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Most reference materials that discuss the historical perspective mention about Indian smoke signals. None of these primitive systems was secure due to the spreading of the unguided light. Ideally, a communication system should be secure and should not require installation of a cumbersome physical media. Fiber optics satisfied these desires, and as early as 1958, fiber-optic equipment was being focused for use in the factory. The fiber-optic cable is an important element in the fiber-optic link. Today, in comparison to the early 1970s, the performance of fiber-optic cable in terms of bandwidth and attenuation is far superior to any electrical cable of similar cost. Some consider it a problem of fiber optics that the electronics of the fiber-optic transceivers are unreliable. This is a false, in that the electronics have the same life as any of the other electronic components used in a network. The need for sharing components or modules is the same for fiber optics as for any other critical factory-level electronics. Optical fiber is used as glass or plastic, to contain and guide light wave.
The fiber cable does not transmit electrical current, so it cannot cause ground loops. Therefore ground differentials caused by lightning-induced transients do not affect the communication cable. This characteristic is quite an advantage because lightning strikes are a common phenomenon. A typical fiber-optic cable can allow up to 200 million bits per second (MBS), while a high-quality coaxial cable is required to achieve the same data rate, but can cover only shorter distances. The reduction in the number of repeaters is a prime reason for the telephone companies increasing use of fiber optics. Many control applications require the operator to perform normal duties in the vicinity of high voltages. The use of fiber allows isolation of the high voltage from the operators. An advantage of fiber-optics is that the light signal used for data communication cannot develop a spark above the ignition point, which could cause ignition in hazardous environments.
The fiber-optic cable is susceptible to noise and it does not generate electromagnetic interference. It is very simple to install because of light and small size and is suitable for rugged environments i.e. it can survive high temperatures and other extreme environments.
1 OPTICAL FIBERS IN COMMUNICATION
Optical fibers are one of the world’s most influential scientific developments from the latter half of the 20th century. Normally we are unaware that we are using them, although many of us do frequently. The majority of telephone calls and internet traffic at some stage in their journey will be transmitted along an optical fiber. Why has the development of fibers been given so much attention by the scientific community when we have alternatives? The main reason is bandwidth – fibers can carry an extremely large amount of information. More indirectly, many of the systems that we either rely on or enjoy in everyday life such as banks, television and newspapers as are themselves dependent on communication systems that are dependent on optical fibers.
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information.
An optical fiber is a glass or plastic fiber that carries light along its length. Optical fibers are widely used in fiber optic communications, which permits transmission over longer distances and at higher bandwidths because light has higher frequency than any other form of radio signal. Light is kept in the core of the optical fiber by total internal reflection. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference.
1.2 Fundamentals of Fibers
The fundamental principle that makes optical fibers possible is total internal reflection. This is described using the ray model of light.
1.3 CONSTRUCTION OF FIBERS
In fibers, there are two significant sections – the core and the cladding. The core is part where the light rays travel and the cladding is a similar material of slightly lower refractive index to cause total internal reflection. Usually both sections are fabricated from silica (glass). The light within the fiber is then continuously totally internally reflected along the waveguide
1.4 CLASSIFICATION OF OPTICAL FIBERS:-
Optical fibers are classified into three types based on the material used, number of modes and refractive index.
1.4.1. Based on the materials used:-
a. Glass fibers:
They have a glass core and glass cladding. The glass used in the fiber is ultra pure, ultra transparent silicon dioxide (SiO2) or fused quartz. Impurities are purposely added to pure glass to achieve the desired refractive index.
b. Plastic clad silica:
This fiber has a glass core and plastic cladding. This performance though not as good as all glass fibers, is quite respectable.
c. Plastic fibers:
They have a plastic core and plastic cladding. These fibers are attractive in applications where high bandwidth and low loss are not a concern.
1.4.2. Based on the number of modes:-
a. Single Mode fiber:
When a fiber wave-guide can support only the HE11 mode, it is referred to as a single mode wave-guide. In a step index structure this occurs when the wave-guide is operating at v<2.4 where v is dimensionless number which relates the propagating in the cladding. These single
mode fibers have small size and low dopant level (typically 0.3% to 0.4% index elevation over the lading index.)
In high silica fibers the wave-guide and the material dispersion are often of opposite signs. This fact can be used conveniently to achieve a single mode fiber of extremely large bandwidth. Reduced dopant level results in lower attenuation than in multimode fibers. A single mode wave guide with its large and fully definable bandwidth characteristics is an obvious candidate for long distance, high capacity transmission applications.
a. Multimode fiber:
It is a fiber in which more than one mode is propagating at the system operating wavelength. Multimode fiber system does not have the information carrying capacity of single mode fibers. However they offer several advantages for specific systems. The larger core diameters result in easier splicing of fibers. Given the larger cores, higher numerical apertures, and typically shorter link distances, multimode systems can use less expensive light sources such as LED s. Multimode fibers have numerical apertures that typically range from 0.2 to 0.29 and have core size that range from 35 to100 micro-meters.
