REVIEW ARTICLE Diode laser absorption sensors for gas

Meas. Sci. Technol. 9 (1998) 545–562. Printed in the UK PII: S0957-0233(98)68747-6

REVIEW ARTICLE

Diode laser absorption sensors forgas-dynamic and combustion flows

Mark G Allen †

Physical Sciences Inc., 20 New England Business Center, Andover, MA 01810,USA

Received 21 April 1997, accepted for publication 15 December 1997

Abstract. Recent advances in room-temperature, near-IR and visible diode lasersources for tele-communication, high-speed computer networks, and optical datastorage applications are enabling a new generation of gas-dynamic andcombustion-flow sensors based on laser absorption spectroscopy. In addition toconventional species concentration and density measurements, spectroscopictechniques for temperature, velocity, pressure and mass flux have beendemonstrated in laboratory, industrial and technical flows. Combined with fibreopticdistribution networks and ultrasensitive detection strategies, compact and portablesensors are now appearing for a variety of applications. In many cases, thesuperior spectroscopic quality of the new laser sources compared with earliercryogenic, mid-IR devices is allowing increased sensitivity of trace speciesmeasurements, high-precision spectroscopy of major gas constituents, and stable,autonomous measurement systems. The purpose of this article is to review recentprogress in this field and suggest likely directions for future research anddevelopment. The various laser-source technologies are briefly reviewed as theyrelate to sensor applications. Basic theory for laser absorption measurements ofgas-dynamic properties is reviewed and special detection strategies for the weaknear-IR and visible absorption spectra are described. Typical sensor configurationsare described and compared for various application scenarios, ranging fromlaboratory research to automated field and airborne packages. Recent applicationsof gas-dynamic sensors for air flows and fluxes of trace atmospheric species arepresented. Applications of gas-dynamic and combustion sensors to research anddevelopment of high-speed flows aeropropulsion engines, and combustionemissions monitoring are presented in detail, along with emerging flow controlsystems based on these new sensors. Finally, technology in nonlinear frequencyconversion, UV laser materials, room-temperature mid-IR materials and broadlytunable multisection devices is reviewed to suggest new sensor possibilities.

1. Introduction

The application of diode lasers to sensing of combustion-generated pollutant emissions began shortly after thedemonstration of direct current injection semiconductorlasers in the late 1960s. By 1974, portable sensorswere being used to detect CO emission from individualautomobiles on US highways (Kuet al 1975). Shortlythereafter, diode lasers found application toin-situmeasurements of combustion gases (Hansonet al 1977).Applications of mid-IR diode lasers to combustion andhigh-speed flow research continue to appear in the scientificand engineering literature, including monitors for ammoniain coal combustion (Silveret al 1991), NO in high-enthalpyshock tunnel flows (Mohammedet al 1996) and aeroengineexhaust analysis (Wiesenet al 1996).

† E-mail address: [email protected]

Today diode lasers dominate the overall laser market,representing over $1.6 billion in 1996 sales (Steele 1997).The majority of these sales are in the telecommunicationsindustry, comprising devices operating between 950 and1500 nm. The requirements of this industry for devicesoperating near room-temperature with wavelength tuning,high spectral purity, long-term stability, and compatibilitywith fibreoptic distribution networks are all consistent withpractical spectroscopic absorption sensors, as well. Thevolume, quality and wavelength range of room-temperaturedevices continue to grow rapidly. The large market drivingthis innovation ensures that rapid progress in laser sourceswill continue for the next few years.

Recently, adaptation of room-temperature diode lasersto measure gas-dynamic properties in reacting and non-reacting flows has received considerable attention. In thisreview, we describe this progress and attempt to outlinethe most promising areas for industrial application and

0957-0233/98/040545+18$19.50 c© 1998 IOP Publishing Ltd 545

M G Allen

further research. Optical sensing of flow phenomenain general grew rapidly during the 1980s and at leastsome of the current interest in diode laser sensorsmay be attributed to the success of these efforts.Using broadly tunable, usually high-power pulsed lasers,researchers demonstrated quantitative measurements ofspecies concentration, temperature, velocity, pressure andother gas-dynamic properties, often with high temporal andspatial resolution, in single-point, two-dimensional or three-dimensional geometries (Eckbreth 1981, Hanson 1986,McKenzie 1993, Kohse-Hoinghaus 1994). The impact ofthis measurement technology on modern research in gas-dynamic and combustion-flow research has been profound.Its utility in research and development environments isnow assumed and has spread to government, university andindustrial research laboratories worldwide.

The diode laser demonstrations of gas sensing in themiddle 1970s did not lead to such immediate growth andimpact. In part, this was due to the nature of the devicesthemselves. The lasers available at that time were generallybased on Pb-salts and operated at multiple wavelengthssimultaneously (multi-longitudinal mode) in the 5–15µmwavelength region. A dispersive spectrometer was usuallyrequired to isolate a single laser mode. They operated at afew tens of K, requiring cumbersome closed-loop cryogeniccoolers, and only generated tens ofµW of optical power.Detectors in the mid-IR also required cooling to liquid N2

temperatures. These operational complexities, coupled withthe generally low device reliability and tedious practicalissues associated with aligning optical systems withµWs ofIR power, cast diode lasers at a considerable disadvantagecompared with the newly emerging high-power tunable dyelasers operating in the visible and UV.

The widespread application of high-power laserdiagnostic techniques to combustion and gas-dynamic flowresearch also advanced the general technology of opticalmeasurements. These advances included practical issuessuch as methods for achieving optical access to oftenharsh flow environments, considerations of beam distortiondue to flow turbulence or shock waves, packaging andmaintenance of sensitive optical components in test facilityor industrial environments, as well as the emergence ofa common vocabulary and mutual understanding betweenthe applied spectroscopist developing new measurementtechniques and the engineer responsible for developmentof the flow or combustion system. Although the progressof measurement technology using large, high-power laserscontinues to evolve, it is now generally recognized that thecost, size and operational complexities of these lasers arelikely to limit their application to the research laboratory orlarge-scale aerospace test facility.

The coincidence of this maturation of understand-ing with the rapidly expanding capabilities in room-temperature, single-mode diode lasers is fortuitous for bothtechnologies. Most of the spectroscopic-based techniquesfor gas-dynamic measurements of flow properties such astemperature and velocity that were developed with thelarger lasers using fluorescence or coherent scattering tech-niques have analogues in pure absorption. Compact sourcesthe size of a typical 14-pin electronic chip are becom-ing available with single-mode tuning ranges approaching

100 nm, optical powers of tens mW, stable operating life-times of 104 h, and costs below $10 000.

As we shall describe in this review, the operationalsimplicity of these lasers allows a much higher level ofautomation in the overall sensor design. This is expectedto result in even wider penetration of the gas-dynamic andcombustion research community since a much lower levelof user sophistication is required. Indeed, commercialindustrial sensors would appear to be genuinely practicaland a number of diode laser-based products are enteringthe commercial sensor marketplace.

We begin with a brief description of the most commonmodern diode lasers and describe the impact of thelaser material and structure on the operation of a typicalsensor. The fundamental photophysics of absorption-basedmeasurements is then reviewed, including extensions togas-dynamic measurements such as temperature, velocity,and mass flux. The near-IR and visible spectroscopy ofimportant gas-dynamic and combustion species is reviewedand sensitive detection strategies for their measurementsdescribed. Typical architectures of diode laser-basedsensors are presented, including fibre-coupled and free-space approaches. Aspects of sensor design important forpractical applications are described in detail.

Specific examples of sensor applications to air flowsand trace atmospheric gas-flux measurements are described.The application of sensors to gas-dynamic, combustion andemissions monitoring is described in detail, focusing onboth fundamental measurement capabilities and practicalissues associated with large-scale engineering applications.The inclusion of diode laser sensor in combustion and flowcontrol applications is described. The field of diode lasersensors is driven by new technologies in diode laser sourcesand fibreoptic components. Emerging developments likelyto affect sensors are also outlined. Finally, we concludewith a summary of the current state of the art andsuggestions for new areas in which further research anddevelopment is likely to have a significant impact on themeasurement community.

