optical digital communication

8/10/2019 optical digital communication

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1. Introduction:

A laser is a device that emits light (electromagnetic radiation) through a processof optical amplification based on the stimulated emission of photons. The term "laser"

originated as an acronym for Light Ampl i f icat ion by S t imulated Emiss ion ofRadiat ion . The emitted laser light is notable for its high degree of spatial andtemporal coherence.

Spatial coherence typically is expressed through the output being a narrow beam whichis diffraction-limited, often a so-called "pencil beam." Laser beams can be focused tovery tiny spots, achieving a very high irradiance, or they can be launched into beams ofvery low divergence in order to concentrate their power at a large distance.

Temporal (or longitudinal) coherence implies a polarized wave at a single frequency

whose phase is correlated over a relatively large distance (the coherence length) alongthe beam. A beam produced by a thermal or other incoherent light source has aninstantaneous amplitude and phase which vary randomly with respect to time andposition, and thus a very short coherence length.

Most so-called "single wavelength" lasers actually produce radiation inseveral modes having slightly different frequencies (wavelengths), often not in a singlepolarization. And although temporal coherence implies mono-chromaticity, there areeven lasers that emit a broad spectrum of light, or emit different wavelengths of light

simultaneously. There are some lasers which are not single spatial mode andconsequently their light beams diverge more than required by the diffraction limit.However all such devices are classified as "lasers" based on their method of producingthat light: stimulated emission. Lasers are employed in applications where light of therequired spatial or temporal coherence could not be produced using simplertechnologies.

Fig1.1 Red (635 nm), green (532 nm), and blue-violet (445 nm) laser
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1.2 Design:

Fig.1.2.1 Design of Laser

Components of a typical laser:1. Gain medium2. Laser pumping energy3. High reflector4. Output coupler5. Laser beam

A laser consists of a gain medium, a mechanism to supply energy to it, and somethingto provide optical feedback. The gain medium is a material with properties that allow itto amplify light by stimulated emission. Light of a specific wavelength that passesthrough the gain medium is amplified (increases in power).

For the gain medium to amplify light, it needs to be supplied with energy. This processis called pumping. The energy is typically supplied as an electrical current, or as light ata different wavelength. Pump light may be provided by a flash lamp or by another laser.

The most common type of laser uses feedback from an optical cavity a pair of mirrorson either end of the gain medium. Light bounces back and forth between the mirrors,passing through the gain medium and being amplified each time. Typically one of thetwo mirrors, the output coupler, is partially transparent. Some of the light escapesthrough this mirror. Depending on the design of the cavity (whether the mirrors are flator curved) , the light coming out of the laser may spread out or form a narrow beam. Thistype of device is sometimes called a laser oscillator in analogy to electronic oscillators, in which an electronic amplifier receives electrical feedback that causes it to produce asignal.

Most practical lasers contain additional elements that affect properties of the emittedlight such as the polarization, the wavelength, and the shape of the beam.
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1.3 Recent Innovations:

Since the early period of laser history, laser research has produced a variety ofimproved and specialized laser types, optimized for different performance goals,including:

new wavelength bands maximum average output power maximum peak pulse energy maximum peak pulse power minimum output pulse duration maximum power efficiency minimum cost

And this research continues to this day.

Fig.1.3.1 Graph showing the history of maximum laser pulse intensity throughout the past 40 years


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1.4 Types of Laser:

There is following types of laser:

Gas laserso Chemical laserso Excimer lasers

Solid-state lasers Fiber lasers Photonic crystal lasers Semiconductor lasers Dye lasers Free electron lasers Bio laser Exotic laser media

2. Fiber laser:

Although fiber lasers are relatively new type of lasers, they have begun to compete forthe applications with many other types of lasers having many active materials such assolid rods, gases or semi conductors. Many of these other lasers have reached a stateof relative maturity. However the applications of the fiber laser are still in the state of

rapid development but already have become important in various fields such as materialprocessing, communication, spectroscopy, medicine and the military. Solid-state lasersor laser amplifiers where the light is guided due to the total internal reflection in a singlemode optical fiber are instead called fiber lasers . Guiding of light allows extremely longgain regions providing good cooling conditions; fibers have high surface area to volumeratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend toreduce thermal distortion of the beam. Erbium and ytterbium ions are common activespecies 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 threeconcentric layers is chosen so that the fiber core acts as a single-mode fiber for thelaser emission while the outer cladding acts as a highly multimode core for the pumplaser. This lets the pump propagate a large amount of power into and through the activeinner core region, while still having a high numerical aperture (NA) to have easylaunching conditions.
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Pump light can be used more efficiently by creating a fiber disk laser, or a stack of suchlasers.

Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannotbe so high that optical nonlinearities induced by the local electric field strength canbecome dominant and prevent laser operation and/or lead to the material destruction ofthe fiber. This effect is called photo darkening. In bulk laser materials, the cooling is notso efficient, and it is difficult to separate the effects of photo darkening from the thermaleffects, but the experiments in fibers show that the photo darkening can be attributed tothe formation of long-living color centers.

Fig.2.1 Schematic diagram of cladding-pumped double-clad fiber laser

A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiberdoped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium,praseodymium, and thulium. They are related to doped fiber amplifiers, which providelight amplification without lasing. Fiber nonlinearities, such as stimulated Raman

scattering or four-wave mixing can also provide gain and thus serve as gain media for afiber laser.

2.2 DESIGN AND MANUFACTURE:

Fiber Lasers are designed with a variety of choices for the laser cavity. The mostcommon type of laser cavity is Fabry- Parot Cavity which is made by placing a gainmedium between two high reflecting mirrors. The mirrors used are generally buttcoupled to the fiber ends to avoid the diffraction losses. This approach was adopted inthe year 1985 for Nd doped fiber. Unlike most other types of lasers, the laser cavity infiber lasers is constructed monolithically by fusion splicing different types of fiber.

Fiber lasers are optically pumped, most commonly with laser diodes but in a few caseswith other fiber lasers. The optics used in these systems are usually fiber components,with most or all of the components fiber-coupled to one another. In some cases, bulk
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optics are used, and sometimes an internal fiber-coupling system is combined withexternal bulk optics.

A diode pump source can be a single diode, an array, or many separate pump diodes,each with a fiber going into a coupler. The doped fiber has a cavity mirror on each end;in practice, these are fiber Bragg gratings, which can be fabricated within the fiber. fiberBragg gratings replace conventional dielectric mirrors to provide optical feedback.

Another type is the single longitudinal mode operation of ultra narrow distributedfeedback lasers (DFB) where a phase-shifted Bragg grating overlaps the gain medium.There are no bulk optics on the end, unless the output beam goes into something otherthan a fiber. The fiber can be coiled, so the laser cavity can be many meters long ifdesired.

A fiber laser can be end- or side-pumped (see Fig. 2.2.1 ). In end-pumping, the light fromone or many pump lasers is fired into the end of the fiber. In side-pumping, pump light iscoupled into the side of the fiber; actually, it is fed into a coupler that couples it into the

outer core. This is different from side-pumping a laser rod, where the light comes inorthogonally to the axis.

FIG 2.2.1 A fiber laser can be end-pumped with one or many lasers, orside-pumped (usually with many lasers) by side-coupling pump light into theouter core.

Many design considerations go into making this work. Considerable attention is spenton coupling the pump light into the core, matching it to the optical absorption, andcoupling that pump light into the inner core to produce a population inversion that willresult in stimulated emission in the inner core. The laser core can have various degreesof gain, depending on the doping in the fiber as well as on the fiber length. These arefactors that the design engineer would adjust to get the performance that is needed.
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Power limitations can arise, particularly from working within a singlemode fiber. Such afiber core has a very small cross-sectional area, and as a result, very high-intensity lightgoing through it. Nonlinear Brillouin scattering becomes increasingly important at thesehigh intensities, and can limit output at multi kilowatt levels. If the output is high enough,the fiber end can be optically damaged.

This arrangement also allows the core to be pumped with much of high power beam.and allows a conversion of pump light with relatively low brightness to a much higherbrightness signal. As a result the fiber las ers are called as brightness convertors .

There is following terms used in above paragraph, which has meaning given below;

2.3. Laser cavity:

An optical cavity or optical resonator is an arrangement of mirrors that forms a standingwave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are alsoused in optical parametric oscillators and some interferometers. Light confined in thecavity reflect multiple times producing standing waves for certain resonancefrequencies. The standing wave patterns produced are called modes; longitudinalmodes differ only in frequency while transverse modes differ for different frequenciesand have differentintensity patterns acrossthe cross section of thebeam.

