Fiber Optics

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.

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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 counterpart. The wavelengths are 850 nm and 1300 nm, again outside the visible spectrum. So as one can see, each type has their advantages. If you want a short-range network without monstrous bandwidth needs, you can save money and get the multimode. On the other hand, if you want a large network with a large bandwidth, you will need to fork out the money for the singlemode.

Fiber optic cable is constructed with an industry standard with respect to that the cladding on all cable, whether single or multimode, must be 125 mm. (slide 8) This is done so the same tools can be used for both types of cables. Fiber cables are also bunched, much like their copper counterparts are. Individual cables are called simplex, double cables are called duplex, and four cables wrapped together are quadplex. Wires with more than four cables are also available. Some of the possible ones are groups of twelve, thirty-six, and forty-eight.

Another important aspect of fiber optics is attenuation, which is decrease in power from one point to another. (Slide 9) Attenuation is measured in dB/km and a 3.0 dB/km loss corresponds to about a 50% loss in 1 km. One of the major advantages of fiber over copper is lower attenuation. Copper wires need repeaters about every three miles, but fiber can go without a repeater for sixty miles.

There are two types of attenuation: intrinsic and extrinsic. One of the two types of intrinsic attenuation is scattering, which makes up about 96% of the intrinsic. This is caused by Rayleigh scattering in which light rays collide with atoms in the cable and scattering is a result. This cannot be avoided. The second type of intrinsic attenuation is absorption. This constitutes about the other 4% of the intrinsic loss, but fortunately this type can go to zero if we can get the fiber pure enough. Therefore it is always a goal to make the purest fiber possible.

The second type of attenuation is extrinsic, or basically all attenuation occurring outside the cable. (Slide 10) This has two types: macrobending and microbending. Macrobending is when the cable has large bends, bends big enough so the critical angle is exceeded so the light is no longer totally internally reflected. Microbending are small-scale bends usually caused by some sort of outside pressure. Here, like in macrobending, the critical angle is exceeded and hence no total internal reflection.
The advantages of fiber over copper are great and many. (Slide 11) The first and most obvious is fiber’s immense bandwidth capabilities. Fiber can carry 40,000 phone conversations and over 250 television channels. Another advantage is fiber needs fewer optical repeaters than copper needs signal regenerators. As said before, copper needs a regenerator every three miles, but a fiber cable does not need a repeater for an entire sixty miles. Fiber cables are also free from electrical interference. Therefore in regions of high electrical activity (stormy areas or power plants), fiber cables are greatly appreciated.

More advantages are that since it is light passing down the cable, and not electricity, it is virtually danger free except for the fact that the light is in the infrared and can harm your eyes. Also fiber cannot be tapped into with traditional electric means such as measuring the changes in magnetic field. They are virtually impossible to tap into optically without doing a tremendous amount of work and making yourself very obvious. Fiber is also very reliable; is immune to high temperature and moisture; has a long life span of approximately thirty years; it is not effected by short circuits, power surges, or static electricity; and is easily installed and upgraded.

One of the few disadvantages (Slide 12) to fiber optics is that to get an optical system operational, you will have to fork out a tremendous amount of money. This technology comes with a price. Another is connecting optical cables together. Connecting copper wires are easy, just solder them together. But with optical cables, the light beam needs to remain continuous, so the connection needs to be extremely precise so that there will be minimal loss. In order to join two fiber cables together, you need some pretty fancy equipment and some well trained people. Because of this difficulty in connecting cable, if something happens to your network, and a line breaks, you’re in trouble.

Since fiber is much better than copper, its beginning to have many applications. (Slide 13) Telephone and cable television companies are both begging to use fiber. Lines connecting central offices in companies are often using fiber. Power companies use it for communication because of its lack of electrical interference. Computing is one of the other main uses. LAN’s usually have their backbones made of fiber. For example, here at Texas A;M we have a fiber optic ring going around campus which connects some of the main buildings such as Blocker, Heldenfels, and the Student Computing Center. Individual computers do not yet have fiber lines going directly into them, but they are probably soon to come.

In conclusion, fiber optics is extremely important in today’s world of technology. It is vastly superior to copper mainly due to its incredible bandwidth. It is used in telephone systems to computer systems. Only time will tell just how important fiber will be in the future.