Abstract: A method and an apparatus for manufacturing a fluoride glass fiber preform are disclosed which preclude the step of pouring a glass melt into a mold from a crucible, and hence permit the fabrication of a long, homogeneous fluoride glass fiber preform free from foreign substances and air bubbles leading to scattering and which also allow ease in the fabrication of a preform having an elliptic core portion for drawing a polarized wave retaining fiber.

Abstract: A fusion splicing method for optical fibers in which optical fibers are fusion spliced in an inert gas atmosphere after water adsorbed on their surfaces is removed by decomposition in a plasma of an inert gas containing a halogen.

Abstract: A method and apparatus is disclosed which ensures highly accurate control of the core-cladding diameter, enabling the fabrication of a preform for the single mode fiber. Moreover, glass refining steps for dehydration, the removal of compound ions, the reduction of the absorption loss by transition metals, etc. and preform manufacturing steps are combined into a series of steps, and the entire manufacturing process can be mechanized and automatically controlled; therefore, the yield rate of product is high and the industrial-scale productivity is also excellent.

Abstract: An optical coupler is formed of at least one polarization maintaining single mode fiber, a polarizing beam splitter connected to the polarization maintaining single mode fiber, and a main optical fiber which is connected with the polarizing beam splitter at one end thereof. This optical coupler is useful as a changeover switch for a light source.

Abstract: In a fluoride glass optical fiber with extremely small transmission loss for infrared light having a core layer and a clad layer each consisting mainly of ZrF.sub.4 - BaF.sub.2 - LaF.sub.3 - AlF.sub.3, NaF and H.sub.f F.sub.4 are used as additives to obtain a desired refractive index difference between the core layer and the clad layer. The mixing ratio of the NaF to the H.sub.f F.sub.4 satisfies the realtions;.vertline.0.25 .DELTA.H.sub.f F.sub.4 -3.DELTA.NaF.vertline..ltoreq.5,where .DELTA.NaF is the difference between the amount (mol %) of the N.sub.a F to be added to the core layer and that to be added to the clad layer, and .DELTA.H.sub.f F.sub.4 to be added to the core layer and that to be added to the clad layer.

Abstract: A fluoride glass for infrared optical transmission fiber is purified for dehydration and deoxidation to reduce transmission loss by reacting melted fluoride glass with NF.sub.3 gas at temperature between 500.degree. C. and 800.degree. C. No scattering loss is increased by the present purification as decomposed product in the reaction does not precipitate.

Abstract: A fault location system for locating the fault point of an optical fiber transmission system has been found. The present system comprises a laser for generating an optical beam, a sweep oscillator for modulating said laser beam, a half mirror mounted between the outlet of the laser and the inlet of the optical fiber to be tested, an optical receiver for converting the optical beam reflected by the fault point to an electrical signal through said half mirror, a modulator for providing the frequency difference between the instantaneous output frequency of said sweep oscillator and the output frequency of said optical receiver, a low pass filter connected to the output of said modulator, and a spectrum analyzer connected to the output of the low pass filter. The present system has the advantage that the S/N of the system is excellent, and plural fault points can be located at the same time.