1.4.3. Based on refractive index:-
a. Step index fiber:
The step index (SI) fiber consists of a central core whose refractive index is n1, surrounded by a lading whose refractive index is n2, lower than that of core. Because of an abrupt index change at the core cladding interface such fibers are called step index fibers.
b. Graded index fibers:
The refractive index of the core in graded index fiber is not constant, but decreases gradually from its maximum value n1 to its minimum value n2 at the core-cladding interface. The ray velocity changes along the path because of variations in the refractive index. The ray propagating along the fiber axis takes the shortest path but travels most slowly, as the index is largest along this path in medium of lower refractive index where they travel faster. It is therefore possible for all rays to arrive together at the fiber output by a suitable choice of refractive index profile.
2 COMPONENTS OF OPTICAL FIBER COMMUNICATION:
2.1 Transmitters: -
Fiber optic transmitters are devices that include an LED or laser source, and signal conditioning electronics, to inject a signal into fiber. The modulated light may be turned on or off, or may be linearly varied in intensity between two predetermined levels.
It is the medium to guide the light form the transmitter to the receiver.
Fiber optic receivers are instruments that convert light into electrical signals. They contain a photodiode semiconductor, signal conditioning circuitry, and an amplifier at the receiver end. Several types of photodiodes include p-n photodiodes, a p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photo detectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.
The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high.
An alternative approach is to use an optical amplifier, which amplifies the optical signal directly without having to convert the signal into the electrical domain. It is made by doping a length of fiber with the rare-earth mineral erbium, and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.
2.5 Wavelength-division multiplexing
Wavelength-division multiplexing (WDM) is the practice of multiplying the available capacity of an optical fiber by adding new channels, each channel on a new wavelength of light. This requires a wavelength division multiplexer in the transmitting equipment and a demultiplexer (essentially a spectrometer) in the receiving equipment. Arrayed waveguide gratings are commonly used for multiplexing and demultiplexing in WDM. Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as 160 channels to support a combined bit rate into the range of terabits per second.
For modern glass optical fiber, the maximum transmission distance is limited not by direct material absorption but by several types of dispersion, or spreading of optical pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the performance of multi-mode fiber. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated.
In single-mode fiber performance is primarily limited by chromatic dispersion (also called group velocity dispersion), which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters necessarily has nonzero spectral width (due to modulation). Polarization mode dispersion, another source of limitation, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. This phenomenon is called fiber birefringence and can be counteracted by polarization-maintaining optical fiber. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver.
Some dispersion, notably chromatic dispersion, can be removed by a 'dispersion compensator'. This works by using a specially prepared length of fiber that has the opposite dispersion to that induced by the transmission fiber, and this sharpens the pulse so that it can be correctly decoded by the electronics.
When a communications link must span a larger distance than existing fiber-optic technology is capable of, the signal must be regenerated at intermediate points in the link by repeaters. Repeaters add substantial cost to a communication system, and so system designers attempt to minimize their use.
Recent advances in fiber and optical communications technology have reduced signal degradation so far that regeneration of the optical signal is only needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost and reliability of repeaters is one of the key factors determining the performance of the whole cable system. The main advances contributing to these performance improvements are dispersion management, which seeks to balance the effects of dispersion against non-linearity; which use nonlinear effects in the fiber to enable dispersion-free propagation over long distances.
2.8 MODES AND PROPAGATION OF LIGHT IN FIBERS
Also crucial to understanding fibers is the principle of modes. A more in-depth analysis of the propagation of light along an optical fiber requires the light to be treated as an electromagnetic wave (rather than as a ray).
The solid line is the lowest order mode shown on figure 4. It is clear that according to the ray model the lowest order mode will travel down a given length of fiber quicker than the others. The electromagnetic field model predicts the opposite – that the highest order mode will travel quicker. However, the overall effect is still the same – if a signal is sent down the fiber as several modes then as it travels along the fiber the pulse will spread out, this can lead to the pulses merging and becoming indistinguishable.
The propagation of light is as shown in figure 5. When light ray enters the core with an angle strikes the surface of cladding whose refractive index is less than that of core. As the incidence angle on surface of the cladding is greater than or equal to critical angle total internal reflection takes place. Hence the ray is reflected back into the core in the forward direction. This process continues until it reaches other end of the cable.
3 PRINCIPLE OF OPTICAL TRANSMISION
3.1 Index of refraction:-
This is the measuring speed of light in respective medium. It is calculated by dividing speed of light in vacuum to the speed of light in material. The RI for vacuum is 1, for the cladding material of optical fiber it is 1.46, the core value of RI is 1.48(core RI must be more than cladding material RI for transmission) it means signal will travel around 200 million meters per second. it will travel 12000 km in only 60 seconds. other delay in communication will be due to communication equipment switching and decoding, encoding the voice of the fiber.
3.2 Snell's Law:-
In order to understand ray propagation in a fiber. We need one more law from high school physics. This is Snell' law. n1 sin Ө1 = n2 sin Ө2
Where n denotes the refractive index of the material. Ө1/ Ө2 are angles in Ө1/ Ө2 respective medium. Higher Refractive Index means denser medium. 1) When light enters in lighter medium from denser it inclines towards normal. 2) When light enters in denser medium from lighter it inclines away to normal.
