Saturday, August 28, 2010

Fiber Optics

What is Fiber Optic Cable?
Fiber Optic Cable consists of thin strands of plastic or glass threads wrapped in a cladding of glass or plastic with a outside coating to protect the inner fibers. The inner core is used to transmit light from one end of the cable to the other with little to no interference at amazing speeds. The light is shot down the inner core in pulses by either an LED or Laser, the light is converted at the end of the cable to a digital signal interpreted by the optical receiver to be either a 1 or 0. The light reflects within the core against the cladding or outer side of the core so that the light stays within the cable with little to no signal degradation. The light reflecting in this way is called total internal reflection.

Fiber Optics has been around a long time but has recently become more popular with telecommunications industry. The principle Fiber Optics use was demonstrated by Jean-Daniel Colladon and Jacques Babinet in the early 1840's at a convention in Paris, France. Jean-Daniel Colladon's work concluded that light will reflect of surfaces at a certain angle would trap the light within the tube of water, this principle would be called Total Internal Reflection. His experiment was supposed to show a tank of water with water jets trapping light until the jets dispersed within the tank, but due to the lighting the audience could not see this happening. Later Jean-Daniel Colladon used a series of pipes to bring the light to the base of the water jets thus trapping the sun's light within the water jet's curved path.

In 1880 Alexander Graham Bell invented the photophone a device that would transmit speech as light pulses that could travel a distance of 200 meters. When you would speak into the photophone it would vibrate a diaphragm inside the mouthpiece and it would reflect off a parabolic dish through a resister that would vary the lights intensity. The changing intensity of the light would alter the current of electricity to the receiver turning the light signals back into speech.

The first fiber optic was invented by Dr. Narinder Singh and Harold Hopkins who in July 1952 after receiving a grant from the Royal Society of England began work on creating glass fiber for use with endoscopes. In 1956 Singh completed his work of creating a glass-coated rod which protected the light within the fiber from outside interference or noise. Singh is refered to as the father of fiber optics since he first coined the term fiber optic. Early fiber scopes were used in many fields such as welding inspection of crucial engine components and for the medical field.

The earliest fiber optic light sources were LED's as the market grew there was more need for faster light source detectors and more direct light sources. Fiber optic cabling had only been used mainly for fiber scopes until in 1957 Gordon Gould invented the LASER – Light Amplification by Stimulated Emission of Radiation. This device is capable of producing intense, spot specific light. It didn't go unnoticed by the communication industry at the time, bell being the first to show interest. Bell then abandoned their research in the infrared spectrum and began to focus on the visible spectrum instead.

Over the years fiber has been developed around the wavelengths available at the time that had the lowest possible data loss. Data loss occurs when there are impurities in the core material. There are two main causes of attenuation within fiber optic cabling scattering and absorption. Scattering or dispersion is caused by light bouncing off of molecules within the core fiber, it is a byproduct of light wavelength the shorter the wavelength the more frequent scattering may occur. Absorption is caused by water bands, water molecules within the glass that absorb the light being transmitted at certain wavelengths. Fiber optic wavelengths are determined by longer wavelengths in the infrared spectrum for lower loss in the core fiber and wavelengths which are between absorption bands. Normal wavelengths are 850nm, 1300nm, and 1550nm. The 800nm to 900nm wavelength range is referred to as the first window, it is widely used in short distance communication because at longer distance it suffers greater attenuation than other windows. In 1977 most of the telecommunication companies switched to the 1300nm range due to the lowered chance of optical loss due to attenuation, this frequency is referred to as the second window. The third window operates on the frequency 1530nm to 1565nm referred to as the C-band or Conventional band, and it is the most commonly used frequency used today. Mainly due to the lessening of attenuation in this frequency range. The newest frequency is called the L-band or Long Band, it is in development and operates at 1565nm to 1625nm and has the same attenuation as the C-band.

Fiber optic cable comes in two different modes; Single-mode fiber, and Multi-mode fiber. Single-mode fiber is when only one ray of light is beamed down a specially designed fiber optic cable made for only one ray of light. Single-mode cable is small between 8 to 10 microns in diameter. Single-mode fiber use two different cables, one for input the other output. The light wavelengths used with Single-mode fiber is 1310nm to 1550nm. Single-mode fiber can carry a higher theoretical bandwidth than Multi-mode fiber. Due to the narrow core of Single-mode fiber a very narrow laser light width is required. Not only is Single-mode fiber faster than Multi-mode fiber and it also has lower attenuation. Single-mode fiber uses Wave Division Multiplexing to transmit different light wavelengths by using multiple colors of laser lights. The main disadvantage is that it has lower tolerances when splicing the cable. In 1990 Bell Labs transmitted 2.5GB/s over 4,660 miles without a repeater.

Multi-mode fiber is Larger than Single-mode fiber, the diameter can be up to 100 microns. Multi-mode fiber is not as fast as Single-mode fiber and is best suited for short distance communication. The limits of transmission distance without a repeater is 1.24miles at 100Mb/s. As the name implies Multi-mode fiber shoots multiple light rays at different angles to transmit data quickly. Because the lights are sent down the channel together there is a higher chance of dispersion. Multi-mode fiber does not use wavelength division multiplexing, it is also cheaper than Single-mode fiber. The light wavelengths used with Multi-mode fiber are 850nm to 1300nm. Plastic cores are sometimes used with Multi-mode fiber as they are only suited for short distance cables, thus lowering the cost of short distance fiber transmissions. There are two ways the light is transmitted through the fiber Step-index and Graded-index. With Step-index the light zigzags as well as straight through the core. Due to the way the light arrives at separate times there is a chance of dispersion. Graded-index has a diminishing core reflective index that allows the light at the center to move slower than the light near the cladding. This way the light takes on a curved helical. The light traveling this way moves slower and has a lowered chance of dispersion with the core light and the helical light arriving at almost the exact same time.

Splicing, or connecting segments of fiber optic cabling is very difficult and must be done with specialized equipment. There are many different kinds of connectors that you can affix to the end of the fiber segment to connect it to another segment of fiber some of the more common connectors are; ST, SC, FC, and MT-RJ.

ST is a keyed bayonet type of connector. Its used with both Single-mode and Multi-mode fiber optic cables. It can be inserted and removed from a fiber optic cable quickly and easily. There are two types of ST connectors ST and ST-II. They can me keyed and push-in or spring loaded and twist types.

SC is used mostly with Single-mode fiber optic cabling. Its low cost, durable, and easy to install. It provides accurate alignment via its ceramic ferrule. It snaps in or off using a locking tab.

FC is used only with Single-mode fiber optic cabling. It offers extremely precise positioning of the Single-mode fiber. It uses a position locatable notch and a threaded receptacle. This maintains the position with absolute accuracy.

MT-RJ or Mechanical Transfer Registered Jack is easier to install than ST/SC connectors and has a similar look to an RJ-45 connector.

Book: Fiber optic installer's field manual, by Bob Chomycz
Fiber optic reference guide: a practical guide to communications technology, by David R Goff, Kimberly S. Hansen

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