1.
Electric discharge
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Electric discharge describes any flow of electric charge through a gas, liquid or solid. Tiny pulses of current are used to detect ionizing radiation in a Geiger–Müller tube, a low steady current can be used to illustrate the spectrum of gases in a Gas-filled tube. A neon lamp is an example of a lamp, useful both for illumination and as a voltage regulator. A flashtube generates a pulse of intense light useful for photography by sending a heavy current through a gas arc discharge. Corona discharges are used in photocopiers, Electric discharges can convey substantial energy to the electrodes at the ends of the discharge. A spark gap is used in combustion engines to ignite the fuel/air mixture on every power stroke. Spark gaps are used to switch heavy currents in a Marx generator. In electric discharge machining, multiple tiny electric arcs are used to erode a conductive workpiece to a finished shape, arc welding is used to assemble heavy steel structures, where the base metal is heated to melting by the heat of the arc. An electric arc furnace sustains arc currents of tens of thousands of amperes and is used for steelmaking and production of alloys, st. Elmos fire Lightning Electric organ Electrical breakdown Electric discharge in gases E/N ratio Lichtenberg figure Space charge Debye sheath
2.
Infrared
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It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
3.
Population inversion
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It is called an inversion because in many familiar and commonly encountered physical systems, this is not possible. The concept is of importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser. To understand the concept of an inversion, it is necessary to understand some thermodynamics. To do so, it is useful to consider a simple assembly of atoms forming a laser medium. Assume there are a group of N atoms, each of which is capable of being in one of two states, either The ground state, with energy E1, or The excited state, with energy E2. The number of atoms which are in the ground state is given by N1. We may calculate the ratio of the populations of the two states at room temperature for an energy difference ΔE that corresponds to light of a frequency corresponding to visible light. In this case ΔE = E2 - E1 ≈2.07 eV, when in thermal equilibrium, then, it is seen that the lower energy state is more populated than the higher energy state, and this is the normal state of the system. In other words, an inversion can never exist for a system at thermal equilibrium. To achieve population inversion therefore requires pushing the system into a non-equilibrated state, the rate of absorption is proportional to the radiation intensity of the light, and also to the number of atoms currently in the ground state, N1. If atoms are in the state, spontaneous decay events to the ground state will occur at a rate proportional to N2. The energy difference between the two states ΔE21 is emitted from the atom as a photon of frequency ν21 as given by the relation above. The photons are emitted stochastically, and there is no fixed relationship between photons emitted from a group of excited atoms, in other words, spontaneous emission is incoherent. If an atom is already in the state, it may be perturbed by the passage of a photon that has a frequency ν21 corresponding to the energy gap ΔE of the excited state to ground state transition. In this case, the excited atom relaxes to the ground state, the original photon is not absorbed by the atom, and so the result is two photons of the same frequency. This process is known as stimulated emission, specifically, an excited atom will act like a small electric dipole which will oscillate with the external field provided. One of the consequences of this oscillation is that it encourages electrons to decay to the lowest energy state. When this happens due to the presence of the field from a photon, a photon is released in the same phase and direction as the stimulating photon
4.
Visible spectrum
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The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm, in terms of frequency, this corresponds to a band in the vicinity of 430–770 THz. The spectrum does not, however, contain all the colors that the human eyes, unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass through the window, the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earths atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, the optical window is also referred to as the visible window because it overlaps the human visible response spectrum. The near infrared window lies just out of the vision, as well as the Medium Wavelength IR window. In the 13th century, Roger Bacon theorized that rainbows were produced by a process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light and he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. The result is red light is bent less sharply than violet as it passes through the prism. Newton divided the spectrum into seven named colors, red, orange, yellow, green, blue, indigo, the human eye is relatively insensitive to indigos frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right. However, the evidence indicates that what Newton meant by indigo, comparing Newtons observation of prismatic colors to a color image of the visible light spectrum shows that indigo corresponds to what is today called blue, whereas blue corresponds to cyan. In the 18th century, Goethe wrote about optical spectra in his Theory of Colours, Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow, the spectrum appears only when these edges are close enough to overlap. Young was the first to measure the wavelengths of different colors of light, the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century
5.
Laser construction
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A laser is constructed from three principal parts, An energy source, A gain medium or laser medium, and Two or more mirrors that form an optical resonator. The pump source is the part that provides energy to the laser system, examples of pump sources include electrical discharges, flashlamps, arc lamps, light from another laser, chemical reactions and even explosive devices. The type of source used principally depends on the gain medium. The gain medium is the determining factor of the wavelength of operation. Gain media in different materials have linear spectra or wide spectra, gain media with wide spectra allow tuning of the laser frequency. There are hundreds if not thousands of different gain media in which laser operation has been achieved, examples of different gain media include, Liquids, such as dye lasers. These are usually organic chemical solvents, such as methanol, ethanol or ethylene glycol, to which are added chemical dyes such as coumarin, rhodamine, the exact chemical configuration of the dye molecules determines the operation wavelength of the dye laser. Gases, such as carbon dioxide, argon, krypton and mixtures such as helium–neon and these lasers are often pumped by electrical discharge. Solids, such as crystals and glasses, the solid host materials are usually doped with an impurity such as chromium, neodymium, erbium or titanium ions. Typical hosts include YAG, YLF, sapphire and various glasses, examples of solid-state laser media include Nd, YAG, Ti, sapphire, Cr, sapphire, Cr, LiSAF, Er, YLF, Nd, glass, and Er, glass. Solid-state lasers are pumped by flashlamps or light from another laser. Semiconductors, a type of solid, crystal with uniform dopant distribution or material with differing dopant levels in which the movement of electrons can cause laser action. Semiconductor lasers are typically small, and can be pumped with a simple electric current. The optical resonator, or optical cavity, in its simplest form is two parallel mirrors placed around the medium which provide feedback of the light. The mirrors are given optical coatings which determine their reflective properties, typically one will be a high reflector, and the other will be a partial reflector. The latter is called the coupler, because it allows some of the light to leave the cavity to produce the lasers output beam. Light from the medium, produced by spontaneous emission, is reflected by the back into the medium. The light may reflect from the mirrors and thus pass through the medium many hundreds of times before exiting the cavity
6.
