A co-doped optical fiber is provided having an attenuation of less than about 0.17 dB/km at a wavelength of 1550 nm. The fiber includes a core region in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a first cladding region in the fiber that surrounds the core region. Further, the core region has a relative refractive index of about −0.10% to about +0.05% compared to pure silica. In addition, the core region includes silica that is co-doped with chlorine at about 1.2% or greater by weight and fluorine between about 0.1% and about 1% by weight.

The present invention relates to an ultra-low loss optical fiber for long haul communications (100) comprising a core region (102) defined by a core relative refractive index and a cladding region surrounding the core region, defined by a cladding relative refractive index. In particular, the core region comprises a relative refractive index in a range of −0.06% to +0.06% and the cladding region is down-doped for entire radial cladding thickness. Moreover, the cladding region further comprises an inner cladding region (104) defined by an inner cladding relative refractive index and an outer cladding region (106) defined by an outer cladding relative refractive index. The inner cladding relative refractive index is less than the outer cladding relative refractive index.

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]

β.sub.P×P.sub.ISM<600  (I)

(S/P)×(S.sub.ISM/P.sub.ISM)≤1000  (II)

Provided is an optical fiber having identification mark, which includes an optical transmission medium (exemplified by glass fiber) including a core part and a cladding part, a primary resin layer coating the optical transmission medium, and a secondary resin layer coating the primary resin layer, in which identification marks for optical fiber identification (exemplified by continuous identification mark) are provided on a surface of the secondary resin layer at a predetermined interval along an axial direction of the optical transmission medium. An effective area of the optical transmission medium at a wavelength of 1550 nm is 90 μm.sup.2 or more, the predetermined interval is 100 mm or more and 500 mm or less, and a Young's modulus of the primary resin layer is 0.9 MPa or less.

In some implementations, a monolithic optical fiber may comprise a tapered core having a first diameter at an input end and a second diameter at an output end. The tapered core may comprise a first tapered region at the input end, a second tapered region at the output end, and a central region having a constant diameter that is larger than the first diameter and the second diameter. The first tapered region expands monotonically from the first diameter to the constant diameter of the central region along a length of the first tapered region, and the second tapered region contracts monotonically from the constant diameter of the central region to the second diameter along a length of the second tapered region. The monolithic optical fiber may be used as a delivery fiber to deliver a laser beam from a fiber laser engine to a process head.

A quasi-single-mode optical fiber with a large effective area is disclosed. The quasi-single-mode fiber has a core with a radius greater than 5 μm, and a cladding section configured to support a fundamental mode and a higher-order mode. The fundamental mode has an effective area greater than 170 μm.sup.2 and an attenuation of no greater than 0.17 dB/km at a wavelength of 1530 nm. The higher-order mode has an attenuation of at least 1.0 dB/km at the wavelength of 1530 nm. The quasi-single-mode optical fiber has a bending loss of less than 0.02 dB/turn for a bend diameter of 60 mm for a wavelength of 1625 nm.

Optical transmission systems and methods are disclosed that utilize a QSM optical fiber with a large effective area and that supports only two modes, namely the fundamental mode and one higher-order mode. The optical transmission system includes a transmitter and a receiver optically coupled by an optical fiber link that includes at least one section of the QSM optical fiber. Transmission over optical fiber link gives rise to MPI, which is mitigated using a digital signal processor. The QSM optical fiber is designed to have an amount of DMA that allows for the digital signal processor to have reduced complexity as reflected by a reduced number of filter taps as compared to if the DMA were zero.

In an optical fiber, a plurality of individual cores extend through a common cladding. Each individual core supports at least one local transverse spatial mode. The individual cores and surrounding cladding are structured to support propagation of plurality of desired signal-carrying modes, while suppressing undesired modes, thereby supporting the propagation of one or more spatially multiplexed signals. The core-to-core spacing of the fiber is configured to maintain an acceptably low level of mode-coupling between cores.

A co-doped optical fiber is provided having an attenuation of less than about 0.17 dB/km at a wavelength of 1550 nm. The fiber includes a core region in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a first cladding region in the fiber that surrounds the core region. Further, the core region has an relative refractive index of about −0.10% to about +0.05% compared to pure silica. In addition, the core region includes silica that is co-doped with chlorine at about 1.2% or greater by weight and fluorine between about 0.1% and about 1% by weight.

An optical fiber connection is provided that includes a first optical fiber defining a first exterior surface and a first effective area. The first fiber defines a first tapered region tapering from a first nominal fiber diameter to a first tapered diameter. A second optical fiber has a second exterior surface and a second effective area less than the first effective area. The second fiber defines a second tapered region tapering from a second nominal fiber diameter to a second tapered diameter and a fiber splice optically coupling the first tapered region of the first fiber to the second tapered region of the second fiber. The first and second tapered regions taper such that the first and second exterior surfaces have a variance from a Gaussian function of less than 25% of the Gaussian function at each point along the first and second exterior surfaces.