IL NUOVO CIMENTO VOL. 14 D, N. 10 Ottobre 1992

Four-Wave Mixing in OH: Comparison between CARS and DFWM (*).

H. BERVAS(1), B. ATTAL-TRt~TOUT(1), L. LABRUNIE (1) and S. LE BOIT~UX(2)

(1) Office National d'Etudes et de Recherches Agrospatiales BP 72, 92322 Chatillon Cedex, France (2) Laboratoire de Physique des Lasers, Institut Galilde, Universitd Paris-Nord Av. J.B. Clgment, 93430 Villetaneuse, France

(ricevuto il 31 Luglio 1992)

Summary. -- A comparison between CARS and Degenerate Four-Wave Mixing experiments on OH is performed in a premixed CHn-air flame. Both optical phase conjugation and forward BOXCARS geometries are studied in DFWM. The relative properties of these two configurations are experimentally investigated from the study of the P and Q branches of the (0--0) band.

PACS 42.65.Dr - Stimulated Raman scattering and spectra; CARS. PACS 42.65.Ma - Nonlinear mixing.

1. - I n t r o d u c t i o n .

Degenerate Four-Wave Mixing (DFWM) has been shown to be a very attractive method for nonintrusive optical diagnostics[I-3]. DFWM experiments have been performed on the OH radical, produced in a flat, high-pressure, premixed CH4-air flame. Resonances in the A - X transition of OH are probed in the P and Q branches of the (0--0) band. A comparison between resonance-enhanced CARS and DFWM is presented. The latter technique has been used both in optical phase conjugation and in a forward geometry [4].

2. - P h a s e c o n j u g a t i o n e x p e r i m e n t .

2"1. Set-up. - The experimental set-up of the phase conjugation arrangement is presented in fig. 1. Three beams, namely Forward (F), Backward (B) and Probe (P), are combined in a conventional geometry[5]. The total output power

(*) Paper presented at the ~,XI European CARS Workshop,, Florence, Italy, 23-25 March, 1992.

71 - Il Nuovo Cimento D 1043

1044 H. BERVAS, B. ATTAL-TRETOUT, L. LABRUNIE and S. LE BOITEUX

e,a er

~ ~ G ' ~t~ ~ R P

r

Fig. 1. - a) Optical-phase-conjugation experimental set-up. RP: half-wave retardation plates; E: electronics; PS: motor power supply; SM: stepping motor; T: trigger; S: 50% beam splitter. @ vertical polarization; //horizontal polarization.

of the frequency-doubled dye laser is i mJ at 310 nm with a linewidth (FWHM) of 0.15cm 1.

The beams are focused into the flame with a 600 mm focal-length lens (L). The crossing angle between F and P is 1 degree at most. The overlap volume has a diameter of about 200 ~m and a 2 cm length which permits a good spatial resolution vertically.

The polarization arrangement selected for the experiment is shown in fig. 1 with beam geometry. We choose crossed polarizations for P and F from which mainly orientational (coherence) gratings will be induced [6]. Since polarizations of P and PC are crossed in this case, a Glan-Taylor polarizer (G) is used to reflect the signal on the probe beam path. The DFWM signal is made to travel over 5 metres through a 200 [~m pinhole placed in front of an optical fibre of 1 mm diameter. The signal is detected by a photomultiplier (PM) (RTC, XP 2020Q). Part of the laser light is directed to a reference PM to monitor the laser power. A computer (PDP) drives the scan of the laser frequency in fine steps of 0.1 cm -1. At each spectral point, an average of 50 shots is taken.

The burner (B) is the same as that used for resonance CARS measurements on the OH radical in high-pressure flames. A complete description of this burner is given in ref. [7].

2"2. R e s u l t s . - Spectral scans of the P1 (7.5)-P2 (5.5) doublet obtained in DFWM are shown in fig. 2 together with the [01PIP1 ] 7.5 CARS line originating from the same rovibrational level [5]. The spectral scanning is performed at a height of 1.0 mm above the burner surface; the methane and air flows are, respectively, 0.53 and 5.21/mn, at standard temperature and pressure. All the spectra are normalized to the

line maxima. The vertical scale gives the peak experimental value of P~/~pc/Pr~e~ in arbitrary units. The numerical calculation of the DFWM spectra (solid line) is performed according to the known expression of the nonlinear susceptibility

FOUR-WAVE MIXING IN OH: COMPARISON BETWEEN CARS AND D F W M 1045

1- (01PIP1)7.5

I I I I 3064 3065 3066 3067

~ov w2 (cm-1)

P1 (7.5) 1- . ~

b) c , l

0 - ' ~ - . . . . - . . . . . . [ ' I l

32 125 32 123 32 121 oJ(cm -1)

Fig. 2. - Spectral profile at i bar: a) [OPP] 7.5 CARS line of OH [7]; b) P1 (7.5) and P2 (5.5) DFWM lines.

