Shot-noise-limited dual-beam detector for atmospherictrace-gas monitoring with near-infrared diode lasers
Georges Durry, Ivan Pouchet, Nadir Amarouche, Theodore Danguy, and Gerard Megie
A dual-beam detector is used to measure atmospheric trace species by differential absorption spectros-copy with commercial near-infrared InGaAs laser diodes. It is implemented on the Spectrometre aDiodes Laser Accordables, a balloonborne tunable diode laser spectrometer devoted to the in situ mon-itoring of CH4 and H2O. The dual-beam detector is made of simple analogical subtractor circuitscombined with InGaAs photodiodes. The detection strategy consists in taking the balanced analogicaldifference between the reference and the sample signals detected at the input and the output of an openoptical multipass cell to apply the full dynamic range of the measurements ~16 digits! to the weakmolecular absorption information. The obtained sensitivity approaches the shot-noise limit. With a56-m optical cell, the detection limit obtained when the spectra is recorded within 8 ms is ;1024
For issues in climatology, chemistry, or dynamics ofthe atmosphere, laser diode infrared absorption spec-troscopy can be of high interest because it providesprecise in situ trace-gas concentration measurementsprecision errors ranging from 5% to 10%! at highemporal resolution ~ranging from 10 ms to 1 s!.herefore several tunable diode laser spectrometersere operated from balloon, aircraft, or rocket plat-
orms to monitor, in the upper troposphere, lowertratosphere, or mesosphere, trace species as CO,O2, O3, NO, NO2, N2O, CH4, HNO3, or HCl, usually
by means of cryogenically cooled lead–salt laserdiodes.1–7 Of particular interest for atmosphericsounding are current telecommunication distributedfeedback ~DFB! InGaAs laser diodes emitting in the
G. Durry [email protected]!, I. Pouchet, and G.egie are with the Institut Pierre-Simon-Laplace, Service
’Aeronomie, Centre National de la Recherche Scientifique, B.P. 3,1371 Verrieres-le-Buisson Cedex, France. N. Amarouche and T.anguy are with the Division Technique, Institut National desciences de l’Univers, Centre National de la Recherche Scienti-que, 77 Avenue Denfert-Rochereau, 75014 Paris, France.Received 9 March 2000; revised manuscript received 5 July
near infrared between 1 and 2 mm, as they can beperated at room temperature and combined withptical fibers to design compact sensors.8–11 In a
previous publication,12 Durry and Megie describedthe Spectrometre a Diodes Laser Accordables~SDLA!, a balloonborne tunable diode laser spectrom-ter devoted to the simultaneous in situ monitoring ofH4 ~in the 1.653-mm region! and H2O ~in the
1.393-mm region! by means of two DFB InGaAs fiberlaser diodes coupled with a multipass optical cellopen to the atmosphere. The instrument was flowntwice from a stratospheric balloon in 1999 at north-ern and midlatitudes in the framework of the ThirdEuropean Stratospheric Experiment on Ozone~THESEO!. Vertical concentration profiles of meth-ane and water vapor were obtained with a 1-s tem-poral resolution, with the main objective ofcontributing to the understanding of the lower strato-spheric water vapor and ozone budgets.13,14
In this paper we focus on the dual-beam detectorused with the SDLA instrument to achieve in situCH4 and H2O monitoring in the upper troposphereand the lower stratosphere ~from ;5 to ;30 km!.Indeed, in that altitude region the atmospheric ab-sorptions probed by the SDLA are weak. RegardingCH4, a pigtailed InGaAsP DFB laser diode ~SensorsUnlimited Inc.! is used to sweep the R~3!, 2n3 rovi-bronic transition at 1.653 mm. The line strength is
20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5609
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;10 cm molecule cm . Assuming a CH4concentration of 1.7 ppm ~parts in 106! at 5 km and0.9 ppm at 30 km, the predicted fractional absorptionobtained with the 56-m optical path length providedby the multipass cell of the SDLA is 4 3 1023 at 5 kmand 2 3 1024 at 30 km.12 Only 0.02% of the laserbeam approximately is absorbed in the cell at 30 km.Indeed, the molecular transitions in the 1–2-mm spec-ral range are much weaker than the fundamentalovibronic transitions suitable for laser probing in theid-infrared with lead–salt laser diodes. However,
o some extent, the weakness of the rotation–ibration transitions in that spectral region is bal-nced by the intrumental advantages provided by theear-infrared lasers ~no cryogeny is required for sta-ilizing the laser emission, and a reliable single-modemission with no mode hops over the tunability ranges obtained!, by the possibility of ensuring light con-ections with optical fibers, or by the availability of
unctional highly linear InGaAs detectors. With theverage concentrations in the upper troposphere andower stratosphere taken into account, the predictedbsorption depths for carbon dioxide ~at 1.57 mm!,ethane ~at 1.65 mm!, or stratospheric water vapor
at 1.39 mm! range from 1022 to 1024. For otherspecies of atmospheric interest, for instance, hydro-gen chloride ~at 1.74 mm!, carbon monoxide ~at 1.58mm! or hydrogen fluoride ~at 1.27 mm!, absorptiondepths in that altitude region are even weaker, by 2orders of magnitude.
