Temperature Measurements in Flames at 1000 Hz Using Femtosecond Coherent Anti-Stokes Raman Spectroscopy Daniel R. Richardson and Robert P. Lucht Purdue University, West Lafayette, Indiana Waruna D. Kulatilaka and Sukesh Roy Spectral Energies, LLC, 2513 Pierce Avenue, Ames, IA James R. Gord Air Force Research Laboratory, Propulsion Directorate, Wright- Patterson AFB, Ohio AIAA Aerospace Sciences Meeting Orlando, FL January 7, 2010

Acknowledgments Funding for this research was provided by the Air Force Office of Scientific Research (Dr. Julian Tishkoff, Program Manager), and by the Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base, under Contract No. F D-2329, by the National Science Foundation, Combustion and Plasmas Program under Award Number CTS, and by the U.S. Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences, under Grant No. DE-FG02-03ER Excellent technical assistance from Kyle Frisch at WPAFB.

Outline of the Presentation Introduction and Motivation Fs CARS Measurements of Temperature in Flames: Frequency-Spread Dephasing Decay of the Initial Raman Coherence Impulsive Interaction of the fs Pump and Stokes Beams to Create Giant Raman Coherence at t=0 Used of Chirped Probe to Map Temporal Domain into Frequency Domain: Single-Shot Temperature Measurements Conclusions and Future Work

Ns CARS for Gas-Phase Diagnostics

Broadband Dye Laser Spectrum Single-Shot Multiplex CARS Spectrum

Nonintrusive Coherent Laser-Like Signal Spatially and Temporally Resolved Excellent Gas-Temperature Measurement Technique Nonresonant Background Collisional/Linewidth Effects Data-Acquisition Rates: No Correlation Between Laser Shots at 10 Hz Broadband Dye Laser Mode Noise Ns CARS for Gas-Phase Diagnostics Good Bad

Fs CARS for Gas-Phase Diagnostics Nanosecond CARS using (typically) a Q-switched Nd:YAG laser and broadband dye laser is a well- established technique for combustion and plasma diagnostics Fs lasers have much higher repetition rates than ns Q-switched Nd:YAG lasers: > 1 kHz versus ~10 Hz

Fsec CARS for Gas-Phase Diagnostics For application as a diagnostic probe for turbulent flames, signal levels must be high enough to extract data on a single laser shot from a probe volume with maximum dimension ~ 1mm. How effectively can Raman transitions with line width ~ 0.1 cm -1 line width be excited by the fs pump and Stokes beams (200 cm -1 bandwidth)? Answer: very effectively. How do we extract temperature and concentration from the measured single-shot fs CARS signals?

Ultrafast Laser System for Fs CARS

Optical System for Fs CARS with Mechanically Scanned Probe

Calculated Time Dependence of CARS Intensity with Time-Delayed Probe Beam At t = 0 psec, all Raman transitions oscillate in phase = giant coherence At t > 20 psec, Raman transitions oscillate with essentially random phases

Calculated Time Dependence of CARS Intensity with Time-Delayed Probe Beam

Temperature can be determined from the decay of the initial Raman coherence due to frequency-spread dephasing – Raman transitions oscillate with different frequencies.

Theory for Fitting Time-Delayed Probe Fs CARS Data Input parameters from spectroscopic database Fitting parameters R. P. Lucht et al., Appl. Phys. Lett. 89, (2006).

Fs CARS Experimental Results: Flame Temperatures S. Roy et al.,Opt. Commun. 281, (2008).

Raman Excitation for Fs Pump and Stokes Pulses There is a drastic difference in laser bandwidth ( cm -1 ) and Raman line width (0.05 cm -1 ) for 100-fs pump and Stokes laser pulses. How effectively do the laser couple with the narrow Raman transitions? Can single-laser-shot fs CARS signals be obtained from flames? (The answer is yes)

Coupling of 70-Fs Pump and Stokes Pulses with the Raman Coherence

Numerical Model of fs CARS in N 2 A model of the CARS process in nitrogen based on direct numerical integration of the time-dependent density matrix equations has been developed [Lucht et al., Journal of Chemical Physics, 127, (2007)]. Model is nonperturbative and is based on direct numerical integration of the time- dependent density matrix equations.

Single-Laser-Shot Fs CARS Measurements Most significant difference for fs CARS compared to ns CARS is the potential for data rates of kHz as compared to Hz for ns CARS.

Optical System for Single-Pulse fs CARS with Chirped Probe Pulse Lang and Motzkus, JOSA B 19, (2002).

