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

High-energy, low-jitter, narrowband ps probe laser for kHz-rate fs/ps coherent anti-Stokes Raman scattering

Hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) thermometry often utilizes ps probe pulses derived from pulse shaping or spectrally filtering the primary laser source or by synchronization with a low repetition rate external laser. This results in limited energy, spectral resolution, and/or repetition rate of the ps probe. In this work, a master-oscillator power-amplifier (MOPA) laser was synchronized to the oscillator of a Ti:sapphire regenerative amplifier to achieve high-energy (600 µJ), narrowband (58 ps) probe pulses at kHz repetition rates. Temporal filtering allows the pulse characteristics to be adjusted for each application. At 25 Torr, relevant to high-speed flows, the kHz-rate MOPA system generated signal-to-noise ratios 3× higher in nitrogen and had improved precision relative to a 10 ps probe derived from spectral filtering and the power-amplifier. The MOPA system also enabled single-shot ro-vibrational hybrid fs/ps CARS thermometry in 650 K heated air.

Hybrid fs/ps vibrational coherent anti-Stokes Raman scattering for simultaneous gas-phase N2/O2/CO2 measurements

Single-shot, 1-kHz measurements of temperatures and mole fraction ratios along with theoretical modeling for gas-phase N₂, O₂, and CO₂ are demonstrated using hybrid femtosecond/picosecond (fs/ps) vibrational coherent anti-Stokes Raman scattering (CARS). The combination of broadband pump and Stokes pulses covers a spectral range over 1800cm^-1 while the narrowband probe pulses generated from a quasi-common-path second harmonic bandwidth compressor (QCP-SHBC) resolves the molecular structures with a bandwidth of ∼7cm^-1. Temperature results of 1700-2000 K in methane/air fuel-lean flames show state-of-the-art inaccuracies of less than ∼3% and precision less than 2%. Mole fraction ratios inaccuracy at room temperature is ∼5%, and precision at flame temperatures are 6%-8%.

Effects of self-phase modulation (SPM) on femtosecond coherent anti-Stokes Raman scattering spectroscopy

The effects of self-phase modulation (SPM) on the power spectra of femtosecond (fs) pulses and the consequent impact on N₂ chirped-probe-pulse (CPP) fs coherent anti-Stokes Raman scattering (CARS) spectra are discussed in this paper. We investigated the pressure dependence of CPP fs CARS for N₂ in a room-temperature gas cell at pressures ranging from 1 to 10 bar, and in our initial experiments the CPP fs CARS spectrum changed drastically as the pressure increased. We found that the spectra of the near-Fourier-transform-limited, 60-fs pump and Stokes pulses at the exit of the gas cell changed drastically as the pressure increased due to self-phase-modulation (SPM). This effect was examined in detail in further experiments where the pulse energies of the pump and Stokes pulses were controlled using a combination of a half-wave plate and a linear polarizer. Along with the generated CARS spectrum, the spectra of pump and Stokes pulses were measured at the entrance and exit of the gas cell. The extent of SPM effects for a particular spectrum was characterized by the least squares difference between that spectrum and a spectrum recorded at low enough pressure and laser intensities that SPM was negligible. SPM effects were investigated for N₂, O₂, CO₂, and CH₄, for pressures ranging from 1 to 10 bar, and for pump and Stokes pulse energies ranging from 10 to 60 µJ. We found that SPM effects in N₂ were much weaker than for O₂, CO₂ and CH₄.

Enhancement of the Raman Effect by Infrared Pumping

We propose a method for increasing Raman scattering from an ensemble of molecules by up to 4 orders of magnitude. Our method requires an additional coherent source of IR radiation with the half-frequency of the Stokes shift. This radiation excites the molecule electronic subsystem that in turn, via Fröhlich coupling, parametrically excites nuclear oscillations at a resonant frequency. This motion is coherent and leads to a boost of the Raman signal in comparison to the spontaneous signal because its intensity is proportional to the squared number of molecules in the illuminated volume.

Gas Temperature Measurement Using Differential Optical Absorption Spectroscopy (DOAS)

A nonintrusive method for flow gas temperature measurement using differential optical absorption spectroscopy (DOAS) was demonstrated. A temperature-dependent spectra (TDS) originated from the DOAS spectra of sulfur dioxide (SO₂) in the wavelength range of 276-310 nm was introduced, and the relationship between the TDS and the temperature was built through experimental calibration process. This relationship is found to be independent of SO₂ concentration and can be used for temperature measurements. The experimental results indicated that the precision of the TDS method is < ± 0.3% for SO₂ concentrations higher than 150 ppm with the optical path length of 170 mm. For lower concentrations, the precision is estimated to be ± 0.4% at 1 ppm. The relative deviation between the temperature measured by the TDS method and that measured by a thermocouple is within 3% in the temperature range of 298-750 K, and the TDS method has a quicker response to the fast-changing temperature than the thermocouple.

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N₂), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H₂) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H₂ S-branch. We demonstrate H₂ thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H₂ S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm^-1. We present a pure-rotational H₂ CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N₂ spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H₂ and N₂ CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H₂ and N₂ temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H₂ CARS thermometry for probing combustion reactions.

