Nonlinear Raman spectroscopy for combustion diagnostics

J. Quam. Spectrosc. Radiat. TransferVol. 40, No. 3, pp. 369-383, 1988 Printed in Great Britain

NONLINEAR RAMAN COMBUSTION

SPECTROSCOPY DIAGNOSTICS

0022-4073/88 $3.00+0.00 Pergamon Press plc

FOR

ALAN C. ECKBRETH United Technologies Research Center, Silver Lane, East Hartford, CT 06108, U.S.A.

(Received 30 September 1986; received for publication 23 September 1987)

Abstract--Nonlinear Raman spectroscopy has emerged as a very powerful tool in combustion analysis due to its high spectral resolution, superior signal/noise and sensitive detection capabilities. Of the myriad of nonlinear Raman techniques, coherent anti-Stokes Raman spectroscopy (CARS) and stimulated Raman gain spectroscopy (SRGS) are experiencing the most utilization in measurement and spectroscopic investigations respectively. This paper traces the diagnostic development of nonlinear Raman approaches largely motivated by the limitations of linear Raman scattering. Primary emphasis is focussed on CARS. The paper discusses its diagnostic advantages, summarizes important advances and highlights some of its many practical combustion applications. The paper concludes with a discussion of the recent trend to multi-color CARS for simultaneous multiple species measurements. INTRODUCTION

In the late 1960s, the utilization of laser-based spectroscopies as potential probes for the study of combustion phenomena began receiving serious attention. The thrust toward laser probing was motivated by the unique attributes of optical techniques. Laser probes are nonintrusive, thus, avoiding perturbation of the delicately stabilized combustion process and amenable to measurements in extremely high temperature environments. They are capable of simultaneously high spatial and temporal resolution. Much of the research effort in the 1970s in this area was spurred by the Office of Naval Research through its Project SQUID program which Professor Penner served in an advisory capacity. Most of the early attention for measurements of combustion temperature and species was directed at spontaneous Raman scattering based upon the forefront work of Lapp mand Lederman 2 and their colleagues. However, in attempting to apply Raman scattering to flames typical of practical devices, i.e. luminous and sooting, severe laser-induced interferences were encountered. 3 These were attributed to the modulated incandescence arising from intense laser heating of the highly absorbing soot particles produced in hydrocarbon-fueled diffusion flames. At soot loadings typical of practical device primary zones, the laser modulated soot incandescence interference far exceeds the Raman scattering rendering linear Raman approaches of little utility in these applications? It was clear that stronger diagnostic processes would be required and the then emerging nonlinear Raman techniques offered a potential solution to practical flame probing. Of many nonlinear Raman techniques, coherent anti-Stokes Raman spectroscopy (CARS) has become the most utilized diagnostic for combustion applications. In this paper, the rationale for this preeminent position will be explored. In the next section, the various major nonlinear Raman spectroscopies will be reviewed and the diagnostic advantages and disadvantages of CARS and stimulated Raman gain spectroscopy (SRGS) will be compared. In the succeeding section, the development history of CARS will be reviewed and some of its more notable applications will be highlighted. The paper concludes with a review of the recent advances into multi-color wave mixing for simultaneous multiple species measurements. NONLINEAR RAMAN TECHNIQUES

Background Before beginning the survey of nonlinear Raman effects, it is instructive to review briefly how these phenomena originate. For simplicity, a classical picture will be employed here although 369

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ALAN C. ECKBREDI

detailed treatments generally require quantum mechanics) Optical phenomena are governed by Maxwell's equations which can be manipulated to yield the wave equation I

1 d 2 -I

v(v ×

d2P a:

(1)

where E is the electric field of the incident electromagnetic light wave; c, the vacuum speed of light; P, the electric polarization induced in the medium; ~ , the permeability of free space, a fundamental physical constant. The polarization physically corresponds to the vector sum of the volume density of electric dipoles induced in the medium by the light wave electric field. The induced polarization can be expressed as a power series of E(o~i), where o~i is the frequency of the input light wave and allowance is made for several frequencies being simultaneously present: P(~oi) = e0~f°)(coi)E(coi) + ~o~ ~fc2)(c°i= coj + ok) E(coj) E (fDk) j,k

+ E0 ~ X°) (COl= coj + cok + con)E(coj) E(C0k) E(col) + ...

