December 1983 / Vol. 8, No. 12 / OPTICS LETTERS 599

Spatially and spectrally resolved multipoint coherent anti-StokesRaman scattering from N2 and 02 flows

Judith B. Snow, Jia-biao Zheng,* and Richard K. Chang

Yale University, Center for Laser Diagnostics and Applied Physics, New Haven, Connecticut 06520

Received August 25, 1983

Single-shot spectrally resolved broadband rotational coherent anti-Stokes Raman scattering (CARS) from multi-ple points along a line intersecting a flow has been observed for N2 and 02. A large-angle phase-matching configu-ration was used to achieve spatial resolution of 0.1 mm X 0.1 mm X 0.1 mm for each of 20 points in the interactionvolume. Single-shot multipoint CARS of N2 at different temperatures and of several species in a coflowing jet wasobserved.

One-dimensional and two-dimensional determinationof species scalar parameters in a turbulent flow usingspontaneous Raman techniques has been demon-strated1 -4 in spite of the small Raman scattering effi-ciency for gaseous molecules. Because of its muchgreater scattering efficiency, vibrational coherentanti-Stokes Raman scattering (CARS) has found wideacceptance as a diagnostic tool for single-species andhigh-temperature determination from a single spatiallyresolved point in a combustion chamber or flow field.5' 6

Multipoint vibrational CARS along a line has been re-ported for spectrally unresolved systems.7'8 By usingan average over many laser shots, the total undispersedvibrational CARS signal from a line across a CH 4 jet wasdetected by using interference filters and an opticalmultichannel analyzer (OMA).8

Rotational CARS, because of its closely spaced buteasily resolvable rotational lines, also has potential asa diagnostic tool for multiple species,9 low tempera-tures,10 and high temperatures.11' 1 2 We have investi-gated the feasibility of using spatially and spectrallyresolved rotational CARS to obtain instantaneoustemperature and species measurements from manypoints along a line. We have achieved single-shot,spectrally resolved broadband rotational CARS from20 individual locations (0.1 mm X 0.1 mm X 0.1 mmsample volume) along a line intersecting a flow of N2and 02 gases. For these gases, the rotational Ramanshifts span a range of 5 to 200 cm-1 , with peak separa-tions of the order of several inverse centimeters.

In an essentially dispersionless gaseous medium,there are various phase-matching configurations forrotational CARS.13'14 For multipoint broadband ro-tational CARS, the following criteria should be con-sidered in selecting the optimal phase-matching con-figuration: (1) energy density of the input beams in theinteraction volume, (2) spatial resolution of each volumeelement, (3) spatial separation of the CARS signal fromthe spectrally similar input beams, and (4) coherencelength within the frequency shifts spanned by all therotational lines involved.

The large-angle nonplanar three-dimensionalBOXCARS geometry shown in Fig. 1 was chosen for thiswork. This nonplanar geometry, in which the propa-

gation direction for one pump beam (kL) is exactlyperpendicular to that of the other pump beam (kL') and'to the Stokes beam (ks), has maximum spatial resolu-tion and adequate spatial separation between the CARSbeam and the input beams. Initially, it was thoughtthat at least two beams (e.g., kL and ks) would have tobe formed into sheets. However, the energy density inthe interaction volume can be significantly increasedby having only one input beam (kL) focused into a sheetby a cylindrical lens while keeping the ks and kL' beamsfocused to a weakly converging cylinder with sphericallenses (see Fig. 1). Since the angle between ks and kL'was small (6 = 5.671°), there was substantial overlap ofthese beams with the sheet formed by kL in the cylin-drical interaction volume. The selection of the anglea as 900 uniquely determines the remainder of the an-gles for a given frequency shift. It follows from thephase-matching condition that ( = 5.671°, A = 5.616°,and 0 = 0.558° for CWR = 91.5 cm-', where WR is the fre-quency shift for the N2 J = 10 peak, which is approxi-mately the spectral center of the rotational CARSspectrum at room temperature. The difficulties asso-ciated with this configuration were the weak intensityof the CARS signal generated within each of the ex-tremely small interaction volume elements (about 30times less than in the small-angle single-point config-uration10) and the experimental problems of achievingthe necessary spatial overlap and proper angularalignment of the three input beams.

The sensitivity of the angles with respect to the phase

Fig. 1. Folded large-angle phase-matching geometry formultipoint CARS. The cylindrical interaction region is de-picted as several spatially resolved volume elements.

