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, 4.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 22m 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.

Rare earth oxides doped multicomponent glass fibers for laser generation and amplification, including a core and a cladding, the core comprising at least 2 weight percent glass network modifier selected from BaO, CaO, MgO, ZnO, PbO, K.sub.2O, Na.sub.2O, Li.sub.2O, Y.sub.2O.sub.3, or combinations; wherein the mode of the core is guided with step index difference between the core and the cladding, a numerical aperture of the fiber is between 0.01 and 0.04; core diameter is from 25 to 120 micron, and a length of the gain fiber is shorter than 60 cm.

Embodiments include a multicore optical fiber including: a common cladding having a refractive index .sub.CC and an outer radius of 70 m to 95 m; a coating having an outer radius of 120 m to 127.5 m; and a plurality of core portions; wherein at least one core portion comprises: an alkali-doped core region having a relative refractive index .sub.1 and a trench region having a relative refractive index .sub.3, wherein .sub.1>.sub.CC>.sub.3; an attenuation less than 0.165 dB/km, and an effective area of 75 m.sup.2 to 135 m.sup.2 at a wavelength of 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm; wherein the multicore optical fiber has a core multiplicity factor of 0.028 to 0.045; and wherein a counter-propagating crosstalk at 1550 nm between adjacent core portions is less than 40 dB per 100 km.

A heterogeneous multicore optical fiber includes two or more core elements and features low counterpropagating crosstalk at large bend radius. At least a pair of the two or more core elements differ in propagation constant and preferably have similar effective area A.sub.eff at 1550 nm. The heterogeneous multicore optical fiber exhibits a critical bend radius corresponding to a maximum in counterpropagating crosstalk and marking a regime of higher bend radius over which counterpropagating crosstalk decreases. The critical bend radius is preferably less than 2000 mm.

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 1251 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.

A single mode optical fiber including a core region doped with an alkali metal. The optical fiber has a total attenuation at 1550 nm of about 0.155 dB/km or less such that extrinsic absorption in the optical fiber contributes to 0.004 dB/km or less of the total attenuation