1.
Tunable laser
–
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, there are many types and categories of tunable lasers. They exist in the gas, liquid, and solid state, among the types of tunable lasers are excimer lasers, gas lasers, dye lasers, transition metal solid-state lasers, semiconductor crystal and diode lasers, and free electron lasers. Tunable lasers find applications in spectroscopy, photochemistry, atomic vapor laser isotope separation, since no real laser is truly monochromatic, all lasers can emit light over some range of frequencies, known as the linewidth of the laser transition. In most lasers, this linewidth is quite narrow, most laser gain media have a number of transition wavelengths on which laser operation can be achieved. For example, as well as the principal 1,064 nm output line, Nd, YAG has weaker transitions at wavelengths of 1,052 nm,1,074 nm,1,112 nm,1,319 nm, and a number of other lines. Usually, these lines do not operate unless the gain of the strongest transition is suppressed, e. g. by use of wavelength-selective dielectric mirrors. If a dispersive element, such as a prism, is introduced into the optical cavity, such schemes are common in argon-ion lasers, allowing tuning of the laser to a number of lines from the ultraviolet and blue through to green wavelengths. For some types of lasers the lasers cavity length can be modified, distributed feedback semiconductor lasers and vertical cavity surface emitting lasers use periodic distributed Bragg reflector structures to form the mirrors of the optical cavity. If the temperature of the laser is changed, the change of the DBR structure causes a shift in its peak reflective wavelength. The tuning range of lasers is typically a few nanometres, up to a maximum of approximately 6 nm. As a rule of thumb the wavelength is tuned by 0.08 nm/K for DFB lasers operating in the 1,550 nm wavelength regime, such lasers are commonly used in optical communications applications such as DWDM-systems to allow adjustment of the signal wavelength. To get wideband tuning using this technique, some such as Santur Corporation or Nippon Telegraph and Telephone contain an array of such lasers on a single chip and concatenate the tuning ranges. Other technologies to achieve wide tuning ranges for DWDM-systems are, External cavity lasers using a MEMS structure for tuning the cavity length, External cavity lasers using multiple-prism grating arrangements for wide-range tunability. DFB laser arrays based on several thermal tuned DFB lasers, Coarse tuning is achieved by selecting the correct laser bar, fine tuning is then done thermally, such as devices commercialized by Santur Corporation. Tunable VCSEL, One of the two stacks is movable. To achieve sufficient output power out of a VCSEL structure, lasers in the 1,550 nm domain are usually either optically pumped or have an optical amplifier built into the device. As of December 2008 there is no widely tunable VCSEL commercially available any more for DWDM-system application and it is claimed that the first infrared laser with a tunability of more than one octave was a germanium crystal laser
Tunable laser
–
CW dye laser based on
Rhodamine 6G. The dye laser is considered to be the first broadly tunable laser.
Tunable laser
–
A typical laser diode. When mounted with external optics these lasers can be tuned mainly in the red and near infrared.
2.
Red
–
Red is the color at the longer-wavelengths end of the spectrum of visible light next to orange, at the opposite end from violet. Red color has a predominant light wavelength of roughly 620–740 nanometers, light with a longer wavelength than red but shorter than terahertz radiation and microwave is called infrared. Red is one of the secondary colors, resulting from the combination of yellow. Traditionally, it was viewed as a primary colour, along with yellow and blue, in the RYB color space and traditional color wheel formerly used by painters. Reds can vary in shade from light pink to very dark maroon or burgundy. Red is the color of cyan. In nature, the red color of blood comes from hemoglobin, the red color of the Grand Canyon and other geological features is caused by hematite or red ochre, both forms of iron oxide. It also causes the red color of the planet Mars, the color of autumn leaves is caused by pigments called anthocyanins, which are produced towards the end of summer, when the green chlorophyll is no longer produced. One to two percent of the population has red hair, the color is produced by high levels of the reddish pigment pheomelanin. Since red is the color of blood, it has historically been associated with sacrifice, danger, modern surveys in the United States and Europe show red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love and joy. In China, India and many other Asian countries it is the color of symbolizing happiness, since the 19th century, red has also been associated with socialism and communism. The word red is derived from the Old English rēad, the word can be further traced to the Proto-Germanic rauthaz and the Proto-Indo European root rewdʰ-. In Sanskrit, the word means red or blood. In the Akkadian language of Ancient Mesopotamia and in the modern Inuit language of Inuit, the words for colored in Latin and Spanish both also mean red. In Portuguese the word for red is vermelho, which comes from Latin vermiculus, in the Russian language, the word for red, Кра́сный, comes from the same old Slavic root as the words for beautiful—красивый and excellent—прекрасный. Thus Red Square in Moscow, named long before the Russian Revolution, in heraldry, the word gules is used for red. Red can vary in hue from orange-red to violet-red, and for each hue there is a variety of shades and tints. Red hematite powder was found scattered around the remains at a grave site in a Zhoukoudian cave complex near Beijing
Red
–
Pure, or solid red, the color of most ripe
strawberries.
Red
–
Red
Red
–
Scarlet is one quarter of the way between the colors red and orange. It is the color worn by a
cardinal of the
Roman Catholic Church..
Red
–
The
cardinal takes its name from the color worn by Roman Catholic
cardinals.
3.
Infrared
–
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
Infrared
–
This infrared space telescope image has (false color) blue, green and red corresponding to 3.4, 4.6, and 12
µm wavelengths, respectively.
Infrared
–
A
false color image of two people taken in long-wavelength infrared (body-temperature thermal) light.
Infrared
–
Materials with higher
emissivity appear to be hotter. In this thermal image, the ceramic cylinder appears to be hotter than its cubic container (made of silicon carbide), while in fact they have the same temperature.
Infrared
–
Active-infrared night vision: the camera illuminates the scene at infrared wavelengths invisible to the
human eye. Despite a dark back-lit scene, active-infrared night vision delivers identifying details, as seen on the display monitor.
4.