To match the sensitivity required for monitoringmethane and water vapor in the 5–30-km altituderegion of the atmosphere, a differential detectionsetup was implemented on the SDLA spectrometer,which is reported in this paper. Indeed, the differ-ential absorption spectroscopy was demonstrated inlaboratory16,17 to be of high sensitivity up to 1026.Furthermore, the baseline ~zero-absorption level! ofthe absorption spectra, which remains a major sourceof uncertainty in the mixing-ratio retrieval, can beobtained accurately.12 The differential detectionives direct access to the full molecular line shapeithout any distorting filtering or derivative effects
n the molecular profile. When the full molecularine shape is reconstructed with a nonlinear least-quares fit in combination with in situ pressure andemperature measurements, it is possible to monitortmospheric species over many orders of magnituden concentration, pressure, and temperature. Theroposed dual-beam detector uses two InGaAs pho-odiodes to detect the optical signals at the inputreference! and the output ~sample! of an optical mul-ipass cell open to the atmosphere. The detectiontrategy then consists in subtracting the referenceignal from the sample signal to remove the slopingackground that is due to the interferring laserower modulation obtained during frequency scan-ing in order to apply the full dynamic range of theeasurements ~16 digits! to the weak atmospheric
bsorption information. The first dual-beam detec-or reported in this paper is currently operated on-oard the SDLA; it is based on a simple analogical
610 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000
ubtractor circuit. However, it requires aelemetry–telecommand link for controlling theatching between the reference and the sample sig-als throughout the complete balloon flight by real-ime observation of the atmospheric spectra.herefore an upgraded version of that dual-beam de-ector capable of automatic balancing has been devel-ped. It is based on the use of a current splitterade of a pair of bipolar junction transistors to main-
ain continuous matching between reference andample signals according to the scheme proposed byobbs.18–20
In Section 2 we briefly describe the principle ofdifferential absorption spectroscopy as implementedonboard the SDLA instrument to explain how mixingratios are retrieved from the signals provided by thedual-beam detector. In Section 3, the designs andthe performances of the dual-beam detector usedwith the SDLA and of the upgraded version capableof automatic balancing are reported. Sensitivity ob-tained in terms of absorbance is discussed with re-gard to atmospheric methane absorption spectrarecorded with the SDLA. Finally, in Section 4 wediscuss the various contributions to the overall noiseand quantify the detection limit obtained by means ofthe proposed substractor circuits.
2. Experimental Method
Figure 1 is a schematic of the SDLA spectrometer.A complete description of the SDLA payload capableof balloonborne operation is found in Ref. 12. Theinstrument is based on a Herriot optical multipasscell.21–23 It is made of two spherical Au-coated mir-rors at a distance of 1 m; it provides a 56-m absorp-tion path length. When operated under astratospheric balloon, the optical cell is open to theatmosphere. The laser beams are absorbed in thecell, providing simultaneous in situ methane and wa-ter vapor absorption spectra. Two DFB InGaAs la-ser diodes devoted to the measurement of CH4 ~in the1.653-mm region! and H2O ~in the 1.393-mm region!re coupled to the optical cell by standard monomodeilica optical fibers. We focus in this paper on CH4
measurements with the purpose of discussing thesensitivity limits achieved by the dual-beam detector.The laser-emitting wavelength is scanned over theselected rotation–vibration molecular transitions bymodulation of the driving current at constant tem-perature. The laser emission frequency can be con-tinuously scanned over an ;1-cm21 spectral rangewith no mode hops. The central wavelength emis-sion ~1.653 mm, 6046.95 cm21! is determined by the
ean driving current and the working temperaturef the semiconductor ~Tlaser 5 26.5 °C!. The latter istabilized within a 1-mK precision error by a Peltierooler–heater located beneath the semiconductor.he obtained output power is ;0.5 mW. The single-ode laser spectrum features a linewidth that is less
han 10 MHz. Figure 2 is a timing diagram thatxplains the timing sequence of a spectrum acquisi-ion over 10 ms with the SDLA instrumental setup.
At the output of the optical fiber used to couple the
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diode laser with the optical cell, the laser beam issplit by a beam splitter, and ;10% of the laser energyis focused on an InGaAs photodiode to give the ref-erence signal B0. The remaining part of the laserbeam is injected into the optical cell, where it is ab-sorbed by atmospheric CH4. At the output of theoptical cell, the laser beam is focused on a secondInGaAs photodiode to give the atmospheric channelA0T~s!. A0 is what the laser flux would be in theabsence of an absorber in the optical cell. T~s! is themolecular transmission that is due to atmosphericCH4. Reference B0 and sample A0T~s! are thencombined in the dual-beam detector ~Fig. 1! to yieldhe differential signal SD that is the balanced analog-cal difference between both signals:
SD 5 G@A0 T~s! 2 B0#, (1)
where G is a gain applied to the differential signal tomaintain a constant voltage at the digitizer input bybalancing the decrease in CH4 absorption with alti-tude.12 The B0, A0T~s!, and SD signals are simulta-neously recorded within 8 ms and digitized over 16digits with 512 sample points taken over each spec-trum. In addition, a reference CH4 spectrum is also
Fig. 1. Schematic of the SDLA balloonborne spectrometer. Theinstrument is based on a Herriott-type optical multipass cell op-erated open to the atmosphere. It provides a 56-m absorptionpath length. A near-infrared InGaAs laser diode is connected tothe optical cell by a monomode silica optical fiber. The dual-beamdetector combines the signals recorded at the input ~B0! and theoutput @A0T~s!# of the optical cell to yield the balanced differentialignal SD 5 G@A0T~s! 2 B0#. A0 is the laser flux at the output ofhe cell in the absence of the absorber. T~s! is the molecularransmission that is due to ambient CH4. A CH4 reference spec-
trum ~Sref! is also recorded at the output of a closed 8-cm cell toperform the wavelength calibration of the atmospheric spectra.
