ROLAND E. LEE, ROBERT TURNER, and RICHARD C. BENSON

OPTICAL MEASUREMENTS FOR RAMJET ENGINE DEVELOPMENT

Two optical instruments, the laser anemometer and the coherent anti-Stokes Raman scattering (CARS) spectrometer, were recently studied at APL's Propulsion Research Laboratory for use as combustion diagnostic tools. Recent test results of the laser anemometer used in a typical exhaust flow environment of a combustion chamber and of CARS in graphite surface oxidation studies are discussed.

INTRODUCTION nautics Division, in collaboration with the Research Advanced propulsion systems involve highly com­ Center, is currently reviewing and test-evaluating se­ plex combustion flow processes. Extensive and accu­ lected optical instruments for use in supersonic ram­ rate flowfield measurements can help to guide and jet combustion flow field measurements. Figure 1 lists support engine development. For example, in the hy­ the techniques considered and the flow parameters personic dual-combustor ramjet, the subsonic com­ they are expected to measure. In some cases, an inter­ bustion chamber operating at fuel-rich conditions mediate parameter is used to obtain the desired flow generates a complex multiphase combustion flow parameter; e.g., the laser anemometer measures par­ comprising liquid fuel droplets, soot particles, and ticle velocity, and the gas flow velocity is deduced reacting gases. The complexity of this flow is intensi­ from modeling of particle drag. fied by its subsequent downstream interaction with At the onset of this program, there was very little air at supersonic speeds. In such reacting flows, con­ information on the size, number density, or index of ventional mechanical probes can disturb the local refraction of particles generated by a fuel-rich hydro­ flow environment as well as be subjected to impact carbon combustor. The first two instruments for and thermal damage. Optical instruments that have measuring particle size were selected on the basis of been developed and used in laboratory experiments laser light transmission at two wavelengths and laser of less complex combustion flows avoid these diffi­ light scattering at two viewing angles. They were culties. However, their application in the harsh flow abandoned early in the program because analysis environment of ramjet engine combustion is more showed that the upper range of particle sizes of inter­ difficult and needs to be approached cautiously. In est could not be measured. Follow-on evaluation and contrast to mechanical probes, optical instruments testing showed the laser anemometer and Raman generally are nonintrusive; produce minimum flow spectrometer to be viable instruments that could disturbance; have a faster response, higher data rate withstand the hostile engine-testing environment and capability, and better spatial resolution; and, in some provide data on a number of the pertinent flow pa­ cases, can measure more than one flow parameter per rameters. The laser anemometer observes laser light instrument. scattered by particulates embedded in the flow to de­ Flow modeling requires accurate flowfield mea­ duce the particle and flow velocities, the particle size, surements of stream velocity, temperature and pres­ and the particle number density. The Raman spec­ sure distributions, and, when chemical reactions are trometer relates the laser light scattered by molecules present, the chemical species concentration distribu­ in the combustion medium to flow temperature and tions. For a reacting flow containing a considerable gaseous species concentrations. amount of particulates, the characteristics and distri­ The evaluation of the laser anemometer instrument butions of these particles need to be measured. The included: (a) analysis with laser light scattering (Mie particulate flow is important because it can strongly theory) and light diffraction computer programs; (b) affect both the performance of optical diagnostic de­ calibration with a cold-air supersonic flow (Mach vices and the combustion efficiency. In relatively 3.5) with controlled particle seeding; and (c) actual clean combustion flows, such as most laboratory measurements in a small-scale combustor that can be flames, the particulates are generally of submicro­ operated at pressures up to 4.5 atmospheres, inlet air meter size. In the fuel-rich ramjet flows, however, temperatures from 650 to 870 K, and fuel-air equi­ soot particles and unburned fuel droplets can extend valence ratios between 0.8 and 4.0, using lP-5 fuel. the particle size range from sub micrometer size to as large as 50 micrometers. PARTICLE SIZING In principle, all of the above flow parameters may Various optical techniques have been developed be determined by optical instruments. APL's Aero- that use scattered light to measure the micrometer-

196 Johns Hopkins A PL Technical Digest Flow parameters Intermediate Techniques parameters Laser light Particulate transmission size

