H chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering thermometry at high temperature and pressure.

This article describes the development and application of hydrogen (H) chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS) in high-temperature and high-pressure conditions. H fs CARS thermometry was performed in Hencken burner flames up to 2300 K, as well as in a heated gas-cell at temperatures up to 1000 K. It was observed that the H fs CARS spectra are highly sensitive to the pump and Stokes chirp.

Two-color polarization spectroscopy measurements of Zeeman state-to-state collision induced transitions of nitric oxide in binary gas mixtures.

We investigated collision induced transitions in the (0, 0) band of the A2Σ+-X2Π electronic transition of nitric oxide (NO) using two-color polarization spectroscopy (TCPS). Two sets of TCPS spectra for 1% NO, diluted in different buffer gases at 295 K and 1 atm, were obtained with the pump beam tuned to the R11(11.5) and OP12(1.

Broadband ultrafast-laser-absorption spectroscopy for multi-hydrocarbon measurements near 3.3 µm in flames.

This paper presents the development and application of a broadband ultrafast-laser-absorption-spectroscopy (ULAS) technique operating in the mid-infrared for simultaneous measurements of temperature, methane ( ), and propane ( ) mole fractions. Single-shot measurements targeting the C-H stretch fundamental vibration bands of and near 3.3 µm were acquired in both a heated gas cell up to ≈650 and laminar diffusion flames at 5 kHz.

Background suppression for CARS thermometry in highly luminous flames using an electro-optical shutter.

An electro-optical shutter (EOS), comprising a Pockels cell located between crossed-axis polarizers, is integrated into a nanosecond coherent anti-Stokes Raman scattering (CARS) system. The use of the EOS enables thermometry measurements in high-luminosity flames through significant reduction of the background resulting from broadband flame emission. A temporal gating ≤100 ns along with an extinction ratio >10,000:1 are achieved using the EOS.

Simultaneous Temperature and Pressure Measurements in Compressible Flow Using Nanosecond O Coherent Anti-Stokes Raman Spectroscopy.

Simultaneous pure-rotational coherent anti-Stokes Raman spectroscopy (PRCARS) and vibrational O CARS spectroscopy (VCARS) were performed at elevated pressure and lowered temperature conditions in non-reacting compressible flow. We applied dual-pump CARS in a three-laser, three-color configuration to simultaneously acquire the PRCARS and VCARS spectra of O PRCARS spectra provide excellent sensitivity to temperature at relatively low temperatures. Pressure was extracted using the differential response of collisional effects in the PRCARS and the VCARS spectra.

0.1-5 MHz ultrahigh-speed gas density distributions using digital holographic interferometry.

Gas density distributions for an underexpanded jet at several different pressure ratios were measured at ultrahigh speeds in this work using digital holographic interferometry (DHI). DHI measurements have generally been performed on the order of several Hz in the literature, although some recent groups report measurements at 10 and 100 kHz. We demonstrate 2D imaging of gas density distributions at imaging rates up to 5 MHz, which is an increase by a factor of 50 compared to the previous DHI literature.

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 N 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 N 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).

The development and performance of a perforated plate burner to produce vitiated flow with negligible swirl under engine-relevant gas turbine conditions.

The development and performance of a perforated plate burner (PPB) operating using premixed natural gas and air at engine-relevant inlet temperatures and combustor pressures with thermal powers up to 1 MW is discussed. A significant benefit of using burners with simplified flow fields, such as the PPB, for experimental studies in the laboratory is the potential for decoupling the complex fluid dynamics in typical combustors from the chemical kinetics. The primary motivation for developing this burner was to use it as a source of vitiated flow with negligible swirl for reacting jet in vitiated crossflow experiments.

Dynamic imaging of the temperature field within an energetic composite using phosphor thermography.

