Saturation and Stark broadening effects in dual-pump CARS of N₂, O₂, and H₂

The dual-pump coherent anti-Stokes Raman spectroscopy (CARS) technique is frequently employed in the study of turbulent flames, but the applicability of this technique is limited by low signal-to-noise ratios at high temperatures. The signal-to-noise ratio can be increased by increasing the energy of the lasers. Unfortunately, there is a limit to the amount of energy that can be added before the signal is affected by high irradiance perturbation effects such as Stark broadening and stimulated Raman pumping (saturation). Such effects alter the spectral shape of the CARS signal and compromise the measurement accuracy. To explore these effects, dual-pump CARS spectra containing the N ₂ and O₂ Q-branch manifolds and three S-branch H ₂ rotational lines were acquired in H₂-air flames while varying the irradiances of the different input beams. For fuel-lean flames at 1180 K and 1 atm., absolute mole fraction measurements are affected by Stark broadening when the total irradiance is above 250 GW/cm², and by saturation when the product of the pump and Stokes irradiances is above 2 × 10³ GW²/cm⁴. The H₂ S-branch is more sensitive to Stark broadening, and small variations in the line amplitude are observed for total irradiance of 170 GW/cm². Temperature measurements are not sensitive to Stark broadening, but they are affected by saturation when the pump-Stokes irradiances product is above 2 × 10⁴ GW²/cm⁴.