Laser light Particulate scattering number density

Laser Flow Particle velocity anemometry velocity Velocity lag

Figure 1 - Optical flow diagnostic Flow parameters and techniques. temperature

Flow Species Raman constituents properties scattering

Species concentration

Top view 90 ~250 mm~250 mml 0mm I focal length I t Flow 00 ? Figure 2 - Laser Doppler velo­ cimeter optical configuration. Photomultiplier Side view 7~L-~_~~\1

size particles found in combustion experiments. In tenuation of a iaser anemometer beam produced by general, all such techniques make assumptions about particles smaller than 100 micrometers. the particle size distribution, particle shape, index of Figure 2 is a schematic diagram of the Thermo­ refraction, and size relative to the wavelength of the Systems, Inc., laser anemometer, which uses a 5-mil­ light source. Some methods use relative amplitude liwatt helium-neon laser, configured for forward 0 measurements; others use absolute measurements. scattering at an elevation angle of 5.8 • The two laser

Some observe the signal from a single particle; others anemometer beams cross at an angle of 2 0 to produce observe the signal produced by a number of particles. an ellipsoidal scattering volume. The measured signal The actual method chosen depends to a large extent is a function of a particle's position in each beam; as on the parameters to be determined and the adapt­ a result, constructive and destructive interference can ability of the instrument to the particular applica­ occur as the particle moves through the beams. It is tion. A standard dual-beam laser anemometer, nor­ often convenient to think of the particle as being illu­ mally used to measure particle velocity, was adapted minated by light and dark fringes, with a spacing of to provide a simultaneous measurement of particle 17.6 micrometers in this case. The minor axis of the size (in the range of 3 to about 100 micrometers) and scattering volume, in the direction of flow, is 250 mi­ speed (550 to 1400 meters per second), the density of crometers; the major axis, perpendicular to the flow, particles greater than about 1 micrometer, and the at- is 2 millimeters. A 250-micrometer aperture in the

Volume 4, N umber 3, 1983 197 R. E. Lee et al. - Optical Measurements/or Ramjet Engine Development collection optics limits the field of view at the center References 1 through 5 describe aspects of the vis­ of the region to 16 fringes. Particles moving at speeds ibility and scattered light measurements that are re­ of 550 to 1400 meters per second will produce scat­ levant to the work described here. tered light pulses from 0.5 to 0.2 microseconds wide Particle sizing by means of the signal visibility is of that are modulated at frequencies from 30 to 80 recent origin, and there has been considerable discus­ megahertz. Figure 3 is a photograph of the laser ane­ sion about its interpretation and application. The vis­ mometer instrument configured for use in the full­ ibility and intensity techniques are both extremely scale ramjet engine combustor. sensitive to the f number of the collection optics and The size of the particle producing the scattered to their location relative to the incident laser beams. light can be derived from the signal visibility (JI), In fact, the visibility and intensity dependence on which is the ratio of the depth of the modulation particle diameter can be tailored by the proper choice of the scattered signal to the mean amplitude of scattering angle and collection aperture. Because [V = (1 max -I min )/(/max + I min ), where I max and I min of this, it is important that the instrument be calibrat­ are the maximum and minimum of the light inten- ed and also that its response function be calculated sities], and from the mean amplitude of the signal for the conditions actually used. directly. The two methods for determining particle Visibility curves computed for spherical particles size are complementary. With the present laser are shown in Fig. 4 for three conditions. Curve A is anemometer, the intensity of the light scattered by calculated for the collection optics centered between small particles « 3 micrometers) is low, but the the laser anemometer beams and in the plane of the visibility is high (about 1); the intensity of light scat­ beams. The visibility is approximated by: tered by large particles (> 20 micrometers) is large, but the visibility is low (less than 0.3). The intensity measurement is absolute; it generally increases monotonically with particle size and can have a large dynamic range. Visibility is a relative measurement, where J , (7rD p / OJ) is the first -order Bessel function of and because the particle size can be multivalued for a (7rD p/oj ), D p is the particle diameter, and OJ is the fr­ given visibility, the dynamic range is generally less inge spacing.' Curve B is calculated using the stan­ than 10. This can present a problem in its application dard diffraction approximation for the detector if the size range of interest is not defined beforehand. mounted 5.8 0 above the plane of the two beams. Cal-

Prism

Beam splitter, reducer, and focusing optics Figure 3 - Photograph of the la­ ser Doppler velocimeter mounted on motorized two-component traverse.

