Abstract: In an optical amplifier for amplifying a signal light by propagating the signal light and a pumping light in a rare earth element doped fiber doped with a rare earth element, a diameter of a rare earth element doped portion of the rare earth element doped fiber is gradually reduced in a direction of propagation of the pumping light. With this construction, an adverse rare earth element doped area which does not contribute to optical amplification, but rather attenuates the pumping light, can be eliminated to thereby provide an optical amplifier having increased amplification efficiency.

Abstract: An optical fiber amplifier is disclosed wherein pumping light and signal light are introduced into an optical fiber doped with a rare-earth element to directly amplify the signal light. The optical fiber amplifier of the invention comprises means constituted from a reflecting film, a fiber loop or the like for causing pumping light to pass by a plurality of times in the doped fiber. Due to the provision of the reflective film or the fiber loop, pumping light can be utilized efficiently, and the amplification factor of the optical fiber amplifier is improved.

Abstract: The present invention relates to an optical fiber cable. In view of narrowing optical fiber cables, the spiral grooves to accommodate optical fiber cables are formed in the core loading material, in the longitudinal direction, by bundling high tensile strength synthetic resin fibers and covering the external circumference of the bundled material with a synthetic resin covering layer. Moreover, a method is disclosed for manufacturing an optical fiber cable which stably loads the optical fibers by holding the core loading material in order to eliminate deviation generated in the rotating direction around the core loading material on the occasion of loading fibers in the spiral grooves of core loading material.

Abstract: An apparatus connects a single-mode optical fiber in a transmitting side to a multi-mode optical fiber in a receiving side without a deterioration of baseband transmission characteristics. The apparatus optically connects the single-mode fiber to the multi-mode fiber with an optical axes displacement therebetween, so that an optical signal to a multi-mode optical fiber from a single mode optical fiber is incident in a steady state mode including not only lower mode, but higher mode. In one embodiment, the optical axes displacement is achieved by axial displacement between the single-mode fiber and multi-mode fiber. In another embodiment, the optical axes displacement is achieved by an angle displacement between the single-mode fiber and multi-mode fiber.

Abstract: An optical waveguide having a first cross section is thinned to constitute a thinned portion having a second cross section, as long as a predetermined length. Surface of the thinned portion is contacted with a laser glass member doped with a rare earth element, such as Er. Due to the thinned diameter, the optical fiber is optically coupled with the laser glass member. A pumping light is input to either into the optical waveguide together with a signal light or directly into the laser glass member. Signal light coupled from the thinned portion to the laser glass member is amplified by stimulated emission of the laser glass. Thus amplified signal light is coupled back to the thinned portion so as to propagate along the optical waveguide. The thinned portion can be as short as several centi-meters compared with ten meters to more than one hundred meters of conventional optical amplifier.