Optical System for Single-Pulse fs CARS with Chirped Probe Pulse

Single-Shot CPP fs CARS +2 ps Probe Delay

4 Single-Shots with Chirped Probe Pulse Probe Delay = +2 ps 300 K 900 K

Theory for CPP fs CARS Input parameters from spectroscopic database Fitting parameters

Linear chirp parameter Envelope function Theory for CPP fs CARS Source: Swamp Optics website

Theory for CPP fs CARS Calculate time-dependent nonresonant signal: Calculate time-dependent resonant signal:

Theory for CPP fs CARS Fourier transform time-dependent CARS signal:

CPP fs CARS Spectrum in Room Temp Air

CPP fs CARS Spectrum at +2 ps Delay for 500 K Theoretical fit parameters same as for 300 K spectra

CPP fs CARS Spectrum at +2 ps Delay for 700 K Theoretical fit parameters same as for 300 K spectra

CPP fs CARS Spectrum at +2 ps Delay for 900 K Theoretical fit parameters same as for 300 K spectra

CPP fs CARS Spectra at +2 ps Delay

Temperature Histograms from Single- Shot fs CARS

Temperature Histograms from Single- Shot fs CARS from Flames

1 kHz Temperature Data from Flame with a 10 Hz Driven Pulsation

1 kHz Temperature Excursion Data from Flame with a 10 Hz Driven Pulsation

1 kHz Temperature Excursion Data from a Turbulent Bunsen Burner Flame

Conclusions Single-shot fs CARS spectra from N 2 recorded at 1000 Hz from atmospheric pressure air at temperatures of 300, 500, 700, and 900 K, and from laminar, unsteady, and turbulent flames. Signals are very strong and reproducible from shot to shot. Precision and accuracy comparble to best single-shot ns CARS. Theoretical model developed to fit CPP fs CARS spectra temperature. Agreement is excellent for +2 ps probe delay at all temperatures. Other probe delays also show promise.

Future Work: fs CARS Analyze recent single-shot data acquired in a turbulent flame at a data rate of 1 kHz. Signals were excellent but data has a long tail to the high-frequency side due to probe spectrum, complicating data analysis. Implement better characterization/phase control of the laser beams. Pulse shaping/control for the pump and Stokes beams may also be quite useful for suppressing nonresonant background. Develop strategies for optimal strategies for temperature and concentration measurements.

Future Work: fs CARS New laser system from Coherent ordered (AFOSR DURIP Program). Laser rep rate of either 5 kHz with 3 mJ per pulse or 10 kHz with 1.2 mJ per pulse. Nominal pulse durations of either 90 fs or 40 fs. Pulse shaper integrated into the system between the oscillator and amplifier.

Potential Advantages of fs CARS Data rate of kHz would allow true time resolution, study of turbulent fluctuations and of transient combustion events. Data rate of kHz as opposed to 10 Hz would decrease test time considerably in practical applications. Fs CARS, unlike ns CARS, is insensitive to collision rates even up to pressure > 10 bars (fs CARS signal increases with square of pressure).

Potential Advantages of fs CARS Fs laser pulses are near-Fourier-transform limited, noise decreased significantly for single- shot measurements. Because collisions do not influence the spectrum, no Raman linewidths are needed for temperature fitting code.

Chirped Probe Pulse Cross-Correlation with Fundamental Beam

Single-Shot CPP fs CARS +2 ps Probe Delay

Conclusions Single-shot fs CARS spectra from N 2 recorded at 1000 Hz from atmospheric pressure air at temperatures of 300, 500, 700, and 900 K, and from flames. Signals are very strong and reproducible from shot to shot. Theoretical model developed to calculate synthetic CPP fs CARS spectra for comparison with experiment. Agreement is excellent for latest set of data at +2 ps probe delay at all temperatures.

Future Work: fs CARS Implement least-squares fitting routine to obtain quantitative agreement between theory and experiment. Implement better characterization/phase control of the laser beams. Develop strategies for optimal strategies for temperature and concentration measurements.

Future Work: fs CARS Fs lasers with mJ output and rep rates of up to 100 kHz will be available soon, use of these fs laser systems has the potential to revolutionize diagnostic measurements in reacting flows – data rate increase of 2-3 orders of magnitude. New methods will be required for signal generation and analysis. Pulse shaping/control for the pump and Stokes beams may be quite useful for suppressing nonresonant background. Pulse shaping/control for the probe beam may be useful for extracting temperatures and concentrations.

Potential Advantages of fs CARS Data rate of kHz would allow true time resolution, study of turbulent fluctuations and of transient combustion events Data rate of kHz as opposed to 10 Hz would decrease test time considerably in practical applications Fs CARS, unlike ns CARS, is insensitive to collision rates even up to pressure > 10 bars (fs CARS signal increases with square of pressure) Fs laser pulses are near-Fourier-transform limited, noise may be decreased significantly for single-shot measurements

Chirped Probe Pulse (CPP) fs CARS in Argon -2 ps Probe Delay +5 ps Probe Delay 0 ps Probe Delay

Temperature Measurements in Flames