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

Abstract

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 O 2 Q-branch manifolds and three S-branch H 2 rotational lines were acquired in H 2 -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/cm 2 , and by saturation when the product of the pump and Stokes irradiances is above 2 Â 10 3 GW 2 /cm 4 . The H 2 S-branch is more sensitive to Stark broadening, and small variations in the line amplitude are observed for total irradiance of 170 GW/cm 2 . Temperature measurements are not sensitive to Stark broadening, but they are affected by saturation when the pump-Stokes irradiances product is above 2 Â 10 4 GW 2 /cm 4 .

Dual-Pump CARS Measurements in the University of Virginia's Dual-Mode Scramjet: Configuration "A"

“…In the dual-pump CARS technique, originally developed by Lucht and coworkers, 6,7 the two pump beams are at different wavelengths and two different ranges of Raman shift may be accessed. The CARS method employed in this work is based on the techniques developed at NASA Langley, 3,8,9,3,10,11 particularly those described in a companion paper by Magnotti. 12 …”

Development of a Dual-Pump CARS System for Measurements in a Supersonic Combusting Free Jet

“…5 and 9 cannot be explained by the choice of line-shape model (or optical diffusion coefficient). Stark broadening 12 of the H 2 lines, was likely responsible of the erroneous measurements of the H 2 /N 2 ratio reported in those previous works. DP-CARS is capable of obtaining absolute mole fractions of all major species in an air-H 2 flame, provided a knowledge of the third order non-resonant susceptibility of all the major species, both resonant and non-resonant.…”

“…The CARS system is the same as described in Ref. 12, with some modifications noted below. An injectionseeded pulsed Nd:YAG laser (Spectra Physics Quanta Ray Pro-350) frequency-doubled to 532 nm, a narrow band dye laser (Spectra Physics PDL-2) centered at 551 nm with a FWHM of ~0.5 cm -1 and a home-built broadband dye laser centered at 603 nm with a FWHM of 265 cm -1 , are used to generate the CARS signal.…”

“…The long focal length of the two lenses insures that the laser irradiances are below the Stark broadening and stimulated Raman pumping thresholds determined in Ref. 12. Two beam viewing systems 14 are used to image a cross section of the three beams in the vicinity of the measurement volume.…”

“…But the long volume is required to produce high signal-to-noise ratio while avoiding Raman pumping and Stark effect. 12 In addition if the temperature in the flow varies along the measurement volume the integrated CARS signal is biased toward lower temperature based on Eq(2). 45 As shown in Fig.…”