Nonlinear susceptibility interference in laser SRS-CARS diagnostics of hydrogen in gas mixtures

We have studied the possibility to measure small hydrogen concentrations in dense gas mixtures by method of coherent anti-Stokes Raman scattering (CARS) in combination with biharmonic laser pumping by means of stimulated Raman scattering (SRS). It was found that the interference of nonlinear susceptibilities of a buffer gas and hydrogen can lead to a parabolic dependence of the signal intensity on the hydrogen concentration, which makes the results of analyses uncertain. The ambiguity can be eliminated by selecting an appropriate composition and pressure of the gas mixture in the cell of the SRS generator of biharmonic laser pumping. Using this approach, hydrogen in air at atmospheric pressure can be detected by the laser SRS-CARS technique on a level of 5 ppm.