Four-color fiber-coupled mid-infrared laser-absorption sensor for temperature, CO, CO, and NO at 5 kHz in internal combustion engine vehicle exhaust.

A mid-infrared (MIR) laser absorption spectroscopy (LAS) sensor was developed for temperature, CO, NO, and measurements at 5 kHz in engine-out exhaust. It used fiber-coupled quantum cascade lasers (QCLs) for measuring CO and NO, and an interband cascade laser (ICL) for measuring . Validation tests in a heated gas cell confirmed that the LAS measurements of CO, , NO, and temperature are accurate to within 4.

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.

Characterization of non-Boltzmann CN X2Σ+ behind shock waves in CH4-N2 via broadband ultraviolet femtosecond absorption spectroscopy.

This article describes the temporal evolution of rotationally and vibrationally non-Boltzmann CN X2Σ+ formed behind reflected shock waves in N2-CH4 mixtures at conditions relevant to atmospheric entry into Titan. A novel ultrafast (i.e.

Near-MHz temperature and HO measurements in post-detonation fireballs of 25 g hemispherical explosives using scanned-wavelength-modulation spectroscopy.

A laser absorption spectroscopy diagnostic integrated within a hardened optical probe was used to measure temperature and water mole fraction at 500 kHz in post-detonation fireballs of explosives. In the experiments, an exploding-bridgewire detonator initiated a 25 g hemisphere of explosive (N5 or PETN). This produced a hemispherical fireball that traveled radially towards a hardened measurement probe.

Quantum-cascade-laser-absorption-spectroscopy diagnostic for temperature, pressure, and NO at 500 kHz in shock-heated air at elevated pressures.

The design, validation, and application of a quantum-cascade-laser-absorption-spectroscopy diagnostic for measuring gas temperature, pressure, and nitric oxide (NO) in high-temperature air are presented. A distributed-feedback quantum-cascade laser (QCL) centered near 1976 was used to scan across two transitions of NO in its ground electronic state ( ). A measurement rate of 500 kHz was achieved using a single QCL by: (1) performing current modulation through a bias-tee, and (2) targeting closely spaced transitions with a large difference in lower-state energy.

Ultrafast-laser-absorption spectroscopy in the mid-infrared for single-shot, calibration-free temperature and species measurements in low- and high-pressure combustion gases.

This manuscript presents an ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic capable of providing calibration-free, single-shot measurements of temperature and CO at 5 kHz in combustion gases at low and high pressures. Additionally, this diagnostic was extended to provide 1D, single-shot measurements of temperature and CO in a propellant flame. A detailed description of the spectral-fitting routine, data-processing procedures, and determination of the instrument response function are also presented.

Spectrally resolved, 1D, mid-infrared imaging of temperature, CO, and HCl in propellant flames.

This work presents a high-speed, spectrally resolved, mid-infrared imaging diagnostic for providing 1D measurements of gas temperature and relative mole fraction of ${{\rm{CO}}_2}$ and HCl in flames. An imaging spectrometer and a high-speed mid-infrared camera were used to provide 1D measurements of ${{\rm{CO}}_2}$ and HCl emission spectra from 2386 to ${{2402}}\;{{\rm{cm}}^{- 1}}$ with a spectral resolution of ${0.46}\;{{\rm{cm}}^{- 1}}$, and simulated emission spectra were least-squares fit to the data to determine the aforementioned gas properties.

Cepstral analysis for baseline-insensitive absorption spectroscopy using light sources with pronounced intensity variations.

This paper presents a data-processing technique that improves the accuracy and precision of absorption-spectroscopy measurements by isolating the molecular absorbance signal from errors in the baseline light intensity () using cepstral analysis. Recently, cepstral analysis has been used with traditional absorption spectrometers to create a modified form of the time-domain molecular free-induction decay (m-FID) signal, which can be analyzed independently from . However, independent analysis of the molecular signature is not possible when the baseline intensity and molecular response do not separate well in the time domain, which is typical when using injection-current-tuned lasers [e.

Simulation technique enabling calibration-free frequency-modulation spectroscopy measurements of gas conditions and lineshapes with modulation frequencies spanning kHz to GHz.

A simulation technique enabling calibration-free measurements of gas properties (e.g., temperature, mole fraction) and lineshapes via wavelength- or frequency-modulation spectroscopy (WMS or FMS) is presented.

2D mid-infrared laser-absorption imaging for tomographic reconstruction of temperature and carbon monoxide in laminar flames.

