This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/abstract.cfm?URI=ol-34-23-3755. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Peer reviewedPublisher PD

Kiefer, Johannes

Kliewer, Christopher J.

Leipertz, Alfred

Patterson, Brian D.

Seeger, Thomas

Settersten, Thomas B.

Optics Letters

English

This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/abstract.cfm?URI=ol-34-23-3755. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Peer reviewe

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December 1, 2009 / Vol. 34, No. 23 / OPTICS LETTERS 3755Picosecond time-resolved pure-rotational coherentanti-Stokes Raman spectroscopyfor N2 thermometryThomas Seeger,1 Johannes Kiefer,1 Alfred Leipertz,1 Brian D. Patterson,2Christopher J. Kliewer,2 and Thomas B. Settersten2,*1Lehrstuhl für Technische Thermodynamik and Erlangen Graduate School in Advanced Optical Technologies(SAOT), Friedrich-Alexander-Universität Erlangen Nürnberg, Am Weichselgarten 8, D-91058, Erlangen, Germany2Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, USA*Corresponding author: tbsette@sandia.govReceived September 9, 2009; revised October 19, 2009; accepted October 19, 2009;posted November 10, 2009 (Doc. ID 116931); published November 30, 2009Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laserpulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces back-ground caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimi-nation of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotationalspectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing ofRaman coherences. The sensitivity decreases, however, with increasing temperature, and accurate tempera-ture measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.© 2009 Optical Society of AmericaOCIS codes: 300.6230, 320.5390, 300.6290, 120.1740, 120.6780, 190.4380.Pure-rotational coherent anti-Stokes Raman spec-troscopy (RCARS) using nanosecond-duration pulsedlaser systems has proven a useful tool for the mea-surement of gas-phase temperatures. In RCARS,S-branch rotational Raman transitions are excitedresonantly by the difference frequency of two pumplasers. A narrowband probe laser is scattered fromthe excited Raman coherence, and the signal is emit-ted as a coherent beam that is Raman shifted to theanti-Stokes side of the probe. Similarly, in rotationalcoherent Stokes Raman spectroscopy (RCSRS), thepump lasers excite O-branch Raman transitions, andthe signal is generated at the Stokes-shifted fre-quency. Temperature is encoded in the relative inten-sities of the rotational lines in the spectra. In thedual-broadband approach [1,2], a single broadbanddye laser serves as both pump beams. Not only is thecomplete RCSRS/RCARS spectrum generated on asingle shot, but because each rotational Raman co-herence is driven by multiple pairs of laser photonsfrom the same laser, the influence of dye laser modefluctuations are reduced and a better temperature ac-curacy and precision can be achieved [1–3]. In prin-ciple the evaluation of pure-rotational spectra ismuch simpler than for vibrational CARS (VCARS)because of the large separation of pure-rotationallines and the negligible effect of coherent line mixing,which becomes increasingly important for VCARS atincreased pressure. Additionally, the interferencefrom C2 that can hinder VCARS measurements insooting flames [4] can be avoided using RCARS.Nevertheless, background from scattering and non-resonant four-wave mixing can still pose technicalchallenges for applied RCARS measurements. Be-cause the RCARS signal is spectrally close to theprobe laser, elastically scattered light often over-whelms the RCARS signal when focused beams ofhigh-energy Q-switched nanosecond lasers are used0146-9592/09/233755-3/$15.00 ©to produce adequate signal. The different approachesthat are commonly used to reduce the background,e.g., holographic notch filters [5] or polarizationschemes [6], either reject part of the RCARS spec-trum or significantly reduce the RCARS signal inten-sity. In diffusion flames, a strong, varying back-ground from nonresonant four-wave mixingcoherently interferes with RCARS. The nonresonantinterference can be suppressed by selecting appropri-ate polarizations for the incoming beams [7,8], butpolarization scrambling from scattering and birefrin-gence in the sample and optical system can be a prob-lem for practical applications. Alternatively the non-resonant contribution can be suppressed by usingultrashort laser pulses and delaying the probe pulse[9]. Roy et al. [10] recently used this approach toeliminate nonresonant interference from broadbandVCARS for gas-phase temperature measurements.The current work investigates time-resolvedRCARS for N2 thermometry. The probe and pumppulses were produced by a regeneratively amplifiedmode-locked Nd:YAG laser and a broadband dye la-ser [11] centered at 633 nm. The