Lasers
–
Lasers is the third studio album by American rapper Lupe Fiasco, released on March 7,2011 by Atlantic Records. Production for the album took place between 2008 and 2010, Lasers features production by The Audibles, The Neptunes, Needlz, Alex da Kid, Syience, and long-time collaborator Soundtrakk, among others. Trey Songz, John Legend, Skylar Grey, Sway, Matt Mahaffey, MDMA, Eric Turner, Lasers was preceded by the lead single The Show Goes On and its follow-up Words I Never Said featuring Skylar Grey. The former became Fiascos highest charting song on the Billboard Hot 100 and has been certified double Platinum in the US, the latter, however, only achieved moderate success on the Hot 100. The album has received mixed or average reviews from most major music critics, Lupe Fiasco was supposedly going to released a triple album, titled LupE. N. D. as his third and final record, but his contract with Atlantic Records prevented him from doing so. Indefinitely and intended on releasing an album tentatively titled The Great American Rap Album in June 2009, instead, the album was also postponed and he announced that a new album was in the works, originally titled We Are Lasers and then changed to Lasers. Lasers is a backronym for Love Always Shines Everytime, Remember 2 Smile, referring to the title of the album, Fiasco has stated, Ive always had that word in my head, Im a word guy. And I thought lasers would be a name for an album, so when I came up with it I just put it in my think-tank and let it evolve. In 2010, to promote the message of the album, Fiasco released a video onto the internet titled The L. A. S. E. R. S. Manifesto, which can be found on both YouTube and the official website, the manifesto reads, On January 4,2011, Billboard revealed two guest appearances on Fiascos album Lasers. The guests confirmed were Trey Songz and British rapper Sway, in the interview with Billboard, Fiasco expressed his thoughts on music piracy and fighting track leaks on various blogs. I definitely think I was part of changing that, and an influence to a lot dudes that are coming out today, some of the records are controversial, but it’s less cohesive than The Cool. Talking about the album with Details, Fiasco has stated that he wanted to make a popular record and he has also stated that the creation of the album was a very painful, dark, fucked-up process. In a February 2011 interview with Complex, Fiasco has stated, Speaking about the album with the Chicago Sun-Times, Fiasco has stated, Im actually happy with the record. I feel I got to say what I wanted even with -- It doesnt make up for what it took to get through it and its still being argued and debated upon. The climate of record was very weird, in some instances surreal. I had to create this commercial art that appeases the corporate side, I had to acquiesce to certain forces. Hopefully within that I snuck in some things I actually wanted to say any way I can, in an interview with The Guardian, Fiasco has expressed that during the recording process of the album he has dealt with depression and suicidal thoughts, It was mentally destructive
Lasers
–
Lasers
5.
Ultrashort pulse
–
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a picosecond or less. Such pulses have an optical spectrum, and can be created by mode-locked oscillators. They are commonly referred to as ultrafast events, amplification of ultrashort pulses almost always requires the technique of chirped pulse amplification, in order to avoid damage to the gain medium of the amplifier. They are characterized by a peak intensity that usually leads to nonlinear interactions in various materials. These processes are studied in the field of nonlinear optics, in the specialized literature, ultrashort refers to the femtosecond and picosecond range, although such pulses no longer hold the record for the shortest pulses artificially generated. Indeed, x-ray pulses with durations on the time scale have been reported. The 1999 Nobel Prize in Chemistry was awarded to Ahmed H. Zewail for using ultrashort pulses to observe chemical reactions on the timescales they occur on, there is no standard definition of ultrashort pulse. A common example is a chirped Gaussian pulse, a wave whose field amplitude follows a Gaussian envelope, the real electric field corresponding to an ultrashort pulse is oscillating at an angular frequency ω0 corresponding to the central wavelength of the pulse. To facilitate calculations, a complex field E is defined, formally, it is defined as the analytic signal corresponding to the real field. Such a chirp may be acquired as a pulse propagates through materials and is due to their dispersion and it results in a temporal broadening of the pulse. The intensity functions—temporal I and spectral S—determine the time duration and spectrum bandwidth of the pulse, as stated by the uncertainty principle, their product has a lower bound. This minimum value depends on the used for the duration and on the shape of the pulse. For a given spectrum, the minimum time-bandwidth product, and therefore the shortest pulse, is obtained by a transform-limited pulse, high values of the time-bandwidth product, on the other hand, indicate a more complex pulse. One of them is the compressor, a device that can be used to control the spectral phase of ultrashort pulses. It is composed of a sequence of prisms, or gratings, when properly adjusted it can alter the spectral phase φ of the input pulse so that the output pulse is a bandwidth-limited pulse with the shortest possible duration. A pulse shaper can be used to more complicated alterations on both the phase and the amplitude of ultrashort pulses. To accurately control the pulse, a full characterization of the spectral phase is a must in order to get certain pulse spectral phase. Then, a spatial light modulator can be used in the 4f plane to control the pulse, Multiphoton intrapulse interference phase scan is a technique based on this concept
Ultrashort pulse
–
A positively chirped ultrashort pulse of light in the time domain.
6.
Lasing medium
–
The active laser medium is the source of optical gain within a laser. The gain results from the emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source. g. 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 requires an external energy source and 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, upper and lower. Within each sub-level system, the fast transitions ensure that equilibrium is reached quickly. The upper level is assumed to be metastable, also, gain and refractive index are assumed independent of a particular way of excitation. For good performance of the medium, the separation between sub-levels should be larger than working temperature, then, at pump frequency ω p. In the case of amplification of signals, the lasing frequency is called signal frequency. However, the term is used even in the laser oscillators. 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 N1 be concentration of active centers in the ground state. Have N2 be concentration of excited centers, in many cases the gain medium works in a continuous-wave or quasi-continuous regime, causing the time derivatives of populations to be negligible. The steady-state solution can be written, n 2 = W u W u + W d, n 1 = W d W u + W d. The dynamic saturation intensities can be defined, I p o = ℏ ω p τ, I s o = ℏ ω s τ, the absorption at strong signal, A0 = N D σ a s + σ e s. The gain at strong pump, G0 = N D σ a p + σ e p, gain never exceeds value G0, and absorption never exceeds value A0 U. The following identities take place, U − V =1, A / A0 + G / G0 =1, the state of gain medium can be characterized with a single parameter, such as population of the upper level, gain or absorption
Lasing medium
–
Fig.1. Simplified scheme of levels a gain medium.
7.
Sapphire
–
Sapphire is a gemstone, a variety of the mineral corundum, an aluminium oxide. It is typically blue in color, but natural fancy sapphires also occur in yellow, purple, orange, the only color which sapphire cannot be is red - as red colored corundum is called ruby, another corundum variety. This variety in color is due to amounts of elements such as iron, titanium, chromium, copper. Commonly, natural sapphires are cut and polished into gemstones and worn in jewelry and they also may be created synthetically in laboratories for industrial or decorative purposes in large crystal boules. Sapphire is the birthstone for September and the gem of the 45th anniversary, a sapphire jubilee occurs after 65 years. Sapphire is one of the two gem-varieties of corundum, the other being ruby, although blue is the best-known sapphire color, they occur in other colors, including gray and black, and they can be colorless. A a pinkish orange variety of sapphire is called padparadscha, significant sapphire deposits are found in Eastern Australia, Thailand, Sri Lanka, China, Madagascar, East Africa, and in North America in a few locations, mostly in Montana. Sapphire and rubies are often found in the geological setting. Every sapphire mine produces a range of quality - and origin is not a guarantee of quality. For sapphire, Kashmir receives the highest premium although Burma, Sri Lanka, the cost of natural sapphires varies depending on their color, clarity, size, cut, and overall quality. For gems of exceptional quality, an independent determination from a respected laboratory such as the GIA, gemstone color can be described in terms of hue, saturation, and tone. Hue is commonly understood as the color of the gemstone, saturation refers to the vividness or brightness of the hue, and tone is the lightness to darkness of the hue. Blue sapphire exists in various mixtures of its primary and secondary hues, various tonal levels, blue sapphires are evaluated based upon the purity of their primary hue. Purple, violet, and green are the most common secondary hues found in blue sapphires, violet and purple can contribute to the overall beauty of the color, while green is considered to be distinctly negative. Blue sapphires with up to 15% violet or purple are generally said to be of fine quality, gray is the normal saturation modifier or mask found in blue sapphires. Gray reduces the saturation or brightness of the hue, and therefore has a negative effect. The 423-carat Logan sapphire in the National Museum of Natural History, in Washington, sapphires in colors other than blue are called fancy or parti colored sapphires. Fancy sapphires are found in yellow, orange, green sapphires, brown, purple
Sapphire
–
The 423-carat (85 g) blue
Logan Sapphire
Sapphire
–
An uncut, rough yellow sapphire found at the Spokane Sapphire Mine near Helena, Montana
Sapphire
–
Pear-shaped blue sapphire
Sapphire
–
Dark blue sapphire, probably of Australian origin, showing the brilliant surface luster typical of faceted corundum gemstones.
