Laser Detection Water Vapour

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Wavelength modulation detection of water

vapor with a vertical cavity surface-emitting laser

David Christian Hovde and Craig A. Parsons

A vertical cavity surface-emitting laser was studied for gas-sensing applications. Properties of the962-nm laser that were measured include side-mode suppression, wavelength tuning with temperatureand current, power versus injection current, and the amplitude noise spectrum. With wavelengthmodulation spectroscopy, a rms noise level of 2 104 absorbance units was achieved. The largecurrent tuning range 25 cm1 and smaller amplitude modulation 11% cm1 of the vertical cavity lasercompare favorably with FabryPerot and distributed feedback diode lasers for spectroscopic gas sensing,especially at atmospheric pressure. 1997 Optical Society of America

Because of their room-temperature operation andfiber-optic compatibility, near-infrared diode lasershave found increasing use in commercial spectro-scopic sensors for trace gases in such applications asatmospheric chemistry, combustion research, emis-sions monitoring, and toxic gas detection. These ap-plications demand high sensitivity and specificity fora particular species with an instrument response thatis rapid 1 s and accurate 10% . To achieve thislevel of performance, laser devices are required thatemit a single frequency that can be tuned across a

spectral absorption feature of the molecule to be de-tected. With frequency or wavelength modulationabsorption spectroscopy WMS techniques that areused to achieve high sensitivity, noise levels can be assmall as 105107 of the laser power, resulting in awide dynamic range with detection limits in the partsper million to parts per billion range for many chem-ical species.17 FabryPerot lasers are of limitedusefulness in commercial instrumentation becausethey exhibit mode hops that can degrade performanceunless the laser output is monitored by a trainedoperator. External cavity lasers extend the single-mode tuning range of FabryPerot lasers and sup-press mode hops, but these benefits are achieved at

the expense of a much larger instrument and at high

cost. Single-mode output is guaranteed in distrib-uted feedback or distributed Bragg reflector lasers,but such structures are difficult to fabricate, resultingin high unit cost. The need for optical isolators addsfurther to the cost.

Vertical cavity surface-emitting lasers VCSELspromise low cost, high manufacturing volume, andsingle-longitudinal and transverse-mode opera-tion.8,9 Despite this favorable combination of priceand performance, there has been little research re-ported on the use of VCSELs for gas detection.10,11

In this paper we present measurements of propertiesimportant in evaluating the suitability of VCSELsfor gas sensing by WMS. The emphasis in our re-search is measuring properties of the laser that havean impact on gas sensing. These properties includecurrent and temperature tuning, side-mode suppres-sion, IV light curves, spectral noise density, and thevariation in noise with injection current. To testthe absorption sensitivity that could be achieved, thespectrum of water vapor was recorded both by directabsorption and by wavelength modulation. Watervapor has discrete spectral lines near the wavelengthof the available laser 962 nm , but because thiswavelength is not well matched to the water vapor

spectrum much stronger lines occur near 940 nm ,no attempt was made to determine the minimumdetectable water concentration. Development ofVCSELs near 940 nm is under way. In a futurepublication we hope to report their use for watervapor detection, including measurementssuch asthe WMS signal at pressures relevant to troposphericdetectionspecific to particular applications; how-ever, such studies are beyond the scope of this re-search. We recently measured oxygen and nitrogen

David Christian Hovde is with Southwest Sciences, Inc., OhioOperations, 6837 Main Street, Cincinnati, Ohio 45244. Craig A.Parsons is with Particle Measuring Systems, 5475 Airport Boule-vard, Boulder, Colorado 80301.

Received 3 November 1995; revised manuscript received 29 July1996.

0003-6935 97 061135-04$10.00 0 1997 Optical Society of America

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dioxide with VCSELs, obtaining results similar tothose presented here.10 Bhadri et al.11 achievedhigher sensitivity than we report here in measure-ments of oxygen with a 760-nm VCSEL by avoidingetalon interference effects.

