Chapter 1: NUMERATION SYSTEMS [ contents: Numbers and symbols ~ Systems of numeration ~ Decimal versus binary numeration ~ Octal and hexadecimal numeration ~ Octal and hexadecimal to decimal conversion ~ Conversion from decimal numeration ]
Chapter 2: BINARY ARITHMETIC [ contents: Numbers versus numeration ~ Binary addition ~ Negative binary numbers ~ Subtraction ~ Overflow ~ Bit groupings ]
Chapter 3: LOGIC GATES [ contents: Digital signals and gates ~ The NOT gate ~ The “buffer” gate ~ Multiple-input gates ~ TTL NAND and AND gates ~ TTL NOR and OR gates ~ CMOS gate circuitry ~ Special-output gates ~ Gate universality ~ Logic signal voltage levels ~ DIP gate packaging ~ Contributors ]
Chapter 4: SWITCHES [ contents: Switch types ~ Switch contact design ~ Contact “normal” state and make/break sequence ~ Contact “bounce” ]
Chapter 5: ELECTROMECHANICAL RELAYS [ contents: Relay construction ~ Contactors ~ Time-delay relays ~ Protective relays ~ Solid-state relays ]
Chapter 6: LADDER LOGIC [ contents: “Ladder” diagrams ~ Digital logic functions ~ Permissive and interlock circuits ~ Motor control circuits ~ Fail-safe design ~ Programmable logic controllers ~ Contributors ]
Chapter 7: BOOLEAN ALGEBRA [ contents: Introduction ~ Boolean arithmetic ~ Boolean algebraic identities ~ Boolean algebraic properties ~ Boolean rules for simplification ~ Circuit simplification examples ~ The Exclusive-OR function ~ DeMorgan’s Theorems ~ Converting truth tables into Boolean expressions ]
Chapter 8: KARNAUGH MAPPING [ contents: Introduction ~ Venn diagrams and sets ~ Boolean Relationships on Venn Diagrams ~ Making a Venn diagram look like a Karnaugh map ~ Karnaugh maps, truth tables, and Boolean expressions ~ Logic simplification with Karnaugh maps ~ Larger 4-variable Karnaugh maps ~ Minterm vs maxterm solution ~ (sum) and (product) notation ~ Don’t care cells in the Karnaugh map ~ Larger 5 & 6-variable Karnaugh maps ]
Chapter 9: COMBINATIONAL LOGIC FUNCTIONS [ contents: Introduction ~ A Half-Adder ~ A Full-Adder ~ Decoder ~ Encoder ~ Demultiplexers ~ Multiplexers ~ Using multiple combinational circuits ]
Chapter 10: MULTIVIBRATORS [ contents: Digital logic with feedback ~ The S-R latch ~ The gated S-R latch ~ The D latch ~ Edge-triggered latches: Flip-Flops ~ The J-K flip-flop ~ Asynchronous flip-flop inputs ~ Monostable multivibrators ]
Chapter 11: COUNTERS [ contents: Binary count sequence ~ Asynchronous counters ~ Synchronous counters ~ Counter modulus ]
Chapter 12: SHIFT REGISTERS [ contents: Introduction ~ Serial-in/serial-out shift register ~ Parallel-in, serial-out shift register ~ Serial-in, parallel-out shift register ~ Parallel-in, parallel-out, universal shift register ~ Ring counters ~ references ]
Chapter 13: DIGITAL-ANALOG CONVERSION [ contents: Introduction ~ The R/2nR DAC ~ The R/2R DAC ~ Flash ADC ~ Digital ramp ADC ~ Successive approximation ADC ~ Tracking ADC ~ Slope (integrating) ADC ~ Delta-Sigma () ADC ~ Practical considerations of ADC circuits ]
Chapter 14: DIGITAL COMMUNICATION [ contents: Introduction ~ Networks and busses ~ Data flow ~ Electrical signal types ~ Optical data communication ~ Network topology ~ Network protocols ~ Practical considerations ]
Chapter 15: DIGITAL STORAGE (MEMORY) [ contents: Why digital? ~ Digital memory terms and concepts ~ Modern nonmechanical memory ~ Historical, nonmechanical memory technologies ~ Read-only memory ~ Memory with moving parts: “Drives” ]
Chapter 16: PRINCIPLES OF DIGITAL COMPUTING [ contents: A binary adder ~ Look-up tables ~ Finite-state machines ~ Microprocessors ~ Microprocessor programming ]
Chapter 17: CONTRIBUTOR LIST [ contents: How to contribute to this book ~ Credits
Download
Saturday, August 8, 2009
Elsevier - Starting Electronics - 3rd Edition
Paperback, 288 pages,
Publication date: SEP-2004
ISBN-13: 978-0-7506-6386-1
ISBN-10: 0-7506-6386-3
Imprint: NEWNES
Description:
Starting Electronics is unrivalled as a highly practical introduction for hobbyists, students and technicians. Keith Brindley introduces readers to the functions of the main component types, their uses, and the basic principles of building and designing electronic circuits. Breadboard layouts make this very much a ready-to-run book for the experimenter; and the use of multimeter, but not oscilloscopes, puts this practical exploration of electronics within reach of every home enthusiast's pocket. The third edition has kept the simplicity and clarity of the original. New material includes sections on transducers and more practical examples of digital ICs.