8.
Titanium
–
Titanium is a chemical element with symbol Ti and atomic number 22. It is a transition metal with a silver color, low density. Titanium is resistant to corrosion in sea water, aqua regia, titanium was discovered in Cornwall, Great Britain, by William Gregor in 1791, and it is named by Martin Heinrich Klaproth for the Titans of Greek mythology. The metal is extracted from its principal mineral ores by the Kroll, the most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments. Other compounds include titanium tetrachloride, a component of smoke screens and catalysts, and titanium trichloride, the two most useful properties of the metal are corrosion resistance and strength-to-density ratio, the highest of any metallic element. In its unalloyed condition, titanium is as strong as some steels, there are two allotropic forms and five naturally occurring isotopes of this element, 46Ti through 50Ti, with 48Ti being the most abundant. Although they have the number of valence electrons and are in the same group in the periodic table. As a metal, titanium is recognized for its high strength-to-weight ratio and it is a strong metal with low density that is quite ductile, lustrous, and metallic-white in color. The relatively high melting point makes it useful as a refractory metal and it is paramagnetic and has fairly low electrical and thermal conductivity. Commercial grades of titanium have ultimate tensile strength of about 434 MPa, equal to that of common, low-grade steel alloys, titanium is 60% denser than aluminium, but more than twice as strong as the most commonly used 6061-T6 aluminium alloy. Certain titanium alloys achieve tensile strengths of over 1400 MPa, however, titanium loses strength when heated above 430 °C. Titanium is not as hard as some grades of heat-treated steel, it is non-magnetic, machining requires precautions, because the material might gall unless sharp tools and proper cooling methods are used. Like steel structures, those made from titanium have a limit that guarantees longevity in some applications. The metal is an allotrope of an hexagonal α form that changes into a body-centered cubic β form at 882 °C. The specific heat of the α form increases dramatically as it is heated to this transition temperature but then falls, similar to zirconium and hafnium, an additional omega phase exists, which is thermodynamically stable at high pressures, but metastable at ambient pressures. This phase is usually hexagonal or trigonal and can be considered to be due to a soft longitudinal acoustic phonon of the β phase causing collapse of planes of atoms, like aluminium and magnesium, titanium metal and its alloys oxidize immediately upon exposure to air. Titanium readily reacts with oxygen at 1,200 °C in air and it is, however, slow to react with water and air at ambient temperatures because it forms a passive oxide coating that protects the bulk metal from further oxidation. When it first forms, this layer is only 1–2 nm thick but continues to grow slowly
Titanium
–
Titanium, 22 Ti
Titanium
–
Titanium(III) compounds are characteristically violet, illustrated by this aqueous solution of
titanium trichloride.
Titanium
–
Martin Heinrich Klaproth named titanium for the
Titans of
Greek mythology.
Titanium
–
Titanium sponge, made by the
Kroll process
9.
Laser pumping
–
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, producing excited states in its atoms, 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, 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. To use the lamps energy most efficiently, the lamps and lasing medium are contained in a cavity that will redirect most of the lamps energy into the rod or dye cell. In the most common configuration, the medium is in the form of a rod located at one focus of a mirrored cavity. The flashlamp is a located at the other focus of the ellipse. In other cases an absorber for the longer wavelengths is used, often, 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. Often, the jacket is given a 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 also serves to protect the rod in the event of a violent lamp failure. Smaller ellipses create fewer reflections, giving higher intensity in the center of the rod. For a single flashlamp, if the lamp and rod are equal diameter, the rod and the lamp are relatively long to minimize the effect of losses at the end faces and to provide a sufficient length of gain medium. Longer flashlamps are also efficient at transferring electrical energy into light. However, if the rod is too long in relation to its diameter a condition called prelasing can occur, rod ends are often antireflection coated or cut at Brewsters angle to minimize this effect. Flat mirrors are often used at the ends of the pump cavity to reduce loss. Variations on this use more complex mirrors composed of overlapping elliptical shapes. This allows greater power, but are less efficient because not all of the light is correctly imaged into the rod and these losses can be minimized by using a close-coupled cavity
Laser pumping
–
A ruby laser head. The photo on the left shows the head unassembled, revealing the pumping cavity, the rod and the flashlamps. The photo on the right shows the head assembled.
Laser pumping
–
Various laser pumping cavity configurations.
Laser pumping
–
Laser pumping lamps. The top three are xenon flashlamps while the bottom one is a krypton arc lamp
Laser pumping
–
External triggering was used in this extremely fast discharge. Due to the very high speed, (3.5 microseconds), the current is not only unable to fully heat the xenon and fill the tube, but is still in direct contact with the glass.
10.
Argon
–
Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the table and is a noble gas. Argon is the third-most abundant gas in the Earths atmosphere, at 0. 934% and it is more than twice as abundant as water vapor,23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earths crust, comprising 0. 00015% of the crust, nearly all of the argon in Earths atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earths crust. In the universe, argon-36 is by far the most common argon isotope, the name argon is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning lazy or inactive, as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet in the atomic shell makes argon stable. Its triple point temperature of 83.8058 K is a fixed point in the International Temperature Scale of 1990. Argon is produced industrially by the distillation of liquid air. Argon is also used in incandescent, fluorescent lighting, and other gas-discharge tubes, Argon makes a distinctive blue-green gas laser. Argon is also used in fluorescent glow starters, Argon has approximately the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas, Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperature. Although argon is a gas, it can form some compounds under extreme conditions. Argon fluorohydride, a compound of argon with fluorine and hydrogen that is stable below 17 K, has been demonstrated. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, ions, such as ArH+, and excited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several more argon compounds that should be stable but have not yet been synthesized, Argon was suspected to be a component of air by Henry Cavendish in 1785. Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide, water and they had determined that nitrogen produced from chemical compounds was 0. 5% lighter than nitrogen from the atmosphere. The difference was slight, but it was important enough to attract their attention for many months and they concluded that there was another gas in the air mixed in with the nitrogen. Argon was also encountered in 1882 through independent research of H. F. Newall, each observed new lines in the color spectrum of air that did not match known elements
Argon
–
Argon, 18 Ar
Argon
–
A small piece of rapidly melting solid argon.
