals are digitized. Using the Cassegrain tele-cope and the optical fibers has significantly simpli-ed the mechanical design of the gondola, as theelescope and the optical cell form an independentechanical module. The weight of the optical mod-
Fig. 6. Schematic of the SDLA gondola. The SDLA is based onthe use of a multipass cell open to the atmosphere. The opticalpath provided by the Herriott-type cell is 56 m. Two laser diodesdevoted to CH4 and H2O monitoring are connected to the multipassell by monomode silica optical fibers. A small Cassegrain tele-cope is used to simplify the injection and detection of both lasereams. The weight of the overall gondola is ;100 kg; the heights ;3 m.
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ule is 20 kg. Resistor heaters are located under theHerriott cell mirrors to avoid the formation of ice.Motorized mechanical shutters are used to cover themirrors when they pass through clouds.
B. Optical Fibers and Laser Diodes
The silica monomode optical fibers that are used toconduct laser emissions to the optical cell are exposedto severe atmospheric temperature gradients duringthe flight ~15 to 260 °C!. Hence we have taken greatcare in adding thermal protection ~polyurethane plusMylar! to minimize fluctuations of the fiber losses orpolarization effects owing to atmospheric tempera-ture variations. Cryogenic Invar ~low coefficients ofthermal expansion at 260 °C! was used for mechan-ical mounting of the Herriott cell to maintain theposition of both mirrors ~dL , 1 mm!. As we havelready mentioned, 1-mm-diameter InGaAs detec-ors are used. The optical fibers are connected to theassegrain telescope by classic frame control–hysical contact connectors. Indeed, an accuracy offew micrometers in positioning of the optical fiber
Fig. 7. Optical layout of the multipass cell. Two different laserdiodes are simultaneously coupled to the optical cell by a smallCassegrain telescope. The principle of the injection detection forone laser diode is shown. Perpendicular to the figure plane, thesame injection–detection scheme is used for the second laser diode.A and B relate to channels A and B of Fig. 2 ~see text for more
etails!. After entering the cell, the laser light produces a seriesof 28 reflex foci lying upon an ellipse on each mirror. The twoellipses, which correspond to the two laser beams, are perpendic-ular. Gold-coated spherical mirrors ~radius, 1080 mm; diameter,100 mm! are used for the Herriott-type multipass cell. The opti-cal path length is 56 m. The weight of the overall optical moduleis ;20 kg.
350 APPLIED OPTICS y Vol. 38, No. 36 y 20 December 1999
utputs is required for optimum optical coupling withhe Herriott cell to be maintained. Therefore, mostarts of the Cassegrain telescope are made from ti-anium to comply with the thermal expansion re-uirements.For the laser diode, care was taken in the thermal
esign of the nonpressurized module that containshe devices and the driving electronics. The laserevice operates at a given temperature stabilized bycooler–heater Peltier thermoelement ~;26 °C for
the 1.653-mm device, ;36 °C for the 1.393-mm laserdiode, ;12 °C for the diode at 1.662 mm!. The Pel-tier element can compensate only for a limited offsetbetween the laser working temperature and the tem-perature of the ambient medium ~the mechanicalmounting!. This problem is even more difficult whenone considers the decrease in thermal convectionwith altitude, which changes the thermal equilib-rium. The temperature of the modules that containthe laser and the driving electronics is maintained inthe range Tlaser 5 615 °C by use of a heat resistor.
C. SDLA Gondola
The gondola is equipped with two global positioningsystems. The accuracy of the spatial resolution is;30 m. The atmospheric pressure is recorded at 1-sintervals by one of two pressure sensors, according tothe altitude. From the ground to 30 km, a Baratrongauge ~Paroscientific, Inc., transducer, Model 215A-01! is used. Another sensor ~Effa-Druck, Inc.,odel AW10-A-105! takes over in the 30–40 km al-
itude range. Both sensors provide ;0.01-hPa pre-ision error. The pressure sensors were calibratedn the laboratory to ensure that they can be operatedn an atmospheric environment. Air at tempera-ures less than 230 °C could cause thermal drift ofhe Baratron’s internal capacitive membrane andhus induce errors in the retrieved pressure. Atmo-pheric temperature is measured with three ther-istors ~VIZ Manufacturing Company, productumber 1266-400! located on the mechanical mount-
ng of the Herriott cell that provide ;0.1 °C precisionrror. Position, pressure, and temperature informa-ion are sampled once per second, synchronously withhe acquisition of the spectra.
