There are two distinct parts of a fiber optic cable—the optical fiber that carries the signal and the protective covering that keeps the fiber safe from environmental and mechanical damage. This section deals specifically with the optical fiber.
An optical fiber has two concentric layers called core and cladding. The core (inner part) is the light carrying part. The surrounding cladding provides the difference in refractive index that allows total internal reflection of light through the core. The index of refraction of the cladding is less than 1% lower than that of the core. Typical values, for example, are a core index of 1.47 and cladding index of 1.46. Fiber manufacturers must carefully control this difference to obtain the desired fiber characteristics.
Fibers have an additional coating around the cladding. This coating, which is usually one or more layers of polymer, protects the core and cladding from shocks that might affect their optical or physical properties. The coating has no optical properties affecting the propagation of light within the fiber. This coating is just a shock absorber.
Figure 6.10-4 shows the light traveling through a fiber. Light injected into the fiber and striking the core-to-cladding interface at a critical angle reflects back into the core. Since the angles of incident and reflection are equal, the light will again be reflected. The light will continue as expected down the length of the fiber.
Light, however, striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly rapidly. The propagation of light is governed by the indices of the core and cladding and by Snell’s Law.
Such total internal reflection forms the basis of light propagation through a simple optical fiber. This analysis considers only meridional rays, the rays that pass through the fiber center axis each time they are reflected. Other rays, called skew rays, travel down the fiber without passing through the axis. The path of the skew ray is typically helical, wrapping around and around the center axis. To simply analyze, skewer rays are ignored in most fiber-optics analysis.
A cone known as the acceptance cone, shown in Figure 6.10-5, defines which light will be accepted and propagated by a total internal reflection. Light that enters the core from within this acceptance cone refracts down the fiber. Light outside the cone will not strike the core-to-cladding interface at the proper angle that allows total internal reflection. This light will not propagate.
The specific characteristics of light propagation through fiber depend on many factors. The factors include the size and composition of the fiber as well as the light source injected into the fiber. An understanding of the interplay between these properties will clarify many aspects of fiber optics.
Fiber itself has a very small diameter. Table 6.10-3 provides the core and cladding diameters of four commonly used fibers.
To realize how small these fibers are, note that human hair has a diameter of about 100 μ. Fiber sizes are usually expressed by first giving the core size, followed by the cladding size. Thus, 50/125 means a core diameter of 50 microns (μm) and a cladding diameter of 125 microns (μm).
Optical fibers are classified in two ways. One way is by the material makeup:
- Glass fiber: Glass fibers have a glass core and glass cladding. They are the most widely used type of fiber. The glass used in an optical fiber is an ultra pure and transparent silicon dioxide or fused quartz. If ocean water was as clear as fiber, one could see to the bottom of the Marianas Trench in the Pacific Ocean, a depth of 36,000 feet. Impurities are purposely added to the pure class to achieve the desired index of refraction. The elements germanium and phosphorus are added to increase the refractive index of the glass. Boron or fluorine is used to decrease the index. There are other impurities that are not removed when the class is purified. These additional impurities also affect the fiber properties by increasing attenuation from scattering or by the absorbing light.
- Plastic-clad silica (PCS): PCS fibers have a glass core and plastic cladding. The performance of PCS fiber is limited compared to a fiber made of all glass.
- Plastic: Plastic fibers have a plastic core and plastic cladding. Plastic fibers are limited by high optical loss and low bandwidth. The very low cost and ease of use make them attractive for applications where low bandwidth or high losses are acceptable. Plastic and PCS fibers do not have the buffer coating surrounding the cladding.
The second way to classify fibers is by the refractive index of the core and the modes that the fiber propagates. Fiber can be categorized into three general types; Figure 6.10-6 shows the three general fiber types and their basic characteristics.
Figure 6.10-6 shows the difference between the input pulse injected into a fiber and the output pulses exiting the fiber. The decrease in the height of the pulse shows the loss of optical signal power. The broadening of the pulse shows the bandwidth limiting effects of the fibers. It also shows the different paths of rays of light traveling down the fiber. And, it shows the relative index of refraction of the core and clad- ding for each type of fiber.