2. Room-temperature diode laser sources

Near room-temperature diode lasers (operating at a tem-perature accessible with thermo-electric coolers, generally∼250–350 K) are presently available over a range of wave-lengths extending from about 630 nm in the visible to about2.2µm in the mid-IR. The available ranges are summarizedin figure 1, along with the diode laser materials and typicalcommercial applications. The shortest wavelength InGaAlPlasers produce a beam easily visible to the eye and are usedfor He–Ne replacements in laser pointers and barcode scan-ners. AlGaAs lasers in the 780–900 nm wavelength regionare the largest volume/lowest cost lasers used for CD datastorage and laser printers. A typical laser in these wave-length regions is a virtual commodity product selling for$5–50 per unit, depending on the volume.

The longest wavelength InGaAs lasers grown on GaAssubstrates are primarily high-power pump sources forerbium-doped fibre amplifiers (EDFAs) used for repeatersin long-haul telecommunications applications. The largest

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Diode laser absorption sensors

0.6 0.8 1.0 1.2 1.4 1.5 1.6 1.8 2.0 2.2

Wavelength (µm)

InGaAlP/GaAs

AlGaAs/GaAs

Entertainment andData Storage

InGaAs/GaAs

EDFA-Pump

Telecommunications

InGaAsP/InP

InGaAsP/InGaAs or/InAsP

AlGaAsSb/InGaAsSb........

Instrumentation and Sensing

Figure 1. Approximate spectral windows of commercially available, room-temperature diode laser sources.

dollar volume of lasers consists of the InGaAsP devicesused for telecommunication transmission. Using InPsubstrates, devices can be grown with lasing wavelengthsbetween about 1100 and 1650 nm, although industrystandards restrict most devices to two spectral windows.The minimum materialdispersion of silica fibreopticwaveguides at 1.31 µm generates one of these windowsand, with improved manufacturing tolerance, most largevolume vendors have now reduced the window of scatterin available devices to less than 10 nm about this centre.The minimum materialattenuation of silica fibreopticwaveguides at 1.55 µm generates the second of thesewindows. However, this window is considerably broaderdue to emerging wavelength-division-multiplexing (WDM)standards and the fact that devices are widely availablewith selectable room-temperature wavelengths over nearly100 nm about this centre. Prices for fibre-coupledtelecommunications lasers range from∼$2000 for standardwavelengths to∼$10 000 for custom wavelengths. Outputpowers from 3 to 10 mW are common.

Sales of diode lasers beyond 1.6 µm represent anegligible component of the overall market, but areimportant for sensor and instrumentation application, aswill be described. Using InGaAs or InAsP substratesallows InGaAsP devices to operate at wavelengths aslong as 2.1 µm. Beyond 2µm, commercial devicesare beginning to appear based on antimonide gain andsubstrate stoichiometries at wavelengths as long as 2.3 µm.Continuous-wave (cw) operation generally requires coolingto about 250 K. Price, performance, and lifetime of thesedevices vary considerably due to the immature fabricationprocesses involved.

The basic diode laser structure consists of a short(≤500 µm), cleaved section of laser material with ohmiccontacts on the top and bottom. Current injected throughthe gain medium creates optical gain along the laser axisand the finite reflectance of the cleaved crystal planes issufficient to create a Fabry–Perot (FP) resonator. TheseFP structures are common in the visible lasers where thegain medium tends to oscillate in a single-longitudinal modedefined by one order of the resonator. This lasing conditionis not necessarily repeatable, however, so that the laser mayoperate on a different longitudinal mode (and hence laser

wavelength) from one day to the next at the same operatingtemperature and current.

All diode lasers can be wavelength tuned in twoways. By adjusting the temperature of the diode, theeffective optical index of the waveguide is changed, therebyadjusting the resonant condition of the laser cavity. Usingthis approach, each laser can typically be tuned from 3 to5 nm between 275 and 325 K. For FP lasers, this tuningrange is typically not continuous and exhibits hysteresisand regions of multimode instability (Franzkeet al 1993).Continuous tuning range between mode hops may be assmall as 0.1 nm. Because of the thermal mass of the diodeand heater/cooler element, this type of tuning is usuallyrestricted to a few Hz maximum bandwidth. The secondmode of wavelength tuning involves the injection currentdensity into the gain section. At any given temperature,the injection current can be varied to modulate the opticalindex. Of course the output power is also modulated,but the laser wavelength may be tuned extremely rapidly.Injection current modulation bandwidths on the order of10 GHz are achieved in telecommunications lasers. TypicalFP InGaAlP and AlGaAs lasers tune at a rate of about0.15 A mA−1 or about 0.25 cm−1 mA−1 near 760 nm. Thetotal continuous tuning range is highly device dependentand a function of the laser gain medium and fabricationtechnique. For visible FP lasers, 1–2 cm−1 is common with5–10 cm−1 of inaccessible wavelengths between mode-hops.

The discontinuous tuning range and lack of repro-ducibility of the operating conditions of FP lasers areunattractive for many sensing applications. In some cases, awavemeter may be required to determine the laser operatingwavelength at the nominal set-up temperature and injec-tion current. The most common method for improving thewavelength control of diode lasers is to form a spatiallyperiodic modulation of the refractive index in the materialadjacent to the gain medium, enhancing the cavity feedbackat a particular wavelength through evanescent coupling ofthe lasing mode. These distributed feedback (DFB) lasershave been grown at wavelengths as short as 760 nm (Morriset al 1995) and are commonly available in InGaAsP devicesout to 2µm. They can be temperature tuned over 3–5 nmabout the nominal operating wavelength although the in-jection current tuning rate is typically below 0.1 A mA−1.

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The injection current tuning range is 1–2 cm−1 and limitedby lasing threshold at the low end and output facet damageat the high end. The long interaction lengths of the DFBgrating provide very narrow linewidths, however, gener-ally on the order of 10 MHz in single-section devices andas low as a few hundred kHz in multisection devices (seesection 6).

The wavelength-selective and gain sections of thelaser may be separated, thereby decoupling the outputpower from the wavelength tuning. Such devices aretermed distributed Bragg reflector (DBR) lasers. Here,the periodic index modulation is applied to sections at therear (usually) of the laser and electrically isolated from thegain medium. High-power, fixed-wavelength lasers usedfor optical pumping are typically DBR structures, althoughmulti-electrode lasers with separate gain and wavelengthcontrol are beginning to be applied to gas sensing (Larsonet al 1997, Upschulteet al 1998) and are expected tohave a broad impact on diode laser sensors. Becausethe wavelength tuning range is no longer restricted by thethreshold/damage limits of the gain medium, wide currenttuning ranges are possible (Yamaguchiet al 1985).

An alternative approach for single-mode wavelengthcontrol is to build an external resonator around the diodelaser, treating the diode as simply a current-pumped gainmedium—analogous to an optically pumped dye cell ina conventional dye laser. Uncontrolled optical feedbackinto diode lasers is typically very deleterious to the diodelaser’s frequency and amplitude stability (Goldberget al1982, Cassidy and Bonnell 1988). Reflections from opticalelements such as lenses, prisms or cleaved fibre facetsas low as−40 dB can induce severe power instabilities,multimode lasing, or linewidth broadening. Controlledfeedback, however, can lock the laser wavelength to theexternal cavity mode. Using an adjustable, wavelength-selective element such as a grating, the laser wavelengthmay be tuned over most of the gain medium. Operationaldetails of the external cavity diode laser (ECDL) aresensitive to the construction of the cavity and the reflectanceproperties of the surfaces within it (Ventrudo and Cassidy1990). Commercially available ECDLs (Nguyenet al1994, Oh and Hovde 1995, Sonnenfroh and Allen 1997a)generally construct a Littman cavity between the rear facetof the diode, a tunable grating, and a high-reflectivitymirror. Such ECDLs are available with centre wavelengthsfrom 760 to 1550 nm and tuning ranges from 30 tonearly 100 nm. The tuning performance of the laser iscritically dependent on the quality of the antireflectivecoating on the front facet of the diode, however, sinceweak reflectance from this facet can set up a second set ofcavity modes leading to mode-hops in the tuning range orcoupled frequency, polarization and amplitude modulationof the output with tuning (Sonnenfroh and Allen 1997a).Tuning rates are limited by the inertia of the tuning elementsto a few kHz and output powers are generally a fewmW. Since the ECDLs are physically much larger thansimple current- and temperature-tuned devices and requiremechanical motion to operate, their suitability for long-term, remote sensor applications is unclear. Their broadtuning range and relatively high output power, however,result in extremely useful laboratory tools.