(2)

j,k,I

where e0 is the permittivity of free space and c2= (/~oE0)-~. Xo) is the linear susceptibility of the medium and ~fc~)are the nth order nonlinear susceptibilities so named because they express how susceptible the medium is to being polarized. For simplicity, plus signs have been employed above. In actuality, the various frequency components add and subtract in all possible combinations. The induced linear polarization modifies the propagation of the light wave through the medium, accounted for by introduction of the refractive index given by x/1 + Zo). Spontaneous Raman scattering also arises from the oscillating polarization induced through the linear susceptibility and may be interpreted as the beat frequency between the incident light frequency and the nuclear vibrations. The higher order susceptibilities lead to wave mixing for multiple input fields or frequency doubling, tripling, etc. for solitary fields. These susceptibilities become progressively weaker with increasing order. However the overall signal efficiencies can become quite large due to Raman resonance terms in the susceptibilities and the use of intensely powerful laser pulses which are commercially available. In isotropic media such as gases, there are no second order effects due to inversion symmetry. The lowest order nonlinearities in a gas are thus third order and arise through the third order nonlinear susceptibility. The third order nonlinear susceptibility is complex and consists of Raman resonant terms, denoted by primed superscripts, and a real nonresonant electronic background, ~nr, namely

~(3)= ~ (~, + iZ#)j+ Zn,

(3)

J

where the summation extends over the ensemble of Raman resonances. The real and imaginary components of the susceptibility display dispersive and resonance behavior about each Raman resonance analogous to the real and imaginary parts of the refractive index. In two color, third order processes, this leads, respectively, to birefringence, i.e. polarization rotation, and gain or loss. Each resonant susceptibility is proportional to the population difference between the states involved in the Raman resonance. The nonlinear Raman signatures essentially are a reflection of the ro-vibrational state population distributions. Thermometry derives from signature analysis and concentration information is obtained from the strength of the signal in general. The various nonlinear Raman methods may depend on different components of the resonant susceptibility or the total susceptibility. If the process exhibits a dependence on the nonresonant susceptibility, the Raman resonances may be undetectable at low species concentrations due to the nonresonant background level. This imposes a lower detectivity threshold unless steps are taken to suppress detection of the background. Due to the coherent nature of the wave mixing process, a "phase-matching" condition must be satisfied for efficient signal generation, namely, k~ = kj + kk + kt

(4)

Nonlinear Raman spectroscopyfor combustion diagnostics

371

StimuLated Roman Gain Spectroscopy (SRGS)

- ' - " 1 PD Pump

L AD (PARS)

Roman-Induced

Kerr Effect (RZKES) T

Pump

v k / 4 (opt)

L

GT

),/2

Coherent Anti-Stokes Roman Spectroscopy (CARS) CARS

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-

- - "

- -

Pump

~

~ L

-

r

t._.~

~

_

--4

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_

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-

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where km is the wave vector at frequency tOmwith magnitude [km I = nmtOm/ C and nm is the refractive index at frequency tom. Phase matching dictates a precise angular orientation for the input wave mixing beams. Two color nonlinear Roman processes, e.g. SRGS, are automatically phase matched due to frequency degeneracy and the laser beams can be arbitrarily oriented. Three and four color processes must be phase matched. Nonlinear Raman processes

Figure 1 summarizes schematically the major third order nonlinear Raman processes. Omitted are several other processes which have little diagnostic utility. For example, stimulated Raman scattering is a process wherein, under intense laser radiation, the spontaneously generated Stokes Raman photons experience exponential growth in the direction of the incident laser propagation and emerge as a coherent beam. Only the strongest Raman transitions experience this growth and, thus, the distribution of state populations is not captured. In some instances, competing processes such as stimulated Brillouin scattering preclude the phenomenon from occurring at all. Hyper Raman scattering is incoherent and may be viewed as two-photon Raman scattering. It is extremely weak and not amenable to diagnostic utilization. The process in the top panel of Fig. 1, stimulated Raman gain spectroscopy (SRGS), may be viewed as an induced emission process at the Stokes frequency, i.e. in the presence of a pump wave, a probe laser, Stokes-shifted in frequency from the pump to coincide with a Raman resonance, experiences gain. An analogous phenomenon exists involving induced absorption at an anti-Stokes shifted probe frequency and is commonly referred to as inverse Raman scattering. In SRGS, the imaginary component of the susceptibility is responsible for the gain and the nonresonant susceptibility of the medium, which is real in general, plays no role. The imaginary component of the resonant susceptibility has resonance character and is directly proportional to the spontaneous Raman cross section. In the small gain limit, the "signal" (i.e. the gain) is linearly proportional to the magnitude of the imaginary susceptibility and, thus, the SRGS signature is identical to the spontaneous Raman spectrum. Since it is a two-color process, phase matching is satisfied regardless of geometry and the laser beams can be arbitrarily crossed. Gas phase gains are generally quite small, on the order of 10-3-10 -5 , requiring very stable probe lasers and sensitive electronic detection strategies. For spectroscopic investigations, an alternate form of detection is to monitor the acoustic wave generated by the relaxation of the upper level excitation created by the pump-probe interaction. Since one is measuring a nascent signal rather than monitoring a small