Sunday, July 26, 2009
Acne
A general information all about the acne scars, effects of acne scars and its proper medication.
Macules and Scars:
Before I jump into the topic of scars, I need to clarify the difference between Macules and Scars. Macules may look like scars, but they are not scars in the sense that a permanent change has occurred. Macules are essentially the final stage of most inflamed acne lesions. They are normally flat, reddish spots that can remain for up to 8 months. But the difference between a Macule and a scar is that a Macule will end up disappearing completely whereas a scar will remain for years or indefinitely.
As for scars, this can also vary from person to person. With some individuals, scars may remain for a lifetime without change but with others, their skin will undergo a form of remodeling that will eventually diminish the scar.
Another factor that needs to be evaluated is the human element of scarring. People simply have different feelings about acne scars. Those who are distressed about their acne scars are much more likely to actively seek out treatment to moderate or remove the scar than those who are more indifferent about the scars.
Cause of Scars:
Let us first gain a better understanding of acne scars by first determining the cause of scars. A scar is a mark left in the skin by the healing of a wound or surgical incision in which the normal functional tissue (skin) is replaced by connective tissue (scar). In the case of acne, the lesion is caused by the body's inflammatory response to sebum, bacteria and dead cells that are trapped in the plugged sebaceous follicle.
When your skin tissue has suffered a lesion of some sorts, your body will attempt to heal the injured site. It does so by increasing the white blood cells in the area along with an array of inflammatory molecules whose function is to repair the damaged tissue and fight infection. In the end, the repair job can be messy, and the site of the lesion is now filled with fibrous scar tissue or eroded tissue. As for the inflammatory molecules and white blood cells, they can remain at the acne lesion for days and even weeks.
Take note of the fact that not everyone functions in the same way, and this holds true with our skin as well. Some people are simply more prone to scarring than others.
Treatment for scars:
Bear in mind that treating acne and treating acne scars are two completely different things. Treating your acne has nothing to do with treating an acne scar. Acne scars can indeed be treated, but it is important that an acne sufferer bring their acne condition under control first if they still suffer from moderate to severe acne.
Once your acne subsides, make an appointment with a dermatologist and discuss the methods (if applicable) of scar treatment(s) he/she recommend you undergo to treat your scars. Keep in mind that there are many methods with which you may treat your scars. These methods vary according to your scar type, size and location, type of skin, and of course, money $$$. All this should be discussed in great detail with your dermatologist.
Before undergoing scar treatment, ask yourself the following questions before having your dermatologist undergo the decided procedure(s).: Are you willing to wait and see if the scars will subside on their own with time? Do your acne scars affect you emotionally and socially? Is your scarring substantial enough to warrant scar treatment? Can you afford the treatment or what treatment options can you afford?
Keep in mind, the objective of scar treatment is not to necessarily rid you of all indications of scars by completely restoring your skin. It very much depends on the severity of your scars, your skin type, your skins ability to regenerate, etc. Significant improvements can definitely be achieved, but complete restoration is often impossible.