3. Absorption spectroscopy usingroom-temperature diode lasers

3.1. Absorption fundamentals

Diode laser sensors are based on absorption of thewavelength-tuned laser intensity as the beam propagatesacross the measurement path. In contrast to somelaser-induced fluorescence, coherent scattering, or othertechniques commonly applied to combustion and flowdiagnostics, the measurement is easily quantified. Theabsorption is described by the Beer–Lambert relation:

Iν = Iν,0 exp[−S(T )g(ν − ν0)N`] (1)

whereIν is the monochromatic laser intensity at frequencyν, measured after propagating a pathlength` through amedium with an absorbing species number densityN . Thestrength of the absorption is determined by the temperature-dependent linestrength,S(T ), and the lineshape function,g(ν − ν0) (Arroyo and Hanson 1993). The lineshapefunction describes the temperature- and pressure-dependentbroadening mechanisms of the fundamental linestrength.The product of the linestrength and lineshape functionis the absorption cross section. The temperaturedependence of the linestrength arises from the Boltzmannpopulation statistics governing the internal energy-levelpopulation distribution of the absorbing species. If themedium is invariant over the absorption pathlength, thenstraightforward inversion of equation (1) yields a quantityproportional to a number of gas-dynamic properties.

The linestrength of the absorption transition is afundamental spectroscopic property of the absorbingspecies (cf Baeret al 1996), although it is usually expressedin relation to tabulated values available in one of a numberof databases. The most commonly used and extensivedatabase for IR transitions of small molecules is the US AirForce HITRAN database (Rothmanet al 1992), originallydeveloped for atmospheric transmission applications. Thelinestrength at any temperatureS(T ) can be calculated fromthe known linestrength at temperatureS(T0) using

S(T ) = S(T0)Q(T0)

Q(T )exp

[−hcE

k

(1

T− 1

T0

)]×[

1− exp(−hcE/kT )1− exp(−hcE/kT0)

](2)

whereQ is the total molecular internal partition function,E is the energy of the lower transition state,h is Planck’sconstant,k is Boltzmann’s constant andc is the speed oflight (Rothmanet al 1992). The last term accounts forstimulated emission and is negligible at wavelengths below2.5 µm and temperatures below 2500 K.

The lineshape function describes the effect ofcollisional and thermal processes on broadening themolecular transition frequency. Detailed discussions oflineshapes relevant to near-IR and visible laser absorptioncan be found in the literature (Arroyo and Hanson 1993,Upschulte and Allen 1997). Forin-situ measurementsnear atmospheric pressure, the lineshape function is usuallydescribed by a Voigt profile containing both collisional andthermal broadening. Typical values of the full width at half

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maximum (FWHM) of lineshapes relevant to gas-dynamicand combustion-flow sensing are generally on the order of0.15 cm−1 (4.5 GHz) at atmospheric pressure. Hence, diodelasers with spectral linewidths below 10 MHz can generallybe considered monochromatic with respect to the absorptionlineshape.

If the gas temperature, line-strength and absorption pathare known, the measured transmission may be directlyrelated to the absorbing species number density. It isusually possible to select a particular absorption transitionsuch that the temperature variation of the linestrength oversome limited range (typically several hundred K) can beneglected. Separate measurements of temperature can beused to correct for variations, if necessary.

Alternatively, two absorption transitions may be probed(using one or two lasers, depending on the target transitionseparation and the laser tuning range). The ratio of theintegrated absorbance of each transition is a pure functionof temperature:

R =(S1

S2

)T0

exp

[−hc1E

k

(1

T− 1

T0

)](3)

where S1 and S2 are the linestrength values at somereference temperature,T0, and1E is the energy separationof the absorbing states. The temperature sensitivity dependson the values of the reference linestrengths and the energyseparation. With the temperature determined, either or bothabsorbances can be used to determine the number density(Arroyo and Hanson 1993, Allen and Kessler 1996, Baeret al 1996).

If the laser beam propagates across a flow with a bulkvelocity V , the molecules in the moving reference frameof the gas observe a laser frequency that is Doppler-shiftedaccording to

1νDoppler= V

cν0 cosθ (4)

where θ is the angle between the laser propagation andbulk flow directions. For near-IR diode lasers, themagnitude of the Doppler-shift is approximately 3×10−5 cm−1 per m s−1 of flow velocity. At typicalatmospheric pressure conditions, a flow velocity of 1 m s−1

represents a shift-to-width ratio of about 10−4. Since theabsorption measurement itself gives the density of the gas,measuring the Doppler-shift allows determination of thedensity–velocity product or the mass flux (Philippe andHanson 1993, Arroyoet al 1994a, Milleret al 1996).

3.2. Overview of near-IR and visible spectroscopy ofgas-dynamic and combustion species

Absorption-based sensors have the highest sensitivity andselectivity when a spectrally narrow source is used to probea spectrally narrow feature. Tuning the wavelength ofthe source across the absorption feature distinguishes theisolated feature from background absorption, scattering, orextinction effects due to obscuration of the optical pathor changes in the total source power coupled onto thereceiver. Thus, most applications relevant to gas-dynamicand combustion flows are based on absorption by low-molecular-weight molecules with well resolved absorption

transitions such as O2, H2O, CO, CO2, NO, NO2, OH, NH3,HF, H2S, and CH4. With the exception of visible transitionsof O2 and NO2, the absorption measurements are performedon overtone and combination vibrational absorption bands.Typical linestrengths of these transitions are between 10−23

and 10−21 cm/molecule, two to three orders of magnitudebelow the fundamental vibrational transitions in the mid-IR.

Water vapour is a trace species in atmospheric air anda major product species of all hydrocarbon combustion. Itpossessesν1+ν3, 2ν1, and 2ν3 absorption bands throughoutthe 1.3–1.4 µm region, including well-resolved lines at thetechnically important wavelengths near 1.31µm (Allen andKessler 1996). Custom-wavelength lasers are available at1.39µm which access transitions approximately 300 timesstronger at room temperature (Allenet al 1995). Otherbands are ubiquitous throughout wavelengths between 1and 2µm and can be particularly important as interferencesin combustion gases at high temperature, where transitionsfrom higher-lying rotational and vibrational energy levelsbecome strong. For example, strongν1 + ν2 and ν2 + ν3

transitions bands are also available between 1.8 and 1.9µm(Sonnenfroh and Allen 1997b). As is typical of therelatively weak absorption transitions in the near-IR, little isknown about the line positions, assignments, and strengthsof some of these transitions. A recently released high-temperature version of HITRAN, the HITEMP database(Rothman et al 1997), appears to address some of theshortcomings of the water database, but has yet to beextensively confirmed with experimental measurementsexcept in the region near 1.31 µm and is only valid to1000 K (Upschulte and Allen 1997).

A number of molecules of interest in combustion flowspossess transitions near the important 1.55 µm spectralwindow. Carbon monoxide has a second overtone bandcentred at 1.575 µm (Cassidy and Bonnell 1988, Hanson1997, Sonnenfroh and Allen 1997a, Mihalceaet al 1997,Gabrysch Met al 1997). The 3ν1 + ν3 band of CO2 iscentred near the CO second overtone at 1.575µm and the2ν1 + ν3 band is centred at 2.01 µm with transitions twoorders of magnitude stronger (Hanson 1997, Sonnenfrohand Allen 1997a). Recent measurements of the high-temperature CO2 bands near 1.57 and 2.01 µm suggestthat the HITEMP database may substantially over-predictthe absorption strengths at flame temperatures (Kessleret al 1997). The OH radical’s first overtone band extendsthroughout this region with linestrengths at typical flameconditions of ∼5 × 10−21 cm/molecule and is easilydetected in laboratory flames (Sonnenfroh and Allen 1996a,Upschulteet al 1998). The methaneν2+ 2ν3 combinationband near 1.3 µm has been used for absorption sensors(Silveira and Grasdepot 1996), as well as the stronger 2ν3

band near 1.65 µm (Shimoseet al 1991, Uehara and Tai1992, Hovdeet al 1995, Nagaliet al 1996a, Chouet al1997, Mihalceaet al 1997). Second overtone transitionsof NO near 1.8 µm have recently been identified anddetected in ambient and combustion gases using a DFBlaser (Sonnenfroh and Allen 1997b).

