# Homework Help: Science: Physics: Fiber Optics

by Emily McPherson

What are fiber optics?

Since the earliest times, people have always wanted to send messages to each other over distances using light. A clever way of using light to send messages is to push it through a pipe, much in the same way as water is moved through pipes. This is where fiber optics comes in. This is a branch of physics based on the transmission of light through long strands of transparent material with a high refractive index. If light it admitted at one end of the fiber, it can travel through the fiber extremely fast, with a very low loss, even if the fiber is curved.

The process is extremely quick as light travels at a speed of about 300,000km, so, in theory, a torch could be used to flash a signal right around the world in next to no time.

Total Internal Reflection

The principle on which this transmission of light depends is called total internal reflection. Light traveling inside the center of the fiber strikes the outside surface at an angle of incidence greater than the critical angle, so that all the light is reflected towards the inside of the fiber without loss (see diagram). This important technical application of total internal reflection means that the light can be transmitted over long distances by being reflected thousands of times. In this way, nearly all the light that enters the fiber emerges thousands of km away at the other end.

What is an optic fiber?

Modern optical fibers are hair-thin strands of ultra pure glass (see samples), which is about 10 micrometers thick. A single pair of optical fibers can carry hundreds of thousands of two-way conversations at once. Many single glass fibers form the center of a fiber optic cable, shown at the bottom of this poster.

How does the light behave?

Light travels along the fiber as the glass is transparent, but it has been designed so that any ray meeting the outer surface of the glass fiber is totally internally reflected back into the fiber, as the glass has a much higher refractive index than the surrounding air around the fiber. The critical angle traveling from glass to air is 42*, which is quite small. Therefore, any angle of incidence greater than this will produce total internal reflection, and as the fiber is very narrow, this angle is always achieved.

Although it is the light refracting from the glass which transmits the signal within the individual fibers, to avoid loss of signal through scattering of light by impurities on the surface of the fiber, every fiber has an outer microscopic layer called cladding, usually made of a separate glass layer of much lower refractive index. This means that if light does escape from the core glass fiber, total internal reflection occurs at the interface of the glass fiber and the cladding. Because total internal reflection occurs, the light can never go back upon itself, and the signal within the fibers always continues in the same direction.

What kind of light is used?

The optical fiber communications network use lasers to generate a suitable and efficient light source as lasers can create very tightly focused pulses of light, which don't disperse or radiate away, as all the rays travel at the same frequency. If plain white light was used, the different frequencies of light would travel and reflect at slightly different speeds within the wire, and over a long distance, the signals would begin to overlap and lead to a distorted image or sound.

Using standard commercial systems, it is possible to send the entire 32 volumes of the Encyclopedia Britannica through an optical fiber across the Southern Hemisphere in less than one second.

Uses

Optical fibers have now become a feature of all our lives, as they have a wide variety of uses, especially in telecommunication systems, and medical institutes. The simplest application of optical fibers is the transmission of light to and from locations otherwise hard to reach, for example, the bore of a dentist's drill. Image transmission by optical fibers is also widely used in medical instruments for viewing inside the human body, especially in childbirth; for laser surgery; in facsimile systems; and computer graphics.

Another growing application of optical fibers is the telecommunications system, as the fibers can be easily laid under ground, and under the sea. This is a great means on transmitting signals over long distances with minimal loss, and it is surprisingly cheap to build, lay, and use. Both Telstra and Optus have realized these capabilities, and are researching and laying fiber optic cables, for use in telephone, Internet, and pay television systems. Underwater fiber optic cables currently carry telephone and Internet signals across the Atlantic and Pacific oceans.

The potential of the applications of optical fibers is nearly unlimited, because of the great ability to bend the fiber, and place it under extreme conditions, without distorting the signals being sent through them. So the next time you pick up the phone to speak, you may well be using an optical fiber system to do it.

Technology

It is very hard to overstate the importance of this technology. These human hair-sized glass pipes have a stupendous capacity for carrying information. A single pair of optical fiber can carry the equivalent of 37000 simultaneous telephone conversation, or 800 video channels. However in practice, few cables contain a single pair. The Pentel pen-sized cables now being laid in our street by Telstra contain at least 8 fiber pairs- a massive capacity.