3.3 Critical Angle:-
If we consider we notice above that as the angle Ө1 becomes larger and larger so does the angle Ө2. Because of the refraction effect Ө2.becomes larger more quickly than Ө1 .At some point Ө2 will reach 90° while Ө1 is still well less than that. This is called the “critical angle”. When Ө1is increased further then refraction ceases and the light starts to be reflected rather than refracted. Thus light is perfectly reflected at an interface between two materials of different refractive index if:
1. The light is incident on the interface from the side of higher refractive index.
2. The angle is greater than a specific value called the “critical angle”. Glass refractive index is 1.50 (critical angle is 41.8); Diamond critical angle is 24.4 degree.
3.4Total Internal reflection (TIR):-
When light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical angle “for the boundary), the light will be completely reflected. This phenomenon is called total internal reflection. This effect is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary; because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. Total internal. Reflection occurs when light enters from higher refractive index to lower refractive index material, i.e. from glass to air total internal reflection is possible but it is not possible in air to glass.
If we now consider above Figures we can see the effect of the critical angle within the fiber. In Figure 2 we see that for rays where angle Ө1 is less than a Critical value then the ray will propagate along the fiber and will be “bound” within the fiber. In Figure 1 we see that where the angle Ө1 is greater than the critical value the ray is refracted into the cladding and will ultimately be lost outside the fiber. This is loss.
3.5 Acceptance Cone:-
When we consider rays entering the fiber from the outside (into the end face of the Fiber) we see that there is a further complication. The refractive index difference between the fiber core and the air will cause any arriving ray to be refracted. This means that there is a maximum angle for a ray arriving at the fiber end face at which the ray will propagate. Rays arriving at an angle less than this angle will propagate but rays arriving at a greater angle will not. This angle is not a “critical angle” as that term is reserved for the case where light arrives from a material of higher RI to one of lower RI. (In this case, the critical angle is the angle within the fiber.) Thus there is a “cone of acceptance” at the end face of a fiber. Rays arriving within the cone will propagate and ones arriving outside of it will not. The size of acceptance cone is function of difference of RI of core and cladding.
[b]4 ADVANTAGES OVER CONVENTIONAL CABLES[/b]
The choice between optical fiber and electrical (or copper) transmission for a particular system is made based on a number of trades-offs. Optical fiber is generally chosen for systems requiring higher bandwidth or spanning longer distances than electrical cabling can accommodate. The main benefits of fiber are its exceptionally low loss, allowing long distances between amplifiers or repeaters; and its inherently high data-carrying capacity, such that thousands of electrical links would be required to replace a single high bandwidth fiber. Another benefit of fiber is that even when run alongside each other for long distances, fiber cables experience effectively no crosstalk, in contrast to some types of electrical transmission lines.
In short distance and relatively low bandwidth applications, electrical transmission is often preferred because of its
• Lower material cost, where large quantities are not required.
• Lower cost of transmitters and receivers.
• Ease of splicing.
• Capability to carry electrical power as well as signals.
• Ease of operating transducers in linear mode.
a. Wide Bandwidth:
Optical fibers offer greater bandwidth due to the use of light as carrier. The frequency range used for glass fiber communication extends from 2*e14Hz to 4*e14Hz. Hence optical fibers are suitable for high speed, large capacity telecommunication lines.
b. Low Loss:
In a coaxial cable attenuation increases with frequency. The higher the frequency of information signals the greater the loss, whereas in an optical fiber the attenuation is independent of frequency. They offer a loss of0.2 dBm/km, allowing repeater separation up to 50Km or more.
c. Freedom from electromagnetic interference:
Optical fibers are not affected by interference originating from power cables, railways and radio waves. They do not limit unwanted radiation and no cross talk between fibers exists. These fibers make an ideal transmission medium when EMI (Electro Magnetic Immunity) is increased.
d. Non conductivity:
Optical fibers are non-conductive and are not effective by strong electromagnetic interference such as lighting. These are usable in explosive environment.
e. Small diameters and less weight:
Even multi fiber optical cables have a small diameter and are light weight, and flexible optical fiber cables permit effective utilization of speech and can also be applicable to long distance use are easier to handle and install than conventional cables.
Fiber optic is a highly source transmission medium. It does not radiate energy that can be received by a nearby antenna, and it is extremely difficult to tap a fiber and virtually impossible to make the tap undetected.
Fiber is a dielectric and does not carry electricity. It presents no sparks or fire hazards. It does not cause explosions, which occur due to faulty copper cable.
5 LIMITATIONS OF OPTICAL FIBER:
1. The terminating equipment is still costly as compared to copper wire.
2. Delicate so has to be handled carefully.
3. Communication is not totally in optical domain, so repeated electric to optical to electrical conversion is needed.
4. Optical amplifiers, splitters, MUX-DEMUX are still in development stages.
5. Tapping is not possible. Specialized equipment is needed to tap a fiber.
6. Optical fiber splicing is a specialized technique and needs expertly trained manpower.
7. The splicing and testing equipments are very expensive as compared to copper equipments.
8. Bending Cables
9. Gamma Radiation
10. Electrical Fields
• Military applications
• Mobile applications
• Satellite communications
• Under sea transmission cable
• Internet & Broadband applications
• Computer applications
• Electrical power companies
• Optical sensor system
• Local area networks
• Electronic media
• Public network applications
• Civil application
• Consumer application
• Industrial application
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02-03-2011, 12:38 PM
Fiber.doc (Size: 197.5 KB / Downloads: 131)
In fiber-optic communications, information is transmitted by sending light through optical fibers.
