A polarized wave holding optical fiber (10) of the present invention includes a plurality of core portions (12a, 12b) in a high refractive index region that are juxtaposed in a radial direction of an optical fiber, and three core portions (12a, 12b) integrally propagate one fundamental mode. A method of producing a polarized wave holding optical fiber according to the present invention comprises forming a plurality of holes (22) in parallel with one another on a diameter of a glass rod (21) having a low refractive index as a clad, inserting glass rods (23) having a high refractive index as core portions into these holes (22), heating and integrating them together to form a base material, and melt-spinning the base material. A polarized wave holding optical fiber containing a rare earth element according to the present invention is formed by adding a rare earth element to a waveguide portion of the polarized wave holding optical fiber, and an optical amplifier and a laser oscillator utilize this polarized wave holding optical fiber. A polarized wave holding optical fiber coupler according to the present invention connects at least two polarized wave holding optical fibers, and heats, fuses and stretches them. The connection method comprises heating portions near the coupling point before and after connection.

A low loss fiber optic coupler is fabricated by forming a coupler preform (10) having a plurality of spaced glass cores (12,13) extending longitudinally through a matrix of glass having a refractive index lower than that of the cores. The preform is heated and stretched to form a glass rod (20) which is then severed into a plurality of units (21). Heat is applied to the central region (23) of each unit while the ends of the unit are pulled apart to elongate the taper inwardly the heated central region. The unit is then provided with a plurality of optical fibers (69, 70, 75, 76°, 82, 83; 89), one of which extends from each of the cores at the endfaces of the unit. A preferred method of providing the optical fibers involves forming the coupler preform of a matrix glass (46) that is easily dissolves in a solvent. Each of the fiber cores within the matrix is surrounded by a layer of cladding glass (54) that is relatively resistant to disolving by the solvent.

A continuously drawn optical fiber (25) comprising a core (26) and cladding (27) having different refractive indices and forming a single-mode guiding region, and the outer surface of the fiber (25) having a cross-section forming a pair of orthogonal exterior fiat surfaces (29, 30) so that the location of the guiding region can be ascertained from the exterior geometry of the fiber (25), the guiding region being offset from the center of gravity of the transverse cross-section of the fiber (25) and located sufficiently close to at least one of the flat surfaces (29, 30) to allow coupling to a guided wave through that surface by exposure or expansion of the field of the guiding region.

Provided is a method for producing an optical fiber having low attenuation and including a core that contains an alkali metal element. An optical fiber preform that includes a core part and a cladding part is drawn with a drawing apparatus 1 to form an optical fiber 30, the core part having an average concentration of an alkali metal element of 5 atomic ppm or more and the cladding part containing fluorine and chlorine. The optical fiber includes a glass portion and resin coating portion and the glass portion is under residual stress that is a compressive stress of 130 MPa or less. During the drawing, the time during which an individual position of the optical fiber preform is maintained at 1500°C or higher is 110 minutes or less. The drawing speed is preferably 1200 m/min or more and more preferably 1500 m/min to 2300 m/min. The optical fiber preform preferably has a diameter of 70 mm to 170 mm and more preferably 90 mm to 150 mm.

A single-mode optical fiber suitable for the transmission of solitons has a refractive index profile that changes along the length of the fiber to provide a fiber dispersion that monotonically decreases along the fiber from one end thereof to the other. The (DDF) fiber includes a core of maximum refractive index n₁ and a radius a, surrounded by cladding material having a refractive index n₂ which is less than n₁. The fiber core includes a central region that extends to the longitudinal axis of the fiber and an outer region, the inner and outer regions being separated by a region of depressed refractive index. The inner radius a₁ of the region of depressed refractive index is greater than zero and the maximum radius a₀ of the region of depressed refractive index is less than a. The fiber preform can be made by depositing layers of glass particles on an elongated mandrel, the composition of glass particles being varied with respect to longitudinal position along the preform during the deposition of some of the layers.