About the Author: Kerwin Chang writes for http://www.acnestuff.net where you can find out more about acne and other skin care topics
Bioplastics
Bioplastics are a form of plastics derived from renewable biomass sources, such as vegetable oil, corn starch, pea starch , microbiota , rather than traditional plastics which are derived from petroleum. They are used either as a direct replacement for traditional plastics or as blends with traditional plastics. There is no international agreement on how much bio-derived content is required to use the term bioplastic.
Bioplastics and biodegradation
The terminology used in the bioplastics sector is sometimes misleading. Most in the industry use the term bioplastic to mean a plastic produced from a biological source. One of the oldest plastics, cellulose film, is made from wood cellulose. All bio- and petroleum-based plastics are technically biodegradable, meaning they can be degraded by microbes under suitable conditions. However many degrade at such slow rates as to be considered non-biodegradable. Some petrochemical-based plastics are considered biodegradable, and may be used as an additive to improve the performance of many commercial bioplastics. Non-biodegradable bioplastics are referred to as durable. The degree of biodegradation varies with temperature, polymer stability, and available oxygen content. Consequently, most bioplastics will only degrade in the tightly controlled conditions of commercial composting units. An internationally agreed standard, EN13432, defines how quickly and to what extent a plastic must be degraded under commercial composting conditions for it to be called biodegradable. This is published by the International Organisation for Standardization ISO and is recognised in many countries, including all of Europe, Japan and the US. However, it is designed only for the aggressive conditions of commercial composting units. There is no standard applicable to home composting conditions.
The term "biodegradable plastic" is often also used by producers of specially modified petrochemical-based plastics which appear to biodegrade. Traditional plastics such as polyethylene are degraded by ultra-violet (UV) light and oxygen. To prevent this process manufacturers add stabilising chemicals. However with the addition of a degradation initiator to the plastic, it is possible to achieve a controlled UV/oxidation disintegration process. This type of plastic may be referred to as degradable plastic or oxy-degradable plastic or photodegradable plastic because the process is not initiated by microbial action. While some degradable plastics manufacturers argue that degraded plastic residue will be attacked by microbes, these degradable materials do not meet the requirements of the EN13432 commercial composting standard.
Environmental impacts
The production and use of bioplastics is generally regarded as a more sustainable activity when compared with plastic production from petroleum, because it relies less on fossil fuel as a carbon source and also introduces less, net-new greenhouse emissions if it biodegrades. However, manufacturing of bioplastic materials is often still reliant upon petroleum as an energy and materials source. This comes in the form of energy required to power farm machinery and irrigate growing crops, to produce fertilisers and pesticides, to transport crops and crop products to processing plants, to process raw materials, and ultimately to produce the bioplastic.
Italian bioplastic manufacturer Novamont states in its own environmental audit producing one kilogram of its starch-based product uses 500g of petroleum and consumes almost 80% of the energy required to produce a traditional polyethylene polymer. Environmental data from NatureWorks, the only commercial manufacturer of PLA (polylactic acid) bioplastic, says that making its plastic material delivers a fossil fuel saving of between 25 and 68 per cent compared with polyethylene, in part due to its purchasing of renewable energy certificates for its manufacturing plant.
A detailed study the process of manufacturing a number of common packaging items in several traditional plastics and polylactic acid carried out by US-group published by the Athena Institute the bioplastic to be less environmentally damaging for some products, but more environmentally damaging for others.
While production of most bioplastics results in reduced carbon dioxide emissions compared to traditional alternatives, there are some real concerns that the creation of a global bioeconomy could contribute to an accelerated rate of deforestation if not managed effectively. There are associated concerns over the impact on water supply and soil erosion.
Cost
With the exception of cellulose, most bioplastic technology is relatively new and is currently not as cost competitive with petroleum-based plastics. Many bioplastics are reliant on fossil fuel-derived energy for their manufacturing, reducing the cost advantage over petroleum-based plastic.
Performance and usage
Many bioplastics lack the performance and ease of processing of traditional materials, albeit materials such as Bioplast from Stanelco closed this performance gap. Polylactic acid plastic is being used by a handful of small companies for water bottles. But shelf life is limited because the plastic is permeable to water - the bottles lose their contents and slowly deform.[citation needed] However, bioplastics are seeing some use in Europe, where they account for 60% of the biodegradable materials market. The most common end use market is for packaging materials. Japan has also been a pioneer in bioplastics, incorporating them into electronics and automobiles.