An improved understanding of energy localization ("hot spots") is needed to improve the safety and performance of explosives. We propose a technique to visualize and quantify the properties of a dynamic hot spot from within an energetic composite subjected to ultrasonic mechanical excitation. The composite is composed of an optically transparent binder and a countable number of octahydro 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals.

Innovative scheme for high-repetition-rate imaging of CN radical.

We have employed, to the best of our knowledge, a novel excitation scheme to perform the first high-repetition-rate planar laser-induced fluorescence (PLIF) measurements of a CN radical in combustion. The third harmonic of a Nd:YVO laser at 355 nm due to its relatively large linewidth overlaps with several R branch transitions in a CN ground electronic state. Therefore, the 355 nm beam was employed to directly excite the CN transitions with good efficiency.

Technique developments and performance analysis of chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering combustion thermometry.

This work characterizes the state of the art in the analysis of high-repetition-rate, ultrafast combustion thermometry using chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS). Several key aspects of the CARS spectroscopy system are described, including: (1) the ultrafast laser source, (2) use of the frequency-doubled idler versus signal from the optical parametric amplifier, (3) the geometry constraints for phase matching, and (4) spectral fitting for single-shot temperature measurements. A frequency-dependent instrument response function (IRF) for the detection system was modeled as a variable-width Gaussian and implemented through a frequency convolution of synthetic spectra.

A model combustor for studying a reacting jet in an oscillating crossflow.

This paper discusses a novel model combustion experiment that was built for studying the structure and dynamics of a reacting jet in an unsteady crossflow. A natural-gas-fired dump combustor is used to generate and sustain an acoustically oscillating vitiated flow that serves as the crossflow for transverse jet injection. Unlike most other techniques that are limited in operating pressure or acoustic amplitude, this method of generating an unsteady flow field is demonstrated at a pressure of 10 atm with peak-to-peak oscillation amplitudes approaching 20% of the mean pressure.

Two-photon-absorption line strengths for nitric oxide: Comparison of theory and sub-Doppler, laser-induced fluorescence measurements.

We discuss the results of high-resolution, sub-Doppler two-photon-absorption laser-induced fluorescence (TPALIF) spectroscopy of nitric oxide at low pressure and room temperature. The measurements were performed using the single-longitudinal mode output of a diode-laser-seeded optical parametric generator (OPG) system with a measured frequency bandwidth of 220 MHz. The measurements were performed using a counter-propagating pump beam geometry, resulting in sub-Doppler TPALIF spectra of NO for various rotational transitions in the (0,0) vibrational band of the AΣ - XΠ electronic transition.

Simultaneous high-speed gas property measurements at the exhaust gas recirculation cooler exit and at the turbocharger inlet of a multicylinder diesel engine using diode-laser-absorption spectroscopy.

A diode-laser-absorption-spectroscopy-based sensor system was used to perform high-speed (100 Hz to 5 kHz) measurements of gas properties (temperature, pressure, and H(2)O vapor concentration) at the turbocharger inlet and at the exhaust gas recirculation (EGR) cooler exit of a diesel engine. An earlier version of this system was previously used for high-speed measurements of gas temperature and H(2)O vapor concentration in the intake manifold of the diesel engine. A 1387.

Focus issue introduction: Laser Ignition Conference.

The purpose of this feature issue is to share information on laser ignition and related sciences and technologies. This feature offers five papers in the field that cover aspects of laser-induced laser ignition, including novel giant pulse micro-lasers, new phenomena of laser breakdown, advanced combustion systems and applications. These topics were chosen from the first Laser Ignition Conference (LIC) covering the topics of high brightness lasers for ignition and diagnostics, laser ignited engines for power generators and vehicles, and from a joint symposium with the Laser Display Conference covering applications of high brightness lasers.

The development of an optically accessible, high-power combustion test rig.

This work summarizes the development of a gas turbine combustion experiment which will allow advanced optical measurements to be made at realistic engine conditions. Facility requirements are addressed, including instrumentation and control needs for remote operation when working with high energy flows. The methodology employed in the design of the optically accessible combustion chamber is elucidated, including window considerations and thermal management of the experimental hardware under extremely high heat loads.

