Among the qualities desired of an optical waveguide, mechanical strength as well as low transmission loss and low signal distortion are especially required. A number of manufacturing processes such as the M-CVD process, VAD process and molecular stuffing process are well known as methods for producing such fibers. Also the double crucible process and rod-in-tube process are also well known.
It is a general practice to apply a primary coating layer of thermally hardened resins such as silicone or epoxy-resins immediately after fiber drawing before contacting the same with any other substances for reinforcing and maintaining the initial mechanical strength of the fiber. In order to produce mechanically strong fibers, the surface of the drawing preform should be clean and smooth and the preform should be placed in a clean atmosphere when heated in a furnace. Apart from this, it is also required that the fiber be rapidly quenched immediately after the surface has been sufficiently smoothened by heat. Such a drawing condition is realized by making the longitudinal temperature gradient abrupt (a large, reduction ratio, is required for high temperature drawing). When the whole glass rod is softened and drawn into a fiber, the surface of the fiber is usually not smooth enough. CO.sub.2 -laser flame, and Joule or induction electric heaters of small diameter and height are examples of means for achieving these thermal drawing conditions.
The above thermal conditions are particularly required for the rod-in-tube process.
The present invention is based on the molecular stuffing process. Detailed information concerning the molecular stuffing process may be found in Japanese Patent Published Specifications No. 50-28339, 51-135915, 51-126207 and 53-102324, respectively equivalent to U.S. Pat. Nos. 3,938,974; 4,110,096; 4,110,093 and divisionals thereof U.S. Pat. Nos. 4,110,096; 4,183,620; 4,188,198; 4,229,608. These are referred to herein below as the "stuffing" patents. In such processes, a porous glass rod, which consists of SiO.sub.2 and several percent of B.sub.2 O.sub.3 produced by phase-separation, is used as the starting material. The above patents refer only to porous glass rods made by phase-separation; however, the invention also relates to and is applicable to the porous glass rods produced by half-sintering of CVD glass powders or by half-sintering of fine glass fibers.
In the prior art such as is disclosed in Japanese published specification (U.S. Pat. No. 4,110,093), a porous silicate glass including a small amount of B.sub.2 O.sub.3 is doped with a dopant material in the manner described so that the dopant distribution produces a desired refractive-index distribution and gradient in the porous glass rod. The method is described in detail as follows:
The porous glass rod is immersed in an aqueous solution of a compound which will later be converted to an oxide dopant to enhance the refractive index of the glass. For example, an aqueous solution of CsNO.sub.3, which decomposes at high temperature to Cs.sub.2 O, is stuffed in the pores of the rod, and the rod is then immersed in an alcoholic solution in order to reduce the temperature of the rod and/or the solubility of the compound, whereby the compound, such as CsNO.sub.3 etc. is deposited or precipitated on the surface of micropores. The rod is then immersed in a solution so that the "stuffed" material that was deposited is gradually removed from the peripheral portions of the rod by dissolution whereby the concentration of the deposited material is varied according to desired radial gradient distribution. Then the rod is immersed in a solution in order to deposit the complete dopant in the micropores. The rod is then dried in a vacuum and the remaining solvent or water absorbed on the surface of the micro pores is removed by heating. As the temperature is raised further, the deposited compound CsNO.sub.3 is decomposed to the dopant Cs.sub.2 O according to reaction; EQU 2CsNO.sub.3 .fwdarw.Cs.sub.2 O+N.sub.2 O.sub.5.
The nitrogen oxide is a gas and diffuses away. The porous doped rod is then still further heated in a suitable atmosphere until the pores collapse, and a transparent glass preform, doped with dopant in a desired radial distribution gradient is obtained.
As described with respect to conventional practices, the core of the preform prepared in accordance with the conventional method consists of Cs.sub.2 O--B.sub.2 O.sub.3 --SiO.sub.2 glass, containing a large amount of Cs.sub.2 O and having a comparatively low melting point; and the cladding of such a preform consists of B.sub.2 O.sub.3 --SiO.sub.2 glass which, by contrast, has a considerably high melting point. This is also the case with commercially available "Vycor" which is commercially used as an economical substitute for the porous pure silica glasses.
Optimally, the drawing temperature is at the softening point of the hard cladding glass when the preform is drawn alone or when it is inserted into a glass tube or pipe which usually has a lower softening temperature than the cladding portion. In other words, the drawing temperature of such preforms is high. However, the lower melting core glass softens so much at such high drawing temperatures that bubbling, believed due to due to decomposition of the glass oxide, takes place in the core and the fiber is subject to large diameter variation due to the bubbles, which obstruct the passage of the fiber through the die used for applying the resin, as described above. Such obstructing and discontinuities cause fracture of the fibers. Accordingly, it is difficult in conventional practice to produce optical waveguides in continuous lengths of high mechanical strength with smooth outer surfaces and quenched rapidly from high temperature.
It is necessary in the step-index type optical fiber for use as an optical waveguide that the refractive-index of the fiber core be higher than that of the outer cladding or jacket layer.
The refractive index of the glasses of pure SiO.sub.2 or SiO.sub.2 containing small quantities of B.sub.2 O.sub.3 is lower than that of such SiO.sub.2 glasses containing the refractive index increasing (Rii) dopants, such as Cs.sub.2 O.
In the case of employing such SiO.sub.2 glasses as the basic materials for the core and the cladding layer, the cladding layer consists of substantially pure SiO.sub.2 and the Rii-dopant has been added to the core, to raise the refractive-index of the SiO.sub.2 for the formation of the predetermined refractive-index distribution.
The preforms of the prior art "stuffing" patents belong to the above-mentioned type. Namely, a Rii-dopant is stuffed in a porous glass rod to raise the refractive-index of the porous rod and this forms the glass rod for the core by the molecular stuffing method. The glass rod is subsequently inserted into a SiO.sub.2 tube or pipe and they are heat collapsed into an integrated unit assembly of transparent glass rod as a preform. The tubing is made of pure SiO.sub.2 or SiO.sub.2 containing several percent of B.sub.2 O.sub.3 for slightly decreasing the refractive-index of the glass.
In the preforms of the "stuffing" patents, the cores contain from several to some ten percent of the Rii-dopant to raise the refractive-index. Such large amounts of the Rii-dopant decreases the melting temperature of the core. On the other hand, the cladding layer, which is made of pure SiO.sub.2 or SiO.sub.2 containing a small quantity of B.sub.2 O.sub.3, has therefore a higher melting temperature. To permit the drawing, the cladding layer must be heated to a temperature sufficiently higher than that needed for the core in order to decrease the viscosity of the cladding layer when the preform is drawn to a fiber. An optical fiber having high mechanical strength thus can not be obtained either because of insufficient melting of the cladding layer in the case of drawing at temperatures merely sufficient to melt the core; or if the drawing temperature of the preform is matched to the softing temperature of the hard cladding glass, the core glass is softened so much at such high temperature that bubbling takes place in the core.