0146-9592/83/120599-03$1.00/0 C) 1983, Optical Society of America

600 OPTICS LETTERS / Vol. 8, No. 12 / December 1983

mismatch Ak and to the coherence length (lcah = 7r/Ak)were calculated for angular displacements of ks in theyz plane and in the xy plane (see Fig. 1). An angulardisplacement of +0.5' in the yz plane results in 1coh =

0.20 cm, whereas a +0.50 displacement in the xy planeresults in lcoh = 0.02 cm. Thus angular misalignmentin the xy plane is 10 times more critical than in the yzplane. To achieve the correct angular alignment of theinput beams, the appropriate targets and masks weredesigned and fabricated. A fine adjustment to theoverlap of the beams was performed by using a micro-scope objective (1OX) to view the interaction region atlow energies. After this adjustment, the laser energywas increased and single-shot burn patterns were madeon transparent tape in order to confirm completeoverlap of the three input beams. After this final ad-justment, very little improvement in the signal (lessthan a factor of 2) was achieved by further fine adjust-ments of the detected signal. In fact, further adjust-ments most often degraded rather than increased thesignal.

The second harmonic (532 nm) of a Molectron MY-34Nd:YAG laser (-1 cm-' FWHM) provided the twopump beams, (OL and CVL'. The third harmonic (355nm) was used to pump a broadband coumarin 500 dyelaser, which provided the broadband Stokes radiation(wos) of -200 cm-'. The dye-laser output was 14mJ/pulse, and the total energy in the two pump beamswas 100 mJ/pulse. A simple calculation demonstratedthat the maximum CARS signal is generated with equalenergy partitioning for the two pump beams kL and kL'shown in Fig. 1.

The detection system consisted of a 0.5-m spectro-graph (Spex 1870) with vertical slits and a 2400-groove/mm holographically ruled grating. For themultipoint broadband CARS, it was necessary that thespectrograph provide maximum spectral resolutionwhile preserving spatial resolution. Initially, the inputwindow of the PAR 1254 silicon-intensified target (SIT)vidicon was placed at the exit focal plane of the spec-trograph at a position optimized for the maximumspectral resolution in the horizontal dimension. Be-cause of the spectrograph astigmatism, it was necessaryto place a cylindrical lens between the final spectro-graph mirror and the exit plane to achieve good reso-lution in both dimensions. The spatial resolution of oursystem corresponded to 0.1 mm in the interaction re-gion, and the spectral resolution was 3 cm-1 .

To achieve single-shot spatially and spectrally re-solved multipoint broadband CARS spectra of N2 and02, it was necessary to increase the detectivity of theSIT vidicon. A single-stage electrostatic image inten-sifier (Varo 510-1248 with a 40-mm input diameter) wascoupled to the vidicon by a fiber-optics bundle (GalileoElectro-Optics, 100 mm long X 17 mm in diameter).We determined that the overall gain of the intensifiedsystem relative to that of the unintensified SIT wasapproximately 50. The image intensifier introducedadditional pincushion distortion and thus affected theoverall spectral and spatial resolution of our detectionsystem.

Laser-burn patterns indicate that the sphericallyfocused Stokes (ks) and pump beams have diametersof -0.1 mm, whereas the cylindrically focused pump

beam (kL) has a cross section of -0.1 mm X 2 mm (seeFig. 1). The beam profiles were measured at lowerenergies by using a 512-element photodiode array. Theinteraction region for this large-angle three-dimensionalgeometry is a cylindrical volume approximately 0.1 mmin diameter and 2 mm in length (see Fig. 1). This esti-mate is confirmed by the results obtained from theimage size of the multipoint CARS signal on the in-tensified vidicon.

The scan format of the SIT vidicon divided the imageon the vidicon into 20 separate tracks of data, with eachtrack corresponding to -0.1 mm so that all 20 tracksrepresented 2 mm. The first and last tracks of datafrequently have an unacceptable level of noise resultingfrom the way in which the signal is read off the vidicon.For this reason, the multipoint spectra shown in Figs.2-4 have only 17 or 18 tracks of data plotted, althoughall 20 tracks were recorded. Because of the spatialprofile of the kL sheet and because of the diminishedoverlap among the three beams at the edges of the in-teraction cylinder shown in Fig. 1, the intensity of theCARS signal tends to decrease at the outlying tracks.This is not a problem per se for temperature determi-nation, since the relative distribution of the rotationallines is unchanged by the total decrease in intensity atthe two edges in the vertical dimension. However, forconcentration measurements, the overall decrease inintensity has to be taken into account. The asymmetricspike that occurs in Figs. 3(a) and 4 is caused by elec-trical interference from the pulsed laser.