Photomultiplier

Supporting base

198 Johns Hopkins A PL Technical Digest R. E. Lee et al. - Optical Measurements/or Ramjet Engine Development

0.8

~ 0.6 :.0 1x10-7 10-5 'Vi :> 0.4 .~ .;;;;~ E c: ...Q) Q) 0.2 .: ~ "C "C ~ 1x10-8 6 Q) 10- ~ 20 40 80 ... ;: ~ ~ Particle diameter (11m) 1i ~ c: c: ~ ~ Figure 4 - Calculated and measured visibility as a func­ Q) Q) tion of particle diameter. The plotted data, except for A, are ~ ~ 9 for a detector with a 250-millimeter focal length lens, an ap­ 1x10- 10 -7 0 erture of 34 millimeters, and a scattering angle of 5.8 • A is the visibility using the approximation of Ref. 1. B is calcu­ lated using the diffraction approximation. C, shown by the symbol 0, is the calculation using the Mie theory for dioc­ tyl phthalate. 0 indicates points calculated using Mie the­ ory for soot-hydrocarbon particles (n = 1.67-iO.33); • indi­ cates measured values for polystyrene (n = 1.59-iO.00); Particle diameter (micrometers) and • indicates measured values for dioctyl phthalate (n = Figure 5 - Calculated and measured values of the mean 1.49- iO .OO) . scattered intensity as a function of particle size. A is the in­ tensity calculated using the diffraction approximation with the detector centered between the beams in the plane of culations using the diffraction approximation are the laser Doppler velocimeter beams. B is the intenSity cal­ suspect when the measurements are made off-axis culated with the diffraction approximation at an angle of 0 and the index of refraction (n) of the particle be­ 5.8 • C, shown by the symbol 0 , is the intensity calculated comes a factor. In this case, the more exact Mie theo­ with the Mie theory for dioctyl phthalate. 0 indicates ry must be used. 6 The visibility calculated for se­ points calculated for soot-hydrocarbon particles (n = 1.67 -iO.33); • indicates measured values (relative) for lected points using the Mie theory for dioctyl phtha­ polystyrene (n = 1.59- iO.00) ; and • indicates measured val­ late (n = 1.59-iO.00) and for a soot-hydrocarbon ues (relative) for dioctyl phthalate (n = 1.49-iO.00). mixture (n = 1.67-iO.33), representative of partially burned fuel, are indicated in Fig. 4. The diffraction approximation is reasonable for the expected com­ by smaller particles. The erratic results are attributed bustion products, but the effect of the index of to the breakup of the large dioctyl phthalate drops in refraction must be considered in the calibration and the high-speed flowfield. for unburned fuel drops. Figure 5 shows the calcu­ Two selection processes are applied to each laser lated intensities for the same two positions of the col­ anemometer pulse to ensure, as far as practical, that lection optics using the diffraction approximation data from only one particle are being collected and and the Mie theory for dioctyl phthalate and the that the particle is illuminated by the maximum in­ soot-hydrocarbon mixture. tensity of each incident beam. Only scattered light The laser anemometer was calibrated with solid pulses that are less than 0.5 microseconds wide and polystyrene spheres and liquid aerosols. Single 5- to have exactly 16 cycles of modulation are processed. 100-micrometer polystyrene spheres were mounted Figure 6 is a simplified block diagram of the signal on a rotating disc that provided a particle speed of 20 processing used. The laser anemometer signal is fil­ meters per second. In this case, the laser anemometer tered with a low-pass filter to provide the mean scat­ output could be identified with a particle whose size, tered intensity (/max + / min )/2; it is also filtered with a shape, index of refraction, and position in the scat­ bandpass filter and rectified to give (lma x - / min )/2. tering volume were known. The results of individual The two outputs are stretched, sampled at their measurements, plotted in Figs. 4 and 5, are in reason­ peaks, and held for 20 microseconds. The output of able agreement with the Mie theory calculations. A the bandpass filter is also used by two fast discrimin­ Bergulund-Liu aerosol generator was used to provide ators. One provides a pulse for each laser anemome­ dioctyl phthalate aerosols up to 50 micrometers in di­ ter output, above a threshold equivalent to a particle ameter, moving at 3 meters per second (the normal larger than 1 micrometer, to start a time-to-pulse velocity of the aerosols from the generator) or up to height converter and to provide a 0.5-microsecond 400 meters per second when they were injected into a gate and a 20-microsecond hold/reset pulse for the small supersonic wind tunnel. The results at 3 meters peak sense-and-hold circuits. The output of the sec­ per second are plotted in Figs. 4 and 5. Measure­ ond discriminator is a train of pulses equal to the ments in the small wind tunnel agreed with the low­ number of cycles of modulation produced by the par­ speed results for particles of up to 12 micrometers in ticle. If there are exactly 16 pulses, the maximum diameter. The measurements with the larger aerosols number at the center of the scattering volume, the were erratic but generally similar to those produced single channel analyzer stops the time-to-pulse height