Fig.2.3.1 Types of two-mirror optical cavities,with mirrors of variouscurvatures, showing theradiation pattern insideeach cavity

Different resonatortypes are distinguishedby the focal lengths ofthe two mirrors and thedistance between them.(Flat mirrors are notoften used because ofthe difficulty of aligningthem to the needed
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precision.) The geometry (resonator type) must be chosen so that the beam remainsstable (that the size of the beam does not continually grow with multiple reflections).Resonator types are also designed to meet other criteria such as minimum beam waistor having no focal point (and therefore intense light at that point) inside the cavity.

Optical cavities are designed to have a large Q factor; a beam will reflect a very largenumber of times with little attenuation. Therefore the frequency line width of the beam isvery small indeed compared to the frequency of the laser.

2.3.1. Stability of Laser Cavity:

Only certain ranges of values for R 1, R 2, and L produce stable resonators in whichperiodic refocusing of the intra-cavity beam is produced. If the cavity is unstable, thebeam size will grow without limit, eventually growing larger than the size of the cavitymirrors and being lost. By using methods such as ray transfer matrix analysis, it is

possible to calculate a stability criterion:

Values which satisfy the inequality correspond to stable resonators.

Fig.2.3.1.1 Stability diagram for a two-mirror cavity. Blue-shaded areas correspond tostable configurations.
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The stability can be shown graphically by defining a stability parameter, g for eachmirror:

,and plotting g 1 against g 2 as shown. Areas bounded by the line g 1 g 2 = 1 and the axesare stable. Cavities at points exactly on the line are marginally stable; small variations incavity length can cause the resonator to become unstable, and so lasers using thesecavities are in practice often operated just inside the stability line.

2.4. Fusion Splicing:

Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal isto fuse the two fibers together in such a way that light passing through the fibers isnot scattered or reflected back by the splice, and so that the splice and the regionsurrounding it are almost as strong as the virgin fiber itself. The source of heat is usuallyan electric arc, but can also be a laser, or a gas flame, or a tungsten filament throughwhich current is passed.

The process of fusion splicing involves using localized heat to melt or fuse the ends oftwo optical fibers together. The splicing process begins by preparing each fiber end forfusion. Fusion splicing requires that all protective coatings be removed from the ends ofeach fiber, a process called stripping. The fiber is then cleaved using the score-and-break method so that its end face is perfectly flat and perpendicular to the axis of thefiber. The quality of each fiber end is inspected using a microscope. In fusion splicing,splice loss is a direct function of the angles and quality of the two fiber-end faces. Thetwo end faces of the fibers are aligned, then are fused together. The bare fiber area isprotected either by recoating or with a splice protector. It is often desirable to perform aproof-test to ensure that the splice is strong enough to survive handling, packaging andextended use.

The basic fusion splicing apparatus consists of two fixtures on which the fibers aremounted and two electrodes. Inspection microscope assists in the placement of theprepared fiber ends into a fusion-splicing apparatus. The fibers are placed into theapparatus, aligned, and then fused together. Initially, fusion splicing used nichrome wireas the heating element to melt or fuse fibers together. New fusion-splicing techniques

have replaced the nichrome wire with carbon dioxide (CO 2) lasers, electric arcs, or gasflames to heat the fiber ends, causing them to fuse together. The small size of thefusion splice and the development of automated fusion-splicing machines have madeelectric arc fusion (arc fusion) one of the most popular splicing techniques incommercial applications.
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Alternatives to fusion splicing include using optical fiber connectors or mechanicalsplices both of which have higher insertion losses, lower reliability and higher returnlosses than fusion splicing.

Fig.2.4.1 A modern fusion splicer

2.5. Fiber Bragg grating (FBG):

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a shortsegment of optical fiber that reflects particular wavelengths of light and transmits allothers. This is achieved by creating a periodic variation in the refractive index of thefiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg gratingcan therefore be used as an inline optical filter to block certain wavelengths, or as awavelength-specific reflector.
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Fig.2.5.1 A Fiber Bragg Grating structure, with refractive index profile and spectralresponse