Comparison of chirped-probe-pulse and hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering for combustion thermometry

A comparison is made between two ultrashort-pulse coherent anti-Stokes Raman scattering (CARS) thermometry techniques-hybrid femtosecond/picosecond (fs/ps) CARS and chirped-probe-pulse (CPP) fs-CARS-that have become standards for high-repetition-rate thermometry in the combustion diagnostics community. These two variants of fs-CARS differ only in the characteristics of the ps-duration probe pulse; in hybrid fs/ps CARS a spectrally narrow, time-asymmetric probe pulse is used, whereas a highly chirped, spectrally broad probe pulse is used in CPP fs-CARS. Temperature measurements were performed using both techniques in near-adiabatic flames in the temperature range 1600-2400 K and for probe time delays of 0-30 ps. Under these conditions, both techniques are shown to exhibit similar temperature measurement accuracies and precisions to previously reported values and to each other. However, it is observed that initial calibration fits to the spectrally broad CPP results require more fitting parameters and a more robust optimization algorithm and therefore significantly increased computational cost and complexity compared to the fitting of hybrid fs/ps CARS data. The optimized model parameters varied more for the CPP measurements than for the hybrid fs/ps measurements for different experimental conditions.

Two-color vibrational, femtosecond, fully resonant electronically enhanced CARS (FREE-CARS) of gas-phase nitric oxide

A resonantly enhanced, two-color, femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) approach is demonstrated and used to explore the nature of the frequency- and time-dependent signals produced by gas-phase nitric oxide (NO). Through careful selection of the input pulse wavelengths, this fully resonant electronically enhanced CARS (FREE-CARS) scheme allows rovibronic-state-resolved observation of time-dependent rovibrational wavepackets propagating on the vibrationally excited ground-state potential energy surface of this diatomic species. Despite the use of broadband, ultrafast time-resolved input pulses, high spectral resolution of gas-phase rovibronic transitions is observed in the FREE-CARS signal, dictated by the electronic dephasing timescales of these states. Analysis and computational simulation of the time-dependent spectra observed as a function of pump-Stokes and Stokes-probe delays provide insight into the rotationally resolved wavepacket motion observed on the excited-state and vibrationally excited ground-state potential energy surfaces of NO, respectively.

Temperature measurements in metalized propellant combustion using hybrid fs/ps coherent anti-Stokes Raman scattering

We apply ultrafast pure-rotational coherent anti-Stokes Raman scattering (CARS) for temperature and relative oxygen concentration measurements in the plume emanating from a burning, aluminized ammonium-perchlorate propellant strand. Combustion of these metal-based propellants is a particularly hostile environment for laser-based diagnostics, with intense background luminosity and scattering from hot metal particles as large as several hundred micrometers in diameter. CARS spectra that were previously obtained using nanosecond pulsed lasers in an aluminum-particle-seeded flame are examined and are determined to be severely impacted by nonresonant background, presumably as a result of the plasma formed by particulate-enhanced laser-induced breakdown. Introduction of femtosecond/picosecond (fs/ps) laser pulses improves CARS detection by providing time-gated elimination of strong nonresonant background interference. Single-laser-shot fs/ps CARS spectra were acquired from the burning propellant plume, with picosecond probe-pulse delays of 0 and 16 ps from the femtosecond pump and Stokes pulses. At zero delay, nonresonant background overwhelms the Raman-resonant spectroscopic features. Time-delayed probing results in the acquisition of background-free spectra that were successfully fit for temperature and relative oxygen content. Temperature probability densities and temperature/oxygen correlations were constructed from ensembles of several thousand single-laser-shot measurements with the CARS measurement volume positioned within 3 mm or less of the burning propellant surface. The results show that ultrafast CARS is a potentially enabling technology for probing harsh, particle-laden flame environments.

Rovibrational hybrid fs/ps CARS using a volume Bragg grating for N₂ thermometry

Coherent anti-Stokes Raman scattering (CARS) spectra of N2 in the hybrid femtosecond/picosecond regime have been recorded with 0.7  cm(-1) resolution. The Q-branch rovibrational structure has been resolved, making it suitable for gas-phase simultaneous rotational and vibrational thermometry applications. Resolving this spectral structure requires synchronization of a narrowband picosecond probe pulse with a broadband femtosecond pair of pump and Stokes pulses. It is achieved using a single femtosecond ytterbium-laser source and a volume Bragg grating in a compact experimental arrangement.

Direct Coherent Raman Temperature Imaging and Wideband Chemical Detection in a Hydrocarbon Flat Flame

A single-shot coherent Raman imaging technique has been developed for spatially correlated one-dimensional high-fidelity gas-phase thermometry and multiplex chemical detection in flames. The technique utilizes two-beam phase matching, operating a single ultrashort pump/Stokes excitation pulse (7 fs) and a narrowband picosecond probe pulse (70 ps), interrogating a Raman active window of ∼4200 cm(-1) with ∼0.3 cm(-1) spectral resolution. The measurement geometry is formed intersecting the two beams shaped as laser-sheets and the one-coordinate spatial information is retrieved with a linespread function of <40 μm. The advance provides the possibility for the multiplexed measurement of all combustion relevant major species simultaneously with gaseous temperature monitored over a several millimeter field of view. The current technique is optimized in a premixed hydrocarbon flat-flame. At the flame-front, it is shown that direct imaging renders the temperature profile within ∼1% inaccuracy, whereas typical point-wise raster scanning may have relative systematic deviations up to 15% due to spatial averaging effects.