Z(3)(w; ~, co , -o J). This calculation takes into account the collisional and Doppler effects and the spectral broadening by the laser [5]. Because of the high selectivity of resonance CARS which uses three different laser frequencies, only one spin component is observed in fig. 2, namely O1 (7.5) Raman line [8]. The CARS linewidth appears to be 0.43 cm 1 while the DFWM linewidth is about 0.29 cm -1 mainly due to a slightly broader laser linewidth function [7]. Comparison can be made from the above spectra. First, we noticed that signals are more instable in DFWM. This has ' two origins: i) the fluctuations of the dye laser modes [9] and ii) the beam steering in the flame which results in a disruption of the pump beams superposition varying from shot to shot. These effects have critical consequences in phase conjugation compared to CARS. Secondly, we have compared signal-to-noise ratios. In DFWM, the input power densities applied to the flame were close to the saturation limits with PF----

80kW/cm 2, Pp ~ 40kW/cm 2 and PB ~ 400kW/cm2- The product PFPBPp which drives the signal amplitude is of the same order of magnitude (within a factor 3 to 5) as the P1P2P3 product of resonance CARS experiment[7]. It is thus found that the amplitude of the resonant signal measured in CARS is comparable to that measured in DFWM. However, the background level caused by stray light in DFWM can reach very high levels. Since the origin of this noise obviously is the scattering of the laser light by the optics, all sorts of precautions must be taken. The most efficient background suppression is obtained using crossed polarizations for P and PC which are collinear in conjunction with an analyser on the detection path. Since B is travelling in the same sense as PC, it also provides a major source of noise. Therefore, crossed polarizations for B and PC lead compulsorily to the final polarization arrangement of fig. 1. Nevertheless, the background level is higher in optical phase conjugation than in CARS (fig. 2).

The flame investigation at higher pressure has been pursued by registrating i) the variation of the line profile vs. pressure and ii) OH spatial distribution above the flame front. The spectral evolution of the line profile vs. pressure is studied at known flame conditions and at 1 mm above the burner surface (fig. 3). Spectral scans of the P doublet are collected from 1 to 15 bar. As pressure is increased, careful attention is paid to the alignment of the pump beams in order to partly correct the beam steering effects. Moreover, the decrease in line intensity with pressure is compensated by enhancing the laser power. Nevertheless, this power is kept under the saturation threshold which increases with pressure; it reaches 4.4 MW/cm 2 at the centreline at

1046 H. BERVAS, B. ATTAL-TRETOUT, L. LABRUNIE and S. LE BOITEUX

I-

... 7 bar

I I I I I

1 - 0 . 1 5 ~

I I I ] I

2"i r" I I i I I

01 I I I I I

1 - 6.6

I I

32 125

3 bar i I

32 '123 32 121

%~

.... J ~..j9.6 bar

ol " " '125 ' ' J , 32 32 123 32 121

~o(cm -1)

Fig. 3. - OH spectra of the P] (7.5)-P2 (5.5) doublet between 1 bar and 15 bar in the flame.

20 bar (FFWHM = 1.6 cm -1 ). In fig. 3, the noise on the spectral profile increases from 1 to 15 bar, since beam steering becomes more drastic. In spite of this inconvenient, the signal-to-noise ratio is still about 5 at 15 bar. Here again, noticeable differences with CARS can be pointed out for which the main source of noise is the nonresonant and coherent background due to the flame nitrogen. The latter mainly appears at 10 bar and increases rapidly up to 20 bar [10]. In DFWM, the stray-light level is obviously independent of pressure and constant if the laser is kept at the same power. However, in fig. 3, the background increases with pressure because of the larger laser power used from 1 to 15 bar. This represents the intrinsic difference between the origins of background in DFWM and CARS.

