An optical fiber has a core region that is doped with one or more viscosity-reducing dopants in respective amounts that are configured, such that, in a Raman spectrum with a frequency shift of approximately 600 cm.sup., the fiber has a nanoscale structure having an integrated D2 line defect intensity of less than 0.025. Alternatively, the core region is doped with one or more viscosity-reducing dopants in respective amounts that are configured such that the fiber has a residual axial compressive stress with a stress magnitude of more than 20 MPa and a stress radial extent between 2 and 7 times the core radius.

According to another aspect of the invention a majority of the optical propagation through the fiber is supported by an identified group of fiber regions comprising the core region and one or more adjacent cladding regions. The fiber regions are doped with one or more viscosity-reducing dopants in respective amounts and radial positions that are configured to achieve viscosity matching among the fiber regions in the identified group.

Proportions of the element erbium, the element aluminum, and the element phosphorus are adjusted during optical fiber preparation so that aluminum phosphate is formed around the element erbium in a prepared optical fiber, a probability that the element erbium in the optical fiber transits to a high energy level is reduced, and an excited-state absorption effect of the element erbium in the optical fiber on an optical signal is suppressed.

A manufacturing method of porous glass base material for optical fiber with suppressed characteristic fluctuation in the longitudinal direction is provided. A manufacturing method of optical fiber base material by the VAD method includes: detecting a tip position of a porous glass base material during deposition; controlling the pull-up speed to keep the tip position constant; in an early stage of deposition, a target pull-up speed is set for each deposition time, and a raw material gas flow rate to the burner is adjusted and corrected at a predetermined correction interval to achieve the target pull-up speed, thereby depositing while gradually changing the pull-up speed until a predetermined time; in a steady state, the deposition is performed such that the pull-up speed is kept constant to form the porous glass base material; and the porous glass base material is dehydrated and vitrified into transparent glass in a heating furnace.

A method of manufacturing including: (a) a first vapor deposition step comprising vapor depositing a first porous glass body of a glass former and a doping constituent onto a substrate; (b) a cleaning step after the first vapor deposition step, the cleaning step comprising exposing the first porous glass body to a cleaning gas at a cleaning temperature for a cleaning period of time, the cleaning gas (i) removing a metal or metal oxide from the first porous glass body, (ii) changing an oxidation state of a metal or metal oxide within the first porous glass body, or (iii) a combination of (i) and (ii); and (c) a second vapor deposition step after the cleaning step, the second vapor deposition step comprising vapor depositing a second porous glass body of the glass former onto the first porous glass body resulting in a porous preform for an optical fiber.