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networks in the developed world.
The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.
Optical fiber is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation and interference, optical fiber has large advantages over existing copper wire in long-distance and high-demand applications. However, infrastructure development within cities was relatively difficult and time-consuming, and fiber-optic systems were complex and expensive to install and operate. Due to these difficulties, fiber-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. Since 2000, the prices for fiber-optic communications have dropped considerably. The price for rolling out fiber to the home has currently become more cost-effective than that of rolling out a copper based network. Prices have dropped to $850 per subscriber in the US and lower in countries like The Netherlands, where digging costs are low.
Since 1990, when optical-amplification systems became commercially available, the telecommunications industry has laid a vast network of intercity and transoceanic fiber communication lines. By 2002, an intercontinental network of 250,000 km of submarine communications cable with a capacity of 2.56 Tb/s was completed, and although specific network capacities are privileged information, telecommunications investment reports indicate that network capacity has increased dramatically since 2004.
In 1966 Charles K. Kao and George Hockham proposed optical fibers at STC Laboratories (STL), Harlow, when they showed that the losses of 1000 db/km in existing glass (compared to 5-10 db/km in coaxial cable) was due to contaminants, which could potentially be removed.
Optical fiber was successfully developed in 1970 by Corning Glass Works, with attenuation low enough for communication purposes (about 20dB/km), and at the same time GaAs semiconductor lasers were developed that were compact and therefore suitable for transmitting light through fiber optic cables for long distances.
After a period of research starting from 1975, the first commercial fiber-optic communications system was developed, which operated at a wavelength around 0.8 µm and used GaAs semiconductor lasers. This first-generation system operated at a bit rate of 45 Mbps with repeater spacing of up to 10 km. Soon on 22 April, 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6 Mbps throughput in Long Beach, California.
The second generation of fiber-optic communication was developed for commercial use in the early 1980s, operated at 1.3 µm, and used InGaAsP semiconductor lasers. Although these systems were initially limited by dispersion, in 1981 the single-mode fiber was revealed to greatly improve system performance. By 1987, these systems were operating at bit rates of up to 1.7 Gb/s with repeater spacing up to 50 km.
The first transatlantic telephone cable to use optical fiber was TAT-8, based on Desurvire optimized laser amplification technology. It went into operation in 1988.
Third-generation fiber-optic systems operated at 1.55 µm and had losses of about 0.2 dB/km. They achieved this despite earlier difficulties with pulse-spreading at that wavelength using conventional InGaAsP semiconductor lasers. Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at 1.55 µm or by limiting the laser spectrum to a single longitudinal mode. These developments eventually allowed third-generation systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of 100 km.
The fourth generation of fiber-optic communication systems used optical amplification to reduce the need for repeaters and wavelength-division multiplexing to increase data capacity. These two improvements caused a revolution that resulted in the doubling of system capacity every 6 months starting in 1992 until a bit rate of 10 Tb/s was reached by 2001. Recently, bit-rates of up to 14 Tbit/s have been reached over a single 160 km line using optical amplifiers.
The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which a WDM system can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1.53-1.57 µm, and the new dry fiber has a low-loss window promising an extension of that range to 1.30-1.65 µm. Other developments include the concept of "optical solitons, " pulses that preserve their shape by counteracting the effects of dispersion with the nonlinear effects of the fiber by using pulses of a specific shape.
In the late 1990s through 2000, industry promoters, and research companies such as KMI and RHK predicted vast increases in demand for communications bandwidth due to increased use of the Internet, and commercialization of various bandwidth-intensive consumer services, such as video on demand. Internet protocol data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased under Moore's Law. From the bust of the dot-com bubble through 2006, however, the main trend in the industry has been consolidation of firms and offshoring of manufacturing to reduce costs. Recently, companies such as Verizon and AT&T have taken advantage of fiber-optic communications to deliver a variety of high-throughput data and broadband services to consumers' homes.
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 cable television companies.
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Optical fibers are one of the world’s most influential scientific developments from the latter half of the 20th century. Normally we are unaware that we are using them, although many of us do frequently.These single mode fibers have small size and low dopant level (typically 0.3% to 0.4% index elevation over the lading index.)In high silica fibers the wave-guide and the material dispersion are often of opposite signs.
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fibre optics 2.pptx (Size: 567.44 KB / Downloads: 122)
• Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fibre.
• The light forms an electromagnetic carrier wave that is modulated to carry information.
• First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunication industry and have played a major role in the advent of the information age.
• The process of communicating using fiber-optics involves the following basic steps:
a. Creating the optical signal involving the use of a transmitter,
b. Relaying the signal along the fiber,
c. Ensuring that the signal does not become too distorted or weak,
d. Receiving the optical signal,
e. Converting it into an electrical signal.
Physics of Light
• Fig. 3A shows how a light ray passing from material 1 to material 2 is refracted in material 2 when A1 is less than the critical angle.
• Fig. 3B shows the condition that exists when A1 is at the critical angle and angle A2 is at 900. The light is directed along the boundary between the 2 materials.