ecorded at the output of a closed 8-cm cell filled with0 Torr of CH4 for wavelength calibration of the at-
mospheric spectra.Regarding the mixing-ratio retrieval, the molecu-
lar transmission T~s! @or absorption Abs~s!# can bebtained from the recorded spectra. Introducing theisbalancing term derr and using Eq. ~1!, we obtain
derr 5 A0 2 B0, (2)
Abs~s! 5Gderr 2 SD
GA0. (3)
The misbalancing term Gderr is directly retrievedfrom the SD differential signal by a 3° polynomialinterpolation over the full-transmission region. A0is determined from the spectrum A0T~s! by a 3° poly-nomial interpolation over full-transmission regions@or, alternatively, B0 instead of A0 can be used in Eq.~3!#. Applied gain G is known with a precision errorof within 0.1%.12 The CH4 mixing ratio is related tothe molecular absorption through the Beer–Lambertlaw:
Abs~s! < rmolN~Tatm, Patm!L
3 (lines
knN~Tatm!F~Tatm, Patm, s!, (4)
where rmol is the mixing ratio, N~T, P! is the totalmolecular density determined by the Mariotte law, Lis the absorption path length ~56 m!, kn~T! is themolecular line strength, and F~T, P, s! is the molec-ular normalized profile that is a Voigt model com-puted with the Humlicek algorithm. The effect inapproximation ~4! of the spectrometer apparatusfunction is neglected, as the laser linewidth ~;10MHz! is much less than the CH4 linewidth ~;1 GHz!.The molecular parameters used in this paper areextracted from the HITRAN database.15 The atmo-spheric pressure Patm and temperature Tatm are pro-vided by onboard sensors that yield 1-s in situmeasurements with matched precision.12
The mixing ratio is retrieved by a nonlinear least-squares fit to the full molecular line shape Abs~s! inconjunction with the in situ temperature and pres-sure measurements. The retrieval involves manyusual procedures: wavelength calibration by meansof the reference 40-Torr CH4 spectrum, baseline de-termination with a polynomial interpolation overfull-transmission regions, detector dark-current sub-traction, correction of nonlinearities in laser fre-quency tuning, and regular laser turning-off duringflight to determine the zero trace. A detailed de-scription of the retrieval technique and the variouscontributions to the overall precision error ~within5%–10%! are found in Ref. 12. In Section 3 we dis-uss the realization of the dual-beam detector.
3. Dual-Beam Detector
A. Subtractor Circuit
The dual-beam detector presently implemented on-board the SDLA balloonborne spectrometer is shown
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in Fig. 3. Channels A and 2B are combined bymeans of a summing amplifier to give the differentialsignal SD according to Eq. ~1!. We use 1-mm-diameter InGaAs photodiodes from Epitaxx Inc.~ETX 1000T! and from Sensors Unlimited ~SU 1000-.2! operated in the photoconductive mode. Theandwith is ;300 kHz at the preamplifier level.he balancing between channels A and B is main-
ained throughout the complete flight by applicationf a gain G1 to the reference signal B0; G1 can take thealue 2ky256, with k 5 1, 2, . . . , 256. B0 is de-
creased to balance a possible decrease in signal A0during the balloon flight. Indeed, as the optical cellis operated open to the atmosphere, it is exposed to asevere environment ~temperature gradients, dust,tc.!; reflectivity coefficients of the mirrors are likelyo degrade, producing a decrease in signal A0. Prac-
tically, gain G1 is changed by observation of the av-erage atmospheric spectra A0T~s!, B0, and SD
displayed in real time once per second during theballoon flight. Therefore a telecommand–telemetrylink is required.