2.6.1. Dielectric Mirror:

Fig.2.6.1 Two broadband dielectric mirrors being used in an optics experiment

A dielectric mirror, also known as a Bragg mirror, is a type of a mirror composed ofmultiple thin layers of dielectric material, typically deposited on a substrate of glass orsome other optical material. By careful choice of the type and thickness of the dielectriclayers, one can design an optical coating with specified reflectivity atdifferent wavelengths off -light. Dielectric mirrors are also used to produce ultra-highreflectivity mirrors: values of 99.999% or better over a narrow range of wavelengths canbe produced using special techniques. Alternatively, they can be made to reflect abroad spectrum of light, such as the entire visible range or the spectrum of the Ti-
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sapphire laser. Mirrors of this type are very common in optics experiments, due toimproved techniques that allow inexpensive manufacture of high-quality mirrors.Examples of their applications include laser cavity end mirrors, hot and cold mirrors, thin-film beam splitters, and the coatings on modern mirror shades.

2.6.2. Mechanism of Dielectric Mirror

Dielectric mirrors function based on the interference of light reflected from the differentlayers of dielectric stack. This is the same principle used in multi-layer anti-reflectioncoatings, which are dielectric stacks which have been designed to minimize rather thanmaximize reflectivity. Simple dielectric mirrors function like one-dimensional photoniccrystals, consisting of a stack of layers with a high refractive index interleaved with

layers of a low refractive index (see diagram). The thicknesses of the layers are chosensuch that the path-length differences for reflections from different high-index layers areinteger multiples of the wavelength for which the mirror is designed. The reflections fromthe low-index layers have exactly half a wavelength in path length difference, but thereis a 180-degree difference in phase shift at a low-to-high index boundary, compared to ahigh-to-low index boundary, which means that these reflections are also in phase. In thecase of a mirror at normal incidence, the layers have a thickness of a quarterwavelengths.

Other designs have a more complicated structure generally produced by numericaloptimization. In the latter case, the phase dispersion of the reflected light can also becontrolled. In the design of dielectric mirrors, an optical transfer-matrix method can beused.
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Fig.2.6.2.1 Diagram of a dielectric mirror. Thin layers with a high refractive index n 1 areinterleaved with thicker layers with a lower refractive index n 2 . The path lengths l A andl B differ by exactly one wavelength, which leads to constructive interference.

2.7. Optical Feedback:

Optical feedback is the optical equivalent of acoustic feedback. A simple example isthe feedback that occurs when a loop exists between an optical input, e.g., a videocamera, and an optical output, e.g., a television screen or monitor. (A simple example ofoptical feedback is also an image cast between mirrors. )

In the animation and the still image examples (right), light from a candle is received by avideo camera, amplified and then sent by cable to a monitor projecting electron beamsto a monitor screen. The image on the monitor is then captured by the video cameraagain, and fed back to the monitor in a continuous loop.

The original light source, in this case from the candle, can then be extinguished, whilethe feedback loop continues. For each loop the image is doubled and theimage interferes with itself. The electronic loop moves with near light speed, but as theresulting image is projected onto the phosphor dots on the inside of the screen by theelectron beam, the phosphor points take time to begin and stop glowing, and thiscreates a persistence which prevents the patterns changing too fast, and thus they
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survive long enough to be perceived. (More recent types of screens, such as plasmadisplay, LCD and LED, can also be used)).

The resulting images depend on different camera and monitor settings, such as lightamplification, contrast, distance, angle and physical vibrations. Optical feedback can becombined with music, or other sound sources, to influence the image loop.

Fig.2.7.1 Optical feedback

2.8. Distributed Feedback Laser:

A distributed feedback laser (DFB) is a type of laser diode, quantum cascadelaser or optical fiber laser where the active region of the device is periodically structuredas a diffraction grating. The structure builds a one dimensional interference grating(Bragg scattering) and the grating provides optical feedback for the laser.