• Fig. 3C shows that any light ray incident at an angle greater than A1 of Fig. 3B will be reflected back into material 1 with A2 equal to A1.
• Reflection in Optical Fiber
Reflection in Optical Fiber
• From fig. 1, the light rays are reflected from the inner walls as they propagate lengthwise along the fiber.
• A single light beam can be modulated simultaneously by hundreds, or even thousands, of independent signals.
Total Internal Reflection of Light
Total internal reflection in optical fibers
• Total internal reflection forms the basis for light propagation in optical fibers.
Mode of Propagation
• Mode simply means path from which light is propagated.
• If there is only one path for light to take down the cable, it is called single mode.
• If there is more than one path, it is called multi-mode.
Single-Mode Step-Index Fiber
• It has a central core that is sufficiently small so that there is essentially only one path that light may take as it propagates down the cable.
• The refractive index of the cladding is slightly less than that of the central core and is uniform throughout the cladding.
• Consequently, all light rays follow approximately the same path down the cable and take approximately the same amount of time to travel the length of the cable.
Multi-mode Step-Index Fiber
• The light rays that strike the core/cladding interface at an angle greater than the critical angle are propagated down the core in a zigzag fashion, continuously reflecting off the interface boundary.
• There are many paths that a light ray may follow as it propagates down the fiber. As a result, all light rays do not follow the same path and hence do not take the same amount of time to travel the length of the fiber.
Advantages of fiber optics
• Optical Fibre In Telecommunication
BSNL has the largest optical fibre cable network in the country, comprising at least 600,000 route kilometres covering all state capitals and district headquarters. The firm also operates around 42,000 telecom towers.
Tata Teleservices plans to spend Rs.2 billion to raise its optical fibre network to 5,000 km in the east by March 2006, the company.
Reliance fully-owned subsidiary of the his Industries, will lay a 4000 route km optical fiber network in Uttar Pradesh, investing 40 billion.
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FIBRE OPTICS.pptx (Size: 599.43 KB / Downloads: 87)
WHAT IS FIBRE OPTICS?
Optical fiber is a thin & transparent guiding medium or material which guides the information carrying light.
A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.
History of Optical Fiber:
In 1854, John Tyndall, a British physicist, demonstrated that light could travel through a curved stream of water thereby proving that a light signal could be bent.
The general use of fiber optics did not begin until the 1970s.
Robert Maurer of Corning Glass Works developed a fiber with a loss of 20 dB/km ,promoting the commercial use of fiber.
WHAT IS THE PRINCIPLE USED IN FIBRE OPTICS?
It Works On the Principle of Total Internal Reflection.
This below fig gives clear idea about the process taking place in total internal reflection.
TOTAL INTERNAL REFLECTION:
when a ray of light in a medium of higher index of refraction approaches another medium of lower index of refraction at more than the critical angle. Then total internal reflection takes place
When i> ,then the light ray will be reflected back in to the denser medium.
According to Snell's law,
When =critical angle then r= .
If the rare medium is air then ,as the refractive index of air is 1
Condition for total internal reflection:
1.The light ray should move from denser to rarer medium.
2.When the angle of incidence is greater than angle of refraction then the light undergoes total internal reflection.
The critical angle
If the rarer medium is air then,
Difference between copper wire and fiber optics:
Fiber optics are gradually replacing traditional copper wire
The major difference is use of light as a transmission medium.
“Total internal reflection” principle makes possible transmission of information.
Construction of fiber optics:
Core: The core is a cylindrical rod of dielectric material.
Dielectric material conducts no electricity.
Light propagates mainly along the core of the fiber. The core is generally made of glass. The core is described as having a radius of (a) and an index of refraction n1. The core is surrounded by a layer of material called the cladding.
Cladding: The cladding layer is made of a dielectric material with an index of refraction n2. The index of refraction of the cladding material is less than that of the core material. The cladding is generally made of glass or plastic.
The cladding performs the following functions:
Reduces loss of light from the core into the surrounding air.
Reduces scattering loss at the surface of the core .
Protects the fiber from absorbing surface contaminants.
Adds mechanical strength.
Coating or buffer: The coating or buffer is a layer of material used to protect an optical fiber from physical damage. The material used for a buffer is a type of plastic.
The buffer is elastic in nature and prevents abrasions. The buffer also prevents the optical fiber from scattering losses caused by micro bends.
Micro bends occur when an optical fiber is placed on a rough and distorted surface.
The core & cladding are made up of either glass or plastics. so there are two types of optical fibers.
If the optical fibers are made up by fusing mixtures of metal oxides & silica glasses, then it is known as glass fiber.
The most commonly used material in glass fiber is silica. It has a refractive index 1.458 .
Examples of fiber compositions:
Geo2 -sio2 core; sio2 cladding.
P2 o5 -sio2 core; sio2 cladding.
The plastic fibers are typically made up of plastics.
They acquire low cost & high durability, toughness.
Examples of plastic fibers,
A polystyrene core and a metylmethacrylate cladding
Consider a cross sectional view of an optical fiber having core and cladding of refractive indices n1 & n2 respectively.
The incident light makes an angle of with fiber axis.
