A single mode optical fiber comprising: a central core region having outer radius r 1 and relative refractive index Δ 1; a cladding region comprising: (i) a first inner cladding region, situated directly adjacent to said central core region, having an outer radius r 2 > 6 microns and relative refractive index Δ 2 and 0.3 ≤r 1 /r 2 ≤0.85; (ii) a second inner cladding region situated immediately adjacent to said first inner cladding region and having an outer radius r 3 > 9 microns and comprising a minimum relative refractive index Δ 3min, the absolute value of the volume V 3 of the second inner cladding region being 35% Δμm 2 ≤ V 3 ≤ 105 % Δμm 2; and (iii) an outer cladding region surrounding the second inner cladding region and comprising relative refractive index Δ 4, wherein Δ 1 > Δ 2 > Δ 3, Δ 3 < Δ 4; and 0.25 ≤ | Δ 4 -Δ 3,min | ≤0.7 characterized in that: the second inner cladding region has at least one region with a relative refractive index delta that becomes more negative with increasing radius such that 1.5≥β ≥0.5, where β = R 3 − R 2 Δ 3, min dΔ dr average, where dΔ dr average is the average index slope in the second inner cladding region. a central core region having outer radius r 1 and relative refractive index Δ 1; a cladding region comprising: (i) a first inner cladding region, situated directly adjacent to said central core region, having an outer radius r 2 > 6 microns and relative refractive index Δ 2 and 0.3 ≤r 1 /r 2 ≤0.85; (ii) a second inner cladding region situated immediately adjacent to said first inner cladding region and having an outer radius r 3 > 9 microns and comprising a minimum relative refractive index Δ 3min, the absolute value of the volume V 3 of the second inner cladding region being 35% Δμm 2 ≤ V 3 ≤ 105 % Δμm 2; and (iii) an outer cladding region surrounding the second inner cladding region and comprising relative refractive index Δ 4, wherein Δ 1 > Δ 2 > Δ 3, Δ 3 < Δ 4; and 0.25 ≤ | Δ 4 -Δ 3,min | ≤0.7 (i) a first inner cladding region, situated directly adjacent to said central core region, having an outer radius r 2 > 6 microns and relative refractive index Δ 2 and 0.3 ≤r 1 /r 2 ≤0.85; (ii) a second inner cladding region situated immediately adjacent to said first inner cladding region and having an outer radius r 3 > 9 microns and comprising a minimum relative refractive index Δ 3min, the absolute value of the volume V 3 of the second inner cladding region being 35% Δμm 2 ≤ V 3 ≤ 105 % Δμm 2; and (iii) an outer cladding region surrounding the second inner cladding region and comprising relative refractive index Δ 4, wherein Δ 1 > Δ 2 > Δ 3, Δ 3 < Δ 4; and 0.25 ≤ | Δ 4 -Δ 3,min | ≤0.7 characterized in that: the second inner cladding region has at least one region with a relative refractive index delta that becomes more negative with increasing radius such that 1.5≥β ≥0.5, where β = R 3 − R 2 Δ 3, min dΔ dr average, where dΔ dr average is the average index slope in the second inner cladding region. the second inner cladding region has at least one region with a relative refractive index delta that becomes more negative with increasing radius such that 1.5≥β ≥0.5, where β = R 3 − R 2 Δ 3, min dΔ dr average, where dΔ dr average is the average index slope in the second inner cladding region.