Volume 4, NUlI1 ber 3, 1983 199 R. E. Lee et al. - Optical Measurementsfor Ramjet Engine Development

Low-pass filter 0-5 MHz

Bandpass Photomultiplier filter tube 20-100 MHz

0.5 gate

Reset gate

Single­ channel~------~------~ analyzer

Figure 6 - Schematic of particle sizing instrument. converter. The time-to-pulse height converter output, intended application and to uncover problems, re­ which is a measure of particle speed, is also sampled sulting from refractive effects or loss of coherence, and held. The (lmax + I min )/2, (lmax - I min )/2, and associated with the environment. Measurements were time-to-pulse height converter voltages are recorded made with the laser anemometer scattering volume if a single channel analyzer pulse is present. From located on the centerline of the exhaust jet at selected these data, the particle's speed and two measures of distances ranging from 6.4 to 229 millimeters from its size are derived. The output of discriminator No. the combustor nozzle exit. The combustor was oper­ 1 is also counted to indicate the flux of particles pass­ ated at a nominal pressure of 240 kilopascals (35 ing through the scattering volume. pounds per square inch, absolute), an air inlet tem­

The transmission ratio (1110 ), where I is the mea­ perature of 650 to 870 K, and fuel-air equivalence ra­ sured intensity with the particles present and 10 is the tios of 1.6 and 2.6 for the results described here. A intensity when no particles are present, of the laser typical run lasted 30 to 60 seconds, during which time beam through the flowfield was measured to indicate particle size and velocity information was obtained the amount of material with diameters of less than on more than 1000 particles. about 100 micrometers in the flow, evidence of mul­ The spread in the measured particle diameters and tiple scattering or refractive effects, and to reinforce speeds was large; in particular, there were large parti­ the sizing results obtained from the visibility and in­ cles moving at much higher speeds than expected. tensity measurements. The transmission ratio is de­ Figure 7 contains plots of the average diameter, the termined by the integrated effect of particle density average particle speed, and the attenuation coeffi­ (N), extinction efficiency (Qex J, particle area (S), and cient at selected distances from the nozzle for an path length (I) thro~gh the particulate flow: equivalence ratio of 1.6. In the first 50 millimeters, the speed of the particles increased from 260 to 820 I! 10 = exp( - al) (2) meters per second; this is the region of accelerating flow. The particle speed then decreased with increas­ where a, the attenuation coefficient per centimeter, ing distance to 528 meters per second, the lower limit equals NQex! S. The extinction efficiency is close to 2 of the instrument. The particle diameter increased in for the large particles of interest; it is near 1 for the the first 50 millimeters from the nozzle, decreased for great number of smaller particles that contribute to about 100 millimeters, and then increased again far­ the beam attenuation. Because this measurement is ther downstream. an average along the path of the beam through the Since small particles accelerate faster and slow flame, average values have been used, and an extinc­ down faster than large particles, and so enter or leave tion efficiency of 2 has been assumed for all calcula­ the velocity selection window at different times, the tions. The path length was derived from photographs size distribution will be skewed to the smaller-sized of the scattered light. particles near the nozzle and to the larger-sized par­ Tests were made on the fuel-rich ramjet combustor ticles at greater distances. This is clearly the case at to evaluate the suitability of the instrument for the distances greater than 125 centimeters from the noz-

200 Johns Hopkins A PL Technical Digest R. E. Lee et at. - Optical Measurements/or Ramjet Engine Development

=g 1500.------.------.------.------.------.----,- ---,-----,.---, o CJ ....> ~ Vg 'u .2- 1000 Q)"C * > c: E 900~ 8 .... CJ Q) ....> c: Q) 700 'f~ 'u :~~ "' ... ~ 500 ,+-Q) '+- .... 500 ~~ > Q)Q) r-r-l OlE Q) Less than 528 m/s 8.§ 0.08 Q)~- .:: '> -.:: g ~ 0.04

