A method of forming a longitudinal solid body within at least one longitudinal hole formed in an optical fiber, comprising the steps of placing a first end and a portion of the optical fiber in a heating chamber; keeping a second end of said optical fiber outside of the heating chamber; forcing a molten material into the longitudinal hole of the fiber from the first end; and thereupon longitudinally extrancting the optical fiber from the heating chamber at a controlled rate. Preferably, the fiber is extracted while molten material is being urged into said fiber.
