The present invention relates to a process for producing a rod glass having an arbitrary refractive index distribution.
For example, in the conventional multimode optical fiber, the presence of a discontinuous difference in the refractive index between the core and the clad has frequently caused a mode transformation and a large loss in the bending of the optical fiber.
This problem has been solved through the use of an optical fiber wherein the refractive index of the core distributes in a parabolic manner.
Examples of the method of distributing the refractive index in a parabolic manner known in the art include a method wherein the core is laminated by the CVD process while changing the components of the reaction gas to distribute the refractive index, a method wherein a glass rod doped with an ion such as Tl.sup.+ having a large electron polarizability is immersed in a melt of KNO.sub.3 or the like to cause ion exchange between Tl.sup.+ and K.sup.+, thereby distributing the concentration of the dopant ions in a parabolic manner, and a method wherein the ion concentration is distributed by means of an electric field.
In the CVD process, however, the distribution is provided by controlling the flow rate of the reaction gas, which requires a very high level of technique.
In the method wherein use is made of the ion exchange, it is necessary to use a combination of ions having different diffusion rates, which requires much time and, at the same time, makes it difficult to control the distribution. Further, in the method of distributing the ions by means of an electric field through the change in the composition of the glass, it is difficult to provide a gradient electric field, so that the provision of a refractive index distribution as desired becomes very difficult.