Water vapour, acetylene and other molecules possessweak, higher-order overtones in the visible spectrum andhave been detected using AlGaAs lasers, although near-IR sensors achieve a lower detection limit. Elsewhere

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in the red portion of the visible, electronic transitionsof some molecules have been used for diode lasersensors. Molecular oxygen possess a spin-forbiddenelectronic system whose(0, 0) band near 763 nm hasproved important for many gas-dynamic and combustionapplications (Bruce and Cassidy 1990, Philippe and Hanson1993, Nguyenet al 1994, Miller et al 1996). A minorconstituent of combustion-generated NOx , NO2, has a broadand poorly understood electronic system throughout thevisible spectrum. The weak red end of this spectrumis accessible with∼635 and∼670 nm lasers, both ofwhich have been used to detect trace quantities of the gas(Mihalceaet al 1996, Sonnenfroh and Allen 1996b).

Many other molecules of possible interest incombustion or gas-dynamic measurements have beendetected using visible and near-IR diode lasers. Therecent review by Feher and Martin (1995) provides a goodoverview of the spectroscopy of these species.

3.3. High-sensitivity absorption measurements

Because of the weak absorption strengths of the near-IR andvisible features described above, most diode laser sensorsmust have extremely high sensitivity to small changes in thetransmitted power—typically less than 1% attenuation formajor species like O2 and H2O and on the order of partsin 10 000 for minor species such as CO and NO. Directabsorption, even using two detectors to compare commonchanges in the incident and transmitted intensity is generallylimited to absorptions greater than 1%. The oldest and mostcommon approach for sensitive absorption measurementswith diode lasers involves high-speed modulation of thelaser injection current, introducing amplitude and frequencymodulation on the output beam. These techniques arebroadly termed frequency-modulating (FM) spectroscopy.Using phase-sensitive detection at some harmonic of themodulation frequency, background-equivalent absorbancesof ∼ 10−7 have been demonstrated (Bjorklund 1980, Lenth1983, Carlisle and Cooper 1989).

If the average wavelength of the modulated laser isfixed at the centre of the absorption lineshape, the peakFM signal contains contributions from laser modulationparameters as well as pressure- and temperature-dependentline-broadening mechanisms. Scanning the diode laserwavelength across a fully resolved lineshape and integratingthe resulting absorption signal can be used to remove thepressure and temperature contributions to the lineshape,although the integrated absorbance may still be a function oflaser parameters such as modulation depth and frequency.Such scanning is usually desirable in any case, sinceit establishes a reliable baseline which may be varyingacross the laser scan range. The total range over whichthe centre frequency of the modulated signal may becontinuously tuned is highly device dependent. This rangeis reduced in the presence of frequency modulation. Forthis reason, many ultrasensitive FM measurements are madein temperature- and pressure-controlled cells which operatebelow atmospheric pressure, although field sensors based onFM spectroscopy have been deployed by several groups forground-based monitors at atmospheric pressure (cf Silverand Hovde 1994).

In principle, dual-beam approaches which divide thesignal beam (that which passes through the absorbingmedium) by a reference beam (which does not passthrough the absorbing medium) should allow cancellationof common-mode excess laser amplitude noise. Inpractice, however, conventional dual-beam measurementsare severely limited by the balancing requirements inthe two detection channels. Generally speaking, thecommon-mode noise on the laser source will be transmittedthrough the balancing circuit at a level proportional tothe product of the noise-to-power ratio and the percentof imbalance (Houser and Garmire 1994). This willeffectively determine the minimum resolvable absorptionlevel. For example, a laser source with a 1% amplitudenoise in the target detection bandwidth must have signaland reference channels balanced to 10−4 to achieve a noiselevel, and minimum detectable absorption, of 10−6. Evenif this balance could be achieved in a laboratory set-up,it would be impractical to maintain in a field instrument.Furthermore, spatial variations in detector responsivity,drifts in amplifier or other circuit characteristics, changingintensities in the two beams due to polarization orreflectance variations during a laser scan, and lack of high-bandwidth temporal coherence in transimpedance amplifiersall contribute to additional noise on the voltage ratio.

An alternative approach invented by Hobbs (1997)replaces this optical balancing limitation with an electronicphotocurrent balancing. This approach has been adaptedfor a variety of diode laser sensors (Allenet al 1995,Allen and Kessler 1996, Milleret al 1996, Sonnenfroh andAllen 1996b) and has demonstrated equivalent sensitivityto FM-based schemes, with over 50 dB of common-modenoise cancellation. It has advantages in some applicationsin that the pure absorption lineshape is recovered and nohigh-frequency electronics are required. With either FMor balanced-detection approaches, the practical sensitivitylimit is usually determined by optical interferences or weaketalon effects from optical elements or fibre components inthe sensor configuration.

4. Sensor architecture

A typical fibre-coupled, multiwavelength diode laser sensorconfiguration is shown in figure 2. The temperature ofeach laser is controlled using thermo-electrically stabilizedmounts and commercially available closed-loop controllers.The injection current into each laser is also controlledusing commercially available, highly stable current sources.These units can be purchased in either benchtop or single-card versions suitable for integration in standard computeror electronic racks. The most convenient packaging for thelaser itself is the communications-style fibre pigtail packagewherein the laser chip, Faraday isolator, antireflectioncoated lens, and fibre are all pre-aligned and soldered into acommon package about the size of typical 14-pin electronicchip. The entire assembly is mounted on a common thermo-electrically cooled mount. These packages are routinelyavailable for InGaAsP lasers and can be assembled on acustom basis for some other wavelengths.

550


Fused N x 2Fiber-Coupler

Process Piping

SignalDetector

ReferenceDetector

Current andTemperatureControllers

FiberPig-Tailed

Lasers

D-4909

Multiplexed FiberSignal Channel

Multiplexed FiberReference Channel

Data AcquisitionSystem

Figure 2. Schematic of typical multiwavelength diode laser sensor configuration.

The details of the laser packaging can be criticallyimportant for many applications. Fibre-coupled systemsare often preferred for their simplicity and the ease withwhich the laser light can be transported over distancesof hundreds of metres. For stable transmission, single-mode fibres, where a single transverse mode structure issupported in the waveguide, are superior. Larger diameter,multimode fibres tend to exhibit uncontrolled bendinglosses and interference effects associated with multipletransverse modes. Depending on the guided wavelength,the core diameter for single mode fibres ranges between 5and 10µm, introducing extreme alignment tolerances at thecoupler. Careful attention by the manufacturer to alignmentand thermal stabilization of this alignment is critical for awell-behaved device.

An optical isolator is also critical to good performance.Because of the laser’s high sensitivity to small levels ofoptical feedback and the efficiency with which the fibrestransmit back reflections, some method of rejection ofthe backward propagating light is necessary to maintainstable laser performance. Optical isolators are based oncross-polarizers with a Faraday rotator between. TheFaraday rotator uses a fixed magnetic field to rotate thepolarized transmission of the first polarizer into the planeof transmission of the second polarizer for light propagatingout of the diode laser. Backward propagating light is rotatedby the same amount and in the same direction as the forwardpropagating radiation so that it is not transmitted by thepolarizer nearest to the diode itself. A combination ofisolators and angle-cleaved fibre end faces with at least−45 dB suppression are preferred. These are generallyavailable for wavelengths between 760 nm and 1.8 µm, butless commonly for shorter or longer wavelength sources atthe present time. We find that angle-cut, physical contact(APC-type) connectors give the best results for other fibre–fibre connections in the remainder of the system.

With short or long wavelength devices where isolatorsor fibre pigtails are not available, the astigmatic, non-circular and highly diverging output of the diode laser must

be collected, circularized and collimated (to the extent thatthe device astigmatism allows) using specially designedaspheric lenses. Antireflection coatings of these lenses iscritically important due to the lack of an isolator.

