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 major component of lasers, surrounding the gain medium and providing feedback of the laser light, they are used in optical parametric oscillators and some interferometers. Light confined in the cavity reflects multiple times producing standing waves for certain resonance frequencies; the standing wave patterns produced. Different resonator types are distinguished by the focal lengths of the two mirrors and the distance between them; the geometry must be chosen. Resonator types are 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. Therefore, the frequency line width of the beam is small indeed compared to the frequency of the laser. Light confined in a resonator will reflect multiple times from the mirrors, due to the effects of interference, only certain patterns and frequencies of radiation will be sustained by the resonator, with the others being suppressed by destructive interference.
In general, radiation patterns which are reproduced on every round-trip of the light through the resonator are the most stable, these are the eigenmodes, known as the modes, of the resonator. Resonator modes can be divided into two types: longitudinal modes, which differ in frequency from each other; the basic, or fundamental transverse mode of a resonator is a Gaussian beam. The most common types of optical cavities consist of two facing spherical mirrors; the simplest of these is the plane-parallel or Fabry–Pérot cavity, consisting of two opposing flat mirrors. While simple, this arrangement is used in large-scale lasers due to the difficulty of alignment. However, this problem is much reduced for short cavities with a small mirror separation distance. Plane-parallel resonators are therefore used in microchip and microcavity lasers and semiconductor lasers. In these cases, rather than using separate mirrors, a reflective optical coating may be directly applied to the laser medium itself; the plane-parallel resonator is 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 radii are equal to half the cavity length, a concentric or spherical resonator results; 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 hemispherical cavity, with one plane mirror and one mirror of radius equal to the cavity length. A common and important design is the confocal resonator, with mirrors of equal radii to the cavity length; this design produces the smallest possible beam diameter at the cavity mirrors for a given cavity length, is used in lasers where the purity of the transverse mode pattern is important. A concave-convex cavity has one convex mirror with a negative radius of curvature; this design produces no intracavity focus of the beam, is thus useful in high-power lasers where the intensity of the intracavity light might be damaging to the intracavity medium if brought to a focus.
A transparent dielectric sphere, such as a liquid droplet forms an interesting optical cavity. In 1986 Richard K. Chang et al. demonstrated lasing using ethanol microdroplets doped with rhodamine 6G dye. This type of optical cavity exhibits optical resonances when the size of the sphere or the optical wavelength or the refractive index is varied; the resonance is known as morphology-dependent resonance. Only certain ranges of values for R1, R2, L produce stable resonators in which periodic refocussing of the intracavity beam is produced. If the cavity is unstable, the beam size will grow without limit growing larger than the size of the cavity mirrors and being lost. By using methods such as ray transfer matrix analysis, it is possible to calculate a stability criterion: 0 ⩽ ⩽ 1. Values which satisfy the inequality correspond to stable resonators; the stability can be shown graphically by defining a stability parameter, g for each mirror: g 1 = 1 − L R 1, g 2 = 1 − L R 2,and plotting g1 against g2 as shown.
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be chemical, electric power, mechanical work, light, or heat. Energy conversion efficiency depends on the usefulness of the output. All or part of the heat produced from burning a fuel may become rejected waste heat if, for example, work is the desired output from a thermodynamic cycle. Energy converter is an example of an energy transformation. For example a light bulb falls into the categories energy converter. Η = P o u t P i n Even though the definition includes the notion of usefulness, efficiency is considered a technical or physical term. Goal or mission oriented terms include efficacy. Energy conversion efficiency is a dimensionless number between 0 and 1.0, or 0% to 100%. Efficiencies may not exceed e.g. for a perpetual motion machine. However, other effectiveness measures that can exceed 1.0 are used for heat pumps and other devices that move heat rather than convert it.