Helium
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Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and its boiling point is the lowest among all the elements. Its abundance is similar to figure in the Sun and in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have formed during the Big Bang. Large amounts of new helium are being created by fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios and it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer, Janssen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in gas fields in parts of the United States. Liquid helium is used in cryogenics, particularly in the cooling of superconducting magnets, a well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre, on Earth it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the radioactive decay of heavy radioactive elements. Previously, terrestrial helium—a non-renewable resource, because once released into the atmosphere it readily escapes into space—was thought to be in short supply. The first evidence of helium was observed on August 18,1868, the line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was assumed to be sodium. He concluded that it was caused by an element in the Sun unknown on Earth, Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος
7.
Neon
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Neon is a chemical element with symbol Ne and atomic number 10 and is a noble gas. Neon is a colorless, odorless, inert gas under standard conditions. It was discovered in 1898 as one of the three residual rare inert elements remaining in dry air, after nitrogen, oxygen, argon and carbon dioxide were removed. Neon was the second of three rare gases to be discovered, and was immediately recognized as a new element from its bright red emission spectrum. The name neon is derived from the Greek word, νέον, neuter singular form of νέος, Neon is chemically inert and forms no uncharged chemical compounds. The compounds of neon include ionic molecules, molecules held together by van der Waals forces and clathrates, during cosmic nucleogenesis of the elements, large amounts of neon are built up from the alpha-capture fusion process in stars. Although neon is a common element in the universe and solar system. It composes about 18.2 ppm of air by volume, the reason for neons relative scarcity on Earth and the inner planets is that neon is highly volatile and forms no compounds to fix it to solids. As a result, it escaped from the planetesimals under the warmth of the newly ignited Sun in the early Solar System, even the atmosphere of Jupiter is somewhat depleted of neon, presumably for this reason. It is also lighter than air, causing it to escape even from Earths atmosphere, Neon gives a distinct reddish-orange glow when used in low-voltage neon glow lamps, high-voltage discharge tubes and neon advertising signs. The red emission line from neon also causes the well known red light of helium–neon lasers, Neon is used in some plasma tube and refrigerant applications but has few other commercial uses. It is commercially extracted by the distillation of liquid air. Since air is the source, it is considerably more expensive than helium. Neon, was discovered in 1898 by the British chemists Sir William Ramsay, Neon was discovered when Ramsay chilled a sample of air until it became a liquid, then warmed the liquid and captured the gases as they boiled off. The gases nitrogen, oxygen, and argon had been identified, first to be identified was krypton. The next, after krypton had been removed, was a gas which gave a brilliant red light under spectroscopic discharge and this gas, identified in June, was named neon, the Greek analogue of novum, suggested by Ramsays son. A second gas was reported along with neon, having approximately the same density as argon but with a different spectrum – Ramsay. However, subsequent spectroscopic analysis revealed it to be contaminated with carbon monoxide
8.
Anode
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An anode is an electrode through which conventional current flows into a polarized electrical device. A common mnemonic is ACID for anode current into device, the direction of electric current is opposite to the direction of electron flow, electrons flow out the anode to the outside circuit. The terms anode and cathode do not relate to the polarity of those electrodes but the direction of the current. Conventional current quantifies the flow of positive charge, in most cases, positive charge leaves the device via the cathode, and positive charge flows into the device via the anode. Conventional current depends not only on the direction the charge carriers move, the currents outside the device are usually carried by electrons in a metal conductor. The flow of electrons is opposite to conventional current because electrons have a negative charge, consequently, electrons leave the device via the anode, and electrons enter the device through the cathode. The anode and cathode have slightly different definitions for electrical devices such as diodes and vacuum tubes where the naming is fixed. These devices usually allow substantial current flow in one direction but negligible current in the other direction, consequently, the electrode names use the terms that have substantial ordinary currents. An ideal diode allows current in one direction but not in the other, for the ideal diode, it makes sense to always call that terminal the anode. For non-ideal diodes, the electrodes are also given fixed names even though such a diode under reverse bias would have a positive charge out of the anode. For some operating conditions and devices, such as diode breakdown, Zener diodes, or photodiodes, the polarity of voltage on an anode with respect to an associated cathode varies depending on the device type and on its operating mode. This inward current is carried externally by electrons moving outwards, negative charge flowing in one direction being electrically equivalent to positive charge flowing in the opposite direction. In a recharging battery, or a cell, the anode is the positive terminal. In a diode, the anode is the terminal at the tail of the arrow symbol. Note electrode naming for diodes is always based on the direction of the forward current, in a cathode ray tube, the anode is the positive terminal where electrons flow out of the device, i. e. where positive electric current flows in. The use of East to mean the in direction may appear contrived, previously, as related in the first reference cited above, Faraday had used the more straightforward term eisode. The reference he used to effect was the Earths magnetic field direction. In electrochemistry, the anode is where oxidation occurs and is the positive polarity contact in an electrolytic cell, at the anode, anions are forced by the electrical potential to react chemically and give off electrons which then flow up and into the driving circuit
9.