Argon
–
Cylinders containing argon gas for use in extinguishing fire without damaging server equipment
Argon
–
A sample of
caesium is packed under argon to avoid reactions with air
11.
Ion laser
–
An ion laser is a gas laser that uses an ionized gas as its lasing medium. Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium, unlike helium–neon lasers, the energy level transitions that contribute to laser action come from ions. A small air-cooled ion laser might produce, for example,130 mW of light with a current of 10 A at 105 V. This is a total power draw over 1 kW, which translates into an amount of heat that must be dissipated. A krypton laser is an ion laser using krypton ions as a gain medium, krypton lasers are used for scientific research, or when krypton is mixed with argon, for creation of white-light lasers, useful for laser light shows. Krypton lasers are used in medicine, for manufacture of security holograms. Krypton lasers emit at several wavelengths through the spectrum, at 406.7 nm,413.1 nm,415.4 nm,468.0 nm,476.2 nm,482.5 nm,520.8 nm,530.9 nm,568.2 nm,647.1 nm,676.4 nm. The argon-ion laser was invented in 1964 by William Bridges at Hughes Aircraft and is one of a family of ion lasers that use a gas as the active medium. Argon-ion lasers are used for retinal phototherapy, lithography, and pumping other lasers, common argon and krypton lasers are capable of emitting continuous-wave output of several milliwatts to tens of watts. Their tubes are made from nickel end bells, kovar metal-to-ceramic seals, beryllium oxide ceramics. The earliest tubes were simple quartz, followed by quartz with graphite disks, in comparison with the helium–neon lasers that require just a few milliamperes, the current used for pumping the krypton laser is several amperes, as the gas has to be ionized. The ion laser tube produces a lot of heat and requires active cooling. The typical noble-gas ion-laser plasma consists of a high-current-density glow discharge in a gas in the presence of a magnetic field. Ar/Kr, A mix of argon and krypton can result in a laser with output wavelengths that appear as white light, helium–cadmium, blue laser emission at 442 nm and ultraviolet at 325 nm. Copper vapor, yellow and green emission at 578 nm and 510 nm, xenon Iodine Oxygen Confocal laser scanning microscopy Surgical. Direct write high density PCB lithography, long coherence length models can be used for holography. Laser List of laser types List of plasma articles
Ion laser
–
From left to right: 1 mW Uniphase
HeNe on alignment-rig, 2 Watt Lexel 88 Argon Ion laser, and power-supply. To the rear are hoses for
water cooling.
Ion laser
–
This argon-ion laser emits blue-green light at 488/514 nm.
12.
Second harmonic generation
–
Second harmonic generation, as an even-order nonlinear optical effect, is only allowed in media without inversion symmetry. It is a case of sum frequency generation and is the inverse of half-harmonic generation. Second harmonic generation was first demonstrated by Peter Franken, A. E. Hill, C. W. Peters, the demonstration was made possible by the invention of the laser, which created the required high intensity coherent light. They focused a ruby laser with a wavelength of 694 nm into a quartz sample and they sent the output light through a spectrometer, recording the spectrum on photographic paper, which indicated the production of light at 347 nm. Famously, when published in the journal Physical Review Letters, the copy editor mistook the dim spot on the paper as a speck of dirt. The formulation of SHG was initially described by N. Bloembergen, in their extensive evaluation of Maxwells equations at the planar interface between a linear and nonlinear medium, several rules for the interaction of light in non-linear mediums were elucidated. It is a case of frequency multiplication. Second harmonic generation occurs in three types, denoted 0, I and II, in Type 0 SHG two photons having extraordinary polarization with respect to the crystal will combine to form a single photon with double the frequency/energy and extraordinary polarization. In Type I SHG two photons having ordinary polarization with respect to the crystal will combine to form one photon with double the frequency, in Type II SHG, two photons having orthogonal polarizations will combine to form one photon with double the frequency and extraordinary polarization. For a given orientation, only one of these types of SHG occurs. In general to utilise Type 0 interactions a quasi-phase-matching crystal type will be required, since media with inversion symmetry are forbidden from generating second harmonic light, surfaces and interfaces make interesting subjects for study with SHG. In fact, second generation and sum frequency generation discriminate against signals from the bulk. In 1982, T. F. Heinz and Y. R. Shen explicitly demonstrated for the first time that SHG could be used as a technique to probe molecular monolayers adsorbed to surfaces. Heinz and Shen adsorbed monolayers of laser dye rhodamine to a fused silica surface. In SHG spectroscopy, one focuses on measuring twice the incident frequency 2ω given an electric field E in order to reveal information about a surface. Early experiments in the field demonstrated second harmonic generation from metal surfaces, eventually, SHG was used to probe the air-water interface, allowing for detailed information about molecular orientation and ordering at one of the most ubiquitous of surfaces. Additionally SHG at planar surfaces has revealed differences in pKa and rotational motions of molecules at interfaces, second harmonic light can also be generated from surfaces that are ‘locally’ planar, but may have inversion symmetry on a larger scale. Specifically, recent theory has demonstrated that SHG from small particles is allowed by proper treatment of Rayleigh scattering
Second harmonic generation
–
Energy level scheme of SHG process.
13.
Modelocking
–
Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds or femtoseconds. The basis of the technique is to induce a fixed-phase relationship between the modes of the lasers resonant cavity. The laser is said to be phase-locked or mode-locked. Interference between these causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of brief duration. Although laser light is perhaps the purest form of light, it is not of a single, all lasers produce light over some natural bandwidth or range of frequencies. For example, a typical helium–neon laser has a bandwidth of about 1.5 GHz. The second factor to determine a lasers emission frequencies is the cavity of the laser. In the simplest case, this consists of two plane mirrors facing each other, surrounding the gain medium of the laser and these standing waves form a discrete set of frequencies, known as the longitudinal modes of the cavity. These modes are the frequencies of light which are self-regenerating and allowed to oscillate by the resonant cavity. In practice, L is usually greater than λ, so the relevant values of q are large. Of more interest is the separation between any two adjacent modes q and q+1, this is given by Δν, Δ ν = c 2 L where c is the speed of light. Using the above equation, a laser with a mirror separation of 30 cm has a frequency separation between longitudinal modes of 0.5 GHz. When more than one mode is excited, the laser is said to be in multi-mode operation. When only one mode is excited, the laser is said to be in single-mode operation. The individual phase of the waves in each mode is not fixed. If instead of oscillating independently, each operates with a fixed phase between it and the other modes, the laser output behaves quite differently. Instead of a random or constant output intensity, the modes of the laser will periodically all constructively interfere with one another, such a laser is said to be mode-locked or phase-locked
Modelocking
–
Laser mode structure
14.