DFB laser diodes do not use two discrete mirrors to form the optical cavity (as they areused in conventional laser designs). The grating acts as the wavelength selective

element for at least one of the mirrors and provides the feedback, reflecting light backinto the cavity to form the resonator. The grating is constructed so as to reflect only anarrow band of wavelengths, and thus produce a single longitudinal lasing mode. This isin contrast to a Fabry-Perot Laser, where the facets of the chip form the two mirrors andprovide the feedback. In that case, the mirrors are broadband and either the laserfunctions at multiple longitudinal modes simultaneously or easily jumps betweenlongitudinal modes. Altering the temperature of the device causes the pitch of thegrating to change due to the dependence of refractive index on temperature. Thisdependence is caused by a change in the semiconductor laser's band gap withtemperature and thermal expansion. A change in the refractive index alters thewavelength selection of the grating structure and thus the wavelength of the laseroutput, producing a wavelength tunable laser or TDL (Tunable Diode Laser). The tuningrange is usually of the order of 6 nm for a ~50 K (90 F) change in temperature, whilethe line width of a DFB laser is a few megahertz. Altering of the current powering thelaser will also tune the device, as a current change causes a temperature change insidethe device. Integrated DFB lasers are often used in optical communication applications,such as DWDM where a tunable laser signal is desired as well as in sensing where
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extreme narrow line width is required, or in gas sensing applications, where the signal ofthe absorbing gas is detected while wavelength tuning the DFB laser.

There are alternatives to traditional types of DFB lasers. Traditionally, DFBs areantireflection coated on one side of the cavity and coated for high reflectivity on theother side (AR/HR). In this case the grating forms the distributed mirror on theantireflection coated side, while the semiconductor facet on the high reflectivity sideforms the other mirror. These lasers generally have higher output power since the lightis taken from the AR side, and the HR side prevents power being lost from the backside. Unfortunately, during the manufacturing of the laser and the cleaving of the facets,it is virtually impossible to control at which point in the grating the laser cleaves to formthe facet. So sometimes the laser HR facet forms at the crest of the grating, sometimeson the slope. Depending on the phase of the grating and the optical mode, the laseroutput spectrum can vary. Frequently, the phase of the highly reflective side occurs at apoint where two longitudinal modes have the same cavity gain, and thus the laseroperates at two modes simultaneously. Thus such AR/HR lasers have to be screened at

manufacturing and parts that are multimode or have poor side mode suppression ratio(SMSR) have to be scrapped. Additionally, the phase of the cleave affects thewavelength, and thus controlling the output wavelength of a batch of lasers inmanufacturing can be a challenge.

An alternative approach is a phase-shifted DFB laser. In this case both facets are anti-reflection coated and there is a phase shift in the cavity. This could be a single 1/4 waveshift at the center of the cavity, or multiple smaller shifts distributed in the cavity. Suchdevices have much better reproducibility in wavelength and theoretically all lies in singlemode.

In DFB fiber lasers the Bragg grating (which in this case forms also the cavity of the

laser) has a phase-shift centered in the reflection band akin to a single very narrowtransmission notch of a Fabry Prot interferometer. When configured properly, theselasers operate on a single longitudinal mode with coherence lengths in excess of tens ofkilometers, essentially limited by the temporal noise induced by the self-heterodynecoherence detection technique used to measure the coherence. These DFB fiber lasersare often used in sensing applications where extreme narrow line width is required.

2.9. Laser pumping:

Laser pumping is the act of energy transfer from an external source into the gainmedium of a laser. The energy is absorbed in the medium, producing excited states inits atoms. When the number of particles in one excited state exceeds the number ofparticles in the ground state or a less-excited state, population inversion is achieved. Inthis condition, the mechanism of stimulated emission can take place and the mediumcan act as a laser or an optical amplifier. The pump power must be higher thanthe lasing threshold of the laser.
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The pump energy is usually provided in the form of light or electric current, but moreexotic sources have been used, such as chemical or nuclear reactions.

Fig2.9.1. Various laser pumping cavity configurations

2.10. Laser Diode:

A laser diode is a laser whose active medium is a semiconductor similar to that found ina light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current. The former devices are sometimesreferred to as injection laser diodes to distinguish them from optically pumped laserdiodes.

Fig2.10.1. A packaged laser diode shown with a penny for scale.
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Fig.2.10.2. The laser diode chip is removed from the above package and placed on theeye of a needle for scale.

2.10.2. Theory of operation of Laser Diode:

A laser diode is formed by doping a very thin layer on the surface of a crystal wafer. Thecrystal is doped to produce an n-type region and a p-type region, one above the other,resulting in a p -n junction, or diode.

Laser diodes form a subset of the larger classification of semiconductor p -n junctiondiodes. Forward electrical bias across the laser diode causes the two species of chargecarrier holes and electrons to be "injected" from opposite sides of the p -n junctioninto the depletion region. Holes are injected from the p -doped, and electrons from the n-doped, semiconductor. (A depletion region, devoid of any charge carriers, forms as aresult of the difference in electrical potential between n- and p -type semiconductorswherever they are in physical contact.) Due to the use of charge injection in poweringmost diode lasers, this class of lasers is sometimes termed "injection lasers orinjection laser diode" (ILD). As diode lasers are semiconductor devices, they may alsobe classified as semiconductor lasers. Either designation distinguishes diode lasersfrom solid-state lasers.

Another method of powering some diode lasers is the use of optical pumping. OpticallyPumped Semiconductor Lasers (OPSL) uses an III-V semiconductor chip as the gainmedia, and another laser (often another diode laser) as the pump source. OPSL offerseveral advantages over ILDs, particularly in wavelength selection and lack ofinterference from internal electrode structures.

When an electron and a hole are present in the same region, they may recombine or"annihilate" with the result being spontaneous emission i.e., the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to thedifference between the electron and hole states involved. (In a conventionalsemiconductor junction diode, the energy released from the recombination of electronsand holes is carried away as phonons, i.e., lattice vibrations, rather than as photons.)Spontaneous emission gives the laser diode below lasing threshold similar properties toan LED. Spontaneous emission is necessary to initiate laser oscillation, but it is oneamong several sources of inefficiency once the laser is oscillating.

The difference between the photon-emitting semiconductor laser and conventionalphonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use of a
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different type of semiconductor, one whose physical and atomic structure confers thepossibility for photon emission. These photon-emitting semiconductors are the so-called "direct band gap" semiconductors. The properties of silicon and germanium,which are single-element semiconductors, have band gaps that do not align in the wayneeded to allow photon emission and are not considered "direct." Other materials, the

so-called compound semiconductors, have virtually identical crystalline structures assilicon or germanium but use alternating arrangements of two different atomic species ina checkerboard-like pattern to break the symmetry. The transition between the materialsin the alternating pattern creates the critical "direct band gap" property. Galliumarsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples ofcompound semiconductor materials that can be used to create junction diodes that emitlight.

Fig.2.10.2.1 Diagram of a simple laser diode, such as shown above; not to scale

In the absence of stimulated emission (e.g., lasing) conditions, electrons and holes maycoexist in proximity to one another, without recombining, for a certain time, termed the"upper-state lifetime" or "recombination time" (about a nanosecond for typical diodelaser materials), before they recombine. Then a nearby photon with energy equal to therecombination energy can cause recombination by stimulated emission. This generatesanother photon of the same frequency, travelling in the same direction, with thesame polarization and phase as the first photon. This means that stimulated emission

causes gain in an optical wave (of the correct wavelength) in the injection region, andthe gain increases as the number of electrons and holes injected across the junctionincreases. The spontaneous and stimulated emission processes are vastly moreefficient in direct band-gap semiconductors than in indirect band-gap semiconductors;therefore silicon is not a common material for laser diodes.

As in other lasers, the gain region is surrounded with an optical cavity to form a laser. Inthe simplest form of laser diode, an optical waveguide is made on that crystal surface,
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such that the light is confined to a relatively narrow line. The two ends of the crystal arecleaved to form perfectly smooth, parallel edges, forming a Fabry Prot resonator.Photons emitted into a mode of the waveguide will travel along the waveguide and bereflected several times from each end face before they are emitted. As a light wavepasses through the cavity, it is amplified by stimulated emission, but light is also lost

due to absorption and by incomplete reflection from the end facets. Finally, if there ismore amplification than loss, the diode begins to "lase" .

Some important properties of laser diodes are determined by the geometry of the opticalcavity. Generally, in the vertical direction, the light is contained in a very thin layer, andthe structure supports only a single optical mode in the direction perpendicular to thelayers. In the transverse direction, if the waveguide is wide compared to the wavelengthof light, then the waveguide can support multiple transverse optical modes, and thelaser is known as "multi-mode". These transversely multi-mode lasers are adequate incases where one needs a very large amount of power, but not a small diffraction-limitedbeam; for example in printing, activating chemicals, or pumping other types of lasers.

In applications where a small focused beam is needed, the waveguide must be madenarrow, on the order of the optical wavelength. This way, only a single transverse modeis supported and one ends up with a diffraction-limited beam. Such single spatial modedevices are used for optical storage, laser pointers, and fiber optics. Note that theselasers may still support multiple longitudinal modes, and thus can lase at multiplewavelengths simultaneously.

The wavelength emitted is a function of the band-gap of the semiconductor and themodes of the optical cavity. In general, the maximum gain will occur for photons withenergy slightly above the band-gap energy, and the modes nearest the gain peak willlase most strongly. If the diode is driven strongly enough, additional side modes may

also lase. Some laser diodes, such as most visible lasers, operate at a singlewavelength, but that wavelength is unstable and changes due to fluctuations in currentor temperature.

Due to diffraction, the beam diverges (expands) rapidly after leaving the chip, typicallyat 30 degrees vertically by 10 degrees laterally. A lens must be used in order to form acollimated beam like that produced by a laser pointer. If a circular beam is required,cylindrical lenses and other optics are used. For single spatial mode lasers, usingsymmetrical lenses, the collimated beam ends up being elliptical in shape, due to thedifference in the vertical and lateral divergences. This is easily observable with ared laser pointer.

3. Double Clad Fiber:

Double-clad fiber (DCF) is a class of optical fiber with a structure consisting of threelayers of optical material instead of the usual two. The inner-most layer is calledthe core. It is surrounded by the inner cladding, which is surrounded by the outercladding. The three layers are made of materials with different refractive indices.
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Fig.3.1. Refractive index profile of dispersion-compensating double-clad fiber. c:core,i:inner cladding, o:outer cladding.

Fig.3.2. Refractive index profile of double-clad fiber for high power fiber lasers andamplifiers. c:core, i:inner cladding, o:outer cladding.

There are two different kinds of double-clad fibers. The first was developed early inoptical fiber history with the purpose of engineering the dispersion of optical fibers. Inthese fibers, the core carries the majority of the light, and the inner and outer claddingalter the waveguide dispersion of the core-guided signal. The second kind of fiber wasdeveloped in the late 1980s for use with high power fiber amplifiers and fiber lasers. Inthese fibers, the core is doped with active dopant material; it both guides and amplifiesthe signal light. The inner cladding and core together guide the pump light, whichprovides the energy needed to allow amplification in the core. In these fibers, the corehas the highest refractive index and the outer cladding has the lowest. In most casesthe outer cladding is made of a polymer material rather than glass.
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Fig.3.3. Cross-section of circular DCF with offset core

Fig.3.4. Cross-section of DCF with rectangular inner cladding

4. Power scaling:

Recent developments in fiber laser technology have led to a rapid and large rise inachieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due tothe introduction of large mode area (LMA) fibers as well as continuing advances in highpower and high brightness diodes, continuous-wave single -transverse-mode powersfrom Yb-doped fiber lasers have increased from 100 W in 2001 to >20 kW. Commercialsingle-mode lasers have reached 10 kW in CW power.
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Fig.4.1. 10,000W SM Laser

5. Mode Locking:

Mode-locking is a technique in optics by which a laser can be made to produce pulsesof light of extremely short duration, on the order of picoseconds (10 12 s) orfemtoseconds (10 15 s).

The basis of the technique is to induce a fixed phase relationship between the modes ofthe laser's resonant cavity. The laser is then said to be phase-locked or mode-locked.Interference between these modes causes the laser light to be produced as a train ofpulses. Depending on the properties of the laser, these pulses may be of extremely brief

duration, as short as a few femtoseconds.

5.2. Laser Cavity Modes:

Although laser light is perhaps the purest form of light, it is not of a single,pure frequency or wavelength. All lasers produce light over some natural bandwidth orrange of frequencies. A laser's bandwidth of operation is determined primarily bythe gain medium that the laser is constructed from, and the range of frequencies that alaser may operate over is known as the gain bandwidth. For example, a typical helium-neon (HeNe) gas laser has a gain bandwidth of approximately 1.5 GHz (a wavelength

range of about 0.002 nm at a central wavelength of 633 nm), whereas a titanium-dopedsapphire (Ti:Sapphire) solid-state laser has a bandwidth of about 128 THz (a 300 nmwavelength range centred around 800 nm).

The second factor that determines a laser's emission frequencies is the optical cavityor resonant cavity of the laser. In the simplest case, this consists of two plane(flat) mirrorsf acing each other, surrounding the gain medium of the laser (thisarrangement is known as a Fabry Prot cavity). Since light is a wave, when bouncingbetween the mirrors of the cavity the light will constructively and
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destructively interfere with itself, leading to the formation of standingwaves or modes between the mirrors.

