Thermometry for turbulent flames by coherent anti-Stokes Raman spectroscopy with simultaneous referencing to the modeless excitation profile

An optimal system for temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) in turbulent flames and flows is presented. In addition to a single-mode pump laser and a modeless dye laser, an echelle spectrometer with a cross disperser is used. This system permits simultaneous measurement of the N2 CARS spectrum and the broadband dye laser profile. A procedure is developed to use software to transform this profile into the excitation profile by which the spectrum is referenced. Simultaneous shot-to-shot referencing is compared to sequential averaged referencing for data obtained in flat flames and in room air. At flame temperatures, the resultant 1.5% imprecision is limited by flame fluctuations, indicating that the system may have a single-shot imprecision below 1%. At room temperature, the 3.8% single-shot imprecision is of the same order as the best values reported for dual-broadband pure-rotational CARS. Using the unique shot-to-shot excitation profiles, simultaneous referencing eliminates systematic errors. At 2000 and 300 K, the 95% confidence intervals are estimated to be +/- 20 and +/- 10 K, respectively.     

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser

The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.     

High-resolution broadband N2 coherent anti-Stokes Raman spectroscopy: comparison of measurements for conventional and modeless broadband dye lasers

We have performed high-resolution N2 coherent anti-Stokes Raman spectroscopy (CARS) measurements using a modeless dye laser (MDL) as the Stokes beam source to determine the effects of a reduction in mode noise on the accuracy and precision of the method. These results are compared with previous research that employed a conventional broadband dye laser (CBDL) as the Stokes beam source. A new spectral-fitting procedure was developed to avoid starting-point bias in the least-squares fitting results, which possibly had altered the previous measurements. Single-shot measurements of pressure were performed in a static-pressure vessel over the range of 0.1-4.0 atm to examine the pressure sensitivity of the technique. The precision of these measurements is a measure of the baseline noise level of the system, which sets the detection limit for flow-field pressure fluctuations. Centerline measurements of pressure and temperature in an underexpanded jet (Mj = 1.85) were also used to determine the performance of the technique in a compressible flow field. Our study represents the first known application, to our knowledge, of a MDL CARS system in a low-temperature, low-pressure supersonic environment. Improvements in accuracy for mean single-shot measurements and increased precision were found for pressure vessel conditions above 1.0 atm. For subatmospheric pressure vessel conditions (0.1-1.0 atm) and the underexpanded jet measurements, there was a decrease in accuracy and precision compared with the CBDL results. A comparison with the CBDL study is included, along with a discussion of the MDL system behavior.     

Quantitative one-dimensional imaging using picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy

We employ picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional (1D) imaging configuration. Temperature and O(2):N(2) concentration ratios are measured along a 1D line of up to 12 mm in length. The images consist of up to 330 individual rotational CARS (RCARS) spectra, corresponding to 330 spatially resolved volume elements in the probe volume. Signal levels are sufficient for the collection of single-laser-pulse images at temperatures of up to approximately 1200 K and shot-averaged images at flame temperatures, demonstrated at 2100 K. The precision of picosecond pure-rotational 1D imaging CARS is assessed by acquiring a series of 100 single-laser-pulse images in a heated flow of N(2) from 410 K-1200 K and evaluating a single volume element for temperature in each image. Accuracy is demonstrated by comparing temperatures from the evaluated averaged spectra to thermocouple readings in the heated flow. Deviations from the thermocouple of <30 K in the evaluated temperature were found at up to 1205 K. Accuracy and single-shot precision are compared to those reported for single-point nanosecond dual-broadband pure-RCARS and nanosecond 1D vibrational CARS.     

Improved temperature precision in rotational coherent anti-stokes raman spectroscopy with a modeless dye laser

Single-shot thermometry with dual-broadband rotational coherent anti-Stokes Raman spectroscopy that employs amplified spontaneous emission from a broadband modeless dye laser has been examined. Evaluation of single-shot spectra of air, N2, and O2 showed an improved temperature precision at room temperature compared with the precision obtained with a conventional dye laser. A comparison was also made between the use of single-mode and multimode Nd:YAG lasers as sources for narrowband radiation, and in all cases the single-mode Nd:YAG laser resulted in higher precision. The experimental results are compared with theoretical predictions.     

