A single mode optical fiber comprising: a core having outer radius r 1, a maximum refractive index delta percent Δ 1 max and core alpha > 5; a cladding surrounding the core, the cladding comprising (i) inner cladding region having outer radius r 2 and refractive index delta percent Δ 2, wherein Δ 1max > Δ 2; and (ii) a trench region surrounding the inner cladding region, the trench region having an outer radius r 3 > 10 microns and refractive index delta percent Δ 3; and (iii) an outer cladding region surrounding the trench region and comprising refractive index delta percent Δ 4; wherein Δ 1max > Δ 4 and Δ 2 > Δ 3, and Δ 4 > Δ 3 and wherein the Δ 4 - Δ 3 ≥ 0.12 percent, and said fiber exhibits a mode field diameter MFD > 9 microns at 1310 nm wavelength, cable cutoff of less than 1260 nm, and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn, and 1300 nm ≤ λ 0 ≤ 1324 nm, where λ 0 is the zero dispersion wavelength; the fiber being characterized in that the relative refractive index profile is defined relative to the average refractive index of undoped silica, and the outer cladding region comprises a first outer cladding region with refractive index delta percent Δ4 and chlorine concentration larger than 1.2 wt.%; and a second outer cladding region surrounding the first outer cladding region and comprising refractive index delta percent Δ5 such that Δ4> Δ5, the second outer cladding region having chlorine concentration of less than 0.5 wt.%, wherein viscosity of glass of said second outer cladding region is greater than the viscosity of glass of said first outer cladding region.. a core having outer radius r 1, a maximum refractive index delta percent Δ 1 max and core alpha > 5; a cladding surrounding the core, the cladding comprising (i) inner cladding region having outer radius r 2 and refractive index delta percent Δ 2, wherein Δ 1max > Δ 2; and (ii) a trench region surrounding the inner cladding region, the trench region having an outer radius r 3 > 10 microns and refractive index delta percent Δ 3; and (iii) an outer cladding region surrounding the trench region and comprising refractive index delta percent Δ 4; wherein Δ 1max > Δ 4 and Δ 2 > Δ 3, and Δ 4 > Δ 3 and wherein the Δ 4 - Δ 3 ≥ 0.12 percent, and said fiber exhibits a mode field diameter MFD > 9 microns at 1310 nm wavelength, cable cutoff of less than 1260 nm, and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn, and 1300 nm ≤ λ 0 ≤ 1324 nm, where λ 0 is the zero dispersion wavelength; the fiber being characterized in that the relative refractive index profile is defined relative to the average refractive index of undoped silica, and the outer cladding region comprises a first outer cladding region with refractive index delta percent Δ4 and chlorine concentration larger than 1.2 wt.%; and a second outer cladding region surrounding the first outer cladding region and comprising refractive index delta percent Δ5 such that Δ4> Δ5, the second outer cladding region having chlorine concentration of less than 0.5 wt.%, wherein viscosity of glass of said second outer cladding region is greater than the viscosity of glass of said first outer cladding region.. (i) inner cladding region having outer radius r 2 and refractive index delta percent Δ 2, wherein Δ 1max > Δ 2; and (ii) a trench region surrounding the inner cladding region, the trench region having an outer radius r 3 > 10 microns and refractive index delta percent Δ 3; and (iii) an outer cladding region surrounding the trench region and comprising refractive index delta percent Δ 4; wherein Δ 1max > Δ 4 and Δ 2 > Δ 3, and Δ 4 > Δ 3 and wherein the Δ 4 - Δ 3 ≥ 0.12 percent, and said fiber exhibits a mode field diameter MFD > 9 microns at 1310 nm wavelength, cable cutoff of less than 1260 nm, and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn, and 1300 nm ≤ λ 0 ≤ 1324 nm, where λ 0 is the zero dispersion wavelength; the fiber being characterized in that the relative refractive index profile is defined relative to the average refractive index of undoped silica, and the outer cladding region comprises a first outer cladding region with refractive index delta percent Δ4 and chlorine concentration larger than 1.2 wt.%; and a second outer cladding region surrounding the first outer cladding region and comprising refractive index delta percent Δ5 such that Δ4> Δ5, the second outer cladding region having chlorine concentration of less than 0.5 wt.%, wherein viscosity of glass of said second outer cladding region is greater than the viscosity of glass of said first outer cladding region..