When talking about the efficiency of heat engines and power stations the convention should be stated, i.e. HHV or LCV, whether gross output or net output are being considered; the two are separate but both must be stated. Failure to do so causes endless confusion. Related, more specific terms include Electrical efficiency, useful power output per electrical power consumed. Same as the thermal efficiency. Luminous efficiency, that portion of the emitted electromagnetic radiation is usable for human vision. In Europe the usable energy content of fuel is calculated using the lower heating value of that fuel, the definition of which assumes that the water vapor produced during fuel combustion, remains gaseous, is not condensed to liquid water so the latent heat of vaporization of that water is not usable. Using the LHV, a condensing boiler can achieve a "heating efficiency" in excess of 100%; this is because the apparatus recovers part of the heat of vaporization, not included in the definition of the lower heating value of fuel.
In the U. S. and elsewhere, the higher heating value is used, which includes the latent heat for condensing the water vapor, thus the thermodynamic maximum of 100% efficiency cannot be exceeded with HHV's use. In optical systems such as lighting and lasers, the energy conversion efficiency is referred to as wall-plug efficiency; the wall-plug efficiency is the measure of output radiative-energy, in watts, per the total of the input electrical-energy in watts. The output-energy is measured in terms of absolute irradiance and the wall-plug efficiency is given as a percentage of the total input-energy, with the inverse percentage representing the losses; the wall-plug efficiency differs from the luminous efficiency in that wall-plug efficiency describes the direct output/input conversion of energy whereas luminous efficiency takes into account the human eye's varying sensitivity to different wavelengths. Instead of using watts, the power of a light source to produce wavelengths proportional to human perception is measured in lumens.
The human eye is most sensitive to wavelengths of 555 nanometers but the sensitivity decreases to either side of this wavelength, following a Gaussian power-curve and dropping to zero sensitivity at the red and violet ends of the spectrum. Due to this the eye does not see all of the wavelengths emitted by a particular light-source, nor does it see all of the wavelengths within the visual spectrum equally. Yellow and green, for example, make up more than 50% of what the eye perceives as being white though in terms of radiant energy white-light is made from equal portions of all colors. Therefore, the radiant intensity of a light source may be much greater than its luminous intensity, meaning that the source emits more energy than the eye can use; the lamp's wall-plug efficiency is greater than its luminous efficiency. The effectiveness of a light source to convert electrical energy into wavelengths of visible light, in proportion to the sensitivity of the human eye, is referred to as luminous efficacy, measured in units of lumens per watt of electrical input-energy.
Unlike efficacy, a unit of measurement, efficiency is unitless number expressed as a percentage, requiring only that the input and output units be of the same type. Therefore, the luminous efficiency of a light source is the percentage of luminous efficacy per the theoretical-maximum efficacy at a specific wavelength; the amount of energy carried by a photon of light is determined by its wavelength. In lumens, this energy is offset by the eye's sensitivity to the selected wavelengt
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state populated by a pump source. Examples of active laser media include: Certain crystals doped with rare-earth ions or transition metal ions. Liquids, in the form of dye solutions as used in dye lasers. In order to fire a laser, the active gain medium must be in a nonthermal energy distribution known as a population inversion; the preparation of this state is known as laser pumping. Pumping may be achieved with electrical currents or with light, generated by discharge lamps or by other lasers. More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams. A universal model valid for all laser types does not exist; the simplest model includes two systems of sub-levels: lower. Within each sub-level system, the fast transitions ensure that thermal equilibrium is reached leading to the Maxwell–Boltzmann statistics of excitations among sub-levels in each system.
