Weakly-coupled few mode optical fibers for mode division multiplexing and corresponding optical transmission system

An optical fiber comprising an optical core surrounded by an optical cladding, wherein the optical core has a refractive-index profile Δn(r) of trapezoid-like shape, as a function of a radial distance r from the center of the optical core, the optical core having a center part of radius R1 and a transition part ranges from the radius R1 to a radius R2, such that R2>R1, said refractive-index profile being defined by:

• a surface integral Acore defined as follows:
• Acore=2∫0R2Δn(r)·dr
• a transition slope S defined as follows:
• S
• =
• 
• Δ
• ⁢
• n
• ⁢
• 2
• -
• Δ
• ⁢
• n
• ⁢
• 1
• R
• ⁢
• 2
• -
• R
• ⁢
• 1
• 
• with:
• Δn1, the refractive-index difference of the center part of the optical core with respect to the outer optical cladding;
• Δn2, the refractive-index difference of a cladding part, adjacent to the optical core, with respect to the outer optical cladding;
• and wherein:
• the surface integral Acore is between 180×10−3 and 270×10−3 μm;
• the radius R2 is between 6.8 and 11.5 μm;
• the transition slope S is between 1.7×10−3 and 12×10−3 μm−1;
• the center part of the optical core comprises a region of depressed refractive index, called inner depressed core, ranging from the center of the optical core to radius R0 and having a refractive-index difference with respect to the outer optical cladding Δn0 such that Δn1>Δn0≥(Δn1−6×10−3), said inner depressed core having a surface integral A0 and assuming a surface integral Atrap as the surface integral of depressless core's refractive-index profile, said surface integrals A0 and Atrap being defined as follows:
• A0=2∫0R0(Δn1−Δn(r))dr
• Atrap=2(Δn1·R1+∫R1R2Δn(r)dr)
• with a ratio A0/Atrap lower than or equal to 0.01.