Figure 4 shows a stratospheric CH4 measurementobtained within 40 3 8 ms by use of the subtractorcircuit during the flight of the SDLA on 10 May1999 from southern France. The absorption depthis 2.5 3 1023. At 17 km, ;0.25% of the laser lights absorbed in the optical cell over the 56-m optical
Fig. 2. Timing diagram of the SDLA spectrometer. ~a! A 64-kHzrecorded within 8 ms. The sampling frequency is 64 kHz, and 512SD, A0T~s!, B0, and Sref ~see Fig. 1!, by means of a 16-bit digitizer ~Aworks in average mode: 40 successive 8-ms laser scans are coaddis modulated at 50 Hz; half the modulation period is devoted to reby the onboard processors to undertake different tasks, such as htelecommand operations or data preprocessing. At the beginningof the driving current provokes the scanning of the laser frequenc
612 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000
ath length by ambient stratospheric CH4. In-deed, ramping the driving current results in scan-ning the frequency but also produces an amplitudemodulation that results in a sloping backgroundwith weak absorption information lying on top of it,as can be seen in Fig. 4~b!. Taking the balanced
nalogical difference between reference and signalermits removal of that sloping background andpplication of the full dynamic range of the mea-urements ~16-digit digitizer! to the absorption in-ormation, drastically reducing the effects ofumerical noise.12 The noise obtained in the spec-
trum is a few 1025 ~expressed in absorbance units!ith a measurement time of 8 ms 3 40 5 320 ms.he various contributions to the overall noise areiscussed in Section 4.That very simple subtractor in combination with a
6-digit digitizer is well adapted to the monitoring oftmospheric trace species with absorptions rangingrom 1022 to 1024, for example, CH4 or CO2 in the
upper troposphere and lower stratosphere or strato-spheric H2O in the stratosphere. An important fea-ture is that stronger absorption in the 1021 absorptionrange can be processed as well by retrieval of the mo-lecular absorption from the direct spectrum A0T~s!; itis highly desirable, for example, to perform additionalmonitoring of water vapor in the upper troposphere,where larger concentrations ~several hundred parts
lator is used as a timing reference. ~b! The absorption spectra areple points are taken simultaneously over each of the four spectra,Inc.!. When operated under the stratospheric balloon, the SDLA
o improve the signal-to-noise ratio. ~c! The laser driving currentng absorption spectra. The remaining time in one period is usedng the instrument housekeeping signals, carrying out telemetry–ser scan, 2 ms are used to reset the modulation. Then a rampingr the molecular line shape within 8 ms.
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per million compared with a few parts per million inthe stratosphere! lead to strong absorption depthswhen the transitions of water vapor at 1.39 mm arescanned.24 The dual-beam detector gives direct ac-ess to the full molecular line shape with no distortingerivation or electronic filtering. When the molecularrofile is reconstructed with a nonlinear least-squarest to retrieve the mixing ratios, atmospheric speciesan be monitored over large ranges at ambient pres-ures, temperatures, or concentrations. Figure 5hows CH4 vertical concentration profiles obtained
with the SDLA spectrometer at a 1-s temporal resolu-tion during recent balloon flights.
However, a telecommand–telemetry link is neededto achieve the balancing in real time by observation ofthe atmospheric spectra. That telecommand–telemetry capability was indeed valuable for the firstballoon flights of the SDLA, it was relatively unknownhow the lasers, the overall electronics, or the opticalmultipass cell open to the atmosphere would behavewhen exposed to a severe environment. In the future,we plan to develop a balloonborne sensor capable offully automatic unattended operations; therefore wehave explored and developed an upgraded differentialsubtractor circuit based on the feedback noise cancel-ler proposed by Hobbs18–20 that was modified with theurpose of in situ laser atmospheric sensing. The re-ulting feedback subtractor circuit, which we plan tomplement on the SDLA for the next flight, is describedn the following subsection.
Fig. 3. Design of the dual-beam detector used on the SDLA balloonat the input ~reference! and the output ~sample! of the optical cell amplified, and digitized over 16 digits. The value of gain G1 is chaight by observation in real time of the A, B, and SD spectra ~Fig. 1ownlinking the in situ spectra and adapting the balancing gain.nlimited Inc. are operated in photoconductive mode in combinaterformances. DAC, digital-to-analog converter; PGA, programm
B. Feedback Subtractor Circuit
Figure 6 shows the feedback subtractor circuit. It isbased on the use of a matched pair of bipolar junctiontransistors ~Q1 and Q2 in Fig. 6! that work as a cur-ent splitter to achieve an automatic balancing be-ween the sample ~A! and the reference ~B! signals.
Indeed, when the error current ε is introduced as thedifference between the reference photocurrent IB andthe collector current IC2 of transistor Q2, the follow-ing relations are obtained:
ε 5 IB 2 IC2, (5)
IA 5 IC1 1 IC2, (6)
IC2yIC1 5 exp~qVbykT!, (7)
where q is the electron charge, k is Boltzmann’s con-stant, and T is the absolute temperature. IA and IBare the sample and the reference photocurrents, re-spectively. Equation ~6! holds, assuming weak basecurrents in the transistors ~i.e., emitter currents closeo collector currents!. IC1 and IC2 are the collector
currents of transistors Q1 and Q2, respectively.Their ratio is controlled by the difference Vb in theirbase–emitter voltages according to Eq. ~7!, assumingthat the transistors work in the Ebers–Moll regime.When Eqs. ~6! and ~7! are combined, the collector
e spectrometer. A simple subtractor circuit is used: both signalslanced by means of gain G1, and the analogical difference is taken,to maintain a good balance throughout the complete stratosphericherefore a telemetry–telecommand capability is required here form-diameter InGaAs photodiodes from Epitaxx Inc. and Sensorsith low-noise Burr–Brown OPA-627 preamplifiers. See text forgain amplifier.
bornre banged!. T
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20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5613
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current IC2 is written as a fraction of the samplephotocurrent:
IC2 5 IA a~Vb!. (8)
The coefficient a, which relates both currents, is fixedby the current splitter controlled by the feedbackloop. Hence, by nulling the error current ε, the feed-back loop makes it possible to balance A and B; whenEqs. ~5! and ~8! are taken into account, the referencephotocurrent IB is found to be equal to a fraction ofthe sample photocurrent IA.