Frequency
–
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as frequency, which emphasizes the contrast to spatial frequency. The period is the duration of time of one cycle in a repeating event, for example, if a newborn babys heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as vibrations, audio signals, radio waves. For cyclical processes, such as rotation, oscillations, or waves, in physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by the Greek letter ν or ν. For a simple motion, the relation between the frequency and the period T is given by f =1 T. The SI unit of frequency is the hertz, named after the German physicist Heinrich Hertz, a previous name for this unit was cycles per second. The SI unit for period is the second, a traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. As a matter of convenience, longer and slower waves, such as ocean surface waves, short and fast waves, like audio and radio, are usually described by their frequency instead of period. Spatial frequency is analogous to temporal frequency, but the axis is replaced by one or more spatial displacement axes. Y = sin = sin d θ d x = k Wavenumber, in the case of more than one spatial dimension, wavenumber is a vector quantity. For periodic waves in nondispersive media, frequency has a relationship to the wavelength. Even in dispersive media, the frequency f of a wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave. In the special case of electromagnetic waves moving through a vacuum, then v = c, where c is the speed of light in a vacuum, and this expression becomes, f = c λ. When waves from a monochrome source travel from one medium to another, their remains the same—only their wavelength. For example, if 71 events occur within 15 seconds the frequency is, the latter method introduces a random error into the count of between zero and one count, so on average half a count. This is called gating error and causes an error in the calculated frequency of Δf = 1/, or a fractional error of Δf / f = 1/ where Tm is the timing interval. This error decreases with frequency, so it is a problem at low frequencies where the number of counts N is small, an older method of measuring the frequency of rotating or vibrating objects is to use a stroboscope
Frequency
–
A resonant-reed frequency meter, an obsolete device used from about 1900 to the 1940s for measuring the frequency of alternating current. It consists of a strip of metal with reeds of graduated lengths, vibrated by an
electromagnet. When the unknown frequency is applied to the electromagnet, the reed which is
resonant at that frequency will vibrate with large amplitude, visible next to the scale.
Frequency
–
As time elapses – represented here as a movement from left to right, i.e. horizontally – the five
sinusoidal waves shown vary regularly (i.e. cycle), but at different
rates. The red
wave (top) has the lowest frequency (i.e. varies at the slowest rate) while the purple wave (bottom) has the highest frequency (varies at the fastest rate).
Frequency
Frequency
–
Modern frequency counter
15.
Chirped pulse amplification
–
Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification. CPA is the current state of the art technique which all of the highest power lasers in the world currently utilize, apart from these state-of-the-art research systems, a number of commercial manufacturers sell Ti, sapphire-based CPAs with peak powers of 10 to 100 gigawatts. Chirped-pulse amplification was introduced as a technique to increase the available power in radar in 1960. CPA for lasers was invented by Gérard Mourou and Donna Strickland at the University of Rochester in the mid 1980s, in addition to the higher peak power, CPA makes it possible to miniaturize laser systems. A compact high-power laser, known as a tabletop terawatt laser, there are several ways to construct compressors and stretchers. The most practical way to achieve this is with grating-based stretchers and compressors, stretchers and compressors are characterized by their dispersion. With negative dispersion, light with higher frequencies takes less time to travel through the device than light with lower frequencies, with positive dispersion, it is the other way around. In a CPA, the dispersions of the stretcher and compressor should cancel out, because of practical considerations, the stretcher is usually designed with positive dispersion and the compressor with negative dispersion. In principle, the dispersion of a device is a function τ. Each component in the chain from the seed laser to the output of the compressor contributes to the dispersion. It turns out to be hard to tune the dispersions of the stretcher, for this, additional dispersive elements may be needed. Figure 1 shows the simplest grating configuration, where long-wavelength components travel a distance than the short-wavelength components. Often, only a single grating is used, with extra mirrors such that the hits the grating four times rather than two times as shown in the picture. This setup is used as a compressor, since it does not involve components that could lead to unwanted side-effects when dealing with high-intensity pulses. The dispersion can be tuned easily by changing the distance between the two gratings, Figure 2 shows a more complicated grating configuration that involves focusing elements, here depicted as lenses. The lenses are placed at a distance 2 f from each other, if L < f, the setup acts as a positive-dispersion stretcher and if L > f, it is a negative-dispersion stretcher. And the case L = f is used in the pulse shaper, usually, the focusing element is a spherical or cylindrical mirror rather than a lens. As with the configuration in Figure 1, it is possible to use an additional mirror and this setup requires that the beam diameter is very small compared to the length of the telescope, otherwise undesirable aberrations will be introduced
Chirped pulse amplification
–
Figure 1. Schematic layout of a grating-based compressor with negative dispersion, i.e. the short wavelengths (in blue) come out first.
16.
Joule
–
The joule, symbol J, is a derived unit of energy in the International System of Units. It is equal to the transferred to an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre. It is also the energy dissipated as heat when a current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule, one joule can also be defined as, The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb volt. This relationship can be used to define the volt, the work required to produce one watt of power for one second, or one watt second. This relationship can be used to define the watt and this SI unit is named after James Prescott Joule. As with every International System of Units unit named for a person, note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, section 5.2. The CGPM has given the unit of energy the name Joule, the use of newton metres for torque and joules for energy is helpful to avoid misunderstandings and miscommunications. The distinction may be also in the fact that energy is a scalar – the dot product of a vector force. By contrast, torque is a vector – the cross product of a distance vector, torque and energy are related to one another by the equation E = τ θ, where E is energy, τ is torque, and θ is the angle swept. Since radians are dimensionless, it follows that torque and energy have the same dimensions, one joule in everyday life represents approximately, The energy required to lift a medium-size tomato 1 m vertically from the surface of the Earth. The energy released when that same tomato falls back down to the ground, the energy required to accelerate a 1 kg mass at 1 m·s−2 through a 1 m distance in space. The heat required to raise the temperature of 1 g of water by 0.24 °C, the typical energy released as heat by a person at rest every 1/60 s. The kinetic energy of a 50 kg human moving very slowly, the kinetic energy of a 56 g tennis ball moving at 6 m/s. The kinetic energy of an object with mass 1 kg moving at √2 ≈1.4 m/s, the amount of electricity required to light a 1 W LED for 1 s. Since the joule is also a watt-second and the unit for electricity sales to homes is the kW·h. For additional examples, see, Orders of magnitude The zeptojoule is equal to one sextillionth of one joule,160 zeptojoules is equivalent to one electronvolt. The nanojoule is equal to one billionth of one joule, one nanojoule is about 1/160 of the kinetic energy of a flying mosquito
Joule
–
Base units
17.