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ALAN C. ECKBRETH

change in a large background, this approach is extremely sensitive. It is known as photoacoustic Raman spectroscopy or PARS. 6 In the Raman-induced Kerr effect (RIKES), a pump laser is used to induce a Kerr effect, i.e. polarization rotation, on a probe beam when the pump-probe frequency difference coincides with a Raman resonance. Thus a probe beam, Raman-shifted from the pump and with its polarization appropriately oriented, experiences anisotropic changes in refractive index. This results in a rotation of the plane of polarization of the probe laser and transmission of a "signal" through a polarizer which normally blocks the probe as seen in the middle panel of Fig. 1. Phase matching is automatically satisfied as in SRGS and arbitrary beam arrangements can be employed. One may regard RIKES as a polarization form of SRGS. Due to the anisotropy of Raman scattering and with proper polarization orientation, different vector components of the probe beam will experience different gains which results in rotation of the plane of polarization. The real component of the nonlinear susceptibility also induces polarization rotation by imposing different phase changes on probe laser components whose polarizations are aligned parallel and perpendicular to the pump field. In RIKES, the signal contains contributions from both the real and imaginary components of the nonlinear susceptibility and the spectral signature depends on IXe~l2 where Xe~is the difference between two of the specific tensor elements of the susceptibility. If the pump beam is circularly polarized however, the real nonresonant contribution can be greatly suppressed. The major limitation of RIKES resides in the extinction achievable with crossed polarizers which, at best, is about 1 part in 106. Optical components such as lenses, windows, etc. invariably possess some strain-induced birefringence which diminishes the extinction ratio. If the signal induced by the polarization rotation is less or comparable to the probe laser leakage, it will not be detectable or will be very noisy. This is generally the situation in moderate-pressure gas phase work, and most RIKES demonstrations have been performed in liquids. To overcome these limitations, one may resort to optically heterodyning the RIKES signal by purposely allowing the polarizer to pass a small portion of the probe laser, a technique known as OHD-RIKES. With this approach, quantum-noise limited signal to noise is achievable and, furthermore, a pure spectrum can be obtained which depends linearly on either the real or imaginary component of the nonlinear resonant susceptibility. Unfortunately, these gains are realized at considerable expense to the signal level degrading sensitivity and decreasing the capability to discriminate against spurious background radiations. Hence RIKES and OHD-RIKES have seen very little use spectroscopically or analytically. To this point, attention has been placed on two-color techniques wherein the modulation of a probe beam in one form or another is monitored. CARS is a three or four color technique wherein a signal, at a new frequency, is generated by the nonlinear interaction. Specifically, as seen in Fig. 1, a pump beam at o~ and a probe beam at o~2, Stokes shifted from the pump, mix to generate the CARS beam at o~3 = 2~ol -o~2. An analogous process, coherent Stokes Raman spectroscopy (CSRS) occurs when an anti-Stokes shifted probe is mixed with the pump. The CARS signal is proportional to IX°)l 2 and, hence, the spectra are more complicated than spontaneous Raman or SRGS signatures. CARS spectra exhibit constructive and destructive interference effects and also contain contributions from the nonresonant background susceptibility. The latter can be suppressed, albeit with loss of signal, by appropriate polarization orientation of the input wave mixing and CARS detection fields. With the background present, CARS spectral signatures become concentration sensitive at low resonant species mole fractions. This permits concentration measurements from spectral shapes. However, detectivity is limited and at low concentrations, the signal disappears into the background level generated by the nonresonant susceptibility. With polarization suppression of the background, detectivity is limited by signal level shot noise considerations. Unfortunately, a substantial reduction (16 x ) in resonant mode signal accompanies suppression and combustion detectivities at atmospheric pressure are approximately comparable with either approach. 7 Since CARS is at least a three-color process, phase matching is not automatically satisfied and in media with significant dispersion, precise angular alignment of the pump and Stokes fields is mandated with the orientation dependent on the Raman resonance magnitude, i.e. pump-Stokes frequency separation. Gases are nearly dispersionless and phase matching is trivially satisfied by merely overlapping the laser beams. For diagnostic purposes, collinear arrangements often result

Nonlinear Raman spectroscopyfor combustion diagnostics

373

in poor spatial resolution and a variety of crossed-beam arrangements has been devised to limit the region over which wave mixing occurs. These will be discussed subsequently when some of the important diagnostic developments in CARS are examined.

