On Tore Supra, a far infrared polarimeter diagnostic has been routinely used for diagnosing the current density by measuring the Faraday rotation angle. As this angle is a few dozen of degrees on present plasma fusion devices, a high precision is needed to correctly reconstruct the current profile. In this article, we present numerical methods to calculate the phase and the amplitude. A new method of phase shifting calculation is described. The validations have been made using the Tore Supra and JET polarimetry signals in comparing the calculated signals with the signals coming from analogue cards. These methods have been developed to be used on the future numerical cards that will replace the Tore Supra present analogue ones.