Recycling
There are also fears that bioplastics will damage existing recycling projects. Packaging such as HDPE milk bottles and PET water and soft drinks bottles is easily identified and hence setting up a recycling infrastructure has been quite successful in many parts of the world. Polylactic acid and PET do not mix - as bottles made from polylactic acid cannot be distinguished from PET bottles by the consumer there is a risk that recycled PET could be rendered unusable. This could be overcome by ensuring distinctive bottle types or by investing in suitable sorting technology. However, the first route is unreliable and the second costly.
Genetically modified bioplastics
Genetic modification (GM) is also a challenge for the bioplastics industry. None of the currently available bioplastics - which can be considered first generation products - require the use of GM crops. However, it is not possible to ensure corn used to make bioplastic in North America is GM-free.
European consumers are hostile to any products that are linked to the GM industry. As a result, some UK retailers such as Sainsbury's not use bioplastic manufactured in the US, such as Natureworks polylactic acid. There is currently no commercial European source of polylactic acid bioplastic.
There is also concern that the route from corn to bioplastics is not the most efficient. Looking further ahead, some of the second generation bioplastics manufacturing technologies under development employ the "plant factory" model, using genetically modified crops or genetically modified bacteria to optimise efficiency. However, a change in consumer perception of GM technology in Europe will be required for these to be widely accepted.
Market size
Because of the fragmentation in the market it is difficult to estimate the total market size for bioplastics, but estimates by SRI global consumption in 2006 at around 85,000 tonnes. In contrast, global consumption of all flexible packaging is estimated at around 12.3 million tonnes.
COPA (Committee of Agricultural Organisation in the European Union) and COGEGA (General Committee for the Agricultural Cooperation in the European Union) have made an assessment of the potential of bioplastics in different sectors of the European economy:
Catering products: 450,000 tonnes per year
Organic waste bags: 100,000 tonnes per year
Biodegradable mulch foils: 130,000 tonnes per year
Biodegradable foils for diapers 80,000 tonnes per year
Diapers, 100% biodegradable: 240,000 tonnes per year
Foil packaging: 400,000 tonnes per year
Vegetable packaging: 400,000 tonnes per year
Tyre components: 200,000 tonnes per year
Total 2,000,000 tonnes per year
Certification
Biodegradability - EN 13432, ASTM D6400
The EN 13432 industrial standard is arguably the most international in scope and compliance with this standard is required to claim that a product is compostable in the European marketplace. In summary, it requires biodegradation of 90% of the materials in a commercial composting unit within 90 days. The ASTM 6400 standard is the regulatory framework for the United States and sets a less stringent threshold of 60% biodegradation within 180 days, again within commercial composting conditions.
The "compostable" marking found on many items of packaging indicates that the package complies with either of the two standards mentined above. However, the marking is not owned by either regulatory body but by third party trade associations representing companies making or selling biodegradable plastics. In Europe, this is European Bioplastics, in the U.S. it is the Biodegradable Products Institute.
Many starch based plastics, PLA based plastics and certain aliphatic-aromatic co-polyester compounds such as succinates and adipates, have obtained these certificates. Additivated plastics sold as fotodegradable or oxobiodegradable do not comply with these standards in their current form.
Biobased - ASTM D6866
The ASTM D6866 method has been developed to certify the biologically derived content of bioplastics. There is an important difference between biodegradability and biobased content. A bioplastic such as high density polyethylene (HDPE) can be 100% biobased (i.e. contain 100% renewable carbon), yet be non-biodegradable. These bioplastics such HDPE play nonetheless an important role in greenhouse gas abatement, particularly when they are combusted for energy production. The biobased component of these bioplastics is considered carbon-neutral since their origin is from biomass.
Applications
Because of their biological biodegradability, the use of bioplastics is especially popular for disposable items, such as packaging and catering items (crockery, cutlery, pots, bowls, straws). The use of bioplastics for shopping bags is already very common[citation needed]. After their initial use they can be reused as bags for organic waste and then be composted. Trays and containers for fruit, vegetables, eggs and meat, bottles for soft drinks and dairy products and blister foils for fruit and vegetables are also already widely manufactured from bioplastics.
