In recent years there has been a major upgrade of the JET Far InfraRed (FIR) diagnostic system consisting of a new laser system with the wavelength at 118.8mm at and more advanced processing electronics for phase counting. This provides a second colour measurement of the electron plasma density on the vertical system, Due to the shorter wavelength, the plasma induced laser beam refraction is reduced by a factor of three alleviating density errors caused by loss of signal (so-called "fringe-jumps"), in particular during high performance plasmas experiments in JET."