Fiber optics is a technology in which signals are converted from electrical into optical signals, transmitted through a thin glass fiber, and re-converted into electrical signals. The basic optical fiber consists of three concentric layers differing in optical properties: Core: the inner light-carrying member. Cladding: the middle layer, which serves to confine the light to the core. Buffer: the outer layer, which serves as a "shock absorber" to protect the core and cladding from damage

Total Internal Reflection

Light injected into the core and striking the core-to-cladding interface at an angle greater than the critical angle will be reflected back into the core. Since angles of incidence and reflection are equal, the light ray continues to zig-zag down the length of the fiber. The light is trapped within the core. Light striking the interface at less than the critical angle passes into the cladding and is lost. This trapping of light and reflecting it down the core of the fiber through total internal reflection describes the operation of a multimode fiber.

Rays of light do not travel randomly. They are channeled into modes, which are possible paths for a light ray traveling down the fiber. A fiber can support as few as one mode and as many as tens of thousands of modes. While we are normally not interested in modes per se, the number of modes in a fiber is significant because it helps determine the fiber's bandwidth. More modes typically means lower bandwidth. The reason is dispersion.

As a pulse of light travels through the fiber, it spreads out in time. While there are several reasons for such dispersion, two are of principal concern. The first is modal dispersion, which is caused by different path lengths followed by light rays as they bounce down the fiber. Some rays follow a more direct route than others. The second type of dispersion is material (or chromatic) dispersion: different wavelengths of light travel at different speeds. By limiting the number of wavelengths of light, you limit the material dispersion.

Dispersion limits the bandwidth of the fiber. At high data rates, dispersion will allow pulses to overlap so that the receiver can no longer distinguish where one pulse begins and another ends.

Types of Fibers: Singlemode or Multimode?

In the simplest optical fiber, the relatively large core has uniform optical properties. Termed a step-index multimode fiber, this fiber supports thousands of modes and offers the highest dispersion - and hence the lowest bandwidth.

By varying the optical properties of the core, the graded-index multimode fiber reduces dispersion and increases bandwidth. Grading makes light following longer paths travel slightly faster than light following a shorter path. Put another way, light traveling straight down the core without reflecting travels slowest. The net result is that the light does not spread out nearly as much. Nearly all multimode fibers used in networking and data communications have a graded index.

But the ultimate in high-bandwidth, low-loss performance is singlemode fiber. Here the core is so small—about 9 m—that only a single mode of light is supported. The bandwidth of a singlemode fiber far surpasses the capabilities of today's network electronics. Indeed, the information-carrying capacity of the fiber is essentially infinite. Not only can the fiber support speeds tens of gigabits per second, it can carry many gigabit channels simultaneously. This is done by having each channel carried by a different wavelength of light. The wavelengths do not interfere with one another. Singlemode fiber is the preferred medium for long distance telecommunications.

The most popular fibers for networking is are 50/125 and 62.5/125 multimode fibers. The numbers mean that the core diameter is 50 or 62.5 m and the cladding is 125 m. Single-mode fibers find use in networks for interbuilding runs and are popular for high-speed network backbones.

Cables

The cable structure protects the fibers mechanically and isolates them from external forces. Most cables contain strength members of steel and fiberglass in multifiber cables or aramid yarn in individually jacketed cables. The strength members are the load-bearing part of the cable, handling tensile stresses and decoupling the fiber from stress. Basic structures include loose-tube and tight-buffered designs. Loose-tube designs, which use a hard plastic with a diameter several times larger than the fibers, accommodates the different coefficients of thermal expansion of the cable elements and fiber. Tight-buffered cables, popular with indoor applications where extreme temperature cycling is not an issue, have a jacket placed directly over the fiber and strength members. Ribbon cables, which have up to 16 fibers laying in parallel, are also popular.