Each second, 8 3 512 spectral sample points, po-ition ~GPS information!, pressure, and tempera-ure data are transmitted to the ground andimultaneously saved onboard, on a hard flash diskith a 300-Mbyte storage capacity. A real-timeisplay of the reference spectrum ~Sref! and the dif-
ferential spectrum ~SD! is performed on a computerat the ground station during the flight to facilitatechecking the laser diodes and adapting various pa-rameters ~G, G1, Ds! according to the altitude. Theelectronics package, containing microprocessorunits, an analog-to-digital converter, a power dis-tribution box, and battery packs, is located 1 mabove the optical module. This distance betweenthe two modules minimizes interference betweenatmospheric water vapor and water vapor that isoutgassed by the gondola during flight. Conse-
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quently the gondola is 3 m high. The completelectronics and laser diodes work at atmosphericressure; no pressurized arrangements, whichould increase the overall weight of the instru-
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ment, are needed. The overall weight of the gon-dola is 100 kg, including the telemetry–telecomandtransmitter–receiver system provided by CNES andthe power-supply. Lithium battery packs supply
Fig. 8. Atmospheric methane spectra recorded during the balloon test flight of SDLA on 23 October 1998 from Aire-sur-l’Adour ~southernFrance!. The 2n3, R~3! multiplet of CH4 is recorded in 1-s intervals with a Sensors Unlimited laser diode at 1.653 mm. Differential channelSD is plotted with the corresponding reference spectrum. The Gderr misbalancing term, obtained by the interpolation technique, wassubtracted from the SD spectrum to demonstrate that the line basis is clearly defined. ~a! Absorption spectrum at 3 km. The atmosphericine shape is strongly broadened by the collisional effects. ~b! Absorption spectrum at 17 km. The line shape is typical of the multiplet R~3!,hich consists of three individual lines not resolved at atmospheric pressure. One of the lines has a stronger line strength. ~c!, ~d! Spectra
obtained at higher altitudes, where noise is more important. The line shape converges toward the Doppler profile.
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the required 100 W of total power, which provides a6-h operational capability for the SDLA.
4. Results
The SDLA was test flown from a stratospheric bal-loon launched on 23 October 1998 from the CNESballoon facility at Aire-sur-l’Adour ~in southernFrance!. The balloon was of medium size, inflatedwith 35,000 m3 of hydrogen to provide a float altitudeof ;30.5 km. The balloon was equipped with a valveto permit slow descent by adjustment of the balloon’shydrogen capacity, as planned for future trajectoriesfor the THESEO campaign. This slow-descent ca-pability is important, as H2O measurements under-taken during a slow descent should be less pollutedby water vapor outgassed by the balloon envelope.The flight lasted 3.5 h and terminated at a pressureof 140 hPa ~;14 km!. After a descent under para-chutes the gondola was recovered and showed limiteddamage.
The main goal of the flight was to test the instru-ment. For the test flight, both channels of the SDLAwere equipped with laser diodes, a Sensors Unlimiteddevice at 1.653 mm and a Thomson-LCR device at.662 mm, for methane measurements. Methaneeasurements started on the ground just before the
alloon was launched, with the Herriott cell open tohe atmosphere, which was made possible by goodeather conditions with no clouds and no rain. Ap-roximately 4000 methane spectra were recordeduring ascent and descent. Figure 8 shows the spec-ra recorded on differential channel SD. We plotted
the spectra against the corresponding reference spec-tra to check out the narrowing of the line shape withaltitude. The interpolated misbalancing term derrwas subtracted from the SD channel to demonstratethat the baseline is well defined. The signal-to-noiseratio degrades with altitude. Figure 9 shows molec-ular absorption !~s! obtained after the recordedspectra were processed. The absorption depth andlinewidth are in good agreement with the predictionsmade with U.S. Standard Atmosphere 76 and theHITRAN database ~see Table 1!. The noise is of theorder of a few times 1025 ~expressed in absorbanceunits! and remains constant over the duration of theflight ~the spectra displayed in Fig. 9 also correspondto different measurement times during the flight!.The signal-to-noise ratio can be improved further byapplication of computerized low filtering to the exper-imental spectra; the same filtering must then be ap-plied to the synthetic spectrum when the iterativeretrieval is made, to take into account the convolutionof the line shape by the numeric filter’s impulse re-sponse. Considering the signal-to-noise ratioachieved at present and the uncertainty in total at-mospheric pressure, the error in methane mixing ra-tio retrieval is better than 5%, degrading 10% ataltitudes higher than 25 km. The frequency of mea-surements and the achieved precision fully match therequirements set for scientific applications in atmo-spheric dynamics and chemistry described in Section1. Furthermore, the precision can be enhanced fur-
352 APPLIED OPTICS y Vol. 38, No. 36 y 20 December 1999
ther by coadding successive methane mixing ratiovalues, at the cost of lower spatial resolution in thevertical concentration profile. The retrieved meth-ane mixing ratio is 1.7 parts in 106 ~ppm! in thetroposphere, decreasing in the stratosphere to 0.9ppm at 30 km. The increase in methane concentra-tion observed near 28 km ~to as much as 1.3 ppm; Fig.9! can be interpreted as being due to transport pro-cesses that affect methane at mid-latitudes at thetime of the sounding.
