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OPTICAL FIBER COMMUNICATION
Principles of operation
How optical fiber works
Index of refraction
Total internal reflection
Multi mode fiber $ single mode fiber
Special purpose fiber
Mechanism of attenuation
An optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.
The use of optic fibers for communication purposes were first carried out in Western Europe in the late 19th and early 20th century, such as they were used to diagnose a patient's stomach by a doctor, and those communications within short ranges. Especially, the transfer of images by optical fibers was largely popularized at the beginning of 21st century, due to the growing medical and television demands In 1991, the emerging field of photonic crystals led to the development of photonic-crystal fiber which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 2000
PRINCIPLES OF OPERATION
In 1621, a Dutch physicist named Willebrord Snell derived the relationship between the different angles of light as it passes from one transparent medium to another. When light passes from one transparent material to another, it bends according to Snell's law which is defined as:
n1sin(1) = n2sin(2)
n1 is the refractive index of the medium the light is leaving
1 is the incident angle between the light beam and the normal (normal is 90Ã‚Â° to the interface between two materials)
n2 is the refractive index of the material the light is entering
2 is the refractive angle between the light ray and the normal
HOW OPTICAL FIBER WORKS
An optical fiber is a cylindrical dielectric waveguide (nonconducting waveguide) that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.
Optical fiber is a long, thin strand of very pure glass about the diameter of a human hair. Optical fibers are arranged in bundles called optical cables and used to transmit light signals over long distances.
Optical fibers are based entirely on the principle of total internal reflection. This is explained in the following picture.
Modern fiber optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber , a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings , multiple kinds of amplifiers , and an optical receiver to recover the signal as an electrical signal The information transmitted is typically digital information generated by computers , telephone systems , and cables television companies. The most commonly used optical transmitters are semi conductor devices such as Light emitting diodes [LED] and laser diodes
The main component of an optical receiver is a photodetector , which converts light into electricity using the photoelectric effect. The photodetector is typically a semiconductor based photodiode. Several types of photidiode include p-n photodiodes , and avalanchae photodiodes. Metal-semiconductor-metal[MSM]photodetectors are also used due to there suitability for circuit integration in regenerators and wavelengh-division multipexers
REFRACTION OF LIGHT
As a light ray passes from one transparent medium to another, it changes direction; this phenomenon is called refraction of light. How much that light ray changes its direction depends on the refractive index of the mediums.
Refractive index is the speed of light in a vacuum (abbreviated c, c=299,792.458km/second) divided by the speed of light in a material (abbreviated v). Refractive index measures how much a material refracts light. Refractive index of a material, abbreviated as n, is defined as
TOTAL INTERNAL REFLECTION
When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal.
If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection.
STRUCTURE OF OPTICAL FIBER
For the most common optical glass fiber types,
which includes 1550nm single mode fibers and
850nm or 1300nm multimode fibers, the core
diameter ranges from 8 ~ 62.5 Ã‚Âµm. The most
common cladding diameter is 125 Ã‚Âµm.
The material of buffer coating usually is
soft or hard plastic such as acrylic, nylon and with diameter ranges from 250 Ã‚Âµm to 900 Ã‚Âµm. Buffer coating provides mechanical protection and bending flexibility for the fiber.
Typical optical fibers are composed of core, cladding and buffer coating.
The core is the inner part of the fiber, which guides light. The cladding surrounds the core completely. The refractive index of the core is higher than that of the cladding, so light in the core that strikes the boundary with the cladding at an angle shallower than critical angle will be reflected back into the core by total internal reflection.
OPTICAL FIBER MODE
An optical fiber guides light waves in distinct patterns called modes. Mode describes the distribution of light energy across the fiber. The precise patterns depend on the wavelength of light transmitted and on the variation in refractive index that shapes the core. In essence, the variations in refractive index create boundary conditions that shape how light waves travel through the fiber, like the walls of a tunnel affect how sounds echo inside.
We can take a look at large-core step-index fibers. Light rays enter the fiber at a range of angles, and rays at different angles can all stably travel down the length of the fiber as long as they hit the core-cladding interface at an angle larger than critical angle. These rays are different modes.
Fibers that carry more than one mode at a specific light wavelength are called multimode fibers. Some fibers have very small diameter core that they can carry only one mode which travels as a straight line at the center of the core. These fibers are single mode fibers. This is illustrated in the following picture.
OPTICAL FIBER INDEX PROFILE
Index profile is the refractive index distribution across the core and the cladding of a fiber. Some optical fiber has a step index profile, in which the core has one uniformly distributed index and the cladding has a lower uniformly distributed index. Other optical fiber has a graded index profile, in which refractive index varies gradually as a function of radial distance from the fiber center. Graded-index profiles include power-law index profiles and parabolic index profiles. The following figure shows some common types of index profiles for single mode and multimode fibers.
MECHANISM OF ATTENUATION
Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is typically usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption.
Glass optical fibers are almost always made from silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for longer-wavelength infrared applications. Like other glasses, these glasses have a refractive index of about 1.5. Typically the difference between core and cladding is less than one percent.
Plastic optical fibers (POF) are commonly step-index multi-mode fibers with a core diameter of 0.5 millimeters or larger. POF typically have higher attenuation coefficients than glass fibers, 1 dB/m or higher, and this high attenuation limits the range of POF-based systems.
The amorphous structure of glassy silica (SiO2). No long-range order is present, however there is local ordering with respect to the tetrahedral arrangement of oxygen (O) atoms around the silicon (Si) atoms.