Hertz
–
The hertz is the unit of frequency in the International System of Units and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide proof of the existence of electromagnetic waves. Hertz are commonly expressed in SI multiples kilohertz, megahertz, gigahertz, kilo means thousand, mega meaning million, giga meaning billion and tera for trillion. Some of the units most common uses are in the description of waves and musical tones, particularly those used in radio-. It is also used to describe the speeds at which computers, the hertz is equivalent to cycles per second, i. e. 1/second or s −1. In English, hertz is also used as the plural form, as an SI unit, Hz can be prefixed, commonly used multiples are kHz, MHz, GHz and THz. One hertz simply means one cycle per second,100 Hz means one hundred cycles per second, and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at 1 Hz, the rate of aperiodic or stochastic events occur is expressed in reciprocal second or inverse second in general or, the specific case of radioactive decay, becquerels. Whereas 1 Hz is 1 cycle per second,1 Bq is 1 aperiodic radionuclide event per second, the conversion between a frequency f measured in hertz and an angular velocity ω measured in radians per second is ω =2 π f and f = ω2 π. This SI unit is named after Heinrich Hertz, as with every International System of Units unit named for a person, the first letter of its symbol is upper case. Note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, the hertz is named after the German physicist Heinrich Hertz, who made important scientific contributions to the study of electromagnetism. The name was established by the International Electrotechnical Commission in 1930, the term cycles per second was largely replaced by hertz by the 1970s. One hobby magazine, Electronics Illustrated, declared their intention to stick with the traditional kc. Mc. etc. units, sound is a traveling longitudinal wave which is an oscillation of pressure. Humans perceive frequency of waves as pitch. Each musical note corresponds to a frequency which can be measured in hertz. An infants ear is able to perceive frequencies ranging from 20 Hz to 20,000 Hz, the range of ultrasound, infrasound and other physical vibrations such as molecular and atomic vibrations extends from a few femtoHz into the terahertz range and beyond. Electromagnetic radiation is described by its frequency—the number of oscillations of the perpendicular electric and magnetic fields per second—expressed in hertz. Radio frequency radiation is measured in kilohertz, megahertz, or gigahertz
Hertz
–
Details of a
heartbeat as an example of a non-
sinusoidal periodic phenomenon that can be described in terms of hertz. Two complete cycles are illustrated.
Hertz
–
A
sine wave with varying frequency
18.
Laser construction
–
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
Laser construction
–
Schematic diagram of a typical laser, showing the three major parts
19.
Chirp
–
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal and it is commonly used in sonar and radar, but has other applications, such as in spread-spectrum communications. In spread-spectrum usage, surface acoustic wave devices such as reflective array compressors are used to generate and demodulate the chirped signals. The name is a reference to the sound made by birds. K = f 1 − f 0 T, where f 1 is the final frequency, T is the time it takes to sweep from f 0 to f 1. Thus this is also called quadratic-phase signal. In other words, if two points in the waveform are chosen, t 1 and t 2, and the interval between them t 2 − t 1 is kept constant, the frequency ratio f / f will also be constant. In an exponential chirp, the frequency of the signal varies exponentially as a function of time, f = f 0 k t where f 0 is the starting frequency, and k is the rate of exponential change in frequency. Unlike the linear chirp, which has a constant chirpyness, a chirp has an exponentially increasing frequency rate. A chirp signal can be generated with analog circuitry via an oscillator. It can also be generated digitally by a signal processor and digital to analog converter, using a direct digital synthesizer. It can also be generated by a YIG oscillator, a chirp signal shares the same spectral content with an impulse signal. However, unlike in the signal, spectral components of the chirp signal have different phases, i. e. their power spectra are alike. Dispersion of a propagation medium may result in unintentional conversion of impulse signals into chirps. On the other hand, many applications, such as chirped pulse amplifiers or echolocation systems. Chirp modulation, or linear frequency modulation for digital communication, was patented by Sidney Darlington in 1954 with significant later work performed by Winkler in 1962 and this type of modulation employs sinusoidal waveforms whose instantaneous frequency increases or decreases linearly over time. These waveforms are commonly referred to as linear chirps or simply chirps, hence the rate at which their frequency changes is called the chirp rate. In binary chirp modulation, binary data is transmitted by mapping the bits into chirps of opposite chirp rates, for instance, over one bit period 1 is assigned a chirp with positive rate a and 0 a chirp with negative rate −a
Chirp
–
Spectrogram of a Linear Chirp. The Spectrogram plot demonstrates the linear rate of change in frequency as a function of time, in this case from 0 to 7 kHz repeating every 2.3 seconds. The intensity of the plot is proportional to the energy content in the signal at the indicated frequency and time.
Chirp
–
A linear chirp waveform; a sinusoidal wave that increases in frequency linearly over time
Chirp
–
Spectrogram of an exponential chirp. The exponential rate of change of frequency is shown as a function of time, in this case from nearly 0 up to 8 kHz repeating every second. Also visible in this Spectrogram is a frequency fallback to 6 kHz after peaking, likely an artifact of the specific method employed to generate the waveform.
20.
Nonlinear optics
–
The nonlinearity is typically observed only at very high light intensities such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear, in nonlinear optics, the superposition principle no longer holds. However, some effects were discovered before the development of the laser. The theoretical basis for many nonlinear processes were first described in Bloembergens monograph Nonlinear Optics, Nonlinear optics explains nonlinear response of properties such as frequency, polarization, phase or path of incident light. Third-harmonic generation, generation of light with a frequency, three photons are destroyed, creating a single photon at three times the frequency. High-harmonic generation, generation of light with much greater than the original. Sum-frequency generation, generation of light with a frequency that is the sum of two other frequencies, difference-frequency generation, generation of light with a frequency that is the difference between two other frequencies. Optical parametric amplification, amplification of an input in the presence of a higher-frequency pump wave. Optical parametric oscillation, generation of a signal and idler wave using an amplifier in a resonator. Optical parametric generation, like parametric oscillation but without a resonator, spontaneous parametric down-conversion, the amplification of the vacuum fluctuations in the low-gain regime. Optical rectification, generation of electric fields. Nonlinear light-matter interaction with electrons and plasmas. Optical Kerr effect, intensity-dependent refractive index, self-focusing, an effect due to the optical Kerr effect caused by the spatial variation in the intensity creating a spatial variation in the refractive index. Kerr-lens modelocking, the use of self-focusing as a mechanism to mode-lock laser, self-phase modulation, an effect due to the optical Kerr effect caused by the temporal variation in the intensity creating a temporal variation in the refractive index. Optical solitons, a solution for either an optical pulse or spatial mode that does not change during propagation due to a balance between dispersion and the Kerr effect. Cross-phase modulation, where one wavelength of light can affect the phase of another wavelength of light through the optical Kerr effect, four-wave mixing, can also arise from other nonlinearities. Cross-polarized wave generation, a χ effect in which a wave with polarization vector perpendicular to the one is generated. Stimulated Brillouin scattering, interaction of photons with acoustic phonons Multi-photon absorption, multiple photoionisation, near-simultaneous removal of many bound electrons by one photon
Nonlinear optics
–
Reversal of Linear Momentum and Angular Momentum in Phase Conjugating Mirror.
Nonlinear optics
Nonlinear optics
–
Dark-Red Gallium Selenide in its bulk form.
21.
