Small-radius coated optical fibers having large mode field diameter and low bending losses. The coated fiber may have an outer radius of 110 m or less, while providing a mode field diameter of 9.0 m or greater and a bending loss when wrapped about a 15 mm mandrel of 0.5 dB/km or less at wavelength of 1550 nm. The coated fiber may have a mode field diameter of 9.2 m or greater and may have a bending loss at 1550 nm of 0.25 dB/km or less when wrapped about a 20 mm mandrel or a bending loss at 1550 nm of 0.02 dB/km or less when wrapped about a 30 mm mandrel.

In various embodiments, optical fibers have arrangements of core, annular core, and cladding regions enabling variation of beam shape and/or beam parameter product and may be utilized for the processing (e.g., welding, cutting, drilling, etc.) of various workpieces.

An optical fiber includes: a core; a cladding layer that is lower in refractive index than the core; and a depressed layer that lies between the core and the cladding layer and that is lower in refractive index than the cladding layer, wherein: the optical fiber has an effective core area Aeff that is equal to or greater than 100 m.sup.2 and equal to or less than 129 m.sup.2, the core has a radius r1 that is equal to or greater than 5.2 m and equal to or less than 7.4 m, the core has a refractive index volume Vcore that is equal to or greater than 8.5% m.sup.2 and equal to or less than 16.5% m.sup.2, the depressed layer has a refractive index volume Vdep that is equal to or greater than 40% m.sup.2 and less than 0% m.sup.2.

An anti-cracking panda-type polarization-maintaining optical fiber includes a cladding layer, stress layers, and a fiber core. The fiber core is located in the center of the cladding layer. The stress layers are located symmetrically at two sides of the fiber core with a distance away from the fiber core and are located within the cladding layer. Each stress layer is enclosed at edges of their outer sides by a transition layer with a gradient refractive index. By providing the transition layer with the gradient refractive index at the edge of the outer side of the stress layer, the pressure stress at the edge of the stress layer is decomposed and released, so as to avoid cracks at the edge of the polished stress layer on the end of the optical fiber, and thus optimizes the performance of the polarization-maintaining optical fiber by decreasing the room temperature polishing cracking rate.

An optical fiber includes a core portion made of silica-based glass; and a cladding portion made of silica-based glass having lower maximum refractive index than the core portion, the cladding portion surrounding an outer periphery of the core portion. The core portion is doped with an alkali metal element and chlorine. An average concentration of chlorine is higher than 800 atomic ppm on a cross-section perpendicular to a longitudinal direction of the core portion. A region doped with the alkali metal element is larger than a region doped with chlorine at 800 atomic ppm or higher.

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.

An optical fiber is made of silica-based glass and includes a core, a first cladding that surrounds the core and that has a refractive index lower than a refractive index of the core; and a second cladding that surrounds the first cladding and that has a refractive index lower than the refractive index of the core and higher than the refractive index of the first cladding. At least a part of the first cladding contains a photosensitive material whose refractive index increases by irradiation with light having a specific wavelength. A difference n between a refractive index of a portion of the first cladding, the portion being nearest to the core, and the refractive index of the core is in a range of 0.25% to 0.30%. The radius ra of the core is larger than 4.3 m and smaller than or equal to 5.0 m.

The refractive index of the inner core part 11 in a region in contact with the boundary of the outer core part 12 is higher than the refractive index of the outer core part 12. The refractive index of the outer core part 12 is gradually decreased from the inner circumferential side to the outer circumferential side. The refractive index of the inner cladding part 21 is equal to the refractive index of the outermost circumferential part of the outer core part 12 and not greater than the refractive index of the outer cladding part 22.

An optical fiber includes: a core; a cladding layer that is lower in refractive index than the core; and a depressed layer that lies between the core and the cladding layer and that is lower in refractive index than the cladding layer, wherein: the optical fiber has an effective core area Aeff that is equal to or greater than 100 ?m.sup.2 and equal to or less than 129 ?m.sup.2, the core has a radius r1 that is equal to or greater than 5.2 ?m and equal to or less than 7.4 ?m, the core has a refractive index volume Vcore that is equal to or greater than 8.5% ?m.sup.2 and equal to or less than 16.5% ?m.sup.2, the depressed layer has a refractive index volume Vdep that is equal to or greater than ?40% ?m.sup.2 and less than 0% ?m.sup.2.

An optical fiber rod (30) according to the present invention includes a center region (35), an outer region (31) formed around the center region (35), and an intermediate region (33) formed between the center region (35) and the outer region (31), and satisfies nA>nB>nC where nA is the refractive index of a material A produced by polymerization of a monomer ma, nB is the refractive index of a material B produced by polymerization of a monomer mb, and nC is the refractive index of a material C produced by polymerization of a monomer mc. The center region (35) is made of a material produced by polymerization of a monomer mixture containing the monomer ma, the outer region (31) is made of a material produced by polymerization of a monomer mixture containing the monomer mc, and the intermediate region (33) is made of a material produced by polymerization of a monomer mixture containing the monomer mb. The refractive index decreases in the order: the center region (35)>the intermediate region (33)>the outer region (31).