Vo/ullle 4, N Ulllber 3, 1983 201 R. E. Lee et al. - Optical Measurementsfor Ramjet Engine Development

the number of particles in the field of view of the las­ (a) Virtual er anemometer. One 50-micrometer particle is equiv­ / states alent in volume to one thousand 5-micrometer par­ ---- \ ticles, while only ten 5-micrometer particles are needed to give the observed signal. It is not unlikely, + then, that a cluster of 10 particles, all moving at the a same high speed, is in the scattering volume at one 3 "'0 "', = "'0 - "'{ - - ~o time and appears to the instrument as a single large particle. The results of these tests indicate that the instru­ Molecular energy levels W v mented laser anemometer can be used to measure the size and speed of large solid particles in high-speed flows. However, the simultaneous measurement of (b) Inelastic size and speed indicated that particles with large mea­ (Rayleigh) sured diameters were moving faster than expected. These "large" particles are probably a number of small particles moving together in a cluster that re­ mains after the breakup of the initial large fuel drops or the coalescence of the small drops. While the breakup of fuel drops is advantageous for their later combustion, the presence of clusters of small par­ ticles appearing to be large particles makes the inter­ Anti-Stokes pretation of the results more difficult. The simul­ Raman taneous measurement of particle size and speed is essential where particle breakup appears likely. Addi­ tional measurements with optical techniques speci­ Wavelength ---+- fically designed to measure clusters of small-size par­ ticles are required to confirm these conclusions. Figure 9 - Spontaneous Raman scattering. (a) Energy lev­ el diagram and (b) spectrum. The intensities are not to COHERENT ANTI-STOKES RAMAN scale; the Raman bands are actually much weaker. SCATTERING Turning our attention from light scattered by par­ ticles to light scattered by molecules, the most com­ fluorescence. Hence, any incident wavelength can be mon processes are illustrated in Fig. 9. Incident pho­ used, although visible radiation is preferred because tons at frequency Wo interact with molecules and are of the high sensitivity of detectors in the visible re­ scattered over 47r steradians with elastic and inelastic gion and the fact that the intensity of the Raman components. The elastic scattering, referred to in the scattering scales as W 04 . In vibrational Raman scatter­ past as Rayleigh scattering, is the phenomenon that ing, the interaction between the radiation and the results in the blue appearance of the sky and the red sample depends on the vibrational modes of the mol­ sun at sunrise and sunset (because shorter wavelength ecule and is, therefore, species specific. Because the blue light is scattered more than red). Because the energy distribution in the vibrational modes depends process is elastic, the scattered light is of the same on the molecular temperature, the spectral distribu­ frequency as the incident light; it is not specific to the tion of the Raman scattering can be used to measure molecule causing the scattering. Thus, the technique the gas temperature, which is assumed to be equal to can be used for total density measurements but not the molecular temperature. In addition to the W 04 for individual species concentrations. dependence, the Raman scattering intensity is linear­ The inelastic scattering of light by molecules is ly proportional to the species number density and the known as spontaneous Raman scattering and is power of the incident laser. termed rotational, vibrational, or electronic, depend­ Unfortunately, spontaneous Raman scattering is a ing on the nature of the energy change that occurs in very weak effect and usually is not amenable to prac­ the molecule. As illustrated in Fig. 9, Raman scatter­ tical combustion diagnostics. However, the advent of ing consists of weak components at fixed frequency high-power lasers made it possible to devise new separations on both sides of Wo. The frequency sepa­ Raman processes that result in larger signals. One rations are related to the characteristic frequencies of such process is coherent anti-Stokes Raman scatter­ the molecule, as for example, the vibrational fre­ ing (CARS). Although CARS was discovered in the quency, W u ' in Fig. 9. The Raman component that is early '60S, 8 it was not until the mid-seventies when displaced toward a longer wavelength is known as the high-peak-power tunable lasers became generally

Stokes band (ws ) and that toward a shorter wave­ available that investigations of the technique became length as the anti-Stokes band (was). These bands are widespread. Numerous experiments in CARS have the result of a true scattering process; i.e., the inci­ been conducted on laboratory flames, 9- 11 and inten­ dent photons are not absorbed and re-emitted as in sive work is in progress at various development labo-

202 Johns Hopkins A PL Technical Digest R. E. Lee et al. - Optical Measurements/or Ramjet Engine Development ratories to use it as a nonintrusive probe of jet meters. Because frequency doublers are not 100070 ef­ engines. ficient, there is residual 1064-nanometer radiation In CARS two laser beams at the frequencies WI and from the laser that can be utilized by passing it

