Fiber Optics

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Fiber optics is a cable that is quickly replacing out-dated copper wires. Fiber optics is based on a concept known as total internal reflection. It can transmit video, sound, or data in either analog or digital form . Compared to copper wires it can transmit thousands of times more data (slide 2) . Some of its general uses are telecommunications, computing, and medicine.

The very first "fiber" was made in 1870 by the British physicist John Tyndal. In this experiment that he showed to the Royal Society he placed a powerful waterproof lamp inside a tank of water, which had closed pipes coming out the sides. When he opened up the pipes so water could flow, to the amazement of the crowd, the light totally internally reflected inside the beam of water as it fell to the ground.

One of the very first forms of optical communication was done Paul Revere in his famous Paul Revere's ride. Here he used the well-known signal "one if by land, two if by sea." Although primitive, this was still optical communication and we must give him credit for it. Another contender was Alexander Gram Bell and his photophone (slide 3). With this device, one person would speak into a microphone causing a mirror to vibrate. Then sunlight would reflect off the vibrating mirror and hit another mirror 200 meters away. This mirror would then cause a selenium crystal to vibrate and sound would come out the other end. This seems interesting, but unfortunately this did not work very well at night, in the rain, or when someone simply walked in front of it.

In the summer of 1970, scientists at the Corning Glass Works developed a single mode fiber with a loss of 20 dB/km. (Slide 4) This corresponds to over a 99% loss over 1 km, which may seem useless, but at the time it was a spectacular breakthrough. On October 30, 1986, a fiber across the English Channel became operational. In December 1988, the TAT-8, the first transatlantic fiber cable became fully functional. Currently, the standard losses of fiber are within 0.5 – 0.25 dB/km with a data transfer rate of 1 trillion bits per second.

The basic setup for a fiber optical system is that first, a transmitter receives an electrical signal, usually from a copper wire. (Slide 5) The transmitter drives a current on a light source and the light source launches the optical signal into the fiber. Inside the cable, repeaters often amplify the signal due to slight losses in power. Once the signal is through the cable, a light detector receives and converts it back to an electrical signal to send down another copper wire.

There are five layers in almost all fiber optic cables. (Slide 6) The inner most layer is the optical core. This is the light-carrying element typically made of silica or germania with an index of refraction of 1.48. The layer surrounding the central core is the optical cladding made of pure silica and has an index of refraction of 1.46. It is the boundary between these two layers that the light reflects off of, so the light never actually enters the cladding, it just reflects off the boundary. The next layer is the buffer material that shields the core and cladding. Next is the strength material, which prevents stretch problems when cables are being pulled or moved. Finally the outer jacket protects against abrasions and environmental contaminants and is typically make of a polymer.

All fiber optic cable can be divided into two categories: singlemode and multimode. (Slide 7) The big difference is that singlemode has higher bandwidth. Other aspects are singlemode cables have a smaller core (8 – 10 mm), can travel long distances, use lasers as the light source, and are much more expensive. The wavelengths that the light source transmits are 1310 nm or 1550 nm, which are outside the visible spectrum. Multimode cables have a larger core (60 – 62.5 mm) can only travel 2 km, uses LED's as their light source, and are much cheaper than their...
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