AbstractA multi-channel Flit laser interferometer system is being constructed for the measurement of the spatial and temporal behaviour of the electron density profile on the Large Helical Device. The optical configuration is of the Michelson interferometer type with 13 vertical chords. A highly stable twin CH3OH laser pumped by a cw CO2 laser has been developed for the direct measurement of the phase shift. The resolution of the phase comparators is 1100 fringes, and corresponds to a line averaged density of 5.6 × 1016 m−3 at the central chord.