Split-probe hybrid femtosecond/picosecond rotational CARS for time-domain measurement of S-branch Raman linewidths within a single laser shot

Abstract

We introduce a multiplex technique for the single-laser-shot determination of S-branch Raman linewidths with high accuracy and precision by implementing hybrid femtosecond (fs)/picosecond (ps) rotational coherent anti-Stokes Raman spectroscopy (CARS) with multiple spatially and temporally separated probe beams derived from a single laser pulse. The probe beams scatter from the rotational coherence driven by the fs pump and Stokes pulses at four different probe pulse delay times spanning 360 ps, thereby mapping collisional coherence dephasing in time for the populated rotational levels. The probe beams scatter at different folded BOXCARS angles, yielding spatially separated CARS signals which are collected simultaneously on the charge coupled device camera. The technique yields a single-shot standard deviation (1σ) of less than 3.5% in the determination of Raman linewidths and the average linewidth values obtained for N(2) are within 1% of those previously reported. The presented technique opens the possibility for correcting CARS spectra for time-varying collisional environments in operando.

Hybrid fs/ps rotational CARS temperature and oxygen measurements in the product gases of canonical flat flames

“…The technique is particularly well-renowned for high-precision thermometry, robust applicability in luminous sooting flames, and good performance in large-scale technical combustion systems where many other laser diagnostics struggle [2][3][4][5][6][7]. CARS instruments for combustion have most often been based on Nd:YAG and dye laser setups that provide nanosecond-duration laser pulses, but there has been a recent surge in research [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] investigating the utility of femtosecond laser sources for potentially dramatic leaps forward in combustion diagnostics. Commercial femtosecond amplifiers offer kHz-scale repetition rates, allowing researchers to dramatically exceed the $10 Hz barrier posed by high-energy nanosecond laser systems, and approach the energy containing scales of some turbulent flames [23,26].…”

“…The need for detailed Raman linewidth information, which is often uncertain in turbulent flames, is reduced or eliminated because some femtosecond CARS schemes are insensitive to collisional effects at pressures perhaps as high as 30-50 bar [16,29]. Timedelayed probing of femtosecond-prepared Raman coherences additionally permits in situ linewidth determination [3,15,21,[30][31][32], which may be particularly useful in high-pressure flames. Femtosecond sources provide nearly transform-limited pump and Stokes pulses, whose large bandwidth and well-defined spectral phase relationship allow for highly efficient [9] preparation of the Raman coherence with very low noise, which has resulted in single-shot temperature-measurement precision in flames as good as 1% [24] when vibrational CARS of N 2 has been employed.…”

“…Femtosecond sources provide nearly transform-limited pump and Stokes pulses, whose large bandwidth and well-defined spectral phase relationship allow for highly efficient [9] preparation of the Raman coherence with very low noise, which has resulted in single-shot temperature-measurement precision in flames as good as 1% [24] when vibrational CARS of N 2 has been employed. http Pure-rotational CARS provides distinct advantages for combustion monitoring and has been the subject of recent development [14,15,[17][18][19][20][21][22]27,[33][34][35][36][37] for use with femtosecond pump/Stokes laser schemes. Rotational Raman spectra are a highly sensitive thermometer over a wide range of temperatures [38][39][40], and offer a potentially straightforward approach to simultaneous species detection because rotational transitions of many combustionrelevant molecules (N 2 , O 2 , CO, CO 2 ) lie below $300 cm À1 and are accessible with a single broadband laser source [41,42].…”

Bandwidth optimization of femtosecond pure-rotational coherent anti-Stokes Raman scattering by pump/Stokes spectral focusing

“…Femtosecond lasers have enabled significant recent advancements in gas-phase and combustion diagnostics [1,2], particularly with regard to temperature and species detection using coherent anti-Stokes Raman scattering (CARS) [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. High-repetitionrate Ti:sapphire amplifiers increase data rates from ∼10 Hz into the kilohertz regime [14].…”

“…Combustion thermometry has been demonstrated using femtosecond CARS of the vibrational Raman Q branch in N 2 [5,8,12,23] and by utilizing purerotational S and O branch [10,13,[15][16][17][18][19][20]22] transitions. Pure-rotational CARS provides distinct advantages for combustion monitoring.…”