Estimation of spatial averaging of temperatures from coherent anti-Stokes Raman spectroscopy

Spatial averaging is a potential problem in the application of coherent anti-Stokes Raman spectroscopy (CARS) to combustion diagnostics when varying temperatures and composition are present at spatial scales smaller than the typical 1-2-mm spatial resolution of CARS. The observed CARS spectrum is then a mixture of the hot and cold components. We show that simulated, spatially averaged spectra, generated by the incoherent addition of intensities, can be significantly different from those obtained by the coherent addition of the electric field amplitudes of the component spectra. The analyses of these simulated, spatially averaged CARS spectra demonstrate that the use of theoretical CARS spectra, generated by the addition of intensities of the hot and cold components of a binary gas mixture, can lead to errors in the estimated flame temperatures.     

Near-resonance enhanced O2 detection for dual-broadband pure rotational coherent anti-Stokes Raman scattering with an ultraviolet-visible setup at 266 nm

Broadband and dual-broadband coherent anti-Stokes Raman scattering (CARS) are widely established tools for nonintrusive gas diagnostics. Up to now the investigations have been mainly performed for electronic nonresonant conditions of the gas species of interest. We report on the enhancement of the O2-N2 detection limit of dual-broadband pure rotational CARS by shifting the wavelength of the narrowband pump laser from the commonly used 532-266 nm. This enhancement is caused when the Schumann-Runge absorption band is approached near 176 nm. The principal concept of this experiment, i.e., covering the Raman resonance with a single- or dual-broadband combination of lasers in the visible range and moving only the narrowband probe laser near or directly into electronic resonant conditions in the UV range, should also be applicable to broadband CARS experiments to directly exploit electronic resonance effects for the purpose of single-shot concentration measurements of minority species. To quantify the enhancement in O2 sensitivity, comparative measurements at both a 266 and a 532 nm narrowband pump laser wavelength are presented, employing a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyram (DCM) dye laser as a broadband laser source at 635 nm. An increase of approximately equal to 13% in the ratio of the rotational CARS cross sections of O2 and N2 was obtained. The broad spectral width of the CARS excitation profile was approximately equal for both setups. Further enhancement should be achievable by shifting the narrowband pump laser closer toward 176 nm, for example, with a frequency-doubled optical parametric oscillator or an excimer laser. The principal concept of this experiment should also be applicable to broadband CARS experiments to directly exploit electronic resonance effects of the narrowband pump laser with electronic transitions of minority species for the purpose of single-shot concentration measurements of those species.     

Crossed two-beam coherent anti-stokes Raman spectroscopy in dispersive media

Coherent anti-Stokes Raman spectroscopy (CARS) is a nonlinear optical wave mixing process that is used in gas-phase systems to determine the energy distribution of the probed species (usually N2) and, through a fitting procedure, the temperature giving rise to it. CARS signal strengths are maximized when the phase matching condition is met. Because gases are generally non-dispersive, this phase matching condition can be found geometrically as a function of the crossing angles between the CARS beams and their wavelengths. In addition, perfect phase matching in non-dispersive media occurs automatically for collinear beams. To improve spatial resolution, however, intersecting the laser beams is desirable. Being a third-order process, phase matching for CARS in gases typically requires three input laser beams. This paper discusses and demonstrates the issues of phase matching for CARS when the medium is dispersive, and the ability for CARS phase matching to occur with only two crossed laser beams (one pump and one probe). This two-beam X-CARS in dispersive media can be used as an alignment tool for gas-phase CARS and may be relevant as a simpler diagnostic in high-pressure environments. The paper also discusses the effects of non-ideal phase matching in dispersive and non-dispersive media.     

Generation and propagation of a resonant CARS signal from biomolecules: Application to dipicolinic acid

Abstract

We develop a theoretical approach and perform simulations of coherent anti-Stokes Raman scattering (CARS) with ultrashort laser pulses. The signal is generated by biomolecules having subpicosecond dephasing times, from femtosecond pulses on exact resonance with the molecular transitions. All propagation effects are explicitly accounted for, including pump depletion, Raman amplification, parametric generation and pulse reshaping. Our model predicts that a measurable CARS signal can be generated by the dipicolinic acid biomolecule under realistic conditions.     