We have also measured an S/N ratio of about 100 at 1 bar with the previous laser power. We estimate the DFWM detectivity from these spectra to be about 7.1013 cm -3 at 1 bar and 3.1016 cm -3 at 15 bar.

The variation of the concentration as a function of height above the burner is measured in fig. 4. The laser frequency is set to the top of the P1 (7.5) line. The burner is subsequently translated in 0.5 mm steps from 0.5 to 6 ram. A 400-shot average is taken at each position.

The spatial distribution of OH above the flame front is measured at 1, 5 and

FOUR-WAVE MIXING IN OH: COMPARISON BETWEEN C A R S AND D F W M 1047

8

v

i 2-

d'

"R.

�9 �9 �9 �9 �9 10 bar �9 �9

5 b a r

�9 �9

1 b a r �9 � 9 �9 �9 �9 �9 �9 �9 �9 �9 �9 �9

I ! l l ! !

0 2 4 6 h (mm)

Fig. 4. - Vertical profiles of OH relative concentration at 1, 5 and 10 bar.

10 bar. The OH concentrations in fig. 4 are relative to the one measured at i bar and 6mm. The latter value is given by a flame code calculation which predicts 1-1016 cm -3 [5].

3. - Folded BOXCARS experiment in DFWM.

3"1. E x p e r i m e n t a l details. - A forward configuration of the three input beams has been used in a second set of experiments by displacing the backward channel of fig. 1 in the forward direction. The resulting folded BOXCARS arrangement is shown in fig. 5a). The signal (beam 4) is emitted symmetric to beam 1 (probe in phase conjugation) in the forward direction in that particular case as in CARS. In the one bar flame, the methane and air flows are, respectively, 0.40 and 4.2 l/mn.

3"2. Resul ts . - The P doublet obtained with forward geometry is shown in fig. 5b). The lines are broader (N 0.50 cm-1) due to the full one-photon Doppler width which affects all the resonant terms of Z (3) (oJ; o~, co, -co) in that case. It should be noted that the final Doppler effect in forward DFWM is even broader than in CARS [11] due to the differences between Raman and one-photon Doppler widths.

The signal amplitude is similar to that measured in phase conjugation. However, an improved rejection of the stray light is achieved, since the signal path has its own direction. Moreover, efficient filtering of the laser light can be realized using the

1048 H. BERVAS, B. ATTAL-TRETOUT, L. LABRUNIE and s. LE BOITEUX

|

a) 2 ~ ~

P1(7.5) 1 - b)

% ~'~ _

0 I I I I ....... 1""

32 125 32 123 32 121 o)(cm -1)

Fig. 5. - a) Folded BOXCARS geometry and polarization of the three input beams in DFWM; (~ vertical polarization; / /hor i zonta l polarization, b) Pi (7-5)-P2 (5.5) D F W M spectra at 1 bar.

polarization arrangement of fig. 5a). In the forward direction, the beam which has parallel polarization with the signal gives the stronger noise contribution. Therefore, beam 2 also have the lower energy, between 10 and 40 kW/cm 2, to minimize the stray-light level. The residual background level in fig. 5b) becomes even lower than in CARS at i bar.

The forward configuration has another basic advantage, since counterpropagating beams drastically enhance the beam steering effects through the flame. As a result, the spectral profiles are much less noisy in fig. 5b). The spatial distribution of OH measured in forward DFWM is shown in fig. 6. Concentrations calculated from a flame code are also shown (solid line). In these profiles, the experimental concentration maximum has been normalized to the maximum value predicted by the flame code for a cold-gas velocity of 25cm/s, viz. 1.8.1016 cm -3. The standard deviation on a 400-shot average is about 5% and is therefore comparable to that measured in CARS [7]. Fluctuations caused by the beam steering only appear at higher pressure quite -normally- and give a standard deviation of 15% to 20% at 10 bar with an S/N ratio of about 10.

Another intrinsic advantage of BOXCARS is that the level of stray light only increases very near the burner surface. The signal is still significative at about 130 ,~m from the position where the beams start to hit the porous, whereas in optical

FOUR-WAVE MIXING IN OH: COMPARISON BETWEEN CARS AND D F W M 1049

2.0

1.5

1.0 X

0.5

J 0.0 1 2 3 4 5 6

h (ram)

Fig. 6. - Experimental OH concentrations in the flame at I bar obtained using forward DFWM in a folded BOXCARS geometry. The full curve is the calculated OH profile using a flame code. Experimental values have been normalized by the calculated maximum value 1.8.10 ~6 cm -3.

phase conjugation the stray light swamps the signal at 380 l~m from that position. We have thus used a lower step for translating the burner between 0 and 2 mm in fig. 6 in order to properly observe the decrease in signal when the beams are located just under the reaction zone and above the porous.