The feedback subtractor, as implemented on theSDLA instrument with the purpose of laser probing,
Fig. 4. Stratospheric CH4 measurement obtained with the dual-beam detector of Fig. 3 during the flight of the SDLA on 10 May1999 from Aire sur l’Adour ~southern France!. The spectra arerecorded within 320 ms by the coaddition of 40 successive 8-mslaser scans. ~a! A reference spectrum is recorded at the output ofa closed 8-cm cell to perform the wavelength calibration of theatmospheric spectra. It features the R~3!, 2n3 triplet of CH4 at046.9 cm21 ~1.653 mm!. ~b! The spectra at the input ~B! and the
output ~A! of the optical multipass cell are also recorded with thepurpose of intensity calibration. ~c! The differential spectrum ob-tained when the analogical difference A 2 B is taken before digi-talization over 16 digits. The sloping background that is due tothe laser power modulation is removed, and the full dynamic rangeof the measurements is applied to the weak absorption informationin A; at 17 km, ;0.25% of the laser beam is absorbed in the cell bystratospheric CH4. ~d! The CH4 molecular absorption is extractedfrom the differential spectrum SD. The G derr misbalancing terms obtained from the differential spectrum SD by a 3° polynomial
interpolation over zero-absorption regions on both sides of theswept spectral range. The CH4 mixing ratio is then retrieved
hen a nonlinear least-squares fit is applied to the full molecularine shape in conjunction with the in situ pressure and tempera-ure measurements and the HITRAN molecular database.
614 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000
works in two successive phases. At time t0, the bal-ancing phase starts. The feedback loop is closed andforces the current error ε to zero. Therefore refer-ence and sample signals are balanced as explainedabove:
IC2 5 aIA~t0! 5 IB~t0!. (9)
Balancing is followed at time t1 by the scanningphase. The feedback loop is open, and the coefficienta is thus held constant. The measured output cur-rent ISD is then related to the reference and the sam-
le photocurrents through a differential relationccording to Eqs. ~5! and ~8!:
We now refer to the timing sequence of spectrumacquisition by the SDLA, as shown in Fig. 2. Thebalancing phase ~closed loop! lasts 2 ms at the begin-ning of a spectrum acquisition. The scanning phase~open loop! lasts the following 8 ms as the laser emis-ion wavelength sweeps the molecular transition.igure 7 explains the principle of the feedback sub-ractor in the frequency domain.
Figure 8 shows the practical realization of the feed-
Fig. 5. CH4 vertical concentration profiles in the upper tropo-sphere and the lower stratosphere obtained with the SDLA. Theinstrument was flown twice from a stratospheric balloon in 1999 atnorthern latitudes ~in Sweden! and midlatitudes ~in southernFrance! within the framework of the THESEO campaign. In situCH4 spectra were recorded with a 1-s temporal resolution by use ofthe subtractor circuit of Fig. 3. Throughout the complete flight,the balancing between reference ~B! and sample ~A! spectra waschieved by observation of the atmospheric spectra in real time.pproximately 800 spectra at northern latitudes and 1000 at mid-
atitudes over the 10,000 recorded for both flights during the as-ent and the descent of the gondola were processed. The CH4
mixing ratios were retrieved from a nonlinear least-squares fit tothe full molecular line shape from the HITRAN database and thein situ pressure and temperature measurements. At northernlatitudes, lower stratospheric CH4 concentrations were measuredbecause of the diabatic descent of air in the Arctic polar vortex.
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back subtractor circuit. The feedback loop is openedor closed by a simple hold-and-sample circuit. Thedetectors are operated in the photoconductive mode,as by principle photocurrents IA and IB are directlybalanced. By opposition with the former simple sub-tractor, the sample signal ~A! is here decreased tomatch the reference signal ~B!. That choice is ex-plained by the optical arrangement of the SDLA; in-deed, taking into account the splitting of the laserbeam by the beam splitter to construct A and B~;10%! and the loss of laser energy in the multipasscell ~;80%!, we get roughly A . 2B. Therefore, asby principle a fraction of signal 1 is used to matchsignal 2, signal 1 has to be the stronger. Indeed,with the simple subtractor of Subsection 3.A, the sig-nal in channel B was first amplified by a factor ;2and then reduced by gain G1 to match the signal A,which is likely to decrease during the stratosphericflight. With the feedback subtractor based on a di-rect balancing of the photocurrents, amplifying signalB would strongly complicate the electronic scheme.Hence A, instead of B, is decreased to achieve thebalancing.