Cathode
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A cathode is the electrode from which a conventional current leaves a polarized electrical device. A conventional current describes the direction in which positive electronic charges move, Electrons have a negative charge, so the movement of electrons is opposite to the conventional current flow. Consequently, the mnemonic cathode current departs also means that flow into the devices cathode. Cathode polarity with respect to the anode can be positive or negative and this outward current is carried internally by positive ions moving from the electrolyte to the positive cathode. It is continued externally by electrons moving inwards, this negative charge moving inwards constituting positive current flowing outwards, for example, the Daniell galvanic cells copper electrode is the positive terminal and the cathode. In a recharging battery, or an electrolytic cell performing electrolysis, for example, reversing the current direction in a Daniell galvanic cell would produce an electrolytic cell, where the copper electrode is the positive terminal and the anode. In a diode, the cathode is the terminal at the pointed end of the arrow symbol. Note, electrode naming for diodes is always based on the direction of the forward current, an electrode through which current flows the other way is termed an anode. The use of West to mean the out direction may appear unnecessarily contrived, previously, as related in the first reference cited above, Faraday had used the more straightforward term exode. The reference he used to effect was the Earths magnetic field direction. The flow of electrons is almost always from anode to cathode outside of the cell or device, regardless of the cell or device type, an exception is when a diode reverse-conducts, either by accident or by design. In chemistry, a cathode is the electrode of a cell at which reduction occurs. Another mnemonic is to note the cathode has a c, as does reduction, perhaps most useful would be to remember cathode corresponds to cation and anode corresponds to anion. The cathode can be negative like when the cell is electrolytic, the cathode supplies electrons to the positively charged cations which flow to it from the electrolyte. The cathodic current, in electrochemistry, is the flow of electrons from the interface to a species in solution. The anodic current is the flow of electrons into the anode from a species in solution, in an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions, when discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more cathodic with respect to the more easily reduced reagent. When metal ions are reduced from ionic solution, they form a metal surface on the cathode
10.
Continuous wave
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A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency, a sine wave. In mathematical analysis, it is considered to be of infinite duration, continuous wave is also the name given to an early method of radio transmission, in which a sinusoidal carrier wave is switched on and off. Information is carried in the duration of the on and off periods of the signal. Very early radio transmitters used a gap to produce radio-frequency oscillations in the transmitting antenna. The signals produced by these spark-gap transmitters consisted of strings of brief pulses of radio frequency oscillations which died out rapidly to zero. The disadvantage of damped waves was that their energy was spread over a wide band of frequencies. As a result they produced electromagnetic interference that spread over the transmissions of stations at other frequencies and this motivated efforts to produce radio frequency oscillations that decayed more slowly, had less damping. Manufacturers produced spark transmitters which generated long ringing waves with minimal damping and it was realized that the ideal radio wave for radiotelegraphic communication would be a sine wave with zero damping, a continuous wave. An unbroken continuous sine wave theoretically has no bandwidth, all its energy is concentrated at a single frequency, continuous waves could not be produced with an electric spark, but were achieved with the vacuum tube electronic oscillator, invented around 1913 by Edwin Armstrong and Alexander Meissner. Damped wave spark transmitters were replaced by continuous wave vacuum tube transmitters around 1920, what is transmitted in the extra bandwidth used by a transmitter that turns on/off more abruptly is known as key clicks. Certain types of power used in transmission may increase the effect of key clicks. The first transmitters capable of producing continuous wave, the Alexanderson alternator and vacuum tube oscillators, early radio transmitters could not be modulated to transmit speech, and so CW radio telegraphy was the only form of communication available. Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a switch to transmit Morse code. However, instead of controlling the electricity in a cross-country wire and this mode is still in common use by amateur radio operators. In military communications and amateur radio, the terms CW and Morse code are used interchangeably. Morse code may be sent using direct current in wires, sound, or light, a carrier wave is keyed on and off to represent the dots and dashes of the code elements. The carriers amplitude and frequency remains constant during each code element, at the receiver, the received signal is mixed with a heterodyne signal from a BFO to change the radio frequency impulses to sound. Though most commercial traffic has now ceased operation using Morse it is popular with amateur radio operators
11.
Optical cavity
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An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a component of lasers, surrounding the gain medium. They are also used in optical parametric oscillators and some interferometers, light confined in the cavity reflects multiple times producing standing waves for certain resonance frequencies. Different resonator types are distinguished by the lengths of the two mirrors and the distance between them. The geometry must be chosen so that the beam remains stable, resonator types are also designed to meet other criteria such as minimum beam waist or having no focal point inside the cavity. Optical cavities are designed to have a large Q factor, a beam will reflect a large number of times with little attenuation. Therefore, the line width of the beam is very small indeed compared to the frequency of the laser. In general, radiation patterns which are reproduced on every round-trip of the light through the resonator are the most stable, the basic, or fundamental transverse mode of a resonator is a Gaussian beam. The most common types of optical cavities consist of two facing plane or spherical mirrors, the simplest of these is the plane-parallel or Fabry–Pérot cavity, consisting of two opposing flat mirrors. However, this problem is reduced for very short cavities with a small mirror separation distance. Plane-parallel resonators are therefore used in microchip and microcavity lasers. In these cases, rather than using separate mirrors, an optical coating may be directly applied to the laser medium itself. The plane-parallel resonator is also the basis of the Fabry–Pérot interferometer, for a resonator with two mirrors with radii of curvature R1 and R2, there are a number of common cavity configurations. If the two curvatures are equal to half the cavity length, a concentric or spherical resonator results and this type of cavity produces a diffraction-limited beam waist in the centre of the cavity, with large beam diameters at the mirrors, filling the whole mirror aperture. Similar to this is the cavity, with one plane mirror. A common and important design is the confocal resonator, with equal curvature mirrors equal to the cavity length. This design produces the smallest possible beam diameter at the cavity mirrors for a given cavity length, a concave-convex cavity has one convex mirror with a negative radius of curvature. A transparent dielectric sphere, such as a liquid droplet, also forms an optical cavity
12.