W 2 interact with the sample molecules in a nonlinear through a second frequency doubler (KD*P crystal). manner to generate a new coherent beam at frequen­ This additional 532-nanometer beam pumps a dye

cy W 3 = 2wI - W 2 ' as illustrated in Fig. 10. When the laser that produces a broadband output centered at frequency difference WI - W 2 is close to the vibra­ W 2 • The dye laser beam and the pump beam are ad­

' tional frequency, W V of a Raman active mode, the justed to the same diameter using Galilean tele­ magnitude of the CARS signal at W 3 becomes very scopes, ensuring optimum focusing of the beams. large. The incident beams, however, must be aligned (The prisms and half-wave plates are used in polar­ so that the nonlinear interaction is properly phased. ization-dependent experiments.) The pump beam is In gases, phase matching can occur when the beams split into two components that can be directed inde­ are collinear. Although this is easy to produce op­ pendently to the focusing lens; a rotatable optical flat tically, the interaction length is rather long, leading allows for minor steering of the dye laser beam. This to poor spatial resolution. The problem is overcome by separating the incident beams and crossing them at the correct phase-matching angles. A narrowband laser at a fixed frequency is used to Frequency-doubled ...... -.l ~---l generate WI' and, since WI - W 2 must be varied to Nd: Y AG laser coincide with a molecular resonance, W 2 must be ob­ tained from a tunable source such as a dye laser.

There are two methods of tuning W 2 to generate the CARS spectrum. In one, W 2 is generated from a nar­ rowband dye laser, and the CARS spectrum is ob­ tained by scanning the dye laser frequency and using a single channel detector. In the other method, W 2 is generated from a broadband dye laser and the entire CARS spectrum can be obtained simultaneously for several species using a multichannel detector. Al­ though the latter approach is not as sensitive, it al­ lows fast time-resolved measurements to be made on fluctuating phenomena. The CARS instrument being tested at APL is mod­ eled after that developed by Eckbreth at the United IO Technologies Research Center. , 12 A schematic of the optical configuration is shown in Fig. 11. A fre­ quency-doubled Nd: YAG laser is used to generate the primary beam (WI) at a wavelength of 532 nano-

(a) 6.) ,-----1 6.) 2 ----~~~=== Sample

(b) Virtual CARS

Molecular energy states

Figure 10 - Coherent anti-Stokes Raman scattering. (a) General approach and (b) energy level diagram. Figure 11 - Schematic of CARS system.