The upper level is assumed to be metastable. Gain and refractive index are assumed independent of a particular way of excitation. For good performance of the gain medium, the separation between sub-levels should be larger than working temperature. In the case of amplification of optical signals, the lasing frequency is called signal frequency. However, the same term is used in the laser oscillators, when amplified radiation is used to transfer energy rather than information; the model below seems to work well for most optically-pumped solid-state lasers. The simple medium can be characterized with effective cross-sections of absorption and emission at frequencies ω p and ω s. Have N be concentration of active centers in the solid-state lasers. Have N 1 be concentration of active centers in the ground state. Have N 2 be concentration of excited centers. Have N 1 + N 2 = N; the relative concentrations can be defined as n 1 = N 1 / N and n 2 = N 2 / N. The rate of transitions of an active center from ground state to the excited state can be expressed with W u = I p σ a p ℏ ω p + I s σ a s ℏ ω s and The rate of transitions back to the ground state can be expressed with W d = I p σ e p ℏ ω p + I s σ e s ℏ ω s + 1 τ, where σ a s and σ a p are effective cross-sections of absorption at the frequencies of the signal and the pump.
Σ e s and σ e p are the same for stimulated emission. The kinetic equation for relative populations can be written as follows: d n 2 d t = W u n 1 − W d n 2, d n 1 d t = − W
The Electric Oxygen Iodine Laser, or ElectricOIL, or EOIL, is an infrared hybrid electrical / chemical laser. Its output wavelength is µm, the wavelength of transition of atomic iodine; the lasing state I* is produced by near-resonant energy transfer with the singlet oxygen metastable O2. EOIL technology represents a unique class of hybrid electric gas high-energy laser with the potential to have inherently higher beam quality than solid state systems, while being more logistically friendly than current Chemical Oxygen Iodine Laser systems; the principal advantage of such an inherently high beam quality system is the trade of a small fixed mass in electrical generation and heat exchanger hardware for the massive fluid supply and large tankage associated with COIL devices. Since the first reporting of a viable electric discharge-driven oxygen-iodine laser system by CU Aerospace and the University of Illinois at Urbana Champaign, there have been a number of other successful demonstrations of gain and laser power.
Computational modeling of the discharge and post-discharge kinetics has been an invaluable tool in EOIL development, allowing analysis of the production of various discharge species and determination of the influence of NOX species on system kinetics. Ionin et al. and Heaven provide comprehensive topical reviews of discharge production of O2 and various EOIL studies. The highest gain in an EOIL device reported to date is 0.30% / cm, the highest output power reported is 538 W. Over the past five years of research and development of the EOIL device, higher performance and efficiency have been obtained by moving towards higher operating flow rates and pressures
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, tasteless, non-toxic, monatomic gas, the first in the noble gas group in the periodic table, its boiling point is the lowest among all the elements. After hydrogen, helium is the second lightest and second most abundant element in the observable universe, being present at about 24% of the total elemental mass, more than 12 times the mass of all the heavier elements combined, its abundance is similar 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 accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, the vast majority of, formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars. Helium is named for the Greek Titan of the Sun, Helios, it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson, Lieutenant John Herschel, was subsequently confirmed by French astronomer Jules Janssen.
Janssen is jointly credited with detecting the element along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868 while Lockyer observed it from Britain. Lockyer was the first to propose; 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 natural gas fields in parts of the United States, by far the largest supplier of the gas today. Liquid helium is used in cryogenics in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in airships; as with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice.
In scientific research, the behavior of the two fluid phases of helium-4 is important to researchers studying quantum mechanics and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. On Earth it is rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements, as the alpha particles emitted by such decays consist of helium-4 nuclei; this radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Terrestrial helium—a non-renewable resource, because once released into the atmosphere it escapes into space—was thought to be in short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases having been released by volcanic activity.
The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India; this line was assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 because it was near the known D1 and D2 Fraunhofer line lines of sodium, he concluded. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος. In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material, sublimated during a recent eruption of Mount Vesuvius. On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun.
These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, who collected enough of the gas to determine its atomic weight. Helium was isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, attributed the lines to nitrogen, his letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science. In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium; when the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited state, population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier; the pump power must be higher than the lasing threshold of the laser. The pump energy is provided in the form of light or electric current, but more exotic sources have been used, such as chemical or nuclear reactions. A laser pumped with an arc lamp or a flashlamp is pumped through the lateral wall of the lasing medium, in the form of a crystal rod containing a metallic impurity or a glass tube containing a liquid dye, in a condition known as "side-pumping." To use the lamp's energy most efficiently, the lamps and lasing medium are contained in a reflective cavity that will redirect most of the lamp's energy into the rod or dye cell.
