As described herein, a mode field adapter (MFA) comprises a first fiber including a core associated with a fundamental mode field diameter and a cladding with a diameter that decreases toward a waist. The MFA comprises a second fiber including a core associated with a fundamental mode field diameter that matches the fundamental mode field of the first fiber at the waist and a cladding with a diameter that matches the diameter of the cladding of the first fiber at the waist and increases from the waist of the second fiber. The cladding of the first fiber may be adiabatically etched such that a core-to-cladding ratio of the first fiber changes over a length of the first fiber, and the core and the cladding of the second fiber may be adiabatically tapered such that a core-to-cladding ratio of the second fiber is constant over a length of the second fiber.

An optical fiber according to an embodiment includes a core, a cladding, and a coating layer. At the boundary between the core and the cladding, the local sound velocity decreases in the direction from the core side toward the cladding side. At least in the cladding, the local sound velocity changes continuously in a radial direction. Further, the line width of the Brillouin gain of the light beam guided by the fundamental mode is 60 MHz or more.

An optical fiber according to an embodiment includes a core, a cladding, and a coating layer. At the boundary between the core and the cladding, the local sound velocity decreases in the direction from the core side toward the cladding side. At least in the cladding, the local sound velocity changes continuously in a radial direction. Further, the line width of the Brillouin gain of the light beam guided by the fundamental mode is 60 MHz or more.

An object of the present invention is to provide an HAF having a structure in which the number of air holes is decreased to be smaller than that of a PCF and Rayleigh scattering loss may be more reduced than that in the existing HAF. The HAF according to the present invention includes a core portion having a uniform optical refractive index; a cladding portion having a uniform optical refractive index and surrounding the core portion; and a plurality of air holes arranged in two layers at positions configuring hexagonal closest packing excluding the core portion within the cladding portion along a longitudinal direction of the hole-assisted fiber, wherein a center-to-center spacing of the air holes is a sum of a radius Rin of an inscribed circle inscribed in the air holes in an inner layer and a radius d/2 of the air hole, and a radius “a” of the core portion and a relative refractive index difference Δ between the core portion and the cladding portion are present within a range where Rn, which is a ratio of a Rayleigh scattering coefficient Rsmf of a single mode optical fiber and an effective Rayleigh scattering coefficient Reff of the hole-assisted fiber, is equal to or less than 0.92.

An object is to obtain an optical fiber having a small diameter and suppressing the increase of a microbending loss of the optical fiber. The optical fiber includes: a core portion made of silica glass; a cladding portion made of silica glass, the cladding portion covering the outer periphery of the core portion and having a refractive index smaller than a maximum refractive index of the core portion; and a coating portion covering the outer periphery of the cladding portion. The outer diameter of the cladding portion is 100 μm or smaller, the relative refractive-index difference Δ1 of the core portion is 0.5% or smaller, and the thickness of the coating portion is 10 μm or larger.

An object is to obtain an optical fiber having a small diameter and suppressing the increase of a microbending loss of the optical fiber. The optical fiber includes: a core portion made of silica glass; a cladding portion made of silica glass, the cladding portion covering the outer periphery of the core portion and having a refractive index smaller than a maximum refractive index of the core portion; and a coating portion covering the outer periphery of the cladding portion. The outer diameter of the cladding portion is 100 μm or smaller, the relative refractive-index difference Δ1 of the core portion is 0.5% or smaller, and the thickness of the coating portion is 10 μm or larger.

The optical fibers disclosed have single mode and few mode optical transmission for VCSEL-based optical fiber transmission systems. The optical fibers have a cable cutoff wavelength λ.sub.C of equal to or below 1260 nm thereby defining single mode operation at wavelengths greater than 1260 nm and few-mode operation at wavelengths in a wavelength range from 800 nm and 1100 nm. The mode-field diameter is in the range from 8.0 microns to 10.1 microns at 1310 nm. The optical fibers have an overfilled bandwidth OFL BW of at least 1 GHz.Math.km at at least one wavelength in the wavelength range. The optical fibers have a single-step or two-step core and can have a trench refractive index profile. VCSEL based optical transmission systems and methods are disclosed that utilize both single core and multicore versions of the optical fiber.

The production and new type of preforms are presented which yield, upon drawing, new, class of optical fibers, improved, speckle-free output optical fibers. Useful fibers, providing speckle-free, smooth output with flat top transmission of light from gaussian or few mode sources are produced from preforms introduced herein. The unique production of these improved preforms is also presented. The preforms, and thus the fibers produced in varying core dimensions from about 100 μm to above 1000 μm, are based on a structured silica section of mode mixing area adjacent to the inner core, or in the case of non-circular core, within the core. Plasma Vapor Deposition process is modified to achieve the structured sections in a well-controlled manner. The structured sections are composed of a number of pairs of layers, where a thin down-doped layer is alternated with a much thicker core material layer. The ratio of the thickness of the core layer to the thickness of the down-doped layer is about 3 to 25. The number of paired layers is typically between about 8 to 30-layer pairs. The effective NA of the structured section is dependent on the particulars of the structured silica section and of the individual down-doped layer. Both circular inner core examples and non-circular core examples are possible and are discussed, herein.

A gain flattening filter includes a first optical fiber that has a core, a first cladding, and a second cladding and that has a uniform composition in a length direction; and a pair of second optical fibers fused to both ends of the first optical fiber. The first optical fiber has a first section in which a slanted refractive index grating is formed and a pair of second sections connecting both ends of the first section to the pair of second optical fibers. The first cladding contains a photosensitive material whose refractive index increases upon irradiation with light having a specific wavelength. In the core, a tensile stress remains in the first section. An average MFD of the second sections is larger than an average MFD of the second optical fibers and smaller than an average MFD of the first section.

An optical combiner includes: first optical input portions each including a first optical input waveguide; and an optical output portion to which the first optical input portions are connected and that includes a first core that allows light to propagate therethrough, and a cladding layer disposed outside of the first core and that has a refractive index lower than a refractive index of the first core. The first optical input portions are connected to a connection end face of the optical output portion such that the first optical input waveguide of at least one of the first optical input portions is optically coupled to the first core of the optical output portion.