High-repetition-rate three-dimensional OH imaging using scanned planar laser-induced fluorescence system for multiphase combustion.

Imaging dynamic multiphase combusting events is challenging. Conventional techniques can image only a single plane of an event, capturing limited details. Here, we report on a three-dimensional, time-resolved, OH planar laser-induced fluorescence (3D OH PLIF) technique that was developed to measure the relative OH concentration in multiphase combustion flow fields.

Development of a combined pure rotational and vibrational coherent anti-Stokes Raman scattering system.

A combined pure rotational coherent anti-Stokes Raman scattering (PRCARS) and vibrational CARS (VCARS) system has been developed. In this system two beams, a broadband beam centered at 607 nm and the frequency-doubled Nd:YAG output at 532 nm is used to generate the PRCARS signal. A second 532 nm beam is used along with the other two beams to simultaneously generate the N(2) VCARS signal using a standard phase-matching scheme.

Polarization suppression of the nonresonant background in femtosecond coherent anti-Stokes Raman scattering for flame thermometry at 5 kHz.

Coherent anti-Stokes Raman scattering (CARS) spectra are acquired at 5 kHz in steady and unsteady flames while suppressing the nonresonant background by polarization techniques. Broadband femtosecond (fs) pump and Stokes pulses efficiently excite many Raman transitions in diatomic nitrogen which subsequently interfere and decay. Single-laser-shot measurements are performed as the decay of the Raman coherence is mapped to the frequency of the CARS signal by a chirped-probe pulse (CPP).

Label-free bond-selective imaging by listening to vibrationally excited molecules.

We report the realization of vibrational photoacoustic (VPA) microscopy using optical excitation of molecular overtone vibration and acoustic detection of the resultant pressure transients. Our approach eliminates the tissue scattering problem encountered in near-infrared spectroscopy and enables depth-resolved signal collection. The 2nd overtone of the CH bond stretch around 8300  cm(-1), where blood interference is minimal, is excited.

Electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide: saturation and Stark effects.

A theoretical analysis of electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) of NO is described. The time-dependent density-matrix equations for the nonlinear ERE-CARS process are derived and manipulated into a form suitable for direct numerical integration. In the ERE-CARS configuration considered in this paper, the pump and Stokes beams are far from electronic-resonance.

Gas-phase single-shot thermometry at 1 kHz using fs-CARS spectroscopy.

Single-laser-shot temperature measurements at a data rate of 1 kHz employing femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of N(2) are demonstrated. The measurements are performed using a chirped-probe pulse to map the time-dependent frequency-spread dephasing of the Raman coherence, which is created by approximately 80-fs pump and Stokes beams, into the spectrum of the coherent anti-Stokes Raman scattering signal pulse. Temperature is determined from the spectral shape of the fs-CARS signal for probe delays of approximately 2 ps with respect to the pump-Stokes excitation.

Effects of collisions on electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide.

A six-level model is developed and used to study the effects of collisional energy transfer and dephasing on electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) in nitric oxide. The model includes the three levels that are coherently coupled by the three applied lasers as well as three additional bath levels that enable inclusion of the effects of electronic quenching and rotational energy transfer. The density-matrix equations that describe the evolution of the relevant populations and coherences are presented.

Two-color planar laser-induced fluorescence thermometry in aqueous solutions.

We demonstrate a two-color planar laser-induced fluorescence technique for obtaining two-dimensional temperature images in water. For this method, a pulsed Nd:YAG laser at 532 nm excites a solution of temperature-sensitive rhodamine 560 and temperature-insensitive sulforhodamine 640. The resulting emissions are optically separated through filters and detected via a charged-couple device (CCD) camera system.

Perturbative theory and modeling of electronic-resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy of nitric oxide.

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal.