ALL LASER USES:LPKF:LASIK:LASER HAIR REMOVEL AND LASER DIODES: November 2009

LASERDIS TECHNICAL INFORMATION

Technical information

The standard home video laserdisc is 30 cm (11.81 inches) in diameter and made up of two single-sided aluminum discs layered in plastic. Although appearing similar to compact discs or DVDs, Laserdiscs use analog video stored in the composite domain with analog sound and/or some form of digital audio. However, despite its analog nature, the Laserdisc at its most fundamental level is still recorded as a series of pits and lands much like DVDs and CDs are today.[10] The first Laserdiscs featured in 1978 were entirely analog but the format evolved to incorporate digital stereo sound in CD format (sometimes with a TOSlink or coax output to feed an external DAC), and later multi-channel formats such as Dolby Digital and DTS.
Since digital encoding and compression schemes were either unavailable or impractical in 1978, three encoding formats based on the rotation speed were used:
CAV (Constant Angular Velocity) or Standard Play discs supported several unique features such as freeze frame, variable slow motion and reverse. CAV discs were spun at a constant rotational speed during playback, with one video frame read per revolution and in this mode, 54,000 individual frames or 30 minutes of audio/video could be stored on a single side of a CAV disc. Another unique attribute to CAV was to reduce the visibility of crosstalk from adjacent tracks, since on CAV discs any crosstalk at a specific point in a frame is simply from the same point in the next or previous frame. CAV was used less frequently than CLV, reserved for special editions of feature films to highlight bonus material and special effects. One of the most intriguing advantages of this format was the ability to reference every frame of a film directly by number—a feature of particular interest to film buffs, students and others intrigued by the study of errors in staging, continuity, etc.
CLV (Constant Linear Velocity) or Extended Play discs do not have the "trick play" features of CAV, offering only simple playback on all but the high-end Laserdisc players incorporating a digital frame store. These high-end Laserdisc players could add features not normally available to CLV discs such as variable forward and reverse, and a VCR-like "pause". CLV encoded discs could store 60 minutes of audio/video per side, or 2 hours per disc. For films with a run–time less than 120 minutes, this meant they could fit on a single disc, lowering the cost of the title and eliminating the distracting exercise of "getting up to change the disc"—at least for those who owned a dual-sided player. The vast majority of titles were only available in CLV. (A few titles were released partly CLV, partly CAV. For example, a 140-minute movie could fit on two CLV sides, and one CAV side, thus allowing for the CAV-only features during the climax of the film.)
CAA (Constant Angular Acceleration). In the early 1980s, due to problems with crosstalk distortion on CLV extended play Laserdiscs, Pioneer Video introduced CAA formatting for extended play discs. Constant Angular Acceleration is very similar to Constant Linear Velocity save for the fact that CAA varies the angular rotation of the disc in controlled steps instead of gradually slowing down in a steady linear pace as a CLV disc is read. With the exception of 3M/Imation, all Laserdisc manufacturers adopted the CAA encoding scheme, although the term was rarely (if ever) used on any consumer packaging.
As Pioneer introduced Digital Audio to Laserdisc in 1985, they further refined the CAA format. CAA55 was introduced in 1985 with a total playback capacity of 55 minutes 5 seconds, and was necessary to resolve technical issues with the inclusion of Digital Audio. Several titles released between 1985 and 1987 were analog audio only due to the length of the title and the desire to keep the film on 1 disc (e.g., "Back to the Future"). By 1987, Pioneer had overcome the technical challenges and was able to once again encode in CAA60—allowing a total of 60 minutes, 5 seconds. Pioneer further refined CAA, offering CAA45—encoding 45 minutes of material, but filling the entire playback surface of the side. Used on only a handful of titles, CAA65 offered 65 minutes 5 seconds of playback time. The final variant of CAA is CAA70, which could accommodate 70 minutes of playback time. There are not any known uses of this format on the consumer market.

History OF LASERDISC

Laserdisc technology, using a transparent disc,[1] was invented by David Paul Gregg in 1958 (and patented in 1961 and 1990).[2][3] By 1969 Philips had developed a videodisc in reflective mode, which has great advantages over the transparent mode. MCA and Philips decided to join their efforts. They first publicly demonstrated the videodisc in 1972. Laserdisc was first available on the market, in Atlanta, on December 15, 1978, two years after the VHS VCR and four years before the CD, which is based on Laserdisc technology. Philips produced the players and MCA the discs. The Philips/MCA cooperation was not successful, and discontinued after a few years. Several of the scientists responsible for the early research (Richard Wilkinson, Ray Dakin and John Winslow) founded Optical Disc Corporation (now ODC Nimbus).
In 1979, the Museum of Science and Industry in Chicago opened their "Newspaper" exhibit which used interactive Laserdiscs to allow visitors to search for the front page of any Chicago Tribune newspaper. This was a very early use of digitally interactive technology in museums and could even be among the first.[citation needed]
The first Laserdisc title marketed in North America was the MCA DiscoVision release of Jaws in 1978. The last two titles released in North America were Paramount's Sleepy Hollow and Bringing Out the Dead in 2000. The last Hong Kong-released Laserdisc format-movie was Tokyo Raiders. A dozen or so more titles continued to be released in Japan, until the end of 2001. Production of Laserdisc players continued until January 14, 2009 when Pioneer stopped making them.[4][5][6]
It was estimated that in 1998, Laserdisc players were in approximately 2% of US households (roughly two million).[7] By comparison, in 1999, players were in 10% of Japanese households.[8] Laserdisc was released on June 10, 1981 and a total of 3.6 million Laserdisc players were sold in Japan.[9] A total of 16.8 million Laserdisc players were sold worldwide of which 9.5 million of them were sold by Pioneer.[4][5][6]
Laserdisc has been completely replaced by DVD in the North American retail marketplace, as neither players nor software are now produced there. Laserdisc has retained some popularity among American collectors and, to a greater degree, in Japan, where the format was better supported and more prevalent during its life. In Europe, the Laserdisc has always remained an obscure format. The format was, however, chosen by the British Broadcasting Corporation (BBC) for the BBC Domesday Project in the mid-1980s, a school-based project to commemorate 900 years since the original Domesday Book in England.

Laserdisc

The Laserdisc (LD) is an obsolete home video disc format, and was the first commercial optical disc storage medium. Initially marketed as Discovision in 1978, the technology was licensed and sold as Reflective Optical Videodisc, Laser Videodisc, Laservision, Disco-Vision, DiscoVision, and MCA DiscoVision until Pioneer Electronics purchased the majority stake in the format and marketed LaserDisc in the mid to late 1980s.
While LaserDisc produced a consistently higher quality image than its rivals, the VHS and Betamax systems, the laserdisc never obtained more than a niche market with videophiles in America. In Europe, it remained largely an obscure format. It was, however, much more popular in Japan and in the more affluent regions of South East Asia, such as Hong Kong and Singapore. Laserdisc was the prevalent rental video medium in Hong Kong during the 1990s.
The technology and concepts provided with the Laserdisc would become the forerunner to Compact Discs and DVDs.

X-ray

When X-rays of sufficient frequency (energy) interact with a substance, inner shell electrons in the atom are excited to outer empty orbitals, or they may be removed completely, ionizing the atom. The inner shell "hole" will then be filled by electrons from outer orbitals. The energy available in this de-excitation process is emitted as radiation (fluorescence) or will remove other less-bound electrons from the atom (Auger effect). The absorption or emission frequencies (energies) are characteristic of the specific atom. In addition, for a specific atom small frequency (energy) variations occur which are characteristic of the chemical bonding. With a suitable apparatus, these characteristic X-ray frequencies or Auger electron energies can be measured. X-ray absorption and emission spectroscopy is used in chemistry and material sciences to determine elemental composition and chemical bonding.
X-ray crystallography is a scattering process; crystalline materials scatter X-rays at well-defined angles. If the wavelength of the incident X-rays is known, this allows calculation of the distances between planes of atoms within the crystal. The intensities of the scattered X-rays give information about the atomic positions and allow the arrangement of the atoms within the crystal structure to be calculated.

Spectroscopy

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ). In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise any measurement of a quantity as function of either wavelength or frequency. Thus it also can refer to a response to an alternating field or varying frequency (ν). A further extension of the scope of the definition added energy (E) as a variable, once the very close relationship E = hν for photons was realized (h is the Planck constant). A plot of the response as a function of wavelength—or more commonly frequency—is referred to as a spectrum; see also spectral linewidth.
Spectrometry is the spectroscopic technique used to assess the concentration or amount of a given species. In this case, the instrument that performs such measurements is a spectrometer or spectrograph.
Spectroscopy/spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them.
Spectroscopy/spectrometry is also heavily used in astronomy and remote sensing. Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines.

Semiconductor lasers

Semiconductor lasers are also solid-state lasers but have a different mode of laser operation.
Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm, and wavelengths of over 3 µm have been demonstrated. Low power laser diodes are used in laser printers and CD/DVD players. More powerful laser diodes are frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 10 kW (70dBm), are used in industry for cutting and welding. External-cavity semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses.

A 5.6 mm 'closed can' commercial laser diode, probably from a CD or DVD player.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam than conventional laser diodes, and potentially could be much cheaper to manufacture. As of 2005, only 850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,[19] and 1550 nm devices an area of research. VECSELs are external-cavity VCSELs. Quantum cascade lasers are semiconductor lasers that have an active transition between energy sub-bands of an electron in a structure containing several quantum wells.
The development of a silicon laser is important in the field of optical computing. Silicon is the material of choice for integrated circuits, and so electronic and silicon photonic components (such as optical interconnects) could be fabricated on the same chip. Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which block lasing. However, recently teams have produced silicon lasers through methods such as fabricating the lasing material from silicon and other semiconductor materials, such as indium(III) phosphide or gallium(III) arsenide, materials which allow coherent light to be produced from silicon. These are called hybrid silicon laser. Another type is a Raman laser, which takes advantage of Raman scattering to produce a laser from materials such as silicon.

Fiber-hosted lasers

Solid-state lasers where the light is guided due to the total internal reflection in an optical fiber are called fiber lasers. Guiding of light allows extremely long gain regions providing good cooling conditions; fibers have high surface area to volume ratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce thermal distortion of the beam. Erbium and ytterbium ions are common active species in such lasers.
Quite often, the fiber laser is designed as a double-clad fiber. This type of fiber consists of a fiber core, an inner cladding and an outer cladding. The index of the three concentric layers is chosen so that the fiber core acts as a single-mode fiber for the laser emission while the outer cladding acts as a highly multimode core for the pump laser. This lets the pump propagate a large amount of power into and through the active inner core region, while still having a high numerical aperture (NA) to have easy launching conditions.
Pump light can be used more efficiently by creating a fiber disk laser, or a stack of such lasers.
Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and/or lead to the material destruction of the fiber. This effect is called photodarkening. In bulk laser materials, the cooling is not so efficient, and it is difficult to separate the effects of photodarkening from the thermal effects, but the experiments in fibers show that the photodarkening can be attributed to the formation of long-living color centers

LASER MODE OF OPERATIONS

Modes of operation
The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.
Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds (10-15 s).
Continuous wave operation
In the continuous wave (CW) mode of operation, the output of a laser is relatively constant with respect to time. The population inversion required for lasing is continually maintained by a steady pump source.
Pulsed operation
In the pulsed mode of operation, the output of a laser varies with respect to time, typically taking the form of alternating 'on' and 'off' periods. In many applications one aims to deposit as much energy as possible at a given place in as short time as possible. In laser ablation for example, a small volume of material at the surface of a work piece might evaporate if it gets the energy required to heat it up far enough in very short time. If, however, the same energy is spread over a longer time, the heat may have time to disperse into the bulk of the piece, and less material evaporates. There are a number of methods to achieve this.
Q-switching
Main article: Q-switching
In a Q-switched laser, the population inversion (usually produced in the same way as CW operation) is allowed to build up by making the cavity conditions (the 'Q') unfavorable for lasing. Then, when the pump energy stored in the laser medium is at the desired level, the 'Q' is adjusted (electro- or acousto-optically) to favourable conditions, releasing the pulse. This results in high peak powers as the average power of the laser (were it running in CW mode) is packed into a shorter time frame.

LASER PHYSICS

LASER DESIGN

A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

LASER TERMINOLOGY

The word laser was originally spelled LASER and is an acronym for light amplification by stimulated emission of radiation. The word light in this phrase is used in the broader sense, referring to electromagnetic radiation of any frequency, not just that in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser.[2]
The back-formed verb to lase means "to produce laser light" or "to apply laser light to."[3] The word "laser" is sometimes used to describe other non-light technologies. For example, a source of atoms in a coherent state is called an "atom laser."

LASER

A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses. More generally, coherent light typically means the source produces light waves that are in step. They have the same frequencies and identical phase[1]. The coherence of typical laser emission is a distinctive characteristic of lasers. Most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Typically, lasers are thought of as emitting light with a narrow wavelength spectrum ("monochromatic" light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously.

Laser printer maintenance

Most consumer and small business laser printers use a toner cartridge that combines the photoreceptor (sometimes called "photoconductor unit" or "imaging drum") with the toner supply bin, the waste toner hopper, and various wiper blades. When the toner supply is consumed, replacing the toner cartridge automatically replaces the imaging drum, waste toner hopper, and wiper blades.
Some laser printers maintain a page count of the number of pages printed since last maintenance. On these models, a reminder message will appear informing the user it is nearing time to replace standard maintenance parts. On other models, no page count is kept or no reminder is displayed, so the user must keep track of pages printed manually or watch for warning signs like paper feed problems and print defects.
Manufacturers usually provide life expectancy charts for common printer parts and consumables. Manufacturers rate life expectancy for their printer parts in terms of "expected page-production life" rather than in units of time.
Consumables and maintenance parts for Business-class printers will generally be rated for a higher page-production expectancy than parts for personal printers. In particular, toner cartridges and fusers usually have a higher page production expectancy in business-class printers than personal-class printers. Color laser printers can require more maintenance and parts replacement than monochrome laser printers since they contain more imaging components.
For rollers and assemblies involved in the paper pickup path and paper feed path, typical maintenance is to vacuum toner and dust from the mechanisms, and replace, clean, or restore the rubber paper-handling rollers. Most pickup, feed, and separation rollers have a rubber coating which eventually suffers wear and becomes covered with slippery paper dust. In cases where replacement rollers are discontinued or unavailable, rubber rollers can be cleaned safely with a damp lint-free rag. Commercial chemical solutions are also available which may help temporarily restore the traction of the rubber.
The fusing assembly (also called a "fuser") is generally considered a replaceable consumable part on laser printers. The fusing assembly is responsible for melting and bonding the toner to the paper. There are many possible defects for fusing assemblies: defects include worn plastic drive gears, electronic failure of heating components, torn fixing film sleeves, worn pressure rollers, toner buildup on heating rollers and pressure rollers, worn or scratched pressure rollers, and damaged paper sensors.
Some manufacturers offer preventative maintenance kits specific to each printer model; such kits generally include a fuser and may also include pickup rollers, feed rollers, transfer rollers, charge rollers, and separation pads.

color laser printers

Color laser printers use colored toner (dry ink), typically cyan, magenta, yellow, and black (CMYK).
While monochrome printers only use one laser scanner assembly, color printers often have two or more scanner assemblies.
Color printing adds complexity to the printing process because very slight misalignments known as registration errors can occur between printing each color, causing unintended color fringing, blurring, or light/dark streaking along the edges of colored regions. To permit a high registration accuracy, some color laser printers use a large rotating belt called a "transfer belt". The transfer belt passes in front of all the toner cartridges and each of the toner layers are precisely applied to the belt. The combined layers are then applied to the paper in a uniform single step.
Color printers usually have a higher "cents-per-page" production cost than monochrome printers.
DPI Resolution

1200 DPI printers are commonly available during 2008.
2400 DPI electrophotographic printing plate makers, essentially laser printers that print on plastic sheets, are also available.

developing of laser printers

How it works

Main article: Xerography
There are typically seven steps involved in the laser printing process:
[edit]Raster image processing

Generating the raster image data
Each horizontal strip of dots across the page is known as a raster line or scan line. Creating the image to be printed is done by a Raster Image Processor (RIP), typically built into the laser printer. The source material may be encoded in any number of special page description languages such as Adobe PostScript (PS) , HP Printer Command Language (PCL), or Microsoft XML Page Specification (XPS) , as well as unformatted text-only data. The RIP uses the page description language to generate a bitmap of the final page in the raster memory. Once the entire page has been rendered in raster memory, the printer is ready to begin the process of sending the rasterized stream of dots to the paper in a continuous stream.
[edit]Charging

Applying a negative charge to the photosensitive drum
A corona wire (in older printers) or a primary charge roller projects an electrostatic charge onto the photoreceptor (otherwise named the photoconductor unit), a revolving photosensitive drum or belt, which is capable of holding an electrostatic charge on its surface while it is in the dark.
An AC bias is applied to the primary charge roller to remove any residual charges left by previous images. The roller will also apply a DC bias on the drum surface to ensure a uniform negative potential. The desired print density is modulated by this DC bias. [4]
Numerous patents describe the photosensitive drum coating as a silicon sandwich with a photocharging layer, a charge leakage barrier layer, as well as a surface layer. One version uses amorphous silicon containing hydrogen as the light receiving layer, Boron nitride as a charge leakage barrier layer, as well as a surface layer of doped silicon, notably silicon with oxygen or nitrogen which at sufficient concentration resembles machining silicon nitride; the effect is that of a light chargeable diode with minimal leakage and a resistance to scuffing.

working of laser printers

history of laser printers

History

Gary Starkweather, inventor of the laser printer, in 2009.
The laser printer was invented at Xerox in 1969 by researcher Gary Starkweather, who had an improved printer working by 1971[1] and incorporated into a fully functional networked printer system by about a year later.[2] The prototype was built by modifying an existing xerographic copier. Starkweather disabled the imaging system and created a spinning drum with 8 mirrored sides, with a laser focused on the drum. Light from the laser would bounce off the spinning drum, sweeping across the page as it traveled through the copier. The hardware was completed in just a week or two, but the computer interface and software took almost 3 months to complete.[citation needed]
The first commercial implementation of a laser printer was the IBM model 3800 in 1976, used for high-volume printing of documents such as invoices and mailing labels. It is often cited as "taking up a whole room," implying that it was a primitive version of the later familiar device used with a personal computer. While large, it was designed for an entirely different purpose. Many 3800s are still in use.[citation needed]
The first laser printer designed for use with an office setting was released as the Xerox Star 8010 in 1981. Although it was innovative, the Star was an expensive ($17,000) system that was purchased by only a relatively small number of businesses and institutions. After personal computers became more widespread, the first laser printer intended for a mass market was the HP LaserJet 8ppm, released in 1984, using a Canon engine controlled by HP software. The HP LaserJet printer was quickly followed by laser printers from Brother Industries, IBM, and others. First-generation machines had large photosensitive drums, of circumference greater than the paper length. Once faster-recovery coatings were developed, the drums could touch the paper multiple times in a pass, and could therefore be smaller in diameter.

As with most electronic devices, the cost of laser printers has fallen markedly over the years. In 1984, the HP LaserJet sold for $3500[3], had trouble with even small, low resolution graphics, and weighed 71 pounds (32 kg). Low end monochrome laser printers often sell for less than $75 as of 2008. These printers tend to lack onboard processing and rely on the host computer to generate a raster image (see Winprinter), but still will outperform the LaserJet Classic in nearly all situations.

laser printers advantage

Laser printers have many significant advantages over other types of printers. Unlike impact printers, laser printer speed can vary widely, and depends on many factors, including the graphic intensity of the job being processed. The fastest models can print over 200 monochrome pages per minute (12,000 pages per hour). The fastest color laser printers can print over 100 pages per minute (6000 pages per hour). Very high-speed laser printers are used for mass mailings of personalized documents, such as credit card or utility bills, and are competing with lithography in some commercial applications.
The cost of this technology depends on a combination of factors, including the cost of paper, toner, and infrequent drum replacement, as well as the replacement of other consumables such as the fuser assembly and transfer assembly. Often printers with soft plastic drums can have a very high cost of ownership that does not become apparent until the drum requires replacement.
A duplexing printer (one that prints on both sides of the paper) can halve paper costs and reduce filing volumes. Formerly only available on high-end printers, duplexers are now common on mid-range office printers, though not all printers can accommodate a duplexing unit. Duplexing can also give a slower page-printing speed, because of the longer paper path.
In comparison with the laser printer, most inkjet printers and dot-matrix printers simply take an incoming stream of data and directly imprint it in a slow lurching process that may include pauses as the printer waits for more data. A laser printer is unable to work this way because such a large amount of data needs to output to the printing device in a rapid, continuous process. The printer cannot stop the mechanism precisely enough to wait until more data arrives, without creating a visible gap or misalignment of the dots on the printed page.
Instead the image data is built up and stored in a large bank of memory capable of representing every dot on the page. The requirement to store all dots in memory before printing has traditionally limited laser printers to small fixed paper sizes such as letter or A4. Most laser printers are unable to print continuous banners spanning a sheet of paper two meters long, because there is not enough memory available in the printer to store such a large image before printing begins.

laser printers

A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and multifunction printers (MFPs), laser printers employ a xerographic printing process but differ from analog photocopiers in that the image is produced by the direct scanning of a laser beam across the printer's photoreceptor.
Overview

A laser beam projects an image of the page to be printed onto an electrically charged rotating drum coated with selenium. Photoconductivity removes charge from the areas exposed to light. Dry ink (toner) particles are then electrostatically picked up by the drum's charged areas. The drum then prints the image onto paper by direct contact and heat, which fuses the ink to the paper.

laser Fictional predictions

For lasers in fiction, see also the ray gun.
Before stimulated emission was discovered, novelists used to describe machines that we can identify as "lasers".
A laser-like device was described in Alexey Tolstoy's sci-fi novel The Hyperboloid of Engineer Garin in 1927.
Mikhail Bulgakov exaggerated the biological effect (laser bio stimulation) of intensive red light in his sci-fi novel Fatal Eggs (1925), without any reasonable description of the source of this red light. (In that novel, the red light first appears occasionally from the illuminating system of an advanced microscope; then the protagonist Prof. Persikov arranges the special set-up for generation of the red light.)

Lasers as weapons

Laser beams are famously employed as weapon systems in science fiction, but actual laser weapons are only beginning to enter the market. The general idea of laser-beam weaponry is to hit a target with a train of brief pulses of light. The rapid evaporation and expansion of the surface causes shockwaves that damage the target.
The power needed to project a high-powered laser beam of this kind is difficult for current mobile power technology. Public prototypes are chemically-powered gas dynamic lasers.
Lasers of all but the lowest powers can potentially be used as incapacitating weapons, through their ability to produce temporary or permanent vision loss in varying degrees when aimed at the eyes. The degree, character, and duration of vision impairment caused by eye exposure to laser light varies with the power of the laser, the wavelength(s), the collimation of the beam, the exact orientation of the beam, and the duration of exposure. Lasers of even a fraction of a watt in power can produce immediate, permanent vision loss under certain conditions, making such lasers potential non-lethal but incapacitating weapons. The extreme handicap that laser-induced blindness represents makes the use of lasers even as non-lethal weapons morally controversial.
In the field of aviation, the hazards of exposure to ground-based lasers deliberately aimed at pilots have grown to the extent that aviation authorities have special procedures to deal with such hazards.[32]
On March 18, 2009 Northrop Grumman announced that its engineers in Redondo Beach had successfully built and tested an electric laser capable of producing a 100-kilowatt ray of light, powerful enough to destroy an airplane or a tank. An electric laser is theoretically capable, according to Brian Strickland, manager for the United States Army's Joint High Power Solid State Laser program, of being mounted in an aircraft, ship, or vehicle because it requires much less space for its supporting equipment than a chemical laser.[33]
Applications

In manufacturing, lasers are used for cutting, bending, and welding metal and other materials, and for "marking"—producing visible patterns such as letters by changing the properties of a material or by inscribing its surface. In science, lasers are used for many applications. One of the more common is laser spectroscopy, which typically takes advantage of the laser's well-defined wavelength or the possibility of generating very short pulses of light. Lasers are used by the military for range-finding, target designation, and illumination. Lasers have also begun to be tested for directed-energy weapons. Lasers are used in medicine for surgery, diagnostics, and therapeutic applications.

Laser safety

Main article: Laser safety
Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as having a power of one "Gillette" as it could burn through one Gillette razor blade. Today, it is accepted that even low-power lasers with only a few milliwatts of output power can be hazardous to human eyesight, when the beam from such a laser hits the eye directly or after reflection from a shiny surface. At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localized burning and permanent damage in seconds or even less time.
Lasers are usually labeled with a safety class number, which identifies how dangerous the laser is:
Class I/1 is inherently safe, usually because the light is contained in an enclosure, for example in CD players.
Class II/2 is safe during normal use; the blink reflex of the eye will prevent damage. Usually up to 1 mW power, for example laser pointers.
Class IIIa/3R lasers are usually up to 5 mW and involve a small risk of eye damage within the time of the blink reflex. Staring into such a beam for several seconds is likely to cause (minor) eye damage.
Class IIIb/3B can cause immediate severe eye damage upon exposure. Usually lasers up to 500 mW, such as those in CD and DVD writers.
Class IV/4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class.
The indicated powers are for visible-light, continuous-wave lasers. For pulsed lasers and invisible wavelengths, other power limits apply. People working with class 3B and class 4 lasers can protect their eyes with safety goggles which are designed to absorb light of a particular wavelength.
Certain infrared lasers with wavelengths beyond about 1.4 micrometres are often referred to as being "eye-safe". This is because the intrinsic molecular vibrations of water molecules very strongly absorb light in this part of the spectrum, and thus a laser beam at these wavelengths is attenuated so completely as it passes through the eye's cornea that no light remains to be focused by the lens onto the retina. The label "eye-safe" can be misleading, however, as it only applies to relatively low power continuous wave beams and any high power or Q-switched laser at these wavelengths can burn the cornea, causing severe eye damage.

laser Hobby uses

In recent years, some hobbyists have taken interests in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb, although some have made their own class IV types.[30] However, compared to other hobbyists, laser hobbyists are far less common, due to the cost and potential dangers involved. Due to the cost of lasers, some hobbyists use inexpensive means to obtain lasers, such as extracting diodes from DVD burners.[31]
Hobbyists also have been taking surplus pulsed lasers from retired military applications and modifying them for pulsed holography. Pulsed Ruby and Pulsed YAG lasers have been used.

Examples by power

Different uses need lasers with different output powers. Lasers that produce a continuous beam or a series of short pulses can be compared on the basis of their average power. Lasers that produce pulses can also be characterized based on the peak power of each pulse. The peak power of a pulsed laser is many orders of magnitude greater than its average power. The average output power is always less than the power consumed.
The continuous or average power required for some uses:
less than 1 mW – laser pointers
5 mW – CD-ROM drive
5–10 mW – DVD player or DVD-ROM drive
100 mW – High-speed CD-RW burner
250 mW – Consumer DVD-R burner
1 W – green laser in current Holographic Versatile Disc prototype development
1–20 W – output of the majority of commercially available solid-state lasers used for micro machining
30–100 W – typical sealed CO2 surgical lasers[26]
100–3000 W (peak output 1.5 kW) – typical sealed CO2 lasers used in industrial laser cutting
1 kW – Output power expected to be achieved by a prototype 1 cm diode laser bar[27]
Examples of pulsed systems with high peak power:
700 TW (700×1012 W) – National Ignition Facility, a 192-beam, 1.8-megajoule laser system adjoining a 10-meter-diameter target chamber.[28]
1.3 PW (1.3×1015 W) – world's most powerful laser as of 1998, located at the Lawrence Livermore Laboratory[29]

ALL LASER USES

Main article: Laser applications
When lasers were invented in 1960, they were called "a solution looking for a problem".[23] Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military.
The first application of lasers visible in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser, but the compact disc player was the first laser-equipped device to become truly common in consumers' homes, beginning in 1982, followed shortly by laser printers.
Some of the other applications include:
Medicine: Bloodless surgery, laser healing, surgical treatment, kidney stone treatment, eye treatment, dentistry
Industry: Cutting, welding, material heat treatment, marking parts
Defense: Marking targets, guiding munitions, missile defence, electro-optical countermeasures (EOCM), alternative to radar, blinding enemy troops.
Research: Spectroscopy, laser ablation, Laser annealing, laser scattering, laser interferometry, LIDAR, Laser capture microdissection
Product development/commercial: laser printers, CDs, barcode scanners, thermometers, laser pointers, holograms, bubblegrams.
Laser lighting displays: Laser light shows
Laser skin procedures such as acne treatment, cellulite reduction, and hair removal.
In 2004, excluding diode lasers, approximately 131,000 lasers were sold worldwide, with a value of US$2.19 billion.[24] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.[25]

Exotic laser media

In September 2007, the BBC News reported that there was speculation about the possibility of using positronium annihilation to drive a very powerful gamma ray laser.[20] Dr. David Cassidy of the University of California, Riverside proposed that a single such laser could be used to ignite a nuclear fusion reaction, replacing the hundreds of lasers used in typical inertial confinement fusion experiments.[20]
Space-based X-ray lasers pumped by a nuclear explosion have also been proposed as antimissile weapons.[21][22] Such devices would be one-shot weapons.

Free electron lasers

Free electron lasers, or FELs, generate coherent, high power radiation, that is widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free electron.

Dye lasers

Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order of a few femtoseconds)

Semiconductor lasers

Photonic crystal lasers

Photonic crystal lasers are lasers based on nano-structures that provide the mode confinement and the density of optical states (DOS) structure required for the feedback to take place[clarification needed]. They are typical micrometre-sized and tunable on the bands of the photonic crystals. [2][clarification needed]

Fiber-hosted lasers

Solid-state lasers

Solid-state laser materials are commonly made by "doping" a crystalline solid host with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby (chromium-doped corundum). The population inversion is actually maintained in the "dopant", such as chromium or neodymium. Formally, the class of solid-state lasers includes also fiber laser, as the active medium (fiber) is in the solid state. Practically, in the scientific literature, solid-state laser usually means a laser with bulk active medium, while wave-guide lasers are caller fiber lasers.
"Semiconductor lasers" are also solid-state lasers, but in the customary laser terminology, "solid-state laser" excludes semiconductor lasers, which have their own name.
Neodymium is a common "dopant" in various solid-state laser crystals, including yttrium orthovanadate (Nd:YVO4), yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. These lasers are also commonly frequency doubled, tripled or quadrupled to produce 532 nm (green, visible), 355 nm (UV) and 266 nm (UV) light when those wavelengths are needed.
Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-1050 nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.
Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy as well as the most common ultrashort pulse laser.
Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat and phonon energy. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diode-pumped solid-state laser, the diode-pumped thin disk laser. The thermal limitations in this laser type are mitigated by using a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kilowatt levels of power.[18]

Excimer lasers

Excimer lasers are powered by a chemical reaction involving an excited dimer, or excimer, which is a short-lived dimeric or heterodimeric molecule formed from two species (atoms), at least one of which is in an excited electronic state. They typically produce ultraviolet light, and are used in semiconductor photolithography and in LASIK eye surgery. Commonly used excimer molecules include F2 (fluorine, emitting at 157 nm), and noble gas compounds (ArF [193 nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).[17]

Chemical lasers

Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, in the Hydrogen fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. They were invented by George C. Pimentel.

