The Fiber Optic Cable

In recent years it has become more than evident that fiber optic cable will definitively replace copper cables as the appropriate means of transmitting communication signals. It is used in long-distance stretches, just as it is the dorsal network platform of many telecommunications systems. Fiber optic networks include cable television services, university campuses, office buildings, industrial plants and electrical service companies.

A fiber optic system is similar to the copper cable system. The difference is that the fiber optic uses pulses of light to transmit information instead of using electrical pulses to transmit information as do the copper lines.

Basic Structure of an Optical System

At one end of the system you have a transmitter. This is the place of origin of the information that reaches the fiber optic lines. The transmitter accepts the information and encodes the electrical pulse. It then processes it and translates that information into its equivalent form of encoded light pulses. A light-emitting diode (LED) or an injection laser diode (ILD) can be used to generate pulses of light. Through the use of a lens, the pulses of light are channeled towards the optical medium in which they travel along the cable. The most used (near infrared) light is 850nm for short distances and 1,300nm for long distances over multimode and singlemode fiber and 1,550nm is used for longer distances.

Think of the fiber optic cable as if it were a very long cardboard roll (like a very long roll of paper towel) that is covered with a mirror inside. If a flashlight is placed on one end you can see the light coming out at the other end – even if it has been bent into a corner.

The pulses of light move easily through the optical fiber, because of a principle known as total internal reflection. “This principle of total internal reflection occurs when the angle of incidence exceeds a critical value, light can not get out and is trapped, light bounces”. When this principle is applied for the construction of the fiber optic strands, it is possible to transmit information in the form of pulses of light. The core must be of a transparent and pure material. The core can be made of plastic (used for very short distances), but most are made of glass. Optical glass fibers are almost always pure silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for applications of longer infrared wavelengths.

There are three types of commonly used fiber optic cables: singlemode, multimode and plastic (POF).

Both transparent glass and plastic fibers allow light to enter at one end reaches the other with minimal loss.

The Fiber optic cable works like a “light guide”, guiding the light introduced at one end of the cable to the other end. The light source can be a light-emitting diode (LED) or a laser.

The light source emits a pulse on and off, and a light-sensitive receiver at the other end of the cable converts the pulses back into digital signals of ones and zeros of the original signals.

Even laser light traveling through a fiber optic cable is subject to loss of power, mainly as a result of light scattering, on the cable itself. The faster the laser pulses move, the greater the risk of dispersion. Light boosters, called repeaters, may be required to reload the signal.

The single-mode cable: it is a fiberglass with a diameter of 8.3 to 10 microns that has a single mode of transmission. The single-mode fiber has a relatively narrow diameter, through which a single mode will typically propagate is 1310 or 1550 nm. It allows having a greater bandwidth than multimode fiber, but it requires a light source with a narrow spectral width. They are Synonyms single mode fiber optic, single mode fiber, single-mode waveguide, uni-mode fiber.

Single-mode fibers have become standard and are used in many applications where data is sent in multi-frequency (WDM Wave-Division Multiplexing-) so only one fiber is needed to carry multiple signals.

The single mode fiber gives a higher transmission speed and supports distances up to 50 times more than a multimode, but it is also less economical. Singlemode fiber has a much smaller core than multimode. Its small core allows transmitting a single wave of light, practically eliminating any distortion that may result from the superposition of light pulses, providing less attenuation of the signal and providing the highest transmission speeds than any other type of fiber cable.

The Multimode fiber cable has a slightly larger diameter, with standard diameters in the range of 50 to 100 microns. Most applications where multi-mode fiber is used, 2 fibers are used (the WDM is not normally used in multimode fiber).

Multimode fiber at Sopto gives high bandwidth at high speeds for medium distances. Light waves are scattered in numerous paths, or modes, as they travel through the cable core normally 850 or 1300 nm pulses are used. Typical diameters of the multimode fiber core are 50, 62.5, and 100 micrometers. For long stretches, the multiple light beams can cause distortion of the signal at the receiving end, resulting in unclear and incomplete transmission of data, which is why only single-fiber fibers are used today for long stretches.