Hybrid femtosecond/picosecond coherent anti‐Stokes Raman scattering for high‐speed CH₄/N₂ measurements in binary gas mixtures

Abstract

Hybrid femtosecond/picosecond (fs/ps) vibrational coherent anti‐Stokes Raman scattering was demonstrated for high‐speed, in‐situ measurements of CH4/N2 mole fractions in binary gas mixtures with temporal nonresonant suppression. A single broadband, 100‐fs laser source was used to simultaneously excite CH4 and N2 vibrational Raman transitions, characterized by a spectral separation of ~584 cm−1, followed by frequency‐domain detection with a time‐delayed ps probe pulse. In this manner, the temporal evolution of the nonresonant background and vibrational wavepackets of CH4 and N2 could be evaluated to maximize signal and minimize interferences. A typical accuracy of ~95% was achieved for experimental CH4 mole fractions between 0.02 and 0.70, and a precision of ~4% was achieved by maximizing relative excitation energy through induced chirp in the pump and Stokes pulses. It was further shown that mole‐fraction measurements were not affected by variations in bi‐molecular broadened linewidths over a wide range of mixture fractions. Copyright © 2013 John Wiley & Sons, Ltd.

Time- and frequency-dependent model of time-resolved coherent anti-Stokes Raman scattering (CARS) with a picosecond-duration probe pulse

“…1,3 As such, this detection approach retains the chemical specificity associated with Raman spectroscopy 1,3 with excellent detection sensitivity. 4 The spectral resolution of hybrid fs/ps CARS signal, furthermore, allows single-shot thermometry and multispecies measurements in reacting flows at repetition rates in the kHz range, 5,11,12,28 and several hybrid fs/ps CARS schemes have been used to probe gas-phase vibrational 5,6 and a) Author to whom correspondence should be addressed. Electronic mail: hans.stauffer@gmail.com rotational [7][8][9][10][11][12][29][30][31] CARS spectra.…”

Dual-pump vibrational/rotational femtosecond/picosecond coherent anti-Stokes Raman scattering temperature and species measurements

“…Additionally, purerotational CARS has been implemented to measure temperature with errors of 1.8% and 2.5% at 1400 and 2283 K, respectively, with spectral focusing and secondharmonic bandwidth compression of the ps probe [16]. Measurements of relative species concentrations have been accomplished using rotational or vibrational transitions of N 2 ∕O 2 [16,17] or N 2 ∕CH 4 [18], respectively, and the ability to excite multiple species, such as N 2 , H 2 , CH 4 , and CO 2 , has been demonstrated using ultra-broadband CARS with a supercontinuum source [19].…”

“…Figure 1 only displays the excitation of vibrational and rotational N 2 transitions, but the same pulse arrangement also excites pure-rotational transitions of O 2 , the S0 pure-rotational transition of H 2 at 354 cm −1 , and the ν 1 vibrational transition of CH 4 at 2914 cm −1 because of the broadband pulses used. The frequency difference between the 660 nm pump and 798 nm Stokes pulses is centered between the N 2 and CH 4 Q-branch transitions [18].…”

“…As discussed by Engel et al [18], the molecular number density can be related to the CARS signal by Eq. (1) for well-separated transitions:…”

Single-shot hyperspectral coherent Raman planar imaging in the range 0–4200 cm−1

“…One prerequisite for the advance was to implement near-transform-limited femtosecond (fs) pulses for the pump-Stokes interaction and narrowband picosecond (ps) pulses for the probe process. This time-resolved approach, termed hybrid fs/ps CARS, [14][15][16][17][18] combines highly efficient impulsive excitation in the time-domain 19 with highresolution detection in the frequency domain. Indeed, for planar CARS imaging in the gas-phase, a clear benefit of hybrid fs/ps CARS is the possibility of probing narrow wellisolated transitions, where the resonant part of the third order susceptibility dominates the signal.…”