Non-disposable applications include mobile phone casings (NEC), carpet fibres (Dupont Sorona), and car interiors (Mazda). The French company, Arkema, produces a grade of bioplastic called Rilsan, which is being used in fuel line and plastic pipe applications. In these areas, the goal is obviously not biodegradability, but to create items from sustainable resources.
Plastic types
Starch based plastics
Constituting about 50 percent of the bioplastics market, thermoplastic starch, such as Plastarch Material, currently represents the most important and widely used bioplastic. Pure starch possesses the characteristic of being able to absorb humidity and is thus being used for the production of drug capsules in the pharmaceutical sector. Flexibiliser and plasticiser such as sorbitol and glycerine are added so that starch can also be processed thermo-plastically. By varying the amounts of these additives, the characteristic of the material can be tailored to specific needs (also called "thermo-plastical starch").
Polylactide acid (PLA) plastics
Polylactide acid (PLA) is a transparent plastic produced from cane sugar or corn starch. It not only resembles conventional petrochemical mass plastics (like PE or PP) in its characteristics, but it can also be processed easily on standard equipment that already exists for the production of conventional plastics. PLA and PLA-Blends (such as the CompostablesTM by Cereplast,Inc. ) generally come in the form of granulates with various properties and are used in the plastic processing industry for the production of foil, moulds, tins, cups, bottles and other packaging.
Poly-3-hydroxybutyrate (PHB)
The biopolymer poly-3-hydroxybutyrate (PHB) is a polyester produced produced by certain bacteria processing glucose or starch. Its characteristics are similar to those of the petrochemical-produced plastic polypropylene. The South American sugar industry, for example, has decided to expand PHB production to an industrial scale. PHB is distinguished primarily by its physical characteristics. It produces transparent film at a melting point higher than 130 degrees Celsius, and is biodegradable without residue.
Polyamide 11 (PA 11)
PA 11 is a biopolymer derived from natural oil. It is also known under the tradename Rilsan B commercialized by Arkema. PA 11 belongs to the technical polymers family and is not biodegradable. Its properties are similar than PA 12 although emissions of greenhouse gases and consumption of non-renewable resources are reduced during its production. Its thermal resistance is also superior than PA 12. It is used in high performance applications as automotive fuel lines, pneumatic airbrake tubing, electrical anti-termite cable sheathing, oil & gas flexible pipes & control fluid umbilicals, sports shoes, electronic device components, catheters, etc.
Bio-derived polyethylene
The basic building block (monomer) of polyethylene is ethylene. This is just one small chemical step from ethanol, which can be produced by fermentation of agricultural feedstocks such as sugar cane or corn. Bio-derived polyethylene is chemically and physically identical to traditional polyethylene - it does not biodegrade but can be recycled. It can also considerably reduce greenhouse gas emissions. Brazilian chemicals group Braskem claims that using its route from sugar cane ethanol to produce one tonne of polyethylene captures (removes from the environment) 2.5 tonnes of carbon dioxide while the traditional petrochemical route results in emissions of close to 3.5 tonnes.
Braskem plans to introduce commercial quantities of its first bio-derived high density polyethylene, used in a packaging such as bottles and tubs, in 2010 and has developed a technology to produce bio-derived butene, required to make the linear low density polethylene types used in film production.
Developments
• In the early 1950s, Amylomaize (>50% starch content corn) was successfully bred and commercial bioplastics applications started to be explored.
• In 2004, NEC developed a flame retardant plastic, polylactic acid, without using toxic chemicals such as halogens and phosphorus compounds
• In 2005, Fujitsu became one of the first technology companies to make personal computer cases from bioplastics, which are featured in their FMV-BIBLO NB80K line.
• In 2007 Braskem of Brazil announced it had developed a route to manufacture high density polyethylene (HDPE) using ethylene derived from sugar cane.
Monday, May 11, 2009
Optical Fibre
An optical fiber (or fibre) 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 data rates (a.k.a "bandwidth"), than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are immune to electromagnetic interference. Fibers are also used for illumination, and in bundles can be used to carry images, allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including as sensors and fiber lasers.