The single-shot multipoint spectrum of room tem-

J02= 7 9 11 13 15 17 19 21

RAMAN SHIFT

Fig. 2. Single-shot multipoint rotational CARS spectrumof room-temperature 02. Each track has a spatial resolutionof 0.1 mmX 0.1mm X 0.1 mm.

toa) (b)

Ix

JN 4

2 6 8 10 12 14 16 JN2 4

6 8 10 12 14 16

RAMAN SHIFT -I RAMAN SHIFT

Fig. 3. Single-shot multipoint rotational CARS spectrumof (a) cold N2 and (b) hot N2, Each track has a spatial reso-lution of 0.1 mm X 0.1 mm X 0.1 mm. Note that the intensitydistribution in the hot-N2 spectrum is shifted to higher Jnumbers (greater Raman shift).

December 1983 / Vol. 8, No. 12 / OPTICS LETTERS 601

0.

JO '7 9 11 13 15 17 19 21 23

RAMAN SHIFT -

Fig. 4. Single-shot multipoint rotational CARS spectrumof coflowing gases in which Freon-12 is the nozzle gas dis-charged from a 1-mm-diameter jet and 02 is the surroundinggas. Each track has a spatial resolution of 0.1 mm X 0.1 mmX 0.1 mm.

perature 02 is presented in Fig. 2. A small nozzle (1 mmin diameter) was used to produce this flow, so the 02concentration is greater in the center tracks. Thisspectrum was not normalized by a reference. Theasymmetry in the rotational envelope is due to the in-fluence of the dye-laser spectral structure and to dis-tortions introduced by the detection system. The sig-nal-to-noise ratio in this single-shot spectrum is cer-tainly sufficient for the temperature determination of02 from a normalized spectrum.

The feasibility of multipoint single-shot CARS of N2at different temperatures was also investigated. Forthe cold-gas experiment, N2 gas was cooled by flowingit through copper tubing immersed in liquid N2. Forthe hot-gas experiment, N2 was heated by flowing itthrough copper tubing wrapped with heating tape. TheCARS spectra of cold N2 and hot N2 are shown in Figs.3(a) and 3(b), respectively. The differences in theseunnormalized spectra are not particularly striking, butit can be seen that the spectrum of the hot N2 is some-what shifted to higher J numbers (larger Raman shift),as expected. No attempt was made to deduce the gastemperature. The signal-to-noise ratio in this spectrumis less than that in the single-shot 02 spectrum becausethe rotational Raman cross section for 02 is almost threetimes greater than the cross section for N2.15

Different species were introduced simultaneously intothe interaction region in order to demonstrate theability of the multipoint CARS technique to spatiallyresolve different volume elements along a line. Thiswas accomplished by using a coflowing jet configurationconsisting of a 1-mm-diameter nozzle surrounded by acoaxial honeycomb. Several combinations of nozzleand surrounding coflowing gases were examined, in-cluding N2 and 02, which have a rotational spectrumoccurring in the same frequency range, and Ar andFreon-12 (CCI2F2 ), which have no spectrum in thisrange. The single-shot CARS spectrum with Freon-12as the nozzle gas and 02 as the coflowing gas is shownin Fig. 4. The absence of a spectrum for the Freon-12nozzle gas is expected to delineate a 1-mm region cor-responding approximately to the central 10 tracks, andthe spectrum of the 02 coflowing gas should appear inthe outermost tracks.

We have demonstrated that it is possible to obtain amultipoint spectrally resolved broadband rotationalCARS spectrum (approximately 10 rotational peaks forN2 and 6 rotational peaks for 02) consisting of 20 spa-tially resolved volume elements (0.1 mm X 0.1 mm X 0.1mm) in a single 10-nsec laser shot. The CARS signalsgenerated from N2 or 02 under these conditions havethe potential of determining scalar parameters (tem-perature and/or species concentration) in a room- orlow-temperature turbulent flow. In particular, themultipoint broadband CARS experiments demon-strated the presence or absence of different species atspecified positions in the interaction volume and thesensitivity to temperature changes. In contrast to vi-brational CARS, pure rotational CARS has the advan-tage of simultaneous detection of multiple specieswithin the spectral range spanned by the OMA (r150cm- 1). The greatest difficulty to overcome is the nor-malization of the single-shot multipoint spectrum sothat absolute gas concentration can be deduced. Mostlikely, a single-point reference would not be adequatefor accurate normalization of a multipoint sample.Therefore, at present, this multipoint broadband CARStechnique is qualitative or, at best, semiquantitative.

We gratefully acknowledge the partial support of thiswork by the National Aeronautics and Space Admin-istration (grant NAG-1-37).

* On leave from the Department of Physics, FudanUniversity, Shanghai, China.

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