The configuration shown in figure 2 is for a balanceddetection set-up where signal and reference channels arerequired. Taking advantage of fibre components forwavelength division multiplexing, multiple lasers may becombined onto common signal and reference fibres usingfused single-modeN × 2 couplers whereN represents thenumber of laser sources. These devices are all-solid-statecomponents where the cores of each fibre are fused togetherover a physical dimension sufficient to couple the guidedmodes onto the output fibres in fixed proportion. Singlewavelength, 1×2 versions of these couplers at wavelengthsbetween 760 nm and 1.55µm have been used in a numberof sensor configurations with no loss of sensitivity. Twoand three wavelength sources have also been demonstratedusing 2× 2 and 3× 2 couplers (Allen and Kessler 1996,Baeret al 1996, Allenet al 1997).

The fibre reference channel carries the unattenuatedlight directly to a reference photodetector, which canbe housed with the laser sources. The fibre signalchannel transmits the light to be used for the absorptionmeasurement directly to the measurement location. Thishas numerous obvious advantages for practical sensors,including the ability to remotely locate the laser and controlelectronics hundreds of metres from the measurementlocation, to use 1×M couplers to simultaneous addressMmeasurement locations with a single source, or to rapidlychange measurement locations or source lasers withoutextensive re-alignment of the optical set-up.

Light from the fibre is launched across the detectionpathlength using a specially designed lens assembly.Typically, these small lenses (diameter∼0.5 cm) aremounted in a housing directly onto the end of the fibre cableby the manufacturer. The launch lens design parameters aredetermined by the distance over which a nearly collimatedbeam is desired, and can include single-pass distances

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of several metres while maintaining a beam diameteron the order of a few mm. Figure 2 shows a typicalexhaust-gas emissions monitor set-up, where the signalchannel is launched across a duct in the exhaust-gas stream.Appropriate windows are required, whose material hasbeen chosen to provide good transmission properties at thewavelength of interest. For high-sensitivity applications,antireflection coating of the window at the laser wavelengthreduces etalon background fringes in the transmission data.Fused-silica windows are satisfactory for most visible andnear-IR applications of moderate temperature. Special IR-grade quartz should be used for applications near 1.4 µmdue to a material absorption band in normal quartz that canaffect the properties of antireflection coatings and lead tosevere etalons.

The transmitted light may be recollected using a capturelens/fibre assembly—essentially the same as the launchoptic except operated in reverse. As mentioned earlier,single-mode fibres provide the most stable transmissionand immunity from fibre vibration and positional changes,but the severe alignment tolerances required to captureall (or a fixed fraction) of the transmitted light in a∼10 µm core make this a very difficult approach inpractical applications. Flow turbulence or facility vibrationscan introduce pointing-instabilities in the launch opticwhich couple to amplitude instabilities in the transmittedradiation and can severely degrade the sensor response.Multimode fibres with 50 or 100µm core diametersalleviate the pointing-instability problem considerably butintroduce other undesirable interference or vibration effects.We find that the most reliable and reproducible response isachieved by directly illuminating the signal photodetector.

Large area Si and InGaAs detectors (∼1–5 mmdiameter) are available which reduce the pointinginstabilities and maximize the collected signal. In single-ended FM approaches, these alignment tolerances arereduced because the absorption signal is detected ina narrow (temporal) frequency band. Both Si andInGaAs detectors are mature technologies with numerouscommercial vendors. Si responsivity is better than0.5 A W−1 from 700 to 1000 nm and InGaAs is onthe order of 1 A W−1 from 1.2 to 1.6 µm. Extended-red response InGaAs devices are becoming available withgood response to 2.1 µm. Compared with Si, however,InGaAs exhibits high dark currents, typically in the nano-amp range compared with subfemto-amps for comparablearea Si. Since the noise floor of most sensors arises fromlaser noise or optical interference effects, this is rarely anissue, although for very low-power sources (<1 µW) itshould be considered.

The remainder of the sensor consists of conventionaldata-acquisition and control electronics. In typicaloperation, the laser temperature is adjusted to tune theoutput wavelength to coincide with the absorption transitionof interest. The laser injection current is swept across∼10–20 mA using a ramp generator, creating a 1–2 cm−1

frequency sweep. This sweep can be accomplished withbandwidths of hundreds of kHz, although typically afew kHz is used to since the overall range of the laserwavelength scan for a given amplitude current ramp is

reduced with high-frequency modulation (Philippe andHanson 1993). For the multiplexed configuration shownin figure 2, each laser is swept at different phases ofcommon ramp function. This provides a time-domainmultiplexing where a single photodetector observes thesequential absorption features as each laser is swept acrossits corresponding lineshape.

Figure 3 is an example absorption scan from a three-wavelength version of the multiplexed configuration shownin figure 2 for simultaneous detection of CH4, CO2, andH2O. The absorption measurements were made in a singlepass through a 50 cm cell filled with 0.5 Torr CH4, 68.1 TorrCO2, and 14.1 Torr H2O. The first peak at the left of thefigure is a group of three unresolved CH4 transitions of the2ν3 band near 1.65µm. The second peak is an isolated CO2

transition of the 3ν1+ν3 band near 1.57µm. The last strongpeak at the right, near data index 8.5, is a water vapourtransition near 1.392 µm and the weak feature at the farright is a neighbouring water transition. The narrow featurenear data index 7 is a repeat of the strong water transitionrecorded during the laser flyback at the beginning of thescan ramp. The mapping between data index and frequencyis different between each of the three lasers according tothe scan rate (MHz mA−1) differences in each laser, so therelative widths are not representative. The entire scan wasrecorded using sweeps of 10 ms duration and 200 sweepswere averaged to reduced the bakcground noise, for a totalmeasurement time of 2 s.

As an alternative to time-domain multiplexing,wavelength-domain multiplexing has also been used indiode laser combustion sensors (Baeret al 1996). Here,the lasers can be ramped across their respective absorptionlines truly simultaneously. Multiple wavelengths in thetransmitted beam are separated onto multiple detectorsusing a frequency-dispersive device such as a conventionalreflection grating. Grating demultiplexers may alsobe useful for isolating the laser wavelengths in high-temperature applications where the measurement gas isemitting at wavelengths that would otherwise be viewed byan unfiltered detector. Wavelength division multiplexingcomponents are beginning to appear commercially fortelecommunications applications which combine fibrecouplers with filters to separate multiple laser frequenciesinto individual channels. Compared with gratings, thesedevices will be much more compact and will require noalignment, but will have a low throughput. Since all ofthe transmitted light is split into each of theM outputchannels, but only one channel will be transmitted, thetotal signal power on any channel is reduced by 1/M.These devices have yet to appear in a diode laser sensorconfiguration; therefore any interference or etalon effectsthey may introduce in the signal are unknown.

Although absorbance sensitivities of 10−5 can beachieved in practical installations, for high-sensitivity trace-gas monitoring or applications requiring a high signal-to-noise ratio, it is often convenient to increase the absorptionpathlength across the usually fixed physical dimension ofthe flow facilitiy. The conventional approach for this isto launch the beam into a multipass cell embedded in theflow or measurment environment. Known as Herriot cells

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Figure 3. Example near-simultaneous detection of CH4, CO2, and H2O using a three-wavelength, time-domain multiplexed,diode laser sensor operating at 1.65, 1.57, and 1.39 µm.

(Herriot et al 1964, Herriot and Schulte 1965), these havebeen widely applied in atmospheric monitoring and arecommercially available with astigmatic optics providingover 100 m of non-overlapping optical pathlength in aphysical cell less than 1 m in length. Simplified versionshave recently been applied in hypersonic shock tunnel flows(Hanson 1997).

As with all optical diagnostics, optical access to themeasurement location is often the most severe engineeringchallenge for a given sensor. Diode laser sensors sufferthe usual constraints, requiring that the windows andwindow mounts sustain the pressure and temperature of theflow gases. Forin-situ combustion measurements, film-cooling of the interior window surface with a purge gasis often used to both cool the window and to keep sootor other particulate from impacting or depositing on thesurface. In supersonic or hypersonic flows, care mustbe taken to ensure that shock waves formed from thewindows or mounting blocks do not excessively heat thewindows, introduce flow-perturbations, or induce severebeam steering effects.