Types and operating principles

Gas lasers
Gas lasers using many gases have been built and used for many purposes.
The helium-neon laser (HeNe) emits at a variety of wavelengths and units operating at 633 nm are very common in education because of its low cost.
Carbon dioxide lasers can emit hundreds of kilowatts[14] at 9.6 µm and 10.6 µm, and are often used in industry for cutting and welding. The efficiency of a CO2 laser is over 10%.
Argon-ion lasers emit light in the range 351-528.7 nm. Depending on the optics and the laser tube a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and 514.5 nm.
A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.[15]
Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy.

HISTORY OF LASER

Foundations
In 1917 Albert Einstein, in his paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation), laid the foundation for the invention of the laser and its predecessor, the maser, in a ground-breaking rederivation of Max Planck's law of radiation based on the concepts of probability coefficients (later to be termed 'Einstein coefficients') for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation.
In 1928, Rudolf W. Ladenburg confirmed the existence of stimulated emission and negative absorption.[5] In 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify "short" waves.[6]
In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission.[7]
In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, which was experimentally confirmed by Brossel, Kastler and Winter two years later.[8]
The first working laser was demonstrated on 16 May 1960 by Theodore Maiman at Hughes Research Laboratories.[9] Since then, lasers have become a multi-billion dollar industry. By far the largest single application of lasers is in optical storage devices such as compact disc and DVD players,[citation needed] in which a semiconductor laser less than a millimeter wide scans the surface of the disc. The second-largest application is fiber-optic communication. Other common applications of lasers are bar code readers, laser printers and laser pointers.
Maser
Main article: Maser
In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave rather than infrared or visible radiation. Townes's maser was incapable of continuous output. Nikolay Basov and Aleksandr Prokhorov of the Soviet Union worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels and produced the first maser. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. In 1955 Prokhorov and Basov suggested an optical pumping of multilevel system as a method for obtaining the population inversion, which later became one of the main methods of laser pumping.
Townes reports that he encountered opposition from a number of eminent colleagues who thought the maser was theoretically impossible—including Niels Bohr, John von Neumann, Isidor Rabi, Polykarp Kusch, and Llewellyn H. Thomas[1].
Townes, Basov, and Prokhorov shared the Nobel Prize in Physics in 1964 "For fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

LASER Modes of operation

The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.
Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds (10-15 s).
Continuous wave operation
In the continuous wave (CW) mode of operation, the output of a laser is relatively constant with respect to time. The population inversion required for lasing is continually maintained by a steady pump source.
Pulsed operation
In the pulsed mode of operation, the output of a laser varies with respect to time, typically taking the form of alternating 'on' and 'off' periods. In many applications one aims to deposit as much energy as possible at a given place in as short time as possible. In laser ablation for example, a small volume of material at the surface of a work piece might evaporate if it gets the energy required to heat it up far enough in very short time. If, however, the same energy is spread over a longer time, the heat may have time to disperse into the bulk of the piece, and less material evaporates. There are a number of methods to achieve this.

Laser physics

The gain medium of a laser is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. It can be of any state: gas, liquid, solid or plasma. The gain medium absorbs pump energy, which raises some electrons into higher-energy ("excited") quantum states. Particles can interact with light both by absorbing photons or by emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an optical amplifier. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.
The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design.
The optical cavity, a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification.
The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are, at best, low order Gaussian beams. However this is rarely the case with powerful lasers. If the beam is not a low-order Gaussian shape, the transverse modes of the beam can be described as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams (for stable-cavity lasers). Unstable laser resonators on the other hand, have been shown to produce fractal shaped beams.[4] The beam may be highly collimated, that is being parallel without diverging. However, a perfectly collimated beam cannot be created, due to diffraction. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well.
Although the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than other light sources. The operation of a free electron laser can be explained without reference to quantum mechanics.

LASER DESIGNS

Main article: Laser construction
A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

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