Comparison of CARS and SRGS From a diagnostic perspective, CARS possesses a number of advantages relative to SRGS. CARS, of course, possesses some disadvantages as well, which often render SRGS the preferred technique in purely spectroscopic investigations. Most practical combustion environments are time-varying in nature being either transient, limited duration or unsteady. Due to the nonlinear dependence of CARS/SRGS on temperature and/or density, time averaging over parameter fluctuations can lead to serious measurement errors. Thus, single pulse ("instantaneous") measurements are required. Averaging errors aside, such approaches are necessary to measure fluctuation magnitudes which are often quite important in addition to mean values. CARS, being signal generative, is easily multiplexed with broadband Stokes lasers and spectrally recorded with optical multichannel detectors. Although SRGS was first demonstrated with a broadband source, s i.e. in its loss variant, inverse Raman, it is difficult to measure the small gas phase gains or losses amidst the background and noise with optical multichannel detectors. Turbulence effects in practical environments generally cause some beam steering and defocussing due to refractive index effects leading to signal loss. Thus, the signal has to be normalized and this generally cannot be accurately performed externally. A reference cell placed after the measurement region will indicate the degree of medium perturbation but does not provide for rigorous correction of the signal. With CARS, the presence of the nonresonant background susceptibility permits in-situ normalization either explicity or implicity through spectral signature analysis. Generally the nonresonant susceptibility is viewed as problematical; diagnostically it is exploited to very important advantage. On the other hand, in SRGS there is no such corresponding normalization scheme. Both techniques are critically dependent on beam overlap at the measurement location. Although the beams can be arbitrarily oriented in SRGS, the phase-matching requirements of CARS are not particularly onerous considering the many approaches available. Both possess the disadvantage of signal generation in the forward direction requiring two optical ports and near line-of-sight optical access. Counterpropagating schemes offer little relief in this regard. Relative to spontaneous Raman scattering, both also possess the disadvantage of generally interrogating just one constitutent at a time. One of the recent research developments in CARS is multi-color wave mixing for simultaneous measurements of multiple species, an area highlighted later in this paper. The spectral complexities of CARS, i.e. constructive and destructive interference effects, nonresonant background, are tolerated diagnostically but are best avoided for spectroscopic studies due to the complicated lineshapes and spectra which result. For spectroscopy, SRGS is the method of choice due to the purity of the individual lineshapes and is providing much of the fundamental data base necessary for the accurate modelling of CARS spectra. 9"~° Nevertheless, CARS can provide important contributions as well. H

DIAGNOSTIC HISTORY OF CARS Here, some of the advances that have contributed to the diagnostic development of CARS will be reviewed. This will also serve as an opportunity to reviewhow CARS is performed in diagnostic applications. After the discovery of anti-Stokes rings in the early 1960s, ~2 four-wave mixing remained primarily in the province of nonlinear optics until Taran's pioneering applications to flames in the early 1970s in which quantitative species measurements ~3 and thermometry ~4 were demonstrated. These early studies were performed with ruby lasers at relatively low repetition rates. In 1974, N d : Y A G lasers were applied to CARS ~5 and much of the progress in the field was accelerated by the commercial emergence of reliable, scientific-quality N d : Y A G lasers in the mid-1970s. This is still the technology in current use employing cylindrical laser rods. Such lasers are limited to repetition rates of 20-30 pps, much lower than necessary to follow medium fluctuations in real time or to map a combustion region rapidly. Most laser alternatives, which have

374

ALAN C. ECKBRETH

~

Stokes

ii

Pumps

" I~ I= niw~/c

wt

• Energy Level diagram

• Spectrum ~

~3

Sconned ....

Broadband

/x.

I

~o~

wt

073

Fig. 2. Coherent anti-Stokes Raman spectroscopy (CARS).

high repetition rate capability, lack sufficient beam energy, beam quality or spectral sharpness to provide a single pulse CARS measurement capability in atmospheric pressure combustion applications. Advances in high energy, good beam quality, and high repetition rate lasers would be clearly desirable. Developments in both temporal and spatial resolution followed shortly after these early studies. As depicted in Fig. 2, the first CARS measurements were performed in a spectrally-scanned manner using narrowband Stokes lasers. Although such approaches maximize signal strength (hence, detectivity) and spectral resolution, they are time consuming and appropriate only to temporally steady situations. In 1976, broadband Stokes sources were introduced to access simultaneously the Raman resonances in a given band region and single pulse recording on an optical multichannel detector was demonstrated.16 Due to the near lack of dispersion in gases, CARS can be collinearly phase matched and was so performed in early investigations. However, due to the integrative nature of the signal generation process, poor spatial resolution can result even with tight focussing. This is particularly so in combustion situations where cold, high density regions often surround the high temperature, low density flame zone. Due to the nonlinear signal dependence on density, cold region contributions can dominate signal growth. Crossed-beam phase matching or BOXCARS 17 was introduced in 1978 and follows the "box-like" phase matching diagram in Fig. 2. in this approach, the pump beam is split into two components and crossed with the Stokes to generate CARS only from the well-defined intersection volume. The laser beams need not reside in a single plane and soon thereafter, folded BOXCARS 18 or three-dimensional phase matching ~9 was introduced. These approaches lead to complete angular and spatial separation of the CARS beam, an experimental convenience, and also permit pure rotational Raman resonances at small frequency shifts to be examined. Pure rotational CARS is attractive for accurate thermometry at low and modest temperatures. :° Three-dimensional spatially-resolved phase matching can also be performed with an annular pump beam and a Stokes laser coaxially aligned along the annular centerline) I This is a simple approach quite attractive for practical device measurements. In the late 1970s, electronic resonance enhancement in gases and flame radicals was demonstrated to extend CARS detection sensitivities to lower levels.22 In these approaches, electronic enhancement of the susceptibility occurs when either the pump (generally the case), Stokes laser or CARS signal itself coincides with an electronic resonance of the constituent being probed. In the area of theory, the incoherent convolution was introduced to deal with finite bandwidth pump laser effects due to multimode sources. 23 Nonresonant background suppression exploiting polarization orientation of the wave mixing and detection fields was demonstrated in flames. 24Of the various methods to suppress background, this is the most practical and effective. In atmospheric pressure flames,

