High-Resolution Broadband N2 Coherent Anti-Stokes Raman Spectroscopy: Comparison of Measurements for Conventional And Modeless Broadband Dye Lasers

Abstract

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.

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

“…A high-resolution CARS system has been designed to measure in high-speed gas flows and to operate over a 90 -295 K temperature range and a 0.1-5 atm pressure range. 25,26 At ambient conditions, a 10 -15 K standard deviation was obtained.…”

Development of a dual-pump coherent anti-Stokes Raman spectroscopy system for measurements in supersonic combustion

“…The instrument precision needs to be carefully characterized, so that fluctuations in the flow can be distinguished from those caused by the instrument. Standard deviation, computed from several singleshot measurements in an oven or a calibration flame, is often used as a metric for the instrument precision [2,18,26,[40][41][42][43][44][45][46]. Commonly reported single-shot temperature standard deviations for N 2 rovibrational CARS are 25-30 K at 300 K [40][41][42]44].…”

“…Vibrational CARS systems with improved precision (10-15 K) at ambient temperature have been developed [18,45,47]. Such systems are characterized by high spectral resolution, and the use of a single mode pump, and modeless Stokes lasers.…”

Experimental cross-correlation nitrogen Q-branch CARS thermometry in a spark ignition engine

“…In the years following these experiments, Ewart et al [28] demonstrated that the use of a modeless dye laser was able to reduce, or even eliminate the effect of dye laser mode competition during the generation of experimental CARS spectra, which resulted in improved measurement precision [29]. Nevertheless, the CARS system described here remained capable of producing distributions of referenced database spectra in the High Pressure High Temperature (HPHT) cell with standard deviations of 20 K to 35 K in the 400 K to 700 K temperature range, and 30 K to 45 K in the 700 K to 1,100 K temperature range, for all pressures in the 1.0 bar to 21.0 bar range.…”

“…The improvement in CARS measurement precision has developed through improvements to rotational CARS, and the development and use of modeless dye lasers (which removes the spectral noise arising from dye laser mode competition) [28,29]. Of course, this level of precision applies to nearly perfectly heated calibrated environments (calibrated heated cells, and calibrated flames).…”