Light is kept in the "core" of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multimode fibers (MMF). Fibers which support only a single mode are called singlemode fibers (SMF). Multimode fibers generally have a large-diameter core, and are used for short-distance communication links or for applications where high power must be transmitted. Singlemode fibers are used for most communication links longer than 200 meters.
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections.
Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the 1840s, with Irish inventor John Tyndall offering public displays using water-fountains ten years later.[1] Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber, based on Tyndall's earlier studies; modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade.[1] Development then focused on fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed.
In 1965, Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables (STC) were the first to promote the idea that the attenuation in optical fibers could be reduced below 20 dB per kilometer, allowing fibers to be a practical medium for communication.[2] They proposed that the attenuation in fibers available at the time was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. The crucial attenuation level of 20 dB was first achieved in 1970, by researchers Robert D. Maurer, Donald Keck, Peter C. Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now Corning Incorporated They demonstrated a fiber with 17 dB optic attenuation per kilometer by doping silica glass with titanium. A few years later they produced a fiber with only 4 dB/km using germanium dioxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet. In 1981, General Electric produced fused quartz ingots that could be drawn into fiber optic strands 25 miles long.
Attenuations in modern optical cables are far less than those in electrical copper cables, leading to long-haul fiber connections with repeater distances of 50–80 km. The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by reducing or even in many cases eliminating the need for optical-electrical-optical repeaters, was co-developed by teams led by David N. Payne of the University of Southampton, and Emmanuel Desurvire at Bell Laboratories in 1986. The more robust optical fiber commonly used today utilizes glass for both core and sheath and is therefore less prone to aging processes. It was invented by Gerhard Bernsee in 1973 of Schott Glass in Germany.
In 1991, the emerging field of photonic crystals led to the development of photonic crystal fiber [5] 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. Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.
Main article: Fiber-optic communication
Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the per channel light signals propagating in the fiber can be modulated at rates as high as 111 Gb/s. (In 2001 the limit was at 40 Gb/s. In today's DWDM systems the net data rate (data rate without overhead bytes) per fiber is the per channel data rate reduced by the FEC overhead multiplied by the number of channels (usually up to 80 channels in commercially available systems as of 2008). (Some communication companies are revealing that net data rates as fast as 1Tb/s are currently being developed.[citation needed]), and each fiber can carry many independent channels, each by a different wavelength of light (wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Also, wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.
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 (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
Main article: Fiber optic sensor
Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiber optic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer.
Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.
Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature inside aircraft jet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.
Fibers are widely used in illumination applications. They are used as light guides in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see non-imaging optics). Optical fiber illumination is also used for decorative applications, including signs, art, and artificial Christmas trees. Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source. Optical fiber is an intrinsic part of the light-transmitting concrete building product, LiTraCon.
Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an endoscope, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
An optical fiber doped with certain rare-earth elements such as erbium can be used as the gain medium of a laser or optical amplifier. Rare-earth doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular (undoped) optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is stimulated emission.
Optical fibers doped with a wavelength shifter are used to collect scintillation light in physics experiments.
Optical fiber can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.
An optical fiber is a cylindrical dielectric 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. 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.
Main article: Refractive index
The index of refraction is a way of measuring the speed of light in a material. Light travels fastest in a vacuum, such as outer space. The actual speed of light in a vacuum is 299,792 kilometers per second, or 186,282 miles per second.[9] Index of refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium. The index of refraction of a vacuum is therefore 1, by definition. The typical value for the cladding of an optical fiber is 1.46. The core value is typically 1.48. The larger the index of refraction, the more slowly light travels in that medium.
Main article: Total internal reflection
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 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 less than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.
In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA.
Multimode fiber
A laser bouncing down an acrylic rod, illustrating the total internal reflection of light in a multimode optical fiber.
Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multimode fiber, from the electromagnetic analysis (see below). 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 high 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 the cladding, and 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 the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.
Optical fiber types.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.
Single mode fiber
Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multimode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.
The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.
The most common type of single-mode fiber has a core diameter of 8 to 10 μm and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometres and as large as hundreds of micrometres.
Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.
Photonic crystal fiber is made with a regular pattern of index variation (often in the form of cylindrical holes that run along the length of the fiber). Such fiber uses diffraction effects instead of or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications.