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background suppressed broadband CARS, due to the factor of 16 resonant mode signal loss, has roughly comparable detectivity to the unsuppressed spectral shape methods. 25 At elevated pressures, detcctivity should be improved with background elimination. Near the end of the 1970s, the first demonstrations of the practical device applicability of CARS began to appear with measurements in internal combustion engines 26 and simulations of gas turbine combustion. 27-29 Practical combustion devices generally operate at elevated pressures and it is important to understand the pressure dependence of the C A R S signatures for accurate high pressure measurements. At the beginning of this decade, collisional narrowing of CARS spectra at elevated pressures was reported, 3° an area still receiving much research emphasisfl Figure 3 displays CARS spectra at ,-, 1600 K which reveal the dramatic signature changes which occur with elevated pressure. As pressure increases and the individual Raman transitions broaden to the point of substantial overlap, band collapse initiates as seen. As pointed our earlier, CARS is a unique spectroscopy in that its signatures are concentration sensitive in certain ranges, typically 0.5-30%, which is common to most of the major flame constituents. This arises due to interference with the nonresonant background which serves as an in-situ reference. With polarization suppression of the

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background, the nonresonant signal can still be captured and employed explicity as an in-situ reference.32 Explicit in-situ referencing could be an important approach in high pressure applications to overcome turbulence effects if implicit referencing (i.e. based upon shapes) loses sensitivity. In the mid-1980s, an exciting new physics has emerged. The incoherent spectral convolution for multi-mode pumps was shown to be inaccurate over certain concentration ranges for laser pump widths in excess of the Raman linewidths necessitating the employment of partically-coherent convolution formalisms.33'34Laser field statistical effects on CARS spectra were observed with some multi-mode pump lasers. 35These effects manifest themselves as a varying resonant to nonresonant background ratio as the two pump beams in crossed-beam phase matching are brought into and out of phase correlation. Another major area of concern involves single pulse spectral quality of the CARS signatures. Due to amplitude ripple on the Stokes laser and spatial and temporal mode effects during wave mixing, single pulse CARS spectra are imperfect and measurement accuracy is impaired. A histogram of measurements from an isothermal source will typically exhibit standard deviations (SDs) on the order of 50-100 K at flame temperatures. Single pulse spectral noise has been observed to exhibit different tendencies for single and multi-mode pump operation. Nonresonant spectra display reduced spectral noise with single mode pump operation, ~ while resonant N2 flame spectra at atmospheric pressure exhibit increased noise with a single mode pump laser. 37In the latter case, the Raman resonances are essentially discrete and not overlapped. With a continuous molecular response, i.e. nonresonant or overlapping resonant transitions, single mode operation might be preferable while multi-mode operation may be necessary for discrete resonances to achieve best results. With injection seeding of multi-mode lasers to obtain reliable single mode operation, 3s selecting the best approach may be as simple as turning the seeding laser on and off. The research reported in Ref. 37 also illustrates the dramatic reduction in histogram widths (nearly 2 x ) which accrue with weighted regression analysis. In fact, the effect of weighting was far more pronounced than the differences exhibited by histograms between single and multi-mode pump operation. Improving the single pulse quality of CARS signatures remains an important problem of current research and applications interest. As noted above, current research interest focusses on spectroscopic modeling, with emphasis on high pressure effects, and on the quantum electronics of single pulse generation. Another area receiving much attention is multi-color wave mixing for simultaneous, multiple species measurements. 39 That area will be reviewed in the next section of the paper. We will conclude this section with a summary of some of the practical demonstrations and applications of CARS. These successes are based upon the preceeding diagnostic developments and have justified the research effort accorded to CARS in the last decade and a half. Indeed, CARS has been demonstrated in many practical environments in the propulsion, automotive and energy conversion fields. CARS has been employed for measurements in gas turbine combustors or simulations thereof,27-29'4o-42in jet engine exhausts,43 in supersonic combustion,44 and over burning solid propellants. 45 It has been applied to numerous internal combustion engine studies 21'26'*s-47 as well as to an actual diesel engine.4s In the energy arena, it has been successfully utilized in furnaces,49'5°a chemical reactor, 51 a coal gasifier, 52 and simulated MHD exhausts) 3 Such listings are, of course, meant to be representative and not all-inclusive since many reports of such applications are pending review or in press. MULTI-COLOR WAVE MIXING One of the disadvantages of CARS, besides its complexity when compared with spontaneous Raman scattering, is the inability as normally implemented to measure more than a single constituent at a time. Recently, there has been a trend toward multi-color CARS techniques to overcome this limitation. This is motivated by CARS applications to phenomena which are of a transient and limited duration. For situations where this is not the case, such approaches can greatly expedite data gathering and, more importantly, permit correlations to be developed between the various parameters being measured. In Fig. 4, various approaches to simultaneous multiple species measurements are summarized. Dual Stokes approaches are a straightforward extension of CARS. 54"5sFor each constituent to be measured, a separate Stokes laser is introduced. Due to the complexity of arranging the beams,

Nonlinear Raman spectroscopyfor combustiondiagnostics

377

" Stokes

CARS

Dual Si'okes • Two ~2 color processos • TWO species

B

DuaL pump • TWO, 3 color processes •Signol, s O.ou f.pectrotLy • Two s p e c i e s

• More thon two species

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• More man two species

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~,

Fig. 4. Summary o f various approaches to multi-color wave mixing for simultaneous measurements o f

multiple species. generally just two Stokes lasers are used. One performs two separate, two-color wave mixing sequences to monitor two different constituents. The CARS signatures are located spectrally where they would normally occur, i.e. at t h e pump frequency plus the Raman shift. In dual pump approaches, 56"s7two narrowband pump lasers, fol and fol, are used in conjunction with a single broadband Stokes laser to monitor two species via two separate three-color wave-mixing sequences. This is merely CARS in its most general form, i.e. three different input waves. In CARS with just two input colors as usually implemented, the two pump waves are frequency degenerate and dual species capability is sacrificed. An interesting aspect of this approach is that the spectra from the two constitutents reside in the same spectral vicinity simplifying optical multichannel detection. The spectra occur at frequencies of (D3 = (.D~ + CO1 - - (.O2 = (.O~ "3I- fOB

(5a)

and /

p

p

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(Sb)

and are separated by fo3 - fog = (fo; - co,) + (cos - COA).

(5c)

By judicious selection of co;, spectral overlap can be avoided or minimized to a significant extent. Both of the foregoing techniques permit measurement of two species simultaneously. They can be extended to a greater number of species by adding more Stokes or pump lasers respectively but clearly at the cost of increased experimental complexity. An approach which permits many species, i.e. at least three, to be measured with no further increase in complexity is dual broadband CARS. 39 Here two broadband Stokes lasers are used in conjunction with a pump laser and a combination of two and three-color wave mixing processes simultaneously occurs assuming all are phasematched. The two, two-color sequences are normal CARS processes as in the dual Stokes method described above. However, in addition, there is the three-color wave mixing sequence for Raman resonances which correspond to the frequency difference between fo2 and coX. Since the frequency difference range spanned by two broadband sources is quite large, Raman resonances over a several hundred wavenumber range can be blanketed. The dual broadband CARS signature resides at the frequency fo3 = fol + (~2 - ~ ) -- fol + foc

(6a)

378

ALAN C. ECKBRETH

and is the same as would pertain in a two-color wave mixing for an o9c Raman resonance. At the same frequency there is also a contribution from col and o96 resonances, i.e. 093 = o92 + (o91 - o96) = o92 + coB-

(6b)