Output coupler
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An output coupler is the component of a optical resonator that allows the extraction of a portion of the light from the lasers intracavity beam. An output coupler most often consists of a reflective mirror. In its most common form, an output coupler consists of a reflective mirror. The reflectance and transmittance of the mirror is usually determined by the gain of the laser medium, in some lasers the gain is very low, so the beam must make hundreds of passes through the medium for sufficient gain. In this case the output coupler may be as high as 99% reflective, in others, such as an excimer laser, the 4% reflectivity of uncoated glass provides enough of a mirror, transmitting nearly 96% of the intracavity beam. Lasers operate by reflecting light between two or more mirrors that have a laser medium between them. The medium amplifies the light by stimulated emission, in other words, the laser must attain threshold. There are two important properties of the coupler, Radii of curvature The output coupler may be either flat or curved. The face of the output coupler facing into the cavity is the side with the partially reflective coating. This is the side which partially determines the laser modal properties, if this inner surface is curved then so must be the outer surface. This will stop the OC performing as a lens, the curvature of the outer surface can be designed to give a collimated laser output. This outer surface generally has a coating applied to maximise the output power. Reflectivity Depending on the gain of the medium, the amount of light the OC needs to reflect back can vary widely, helium–neon lasers require around a 99% reflective mirror to lase, while nitrogen lasers have an extremely high gain and do not require any OC. The reflectivity of any OC will vary with wavelength, metal-coated mirrors generally have good reflectivity over a wide bandwidth, but may not cover an entire portion of the spectrum. Silver has up to 99. 9% reflectivity in the visual range, a dielectric mirror may have a tuning range as low as a 10nm when designed for a specific wavelength, or can be designed with a wide range, spanning as much as 100nm, for tunable lasers. For this reason the properties of the OC are important to consider when a laser cavity is being assembled. The material used as the substrate is also an important consideration. Most glasses have good transmissivity from the near UV to the near IR, for example, zinc selenide is typically used in carbon-dioxide lasers because of its high transmittance to infrared wavelengths
13.
Power (physics)
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In physics, power is the rate of doing work. It is the amount of energy consumed per unit time, having no direction, it is a scalar quantity. In the SI system, the unit of power is the joule per second, known as the watt in honour of James Watt, another common and traditional measure is horsepower. Being the rate of work, the equation for power can be written, because this integral depends on the trajectory of the point of application of the force and torque, this calculation of work is said to be path dependent. As a physical concept, power requires both a change in the universe and a specified time in which the change occurs. This is distinct from the concept of work, which is measured in terms of a net change in the state of the physical universe. The output power of a motor is the product of the torque that the motor generates. The power involved in moving a vehicle is the product of the force of the wheels. The dimension of power is divided by time. The SI unit of power is the watt, which is equal to one joule per second, other units of power include ergs per second, horsepower, metric horsepower, and foot-pounds per minute. One horsepower is equivalent to 33,000 foot-pounds per minute, or the required to lift 550 pounds by one foot in one second. Other units include dBm, a logarithmic measure with 1 milliwatt as reference, food calories per hour, Btu per hour. This shows how power is an amount of energy consumed per unit time. If ΔW is the amount of work performed during a period of time of duration Δt and it is the average amount of work done or energy converted per unit of time. The average power is simply called power when the context makes it clear. The instantaneous power is then the value of the average power as the time interval Δt approaches zero. P = lim Δ t →0 P a v g = lim Δ t →0 Δ W Δ t = d W d t. In the case of constant power P, the amount of work performed during a period of duration T is given by, W = P t
14.
Optical amplifier
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An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, optical amplifiers are important in optical communication and laser physics. There are several different physical mechanisms that can be used to amplify a light signal, in doped fibre amplifiers and bulk lasers, stimulated emission in the amplifiers gain medium causes amplification of incoming light. In semiconductor optical amplifiers, electron-hole recombination occurs, in Raman amplifiers, Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Almost any laser active gain medium can be pumped to produce gain for light at the wavelength of a made with the same material as its gain medium. Such amplifiers are used to produce high power laser systems. Special types such as amplifiers and chirped-pulse amplifiers are used to amplify ultrashort pulses. Doped fibre amplifiers are amplifiers that use a doped optical fibre as a gain medium to amplify an optical signal. They are related to fibre lasers, the signal to be amplified and a pump laser are multiplexed into the doped fibre, and the signal is amplified through interaction with the doping ions. An erbium-doped waveguide amplifier is an amplifier that uses a waveguide to boost an optical signal. Amplification is achieved by stimulated emission of photons from dopant ions in the doped fibre, the pump laser excites ions into a higher energy from where they can decay via stimulated emission of a photon at the signal wavelength back to a lower energy level. The excited ions can also decay spontaneously or even through nonradiative processes involving interactions with phonons of the glass matrix and these last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification. The amplification window of an amplifier is the range of optical wavelengths for which the amplifier yields a usable gain. The amplification window is determined by the properties of the dopant ions, the glass structure of the optical fibre. This broadening is both homogeneous and inhomogeneous, homogeneous broadening arises from the interactions with phonons of the glass, while inhomogeneous broadening is caused by differences in the glass sites where different ions are hosted. Different sites expose ions to different local fields, which shifts the energy levels via the Stark effect. In addition, the Stark effect also removes the degeneracy of states having the same total angular momentum. Thus, for example, the trivalent erbium ion has a state with J = 15/2
15.
Electronvolt
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In physics, the electronvolt is a unit of energy equal to approximately 1. 6×10−19 joules. By definition, it is the amount of energy gained by the charge of an electron moving across an electric potential difference of one volt. Thus it is 1 volt multiplied by the elementary charge, therefore, one electronvolt is equal to 6981160217662079999♠1. 6021766208×10−19 J. The electronvolt is not a SI unit, and its definition is empirical, like the elementary charge on which it is based, it is not an independent quantity but is equal to 1 J/C √2hα / μ0c0. It is a unit of energy within physics, widely used in solid state, atomic, nuclear. It is commonly used with the metric prefixes milli-, kilo-, in some older documents, and in the name Bevatron, the symbol BeV is used, which stands for billion electronvolts, it is equivalent to the GeV. By mass–energy equivalence, the electronvolt is also a unit of mass and it is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/c2, where c is the speed of light in vacuum. It is common to express mass in terms of eV as a unit of mass. The mass equivalent of 1 eV/c2 is 1 eV / c 2 = ⋅1 V2 =1.783 ×10 −36 kg. For example, an electron and a positron, each with a mass of 0.511 MeV/c2, the proton has a mass of 0.938 GeV/c2. In general, the masses of all hadrons are of the order of 1 GeV/c2, the unified atomic mass unit,1 gram divided by Avogadros number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to megaelectronvolts, use the formula,1 u =931.4941 MeV/c2 =0.9314941 GeV/c2, in high-energy physics, the electronvolt is often used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy and this gives rise to usage of eV as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of units are LMT−1. The dimensions of units are L2MT−2. Then, dividing the units of energy by a constant that has units of velocity. In the field of particle physics, the fundamental velocity unit is the speed of light in vacuum c. Thus, dividing energy in eV by the speed of light, the fundamental velocity constant c is often dropped from the units of momentum by way of defining units of length such that the value of c is unity
16.