Figure 9 shows a CH4 measurement at room pres-sure and temperature obtained in the laboratory withthe feedback substractor circuit implemented on theSDLA spectrometer. The channel A0T~s!, B0, andSD are recorded simultaneously within 8 ms. Thevarious contributions to the noise achieved in thespectra are discussed in Section 4. Regarding themixing-ratio retrieval, according to Eq. ~10!, the fol-owing differential relation holds among the spectra, B, and SD:
SD 5 G@aA0 T~s! 2 B0#. (11)
Fig. 6. Schematic diagram of the feedback subtractor. The col-lector current IC2 is a fraction of the sample photocurrent IA. Aspectrum acquisition timing sequence starts with a 2-ms balancingphase during which the feedback loop is closed and forces thecurrent ε to zero, leading to IB 5 IC2 5 aIA. It is followed by an8-ms scanning phase while the laser sweeps the molecular transi-tion by ramping its driving current. The feedback loop is thenopen and the balanced differential signal ISD 5 ε 5 IB 2 aIA isamplified and digitized over 16 digits to yield the differential spec-trum. See text and Fig. 7 for more details.
When the misbalancing term derr, is introduced, aformula similar to Eq. ~3! is obtained:
derr 5 aA0 2 B0, (12)
Abs~s! 5Gderr 2 SD
GaA0. (13)
The mixing-ratio retrieval is similar to processing thespectra with the simple subtractor circuit of Subsec-tion 3.A. The term Gderr can be extracted from thedifferential spectrum SD by a 3° polynomial interpo-lation over full-transmission regions. However, theanalogical coefficient a determined during the bal-ancing phase is not known: A0T~s!, instead ofaA0T~s!, is measured simultaneously with the refer-ence ~B! and the differential ~SD! signals with thescheme proposed in Fig. 8. Hence, to extract themolecular absorption, we assume the approximation
aA0 < B0 (14)
uring the scanning phase and replace aA0 with B0 inEq. ~13!. This approximation contributes to theverall precision error in the absorption retrieval ac-ording to the precision error obtained during thecanning phase in balancing A and B. Indeed, thenalogical coefficient a is determined at a time t0
corresponding to a laser-emitting frequency s1 and isheld constant during the scanning phase when the
Fig. 7. Principle of the feedback subtractor explained in the laserfrequency domain. Both the signals recorded at the input ~refer-ence! and the output ~sample! of the optical multipass cell arebalanced within 2 ms to achieve B~s1! 5 aA~s1!. The laser fre-quency is then scanned from s1 to s2 over the molecular transitionby ramping of its driving current within 8 ms. The differentialsignal SD~s! 5 G@aA~s! 2 B~s!# is used to retrieve the molecularabsorption. Indeed, both sample and reference spectra can alsobe balanced within 2 ms at wavelength s2 to achieve B~s2! 5aA~s2!; a is then maintained constant and used for the next laserscan coming 10 ms later ~Fig. 2!. Balancing at s2 instead of s1
may be of interest if more laser power is available at that fre-quency. Wavelength-dependent effects, for instance those thatare due to the beam splitter used to construct A and B, cause aslight misbalancing Dderr during the scanning phase. See text formore details.
20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5615
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laser frequency s~t! is scanned from s1 to s2 over themolecular transition. The difference aA 2 B canhen be slightly misbalanced because of allavelength-dependent effects, for instance, those
hat are due to the beam splitter used to construct And B ~see Fig. 7!. To quantify approximation ~14!,he error da in coefficient a is introduced and is re-ated to the misbalancing term Dderr 5 SD~s1! 2
SD~s2! measured in the experimental spectra @seeig. 9~c!#. According to Eq. ~11!, we obtain
da <Dderr
GA0. (15)
A numerical application with Dderr equal to 600 mVyields da of ;1023. Hence the contribution of ap-proximation ~14! to the overall precision error in ab-orption retrieval is found to be well within 1%.Figure 10 shows the molecular absorption re-
rieved from the spectra of Fig. 9 by means of Eq. ~13!nd approximation ~14!. Noise is ;1024, expressedn absorbance units; the molecular line shape iscanned by the laser frequency within 8 ms. Figure1 shows the same CH4 molecular profile but aver-
aged over 40 successive laser scans, which is the typ-ical acquisition timing sequence of the balloonborneSDLA for field operation. The average noise is ;2 3025. The achieved sensitivity is comparable withhat obtained with the subtractor of Subsection 3.A.
Fig. 8. Design of the feedback subtractor circuit. 1-mm-diameteto detect the sample ~A! and the reference ~B! signals. The spsimultaneously recorded within 8 ms and digitized over 16 digits.during 8 ms ~scanning phase! by means of a sample-and-hold circuare used. See text for performances.
616 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000
The CH4 mixing ratio at room pressure and temper-ature is ;1.8 ppm, obtained by application of a non-linear least-squares fit to the full molecular lineshape from the HITRAN molecular database.
