Abstract: A bundle of optical fibers each covered by an acid-soluble borosilicate glass is treated with an acid solution, with an alkali solution, and then with an acid solution to completely remove the acid-soluble glass from the middle portion between the end portions of the optical fiber bundle. A plastic material is charged into the interstices among the fibers at the boundary portions between the middle portion and the end portions. These interstices are free of any insoluble residue of the acid-soluble glass. A reinforcement is formed around the periphery of at least a part of the boundary portions by covering the periphery with a flexible plastic material or by winding a linear member around the periphery.

Abstract: A number of three-layer optical fibers each having a core a cladding and an overcoating layer of acid-soluble glass material are bonded together by means of an adhesive which can be removed by applying heat. The bonded optical fibers are heated and stretched to obtain a solid optical fiber bundle. The solid optical fiber bundle is brought into contact with an acid to elute (leach away) the overcoating layer of the acid-soluble glass material.