Laser
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A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term laser originated as an acronym for light amplification by stimulated emission of radiation, the first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow. A laser differs from other sources of light in that it emits light coherently, spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances, Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum, i. e. they can emit a single color of light. Temporal coherence can be used to produce pulses of light as short as a femtosecond, Lasers are distinguished from other light sources by their coherence. Spatial coherence is typically expressed through the output being a narrow beam, Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can have very low divergence in order to concentrate their power at a great distance. Temporal coherence implies a polarized wave at a single frequency whose phase is correlated over a great distance along the beam. A beam produced by a thermal or other incoherent light source has an amplitude and phase that vary randomly with respect to time and position. Lasers are characterized according to their wavelength in a vacuum, most single wavelength lasers actually produce radiation in several modes having slightly differing frequencies, often not in a single polarization. Although temporal coherence implies monochromaticity, there are lasers that emit a broad spectrum of light or emit different wavelengths of light simultaneously, there are some lasers that are not single spatial mode and consequently have light beams that diverge more than is required by the diffraction limit. However, all devices are classified as lasers based on their method of producing light. Lasers are employed in applications where light of the spatial or temporal coherence could not be produced using simpler technologies. The word laser started as an acronym for light amplification by stimulated emission of radiation, in the early technical literature, especially at Bell Telephone Laboratories, the laser was called an optical maser, this term is now obsolete. A laser that produces light by itself is technically an optical rather than an optical amplifier as suggested by the acronym. It has been noted that the acronym LOSER, for light oscillation by stimulated emission of radiation. With the widespread use of the acronym as a common noun, optical amplifiers have come to be referred to as laser amplifiers. The back-formed verb to lase is frequently used in the field, meaning to produce light, especially in reference to the gain medium of a laser. Further use of the laser and maser in an extended sense, not referring to laser technology or devices, can be seen in usages such as astrophysical maser
17.
Transverse mode
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A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam. Transverse modes occur in waves and microwaves confined to a waveguide. Transverse modes occur because of conditions imposed on the wave by the waveguide. For this reason, the supported by a waveguide are quantized. The allowed modes can be found by solving Maxwells equations for the conditions of a given waveguide. Unguided electromagnetic waves in space, or in a bulk isotropic dielectric, can be described as a superposition of plane waves. However in any sort of waveguide where boundary conditions are imposed by a physical structure and these modes generally follow different propagation constants. When two or more modes have a propagation constant along the waveguide, then there is more than one modal decomposition possible in order to describe a wave with that propagation constant. Modes in waveguides can be classified as follows, Transverse electromagnetic modes. Transverse electric modes, no field in the direction of propagation. These are sometimes called H modes because there is only a field along the direction of propagation. Transverse magnetic modes, no field in the direction of propagation. These are sometimes called E modes because there is only a field along the direction of propagation. Hybrid modes, non-zero electric and magnetic fields in the direction of propagation, hollow metallic waveguides filled with a homogeneous, isotropic material support TE and TM modes but not the TEM mode. In coaxial cable energy is transported in the fundamental TEM mode. The TEM mode is usually assumed for most other electrical conductor line formats as well. In an optical fiber or other dielectric waveguide, modes are generally of the hybrid type, in circular waveguides, circular modes exist and here m is the number of full-wave patterns along the circumference and n is the number of half-wave patterns along the diameter. In a laser with cylindrical symmetry, the transverse mode patterns are described by a combination of a Gaussian beam profile with a Laguerre polynomial, the modes are denoted TEMpl where p and l are integers labeling the radial and angular mode orders, respectively
18.
Longitudinal mode
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A longitudinal mode of a resonant cavity is a particular standing wave pattern formed by waves confined in the cavity. The longitudinal modes correspond to the wavelengths of the wave which are reinforced by constructive interference after many reflections from the reflecting surfaces. All other wavelengths are suppressed by destructive interference, a longitudinal mode pattern has its nodes located axially along the length of the cavity. Transverse modes, with nodes located perpendicular to the axis of the cavity, a common example of longitudinal modes are the light wavelengths produced by a laser. In the simplest case, the optical cavity is formed by two opposed plane mirrors surrounding the gain medium. The allowed modes of the cavity are those where the separation distance L is equal to an exact multiple of half the wavelength. In practice, the distance of the mirrors L is usually much greater than the wavelength of light λ. If the cavity is non-empty, the values of L used are the path lengths for each element. The frequency spacing of longitudinal modes in the cavity is then given by, more generally, the longitudinal modes may be found for any type of wave in a cavity by solving the relevant wave equation with the appropriate boundary conditions. Both transverse and longitudinal waves may have longitudinal modes when confined to a cavity, the analysis of longitudinal modes is especially important in lasers with single transversal mode, for example, in single-mode fiber lasers. The number of modes of such a laser can be estimated as ratio of the spectral width of gain to the spectral separation of longitudinal modes. For lasers with single mode, the power per one longitudinal mode can be significantly increased by the coherent addition of lasers. The reduction of the number of longitudinal modes determines the limits of the coherent addition
19.