5. Example diode laser sensor applications

5.1. Air flows

The availability of room-temperature diode lasers near760 nm provides direct access for spectroscopicmeasurements of O2 and, therefore, unseeded air flows.Numerous applications for monitoring air-flow propertiesin wind tunnel and air-breathing combustion systems havebeen demonstrated. Also, despite the fact that diodelaser absorption sensors necessarily provide line-of-sightaverages, tomographic reconstruction techniques are wellestablished for inverting multiple lines of sight into spatiallyresolved measurements (Chunget al 1995, Vargheseet al1997). Tomographic reconstruction using FM detection ofO2 jets has been demonstrated with density reconstructionuncertainties on the order of 5% over the domain (Kaura-nen et al 1994). Extensions of this technique for simulta-neous measurements of air density and temperature usinga multiplexed two-laser system has also been demonstrated

(Kessleret al 1995). Figure 4 is an example reconstruc-tion from this work of the temperature field above a non-axisymmetric jet mixing flow consisting of a 150 K jet ofpure N2 located atx = 5 cm andy = 0 cm in the domainand mixing with a room-temperature jet of pure O2. Thereconstructed temperature field was compared with detailedthermocouple surveys and exhibited an rms uncertainty of14 K over the domain. Temperature uncertainty over single,uniform lines of sight was on the order of 3 K. This typeof sensor has applications to measurements of temperatureand density distortions at aeroengine inlets in large-scaletest facilities.

Simultaneous air density and velocity measurementshave been significantly refined and applied to a full-scaleF-100 aeroengine inlet in ground tests using balanced dualbeam detection (Milleret al 1996). A single FP AlGaAslaser and 1×4 splitter deployed two signal channels acrossthe 0.92 m diameter inlet, as illustrated in figure 5. 60 mfibreoptic cables delivered the light from an equipmentbunker near the test stand and the launched beams directlyilluminated large-area Si photodetectors mounted on theopposite side of the inlet. Figure 6 shows single sweep andaverage O2 lineshapes recorded with the engine operatinga full power (no afterburner) and clearly illustrates theDoppler shift between the two measurement paths. At thispower level, the duct air density was 0.91 kg m−3 and theflow velocity was 160 m s−1. A large set of data acquiredover the full range of engine operating conditions wasstatistically analysed and demonstrated density, velocityand mass-flux precision on the order of 2% or less anda precision-limited velocity of 40 cm s−1 at atmosphericpressure. This corresponds to a Doppler shift/linewidthratio of 3×10−4. Such high-precision, direct measurementsof inlet air flux is required for improved aeroengine controland flight versions are under development.

Diode lasers have been extensively applied tomeasurements of trace gases in the atmosphere (see therecent review by Feher and Martin 1995). In conjunctionwith a multipass cell to achieve extreme sensitivity ina relatively short path (∼50 cm) and a conventionaldirectional anemometer to record wind speed and direction,it is possible to make essentially point measurements ofthe turbulent exchange of trace gases from anthropogenic

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Figure 4. Example tomographic reconstruction of average temperature distribution in a turbulent, mixing O2/N2 jet flow.

Balanced RatiometricDetectors (BRD)

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Figure 5. Diode laser air mass flux sensor configuration on full-scale aeroengine inlet.

sources such as waste dumps, agricultural fields, industrialemission, etc within the atmosphere. Using a 1.65µm DFBlaser diode, researchers have recently determined the fluxof methane from a landfill with a sensitivity below 100 ppb(Hovdeet al 1995). Flight versions of 1.39µm diode lasersensors for trace water vapour measurements have also beendemonstrated on research aircraft with sensitivity on theorder of 10 ppm (Silveret al 1994). Here, the pointlikenature of the measurement is due to the vast scale differencebetween the∼1 m path and the characteristic dimension ofthe atmosphere. Using water-cooled, fibre-coupled probes,similar short-path, quasipointwise absorption measurementshave been demonstrated with UV absorption in flamesusing a frequency-doubled ring-dye laser (Kimball-Linneet al 1986) and should find application to other gas-

dynamic flows using near-IR lasers with either short-pathor evanescent wave absorption.

5.2. High-speed flows

The unique requirements of high-speed flows for fastresponse, non-intrusive instrumentation often drive devel-opments of new optical diagnostic techniques and diodelaser absorption sensors are no exception. One of the ear-liest demonstrations of room-temperature diode laser sen-sors for gas-dynamic monitoring was based on O2 absorp-tion in high-speed air flows in a shock tube (Philippe andHanson 1991, 1993). Using wavelength-modulation tech-niques, simultaneous measurements of temperature, den-sity, velocity and mass flux were demonstrated over a range

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Figure 6. Example O2 absorption lineshapes recorded in the aeroengine inlet at full (mil-spec) power conditions.

of 500–1000 m s−1, 0.4–1.0 atm and 600–1100 K. Themeasurements demonstrated submillisecond temporal reso-lution. Although only a limited set of data was analysed,estimates of experimental uncertainties were 75 m s−1,0.07 atm and 75 K. This basic concept was later extendedto a multiplexed system for two wavelength measurementson H2O (thus determining the flow temperature) and O2

for characterizing combustion in high-speed shock tunnelflows (Baeret al 1994, 1996, Hanson 1997).

Water vapour is a natural constituent of humid airand present in most high-enthalpy test facility flows. Itis also a major product of hydrocarbon/hydrogen com-bustion and is thus a natural target for diode laser sen-sor applications. As with the O2 measurements de-scribed above, simultaneous measurement of temper-ature, density, velocity and mass flux were demon-strated over comparable ranges of these parameterswith comparable uncertainties (Arroyoet al 1994a, b).Recently, this technique has been combined with amultipass probe geometry to determine the operatingcharacteristics of a high-enthalpy shock tunnel facility

(Hanson, 1997). Figure 7 is an example from this workof a single 125µs sweep of two lasers operating onseparate transitions near 1.4 µm for a 10 MJ kg−1 high-enthalpy test condition. The top panel shows the Doppler-shifted absorption line recorded by a path oriented at 54◦

with respect to the main flow velocity and the bottompanel shows an second absorption lineshape recordedsimultaneously but with a laser beam path normal to theflow velocity. The Doppler-width of each line gives thegas translational temperature, which is in good agreementwith the rotational temperature inferred from the ratio of thetwo lines. The small peak in the top panel is from unshiftedabsorption due to room air and conveniently providesthe frequency reference for determining the magnitudeof the Doppler shift and, hence, the gas velocity of4630 m s−1. This result demonstrates the high-speedcapability of laser diode frequency tuning and suggests anumber of related high-enthalpy flow facility diagnosticsfor facility characterization and flow phenomenology.

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Figure 7. Example of simultaneous temperature, watervapour density, and velocity in a 10 MJ kg−1 shock tunnelflow field.

5.3. General flame, combustion and emissionsmeasurements

Other than the water vapour measurements describedpreviously, in-situ measurements of combustion specieswith room-temperature diode lasers are only beginningto appear in the literature. Initial measurements ofCO, CO2, OH, and NO have demonstrated the essentialfeasibility of flame monitoring, but more effort is requiredto understand the high-temperature spectroscopy of thetargeted molecules, as well as interferences from high-temperature water vapour (Sonnenfroh and Allen 1996a,1997a, b). Even water vapour measurements remain subjectto considerable uncertainty in many bands, particularly thetransitions near the technically important 1.31 and 1.55µmbands, where the long-term availability of high-quality,inexpensive laser devices is ensured (Allen and Kessler1996, Sonnenfroh and Allen 1996a, Upschulte and Allen1997). Figure 8 shows a diode laser survey spectrum ofseveral NO and water vapour transitions near 1.8 µm. Themeasurement was made in a cell with 74 Torr of NO in N2 ata total pressure of 740 Torr. The neighbouring water vapourtransitions arise from ambient humidity in the surroundingpath and illustrate the congested nature of this spectralregion for flame sensing applications. Although∼30 ppmdetection limits in ambient atmospheric air would appear

feasible, interference from strong high-temperature watervapour lines limits the sensitivity on the transitions shownto∼140 ppm (Sonnenfroh and Allen 1997b). Further workis required to identify more optimum transitions. Recentresults using a cooled AlGaAsSb/InGaAsSb laser to accessthe stronger first overtone of NO near 2.65 µm achievedlower ambient detection limits but this wavelength regionis expected to find even stronger water vapour interferencesin flame environments (Oh and Stanton 1997).

