Excitation nonlinearities in emission reabsorption laser-induced fluorescence techniques.

The effects of the nonlinear behavior of fluorescent intensity with excitation intensity on emission reabsorption laser-induced fluorescence (ERLIF) are investigated. Excitation nonlinearities arise mainly as a consequence of the depletion of the ground-state population stemming from the finite lifetime of molecules in the excited state. These nonlinearities hinder proper suppression of the excitation intensity information in the fluorescence ratio, degrading measurement accuracy. A method for minimizing this effect is presented. This method is based on the approximation of the fluorescence intensity nonlinearities by a power law. Elevating the two-dimensional fluorescent intensity maps to the appropriate exponent allows for proper suppression of excitation intensity in the fluorescence ratio. An overview of the principles and constitutive equations behind ERLIF film-thickness measurements, along with a characterization of the fluorescence's nonlinear behavior, is presented. The power law approximation and processing scheme used to mitigate this behavior are introduced. Experimental proof of the validity of the approximation and processing scheme is provided.