Pressure broadening
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Spectral lines are often used to identify atoms and molecules from their characteristic spectral lines. Spectral lines are the result of interaction between a system and a single photon. When a photon has about the amount of energy to allow a change in the energy state of the system. A spectral line may be observed either as a line or an absorption line. Which type of line is observed depends on the type of material, an absorption line is produced when photons from a hot, broad spectrum source pass through a cold material. The intensity of light, over a frequency range, is reduced due to absorption by the material. By contrast, a bright, emission line is produced when photons from a hot material are detected in the presence of a spectrum from a cold source. The intensity of light, over a frequency range, is increased due to emission by the material. Spectral lines are highly atom-specific, and can be used to identify the composition of any medium capable of letting light pass through it. Several elements were discovered by means, such as helium, thallium. Mechanisms other than atom-photon interaction can produce spectral lines, depending on the exact physical interaction, the frequency of the involved photons will vary widely, and lines can be observed across the electromagnetic spectrum, from radio waves to gamma rays. In other cases the lines are designated according to the level of ionization by adding a Roman numeral to the designation of the chemical element, so that Ca+ also has the designation Ca II. Neutral atoms are denoted with the roman number I, singly ionized atoms with II, more detailed designations usually include the line wavelength and may include a multiplet number or band designation. Many spectral lines of hydrogen also have designations within their respective series. A spectral line extends over a range of frequencies, not a single frequency, in addition, its center may be shifted from its nominal central wavelength. There are several reasons for this broadening and shift and these reasons may be divided into two general categories – broadening due to local conditions and broadening due to extended conditions. Broadening due to conditions is due to effects which hold in a small region around the emitting element. Broadening due to extended conditions may result from changes to the distribution of the radiation as it traverses its path to the observer
20.
Holography
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Holography is the science and practice of making holograms. The hologram itself is not an image and it is usually unintelligible when viewed under diffuse ambient light and it is an encoding of the light field as an interference pattern of seemingly random variations in the opacity, density, or surface profile of the photographic medium. In its pure form, holography requires the use of light for illuminating the subject. A microscopic level of detail throughout the volume of space can be reproduced. Holograms can now also be entirely computer-generated and show objects or scenes that never existed, stage illusions such as Peppers Ghost and other unusual, baffling, or seemingly magical images are also often incorrectly called holograms. The Hungarian-British physicist Dennis Gabor, was awarded the Nobel Prize in Physics in 1971 for his invention and development of the holographic method. His work, done in the late 1940s, was built on pioneering work in the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and William Lawrence Bragg in 1939. The discovery was a result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England. The word holography comes from the Greek words ὅλος and γραφή, early holograms used silver halide photographic emulsions as the recording medium. They were not very efficient as the produced grating absorbed much of the incident light, various methods of converting the variation in transmission to a variation in refractive index were developed which enabled much more efficient holograms to be produced. Several types of holograms can be made, a later refinement, the rainbow transmission hologram, allows more convenient illumination by white light rather than by lasers. Rainbow holograms are used for security and authentication, for example, on credit cards. They are also capable of multicolour-image reproduction, specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface. It works by reflectively or refractively manipulating bundles of light rays, most holograms produced are of static objects but systems for displaying changing scenes on a holographic volumetric display are now being developed. Holograms can also be used to store, retrieve, and process information optically and it was thought that it would be possible to use X-rays to make holograms of very small objects and view them using visible light. Today, holograms with x-rays are generated by using synchrotrons or x-ray free-electron lasers as radiation sources, the reconstruction is then retrieved via computation. Due to the wavelength of x-rays compared to visible light. As free-electron lasers can provide ultrashort and x-ray pulses in the range of femtoseconds which are intense and coherent, in laser holography, the hologram is recorded using a source of laser light, which is very pure in its color and orderly in its composition
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Spectroscopy
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Spectroscopy /spɛkˈtrɒskəpi/ is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is represented by an emission spectrum, a plot of the response of interest as a function of wavelength or frequency. Spectroscopy and spectrography are terms used to refer to the measurement of radiation intensity as a function of wavelength and are used to describe experimental spectroscopic methods. Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers, daily observations of color can be related to spectroscopy. Neon lighting is an application of atomic spectroscopy. Neon and other noble gases have characteristic emission frequencies, neon lamps use collision of electrons with the gas to excite these emissions. Inks, dyes and paints include chemical compounds selected for their characteristics in order to generate specific colors. A commonly encountered molecular spectrum is that of nitrogen dioxide, gaseous nitrogen dioxide has a characteristic red absorption feature, and this gives air polluted with nitrogen dioxide a reddish-brown color. Rayleigh scattering is a spectroscopic scattering phenomenon that accounts for the color of the sky, Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect, identify and quantify information about the atoms, Spectroscopy is also used in astronomy and remote sensing on earth. The measured spectra are used to determine the composition and physical properties of astronomical objects. One of the concepts in spectroscopy is a resonance and its corresponding resonant frequency. Resonances were first characterized in mechanical systems such as pendulums, mechanical systems that vibrate or oscillate will experience large amplitude oscillations when they are driven at their resonant frequency. A plot of amplitude vs. excitation frequency will have a peak centered at the resonance frequency and this plot is one type of spectrum, with the peak often referred to as a spectral line, and most spectral lines have a similar appearance. In quantum mechanical systems, the resonance is a coupling of two quantum mechanical stationary states of one system, such as an atom, via an oscillatory source of energy such as a photon. The coupling of the two states is strongest when the energy of the matches the energy difference between the two states. The energy of a photon is related to its frequency by E = h ν where h is Plancks constant, spectra of atoms and molecules often consist of a series of spectral lines, each one representing a resonance between two different quantum states
22.
