  Space-borne nulling interferometers have long been considered as the best
option for searching and characterizing extra-solar planets located in the
habitable zone of their parent stars. Solutions for achieving deep starlight
extinction are now numerous and well demonstrated. However they essentially aim
at realizing an achromatic central null in order to extinguish the star. In
this communication is described a major improvement of the technique, where the
achromatization process is extended to the entire fringe pattern. Therefore
higher Signal-to-noise ratios (SNR) and appreciable simplification of the
detection system should result. The basic principle of this Fully achromatic
nulling interferometer (FANI) consists in inserting dispersive elements along
the arms of the interferometer. Herein this principle is explained and
illustrated by a preliminary optical system design. The typical achievable
performance and limitations are discussed and some initial tolerance
requirements are also provided
