Optical fibers are made of silica or plastic. Generally, optical fibers used in telecom field are silica based and those used in sensors and imaging could be both silica and plastic. Silica glass has found its use in making optical fibers due to their inherent nature to allow light to travel. Silica is a transparent medium, meaning it allows light propagation.
Optical fibers have a core, which is covered by an outer layer known as cladding. The cladding material will have low refractive index. The dimensions of core and cladding varies depending on the type of optical fibers. In any case, both core and cladding are circular in structure. The core and surrounding cladding are made of silica material in case of silica glass fiber. Core region has higher refractive index compared to that of cladding to allow total internal reflection.
Typically, a single mode fiber used in fiber optic telecommunications will have a core diameter of approximately 8 micrometers and a cladding diameter of 125 micrometers. Both core and cladding are drawn together. Manufacturing of optical fiber from its base raw material, preform is called drawing. During drawing, the core and cladding draws out of the preform in the same ratio, that is set when making the preform. Whatever be the speed of drawing, the ratio will be same to achieve desired transmission characteristics targeted for each type of fiber.
The solid cylinder of silica glass with core and cladding is called bare fiber. It is called bare fiber as it is not covered and is exposed to outside environment. Bare fibers can not be used for further processing. Bare fibers may have research applications. In order to process further and make products using bare fiber, it must be made flexible and protected from direct exposure to outside environment. In order to provide outside protection and flexibility to bare fiber, thin layer of acrylate is applied.
Optical fibers used in current telecommunication cables have two layers of acrylate coating. First coating is made of soft acrylate material and the second coating that is the outer coating is made of hard acrylate material. Overall, the structure of an optical fiber coated with acrylate material will look like this;