This manuscript presents the design and initial application of a mid-infrared laser-absorption-imaging (LAI) technique for two-dimensional (2D) measurements and tomographic reconstruction of gas temperature and CO in laminar flames. In this technique, the output beam from a quantum-cascade laser (QCL) is expanded, passed through the test gas, and imaged in 2D using a high-speed mid-infrared camera. The wavelength of the QCL is scanned across the P(0,20) and P(1,14) transitions of CO near 4.

Design and application of a high-pressure combustion chamber for studying propellant flames with laser diagnostics.

This manuscript presents the design and initial application of a high-pressure combustion chamber (HPCC). The HPCC exhibits several unique design attributes to enable high-fidelity studies of propellant-combustion physics at high pressures. The HPCC employs a flangeless and weldless design to provide a compact, easy to access, and relatively light weight (for its size and pressure capability) test chamber.

Compact, fiber-coupled, single-ended laser-absorption-spectroscopy sensors for high-temperature environments.

The design and demonstration of a compact single-ended laser-absorption-spectroscopy sensor for measuring temperature and HO in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture that uses a single lens to provide single-ended, alignment-free (after initial assembly) measurements of gas properties in a combustor without windows. We demonstrate that the sensor is capable of sustaining operation at temperatures up to at least 625 K and is capable of withstanding direct exposure to high-temperature (≈1000  K) flame gases for long durations (at least 30 min) without compromising measurement quality.

Tomographic laser absorption imaging of combustion species and temperature in the mid-wave infrared.

In this work, laser absorption spectroscopy techniques are expanded in spatial resolution capability by utilizing a high-speed infrared camera to image flow fields backlit with tunable mid-wave infrared laser radiation. The laser absorption imaging (LAI) method yields spectrally-resolved and spatially-rich datasets from which quantitative species and temperature profiles can be generated using tomographic reconstruction. Access to the mid-wave infrared (3-5 µm) enables imaging of fuels, intermediates, and products of combustion in canonical small-diameter flames (< 1 cm).

Wavelength-modulated planar laser-induced fluorescence for imaging gases.

This work presents the development of wavelength-modulated planar laser-induced fluorescence (WM-PLIF) and its initial application to infrared imaging of carbon monoxide in a laminar flame. A continuous-wave quantum-cascade laser producing 50 mW near 4.8 μm was injection-current modulated at 1 kHz and scanned across the P(20) transition of CO at 20 Hz.

Single-ended mid-infrared laser-absorption sensor for simultaneous in situ measurements of HO, CO, CO, and temperature in combustion flows.

The development and demonstration of a four-color single-ended mid-infrared tunable laser-absorption sensor for simultaneous measurements of HO, CO, CO, and temperature in combustion flows is described. This sensor operates by transmitting laser light through a single optical port and measuring the backscattered radiation from within the combustion device. Scanned-wavelength-modulation spectroscopy with second-harmonic detection and first-harmonic normalization (scanned-WMS-2f/1f) was used to account for variable signal collection and nonabsorption losses in the harsh environment.

Kinetics of Excited Oxygen Formation in Shock-Heated O-Ar Mixtures.

The formation of electronically excited atomic oxygen was studied behind reflected shock waves using cavity-enhanced absorption spectroscopy. Mixtures of 1% O-Ar were shock-heated to 5400-7500 K, and two distributed-feedback diode lasers near 777.2 and 844.

Fiber-coupled diode-laser sensors for calibration-free stand-off measurements of gas temperature, pressure, and composition.

A fiber-coupled near-infrared diode-laser sensor for stand-off measurements of gas temperature, pressure, and composition is presented. This sensor utilizes a fiber bundle with six multimode catch fibers surrounding one single-mode pitch fiber to transmit and receive backscattered laser light in a handheld transmitter/receiver. Scanned-wavelength-modulation spectroscopy with 1f-normalized 2f-detection and fast (80-200 kHz) wavelength modulation were used to provide calibration-free measurements and reduce the influence of spurious cavity noise formed by the overlapping transmitted and reflected laser light.

Shock-tube measurements of excited oxygen atoms using cavity-enhanced absorption spectroscopy.

We report the use of cavity-enhanced absorption spectroscopy (CEAS) using two distributed feedback diode lasers near 777.2 and 844.6 nm for sensitive, time-resolved, in situ measurements of excited-state populations of atomic oxygen in a shock tube.

Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes.

The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques.

Two-color absorption spectroscopy strategy for measuring the column density and path average temperature of the absorbing species in nonuniform gases.

A two-color absorption spectroscopy strategy has been developed for measuring the column density and density-weighted path-average temperature of the absorbing species in nonuniform gases. This strategy uses two transitions with strengths that scale nearly linearly with temperature. In addition, measured lineshapes are used to accurately model absorbance spectra.