Both frequency modulation absorption spectros-copy and WMS have been used effectively by a num-ber of researchers to suppress laser excess noise andachieve high sensitivity.1,2,5 The two techniques arerelated theoretically,6 comparisons have been pub-

lished,4,7 and atmospheric science applications havebeen described that include the incorporation of so-phisticated digital filtering algorithms.3 In diode la-ser WMS, one can vary the laser frequency bymodulating the injection current.6 For sine wavemodulation,

t 0m sin t . (1)

The modulation frequency is typically in the rangefrom 1 kHz to 10 MHz. The modulation amplitudem is chosen to optimize the signal-to-noise ratio.The molecular absorption converts the frequencymodulation to amplitude modulation, so that the ab-

sorption signal can be recovered with standardlock-in demodulation methods at a frequency n n 1, 2, 3, . . . where laser noise is minimal. When the2 signal is recovered, the signal is optimized for m2, where is the linewidth HWHM of an iso-lated absorption feature.6 Thus the current tuningrate of the laser d di, is an important device param-eter. The tuning range that is due to temperatureand current tuning is also a critical property forsensor applications because it determines whichspectral features can be measured and the manufac-turing tolerance for the laser wavelength.

One problem that arises in WMS is overloading theoptical receiver by the strong amplitude modulation

at that is independent of any absorption feature.This power modulation can have a magnitude ofP2 dP di d di 1 . Nonlinearities in the powerversus the current curve can also generate signals atharmonics that will interfere with detection of theabsorption signal.

An array of independent VCSELs was fabricatedand tested at Bandgap Technologies. The laserstructure was grown by molecular beam epitaxy on75-mm-diameter n-type GaAs substrates. Alter-nating quarter-wavelength-thick layers of AlAs andGaAs comprised the mirrors. The bottom mirrorwas n doped with Si; the top mirror was p doped withC. The cavity region was one wavelength thick and

contained three GaAs In0.2Ga0.8As quantum wells.Gain-guided, 10-m-diameter VCSELs were definedby way of proton implantation. Each device pro-duced as much as 1 mW of near-infrared radiationnear 962 nm. In Fig. 1 the light-current curve for atypical device shows a linear region just abovethreshold, but it becomes strongly nonlinear at 12mA. The short cavity operates in a single-longitudinal mode. The largest transverse sidemodes were measured to be32 dB of the main mode

at 10-mA injection current, growing to 25 dB at 15mA.

The remaining measurements were performed atSouthwest Sciences. The laser array was mounted

in an aluminum block thermally stabilized to 0.01 K.An ILX Model LDC 3722 diode laser controller wasused to set the laser current and the block tempera-ture. We obtained static wavelength tuning charac-teristics by directing the beam into a commercialwavemeter. The temperature tuning rate d dTover the range from 12 to 30 C was 0.79 cm1 K1,in good agreement with the results of Bae et al. whoused lasers from the same wafer.12 The dc tuningrate including the effects of self-heating was 3.1cm1 mA1 over the range from 9.7 to 17 mA, butwith systematic deviation from linearity to 0.25cm1.

The noise spectrum of the laser was measured with

an unbiased, 300-m-diameter InGaAs photodiode 0.45 A W at 962 nm mounted directly across the50- input of a video amplifier whose output wasrouted to a Tek 2712 spectrum analyzer. The lasernoise spectrum at 12.77-mA injection current, whichis just above the knee in the power curve, is shown inFig. 2. Above 3 MHz, the noise power in a 1-Hzbandwidth corresponds to 4 107 of the laserpower, near the shot-noise limit of 3.4 107. Atother injection currents, the noise floor was higher, asmuch as 10 dB higher for injection currents justabove threshold and at 17 mA.

The spectrum of water vapor was recorded with alow-pressure cell equipped with multipass optics that

provided a 46-m optical path in 64 passes. The totalpressure in the cell was below 1 Torr, but it could notbe measured accurately because of the effects of out-gassing and the use of a low-resolution pressuregauge. From our measured absorbance and pub-lished line strengths,13 a water vapor pressure of 0.6Torr is inferred. The maximum absorption signalobserved was 8.5% at 10,385.3 cm1. We obtained adirect absorption scan Fig. 3, top across the spectralregion from 10,389 to 10,381 cm1 962.57963.28

Fig. 1. VCSEL front surface output power solid curve and ex-ternal applied voltage dashed curve versus injection current.

