A coated optical fiber includes an optical fiber; and a primary coating encapsulating the optical fiber, the primary coating having an in-situ modulus of about 0.12 MPa or less at a thickness of about 32.5 m, a Young's modulus as a cured film of about 0.7 MPa or less, and a T.sub.g of about 22 C. or below, wherein the primary coating is the cured reaction product of a primary curable composition having a gel-time ratio relative to C1 of less than about 2.

A single-mode optical fiber including a core having a refractive index n, wherein the core comprises a region in which the value of n decreases from a value n.sub.0, to a value n.sub.2; and a cladding having a first layer of cladding in which the refractive index is n.sub.2; and a second layer of cladding in which the refractive index is n.sub.3, lower than n.sub.2; a third layer of cladding in which the refractive index is n.sub.4, higher than n.sub.3;
in which the core radius r.sub.1 is comprised between 2.5 m and 5.5 m; and
in which a refractive-index difference n.sub.0=n.sub.0n.sub.4 is higher than 5.810.sup.3; and
in which a refractive-index difference n.sub.2=n.sub.2n.sub.4 is between 110.sup.3 and 2.510.sup.3; and in which
a refractive-index difference n.sub.3=n.sub.3n.sub.4 is lower than 0.

There is provided a single mode optical fiber including a core layer, a first cladding layer, and a second cladding layer, in which when a refractive index of a center portion of the core layer is set as no, a radial position where a refractive index becomes n.sub.00.45 is set as r.sub.CORE, a minimum value of a refractive index of the first cladding layer is set as n.sub.3, and a radial position where a refractive index becomes the n.sub.3 is set as r.sub.3, a relative refractive index difference of the first cladding layer is reduced continuously and gently from the r.sub.CORE to the r.sub.3.

The present invention provides multicore optical fiber (100, 200, 300, 400, 500, 600, 700, 800) with a cladding region (102), one or more trench regions (104) in the cladding region (102). In particular, the cladding region (102) has at least one core region of one or more core regions (106) on both sides of the one or more trench regions (104). Further, the one or more trench regions (104) is not adjacent to the at least one core region of the one or more core regions (106).