4. Noise Analysis and Discussion
Subtracting a reference signal from the sample signalmakes it possible to remove the background that isdue to the laser power modulation as well as excesslaser intensity noise or interferring modulation, asthe laser beam is split with a beam splitter to yield areference signal with the same fractional noise asthat from the sample signal.18 The time delay be-tween reference and sample signals that is due to the56-m optical path in the cell is not large enough touncorrelate noises in both channels. Anyway, it isimportant to mention, regarding the performance ofthe feedback substractor, that DFB InGaAs diodesare rather quiet lasers. Indeed, there is no unin-tended troublesome optical feedback in the laser cav-ity. Feedback is drastically reduced by means of adouble Faraday isolator ~60 dB! located in the but-terfly package. DFB laser diodes provide reliablesingle-mode spectral emission and do not exhibitmode hoppings while scanning the frequency.Furthermore, we have taken great care in designingthe laser power supply to avoid at-maximum ripplesand additional noise. Another important feature
aAs photodiodes operated in the photoconductive mode are usedA and B and the differential spectrum SD 5 G~aA 2 B! are
e feedback loop is closed during 2 ms ~balancing phase! and open1-mm-diameter InGaAs photodiodes from Epitaxx ~Model 1000T!
r InGectra
Thit.
ot
Bodrs
relevant to the sensitivity is parasitic Fabry–Perotfringing that is due to scattered light in the opticalcell. That interferring optical modulation is ap-parent only in the sample signals and cannot beremoved by subtracting the reference. It is a ma-jor cause of limitation in laser spectroscopic mea-surements. Regarding the SDLA, the multipasscell was found to be free of a fringing pattern in the1024–1025 absorption range12; insights into the op-tical cell design that minimize Fabry–Perot fringingare given in Ref. 12. We now try to quantify thedetection limit obtained with the proposed dual-beam detector.
Regarding the feedback subtractor, Fig. 12 shows asimplified model used for noise analysis in the scan-ning phase. The noise is determined mainly at the
Fig. 9. Ambient CH4 spectrum at room pressure and temperaturebtained with the feedback subtractor. ~a! Reference spectrumhat features the R~3!, 2n3 triplet of CH4 at 6046.9 cm21. ~b!
Signals recorded at the input ~B! and the output ~A! of the opticalmultipass cell. A beam splitter takes ;10% of the laser energy toconstruct the signal in channel B before the laser beam is coupledwith the optical cell ~Fig. 1!. Approximately 80% of the laserenergy is lost in the cell because of the successive reflections onboth cell mirrors. Hence A is roughly equal to twice B. ~c! Bal-anced differential spectrum SD yielded by the feedback subtractor.
y taking the difference between the reference ~B! and a fractionf the sample signals ~aA!, the strong sloping background that isue to the laser power modulation is removed and the full dynamicange of the measurements ~16 digits! is applied to the weak ab-orption information in A. The CH4 spectrum is recorded within
8 ms in the laboratory at room pressure and temperature, and theobtained absorption depth is ;0.4% over the 56-m absorption pathlength. The molecular profile is strongly broadened by collisionaleffects at room pressure compared with the reference spectrum in~a!.
level of the photodiode and amplifier A1 and thenamplified throughout the remaining part of the cir-cuit. Indeed, the noise has different components;one is due to the shot noise of reference ~IB! andsample ~IA! photocurrents, one is due to thermalnoise ~Johnson noise of the feedback resistance Rf !,and the last is the contribution of the rms noise volt-age eamp and current Iamp of the amplifier A1. Thetotal equivalent noise current In @expressed in am-peres per root ~hertz!# is
In2 5 2q@a~IA 1 IA0!# 1 2q~IB 1 IB0!
1eamp
2
Rf2 1 Iamp
2 14kTRf
. (16)
Fig. 10. CH4 absorption at room pressure and temperature re-corded within 8 ms. The molecular absorption Abs~s! is retrievedfrom the spectra in Fig. 9. The noise in the spectrum achievedwith the feedback subtractor is ;1024, expressed in absorbanceunits. The spectrum is recorded within 8 ms during the scanningphase with 512 samples points taken over the fully swept spectralrange by means of a 16-digit digitizer.
Fig. 11. CH4 absorption at room pressure and temperature re-corded within 40 3 8 ms. The ambient CH4 is recorded in thelaboratory within 320 ms by the coaddition of 40 successive 8-mslaser scans. The signal-to-noise ratio is improved by a factor of;6 compared with that of the spectrum in Fig. 10. The CH4
mixing ratio is retrieved from the spectrum with a nonlinear least-squares fit applied to the full line shape and the HITRAN data-base.
20 October 2000 y Vol. 39, No. 30 y APPLIED OPTICS 5617
Oc
soab
amfisao
pt
5
The detectors used to obtain the spectrum shown inFig. 9 are 1-mm-diameter InGaAs photodiodes fromEpitaxx ~Model 1000T!. Dark currents are ;50 nA.Measured reference photocurrent IB ~5aIA! is ;15mA. Therefore the contribution of detector dark cur-rents IA0 and IB0 can be neglected in Eq. ~16!. The
PA-627 amplifier from Burr-Brown was purposelyhosen as amplifier A1 to get low-rms noise voltage
and current @eamp is 6 nV per root ~Hz!; Iamp is 2.5 fAper root ~Hz!#. The feedback resistance Rf is 100 kV.A numerical application shows that all componentsexcept the shot-noise contributions of both referenceand sample photocurrents can be neglected in Eq.~16!. The noise voltage ~expressed in volts! can thenbe deduced from
enoise < Î2Î2qIB Rf GÎB. (17)
The total bandwith B is ;30 kHz with the schematicproposed in Fig. 8. The overall gain G applied to thedifferential signal before digitalization is ;400 forthe spectra displayed in Fig. 9. A numerical appli-cation gives a rms noise voltage enoise of ;20 mV.The measured rms noise in the spectrum of Fig. 9~c!is ;30 mV; the obtained sensitivity approaches theshot-noise limit.
