Construction of an Optical Fiber
The core has a higher retractive index than the cladding. This ensures total internal reflection of the core-cladding boundary and guides the light through the fiber core. For graded index multimode fibers the core is made with progressively changing refractive index:- highest in the center and gradually reducing towards the outside. Multimode fibers have either 50 or 62.5 micrometer cores and are used for shorthaul systems. Single mode fibers have a core of about 8.5 micrometers and are used exclusively for high bandwidth, long distance systems. Both types of fiber are coated with a protective plastic layer to protect the pristine glass surface from damage.
Optical Fiber Cross Sections
The core and the cladding will trap the light ray in the core provided the light ray enters the core at an angle greater than the critical angle. The light ray will then travel down the core of the fiber, with minimal loss in power, by a series of total internal reflections. Figure 1 illustrates this process.
Figure 1. Light
Ray Travelling Through an Optical Fiber
It would in theory be possible to simply have a tube of glass of uniform refractive index acting as the core, with air acting as the outer cladding. This is possible as air has a refractive index lower than glass. This type of implementation does not generally work well because an unprotected core that is covered in scratches, dirt and oil will appear to have an irregular cladding, with a higher refractive index at these irregular points than the core. Therefore a lot of light will not be reflected and will be radiated out of the glass. This is illustrated in Figure 2.
Figure 2. Problems
Associated with a Glass Air Interface
The core is generally constructed of Germania doped silica glass. The cladding is generally constructed of near pure silica glass. The cladding therefore has a lower refractive index than the core (the more impurities there are in glass the higher the density of that glass). The sheath is generally constructed of ultra violet cured plastic which provides protection against abrasion and external forces. The sheath will also be color coded in a similar manner to multicore copper cables to enable the user to distinguish between fibers.