Nonlinear optical tagging and laser induced fluorescence

Optical test‐particle diagnostics based on laser induced fluorescence are typically performed in the regime of weak optical pumping or in the regime of a short tagging pulse. In such cases, it is usually possible to construct a linear transfer function to describe the effect of tag‐beam optical pumping on search beam fluorescence. The regime where particle dynamics plays a role in the optical pumping and laser induced fluorescence processes is explored. In this new regime, the diagnostic signal is nonlinearly related to the pump beam amplitude, and it becomes necessary to consider the time‐dependent process of optical pumping on each particle orbit. The possibilities of techniques based on this extension are illustrated by a simple example which shows how ion Fokker–Planck diffusion can be measured with a single laser beam.