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

Abstract

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.

Development of temperature imaging using two-line atomic fluorescence

“…The inherently low signal associated with Raman does restrict the application of spontaneous Raman approaches to thermometry due to issues with signal quality and is typically limited to line measurements. Offering stronger signal, and more applicable to luminous and particle-laden flows, coherent anti-Stokes Raman spectroscopy (CARS)-based thermometry is highly advantageous [13]. Nonetheless, the necessity for line-of-sight optical access, the experimental complexity (including effects of laser mode), and the lack of spatial fidelity in comparison to planar techniques does restrict the generic application of CARS [7].…”

Accurate temperature measurements in flames with high spatial resolution using Stokes Raman scattering from nitrogen in a multiple-pass cell

“…8 and Nikkor 50 mm f ∕1.4 camera lenses that are used in order to avoid chromatic and spherical aberrations. The position and orientation of the camera lenses and their separation are governed by the requirement that it is desirable to keep the front surface of the collection optics at a reasonable distance from the burner, and the f ∕# at the back of the collection optics should match the f ∕# of the spectrometer.…”

“…More recently, the multiple-passing concept was applied to a silver-coated capillary cell to enhance the Raman signal from CO 2 and CH 4 by up to 20 times [7]. Multiple passing was also used to generate a CARS signal in a flame from as many as nine points simultaneously [8]. A multiple-pass cell, which is similar to the one reported in this paper, with signal gain of ∼45 was also employed for the detection of trace gas in transformer oil [9].…”

High-Data-Rate One-Dimensional Thermometry Using Femtosecond CARS

“…OHERENT anti-Stokes Raman scattering (CARS) spectroscopy has been well established as an accurate thermometry technique in gasphase reacting flows [1]. Although CARS is a third-order nonlinear signal generation process requiring intense laser pulsesparticularly when applied in high-temperature gas-phase medium-its potential for spatially resolved one-dimensional (1-D) line imaging of temperature and species concentration was recognized during the very early stages of CARS development [2].…”