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 multimode fibers with a core diameter of 0.5 mm or larger. POF typically have higher attenuation co-efficients than glass fibers, 1 dB/m or higher, and this high attenuation limits the range of POF-based systems.
Standard optical fibers are made by first constructing a large-diameter preform, with a carefully controlled refractive index profile, and then pulling the preform to form the long, thin optical fiber. The preform is commonly made by three chemical vapor deposition methods: inside vapor deposition, outside vapor deposition, and vapor axial deposition.[10]
With inside vapor deposition, a hollow glass tube approximately 40 cm in length known as a "preform" is placed horizontally and rotated slowly on a lathe, and gases such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4) are injected with oxygen in the end of the tube. The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1900 kelvins, where the tetrachlorides react with oxygen to produce silica or germania (germanium oxide) particles. When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called modified chemical vapor deposition.
The oxide particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outwards (this is known as thermophoresis). The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited. For each layer the composition can be modified by varying the gas composition, resulting in precise control of the finished fiber's optical properties.
In outside vapor deposition or vapor axial deposition, the glass is formed by flame hydrolysis, a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water (H2O) in an oxyhydrogen flame. In outside vapor deposition the glass is deposited onto a solid rod, which is removed before further processing. In vapor axial deposition, a short seed rod is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. The porous preform is consolidated into a transparent, solid preform by heating to about 1800 Kelvin.
The preform, however constructed, is then placed in a device known as a drawing tower, where the preform tip is heated and the optic fiber is pulled out as a string. By measuring the resultant fiber width, the tension on the fiber can be controlled to maintain the fiber thickness.
Practical issues
Main article: Optical fiber cable
In practical fibers, the cladding is usually coated with a tough resin buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-absorbing ("dark") glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle imaging applications.
Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, dual use as power lines, installation in conduit, lashing to aerial telephone poles, submarine installation, or insertion in paved streets. In recent years the cost of small fiber-count pole-mounted cables has greatly decreased due to the high Japanese and South Korean demand for fiber to the home (FTTH) installations.
Fiber cable can be very flexible, but traditional fiber's loss increases greatly if the fiber is bent with a radius smaller than around 30 mm. This creates a problem when the cable is bent around corners or wound around a spool, making FTTX installations more complicated. "Bendable fibers", targeted towards easier installation in home environments, have been standardized as ITU-T G.657. This type of fiber can be bent with a radius as low as 7.5 mm without adverse impact. Even more bendable fibers have been developed.[14] Bendable fiber may also be resistant to fiber hacking, in which the signal in a fiber is surreptitiously monitored by bending the fiber and detecting the leakage.
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 , which melts the fiber ends together with an electric arc. For quicker fastening jobs, a "mechanical splice" is used.
Fusion splicing is done with a specialized instrument that typically operates as follows: The two cable ends are fastened inside a splice enclosure that will protect the splices, and the fiber ends are stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are cleaved (cut) with a precision cleaver to make them perpendicular, and are placed into special holders in the splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical. The complexity of this process makes fiber splicing much more difficult than splicing copper wire.
Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear index-matching gel that enhances the transmission of light across the joint. Such joints typically have higher optical loss and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.
Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely. A fiber-optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be "push and click", "turn and latch" ("bayonet"), or screw-in (threaded). A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used so the fiber is held securely, and a strain relief is secured to the rear. Once the adhesive has set, the fiber's end is polished to a mirror finish. Various polish profiles are used, depending on the type of fiber and the application. For singlemode fiber, the fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores. This is known as a "physical contact" (PC) polish. The curved surface may be polished at an angle, to make an "angled physical contact" (APC) connection. Such connections have higher loss than PC connections, but greatly reduced back reflection, because light that reflects from the angled surface leaks out of the fiber core; the resulting loss in signal strength is known as gap loss. APC fiber ends have low back reflection even when disconnected.
It often becomes necessary to align an optical fiber with another optical fiber or an optical device such as a light-emitting diode, a laser diode, or an optoelectronic device such as a modulator. This can involve either carefully aligning the fiber and placing it in contact with the device to which it is to couple, 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, the fiber end is usually aligned to the device or other fiber with a fiber launch system that 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.
At high optical intensities, above 2 megawatts per square centimetre, 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 1–3 meters per second. 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
Subscribe to:
Posts (Atom)