Finally, the band head of the Q2 branch is scanned over 12 cm -1 spectral range using the two techniques (fig. 7). These spectra illustrate one of the main differences existing between the two techniques. Yet, in optical phase conjugation, because of the two counterpropagating beams, the signal originates only from the zero-velocity class, and exhibits a Doppler selective lineshape. However, as in CARS, forward geometry is submitted to Doppler broadening. This property appears very clearly on the spectra shown in fig. 7, the line spacing between Q~ (2.5) and Q2 (1.5) is just under

1

%

o ~

Q1(7.5)

Q2(2.5)

a)

Q2(1.5)

1-

Q2(3.5)

o 32'356 ' 32 346 32 358

o) (cm -1) 32 358 32 354'

Q2(2.5) b) Q1(7.5) Q2(1.5)

J 32 354'

Q2(3.5)

Q 1 2 ~

'32;5d ' 32346

Fig. 7. - Q2 band head of OH. a) Optical phase conjugation: rotational line strengths used in the calculated spectra (full curve) are tabulated in [12]. b) Forward DFWM in folded BOXCARS geometry.

1050 H. BERVAS, B. ATTAL-TRETOUT, L. LABRUNIE and S. LE BOITEUX

the resolution limit of the apparatus in fig. 7a); in fig. 7b), the laser linewidth function is identical, whereas the total linewidth increases with additional Doppler effect and leads to a broad unresolved peak. Theoretical spectra take into account the rotational line strengths of DFWM using crossed polarizations[12]. A satellite line Q12 (3.5) appears in forward geometry. To fit this latter, we have used an intermediate case between Hund's cases a) and b) for the ground state 2//[13, 14]. Here, the satellite line is much stronger than in the pure Hund's case b) and two times less intense than in the pure case a).

4 . - Conclusion.

We have presented a study of OH in a flat, laminar, premixed CH4-air flame by the DFWM method. Spectra were obtained in the P branch over the pressure range (1 + 15)bar. The fitting of the experimental spectra was done under nonsaturating conditions by calculating the resonant DFWM susceptibility with Doppler and collisional broadening. We measured OH relative concentrations above the reaction zone with a vertical resolution of 200 ,~m.

Differences between optical phase conjugation and forward DFWM have been studied in detail in order to compare signal-to-noise ratios, signal fluctuations and noise levels. The folded BOXCARS arrangement seems to have more advantages for diagnostic purposes in combustion. CARS and this latter DFWM technique allow easy applications for species detection at high pressure.

The authors would like to thank Y. Prior for helpful discussions.

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(1981). [5] H. BERVAS, B. ATTAL-TRI~TOUT, S. LE BOITEUX and J. P. TARAN: J. Phys. B, 25, 949 (1992). [6] R. FISHER: Optical Phase Conjugation (Academic Press, New York, N.Y., 1983). [7] B. ATTAL-TRETOUT, S. C. SCHMIDT, E. CRETE, P. DUMAS and J. P. TARAN: J. Quant.

Spectr. Radiat. Transfer, 43, 351 (1990). [8] B. ATTAL-TRETOUT, P. BERLEMONT and J. P. TARAN: Mol. Phys., 70, 1 (1990). [9] J. COOPER, A. CHARLON, D. R. MEACHER, P. EWART and G. ALBER: Phys. Rev. A, 40, 5705

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TARAN: Coherent Raman Spectroscopy, Proceedings of the Samarkand Conference on Nonlinear Spectroscopy, edited by G. MAROWSKY and V. SMIRNOV (Springer, Heidelberg, 1992).

[11] H. BERVAS: thesis, to be published. [12] H. BERVAS, S. LE BOITEUX, L. LABRUNIE and B. ATTAL-TR]~TOUT: to be published. [13] G. HERZBERG: Molecular Spectra and Molecular Structure: L Spectra of Diatomic

Molecules (Van Nostrand Reinhold, New York, N.Y., 1950). [14] L. T. EARLS: Phys. Rev., 48, 423 (1935).