Optical fiber and transmission system, and wavelength division multiplexing system

An optical fiber comprising a center core and a cladding located at an outer periphery of the center core,

• wherein the center core comprises
• an inner core, located in the vicinity of a radial center of the fiber, the inner core comprising a codoped layer comprising silica glass doped with germanium and fluorine, and
• at least one lower-concentration codoped layer comprising one of:
• silica glass doped with germanium, and
• silica glass doped with germanium and fluorine;
• wherein an amount of fluorine in the at least one lower-concentration codoped layer is smaller than an amount of fluorine in the inner core;
• and
• wherein a concentration of germanium oxide in the inner core is in a range between 4% and 15% by mass, and a concentration of fluorine in the inner core is in a range between 0.2% and 5% by mass.