This application relates generally to an optical fiber for the delivery of infrared light where the polarization state of the light entering the fiber is preserved upon exiting the fiber and the related methods for making thereof. The optical fiber has a wavelength between about 0.9 ?m and 15 ?m, comprises at least one infrared-transmitting glass, and has a polarization-maintaining (PM) transverse cross-sectional structure. The infrared-transmitting, polarization-maintaining (IR-PM) optical fiber has a birefringence greater than 10.sup.?5 and has applications in dual-use technologies including laser power delivery, sensing and imaging.

The single-mode optical fibers have a core region that includes an inner core region having a delta value .sub.1 and a radius r.sub.1 immediately surrounded by an outer core region of radius r.sub.2 and a delta value .sub.2<.sub.1, wherein .sub.1-.sub.2 is in the range from 0.3% to 2%. A cladding region of radius r.sub.3 immediately surrounds the core region. The inner and outer regions define an annular width r=r.sub.2r.sub.1. At least one of r.sub.1, r.sub.2, r and r.sub.3 changes with a period p in the longitudinal direction between first and second values each having a corresponding level distance d.sub.F. The change occurs over a transition distance d.sub.T such that d.sub.T/d.sub.F<0.1. The Brillouin frequency shift f changes by an amount [f] that is least 10 MHz over each period p, thereby allowing for Brillouin frequency-shift management in fiber-based sensor systems.

An optical fiber includes: a core portion including a center core doped with germanium; and a cladding portion having a refractive index lower than a maximum refractive index of the core portion and surrounding an outer periphery of the core portion. The cladding portion has a relative refractive index difference of a positive value equal to or lower than 0.1% with respect to pure silica glass, an alkali metal element is doped in the center core to be diffused, and a peak of a concentration distribution of the alkali metal element in a radial direction is positioned at a distance away from the center of the center core by two times or more a radius of the center core.

There is provided an optical fiber for providing increased sensitivity in sensing applications by increasing the Rayleigh backscatter coefficient of the fiber while maintaining tolerable levels of signal attenuation (e.g., less than 20% over 10 km). Such an optical fiber comprises a core, a first cladding layer and a second cladding layer. The core comprises at least one core dopant selected from the range of: germanium, phosphorus, aluminium, boron, fluorine. The at least one core dopant is used to increase the core refractive index and enhance the core Rayleigh backscatter coefficient. The first cladding layer comprises at least one dopant selected from: germanium, phosphorus, aluminium, boron, fluorine; wherein at least one first cladding layer dopant is used to reduce the first cladding layer refractive index. The signal attenuation generated in the fiber is less than 20% over 1 km.

According to embodiments, an optical fiber may include a core portion comprising an outer radius r.sub.C and a maximum relative refractive index .sub.Cmax. A cladding may surround the core portion and include a low-index trench and an outer cladding. The low index trench may surround the core portion and includes an outer radius r.sub.T and relative refractive index .sub.T. The outer cladding may surround and be in direct contact with the low-index trench. The outer cladding may be formed from silica-based glass comprising greater than 1.0 wt. % bromine and has a relative refractive index .sub.OC, wherein .sub.Cmax>.sub.OC>.sub.T. The optical fiber may have a cable cutoff of less than or equal to 1530 nm. An attenuation of the optical fiber may be less than or equal to 0.185 dB/km at a wavelength of 1550 nm.

The core region of an optical fiber is doped with chlorine in a concentration that allows for the viscosity of the core region to be lowered, approaching the viscosity of the surrounding cladding. An annular interface region is disposed between the core and cladding and contains a concentration of fluorine dopant sufficient to match the viscosity of the core. By including this annular stress accommodation region, the cladding layer can be formed to include the relatively high concentration of fluorine required to provide the desired degree of optical signal confinement (i.e., forming a low loss optical fiber). The inclusion of the annular stress accommodation region allows for the formation of a large effective area optical fiber that exhibits low loss (i.e., <0.19 dB/km) in both the C-band and L-band transmission ranges.

A high-divergence-angle optical fiber apparatus is disclosed that includes a multimode optical fiber having a distal end and a divergence angle . A light-redirecting structure is operably disposed at the distal end and consists of an array of between 1 and 10 layers of fused glass microspheres. The light-redirecting structure defines a divergence angle , wherein 2. A light source system that utilizes the high-divergence-angle optical fiber apparatus is also disclosed.

Provided is an alkali doping process of bringing a melt of an alkali metal compound or an alkaline earth metal compound into contact with a part of the inner circumferential surface of a silica glass tube, and thus doping the silica glass tube with the alkali metal compound or the alkaline earth metal compound, and in the alkali doping process, the contact location between the inner circumferential surface of the silica glass tube and the melt is moved along the longitudinal direction of the silica glass tube while rotating the silica glass tube around its longitudinal axis.

An optical fiber preform of the present embodiment comprises a core portion and a cladding each comprised of silica glass. The core portion has a first dopant region including a central axis of the core portion and a second dopant region away from the central axis. The first dopant region contains a first dopant selected from among Na, K, and their compounds, and a concentration of the first dopant is 10 atomic ppm or more but 2,000 atomic ppm or less. The second dopant region contains a second dopant reducing viscosity of the silica glass. The second dopant has, as a characteristic at a temperature of 2,000 C. to 2,300 C., a diffusion coefficient of 110.sup.12 cm.sup.2/s or higher but lower than that of the first dopant, and a concentration of the second dopant region is 10 atomic ppm or more.

An optical fiber that has an air layer in a clad portion and can suppress a degradation in a transmission characteristic caused by a support member present in the air layer is provided. The optical fiber includes a core (1), tubular layers (22 to 25) concentrically arranged around the core (1) via the air layer, and support members (3a to 3l) that are arranged to the air layer and connect the core (1) with the tubular layers (22 to 25), and in the optical fiber, in a cross-sectional view of a longitudinal direction of the optical fiber, a distance between support members (3a to 3l) in a circumferential direction of the optical fiber is wider than a double thickness of the air layer in which the support members (3a to 3l) are arranged.