Precision of multiplex CARS temperatures using both single-mode and multimode pump lasers

Area of Science:

• Spectroscopy
• Laser-induced fluorescence
• Combustion diagnostics

Background:

• Coherent anti-Stokes Raman scattering (CARS) is a powerful technique for temperature and species concentration measurements in combustion.
• Single-pulse CARS offers high temporal resolution but can be limited by spectral noise.
• Understanding factors influencing CARS spectral quality is crucial for accurate thermometry.

Purpose of the Study:

• To investigate the effect of pump laser bandwidth on noise levels in single-pulse resonant nitrogen CARS.
• To improve the precision of single-pulse CARS thermometry through advanced spectral fitting methods.
• To analyze the impact of collisional narrowing and cross-coherence on CARS temperature measurements.

Main Methods:

• Single-pulse resonant nitrogen CARS spectra were acquired in a flat-flame burner.
• Pump laser bandwidth was systematically varied.
• A weighted fitting procedure was applied to experimental and theoretical CARS spectra using detector noise coefficients.
• CARS temperatures were compared with those obtained via Na line-reversal.

Main Results:

• Noise in resonant nitrogen CARS spectra decreased with increasing pump laser bandwidth.
• This trend is opposite to that observed for nonresonant CARS.
• Weighted spectral fitting significantly enhanced the precision of temperature measurements.
• Inclusion of collisional narrowing and cross-coherence affected best-fit temperatures.

Conclusions:

• Wider pump laser bandwidth is beneficial for reducing noise in resonant nitrogen CARS.
• Weighted spectral fitting is a key advancement for precise single-pulse CARS thermometry.
• Theoretical models incorporating collisional effects improve CARS temperature accuracy in combustion.