Continuous wave laser
–
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
Continuous wave laser
–
United States Air Force laser experiment
Continuous wave laser
–
Red (660 & 635 nm), green (532 & 520 nm) and blue-violet (445 & 405 nm) lasers
Continuous wave laser
–
Laser beams in fog, reflected on a car windshield
Continuous wave laser
22.
Linewidth
–
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
Linewidth
–
Continuous spectrum of a
incandescent lamp (mid) and discrete spectrum lines of a
fluorescent lamp (bottom)
Linewidth
–
Continuous spectrum
23.
ArXiv
–
In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository. Begun on August 14,1991, arXiv. org passed the half-million article milestone on October 3,2008, by 2014 the submission rate had grown to more than 8,000 per month. The arXiv was made possible by the low-bandwidth TeX file format, around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Additional modes of access were added, FTP in 1991, Gopher in 1992. The term e-print was quickly adopted to describe the articles and its original domain name was xxx. lanl. gov. Due to LANLs lack of interest in the rapidly expanding technology, in 1999 Ginsparg changed institutions to Cornell University and it is now hosted principally by Cornell, with 8 mirrors around the world. Its existence was one of the factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists regularly upload their papers to arXiv. org for worldwide access, Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv. The annual budget for arXiv is approximately $826,000 for 2013 to 2017, funded jointly by Cornell University Library, annual donations were envisaged to vary in size between $2,300 to $4,000, based on each institution’s usage. As of 14 January 2014,174 institutions have pledged support for the period 2013–2017 on this basis, in September 2011, Cornell University Library took overall administrative and financial responsibility for arXivs operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it was supposed to be a three-hour tour, however, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. The lists of moderators for many sections of the arXiv are publicly available, additionally, an endorsement system was introduced in 2004 as part of an effort to ensure content that is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, new authors from recognized academic institutions generally receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for allegedly restricting scientific inquiry, perelman appears content to forgo the traditional peer-reviewed journal process, stating, If anybody is interested in my way of solving the problem, its all there – let them go and read about it. The arXiv generally re-classifies these works, e. g. in General mathematics, papers can be submitted in any of several formats, including LaTeX, and PDF printed from a word processor other than TeX or LaTeX. The submission is rejected by the software if generating the final PDF file fails, if any image file is too large. ArXiv now allows one to store and modify an incomplete submission, the time stamp on the article is set when the submission is finalized
ArXiv
–
arXiv
ArXiv
–
A screenshot of the arXiv taken in 1994, using the browser
NCSA Mosaic. At the time,
HTML forms were a new technology.
24.
Semiconductor laser
–
A laser diode is electrically a PIN diode. The active region of the diode is in the intrinsic region. Unlike a regular diode, the goal for a diode is to recombine all carriers in the I region. Thus, laser diodes are fabricated using direct bandgap semiconductors, the active layer most often consists of quantum wells, which provide lower threshold current and higher efficiency. Laser diodes form a subset of the classification of semiconductor p-n junction diodes. Forward electrical bias across the laser diode causes the two species of charge carrier – holes and electrons – to be injected from opposite sides of the p-n junction into the depletion region, holes are injected from the p-doped, and electrons from the n-doped, semiconductor. Due to the use of injection in powering most diode lasers. As diode lasers are semiconductor devices, they may also be classified as semiconductor lasers, either designation distinguishes diode lasers from solid-state lasers. Another method of powering some diode lasers is the use of optical pumping, optically pumped semiconductor lasers use a III-V semiconductor chip as the gain medium, and another laser as the pump source. OPSL offer several advantages over ILDs, particularly in wavelength selection, spontaneous emission below the lasing threshold produces similar properties to an LED. Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating and these photon-emitting semiconductors are the so-called direct bandgap semiconductors. The properties of silicon and germanium, which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered direct, the transition between the materials in the alternating pattern creates the critical direct bandgap property. Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples of compound semiconductor materials that can be used to create junction diodes that emit light, a nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This generates another photon of the frequency, polarization, and phase. This means that stimulated emission will cause gain in a wave in the injection region. As in other lasers, the region is surrounded with an optical cavity to form a laser. In the simplest form of diode, an optical waveguide is made on that crystals surface. The two ends of the crystal are cleaved to form smooth, parallel edges, forming a Fabry–Pérot resonator
Semiconductor laser
–
Top: a packaged laser diode shown with a
penny for scale. Bottom: the laser diode chip is removed from the above package and placed on the eye of an orphan needle for scale.
Semiconductor laser
Semiconductor laser
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A laser diode with the case cut away. The actual laser diode chip is the small black chip at the front; a photodiode at the back is used to control output power.
Semiconductor laser
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Semi-conductor lasers (660 nm, 635 nm, 532 nm, 520 nm, 445 nm, 405 nm)
25.
Yttrium aluminium garnet
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Yttrium aluminium garnet is a synthetic crystalline material of the garnet group. It is also one of three phases of the composite, the other two being yttrium aluminium monoclinic and yttrium aluminium perovskite. YAG, like garnet and sapphire, has no uses as a laser medium when pure, however, after being doped with an appropriate ion, YAG is commonly used as a host material in various solid-state lasers. Rare earth elements such as neodymium and erbium can be doped into YAG as active laser ions, yielding Nd, YAG and Er, YAG lasers, cerium-doped YAG is used as a phosphor in cathode ray tubes and white light-emitting diodes, and as a scintillator. YAG for a period was used in jewelry as a diamond, colored variants and their doping elements include, green, blue, red, yellow, purple, pink, and orange. As faceted gems they are valued for their clarity, durability, high refractive index, the critical angle of YAG is 33 degrees. YAG cuts similar to natural garnet, with polishing being performed with alumina or diamond on common polishing laps, as a synthetic gemstone YAG has numerous varietal and trade names, as well as a number of misnomers. Production for the gem trade decreased after the introduction of synthetic cubic zirconia, some demand exists as synthetic garnet, and for designs where the very high refractive index of cubic zirconia is not desirable. Neodymium-doped YAG was developed in the early 1960s, and the first working Nd, the best absorption band of Nd, YAG for pumping the laser is centered at 807.5 nm, and is 1 nm wide. Most Nd, YAG lasers produce infrared light at a wavelength of 1064 nm, light at this wavelength is rather dangerous to vision, since it can be focused by the eyes lens onto the retina, but the light is invisible and does not trigger the blink reflex. Nd, YAG lasers can also be used with frequency doubling or frequency tripling crystals, to green light with a wavelength of 532 nm or ultraviolet light at 355 nm. The dopant concentration in commonly used Nd, YAG crystals usually varies between 0.5 and 1.4 molar percent, higher dopant concentration is used for pulsed lasers, lower concentration is suitable for continuous-wave lasers. Nd, YAG is pinkish-purple, with lighter-doped rods being less colored than heavier-doped ones. Since its absorption spectrum is narrow, the hue depends on the light under which it is observed, YAG doped with neodymium and chromium has absorption characteristics which are superior to Nd, YAG. This is because energy is absorbed by the absorption bands of the Cr3+ dopant. This material has been suggested for use in solar-pumped lasers, which could form part of a solar power satellite system, erbium-doped YAG is an active laser medium lasing at 2940 nm. Its absorption bands suitable for pumping are wide and located between 600 and 800 nm, allowing for efficient flashlamp pumping, the dopant concentration used is high, about 50% of the yttrium atoms are replaced. Er, YAG is used for monitoring of blood sugar
Yttrium aluminium garnet
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Nd:YAG laser rod 0.5 cm in diameter.
