We have performed high-resolution N<sub>2</sub> 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 (<i>M</i><sub><i>j</i></sub> = 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.
© 2003 Optical Society of America
(280.2490) Remote sensing and sensors : Flow diagnostics
(300.6230) Spectroscopy : Spectroscopy, coherent anti-Stokes Raman scattering
(300.6420) Spectroscopy : Spectroscopy, nonlinear
(300.6450) Spectroscopy : Spectroscopy, Raman
Joel P. Kuehner, Mark A. Woodmansee, Robert P. Lucht, and J. Craig Dutton, "High-Resolution Broadband N2 Coherent Anti-Stokes Raman Spectroscopy: Comparison of Measurements for Conventional And Modeless Broadband Dye Lasers," Appl. Opt. 42, 6757-6767 (2003)