In the most common configuration, the gain medium is in the form of a rod located at one focus of a mirrored cavity, consisting of an elliptical cross-section perpendicular to the rod's axis. The flashlamp is a tube located at the other focus of the ellipse; the mirror's coating is chosen to reflect wavelengths that are shorter than the lasing output while absorbing or transmitting wavelengths that are the same or longer, to minimize thermal lensing. In other cases an absorber for the longer wavelengths is used; the lamp is surrounded by a cylindrical jacket called a flow tube. This flow tube is made of a glass that will absorb unsuitable wavelengths, such as ultraviolet, or provide a path for cooling water which absorbs infrared; the jacket is given a dielectric coating that reflects unsuitable wavelengths of light back into the lamp. This light is absorbed and some of it is re-emitted at suitable wavelengths; the flow tube serves to protect the rod in the event of a violent lamp failure. Smaller ellipses create fewer reflections.
For a single flashlamp, if the lamp and rod are equal diameter, an ellipse, twice as wide as it is high is the most efficient at imaging the light into the rod. The rod and the lamp are long to minimize the effect of losses at the end faces and to provide a sufficient length of gain medium. Longer flashlamps are more efficient at transferring electrical energy into light, due to higher impedance. However, if the rod is too long in relation to its diameter a condition called "prelasing" can occur, depleting the rod's energy before it can properly build up. Rod ends are antireflection coated or cut at Brewster's angle to minimize this effect. Flat mirrors are often used at the ends of the pump cavity to reduce loss. Variations on this design use more complex mirrors composed of overlapping elliptical shapes, to allow multiple flashlamps to pump a single rod; this allows greater power, but are less efficient because not all of the light is imaged into the rod, leading to increased thermal losses.
These losses can be minimized by using a close-coupled cavity. This approach may allow more symmetric increasing beam quality, however. Another configuration uses a rod and a flashlamp in a cavity made of a diffuse reflecting material, such as spectralon or powdered barium sulfate; these cavities are circular or oblong, as focusing the light is not a primary objective. This doesn't couple the light as well into the lasing medium, since the light makes many reflections before reaching the rod, but requires less maintenance than metalized reflectors; the increased number of reflections is compensated for by the diffuse medium's higher reflectivity: 99% compared to 97% for a gold mirror. This approach is more compatible with multiple lamps. Parasitic modes occur when reflections are generated in directions other than along the length of the rod, which can use up energy that would otherwise be available to the beam; this can be a particular problem. Cylindrical laser rods support whispering gallery modes due to total internal reflection between the rod and the cooling water, which reflect continuously around the circumference of the rod.
Light pipe modes can reflect down the length of the rod in a zig-zag path. If the rod has an antireflection coating, or is immersed in a fluid that matches its refractive index, it can reduce these parasitic reflections. If the barrel of the rod is rough ground, or grooved, internal reflections can be dispersed. Pumping with a single lamp tends to focus most of the energy on one side, worsening the beam profile, it is common for rods to have a frosted barrel, to diffuse the light, providing a more distribution of light throughout the rod. This allows more energy absorption throughout the gain medium for a better transverse mode. A frosted flow tube or diffuse reflector, while leading to lowered transfer efficiency, helps increase this effect, improving the gain. Laser host materials are chosen to have a low absorption. Therefore, any light at frequencies not absorbed by the doping will go back into the lamp and reheat the plasma, shortening lamp life. Flashlamps were the earliest energy source for lasers.
