Fabrication of a low-loss plastic optical fiber

In a fabrication method of low-loss plastic optical fibers by using a polymer prepared from methyl methacrylate as principal constituent for core material, and forming a cladding of a synthetic macromolecular compound having a lower refractive index than that of said core material therearound, the improvement comprises, in a sealed system, the steps of:

• adding a polymerization initiator and a chain transfer agent to a methyl methacrylate monomer under a reduced pressure condition in such a manner that the number of dust in said methyl methacrylate monomer to which said polymerization initiator and chain transfer agent have been added is substantially 1 or less per 1 mm.sup.3 at any place in said monomer,
• subjecting said methyl methacrylate monomer to which said polymerization initiator and said chain transfer agent have been added to bulk polymerization at a temperature higher than the glass transition temperature of said methyl methacrylate polymer, thereby preparing said polymer while further maintaining said reduced pressure condition in such a manner that each content of transition metallic ions in said polymer is 50 ppb or less for iron and manganese, 10 ppb or less for copper and nickel, 5 ppb or less for chromium, or 2 ppb or less for cobalt,
• subjecting said polymer to melt spinning at such a temperature that voids are not generated to form a core fiber, and
• applying said cladding of the substantially amorphous synthetic macromolecular compound having a lower refractive index than that of said core fiber around said core fiber.