Volume 4, N umber 3, 1983 203 R. E. Lee et al. - Optical Measurements/or Ramjet Engine Development enables the beams to be aligned at the proper angles dicting the amount of material loss resulting from for phase matching, while achieving good-spatial res­ ablation in supersonic ramjet combustors. Prelimi­ olution in the focal region. After being focused into nary measurements using the existing ablation appa­ the medium, the incident beams and the newly gener­ ratus were encouraging in that carbon monoxide pro­ ated CARS beam are recollimated by a collection duced during the oxidation of graphite was detected lens. The CARS beam, which is spatially separated with moderate spatial resolution. It should be noted from the other beams, passes through an aperture that spontaneous Raman scattering could not be used and a cutoff filter and is then detected by an optical in this application because of the extremely bright multichannel analyzer. incandescence from the hot graphite surface. How­ From a systems standpoint, spontaneous Raman ever, the entire CARS signal can be collected using scattering has several advantages over the CARS small solid-angle optics, which eliminated most of technique, the most important being that it is much the incandescent background. Improvements to the simpler in both optical layout and interpretation of ablation apparatus and the CARS system are in pro­ data. The relationship between the Raman signal and gress so that accurate measurements of species con­ the species concentration and temperature is straight­ centration can be made as a function of distance forward. The spatial resolution, approximately 20 above the ablating surface. micrometers for lenses of moderate focal length, is excellent. Unfortunately, those advantages are often CONCLUSIONS outweighed by low sensitivity and, especially in com­ Because of their nonintrusive nature, fast time re­ bustion applications, by background luminosity and sponse, and high spatial resolution, optical measur­ laser-induced interferences such as fluorescence and ing devices are attractive for measuring complex and particulate incandescence. The CARS technique, al­ unstable combustion flowfields. Because of their lim­ though much more complex than the spontaneous itations, however, their application must be scruti­ Raman method, overcomes most of the disadvan­ nized carefully, and they must be well calibrated and tages of the latter when probing combustion process­ adapted to the environment. The importance of these es. CARS is a rather strong process that results in sig­ points was clearly demonstrated in the application of nallevels many orders of magnitude larger than those a laser anemometer for particle sizing. from spontaneous Raman scattering. Because the CARS radiation is itself essentially a laser beam, all of it can be collected. On the other hand, in spontan­ REFERENCES eous Raman scattering the photons are scattered over I w. M. Farmer, "Measurement of Particle Size, Number Densit y and Ve­ 471" steradians and are collected only over a limited locit y Using a Laser Interferometer," Appl. Opt. 11,2603 (1972). 2W. M. Farmer, "Observations of Large Particles with a Laser Interfero­ solid angle. A more important consequence of the co­ meter," Appl. Opt. 13,610 (1974). herent nature of the CARS radiation is that it can be 3 W. M. Farmer, "Visibility of Large Spheres Observed with a Laser Ve­ spatially separated (normally with just an aperture) locimeter: A Simple Model," Appl. Opt. 19,3660 (1980). 4 D. Holve and S. A. Self, "Optical Particle Sizing for in Situ Measure­ from the incident pump beams, and elaborate double ments," Parts I and 2, Appl. Opt. 18, 1632 (1979). monochromators are not required. Because only a SF. Durst, " Review-Combined Measurements of Particle Velocities, Size Distributions, and Concentrations," 1. Fluids Eng. 104,284 (1982). very small solid angle is required to collect all of the 6 G . Grehan and G. Gouesbet, "Mie Theory Calculations - New Pro­ CARS radiation, interferences from background lu­ gress, with Emphasis on Particle Sizing," Appl. Opt. 18,3489 (1979). minosity and laser-induced particulate incandescence 7W. J. Yanta, "Measurements of Aerosol Size Distribution with a Laser Doppler Velocimeter (LDV)," in Aerosol Measurements, NBS special are small. Furthermore, the CARS beam is in a spec­ publication 412, pp. 73-88 (7 May 1974). tral region (the anti-Stokes region) that is free of 8 p . D. Maker and R. W. Terhune, "Study of Optical Effects due to an In­ duced Polarization Third Order in Electric Field Strength," Phys. Rev. laser-induced fluorescence. 137A, 801 (1965). Despite all of the significant advantages of CARS, 9 W. M. Tolles, J . W. Nibler, J. R. McDonald, and A. B. Harvey, "A Re­ there are a few disadvantages. Although the signal is view of the Theory and Application of Coherent Anti-Stokes Raman Spectroscopy (CARS)," Appl. Spectrosc. 31,253 (1977). rather intense, the sensitivity is limited by interfer­ 10 A. C. Eckbreth, P . A. Bonczyk, and J. F. Verdieck, "Combustion Diag­ ences from the nonresonant background. Polariza­ nostics by Laser Raman and Fluorescence Techniques," Prog. Energy tion-dependent experiments have recently been used II Combust. Sci. S, 253 (1979). G. L. Eesley, Coherent Raman Spectroscopy, Pergamon Press, New to suppress this background but at a cost of losing 12 York (1981). some of the desired CARS signal. The spatial resolu­ A. C. Eckbreth, "BOXCARS: Cross-Beam Phase-Matched CARS Gen­ tion is not as good as in spontaneous Raman scatter­ eration in Gases," Appl. Phys. Lett. 32,421 (1978). ing, with typical interaction regions 1 millimeter long and 0.2 millimeter in diameter. Another disadvan­ tage is the complex dependence of the CARS signal ACKNOWLEDGMENTS - We would like to acknowledge the as­ sistance of the following people in the various aspects of this work: on species concentration and temperature; that de­ R. A. Murphy and S. Favin of the Research Center in the particle pendence requires extensive analysis and calibration. sizing experiment; J. Funk and C. E. Stevens of the Propulsion The initial work on CARS at APL has used the Research Laboratory in the actual combustor measurements in the technique to measure the concentration of gaseous test cell; and H. Y. Chiu in the CARS experiments on graphite ab­ lation. We would also like to thank A. C. Eckbreth of the United species near a hot graphite surface in various oxidiz­ Technology Research Center for help in the design of the CARS ing environments. The experimental results will be system and R. Antcliff and O . Jarrett of NASA Langley for help used to improve the theoretical procedure for pre- in the initial checkout of CARS.

204 Johns Hopkins APL Technical Digest