Single mode fibre with a trapezoid core, showing reduced losses

A single mode optical fibre having a core surrounded by a cladding, the core refractive index profile having a trapezoid-like shape,

• wherein said core comprises (i) a centre part having a radius r0 and a refractive index no and (ii) a transition part ranging from radius r0 to a radius r1>r0, wherein r1 is between 4.5 microns and 7.2 microns;
• wherein a ratio r0/r1 of said centre part's radius r0 to said transition part's radius r1 is between 0.25 and 0.75;
• wherein, to reduce Rayleigh scattering losses at a 1550 nm wavelength, said transition part of the trapezoid-like core refractive index profile is obtained by gradually changing a concentration of at least two dopants from a concentration in said centre part of said core to a concentration in a cladding part adjacent to said core;
• wherein said centre part of said core comprises one or more dopants inducing refractive index higher than silica and wherein said one or more dopants induce a refractive index increase of 0.6×10−3 or more, wherein the concentration of said one or more dopants inducing refractive index higher than silica decreases substantially linearly across said core's transition part between radius r0 and radius r1;
• and
• wherein said cladding comprises one or more dopants inducing refractive index lower than silica and wherein said one or more dopants induce a refractive index decrease of −0.2×10−3 or less, wherein the concentration of said one or more dopants inducing refractive index lower than silica changes substantially linearly across said core's transition part between radius r0 and radius r1.