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

T/C, which is a ratio of an area T of a skirt part outside the boundary to an area C of the core region in a refractive index distribution, is 4% or more and 30% or less, the boundary is defined at a position where an absolute value of a change amount of the index becomes maximum between the center of the core region and the outer peripheral part of the first clad region, the area C of the core region is defined in a range from the center of the core region to the boundary in the radial direction, the area T of the skirt part is defined in a range from the boundary to the outer peripheral part of the first clad region.

A method of manufacturing an optical fiber glass preform, the method comprising depositing glass particles on a base material, the glass particles being generated by glass making feedstock gas being supplied while a burner and the base material that is rotating are reciprocated relatively to each other, wherein when a portion corresponding to an outer diameter equal to or more than 0.80 L and equal to or less than L is deposited, wherein L represents a final outer diameter of a part of the optical fiber glass preform manufactured, the part being formed by the deposition of the glass particles, the deposition is performed under a first condition where an angle formed by a first line extending from a center O of a cross section of the base material to a rotational position r0 at which one round trip of the relative reciprocation starts and a second line extending from the center O to a rotational position r1 at which the one round trip of the relative reciprocation ends is an angle excluding 0, 120, 240, 72, 144, 216, and 288; or the deposition is performed under a second condition where the angle is 120 or 240, thereby to deposit the glass particles to a thickness corresponding to a thickness equal to or less than 0.03 L; or the deposition is performed under a third condition where the angle is 72, 144, 216, or 288, thereby to deposit the glass particles to a thickness corresponding to a thickness equal to or less than 0.02 L; or the deposition is performed under a fourth condition where the angle is 0, thereby to deposit the glass particles to a thickness corresponding to a thickness equal to or less than 0.01 L.

T/C, which is a ratio of an area T of a skirt part outside the boundary to an area C of the core region in a refractive index distribution, is 4% or more and 30% or less, the boundary is defined at a position where an absolute value of a change amount of the index becomes maximum between the center of the core region and the outer peripheral part of the first clad region, the area C of the core region is defined in a range from the center of the core region to the boundary in the radial direction, the area T of the skirt part is defined in a range from the boundary to the outer peripheral part of the first clad region.

Bromine doping of silica glass is demonstrated. Bromine doping can be achieved with SiBr.sub.4 as a precursor. Bromine doping can occur during heating, consolidation or sintering of a porous silica glass body. Doping concentrations of bromine increase with increasing pressure of the doping precursor and can be modeled with a power law equation in which doping concentration is proportional to the square root of the pressure of the doping precursor. Bromine is an updopant in silica and the relative refractive index of silica increases approximately linearly with doping concentration. Bromine can be used as a dopant for optical fibers and can be incorporated in the core and/or cladding regions. Core doping concentrations of bromine are sufficient to permit use of undoped silica as an inner cladding material in fibers having a trench in the refractive index profile. Co-doping of silica glass with bromine and chlorine is also demonstrated.

A hollow-core antiresonant fiber (HC-ARF) with nested supporting rings (NSRs) has a fiber structure that includes from the inside out a fiber core, a first silica antiresonant ring (SARR), a first air antiresonant ring (AARR), a second SARR, a second AARR and an external silica wall. The fiber structure further includes a first NSR within the first AARR and a second NSR within the second AARR. The HC-ARF with NSRs has advantages and benefits of low confined loss (CL), large bandwidth, simple structure and very good bending characteristics. Therefore, the application fields of HC-ARF are greatly expanded.

A hollow-core antiresonant fiber (HC-ARF) with nested supporting rings (NSRs) has a fiber structure that includes from the inside out a fiber core, a first silica antiresonant ring (SARR), a first air antiresonant ring (AARR), a second SARR, a second AARR and an external silica wall. The fiber structure further includes a first NSR within the first AARR and a second NSR within the second AARR. The HC-ARF with NSRs has advantages and benefits of low confined loss (CL), large bandwidth, simple structure and very good bending characteristics. Therefore, the application fields of HC-ARF are greatly expanded.

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 in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a cladding in the fiber that surrounds the core addition, the core includes silica that is co-doped with two or more halogens.

Provided is an optical fiber containing an alkali metal element or the like having a smaller diffusion coefficient than K and having a low Rayleigh scattering loss. An optical fiber is composed of silica glass and includes a core and a cladding arranged to surround the core which has a lower refractive index than the core. The core includes a first core including a central axis and a second core arranged to surround the first core. The average concentration of an alkali metal element or alkaline-earth metal element in the first core is 10 mol ppm or less. The average concentration of chlorine in the first core is 2000 mol ppm or more. The average concentration of an alkali metal element or alkaline-earth metal element in the second core is 10 mol ppm or more. The average concentration of chlorine in the second core is 10 to 600 mol ppm.

An optical fiber having a core comprising silica and greater than 1.5 wt % chlorine and less than 0.5 wt % F, said core having a refractive index .sub.1MAX, and an inner cladding region having refractive index .sub.2MIN surrounding the core, where .sub.1MAX>.sub.2MIN.