Let be the angle of refraction at a point ‘A’ & be the angle of incidence at ‘B’.
If is greater than critical angle ( ) then TIR takes place and the light ray takes the path BD.
At the point ‘B’ we know that
Let the maximum angle at point ‘A’ be “ “. From the critical angle we know that
Therefore from for air medium Here, in the above expression is the maximum angle of incidence , it is also called as “acceptance angle”
Types of optical fibers:
Single Mode Optical Fiber:
Used to transmit one signal per fiber.
Generally Single Mode fibers are used in telephones and cable TV applications.
Single Mode Optical Fiber produce as 8 / 125 and 9 / 125 ( Core / Cladding diameter Microns).
Diameter of core is smaller when compared to the width of cladding as a result so single path is possible.
Multi Mode Optical Fiber:
Used to transmit many signals per fiber.
Multi Mode generally are used in computer networks, lan applications.
The width of the core is greater than the cladding, then large number of paths are available for the light ray through the fiber.
Multimode Optical Fiber produce as 50 / 125 and 62.5 / 125 ( Core / Cladding diameter Microns).
Step-index optical fiber:
Ina a step index optical fiber, The refractive index of the core remains constant through out the core and decreases from step to at the core cladding interface. thus it is known as step-index optical fiber.
For a single mode the step-index optical fiber, a single light ray from the signal enters into the fiber and takes single path and forms the output signal similar to that of input signal.
In a multimode step-index fiber,due to large width of core greater number of light rays from input signals enters into core and takes multi paths.
Light ray which have more acceptance angle suffers more reflections through fiber and takes more time to traverse the optical fiber.
Another light ray having less acceptance angle suffers less reflections and traverses the fiber within a short time.
Due to path difference between the light rays when they superimpose to form the output signals, the signals are overlapped. In this, we get signal distortion known as intermodal dispersion.
Due to intermodal dispersion it is difficult to retrieve the information carried by the distorted out put signal.
Graded index optical fiber:
It is a multimode fiber with huge core diameter and with a core having non-uniform refractive index.
The refractive index of the cladding is uniform. Since the refractive index towards the core-cladding interface is lower than that at the center, the light rays traveling along the edge travels faster. Thus, all rays arrive the end of the fiber at approximately the same time.
FIBER OPTICS IN COMMUNICATION SYSTEM:
Fiber optic communication system consists of three important components.
An optical transmitter converts an analog or digital signal into optical form.
It consists of an encoder , light source and modulator.
The input analog signal is converted into a digital signal by means of an encode.
The converted signal is fed to the source.
The source can be light LED’s.
The optical fiber from the source is modulated based on intensity, amplitude or frequency with the help of a modulator.
This optical fiber by means of couplers. The couplers launch the optical signal fiber into the fiber without any distortion or loss.
The optical signal is connected to the to a repeater with the help of connector.
The optical signal while travelling through very long optical fibers through long distances can suffer transmission losses an fiber losses like dispersion.
As a result we get a weak optical signal.
To minimize the losses, we use fiber repeaters at regular intervals between the fibers.
The repeaters consists of amplifier & regenerator.
The amplifier amplifies the weak optical signal, it is reconstructed to original optical signal with the help of regenerator &transmitted to optical receiver.
Optical receiver :
The receiver unit consists of a photo detector ,amplifier , demodulator & decoder.
The photo detector consists of PIN photodiode .
this works on the principle of creation of an e-h pair at p-n junction diode by successive collisions of the incident optical signal.
The released electrons outputs a current which is direct relation with the incident optical signal.
This electric current is then amplified and demodulated to obtain a digital signal.
Advantages of fiber optics:
Greatly increased bandwidth and capacity .
Lower signal loss .
Immunity to Electrical Noise.
Immune to noise (electromagnetic interference [EMI] and radio-frequency interference [RFI] .
Lower bit error rates .
Difficult to tap.
Nonconductive (does not radiate signals)Electrical Isolation.
No common ground required.
Freedom from short circuit and sparks.
Size and Weight.
Reduced size and weight cables.
Environmental Protection .
Resistant to radiation and corrosion.
Resistant to temperature variations.
Improved ruggedness and flexibility.
Less restrictive in harsh environments.
Overall System Economy.
Lower installation cost.
Disadvantages of fiber optics:
Price - Even though the raw material for making optical fibers, sand, is abundant and cheap, optical fibers are still more expensive per meter than copper. Although, one fiber can carry many more signals than a single copper cable and the large transmission distances mean that fewer expensive repeaters are required.
Fragility - Optical fibers are more fragile than electrical wires.
Affected by chemicals - The glass can be affected by various chemicals including hydrogen gas (a problem in underwater cables.)
Opaqueness - Despite extensive military use it is known that most fibers become opaque when exposed to radiation.
Requires special skills - Optical fibers cannot be joined together as a easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment.
The small size and large information-carrying capacity of optic fibers is much high as compared to copper twisted-pair cables in telephone systems.
Continuous passive links more than 100 km long have been produced. With repeaters, messages over thousands of kilometers of fiber can be sent. Because of low losses, separation between repeaters in a fiber system is greater than in a coaxial link.
