Contrast ratio variation caused by radiation attenuation in pulse width measurement by two-photon absorption fluorescence

In the technique for measuring pulse width by two-photon absorption fluorescence, theoretical calculations indicate that one can hope to obtain a maximum contrast ratio of three between the point at which the two peaks overlap and the point at which there is no superposition. Nevertheless, this calculation has been carried out assuming that the pulse is not attenuated as it traverses the fluorescent medium. In this paper it will be shown that this attenuation gives rise to a decrease in the contrast ratio. As the fluorescence intensity is proportional to the absorption, an increase in the former will be to the detriment of the contrast ratio. We will attempt to give a criterion for the selection of experimental conditions which lead to maximum intensity without an appreciable decrease in the contrast ratio.