pulse widths wereapproximately 100 ps. The broadband output wassplit into two equal pump beams, and the relative de-lay between the pump pulses was zeroed using a me-chanical delay. A mechanical delay in the probe beampath varied the relative delay between the pumppulses and the 532 nm probe pulse from −280 ps to+660 ps. The RCARS probe volume was formed usinga folded BOXCARS geometry with a 750 mm focallength lens. A 1 m spectrometer equipped with a1800 line/mm grating dispersed the RCARS signal,and a backilluminated CCD camera recorded thespectra with a dispersion of 0.44 cm−1/pixel.RCARS spectra from pure N2 at 294 K and 1 barand from N2 at 1100 K on the fuel-rich side of an2009 Optical Society of America3756 OPTICS LETTERS / Vol. 34, No. 23 / December 1, 2009atmospheric-pressure laminar CH4 diffusion flamewere acquired for a range of probe delays. Addition-ally, nonresonant four-wave mixing signals from pureAr at 294 K and 5 bar were recorded. RCARS spectrawere normalized by the nonresonant spectrum withthe probe delay set to zero to account for the spectralprofile of the broadband dye laser. Even moderatepulse energies of 500 J for the probe pulse and200 J for each broadband pump pulse producedspectrally integrated single-shot signals of 2106counts from N2 at 294 K and 1 bar and 5103 countsin the flame. Figure 1 presents spectrally integratedRCARS signals as a function of delay. Because thenonresonant signal is generated only when two pumpphotons and a probe photon simultaneously drive thefour-wave mixing process, the Ar signal representsthe temporal convolution of the probe intensity withthe square of the pump intensity. The FWHM of thenonresonant signal is approximately 125 ps, which isconsistent with Gaussian laser pulses with a FWHMof 100 ps. On the other hand, RCARS signals can begenerated when the probe pulse does not temporallyoverlap the pump pulses. For delays greater than200 ps, the RCARS signals decay exponentially withtime constants of approximately 60 ps for N2 at294 K and 1 bar and 170 ps for N2 in the flame. Theslower decay observed in the flame results fromsmaller collisional dephasing rates in the flame.It is possible to use a probe delay that reduces thenonresonant interference to a negligible level whilecausing only a relatively small RCARS signal loss.For example, a probe delay of 200 ps reduces the non-resonant Ar signal by 3 orders of magnitude from itspeak value, but it reduces the RCARS signal frompure N2 at 294 K and 1 bar only by a factor of 10. TheRCARS signal loss is even less significant at highertemperatures. Figure 2 displays spectra acquired in amixture of 5% N2 in Ar at 294 K and 1 bar. At zerodelay [Fig. 2(a)], interference between the nonreso-nant and RCARS contributions causes the pro-nounced asymmetry in line shapes. Figure 2(b) showsthat a 200 ps delay effectively eliminates the inter-ference.The use of picosecond lasers can also reduce therelative background caused by elastically scatteredprobe photons. Because elastic scattering typicallyoverwhelms the RCARS signal for measurements us-Fig. 1. (Color online) Spectrally integrated RCARS signalplotted as a function of the pump–probe delay.ing nanosecond-duration laser pulses, a double-spectrometer or a Raman filter is often used to sup-press the background. The use of a filter typicallyrejects data for Raman shifts of less than50–100 cm−1. In the current work, a filter was notnecessary, enabling detection of the complete RCARSand RCSRS spectra. In Fig. 2(a), elastic scattering iscomparable with the RCARS signal for a N2 partialpressure of only 0.05 bar; the scattering was negli-gible compared with the signal in room air. Increasedsignal-to-background ratios are possible, because theRCARS signal power varies quadratically with thepump intensity and linearly with the probe intensity,while the background from elastic scattering varieslinearly with the probe pulse energy. For example,time-integrated RCARS signals using lasers with apulse duration of 10 ns and a pulse energy of 10 mJshould be similar to those using lasers with a 100 psduration and an energy of 0.5 mJ, but backgroundfrom elastic scattering will be 20 times larger for thenanosecond-laser-based measurements.It is important to consider the accuracy of tempera-ture measurements based on delayed spectra. Astraightforward analysis employs a frequency-domain contour-fitting procedure that has been usedextensively for nanosecond-laser-based RCARS [12]but does not explicitly account for J-dependentdephasing of coherences that is caused by collisionsoccurring in the time interval between the pump andprobe pulses. Figure 3(a) displays the room-temperature N2 RCARS spectrum for zero delay, andFig. 