Another important application of optical fibres is in sensors. If a fibre is stretched or squeezed, heated or cooled or subjected to some other change of environment, there is usually a small but measurable change in light transmission. Hence, a rather cheap sensor can be made which can be put in a tank of acid, or near an explosion or in a mine and connected back, perhaps through kilometres of fibre, to a central point where the effects can be measured.
Military applications of fiber optics include communications, command and control links on ships and aircrafts, data links for satellite earth stations and transmissions lines for tactical command post communication.
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sem6_2.pdf (Size: 133.85 KB / Downloads: 62)
Historical perspective, basic communication systems, optical frequency range,
advantages of optical fibre communication, application of fibre optic
Electromagnetic spectrum used, Advantages and disadvantages of optical
Principle of light penetration, reflection, critical angle.
2. Optical Fibers and Cables
Fiber types construction, multimedia and monomode fibers, step index and
graded index fibers, acceptance angle and acceptance types of optical fiber
3. Losses in optical fiber cable:
a) Absorption Losses, Scattering Losses, Radiation losses, Compelling losses,
b) Dispersion, Material dispersion, wave guide dispersion, modal dispersion
total dispersion and bit rate.
4. Light Sources and Detectors
a) Characteristics of light source used in optical communication, principle of
operation of LED, different type of LED structures used and their brief
description, LED driving circuitry, Injection Laser diode, principle of
operation, different injection laser diodes, comparison of LED and ILD, non
1. Optical fiber Communication by John M Senior, Prentice Hall of India, New Delhi
2. Optical fiber Communication by J. Gower , Prentice Hall of India, New Delhi
3. Optical fiber Communication by ‘ Gerd Keiser, McGraw Hill International Editions
4. Optical Communications – Components and Systems by JH Franz and VK Jain, Narosa
Publishing House, New Delhi
5. Optical fiber Communication Systems by GP Agrawal, John Wiley & Sons, New Delhi
6. Optical fiber Communication and its Applications by S C Gupta, Prentice Hall of India, New
MICRO CONTROLLERS AND PLCs
In industry, many manufacturing processes demand a sequence of operation, which are to be
performed repetitively. Early automation systems were mechanical in design, timing and
sequencing being effected by gears and cams. Slowly these design concepts were replaced by
electrical drives which were controlled by relays and now by programmable logic controllers
(PLCs). A PLC is a solid state device, designed to operate in noisy industrial environments and
can perform all logic functions. PLCs are widely used in all industries for efficient control
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FIBER OPTICS IN COMMUNICATION
FIBER OPTICS IN COMMUNICATION.ppt (Size: 157.5 KB / Downloads: 68)
An opical fibre (or fibre) is a glass or plastic fiber designed to guide light along its
length by total internal reflection.Fiber optics is the branch of applied science and
engineering concerned with such optical fibers.optical fibers are widely used in
fiber optic communication, which permitts digital data transmission over longer
distances and at higher data rates than other forms of wired and wireless
communications.they are also used to form sensors,and in a variety of other
The operating principle of optical fibers applies to a number of variants including
multi-mode optical fibers, single-mode optical fibers, graded-index optical fibers,
and step index optical fibers.because of the physics of the optical fiber, special.
OPTICAL FIBER COMMUNICATION
The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bounded as cables. Although fibers can be made out of transparent plastic,glass,or a combination of the two, the fibers used in long-distance telecommunications applications are always glass,because of the lower optical attenuation.Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances and single mode fiber used for longer distance links.Because of the tighter tolerance required to couple light into and between single-mode fibers,single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components
Fiber with large (greater than 10) core diameter may be analyzed by geometric
optics. Such fiber is called multimode fiber, from the electromagnetic analysis. In
a step index multimode fiber, rays of light are guided along the fiber core by total
internal reflection. Rays that meet the core-cladding boundary at a higher angle
(measured relative to a line normal to the boundary), greater than the critical angle
for this boundary, are completely reflected. The critical angle (minimum angle for
total internal reflection) is determined by the difference in index of refraction
between the core and cladding materials. Rays that meet the boundary at a low
angle are refracted from the core into cladding; do not convey light and hence
information along the fiber. The critical angle determines the acceptance angle of
the fiber, often reported as a numerical aperture. A high numerical aperture allows
light to propagate down the fiber in rays both close to axis and at various angles,
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FIBER OPTICS COMMUNICATION
1FIBER OPTICS.doc (Size: 187.5 KB / Downloads: 129)
Our current ‘age of technology’ is the result of many brilliant inventions and discoveries, but it is our ability to transmit information, and the media we use to do it, that is perhaps most responsible for its evolution. Progressing from the copper wire of a century ago to today's fiber optic cable, our increasing ability to transmit more information, more quickly and over longer distances has expanded the boundaries of our technological development in all areas. An optical fiber communication is a transmission medium designed to transmit digital signals in the form of pulses of light using optical fibers. In its simplest terms, fiber optics is a medium for carrying information from one point to another in the form of light. Unlike the copper form of transmission, fiber optics is not electrical in nature.
Today's low-loss glass fiber optic cable offers almost unlimited bandwidth and unique advantages over all previously developed transmission media.