Simultaneous vibrational and pure rotational coherent anti-stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames

The potential of measuring temperature and multiple species concentrations (N2, O2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS spectra, O2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique for probing of additional species are discussed.     

Single pulse CARS noise: a comparison between single-mode and multimode pump lasers

An investigation of single pulse coherent anti-Stokes Raman spectroscopy (CARS) noise, determined by the analysis of broadband nonresonant spectra, is described. It is shown that the use of a single-mode rather than a multimode pump laser leads to a significant reduction of CARS noise (40%), down to the level exhibited by the Stokes spectral profile itself. This reduction in noise is attributed to the minimization of the effects due to random variations in the laser temporal profiles by using temporally smooth single-mode laser pumps. A measurement of detector shot noise is presented and its effect on CARS noise is described. The advantages of using a single-mode pump laser in CARS spectroscopy are discussed.     

Comparison of Gas Temperatures Measured by Coherent Anti-Stokes Raman Spectroscopy (CARS) of O(2) and N(2)

We investigate the accuracy of temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) of O(2) and use measurements taken with N(2) CARS and a thermocouple for comparison. Scanning vibrational CARS spectra of O(2) and N(2) were recorded over a broad range of temperatures: between 294 K and 1900 K in air that was heated in a tube furnace and at approximately 2450 K in a fuel-lean CH(4)-O(2)-N(2) flame. Temperatures were derived from least-squares fits of simulated and experimental spectra. Both the fundamental vibrational band and the first hot vibrational band were included in fitting. In the case of the tube furnace, the N(2) and the O(2) CARS temperature measurements agreed to within 3%, and results were similar with the thermocouple; in the flame the agreement was to within 1%. We conclude that, for cases in which O(2) is present in sufficient concentrations ( approximately 10% or greater), the accuracy of O(2) thermometry is comparable with that of N(2).     

Multiplex H2 coherent anti-Stokes Raman scattering thermometry with a modeless laser

The use of a modeless laser as the Stokes source for multiplex coherent anti-Stokes Raman scattering in molecular hydrogen is reported. The elimination of noise associated with mode competition in conventional standing wave lasers is shown to result in reliable and accurate single-shot thermometry of H(2) in a microwave-assisted diamond chemical vapor deposition plasma reactor. Single-shot temperatures are recorded with a precision of 7.3%. Possible improvements to this precision are discussed and applications of the technique for on-line process monitoring are briefly presented.     

Improving signal-to-interference ratio in rich hydrocarbon-air flames using picosecond coherent anti-Stokes Raman scattering

There is growing interest in the use of short-pulse lasers for coherent anti-Stokes Raman scattering (CARS) to minimize non-resonant background (NRB) contributions in a variety of applications. Using time-coincident picosecond (ps) pump and Stokes beams and a time-delayed ps probe beam, we show that a three orders of magnitude reduction in NRB interference can be achieved in rich hydrocarbon-air flames while preserving 60% to 80% of the CARS signal. This represents a significant improvement in signal-to-interference ratio compared with previous measurements in room temperature air and is attributable to reduced rates of collisional dephasing and relaxation at flame temperatures. Measurements within the flame zone of a laminar flat-flame burner are used to investigate the characteristics of time-coincident and probe-delayed broadband ps N(2)-CARS spectra for C(2)H(4)-air equivalence ratios of 0.5 to 1.2. Up to three ro-vibrational bands of N(2) are excited with each laser shot using 135 ps pump and 106 ps Stokes beams, and the CARS signal is generated using a 135 ps probe beam delayed by 165 ps. The enhanced signal-to-interference ratio achieved in the current work is one to two orders of magnitude higher than that previously achieved using polarization-selection techniques without sensitivity to the effects of birefringence caused by density gradients or test cell windows. Moreover, the use of a 135 ps laser source in this study enables frequency domain "broadband" CARS with sufficient resolution to extract ro-vibrational spectral features under various flame conditions. The effect of probe delay and NRB suppression on characteristics of these broadband CARS spectra are investigated, and evidence of preferential collisional dephasing and relaxation of different ro-vibrational transitions is not detected. This is a promising but preliminary result to be investigated further in future work.     