Regarding the subtractor circuit described in Sub-section 3.A, a noise calculation yields the same con-clusions as those for the predominant shot-noisecontribution of reference and sample photocurrents.The noise is determined mainly at the level of eachphotodiode and associated preamplifier ~OPA-627from Burr-Brown; see Fig. 3!; then it is quadraticallyadded by the summing amplifier and amplified bygain G, leading to an expression similar to approxi-mation ~17! for the overall noise voltage enoise. Theobtained noise limit and sensitivity are rather similarfor both circuits, as mentioned in Subsection 3.B.Indeed, the main advantage of the feedback subtrac-tor circuit is the continuous balance obtained be-tween reference and sample signals that is performedautomatically and thereby obviates the need for a
Fig. 12. Schematic diagram of the model used to quantify thenoise limit achieved with the feedback subtractor in the scanningphase ~open loop!. IB and IA are the reference and the sample
hotocurrents, respectively. IB0 and IA0 are the dark currents ofhe used InGaAs photodiodes. Iamp and eamp are the rms noise
current and the voltage of amplifier A1, respectively, and Rf is thefeedback resistance. Gain G is 400 ~see Figs. 6 and 8!.
618 APPLIED OPTICS y Vol. 39, No. 30 y 20 October 2000
telemetry–telecommand link; this is a very importantimprovement for field operation.
Indeed, the sensitivity limit may still be improvedby the coaddition of a larger number N of successivelaser scans. The noise in an elementary 8-ms spec-trum was measured to ;30 mV, or 1024 expressed inabsorbance units ~Figs. 9 and 10!. An elementarytep of digitalization, dstep, is 300 mV ~i.e., 16 digitsver 610 V!. Therefore it is possible to keep theveraging process moving forward to reduce the noisey root~N! until N equals ~enoiseydstep!2. At maxi-
mum, 10,000 successive 8-ms scans could be coaddedover a total measurement time of ;1 min to achievea maximum sensitivity of ;1026 with a 56-m absorp-tion path length. This is not realistic for balloon-borne sensing, as increasing the measurement time isdone at the cost of the spatial resolution in the ver-tical concentration profiles, but increasing the sensi-tivity may be of interest for industrial or laboratoryapplications.
5. Conclusion
The design and performance of a dual-beam detectormade of simple and cheap analog subtractor circuitscombined with InGaAs photodiodes were reported toachieve shot-noise-limited detection of atmospherictrace species by differential absorption spectroscopywith commercial DFB InGaAs laser diodes. Thesubtractor circuits work by taking the analogical dif-ference between the signals detected at the input andthe output of an optical multipass cell to remove thesloping background that is due to the power modula-tion of the laser diode and applying the full dynamicrange of the measurements ~16 digits! to the weakbsorption information. It gives access to the fullolecular line shape with no derivation or electronicltering, making it possible to monitor atmosphericpecies over large ranges of pressures, temperatures,nd concentrations. The sensitivity achieved by usef a 56-m optical multipass cell is in a 1024–1025
absorption range. The acquisition time can be se-lected in a range from 8 ms to 1 s to improve thesensitivity or the temporal resolution as appropriateaccording to the scientific purpose.
The dual-beam detector was implemented on theSDLA balloonborne tunable diode laser spectrometerto achieve in situ monitoring of CH4 in the 1.653-mmregion and water vapor in the 1.393-mm region witha 1-s temporal resolution in the upper troposphereand lower stratosphere. Its operational capability,in combination with a telemetry–telecommand linkto control the balancing between reference and sam-ple signals, was successfully demonstrated duringthe recent balloon flights.
In the future we plan to develop compact balloon-borne sensors devoted to the in situ determination ofH2O in the stratosphere and troposphere and capableof fully unattended operations with the purpose oflong-duration balloon flights; therefore the detectionwill be based on the upgraded feedback subtractorcircuit reported in this paper.
sTstaSsNlm
11. D. M. Sonnenfroh, W. J. Kessler, J. C. Magill, B. L. Upschulte,
Several members of the technical department ofthe Centre National de la Recherche Scientifique ~In-titut National des Sciences de l’Univers—Divisionechnique! and of the Service d’Aeronomie weretrongly involved in the design and field operations ofhe SDLA and should be thanked for their valuablessistance: A. Abchiche, B. Brient, H. Poncet, J. C.amake, P. Schibler, and F. Semelin. The work de-cribed in this paper was supported by the Centreational d’Etudes spatiales, the Centre National de
a Recherche Scientifique, and the European Com-ission.
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