Optical fiber (or "fiber optic") refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic wire or fiber. Optical fiber carries much more information than conventional copper wire and is in general not subject to electromagnetic interference and the need to retransmit signals. Most telephone company long-distance lines are now of optical fiber.Transmission on optical fiber wire requires repeaters at distance intervals. The glass fiber requires more protection within an outer cable than copper. For these reasons and because the installation of any new wiring is labor-intensive, few communities yet have optical fiber wires or cables from the phone company's branch office to local customers (known as local loops).A type of fiber known as single mode fiber is used for longer distances; multimode fiber fiber is used for shorter distances.
Clicksense
Tuesday, May 22, 2007
Advantages of Fiber Optics
Advantages of Fiber Optics
Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibers are:
Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cable TV, Internet) and you money.
Thinner - Optical fibers can be drawn to smaller diameters than copper wire.
Higher carrying capacity - Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.
Less signal degradation - The loss of signal in optical fiber is less than in copper wire.
Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.
Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.
Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibers are:
Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cable TV, Internet) and you money.
Thinner - Optical fibers can be drawn to smaller diameters than copper wire.
Higher carrying capacity - Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.
Less signal degradation - The loss of signal in optical fiber is less than in copper wire.
Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.
Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.
Digital signals -
Optical fibers are ideally suited for carrying digital information, which is especially useful in computer networks.
Non-flammable -
Non-flammable -
Because no electricity is passed through optical fibers, there is no fire hazard.
Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber-optic cables take up less space in the ground.
Flexible -
Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber-optic cables take up less space in the ground.
Flexible -
Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes:
Medical imaging - in bronchoscopes, endoscopes, laparoscopes
Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)
Medical imaging - in bronchoscopes, endoscopes, laparoscopes
Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)
Plumbing - to inspect sewer lines
Because of these advantages, you see fiber optics in many industries, most notably telecommunications and computer networks. For example, if you telephone Europe from the United States (or vice versa) and the signal is bounced off a communications satellite, you often hear an echo on the line. But with transatlantic fiber-optic cables, you have a direct connection with no echoes.
How Are Optical Fibers Made?
Now that we know how fiber-optic systems work and why they are useful -- how do they make them? Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far fewer impurities than window-pane glass. One company's description of the quality of glass is as follows: If you were on top of an ocean that is miles of solid core optical fiber glass, you could see the bottom clearly.
Making optical fibers requires the following steps:
1. Making a preform glass cylinder
2. Drawing the fibers from the preform
3. Testing the fibers
Making the Preform Blank
The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).
In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen: The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2). The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass. The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control (index of refraction).Drawing Fibers from the Preform Blank
Drawing Fibers from the Preform Blank
Once the preform blank has been tested, it gets loaded into a fiber drawing tower.
The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.
The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.
Testing the Finished Optical Fiber
The finished optical fiber is tested for the following:
Tensile strength - Must withstand 100,000 lb/in2 or more
Refractive index profile - Determine numerical aperture as well as screen for optical defects
Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform
Attenuation - Determine the extent that light signals of various wavelengths degrade over distance
Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-mode fibers)
Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)
Operating temperature/humidity range
Temperature dependence of attenuation
Ability to conduct light underwater - Important for undersea cables
Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.
Once the preform blank has been tested, it gets loaded into a fiber drawing tower.
The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.
Testing the Finished Optical Fiber
The finished optical fiber is tested for the following:
Tensile strength - Must withstand 100,000 lb/in2 or more
Refractive index profile - Determine numerical aperture as well as screen for optical defects
Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform
Attenuation - Determine the extent that light signals of various wavelengths degrade over distance
Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-mode fibers)
Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)
Operating temperature/humidity range
Temperature dependence of attenuation
Ability to conduct light underwater - Important for undersea cables
Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.
Physics of Total Internal Reflection
Physics of Total Internal Reflection
When light passes from a medium with one index of refraction (m1) to another medium with a lower index of refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through m2 bends further away from the line.
At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the surface between the two media (sin [critical angle] = n2/n1 where n1 and n2 are the indices of refraction [n1 is less than n2]). If the beam through m1 is greater than the critical angle, then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may be transparent! In physics, the critical angle is described with respect to the normal line. In fiber optics, the critical angle is described with respect to the parallel axis running down the middle of the fiber. Therefore, the fiber-optic critical angle = (90 degrees - physics critical angle).
When light passes from a medium with one index of refraction (m1) to another medium with a lower index of refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through m2 bends further away from the line.
At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the surface between the two media (sin [critical angle] = n2/n1 where n1 and n2 are the indices of refraction [n1 is less than n2]). If the beam through m1 is greater than the critical angle, then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may be transparent! In physics, the critical angle is described with respect to the normal line. In fiber optics, the critical angle is described with respect to the parallel axis running down the middle of the fiber. Therefore, the fiber-optic critical angle = (90 degrees - physics critical angle).
In an optical fiber, the light travels through the core (m1, high index of refraction) by constantly reflecting from the cladding (m2, lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle the fiber itself gets bent at, even if it's a full circle! Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.
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