Single-shot gas-phase thermometry using pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

Abstract

High-repetition-rate, single-laser-shot measurements are important for the investigation of unsteady flows where temperature and species concentrations can vary significantly. Here, we demonstrate singleshot, purerotational, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) thermometry based on a kHz-rate fs laser source. Interferences that can affect nanosecond (ns) and ps CARS, such as nonresonant background and collisional dephasing, are eliminated by selecting an appropriate time delay between the 100-fs pump/Stokes pulses and the pulse-shaped 8.4-ps probe. A time-and frequencydomain theoretical model is introduced to account for rotational-level dependent collisional dephasing and indicates that the optimal probe-pulse time delay is 13.5 ps to 30 ps. This time delay allows for uncorrected best-fit N 2 -RCARS temperature measurements with ~1% accuracy. Hence, the hybrid fs/ps RCARS approach can be performed with kHz-rate laser sources while avoiding corrections that can be difficult to predict in unsteady flows. KeywordsFriedrich-Alexander University Erlangen-Nürnberg, aerodynamics, probes, Raman scattering, Raman spectroscopy, temperature measurement, thermometers, dephasing, femtoseconds, frequency domains, Fs laser, gasphase, high repetition rate, laser sources, species concentration, equipment design, optics and photonics, Fourier analysis Abstract: High-repetition-rate, single-laser-shot measurements are important for the investigation of unsteady flows where temperature and species concentrations can vary significantly. Here, we demonstrate singleshot, pure-rotational, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) thermometry based on a kHz-rate fs laser source. Interferences that can affect nanosecond (ns) and ps CARS, such as nonresonant background and collisional dephasing, are eliminated by selecting an appropriate time delay between the 100-fs pump/Stokes pulses and the pulse-shaped 8.4-ps probe. A time-and frequency-domain theoretical model is introduced to account for rotational-level dependent collisional dephasing and indicates that the optimal probe-pulse time delay is 13.5 ps to 30 ps. This time delay allows for uncorrected best-fit N 2 -RCARS temperature measurements with ~1% accuracy. Hence, the hybrid fs/ps RCARS approach can be performed with kHz-rate laser sources while avoiding corrections that can be difficult to predict in unsteady flows.

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

“…A timeand frequency-resolved model based on fundamental spectroscopic constants and experimental parameters was used to generate N 2 ro-vibrational and N 2 ∕O 2 rotational spectra at various temperatures as implemented by Miller and co-workers [15,20] and described in detail by Stauffer et al [21] and Prince et al [12]. Best-fit temperatures were computed using a spectral library and optimization algorithm to minimize the residual between experimental and modeled spectra.…”

“…The experimental arrangement is modified from previously published studies [15,20]. A Ti:sapphire, regeneratively amplified, 1-kHz-rate laser (Solstice, Spectra-Physics) is used to produce a 2.5 mJ, 150 cm −1 , 100 fs pulse centered around 798 nm.…”

Communication: Simplified two-beam rotational CARS signal generation demonstrated in 1D

“…The hybrid fs/ps CARS spectra were fitted with a time-domain CARS model similar to one previously reported. 18 As can be seen in Fig. 4, as the gas environment heats up, rotational states of higher energy become populated, and signal from rotational transitions at higher Raman shift are observed.…”

“…The CARS experiments were performed using time synchronized femtosecond (fs) and picosecond (ps) laser systems in a hybrid fs pump ps probe setup. 18 The fs laser system consisted of a Halcyon oscillator (KM Labs) synced to an external 100 MHz RF source. The oscillator pumped a fs regenerative amplifier (Wyvern 1000, KM Labs), yielding a maximum of 5 mJ/pulse with 45 fs pulses at a rate of 1-10 kHz.…”

Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

“…These lasers often operate at high repetition rate, i.e., kHz to MHz. [158][159][160][161][162] Hence, correlated data can be acquired and it has become possible to monitor turbulent flows and single, transient phenomena. 163 A big step into this direction was the recent development of burst mode lasers that produce a reasonable number of high energy laser pulses as a burst.…”