3(b) shows the spectrum for a 360 ps delay.Also shown are best-fit theoretical spectra. Althoughthe spectrum for zero delay produces an accuratetemperature measurement, the delayed spectrum isconsiderably hotter. Figure 3(c) demonstrates thattemperature evaluation is relatively accurate294±13 K for short delays of less than 150 ps, forwhich the probe pulse still overlaps the pump pulses.At the longer delays required for significant suppres-sion of nonresonant interference, however, the evalu-ated temperatures increase with increasing delay.This trend is caused by the J dependence of dephas-ing rates, which decrease significantly with increas-ing rotation for N2–N2 collisions at 294 K [13]. Be-cause collisions dephase low-J Raman coherencesFig. 2. Spectra (corrected for dark counts) from 5% N2 inAr at 294 K and 1 bar for delays of (a) 0 ps and (b) 200 ps.faster than high-J coherences, the RCARS spectrumDecember 1, 2009 / Vol. 34, No. 23 / OPTICS LETTERS 3757becomes hotter with increasing probe delay, and theevaluated temperature is too large if this effect is notincluded in the calculation of the spectra.At increased temperatures, the dephasing ratesare not only smaller, but they also are much less de-pendent on J [13]. For illustration, Fig. 4 presentsRCARS spectra from the fuel-rich side of the diffu-sion flame. For zero delay, nonresonant interferenceis clearly evident, but a probe delay of 260 ps sup-presses the interfering background, producing aclean N2 spectrum. Similar spectra were obtained fora range of probe delays greater than 150 ps to ensureadequate rejection of the background, and Fig. 4(c)plots the best-fit temperatures as a function of theprobe delay. In contrast to the 294 K measurements,the evaluated temperatures in the flame show only aFig. 3. (Color online) Fitted experimental spectra from N2at 294 K and 1 bar for delays of (a) 0 ps and (b) 360 ps. (c)Evaluated temperatures as a function of delay.Fig. 4. (Color online) Spectra obtained on the fuel-richside of a CH4 flame for delays of (a) 0 ps and (b) 260 ps. (c)Evaluated temperatures as a function of delay.minor systematic increase 4%  for delays increas-ing from 150 to 500 ps. To check the consistency ofthe delayed-probe temperature measurement with astandard RCARS measurement, the background wasinstead eliminated using a polarization approachwith zero probe delay [8]. The evaluated temperaturewas 1120 K, in excellent agreement with time-delayed-probe measurements.In summary, time-delayed picosecond RCARS is auseful tool for suppression of nonresonant back-ground. Because collisions that occur in the time in-terval between the pump and probe pulses can sig-nificantly change the relative shape of the recordedRCARS spectrum, a time-dependent model will bedeveloped for robust and accurate analysis of time-delayed spectra. The effect of these collisions, how-ever, is mitigated at increased temperatures, andtemperature evaluation with accuracy comparablewith nanosecond RCARS was demonstrated using anestablished frequency-domain analysis at 1100 K ina flame. The use of short pump pulses was alsoshown to reduce significantly the relative back-ground caused by elastic scattering of the probepulse, enabling collection and analysis of the entireRCSRS and RCARS spectra without the use of a Ra-man filter or polarizers.Funding provided by the U.S. Department of En-ergy, Office of Basic Energy Sciences, Division ofChemical Sciences and the Bavaria California Tech-nology Center. Sandia is a multiprogram laboratoryoperated by Sandia Corporation, a Lockheed MartinCompany, for the U.S. Department of Energy’s Na-tional Nuclear Security Administration under con-tract DE-AC04-94AL85000.References1. A. C. Eckbreth and T. J. Anderson, Opt. Lett. 11, 496(1986).2. M. Aldén, P.-E. Bengtsson, and H. Edner, Appl. Opt.25, 4493 (1986).3. T. Seeger and A. Leipertz, Appl. Opt. 35, 2665 (1996).4. S. P. Kearney, K. Frederickson, and T. W. Grasser,Proc. Combust. Inst. 32, 871 (2009).5. J. Bood, P.-E. Bengtsson, F. Mauss, K. Burgdorf, and I.Denbratt, presented at the International Spring Fuelsand Lubricants Meeting and Exposition (SAEInternational, 1997), Paper 971669.6. F. Beyrau, A. Bräuer, T. Seeger, and A. Leipertz, Opt.Lett. 29, 247 (2004).7. L. A. Rahn, L. J. Zych, and P. L. Mattern, Opt.Commun. 30, 249 (1979).8. F. Vestin, M. Afzelius, and P.-E. Bengtsson, Proc.Combust. Inst. 31, 833 (2007).9. A. Laubereau and W. Kaiser, Rev. Mod. Phys. 50, 607(1978).10. S. Roy, T. R. Meyer, and J. R. Gord, Appl. Phys. Lett.87, 264103 (2005).11. S. Roy, T. R. Meyer, and J. R. Gord, Opt. Lett. 30, 3222(2005).12. T. Seeger, F. Beyrau, A. Bräuer, and A. Leipertz, J.Raman Spectrosc. 34, 932 (2003).13. L. Martinsson, P.-E. Bengtsson, M. Aldén, S. Kröll, andJ. Bonamy, J. Chem. Phys. 99, 2466 (1993).

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N-2 thermometry

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Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N-2 thermometry

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