The real time control of various plasma scenarios is one of the most qualifying JET programs on the route to a next step Tokamak. The feasibility of these control schemes depends crucially on the quality and reliability of diagnostics and the plasma density is one of the most frequently required. In this perspective, in JET a fast acquisition system for the interferometer/polarimeter diagnostic has recently been installed. It produces a measurement every millisecond and it has been fully validated in the last campaigns. The main problem faced by the system consists in the fringe jumps, consequence of the strong variations of the density in coincidence with particular events like ELMs or pellets. To overcome this difficulty several methods can be used, which fall mainly in two general categories. The first category includes approaches that use only the interferometric data. The second class of solutions includes methods based on the Faraday rotation or Cotton-Mouton effects that do not suffer from the fringe jump problem.