Silica exhibits fairly good optical transmission over a wide range of wavelengths. In the near-infrared (near IR) portion of the spectrum, particularly around 1.5 Ã‚Âµm, silica can have extremely low absorption and scattering losses of the order of 0.2dB/km. A high transparency in the 1.4-Ã‚Âµm region is achieved by maintaining a low concentration of hydroxyl groups (OH). Alternatively, a high OH concentration is better for transmission in the ultraviolet (UV) region.
Fluoride glass is a class of non-oxide optical quality glasses composed of fluorides of various metals. Because of their low viscosity, it is very difficult to completely avoid crystallization while processing it through the glass transition (or drawing the fiber from the melt). Thus, although heavy metal fluoride glasses (HMFG) exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks. Their best attribute is that they lack the absorption band associated with the hydroxyl (OH) group (3200â€œ3600 cm-1), which is present in nearly all oxide-based glasses.
Phosphate glass constitutes a class of optical glasses composed of metaphosphates of various metals. Instead of the SiO4 tetrahedra observed in silicate glasses, the building block for this glass former is Phosphorus pentoxide (P2O5), which crystallizes in at least four different forms. The most familiar polymorph (see figure) comprises molecules of P4O10.
Phosphate glasses can be advantageous over silica glasses for optical fibers with a high concentration of doping rare earth ions. A mix of fluoride glass and phosphate glass is fluorophosphate glass.
The chalcogensâ€the elements in group 16 of the periodic tableâ€particularly sulfur (S), selenium (Se) and tellurium (Te)â€react with more electropositive elements, such as silver, to form chalcogenides. These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of ions or electrons.
Fiber optic coatings are UV-cured urethane acrylate composite materials applied to the outside of the fiber during the drawing process. The coatings protect the very delicate strands of glass fiberâ€about the size of a human hairâ€and allow it to survive the rigors of manufacturing, proof testing, cabling and installation.
Todayâ„¢s glass optical fiber draw processes employ a dual-layer coating approach. An inner primary coating is designed to act as a shock absorber to minimize attenuation caused by microbending. An outer secondary coating protects the primary coating against mechanical damage and acts as a barrier to lateral forces.
TERMINATION AND SPLICING
Optical fibers are connected to terminal
equipment by optical fiber connectors.
These connectors are usually of a standard type such as FC, SC, ST, LC, or MTRJ.
Optical fibers may be connected to each other by connectors or by splicing, that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is arc fusion splicing, which melts the fiber ends together with an electric arc. For quicker fastening jobs, a "mechanical splice" is used.
FREE SPACE COUPLING
It is often necessary to align an optical fiber with another optical fiber, or with an optoelectronic device such as a light-emitting diode, a laser diode, or a modulator. This can involve either carefully aligning the fiber and placing it in contact with the device, or can use a lens to allow coupling over an air gap. In some cases the end of the fiber is polished into a curved form that is designed to allow it to act as a lens.
In a laboratory environment, a bare fiber end is coupled using a fiber launch system, which uses a microscope objective lens to focus the light down to a fine point. A precision translation stage (micro-positioning table) is used to move the lens, fiber, or device to allow the coupling efficiency to be optimized. Fibers with a connector on the end make this process much simpler: the connector is simply plugged into a pre-aligned fiberoptic collimator, which contains a lens that is either accurately positioned with respect to the fiber, or is adjustable. To achieve the best injection efficiency into single-mode fiber, the direction, position, size and divergence of the beam must all be optimized. With good beams, 70 to 90% coupling efficiency can be achieved.
At high optical intensities, above 2 megawatts per square centimeter, when a fiber is subjected to a shock or is otherwise suddenly damaged, a fiber fuse can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propagates back toward the transmitter at 1â€œ3 meters per second (4-11 km/h, 2â€œ8 mph). The open fiber control system, which ensures laser eye safety in the event of a broken fiber, can also effectively halt propagation of the fiber fuse. In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a "fiber fuse" protection device at the transmitter can break the circuit to prevent any damage
Lower material cost , where large quantities are not required
Lower cost of transmitters and receivers
Much smaller cable size
Not electromagnetically radiating
No sparks-important in flammable or explosive gas environments
Optical fibers are more difficult and expensive to splice
At higher optical powers , optical fibers are susceptible to fiber fuse wherein a bit too much light meeting with an imperfection can destroy several meters per second
Its life period is short
Fiber Optics is a significant technology used in many different areas of communications. With the explosion of the internet, fiber optics can readily provide the capacity of data that is transmitted with its gigabit speeds. As more breakthroughs in technology occur, it will spread to every aspect of the industry. Telephones, Fax Machines, Radios, Television Broadcasting, and even satellites use this highly reliable light wave technology. The telecommunications industry receives the most benefits from fiber optics. It allows for the transmission of audio, video, and data information in high quality.
^ a b Bates, Regis J (2001). Optical Switching and Networking Handbook. New York: McGraw-Hill. p. 10. ISBN 007137356X.
^ Tyndall, John (1870). "Total Reflexion". Notes about Light. http://www.archive.org/details/notesofco...0tyndrich.
^ Tyndall, John (1873). "Six Lectures on Light". http://www.archive.org/details/sixlectur...0tynduoft.
^ The Birth of Fiber Optics
^ Nishizawa, Jun-ichi; Suto, Ken (2004). "Terahertz wave generation and light amplification using Raman effect". in Bhat, K. N.; DasGupta, Amitava. Physics of semiconductor devices. New Delhi, India: Narosa Publishing House. p. 27. ISBN 8173195676. http://books.google.com/booksid=2NTpSnfh...1#PPA27,M1