An optical fiber according to an embodiment comprises, as a structure suitable for large capacity transmission over a long haul, a core, a cladding having an outer diameter of 80 m or more and 130 m or less, a primary coating, and a secondary coating having elasticity higher than that of the primary coating and an outer diameter of 210 m or less. The optical fiber having the structure as described above has an MFD of 10 m or more at a wavelength of 1550 nm, a cable cutoff wavelength longer than 1260 nm, and a microbending loss of 0.6 dB/km or less at a wavelength of 1550 nm.

The optical fiber according to the present disclosure is an optical fiber having a pure silica core which is a step index core through which an LPm1 mode with an effective area of 250 m.sup.2 or more is propagated by setting a relative refractive index difference to 0.60% or less, considering an effective cutoff condition, and only 2 or more modes or the LPm1 mode are propagated, with the optical fiber as the transmission line.

An uncoupled-core multicore optical fiber is disclosed, the fiber including at least two core portions, each core portion including a core and a depressed-index cladding. The core having a radius r.sub.1 and a relative refractive index .sub.1. The depressed-index cladding having a radius r.sub.2 and a relative refractive index .sub.2, the depressed-index cladding surrounding and directly contacting the core, a volume V.sub.2 of the depressed-index cladding being about 15.0% -micron.sup.2 to about 37.0% -micron.sup.2. The fiber further includes a common cladding having a radius r.sub.3 and a relative refractive index .sub.3 such that .sub.2<.sub.3<.sub.1, the common cladding surrounding and directly contacting the depressed-index cladding. Furthermore, a cable cutoff wavelength of each core portion is about 1530 nm or less and a center-to-center spacing between centerlines of adjacent core portions is about 48 microns to about 60 micron.

The present invention satisfies at least one of the condition of the degree of freedom of a primary layer 11 shown in the equation (I) and the condition of the rigidity of a secondary layer 12 shown in the equation (II). Thus, a coated optical fiber 1 capable of suppressing transmission loss in a low temperature environment is provided, in which, even when an optical fiber 10 having a large effective core cross-sectional area A.sub.eff of the optical fiber 10 at a wavelength of 1550 nm and having high microbend sensitivity is used, transmission loss in a low temperature environment can be suppressed.
[Math. 1]
.sub.PP.sub.ISM<600(I)
(S/P)(S.sub.ISM/P.sub.ISM)1000(II)

Embodiments of current disclosure include a multicore optical fiber including a common-cladding region having a refractive index .sub.cc and an outer radius R.sub.CC; and at least two core portions disposed within the common-cladding region, wherein each core portion includes a central axis, a core region extending from the central axis to an outer radius r.sub.i, wherein each of the at least two core portions is doped with a dopant from a group including sodium, potassium, rubidium or combination thereof, an inner-cladding region encircling and directly contacting the core region and extending from the outer radius r.sub.1 to an outer radius r.sub.2, a trench region encircling and directly contacting the inner cladding region and extending from the outer radius r.sub.2 to an outer radius r.sub.3, the trench region having a trench volume greater than or equal to 20% micron.sup.2 and less than or equal to 60% micron.sup.2.

It is an object of the present invention to provide a multicore optical fiber, a design method for the multicore optical fiber and an optical transmission method using the multicore optical fiber including four cores having a standard cladding diameter of 125?1 ?m for an existing single mode optical fiber covering several thousands of kilometers of transmission. The multicore optical fiber according to the present invention disposes two-stage claddings with different refractive indices around each core, and designates as a predetermined range, a core radius a1, a radius a2 of a first cladding region surrounding each core, specific refractive index ?1 relative to the core of the first cladding region and a specific refractive index ?2 relative to the core of a second cladding region including four cores and the first cladding region.

An optical fiber having a core comprising silica and greater than 1.5 wt % chlorine and less than 0.5 wt % F, said core having a refractive index .sub.1MAX, and an inner cladding region having refractive index .sub.2MIN surrounding the core, where .sub.1MAX>.sub.2MIN.

The present disclosure provides a few mode optical fiber (100). The few mode optical fiber (100) includes a core region (102). A core region (102) defined by a region around a central longitudinal axis (116) of the few mode optical fiber (100). In addition, the core region (102) has a first annular region (106) extended from central longitudinal axis (116) to radius r.sub.1, a second annular region (108) extended from radius r.sub.1 to radius r.sub.2, a third annular region (110) extended from radius r.sub.2 to radius r.sub.3, a fourth annular region (112) extended from radius r.sub.3 to radius r.sub.4 and a fifth annular region (114) extended from radius r.sub.4 to radius r.sub.5. Also, the few mode optical fiber (100) has a cladding defined by the sixth annular region (104) extended from radius r.sub.5 to radius r.sub.6.

It is an object of the present invention to provide a multicore optical fiber, a design method for the multicore optical fiber and an optical transmission method using the multicore optical fiber including four cores having a standard cladding diameter of 125?1 ?m for an existing single mode optical fiber covering several thousands of kilometers of transmission. The multicore optical fiber according to the present invention disposes two-stage claddings with different refractive indices around each core, and designates as a predetermined range, a core radius a1, a radius a2 of a first cladding region surrounding each core, specific refractive index ?1 relative to the core of the first cladding region and a specific refractive index ?2 relative to the core of a second cladding region including four cores and the first cladding region.

Provided is a few-mode optical fiber. The optical fiber includes: a core and a cladding enclosing the core. The cladding includes: a first inner cladding surrounding the core; a first high-refractive-index mode filter layer surrounding the first inner cladding; a second inner cladding surrounding the first high-refractive-index mode filter layer; a second high-refractive-index mode filter layer surrounding the second inner cladding; and an outer cladding surrounding the second high-refractive-index mode filter layer.