An optical fiber with low attenuation and methods of making same are disclosed. The optical fiber has a core, an inner cladding surround the core, and an outer cladding surrounding the inner cladding. The outer cladding is chlorine-doped such that the relative refractive index varies as a function of radius. The radially varying relative refractive index profile of the outer cladding reduces excess stress in the core and inner cladding, which helps lower fiber attenuation while also reducing macrobend and microbend loss. A process of fabricating the optical fiber includes doping an overclad soot layer of a soot preform with chlorine and then removing a portion of the chlorine dopant from an outermost region of the overclad soot layer. The soot preform with the modified chlorine dopant profile is then sintered to form a glass preform, which can then be used for drawing the optical fiber.

An object is to provide a beam propagating method capable of satisfying desired output power and a desired propagation distance and a required condition of beam quality and a method of designing an optical fiber designing the structure of an optical fiber. According to the present invention, an effective core cross-sectional area A.sub.eff is calculated based on desired specification values and, by appropriately adjusting the structure of an optical fiber satisfying the effective core cross-sectional area and the number of modes to be propagated, the structure of the optical fiber is determined. In this way, by controlling the excitation ratio of a high-order mode at the time of coupling laser light in the optical fiber designed as above, light of high-output laser can be propagated a long distance with the beam quality maintained.

In some implementations, a cladding light stripper includes an optical fiber that includes a core and an exterior cladding, and a light removal component. The exterior cladding of the optical fiber circumferentially surrounds the core of the optical fiber. The exterior cladding includes a first portion, a second portion, and an intermediate portion disposed between the first portion and the second portion. The intermediate portion is bidirectionally tapered. The light removal component is disposed on one or more regions of a central section of the intermediate portion.

Embodiments of optical fiber may include cladding features that include a material (e.g., fluorine-doped silica glass) that may produce a very low relative refractive index difference with respect to cladding material in which the cladding features are disposed. This relative refractive index difference may be characterized by (n.sub.1n.sub.2)/n.sub.1, where n.sub.1 is the index of refraction of the cladding material in which the cladding features are included, and n.sub.2 is the index of refraction of the cladding features. In certain embodiments, the relative refractive index difference may be less than about 4.510.sup.3. In various embodiments, the configuration of the cladding features including, for example, the size and spacing of the cladding features, can be selected to provide for confinement of the fundamental mode yet leakage for the second mode and higher modes, which may provide mode filtering, single mode propagation, and/or low bend loss.

Embodiments of optical fiber may include cladding features that include a material (e.g., fluorine-doped silica glass) that may produce a very low relative refractive index difference with respect to cladding material in which the cladding features are disposed. This relative refractive index difference may be characterized by (n.sub.1-n.sub.2)/n.sub.1, where n.sub.1 is the index of refraction of the cladding material in which the cladding features are included, and n.sub.2 is the index of refraction of the cladding features. In certain embodiments, the relative refractive index difference may be less than about 4.510.sup.3. In various embodiments, the configuration of the cladding features including, for example, the size and spacing of the cladding features, can be selected to provide for confinement of the fundamental mode yet leakage for the second mode and higher modes, which may provide mode filtering, single mode propagation, and/or low bend loss.

Embodiments of the invention relate to a hydrogen-resistant optical fiber with a core having a central axis. The core may include only silica, or only silica and fluorine, while a cladding region surrounding the core may be made of silica and fluorine, along with at least one of germanium, phosphorus, and titanium.

The optical fiber includes a core, the first cladding, and second cladding. The core is made of silica based glass containing Cl. The first cladding and the second cladding are made of silica based glass containing fluorine. The refractive index of the first cladding is lower than that of the core. The refractive index of the second cladding is lower than that of the core and higher than that of the first cladding. The second cladding is divided into an outer region having a uniform refractive index and an inner region having a refractive index higher than that of the outer region. The difference P between the maximum refractive index of the inner region and the refractive index of the outer region is 0.02% to 0.10% in terms of relative refractive index with respect to pure silica based glass. The radial thickness R of the inner region is 10 m to 25 m.

Multimode beam combiners include at least one gradient-step index optical fiber in which a refractive index difference at a core/cladding interface is selected to provide a numerical aperture so as to provide stable, uniform beam output. One or more such fibers is formed into a tapered bundle than can be shaped to provide a selected illuminated aperture. The fibers in the bundle can be separated by respective tapered claddings so as to be optically coupled or uncoupled. Illumination systems can include a plurality of such fibers coupled to a plurality of laser diodes or other light sources.

An uncoupled multicore optical fiber may include: a common cladding having a refractive index .sub.CC and an outer diameter ranging from about 120 m to about 130 m; and a plurality of core portions disposed within the common cladding. At least one core portion may include: a central axis; an alkali doped core region extending from the central axis and having a relative refractive index .sub.1; a trench region encircling the core region and having a relative refractive index .sub.3, wherein .sub.1>.sub.CC>.sub.3; an attenuation less than 0.165 dB/km at 1550 nm; an effective area ranging from about 75 m.sup.2 to about 135 m.sup.2 at 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm. The common cladding may directly contact the trench region. A counter-propagating crosstalk at 1550 nm between two adjacent core portions may be less than or equal to 40 dB/100 km.

An optical fiber including a core region having an outer radius r.sub.1 in a range from 4.0 m to 8.0 m and a relative refractive index profile .sub.1 with a maximum relative refractive index .sub.1max in a range from 0.20% to 0.50%, a cladding region comprising a trench cladding region having a minimum relative refractive index .sub.3 min greater than 0.60% and less than 0.10%, and a trench volume greater than 30%-m.sup.2 and an outer cladding region having a relative refractive index .sub.4 in a range from 0.10% to 0.10%. The optical fiber also including a primary coating and a secondary coating. The optical fiber has a mode field diameter at 1310 nm of greater than 8.8 microns, a cable cutoff wavelength of less than 1260 nm, a zero dispersion wavelength between 1300 nm and 1324 nm, and low macrobend loss at 1550 nm.