26.
Neodymium
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Neodymium is a chemical element with symbol Nd and atomic number 60. It is a silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach and it is present in significant quantities in the ore minerals monazite and bastnäsite. Neodymium is not found naturally in form or unmixed with other lanthanides. Although neodymium is classed as an earth, it is a fairly common element, no rarer than cobalt, nickel, and copper. Most of the worlds commercial neodymium is mined in China, neodymium compounds were first commercially used as glass dyes in 1927, and they remain a popular additive in glasses. Some neodymium-doped glasses are used in lasers that emit infrared with wavelengths between 1047 and 1062 nanometers. These have been used in applications, such as experiments in inertial confinement fusion. Neodymium is also used various other substrate crystals, such as yttrium aluminum garnet in the Nd. This laser usually emits infrared at a wavelength of about 1064 nanometers, the Nd, YAG laser is one of the most commonly used solid-state lasers. Another important use of neodymium is as a component in the used to make high-strength neodymium magnets—powerful permanent magnets. Larger neodymium magnets are used in electric motors and generators. Neodymium, a rare metal, was present in the classical mischmetal at a concentration of about 18%. Metallic neodymium has a bright, silvery luster, but as one of the more reactive lanthanide rare-earth metals. The oxide layer forms then peels off, exposing the metal to further oxidation, thus, a centimeter-sized sample of neodymium completely oxidizes within a year. Neodymium commonly exists in two forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at about 863 °C. Naturally occurring neodymium is a mixture of five stable isotopes, 142Nd, 143Nd, 145Nd, 146Nd and 148Nd, with 142Nd being the most abundant, and two radioisotopes, 144Nd and 150Nd. In all,31 radioisotopes of neodymium have been detected as of 2010, with the most stable radioisotopes being the naturally occurring ones, 144Nd and 150Nd
Neodymium
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Neodymium, 60 Nd
Neodymium
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Neodymium(III)-sulfate
Neodymium
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Neodymium compounds in
fluorescent tube light—from left to right, the sulfate, nitrate, and chloride
Neodymium
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Neodymium compounds in
CFL light
27.
Gain media
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The active laser medium is the source of optical gain within a laser. The gain results from the emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source. g. 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 requires an external energy source and 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, upper and lower. Within each sub-level system, the fast transitions ensure that equilibrium is reached quickly. The upper level is assumed to be metastable, also, gain and refractive index are assumed independent of a particular way of excitation. For good performance of the medium, the separation between sub-levels should be larger than working temperature, then, at pump frequency ω p. In the case of amplification of signals, the lasing frequency is called signal frequency. However, the term is used even in the laser oscillators. 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 N1 be concentration of active centers in the ground state. Have N2 be concentration of excited centers, in many cases the gain medium works in a continuous-wave or quasi-continuous regime, causing the time derivatives of populations to be negligible. The steady-state solution can be written, n 2 = W u W u + W d, n 1 = W d W u + W d. The dynamic saturation intensities can be defined, I p o = ℏ ω p τ, I s o = ℏ ω s τ, the absorption at strong signal, A0 = N D σ a s + σ e s. The gain at strong pump, G0 = N D σ a p + σ e p, gain never exceeds value G0, and absorption never exceeds value A0 U. The following identities take place, U − V =1, A / A0 + G / G0 =1, the state of gain medium can be characterized with a single parameter, such as population of the upper level, gain or absorption
Gain media
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Fig.1. Simplified scheme of levels a gain medium.
28.
Ruby laser
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A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a laser made by Theodore H. Ted Maiman at Hughes Research Laboratories on May 16,1960. Ruby lasers produce pulses of coherent visible light at a wavelength of 694.3 nm, typical ruby laser pulse lengths are on the order of a millisecond. A ruby laser most often consists of a rod that must be pumped with very high energy, usually from a flashtube. The rod is placed between two mirrors, forming an optical cavity, which oscillate the light produced by the rubys fluorescence. Ruby is one of the few solid state lasers that produce light in the range of the spectrum, lasing at 694.3 nanometers, in a deep red color. The ruby laser is a three level solid state laser, the active laser medium is a synthetic ruby rod that is energized through optical pumping, typically by a xenon flashtube. Ruby has very broad and powerful absorption bands in the spectrum, at 400 and 550 nm. This allows for high energy pumping, since the pulse duration can be much longer than with other materials. While ruby has a very wide absorption profile, its efficiency is much lower than other mediums. The finely polished ends of the rod were silvered, one end completely, the rod, with its reflective ends, then acts as a Fabry–Pérot etalon. Modern lasers often use rods with antireflection coatings, or with the ends cut and this eliminates the reflections from the ends of the rod. External dielectric mirrors then are used to form the optical cavity, curved mirrors are typically used to relax the alignment tolerances and to form a stable resonator, often compensating for thermal lensing of the rod. Ruby also absorbs some of the light at its lasing wavelength, to overcome this absorption, the entire length of the rod needs to be pumped, leaving no shaded areas near the mountings. The active part of the ruby is the dopant, which consists of chromium ions suspended in a sapphire crystal. The dopant often comprises around 0. 05% of the crystal, depending on the concentration of the dopant, synthetic ruby usually comes in either pink or red. One of the first applications for the laser was in rangefinding. By 1964, ruby lasers with rotating prism q-switches became the standard for military rangefinders, until the introduction of more efficient Nd, ruby lasers were used mainly in research
Ruby laser
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Diagram of the first ruby laser.
Ruby laser
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A ruby laser rod. Inset: The view through the rod is crystal clear
Ruby laser
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Maiman's original ruby laser
Ruby laser
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Components of original ruby laser
29.
Yttrium iron garnet
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Yttrium iron garnet is a kind of synthetic garnet, with chemical composition Y3Fe23, or Y3Fe5O12. In YIG, the five iron ions occupy two octahedral and three sites, with the yttrium ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behavior, by substituting specific sites with rare earth elements, for example, interesting magnetic properties can be obtained. These properties make it useful for MOI applications in superconductors, YIG is used in microwave, acoustic, optical, and magneto-optical applications, e. g. microwave YIG filters, or acoustic transmitters and transducers. It is transparent for light wavelengths over 600 nm and it also finds use in solid-state lasers in Faraday rotators, in data storage, and in various nonlinear optics applications. Gadolinium gallium garnet Terbium gallium garnet Yttrium aluminium garnet Yttrium iron garnet filter YIG sphere
Yttrium iron garnet
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Yttrium iron garnet
. doi:10.1007/s00340-014-5846-6.