This contribution will be spectrally smeared since it is the broadband ca2 which is scattering from the excited Raman coherences at cot - o9~ = coB. Depending upon the relative concentrations of the species with COg,COBresonances, this background may have to be accounted for to perform accurate measurements. For hydrocarbon-fueled combustion, the spectral location of the various major species resonances is quite fortuitous. The two Stokes sources are positioned to generate CARS from the major products of combustion, namely CO2 and H20. The H20 bandhead is situated at 3657 cm-' and the major bands of CO2 at 1285 and 1388 cm -~. Centering the COs Stokes laser near 1326cm -~ permits full coverage of the CO2 band system. The frequency difference of 2331 cm-t between the two broadband sources permits excitation of the N2 Raman resonances from which the o9~ pump beam scatters to generate the CARS signature. Despite the use of two broadband sources in the three-color wave mixing, the spectral resolution is governed, as in the two-color processes, by the spectral width of co, and/or the resolution of the spectrograph employed. This is due to the Raman resonances being well defined by the specific molecular constituents and not by the manner in which the resonances are excited. Many other species possess Raman resonances in the broad frequency difference range spanned by the two broadband sources and are detectable if sufficiently abundant. These include CO (2143 cm-~), N20 (2224), HCN (2097) and NO (1876). Experimentally, if the laser dye DCM is employed for the H20 source, lasing bandwidths (FWHH) of 350cm -~ result. Coupled with a bandwidth of 150 cm-~ typical of the Rhodamine dyes used for CO2, a spectral range of 500 cm- ~can be covered with less than a factor of 4 loss in peak signal. An example of simultaneous CARS generation from CO2, N2 and H20 in the post reaction zone of a CH4-air flame is displayed in Fig. 5. Since these signatures reside in their normal spectral locations relative to o9~, they were sequentially recorded due to the high dispersion of the spectrograph employed. Although not yet implemented, optical approaches have been proposed to position all the spectra onto a single optical multichannel detector. 5s Another spectral placement strategy is as follows. The low Raman shift dye laser is positioned at 1472 cm-' to generate CARS from 02 (1556 c m - ' ) and CO2 (1388) nominally at the half-height of a dye laser with a FWHH in the 150-200 cm -~ range. This second dye is maintained at the H20 bandhead (3657). This places the center of the N2 resonance just above the H20 dye half height and the CO resonance slightly off the peak of the profile. Several other comments are in order concerning dual broadband CARS. For all wave mixing combinations to occur, each must be phase-matched. There are a number of schemes for doing this involving planar or folded BOXCARS or combinations thereof. 39There is a particularly simple arrangement which exploits the annular output of the diffractively-coupled unstable resonator often employed for the o9! pump laser. 59 In this simple arrangement, the two Stokes beams are coaxially aligned and placed inside the annular pump beam. With any geometry, the three-color dual broadband process will not be as intense as a normal two-color process due to the intensity difference between the Stokes and pump lasers. On the other hand, it will not be as weak as one might suspect due to the spectral integration aspect of dual broadband CARS. That is, there are many frequency combinations driving each Raman resonance. For a resonance centered between two, equally-broad Stokes lasers, all of the energy in each laser is employed in the wave mixing. This is unlike normal broadband CARS where only a thin spectral slice of the dye laser drives the Raman resonance. Due to the increase of the CARS signal with increasing pressure, dual broadband CARS should be capable of single pulse measurements of the major constituents at the elevated pressures typical of gas turbines, internal combustion engines, and burning propellants. Furthermore, a beneficial spectral averaging may occur resulting in improved single pulse spectral quality. This aspect has yet to receive detailed investigation. The last technique shown in Fig. 4 is a hybrid technique termed dual pump-Stokes due to the fact that the low Raman shift source, co2, can serve as both a pump and a Stokes source. If the low frequency Raman resonance is narrow, the strength of the two-color CARS signal is greatly enhanced by contracting the o92 bandwidth. In so doing, this will sharpen the underlying, normally

Nonlinear Raman spectroscopyfor combustion diagnostics

379

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Fig. 5. Simultaneously generated dual broadband CARS signatures from N 2, CO2 and H20 in the postflame zone of a ~ 1700 K premixed CH4-air flame. The dispersions are respectively 0.53, 0.47 and 0.59 cm- I/pixel.

diffuse background spectrum in dual broadband CARS for oJm-o~ resonances, Eq. (6b), producing two well-defined spectral signatures in close proximity as in the dual pump approaches. This situation arises in examining multi-color strategies for H2-air combustion. ~° One Stokes source is centered at 3657 crn -~ to track the appearance of H~O product in the reaction. Subtracting the N~ Raman shift, one positions the low Raman shift source at 1246 cm -~ to generate H2 CARS from its pure rotational S(4) transition which is very sharp. This permits the disappearance of the fuel to be tracked. By making this laser narrow to enhance the two-color mixing, an H 2 0 signature appears along with Ne in the three-color"dual broadband" process. The H 2 0 bandhead occurs 80 cm-' to the high frequency side of the N2 signature, i.e. (3657-1246) - 2 3 3 1 . An example of the approach is seen in Fig. 6 which displays N~ and H 2 0 signatures at two heights above a symmetric Wolfhard-Parker burner operating on H2 and air.