Sampling the combustion gases into a low-pressure,low-temperature cell eliminates many of the interferenceproblems associated within-situ measurements. Usingthis approach, off-the-shelf commercial instruments havebecome available for combustion applications such as NH3

monitoring in post-combustion NOx clean-up systems forcoal-fired utility boilers (Bomse 1995). Similar extractiontechniques have recently been applied to sampling CO,and CO2 in laboratory flames (Hanson 1997). Othercommercially available industrial diode laser sensorsinclude open-path monitors for HF and H2S fence linemonitoring in petroleum refineries (Bomse 1995).

An interesting example where the unique attributes of afibre-coupled diode laser sensor enable new measurementsis microgravity combustion experiments in drop towers.At a NASA Lewis facility, the entire experimental testrig falls within a 27 m shaft during the∼2 s experiment.Using a multichannel fibreoptic delivery system to createmultiple lines of sight, researchers have measured the watervapour distribution in diffusion flames during a drop (Silveret al 1995). The fibre launch optics were mounted onthe drop platform while the lasers, control electronics anddata acquisition system were located in a remote and fixedlaboratory. Figure 9 is an example of the measured watervapour mole fraction in a microgravity propane jet diffusionflame. The two-dimensional map was constructed from sixseparate drops wherein the radial distribution at a fixedheight was determined. The stated uncertainty in thisdata is between 15 and 30%, due in part to the extremeexperimental difficulty of the measurement and the lack ofcorroborating flame-temperature measurements.

Many of the diode laser combustion sensors arebeing developed for eventual inclusion in aeroengine testarticles where a number of photophysical and engineeringchallenges are being addressed. One basic issue arisesfrom the high pressure associated with the combustorenvironment—typically between 10 and 50 atm. At thesepressures, the collisional broadening of the absorptiontransition blends neighbouring transitions and complicatesdata interpretation since it is not possible to determinea baseline transmission by scanning the laser wavelengthoff the absorption line. Initial results using fixedwavelength (Nagaliet al 1996b, Nagali and Hanson 1997)or partially resolved transitions (Allen and Kessler, 1996)at moderate pressures (<10 atm) and temperature (<500 K)are encouraging, but more work is required to extendexperimental techniques to practical aeroengine combustorconditions. Work is underway in several laboratoriesto develop engineering solutions to optical access incombustor and engine test stands.

Successful implementation of diode laser sensors inpractical aeroengine combustors may enable more sensitive

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Figure 8. Example survey spectra of NO and water vapour obtained with a 1.8 µm diode laser.

Figure 9. Example contour map of water vapour mole fraction measured in a micro gravity jet diffusion flame usingtomographically inverted diode laser absorption data.

control of engine operating conditions or suppression ofdeleterious instabilities which limit their operational ranges.Laboratory demonstrations of prototype sensor/controlsystems are appearing. Using a dual wavelengthmultiplexed sensor for simultaneous measurements ofwater vapour concentration and temperature, researchershave demonstrated suppression of acoustically coupledcombustion instabilities with a closed-loop bandwidth ofseveral hundred Hz (Furlonget al 1997). Figure 10 isan example from this work of continuously recorded gastemperature and water-vapour partial pressure 2 cm abovea flat flame burner subject to an 85 Hz instability. Since theinstability is essentially one-dimensional, the path-averaged

absorption data capture it well and the precision of thetemperature and concentration data is on the order of a fewper cent. An interesting observation made possible by thistype of data is the suppression of the mean temperature andwater vapour concentration, indicating that the combustioninstability is reducing the overall combustor efficiency.

6. Emerging diode laser sources

Progress in the application of diode laser sensors to gas-dynamic and combustion flows continues to be driven byrapid developments in the laser-source technology and wecan anticipate several areas that will have near-term impact

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Figure 10. Measured combustion temperature (top) andwater vapour concentration (bottom) in a premixedlaboratory CH4-air flat flame burner subject to an acousticinstability.

on sensor architecture and applications. Direct currentinjection of green and blue–green visible lasers is a majoraim of the optical storage portion of the diode laser market.Progress in GaN- and ZnSe-based devices continues, butit is likely to be several years before the developmentof room-temperature, cw sources appropriate for sensorapplications. A technology immediately available forextending the short wavelength range of diode laser sensorsis nonlinear frequency conversion in KDP, BBO, orsome other appropriate material. Although the conversionefficiency in conventional materials is low due to thelow cw output power available from diode lasers, theyare particularly interesting because they allow diode lasersources to operate in the 200–300 nm region where strongelectronic transitions of species such as NO and OH areavailable with linestrengths 4–6 orders of magnitude largerthan the near-IR transitions currently used. For example,sum frequency generation in conjunction with a fixedwavelength, high-power source has been used to detect OHat 308 nm with ppt sensitivity, despite only 1.5 µW ofavailable laser power (Oh 1995). The recent availabilityof high-power, master-oscillator power-amplifier (MOPA)diode lasers near 860 nm has allowed researchers togenerate deep-UV light, using mode-locking or pulsing ofthe near-IR source to improve the conversion efficiency forsecond, third, and fourth harmonic generation (Goldbergand Kliner 1995a, b). A tunable source based on fourth

harmonic conversion has been used to detect NO at 215 nmwith a detection limit below 1 ppm (Klineret al 1997).

The efficiency of the frequency conversion may beimproved using periodically poled materials in either bulkor waveguide geometries, allowing quasiphase-matchedconversion over longer interaction lengths (Webjorn et al1997). Frequency-doubled diode lasers in periodicallypoled lithium niobate waveguides have been applied tomeasurements of visible transitions of NO2 (Mihalceaet al1996) and, using Doppler-shift absorption techniques, as adeposition controller for Al sputtering devices (Wanget al1995). A number of other materials for deep UV conversionare under development but presently suffer from gradualoptical degradation at moderate laser power levels.

Alternatively, difference frequency generation inperiodically poled lithium niobate or bulk AgGaSe2 orAgGaS2 has been demonstrated as a relatively simpleapproach for generating tunable radiation from room-temperature sources at wavelengths between 3.2 and8.7µm (Petrovet al 1995, Balakrishnan 1996, Eckhoffet al1996, Kronfeldtet al 1996, Petrovet al 1996a, b). Theselasers return room-temperature sources to the wavelengthregion traditionally accessed with cryogenic Pb-salt lasers.Here, strong fundamental band absorption transitions areavailable with linestrengths 2–3 orders of magnitude largerthan the near-IR transitions described earlier. For example,ppb detection limits for CO have been demonstrated usinga 4.6 µm source derived from difference-frequency mixinga diode-pumped cw Nd:YAG laser at 1.06 µm and atunable 850 nm diode laser (Petrovet al 1996a). Acompact, tunable spectrometer based on this technologyhas been demonstrated for the detection of N2O, CO2, SO2,H2CO, and CH4 (Topfler et al 1997). It seems likely thatthese sources may soon replace cryogenic Pb-salt lasersaltogether.

These sources may in themselves soon be replacedby advances in direct-current injection devices which re-cover the inherent simplicity and tuning range of thenear-IR and visible lasers. Progress in materials technol-ogy for InAsSb/InAlAsSb, InGaAs/InAsSb, InAsSb/InPSb,GaInAsSb/AlGaSb, and InAsSb(P)/InAsSbP devices con-tinues to produce new mid-IR lasers in the 2–4µm region(Wu et al 1997, Hasenberget al 1997). Most of the de-vices are presently multimode and operate at near liquid ni-trogen temperatures, although improvements in fabricationtechniques are expected to produce cw devices operatingat temperatures>250 K—accessible with thermo-electriccoolers. Broad tuning capability has been demonstratedaround 3.85µm using an external cavity configuration (Leet al 1996). Less mature, but potentially more significanttechnology in quantum cascade lasers offers the possibilityof broadly tunable, room-temperature sources throughoutthe mid-IR (Faistet al 1996). In these devices, quantumconfinement effects are exploited to produce photon emis-sion from sub-band transitions within the conduction band,rather than between conduction and valence bands. Thepast year has seen remarkable progress in these devices,including room-temperature pulsed and cw operation in the8–10 µm region (Sirtoriet al 1997a, b) and single-modeDFB devices near 5.4 µm (Faistet al 1997, Gmachlet al1997).