In our treatise we explain the concept of fiber optics, it’s principle, fiber optics data communication link, end¬¬-to-end. This article discusses these standards and protocols, including ATM (Asynchronous Transfer Mode) Ethernet, ISDN (Integrated Service Data Interface) and areas of application.
What is Fiber Optics Communication?
A transmission medium designed to transmit digital signals in the form of pulses of light. These pulses of light are transferred through the optical fibers. The transmission media is the fiber optics itself. They guide visible and infrared light over long distances. The optical fiber made up of an insulating material like glass, plastic etc. Fiber Optics doesn't suffer from either line of sight or absorption under normal circumstances. Fiber optic systems use a laser beam fired into a special glass or plastic fiber with a transparent core. The fiber is designed to allow light to be transmitted along its length. It is specially made to reflect light that reaches the outer edge of its core, and gently deflect the light wave back to the center of the fiber. Thus, the light signal can be bent around corners. The fiber is sealed, thus preventing water from getting in the way. These fibers are tiny. Several fibers together are the width of a human hair, and each fiber can carry thousands of connections (refer figure1.1a). Telecommunications companies use fiber optics most frequently. Most fiber optic cable is buried in the ground, so water sometimes creeps in and causes attenuation, but this is increasingly rare. Because fiber optics are so reliable, and difficult to disrupt (it practically takes a backhoe to stop it), they are the most popular form of long distance communication in use today.
Invention of optical fiber
British physics John Tyndall nearly 120 years ago demonstrated experimentally that light could be guided along a curved steam of water employing optical phenomena of total internal reflection. Based on this concept the first glass fibers were made in 1920’s but the concept of cladding made guiding of light practical with limited success in 1950.
It is the invention of lasers in1960 by Maiman, which created a sensation in optical communication because of its high coherence property. The optical frequencies are of order of 5x1014Hz,as compared to the frequencies of up to 1010 Hz employed in electric communication such as TV, radar, and microwave links. This indicates an increase of information capacity 104 times in optical range.
In recent years it has become apparent that fiber optics are steadily replacing copper wire as an appropriate means of communication signal transmission. They span the long distances between local phone systems as well as providing the backbone for many network systems. Other system users include cable television services, university campuses, office buildings, industrial plants, and electric utility companies.
Think of a fiber cable in terms of very long cardboard roll (from the inside roll of paper towel) that is coated with a mirror. If you shine a flashlight in one you can see light at the far end - even if bent the roll around a corner.
Light pulses move easily down the fiber-optic line because of a principle known as ‘total internal reflection’2. This principle of total internal reflection states ‘that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in’. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses. (Refer figure 1.3a).
Light Emitting Diodes:
An LED is a p-n junction diode in a transparent capsule usually with a lens to let the light escape and to focus it. LED's can be manufactured to operate at 850 nm, 1300 nm, or 1500 nm. These wavelengths are all in the infrared region. LED's have a typical response time of 8 ns, a line width of 40 nm, and an output power of tens of microwatts.
A laser diode is an LED with two important differences
1) The operating current is much higher in order to produce optical gain.
2) Two of the ends of the LD are cleaved parallel to each other. These ends act as perfectly aligned mirrors which reflect the light back and forth through the ‘gain medium’ in order to get as much amplification as possible.
The typical response time of a laser diode is 0.5 ns. The line width is around 2 nm with a typical laser power of 10's of mill watts. The wavelength of a laser diode can be 850 nm, 1300 nm, or 1500 nm.
Fiber Optic Communications Networks
All networks involve the same basic principle: information can be sent to, shared with, passed on, or bypassed within a number of computer stations (nodes) and a master computer (server). In addition to various topologies for networks a number of standards and protocols have been developed, each with their own advantages, topologies, and medium requirements. This discusses these standards and protocols, including: ATM, Ethernet & ISDN.
Asynchronous Transfer Mode (ATM)
Asynchronous transfer mode (ATM) is widely deployed as a network backbone technology. This technology integrates easily with other technologies, and offers sophisticated network management features that allow signal carriers to guarantee Quality Of Service (QOS). ATM may also be referred to as cell relay because the network uses short, fixed length packets or cells for data transport. The information is divided into different cells, transmitted, and re-assembled at the receiving end. Each cell contains 48 bytes of data payload as well as a 5-byte cell header. This fixed size ensures that time critical voice or video data will not be adversely affected by long data frames or packets.
Integrated Services Digital Network (ISDN)
ISDN has been designed to replace the standard telephone system and provide greater numbers of digital services to telephone customers, such as digital audio, interactive information services, fax, e-mail, and digital video. ISDN uses asynchronous transfer mode, which can handle data transmission in both connection-oriented, and packet schemes. As with regular telephone lines, the user must pay a fee for use of the line. Basic rate ISDN offers two simultaneous 64 kb/s data channels as well as a 16 kb/s carrier channel for signaling and control information. The combined data rate, 128 kb/s, allows for videoconferencing capabilities. Multiple ISDN-B connections further increase the data rate and the transmission quality. Primary rate ISDN (PRI) offers 30 channels (of 64 kb/s each), giving a total of 1920 kb/s. As with BRI, each channel can be connected to a different destination, or they can be combined to give a larger bandwidth. These channels, known as ‘bearer’ or ‘B’ channels, give ISDN tremendous flexibility.
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