Barcode reader
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A barcode reader is an electronic device that can read and output printed barcodes to a computer. Like a flatbed scanner, it consists of a source, a lens. Additionally, nearly all barcode readers contain decoder circuitry analyzing the image data provided by the sensor. Barcode readers can be differentiated by technologies as follows, Pen-type readers consist of a light source, to read a bar code, the person holding the pen must move the tip of it across the bars at a relatively uniform speed. The photodiode measures the intensity of the light reflected back from the source as the tip crosses each bar. The photodiode generates a waveform that is used to measure the widths of the bars, dark bars in the bar code absorb light and white spaces reflect light so that the voltage waveform generated by the photodiode is a representation of the bar and space pattern in the bar code. This waveform is decoded by the scanner in a similar to the way Morse code dots. As with the pen type reader, a photo-diode is used to measure the intensity of the light reflected back from the bar code, CCD readers use an array of hundreds of tiny light sensors lined up in a row in the head of the reader. Each sensor measures the intensity of the light immediately in front of it, two-dimensional imaging scanners are a newer type of bar code reader. They use a camera and image processing techniques to decode the bar code, large field-of-view readers use high resolution industrial cameras to capture multiple bar codes simultaneously. Unlike the simpler single-line laser scanners, they produce a pattern of beams in varying orientations allowing them to read barcodes presented to it at different angles, most of them use a single rotating polygonal mirror and an arrangement of several fixed mirrors to generate their complex scan patterns. Omnidirectional scanners are most familiar through the scanners in supermarkets. Omnidirectional scanners are also better at reading poorly printed, wrinkled, cell phone cameras open up a number of applications for consumers, Movies, DVD/VHS movie catalogs. Music, CD catalogs – play MP3 when scanned, groceries, nutrition information, making shopping lists when the last of an item is used, etc. Personal Property inventory ode scanned into personal finance software when entering, later, scanned receipt images can then be automatically associated with the appropriate entries. Later, the bar codes can be used to weed out paper copies not required to be retained for tax or asset inventory purposes. Receipts scanned on a scanner could be identified and associated with the appropriate entries in finance. Consumer tracking from the retailer perspective, a number of enterprise applications using cell phones are appearing, Access control, inventory reporting, asset tracking
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Ring laser gyroscope
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A ring laser gyroscope consists of a ring laser having two independent counter-propagating resonant modes over the same path, the difference in the frequencies is used to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the standing wave pattern in response to angular rotation. Interference between the beams, observed externally, results in motion of the standing wave pattern. The first experimental ring laser gyroscope was demonstrated in the US by Macek, various organizations worldwide subsequently developed ring-laser technology further. Ring laser gyroscopes can be used as the elements in an inertial reference system. The advantage of using an RLG is that there are no moving parts and this means there is no friction, which in turn means there will be no inherent drift terms. Additionally, the unit is compact, lightweight and virtually indestructible. Unlike a mechanical gyroscope, the device does not resist changes to its orientation and these hybrid INS/GPS units have replaced their mechanical counterparts in most applications. Where ultra accuracy is needed however, spin gyro based INSs are still in use today, a certain rate of rotation induces a small difference between the time it takes light to traverse the ring in the two directions according to the Sagnac effect. Therefore, the net shift of that interference pattern follows the rotation of the unit in the plane of the ring, rLGs, while more accurate than mechanical gyroscopes, suffer from an effect known as lock-in at very slow rotation rates. When the ring laser is hardly rotating, the frequencies of the counter-propagating laser modes become almost identical, forced dithering can largely overcome this problem. The ring laser cavity is rotated clockwise and anti-clockwise about its axis using a spring driven at its resonance frequency. This ensures that the velocity of the system is usually far from the lock-in threshold. Typical rates are 400 Hz, with a peak velocity of 1 arc-second per second. If a pure frequency oscillation is maintained, these small lock-in intervals can accumulate and this was remedied by introducing noise to the 400 Hz vibration. A related device is the fibre optic gyroscope which also operates on the basis of the Sagnac effect and this is therefore less sensitive than the RLG in which the externally observed phase shift is proportional to the accumulated rotation itself, not its derivative. However the sensitivity of the gyro is enhanced by having a long optical fiber coiled for compactness. Inertial Navigation – Forty Years of Evolution
24.
Optics
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Optics is the branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light, because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties. Most optical phenomena can be accounted for using the classical description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice, practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines, physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties, explanation of these effects requires quantum mechanics. When considering lights particle-like properties, the light is modelled as a collection of particles called photons, quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics. Optics began with the development of lenses by the ancient Egyptians and Mesopotamians, the earliest known lenses, made from polished crystal, often quartz, date from as early as 700 BC for Assyrian lenses such as the Layard/Nimrud lens. The ancient Romans and Greeks filled glass spheres with water to make lenses, the word optics comes from the ancient Greek word ὀπτική, meaning appearance, look. Greek philosophy on optics broke down into two opposing theories on how vision worked, the theory and the emission theory. The intro-mission approach saw vision as coming from objects casting off copies of themselves that were captured by the eye, plato first articulated the emission theory, the idea that visual perception is accomplished by rays emitted by the eyes. He also commented on the parity reversal of mirrors in Timaeus, some hundred years later, Euclid wrote a treatise entitled Optics where he linked vision to geometry, creating geometrical optics. Ptolemy, in his treatise Optics, held a theory of vision, the rays from the eye formed a cone, the vertex being within the eye. The rays were sensitive, and conveyed back to the observer’s intellect about the distance. He summarised much of Euclid and went on to describe a way to measure the angle of refraction, during the Middle Ages, Greek ideas about optics were resurrected and extended by writers in the Muslim world
25.
Gaussian beam
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This fundamental transverse gaussian mode describes the intended output of most lasers, as such a beam can be focused into the most concentrated spot. When such a beam is refocused by a lens, the transverse dependence is altered. The electric and magnetic field amplitude profiles along any such circular Gaussian beam are determined by a single parameter, at any position z relative to the waist along a beam having a specified w0, the field amplitudes and phases are thereby determined as detailed below. The equations below assume a beam with a circular cross-section at all values of z, arbitrary solutions of the paraxial Helmholtz equation can be expressed as combinations of Hermite–Gaussian modes or similarly as combinations of Laguerre–Gaussian modes. At any point along the beam z these modes include the same Gaussian factor as the fundamental Gaussian mode multiplying the additional geometrical factors for the specified mode, Gaussian beam normally implies radiation confined to the fundamental Gaussian mode. The Gaussian beam is an electromagnetic mode. The mathematical expression for the field amplitude is a solution to the paraxial Helmholtz equation. There is also an understood time dependence e i ω t multiplying such phasor quantities, since this solution relies on the paraxial approximation, it is not accurate for very strongly diverging beams. In most practical cases the form is valid. Then the waves associated magnetic field is directly proportional to the electric field. For free space, η = η0 ≈377 Ω, at a position z along the beam, the spot size parameter w is given by w = w 01 +2. Where z R = π w 02 λ is called the Rayleigh range as further discussed below. The radius of the w, at any position z along the beam, is related to the full width at half maximum at that position according to. The curvature of the wavefronts is zero at the beam waist and it is equal to 1/R where R is the radius of curvature as a function of position along the beam, given by R = z. The so-called Gouy phase of the beam at z is given by, the Gouy phase results in an increase in the apparent wavelength near the waist. The phase velocity near the waist exceeds the speed of light in the medium, the Gouy phase shift along the beam remains within the range ±π/2 and is not observable in most experiments. However it is of importance and takes on a greater range for higher-order Gaussian modes. Many laser beams have an elliptical cross-section, also common are beams with waist positions which are different for the two transverse dimensions, called astigmatic beams
26.