The test flight was followed by two flights as part ofthe THESEO campaign, one flight at northern lati-tudes from Kiruna ~in Sweden! on 23 February 1999followed by a second flight at mid-latitudes on 10 May1999 from Aire-sur-l’Adour ~in southern France!.For these flights the water-vapor channel was oper-ated with the Sensors Unlimited laser device at 1.393mm simultaneously with the methane channel
Fig. 9. Molecular absorption !~s! as a function of altitude. Themolecular absorption for the 2n3, R~3! line of CH4 at 6046.95 cm21
was retrieved from the atmospheric spectra by Eq. ~2!. Wave-length calibration was performed by use of the reference spectrum~see text for more details!. Noise at 30 km is of the order of a fewtimes 1025; it can be improved by use of a numerical low-pass filter.
he linewidth converges toward a pure Doppler profile when theltitude increases ~;0.12 cm21 at 1.4 km to ;0.03 cm21 at 30.6
km!. The absorption depth decreases from 0.32% at 1.4 km to0.025% at 30.6 km ~with a 56-m path length!. Up to ;12 km theabsorption depth increases because of the reduction of collisionaleffects. Mixing ratios were retrieved by the full line shape itera-tive technique. Processing of the ;4000 methane spectra re-corded from ;1- to ;30-km altitude range during the balloon testflight is under way.
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equipped with the Sensors Unlimited laser device at1.653 mm. Approximately 10,000 spectra were ob-tained for both species during ascent and descent ofthe payload. Figure 10 shows simultaneous CH4and H2O measurements performed in the Arctic polarvortex. Concentrations were retrieved from the mo-lecular spectra by means of a full line-shape fit, theHITRAN database, and in situ pressure and temper-ature measurements. Figure 11 illustrates themethane vertical profile obtained in Kiruna duringthe ascent of the payload. Eight hundred of fivethousand spectra recorded have been processed toshow the dispersion of the concentration values overthe probed altitude range. Complete processing ofthe data is continuing.
5. Conclusion and Perspectives
The operational capability of fiber DFB InGaAs laserdiodes for atmospheric sounding under a strato-spheric balloon has been demonstrated for trace spe-cies in a 1024 absorption range by use of a simplebalanced detection scheme. The behavior of thiscommercial DFB InGaAs laser and of the monomodeoptical fibers has been entirely satisfactory duringthe stratospheric balloon flight despite the severetemperature gradients that were encountered.Complete processing of the thousands of spectra to
2
retrieve the vertical methane and water-vapor con-centration profiles is under way.
An issue to address is the effective spatial resolu-tion that is produced by recording methane andwater-vapor spectra at 1-s intervals. We are tryingto determine how to distinguish, in the successivespectra, atmospheric fluctuations from uncertaintiesin the mixing-ratio retrieval by taking into accountall sources of instrumental errors ~uncertainties inatmospheric pressure monitoring, in GPS positioninformation, etc.!.
An upgrading of SDLA that is under way consistsin using a fused-fiber multiplexer to inject two laserdiodes into the same optical channel of the multipasscell to permit monitoring of a third atmospheric spe-cies, for example, CO2. Indeed, H2O ~at 1.39 mm!and CO2 ~at 1.58 mm! could be alternatively moni-ored to ensure CH4–CO2 coverage during the ascent
and CH4–H2O coverage during the descent of thestratospheric balloon.
Several members of the technical department ofthe CNRS ~Division Technique! and of the Service
’Aeronomie have contributed to the design, fabrica-ion, and test flight of the SDLA. T. Danguy wasesponsible for the optics and detection electronics.. Brient, F. Semelin, and J. C. Samake had charge of
he mechanical and functional design of the overall
Fig. 10. Simultaneous methane and water-vapor measurementperformed during the flight of the SDLA at northern latitudes ~in
weden! on 23 February 1999. Both spectra were recorded within1 s. The concentrations were retrieved from the molecular spec-tra by use of a full line-shape fitting technique ~with Voigt model-ing!, the HITRAN database, and the corresponding in situatmospheric pressure and temperature measurements.
Fig. 11. Methane vertical concentration profile obtained duringthe flight of the SDLA in the Arctic vortex from Kiruna ~in Sweden!on 23 February 1999. Approximately 800 spectra of the ;5000recorded during the ascent of the gondola have been processed.The concentrations were retrieved from the molecular spectra froma nonlinear least-squares fit to the spectral line shape, the HIT-RAN database, and the corresponding in situ pressure and tem-perature measurements. Complete processing of the ;10,000spectra obtained for both CH4 and H2O during the ascent and
escent of the payload is under way.
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12. C. R. Webster, R. D. May, C. A. Trimble, R. G. Chave, and J.
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gondola. N. Amarouche and A. Abchiche have beendeeply involved in the command and data-acquisitionsoftware development. H. Poncet was involved inthe calibration of the instrument. P. Schibler hasmanaged the overall SDLA project. The researchdescribed in this paper has been supported by theCNES ~the French space agency! and the CNRS.
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