380

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2500

2100

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Fig. 6. Dual pump-Stokes spectra of N2 and H20 in a laminar Hz-air diffusionflamesustained on a symmetricWolfhard-Parkerburner. A variant of dual broadband CARS has led to a new and very simple approach to pure rotational CARS.6~ Pure rotational CARS is of interest due to its thermometric accuracy at low temperatues, 2° e.g. < 1000 K and the fact that its transitions in N2 remain well separated even at high pressures. Thus collisonal narrowing is avoided and low spectral resolution will suffice. Despite these features pure rotational CARS has not seen widespread diagnostic use since it is cumbersome to implement. To excite pure rotational resonances, the Stokes laser must be placed in close spectral proximity to the pump. However dye laser physics precludes this, Thus with Nd: YAG based CARS systems, one must generate both the third and second harmonics simultaneously at considerable power loss to the latter. The third harmonic is then used to pump a dye laser spectrally adjacent to the pump to excite the rotational resonances. Because of the discreteness of the transitions and spiking in the dye laser amplitude profile, each single pulse spectrum must be referenced to account for the dye profile irregularities. This is done by wave mixing in some standard gas. Hence two spectra must be captured with each laser pulse and spectrally registered to one another, a complexity clearly desirable to avoid. The spectral integration aspect of dual broadband CARS may permit such amplitude irregularities to be averaged out and eliminate the need for referencing at all. In addition, dual broadband CARS for rotational resonances leads to a very simple experimental approach. To perform dual broadband CARS for rotational Raman resonances in the 0-150 cm-~ range, the two Stokes sources have to be considerably overlapped. So overlapped in fact that one can simply employ a single broadband dye laser. Different frequency combinations within the broadband laser excite the rotational Raman coherences from which the a h pump scatters. Quite importantly, as indicated in Fig. 7, the broadband Stokes laser can be arbitrarily positioned in a spectral sense. Thus, third harmonic frequency generation from the Nd: YAG laser is not necessary and precise spectral positioning of the Stokes laser is not required as well. One need know only the amplitude profile shape, not its location. In addition, by employing crossed-beam phase matching according to the Fig. 7 diagrams, either in a plane or folded, very good discrimination against the pump laser can be attained. Discrimination is expedited further by placing the solitary Stokes source at wavelengths beyond 600 nm. This is illustrated in Fig. 8 where the pure rotational CARS spectrum of N: generated using this new approach is displayed. With this method, the CSRS spectrum is also generated simultaneously. Examination of the signal intensity indicates that single shot thermometry in flames should also be feasible. Due to its simplicity, this technique could provide quite useful for spectroscopic investigations. Whether its diagnostic potential is realized vis-a-vis the elimination of referencing is a subject of current investigation.

SUMMARY Nonlinear Raman spectroscopy has received considerable development attention in the last decade due to its extremely high resolution for spectroscopic studies and superior signal/noise ratios

Nonlinear R ~ a n spectroscopy for combustion diagnostics

381

W2

cat

,hhhl, .I.I,h,. CARS

CSRS

• Phase matchlng

• OPticaL arrangement

Fig. 7. New approach to pure rotational CARS in which the pump and Stokes lasers have arbitrary spectral separation.

1.0

"ID

0.5

E o Z

0 50 100 Raman shift [cm-I) Fig. 8. Pure rotational CARS and CSRS spectrum of ambient N2.

-150

-100

- 50

150

in chemical analysis applications. O f the m a n y nonlinear R a m a n techniques, two have emerged as most powerful and useful. F o r diagnostic applications, the primary focus of our attention here, C A R S possesses a number o f characteristics that m a k e it advantageous and preferable to SRGS. The latter is often, but not always, the method of choice for spectroscopic studies. C A R S has utility in this realm as well due to its greater sensitivity at low pressures. The large research investment in C A R S has resulted in m a n y theoretical and experimental advances, s o m e o f which have been reviewed here, and has culminated in its utilization in m a n y fundamental and practical combustion investigations. REFERENCES i. M. Lapp and C. M. Penney eds., Laser Raman Gas Diagnostics, Plenum Press, New York, N Y (1974). 2. S. Lederman, Prog. Energy Combast. Sci. 3, 1 (1977). Q,S.R,T. 40/3---N

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ALANC. ECKBRETI-I

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Nonlinear Ram_an spectroscopy for combustion diagnostics

56. 57. 58. 59. 60. 61.

383

Temperature, Nitrogen and Oxygen Densities in a Turbulent Flame, presented at 22nd JANNAF Combustion Meeting, Pasadena, CA 0985). R. E. Teets, Laser Mode Effects on CARS Spectroscopy, presented at First International Laser Science Conference, Dallas, TX 0985). R. P. Lucht, Three Laser CARS Measurements of Two Species, presented at 1986 APS/OSA International Laser Science Conference, Seattle (1986). J. H. Stufflebram and A. C. Eckbreth, New Concepts for CARS Diagnostics of Solid Propellant Combustion, presented at 23rd JANNAF Combustion Meeting (1986). A. C. Eckbreth and T. J. Anderson, AppL Opt. 25, 1534 (1986). A. C. Eckbreth, T. J. Anderson, and G. M. Dobbs, CARS Approaches to Simultaneous Measurements of H2 and H~O Concentrations and Temperature in H2-Air Combustion Systems, presented at 23rd JANNAF Combustion Meeting 0986). A. C. Eckbreth and T. J. Anderson, Opt. Lett. II, 496 (1986).