Stray Light Rejection in Rotational Coherent Anti-Stokes Raman Spectroscopy by use of a Sodium-Seeded Flame

A common experimental problem with rotational coherent anti-Stokes Raman spectroscopy (CARS) is undesired spectral interference that is due to stray light from the primary laser beams. Also, for the most developed approach, dual-broadband rotational CARS, practical measurements often suffer from stray light interference from the narrow-band laser, inasmuch as the CARS signal is produced inherently in the spectral vicinity of the narrow-band laser beam. An optical filter does not provide a sufficiently sharp transmission profile, thus leading to signal loss and spectral distortion of the rotational CARS signal. An atomic filter consisting of a sodium-seeded flame is presented here as a solution to the problem, and its usefulness was demonstrated in dual-broadband rotational CARS experiments.     

Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed.     

Multimodal coherent anti-Stokes Raman spectroscopic imaging with a fiber optical parametric oscillator

We report on multimodal coherent anti-Stokes Raman scattering (CARS) imaging with a source composed of a femtosecond fiber laser and a photonic crystal fiber (PCF)-based optical parametric oscillator (FOPO). By switching between two PCFs with different zero dispersion wavelengths, a tunable signal beam from the FOPO covering the range from 840 to 930 nm was produced. By combining the femtosecond fiber laser and the FOPO output, simultaneous CARS imaging of a myelin sheath and two-photon excitation fluorescence imaging of a labeled axons in rat spinal cord have been demonstrated at the speed of 20 μs per pixel.     

Measurements of OH mole fraction and temperature up to 20 kHz by using a diode-laser-based UV absorption sensor

Diode-laser-based sum-frequency generation of ultraviolet (UV) radiation at 313.5 nm was utilized for high-speed absorption measurements of OH mole fraction and temperature at rates up to 20 kHz. Sensor performance was characterized over a wide range of operating conditions in a 25.4 mm path-length, steady, C2H4-air diffusion flame through comparisons with coherent anti-Stokes Raman spectroscopy (CARS), planar laser-induced fluorescence (PLIF), and a two-dimensional numerical simulation with detailed chemical kinetics. Experimental uncertainties of 5% and 11% were achieved for measured temperatures and OH mole fractions, respectively, with standard deviations of < 3% at 20 kHz and an OH detection limit of < 1 part per million in a 1 m path length. After validation in a steady flame, high-speed diode-laser-based measurements of OH mole fraction and temperature were demonstrated for the first time in the unsteady exhaust of a liquid-fueled, swirl-stabilized combustor. Typical agreement of approximately 5% was achieved with CARS temperature measurements at various fuel/air ratios, and sensor precision was sufficient to capture oscillations of temperature and OH mole fraction for potential use with multiparameter control strategies in combustors of practical interest.     

Interline Transfer CCD Camera for Gated Broadband Coherent Anti-Stokes Raman-Scattering Measurements

Use of an interline transfer CCD camera for the acquisition of broadband coherent anti-Stokes Raman-scattering (CARS) spectra is demonstrated. The interline transfer CCD has alternating columns of imaging and storage pixels that allow one to acquire two successive images by shifting the first image in the storage pixels and immediately acquiring the second image. We have used this dual-image mode for gated CARS measurements by acquiring a CARS spectral image and shifting it rapidly from the imaging pixel columns to the storage pixel columns. We have demonstrated the use of this dual-image mode for gated single-laser-shot measurement of hydrogen and nitrogen CARS spectra at room temperature and in atmospheric pressure flames. The performance of the interline transfer CCD for these CARS measurements is compared directly with the performance of a back-illuminated unintensified CCD camera.     

Gas-phase thermometry using delayed-probe-pulse picosecond coherent anti-Stokes Raman scattering spectra of H2

We report the development and application of a simple theoretical model for extracting temperatures from picosecond-laser-based coherent anti-Stokes Raman scattering (CARS) spectra of H2 obtained using time-delayed probe pulses. This approach addresses the challenges associated with the effects of rotational-level-dependent decay lifetimes on time-delayed probing for CARS thermometry. A simple procedure is presented for accurate temperature determination based on a Boltzmann distribution using delayed-probe-pulse vibrational CARS spectra of H2; this procedure requires measurement at only a select handful of probe-pulse delays and requires no assumptions about sample environment.