Optical fibre and optical fibre device

optical fiber having a coating layer (30) surrounding the core (20), the coating layer (30) at least one relatively inner first zone (26), a third relatively outer zone (29) and a second comprising zone (27) disposed between the first and the third zone, the second zone (27) an index higher than the first snack and the third zone, characterized in that the maximum difference in refractive index between the first cladding region ( 26) and the core (20) is less than 0, 0030, approximately.