LaserDisc
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LaserDisc is a home video format and the first commercial optical disc storage medium, initially licensed, sold and marketed as MCA DiscoVision in North America in 1978. It was not a format in Europe and Australia when first released but was popular in the 1990s. Its superior video and audio quality made it a choice among videophiles. The technologies and concepts behind LaserDisc were the foundation for later optical disc formats including Compact Disc, DVD, Optical video recording technology, using a transparent disc, was invented by David Paul Gregg and James Russell in 1958. The Gregg patents were purchased by MCA in 1968, by 1969, Philips had developed a videodisc in reflective mode, which has advantages over the transparent mode. MCA and Philips then decided to combine their efforts and first publicly demonstrated the video disc in 1972. LaserDisc was first available on the market, in Atlanta, Georgia, on December 15,1978, Philips produced the players while MCA produced the discs. The Philips-MCA cooperation was not successful, and discontinued after a few years, several of the scientists responsible for the early research founded Optical Disc Corporation. In 1979, the Museum of Science and Industry in Chicago opened its Newspaper exhibit which used interactive LaserDiscs to allow visitors to search for the front page of any Chicago Tribune newspaper and this was a very early example of public access to electronically stored information in a museum. The first LaserDisc title marketed in North America was the MCA DiscoVision release of Jaws in 1978, the last title released in North America was Paramounts Bringing Out the Dead in 2000. The last Japanese released movie was the Hong Kong film Tokyo Raiders from Golden Harvest, a dozen or so more titles continued to be released in Japan, until the end of 2001. Production of LaserDisc players continued until January 14,2009, when Pioneer stopped making them and it was estimated that in 1998, LaserDisc players were in approximately 2% of U. S. households. By comparison, in 1999, players were in 10% of Japanese households, LaserDisc was released on June 10,1981 in Japan, and a total of 3.6 million LaserDisc players were sold there. A total of 16.8 million LaserDisc players were sold worldwide, by the early 2000s, LaserDisc was completely replaced by DVD in the North American retail marketplace, as neither players nor software were then produced. Players were still exported to North America from Japan until the end of 2001, the format has retained some popularity among American collectors, and to a greater degree in Japan, where the format was better supported and more prevalent during its life. In Europe, LaserDisc always remained an obscure format and it was chosen by the British Broadcasting Corporation for the BBC Domesday Project in the mid-1980s, a school-based project to commemorate 900 years since the original Domesday Book in England. From 1991 up until the early 2000s, the BBC also used LaserDisc technology to play out the channel idents, the standard home video LaserDisc was 30 cm in diameter and made up of two single-sided aluminum discs layered in plastic. Although appearing similar to compact discs or DVDs, LaserDiscs used analog video stored in the domain with analog FM stereo sound
27.
Optical disc
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The encoding material sits atop a thicker substrate which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the disc surface. Most optical discs exhibit a characteristic iridescence as a result of the diffraction grating formed by its grooves and this side of the disc contains the actual data and is typically coated with a transparent material, usually lacquer. The reverse side of a disc usually has a printed label, sometimes made of paper. Optical discs are usually between 7.6 and 30 cm in diameter, with 12 cm being the most common size, a typical disc is about 1.2 mm thick, while the track pitch ranges from 1.6 µm to 320 nm. An optical disc is designed to support one of three recording types, read-only, recordable, or re-recordable, write-once optical discs commonly have an organic dye recording layer between the substrate and the reflective layer. Rewritable discs typically contain an alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony, Optical discs are most commonly used for storing music, video, or data and programs for personal computers. The Optical Storage Technology Association promotes standardized optical storage formats, although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage. Libraries and archives enact optical media preservation procedures to ensure continued usability in the optical disc drive or corresponding disc player. For computer data backup and physical data transfer, optical discs such as CDs and DVDs are gradually being replaced with faster, smaller solid-state devices and this trend is expected to continue as USB flash drives continue to increase in capacity and drop in price. Additionally, music purchased or shared over the Internet has significantly reduced the number of audio CDs sold annually. The first recorded use of an optical disc was in the 1884 when Alexander Graham Bell, Chichester Bell. An early optical disc system existed in 1935, named Lichttonorgel, an early analog optical disc used for video recording was invented by David Paul Gregg in 1958 and patented in the US in 1961 and 1969. This form of optical disc was an early form of the DVD. It is of special interest that U. S, patent 4,893,297, filed 1989, issued 1990, generated royalty income for Pioneer Corporations DVA until 2007 —then encompassing the CD, DVD, and Blu-ray systems. In the early 1960s, the Music Corporation of America bought Greggs patents and his company, american inventor James T. Russell has been credited with inventing the first system to record a digital signal on an optical transparent foil which is lit from behind by a high-power halogen lamp. Russells patent application was first filed in 1966 and he was granted a patent in 1970, following litigation, Sony and Philips licensed Russells patents in the 1980s. Both Greggs and Russells disc are floppy media read in transparent mode, in the Netherlands in 1969, Philips Research physicist, Pieter Kramer invented an optical videodisc in reflective mode with a protective layer read by a focused laser beam U. S
28.
Integrated Authority File
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The Integrated Authority File or GND is an international authority file for the organisation of personal names, subject headings and corporate bodies from catalogues. It is used mainly for documentation in libraries and increasingly also by archives, the GND is managed by the German National Library in cooperation with various regional library networks in German-speaking Europe and other partners. The GND falls under the Creative Commons Zero license, the GND specification provides a hierarchy of high-level entities and sub-classes, useful in library classification, and an approach to unambiguous identification of single elements. It also comprises an ontology intended for knowledge representation in the semantic web, available in the RDF format
