A single mode optical fiber, comprising: (i) a silica based core having a graded refractive index profile with an alpha of less than 5, a relative refractive index .sub.1max, and an outer radius r.sub.1, wherein 10 microns>r.sub.16.5 microns, the core comprising Cl, Ge, or a combination thereof; (ii) a first cladding region in contact with and surrounding the core, the first cladding region having a relative refractive index .sub.2min, an inner radius r.sub.1, and an outer radius r.sub.2, wherein r.sub.2<20 microns; and (iii) an outer cladding region surrounding the first cladding region, the outer cladding region having a relative refractive index .sub.3. The fiber has MFD at 1310 nm>than 9 microns, a zero dispersion wavelength <1306 nm; a 22 m cable cutoff wavelength <1260nm; and a bend loss <0.005 dB/turn when the fiber is bent around a 30 mm mandrel; and <0.5dB/turn when the fiber is bent around a 20 mm mandrel.

The single-mode optical fibers have a core region that includes an inner core region having a delta value .sub.1 and a radius r.sub.1 immediately surrounded by an outer core region of radius r.sub.2 and a delta value .sub.2<.sub.1, wherein .sub.1-.sub.2 is in the range from 0.3% to 2%. A cladding region of radius r.sub.3 immediately surrounds the core region. The inner and outer regions define an annular width r=r.sub.2r.sub.1. At least one of r.sub.1, r.sub.2, r and r.sub.3 changes with a period p in the longitudinal direction between first and second values each having a corresponding level distance d.sub.F. The change occurs over a transition distance d.sub.T such that d.sub.T/d.sub.F<0.1. The Brillouin frequency shift f changes by an amount [f] that is least 10 MHz over each period p, thereby allowing for Brillouin frequency-shift management in fiber-based sensor systems.

An optical fiber having both low macrobend loss and low microbend loss. The fiber has a central core region, a first (inner) cladding region surrounding the central core region and having an outer radius r.sub.2>16 microns and relative refractive index .sub.2, and a second (outer) cladding region surrounding the first cladding region having relative refractive index, .sub.3, wherein .sub.1>.sub.3>.sub.2. The difference between .sub.3 and .sub.2 is greater than 0.12 percent. The fiber exhibits a 22 m cable cutoff less than or equal to 1260 nm, and r.sub.1/r.sub.2 is greater or equal to 0.24 and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn.

A single mode optical fiber having a core made from silica and less than or equal to about 6.5 weight % germania and having a maximum relative refractive index .sub.1MAX. The optical fiber also has an inner cladding surrounding the core and having a minimum relative refractive index .sub.2MIN. A difference between a softening point of the core and a softening point of the inner cladding is less than or equal to about 20 C., and .sub.1MAX>.sub.2MIN. The single mode optical fiber may also have an outer cladding surrounding the inner cladding made from silica or SiON. The outer cladding has a maximum relative refractive index .sub.3MAX, and .sub.3MAX>.sub.2MIN. A method for manufacturing an optical fiber includes providing a preform to a first furnace, the preform, drawing the optical fiber from the preform, and cooling the drawn optical fiber in a second furnace.

A multicore fiber for communication 10 which allows propagation of an optical signal includes: a clad 12;a core 11a which is arranged in a center of the clad 12; and seven to ten cores 11b which are arranged at equal intervals surrounding the core 11a, and the cladding diameter is 230 m, distances between centers of the mutually neighboring cores 11a and 11b are 30 m or more, distances between the centers of the cores 11b and an outer peripheral surface of the clad 12 are 35 m or more and a mode field diameter of light propagating in the cores 11a and 11b is 9 m to 13 m.

An optical fiber includes a core having a maximum refractive index n.sub.1, and cladding provided around the core and having a refractive index n.sub.0 that is lower than the maximum refractive index n.sub.1. A radial refractive-index profile of the core is expressed with an exponent that is 1.5 to 10. A relative refractive-index difference .sub.1 at a center of the core that is expressed as .sub.1=100(n.sub.1.sup.2n.sub.0.sup.2)/(2n.sub.1.sup.2) is 0.3% to 0.5%. A diameter 2a of the core is 9 m to 14 m. A zero-dispersion wavelength is 1300 nm to 1324 nm. A cable cutoff wavelength cc is 1260 nm or shorter. A bending loss at a wavelength of 1550 nm in a case where the optical fiber is wound by ten turns with a bend diameter of 30 mm is 0.25 dB or smaller.

A multicore fiber for communication 10 which allows propagation of an optical signal includes: a clad 12; a core 11a which is arranged in a center of the clad 12; and seven to ten cores 11b which are arranged at equal intervals surrounding the core 11a, and the cladding diameter is 230 m, distances between centers of the mutually neighboring cores 11a and 11b are 30 m or more, distances between the centers of the cores 11b and an outer peripheral surface of the clad 12 are 35 m or more and a mode field diameter of light propagating in the cores 11a and 11b is 9 m to 13 m.

An optical fiber having both low macrobend loss and low microbend loss. The fiber has a central core region, a first (inner) cladding region surrounding the central core region and having an outer radius r.sub.2>16 microns and relative refractive index .sub.2, and a second (outer) cladding region surrounding the first cladding region having relative refractive index, .sub.3, wherein .sub.1>.sub.3>.sub.2. The difference between .sub.3 and .sub.2 is greater than 0.12 percent. The fiber exhibits a 22 m cable cutoff less than or equal to 1260 nm, and r.sub.1/r.sub.2 is greater or equal to 0.24 and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn.

An optical fiber includes a core having a maximum refractive index n.sub.1, and cladding provided around the core and having a refractive index n.sub.0 that is lower than the maximum refractive index n.sub.1. A radial refractive-index profile of the core is expressed with an exponent a that is 1.5 to 10. A relative refractive-index difference .sub.1 at a center of the core that is expressed as .sub.1=100(n.sub.1.sup.2n.sub.0.sup.2)/(2n.sub.1.sup.2) is 0.3% to 0.5%. A diameter 2a of the core is 9 m to 14 m. A zero-dispersion wavelength is 1300 nm to 1324 nm. A cable cutoff wavelength cc is 1260 nm or shorter. A bending loss at a wavelength of 1550 nm in a case where the optical fiber is wound by ten turns with a bend diameter of 30 mm is 0.25 dB or smaller.

The present embodiment relates to a CMCF including a structure to achieve more efficient reduction in transmission loss by suppressing decrease in concentration of alkali metal due to diffusion of alkali metal. In the CMCF including a plurality of cores, a power coupling coefficient h between adjacent cores is set to 110.sup.3/m or more, to maintain an optical coupling state between the adjacent cores. In addition, alkali metal contributing to reduction in transmission loss is added to each of the cores such that a stress maximum value _.sub.max between adjacent cores has a negative value.