A single mode optical fiber comprising: a graded index central core region having outer radius r 1, having a relative refractive index Δ 1, a maximum relative refractive index Δ 1max and having an alpha profile, alpha core, of 0.5 ≤ alpha core ≤ 4; a cladding region including: a trench region surrounding said graded index central core region and comprising a relative refractive index delta Δ 3 profile that becomes more negative with increasing radius, said trench region having an inner radius r 2, an outer radius r 3 > 10 microns, and a minimum relative refractive index Δ 3min such that Δ 1max > Δ 3min, r 3 ≥ r 3a, and 0.5 ≤ (r 3a -r 2)/(r 3 -r 2) ≤ 1, where r 3a is a distance from fiber centerline where Δ 3 first reaches the value Δ 3min, said trench region having an alpha profile, alpha t such that 0.5 ≤ alpha t ≤ 5, and an outer cladding region surrounding said trench region and having a relative refractive index Δ 4, and Δ 3min < Δ 4; and characterised by a first inner cladding region having a relative refractive index Δ2, the first inner cladding region surrounding the graded index central core region and situated between the central core region and the trench region, the first inner cladding region having an outer radius r2 ≤ 10 microns, and 0.65 ≤ r1/r2 < 1, wherein the outer radius of said first inner cladding region is equal to the inner radius r2 of the trench region, Δ1max > Δ2 > Δ3min, wherein 0.36 ≤ Δ 1max ≤ 0.44; and wherein the relative refractive index Δ of a layer is defined by: Δ % = 100 × n i 2 − n c 2 / 2 n i 2, where n c is the refractive index of the outer cladding region. a graded index central core region having outer radius r 1, having a relative refractive index Δ 1, a maximum relative refractive index Δ 1max and having an alpha profile, alpha core, of 0.5 ≤ alpha core ≤ 4; a cladding region including: a trench region surrounding said graded index central core region and comprising a relative refractive index delta Δ 3 profile that becomes more negative with increasing radius, said trench region having an inner radius r 2, an outer radius r 3 > 10 microns, and a minimum relative refractive index Δ 3min such that Δ 1max > Δ 3min, r 3 ≥ r 3a, and 0.5 ≤ (r 3a -r 2)/(r 3 -r 2) ≤ 1, where r 3a is a distance from fiber centerline where Δ 3 first reaches the value Δ 3min, said trench region having an alpha profile, alpha t such that 0.5 ≤ alpha t ≤ 5, and an outer cladding region surrounding said trench region and having a relative refractive index Δ 4, and Δ 3min < Δ 4; and a trench region surrounding said graded index central core region and comprising a relative refractive index delta Δ 3 profile that becomes more negative with increasing radius, said trench region having an inner radius r 2, an outer radius r 3 > 10 microns, and a minimum relative refractive index Δ 3min such that Δ 1max > Δ 3min, r 3 ≥ r 3a, and 0.5 ≤ (r 3a -r 2)/(r 3 -r 2) ≤ 1, where r 3a is a distance from fiber centerline where Δ 3 first reaches the value Δ 3min, said trench region having an alpha profile, alpha t such that 0.5 ≤ alpha t ≤ 5, and an outer cladding region surrounding said trench region and having a relative refractive index Δ 4, and Δ 3min < Δ 4; and characterised by a first inner cladding region having a relative refractive index Δ2, the first inner cladding region surrounding the graded index central core region and situated between the central core region and the trench region, the first inner cladding region having an outer radius r2 ≤ 10 microns, and 0.65 ≤ r1/r2 < 1, wherein the outer radius of said first inner cladding region is equal to the inner radius r2 of the trench region, Δ1max > Δ2 > Δ3min, wherein 0.36 ≤ Δ 1max ≤ 0.44; and wherein the relative refractive index Δ of a layer is defined by: Δ % = 100 × n i 2 − n c 2 / 2 n i 2, where n c is the refractive index of the outer cladding region.