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These new mid-IR sources are likely to find their mostvaluable application in the area of trace-gas monitoring inambient atmospheric environments or low-temperature flowfacilities. In high-temperature combustion applications,interference from strong CO2, H2O, or other hydrocarbonsmay make sensitive detection of trace species problematic,as work with mid-IR Pb-salt lasers forin-situ NO detectionhas demonstrated (Wormhoudtet al 1994). Detection ofCO, however, should be straightforward.

Advances in room-temperature near-IR source tech-nology will enable more powerful sensor applicationsin the near future. Single-mode power, linewidth, andprice/performance ratio of these devices continues to im-prove. Multi-electrode lasers are becoming availablein fibre-pigtailed communication-style packages. Thesesources separate the wavelength selection, gain, and phasecontrol regions of the laser cavity and are available in anumber of different configurations (cf Ishiiet al 1996).Injection current tuning over 100 nm around 1.5 µm hasbeen demonstrated (Rigoleet al 1995). Devices have beenrecently applied to high-sensitivity NH3 detection (Larsonet al 1997) andin-situ measurements of CO, OH, and H2Oin combustion gases (Upschulteet al 1998). These tun-ing ranges are equivalent to external cavity devices and ap-proach that of a conventional dye laser, thereby simplifyingpresent multilaser sensor concepts by allowing comparablewavelength coverage with a single device.

Vertical cavity surface-emitting lasers (VCSELs) areanother laser structure fabricated from conventionalmaterials that offer some promise for gas sensingapplication (Morgan and Hibbs-Brenner 1995, Morganet al 1995b). Using AlGaAs materials, devices havebeen grown with laser cavities normal to the chip plane.Low and stable threshold currents operating at>100◦Ctemperature have been exploited to achieve nearly 100 nmof continuous, single-mode tuning near 850 nm. Largearrays of laser devices can also be fabricated on a singlechip and application to water vapour detection has recentlybeen demonstrated (Hovde and Parsons 1997).

7. Summary

Room-temperature diode laser absorption sensors havebeen developed for a variety of gas-dynamic andcombustion flows. Measurements of trace and majorspecies concentration, gas temperature, velocity, pressureand mass flow have all been demonstrated. Thecompact, low-cost sensors appear poised to makethe transition from research instruments to standardinstrumentation in a number of industrial and practicalengineering test-facility applications. Mature laserdevices and fibreoptic components in the near-IR arealready incorporated into commercial sensors for industrialmonitoring. Specialized units for gas temperature andmajor species monitoring, primarily H2O and O2, havebeen successfully demonstrated on full-scale aeroenginesand large-scale aerospace test facilities. Additional facilityintegrations in the near future will further broaden the rangeof demonstrated applications.

As new laser devices emerge with new operatingwavelength windows, additional species detection will bedemonstrated, in both ambient and combustion gases.Using these sensors for closed-loop combustion controlhas already been demonstrated for suppressing acousticinstabilities and several efforts are underway to developemissions sensors (CO and NO) or combustion efficiencysensors, (CO, CO2, and O2) to be used in combustor orengine control systems. Diode laser sensors in the mid- andnear-IR have been flown on aircraft engaged in atmosphericsensing for many years, and a compact diode laser-basedair mass flux sensor is under development for flightdemonstration in 1998. These and related demonstrationsshould provide crucial data regarding the practical utility ofdiode laser absorption sensors for engineering research anddevelopment, as well as routine system operation.

In the next few years, integration of existingnear-IR and visible sensors for O2, H2O, CO, CO2,and temperature with challenging practical gas-dynamicand combustion test facilities will be an importantcomponent of progress in the field. These integrationswill address problems associated with high-pressureflows, large-scale measurements, fibreoptic distribution tomultiple measurement locations, high-temperature opticalaccess, flow-quality effects, multiplexed configurations,compatibility with flight requirements and extendedautonomous operation. As the sensor technology matures,such successful demonstrations will be necessary to transferthe sensors from the laser diagnostic research laboratoryto the practical engineering research and developmentcommunity.

The incomplete absorption database in the near-IRwill continue to require substantial work in fundamentalspectroscopic studies, particularly at high temperature.Broad surveys and spectral assignments are required forH2O, CO2, and small hydrocarbons such as CH4 and C2H2

at wavelengths between 1 and 2µm. The recent release ofthe Air Force HITEMP database is a major improvementfor water assignments, but few validations are available andthe database is only intended for use up to 1000 K. Errorsin the high-temperature CO2 database are already emerging.Extensions of this and additional databases to 2000 K arenecessary for a comprehensive understanding of optimalcombustor sensor strategies.

Multi-electrode near-IR devices with∼100 nmtuning range are particularly intriguing for gas sensingapplications. Their use in sensor configurations needsto be carefully explored in order to understand optimalmethods for incorporating their broad tuning range intopresent sensor configurations. Because of the largemarket potential for dense wavelength division multiplexingin telecommunications, the availability and quality ofthese devices near 1.55 µm can be expected to improveconsiderably in the next two to three years, as with theconventional DFB devices at 1.31 and 1.55 µm. Becauseof this stable and growing vendor base, applications usingthese lasers will always have a cost and reliability advantageover custom-fabricated lasers at other wavelengths.

Frequency conversion in nonlinear materials has al-ready demonstrated improvements in diode laser sensor

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detection limits by accessing stronger UV and mid-IRtransitions. Commercial devices are nearing release forfrequency-doubled (blue–green) and tunable difference-frequency-generated mid-IR sources. Tuning characteris-tics, frequency and amplitude stability and general oper-ational parameters of these devices should receive carefulattention from the diode laser sensor community. Deep UVsources (below 300 nm) continue to require improvementsin the basic properties of the nonlinear materials and meth-ods for rapid and stable frequency tuning, especially forperiodically poled materials.

Despite the excitement associated with new room-temperature mid-IR sources, it is important to recognizethat fibreoptic components and distribution are a majorand valuable attribute of near-IR diode laser sensors andmuch of this advantage is lost beyond 2µm. Fluoride,sapphire, and chalcogenide fibres are available, but not withcost and performance characteristics of telecommunicationpackaging. On-chip, optical waveguides carrying near-IRtunable diode laser light are expected to be incorporateddirectly as sensor elements for gas-dynamic and combustionapplications in the near future. Long evanescent waveabsorption pathlengths can be fabricated into small physicaldimensions, allowing essentially point measurements froma fibre-coupled sensor (Taiet al 1987). Emergingtechnologies in micro-fabrication of electromechanical andelectro-optical systems can incorporate near-IR waveguidesfor distributed sensing in micron-sized chemical reactorsand flow devices. The transparency of the Si and SiO2

materials used in these systems allows near-IR diode laserto penetrate structural elements for measurements in flowswithout special ‘optical’ access. This is potentially anotherexample of a fundamental technology whose engineeringdevelopment will be accelerated using sensitive, non-intrusive optical measurement techniques.

Acknowledgments

The author wishes to acknowledge the contributions ofDr David Sonnenfroh, Dr Bernard Upschulte, Dr MichaelMiller, Dr Steven Davis and Mr William Kessler inthe diode laser sensor work conducted at PSI, as wellas the continuing sponsorship of NASA, the US AirForce Material Command, the US Air Force Officeof Scientific Research, the US Department of Energy,and the US Department of Commerce—National Oceanicand Atmospheric Administration. Contributed figures,manuscript review and helpful comments from ProfessorRonald Hanson of Stanford University, and Dr Joel Silver,of Southwest Sciences Inc. were also gratefully appreciated.

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REVIEW ARTICLE Diode laser absorption sensors for gas