An Introduction to Fiber Optics Technology

An Introduction to Fiber Optics Technology

Throughout time, speed and efficiency in the telecommunications industry has progressed at a rapid pace due to fiber optic technology. In 1979, AT&T revolutionized the telecommunications industry by producing a medium for data transmission which used light, called fiber optic cable. This medium created a bandwidth of 44.736 Mbps and could multiplex 672 trunk circuits onto one fiber (Cole, 2000). However, this invention was only the beginning of a great addition to telecommunications, one that would change the industry forever.

Even though AT&T introduced fiber optic technology in 1979, they weren't the first company to think of such a creative idea. The concept of exchanging data by the use of light was thought of by Alexander Graham Bell in the late 1800's. Bell always thought of possibilities that pulses of light could transmit voice signals, but Bell never had a dependable light source to test the idea (Cheo, 1990). In 1880, Bell patented a phone using optical transmission called the Photophone. Bell's invention failed because it used air as the medium to transmit light, rather than the glass fibers that are used today. Copper wire was simply more reliable than Bell's invention at the time, leading to the failure of his Photophone (Hecht, 1999).

Expanding on Bell's idea, English scientist John Logie Bard and United States scientist Clarence W. Hansell patented the idea of using hollow glass pipes to transmit television images in the 1920's. However, the tubes patented were very poor quality and experienced signal loss very easily. Bard and Hansell also ran into the same problem Bell did, not having a constant, intense light source (Hecht, 1999).

Solving Bard and Hansell's problem, engineers at Laser Diode Labs invented the continuous wave laser in 1975. This laser was smaller than a grain of sand, but made the use of fiber optics in telephony possible. In 1987, another great achievement was made in the fiber optics industry; this achievement was the erbium-doped fiber amplifier, which allowed multiple channels of light to coexist on a single circuit. This fiber amplifier provided enough channels for one fiber cable to handle 80 million telephone calls simultaneously (Greatest, 2000).

Today, fiber optic technology transmits data by sending light pulses down thin strands of glass or plastic fiber using a laser or light emitting diode (LED). Strands of fiber are composed of three main elements: the core, cladding, and buffer coating. The inside piece of the fiber is called the core as can be seen from the picture below. A fiber's core is the path where the light travels. Surrounding the core is optical material called cladding. Cladding continually reflects light pulses causing the pulses to travel smoothly through the fiber core. The buffer coating serves as a protection for the cladding and the core by protecting it from outside elements such as moisture (Fotec, 1996).

The glass fibers that compose the core of the fiber strands used in present-day fiber optic systems are mostly based on extremely pure sand. Fiber made from ordinary glass used in windows is so dirty that impurities reduce signal intensity by a factor of one million in only about 16 feet of fiber. These impurities must be removed before useful long-haul fibers can be made (Stafford, 1988).

Even perfectly pure glass is not completely transparent. Fiber optic loss is much lower than copper wire loss, yet some loss does still exist. Light pulses can be lossed during transmission by one of two ways. The first way, occurring at shorter wavelengths, is a scattering caused by unavoidable density changes within the fiber. When the light changes mediums, the change in density causes interference. The other is a longer wavelength absorption caused by atomic vibrations within the glass fiber (Stafford, 1988).

The two main types of fiber in use today are single-mode and multi-mode fiber. The difference in single-mode and multi-mode fiber is in the size of the core. Single-mode fiber has a core with a diameter of 9 microns. Single-mode fiber typically is used to transmit light pulses that have wavelengths of 1300 to 1500 nanometers. This type of fiber is used primarily for the transmission of sound. Multi-mode fiber has a core with a diameter of approximately 62.5 microns. This type of fiber is used primarily in local area network (LAN) connections and carries wavelengths of 850-1300 nanometers (Fotec, 1996).

Presently, wave division multiplexing (WDM) technology is used in fiber optics system. WDM breaks the different colors of light into multiple frequencies so they can coexist on the same channel. Each color of light has it's own frequency. Currently, WDM has 16 different frequencies that are used (Cole, 2000).

Fiber optics technology has many advantages. One of the main advantages fiber optic cable has over electrical wires is the distance the repeaters are spaced apart. In electrical systems, repeaters are needed approximately every mile, whereas a fiber optics system only needs a repeater about every 4-7 miles. Low speed systems can have repeaters spaced up to 62 miles apart (Microsoft, 1997). Fiber can also handle many more calls that copper wire. Two fibers in a fiber cable can handle more calls that a entire single copper wire can (Stafford, 1988). "A typical fiber optic cable, approximately 1.25 cm in diameter can carry in excess of 2.3 million simultaneous

...