They are used for high pulsed energies in both dye lasers. They produce a broad spectrum of light, causing most of the energy to be wasted as heat in the gain medium. Flashlamps tend to have a short lifetime; the first laser consisted of a helical flashlamp surrounding a ruby rod. Quartz flashla
Nitrogen is a chemical element with symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is accorded the credit because his work was published first; the name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790, when it was found that nitrogen was present in nitric acid and nitrates. Antoine Lavoisier suggested instead the name azote, from the Greek ἀζωτικός "no life", as it is an asphyxiant gas. Nitrogen is the lightest member of group 15 of the periodic table called the pnictogens; the name comes from the Greek πνίγειν "to choke", directly referencing nitrogen's asphyxiating properties. It is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System. At standard temperature and pressure, two atoms of the element bind to form dinitrogen, a colourless and odorless diatomic gas with the formula N2.
Dinitrogen forms about 78 % of Earth's atmosphere. Nitrogen occurs in all organisms in amino acids, in the nucleic acids and in the energy transfer molecule adenosine triphosphate; the human body contains about 3% nitrogen by mass, the fourth most abundant element in the body after oxygen and hydrogen. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds back into the atmosphere. Many industrially important compounds, such as ammonia, nitric acid, organic nitrates, cyanides, contain nitrogen; the strong triple bond in elemental nitrogen, the second strongest bond in any diatomic molecule after carbon monoxide, dominates nitrogen chemistry. This causes difficulty for both organisms and industry in converting N2 into useful compounds, but at the same time means that burning, exploding, or decomposing nitrogen compounds to form nitrogen gas releases large amounts of useful energy. Synthetically produced ammonia and nitrates are key industrial fertilisers, fertiliser nitrates are key pollutants in the eutrophication of water systems.
Apart from its use in fertilisers and energy-stores, nitrogen is a constituent of organic compounds as diverse as Kevlar used in high-strength fabric and cyanoacrylate used in superglue. Nitrogen is a constituent including antibiotics. Many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules: for example, the organic nitrates nitroglycerin and nitroprusside control blood pressure by metabolizing into nitric oxide. Many notable nitrogen-containing drugs, such as the natural caffeine and morphine or the synthetic amphetamines, act on receptors of animal neurotransmitters. Nitrogen compounds have a long history, ammonium chloride having been known to Herodotus, they were well known by the Middle Ages. Alchemists knew nitric acid as aqua fortis, as well as other nitrogen compounds such as ammonium salts and nitrate salts; the mixture of nitric and hydrochloric acids was known as aqua regia, celebrated for its ability to dissolve gold, the king of metals. The discovery of nitrogen is attributed to the Scottish physician Daniel Rutherford in 1772, who called it noxious air.
Though he did not recognise it as an different chemical substance, he distinguished it from Joseph Black's "fixed air", or carbon dioxide. The fact that there was a component of air that does not support combustion was clear to Rutherford, although he was not aware that it was an element. Nitrogen was studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephitic air" or azote, from the Greek word άζωτικός, "no life". In an atmosphere of pure nitrogen, animals died and flames were extinguished. Though Lavoisier's name was not accepted in English, since it was pointed out that all gases are mephitic, it is used in many languages and still remains in English in the common names of many nitrogen compounds, such as hydrazine and compounds of the azide ion, it led to the name "pnictogens" for the group headed by nitrogen, from the Greek πνίγειν "to choke".
The English word nitrogen entered the language from the French nitrogène, coined in 1790 by French chemist Jean-Antoine Chaptal, from the French nitre and the French suffix -gène, "producing", from the Greek -γενής. Chaptal's meaning was that nitrogen is the essential part of nitric acid, which in turn was produced from nitre. In earlier times, niter had been confused with Egyptian "natron" – called νίτρον in Greek – which, despite the name, contained no nitrate; the earliest military and agricultural applications of nitrogen compounds used saltpeter, most notably in gunpowder, as fertiliser. In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", a monatomic allotrope of nitrogen; the "whirling cloud of brilliant yellow light