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nm by sweeping the injection current from 11.9 to14.4 mA while holding the heat sink temperaturefixed at 28.3 C. The laser beam was chopped anddetected with a lock-in amplifier. The observed line

positions agree with the spectral positions predictedfrom the HITRAN database13 to within 0.15 cm1. Therelative amplitudes are also in good agreement.

To obtain a rough estimate of the laser linewidth,the strong water absorption feature at 10,385.3 cm1

was fit to a Gaussian line shape with a HWHM of870 150 MHz on top of a weak, broad absorption asa result of water vapor in room air. This fitted valueis substantially greater than the expected Dopplerlinewidth for water vapor of 455 MHz. Pressurebroadening, calculated with the self-broadening coef-ficient of 0.443 cm1 atm1 tabulated in the HITRAN

database,13 contributes only 10.5 MHz to the line-width at the estimated water vapor pressure and isneglected in the fit. This implies a broad laser line-width of750 MHz over the course of3 s to sweepacross the line, assuming the widths add in quadra-ture. Other researchers have measured VCSELlinewidth power products in the range of 595 MHzmW.1416 Possible reasons for the excessive broad-ening that we observed include current supply jitterover the time scale of the measurement the high

tuning rate of the laser demands a quiet current sup-ply and feedback from the collimating lens or otheroptics. More research is required to determine thecause of the apparent laser broadening.

The WMS spectrum of water vapor was recordedunder the same conditions as the direct absorptionspectrum Fig. 3, bottom . Instead of chopping thelaser beam, we modulated the injection current at 10kHz. A commercial lock-in amplifier with a timeconstant of 100 ms was used to demodulate the pho-tocurrent at a detection frequency of 20 kHz. Therms noise level measured over a 1-cm1 interval wasequivalent to an absorbance peak of the demodu-

lated 2 signal of 2 104

absorbance units. Thisnoise results from accidental etalon fringes formed inthe optical path with a free spectral range of 0.5cm1. The WMS technique offers higher sensitivityand flatter baselines than direct absorption, as ex-pected from results with edge-emitting lasers.

VCSELs exhibit several properties that are attrac-tive for sensor applications. The current tuningrange is wide: 25 cm1 for the device tested com-pared with 2 cm1 for a typical distributed feed-back laser and 12 cm1 for a FabryPerot laser.

VCSELs near 760 nm also exhibit a large tuningrange. The fractional amplitude modulation per

unit frequency change was 11% cm

1

near the centerof the tuning range currents 12 mA . For com-parison, distributed feedback lasers tested in our lab-oratory exhibited amplitude modulation valuesapproximately six times larger. However, this VC-SEL exhibits nonlinearities in the power versus thecurrent curve that can introduce an offset in the de-modulated WMS spectrum. The large apparentlinewidth of the VCSEL in the present setup canreduce sensitivity in low-pressure measurements inwhich Doppler broadening is dominant, but for sam-ples at 1-atm pressure, pressure broadening can re-sult in a linewidth of 3 GHz, and no loss of sensitivityis expected. At present, VCSELs are available in

the range of 6501000 nm.9 Important species thatcan be detected in this spectral region include O2,H2O, NO2 and numerous atoms and radicals. On-going development of VCSELs in the 10002000-nmspectral window will permit a wide range of com-pounds to be monitored with these laser sources.

Our research at Southwest Sciences was supportedby the U.S. Department of Energy under grant DE-FG03-91ER81211.

Fig. 2. Noise spectrum at 12.5-mA injection current. The noisefloor is essentially the quantum shot-noise limit.

Fig. 3. Water vapor spectrum from 10,381 to 10,389 cm1 ob-tained by sweeping the injection current from 14.4 to 11.9 mA.Top panel: direct absorption spectrum. The strongest featurehas an absorbance of 8.5% and an apparent linewidth HWHM of870 MHz. Bottom panel: same current range recorded withWMS by modulating the injection current at 10 kHz and demod-ulating at 20 kHz.

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Laser Detection Water Vapour