Low loss optical fiber designs

An optical fiber for transmitting optical power in the 1550 nm wavelength region, wherein the optical fiber has a refractive index profile expressed as refractive index deviation from the refractive index of silica, comprises an inner core (12), an outer core (13) having a minimum radius of 5 microns, a trench (14) having a maximum radius of 25 microns. The refractive index of the outer core is less than the refractive index of the inner core. The refractive index profile is shaped to confine (i) 50-80 percent of the optical power within the inner core, (ii) 20-40 percent of the optical power within the outer core, and (iii) less than 5 percent of the optical power in the trench.