Fig.5.2.1. Laser mode structure.

These standing waves form a discrete set of frequencies, known as the longitudinalmodes of the cavity. These modes are the only frequencies of light which are self-regenerating and allowed to oscillate by the resonant cavity; all other frequencies oflight are suppressed by destructive interference. For a simple plane-mirror cavity, theallowed modes are those for which the separation distance of the mirrors L is an exactmultiple of half the wavelength of the light , such that L = q /2, where q is an integerknown as the mode order.

In practice, the separation distance of the mirrors L is usually much greater than thewavelength of light , so the relevant values of q are large (around 10 5 to 10 6). Of moreinterest is the frequency separation between any two adjacent modes q and q+1; this isgiven (for an empty linear resonator of length L ) by :

where c is the speed of light (310 8 ms 1).
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Using the above equation, a small laser with a mirror separation of 30 cm has afrequency separation between longitudinal modes of 0.5 GHz. Thus for the two lasersreferenced above, with a 30 cm cavity the 1.5 GHz bandwidth of the He-Ne laser wouldsupport up to 3 longitudinal modes, whereas the 128 THz bandwidth of the Ti: sapphirelaser could support approximately 250,000 modes. When more than one longitudinal

mode is excited, the laser is said to be in "multi-mode" operation. When only onelongitudinal mode is excited, the laser is said to be in "single-mode" operation.

Each individual longitudinal mode has itself some bandwidth or narrow range offrequencies over which it operates, but typically this bandwidth, determined by the Qfactor (see Inductor) of the cavity (see Fabry Prot interferometer) , is much smallerthan the inter-mode frequency separation.

6. Features Of Fiber Lasers:

Using a fiber as a laser medium gives a long interaction length, which works well fordiode-pumping. This geometry results in high photon conversion efficiency, as well as arugged and compact design. When fiber components are spliced together, there are nodiscrete optics to adjust or to get out of alignment.The fiber-based laser design is highly adaptable. It can be adapted to do anything fromwelding heavy sheets of metal to producing femtosecond pulses. Many variations existon the fiber-laser theme, as well as some configurations that are not, strictly speaking,fiber lasers. Fiber amplifiers provide single-pass amplification; they're used intelecommunications because they can amplify many wavelengths simultaneously. Fiberamplification is also used in the master-oscillator power-amplifier (MOPA) configuration,where the intent is to generate a higher output from a fiber laser. In somecircumstances, an amplifier is used even with a continuous-wave (CW) laser.

Another example is fiber-amplified spontaneous-emission sources, in which thestimulated emission is suppressed. Yet another example is the Raman fiber laser, whichrelies on Raman gain that essentially Raman-shifts the wavelength. This is anapplication that's not in wide use, although it's certainly being used in research. In fact,some new research is going on into using fluoride glass fibers for Raman lasing andamplification rather than the standard silica fibers.However, the fiber host is usually silica glass with a rare earth dopant in the core. Theprimary dopants are ytterbium and erbium. Ytterbium has center wavelengths rangingfrom about 1030 to 1080 nm and can emit in a broader range of wavelengths if pushed.Using pump diodes emitting in the 940 nm range can make the photon deficit verysmall. Ytterbium has none of the self-quenching effects that occur in neodymium at highdensities, which is why neodymium is used in bulk lasers and ytterbium is used in fiberlasers (they both provide roughly the same wavelength).Erbium fiber lasers emit at 1530 to 1620 nm, which is an eye-safe wavelength range.This can be frequency-doubled to generate light at 780 nm a wavelength that's not
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available from fiber lasers in other ways. And finally, ytterbium can be added to erbiumso that the

ytterbium absorbs pump light and transfers that energy to erbium. Thulium is anotherdopant that emits even deeper into the near-infrared (NIR; 1750 to 2100 nm), and isthus another eye-safe material.

7. Advantages Of Fiber Laser:

As compared to other lasers fiber lasers has many advantages some of them aredescribed below:

7.1. High Efficiency:

Fibre lasers offer higher efficiency than other lasers used for similar applications,like the Nd:YAG laser and the carbon dioxide laser. The efficiency is the fractionof the electrical power input that emerges as laser power output. For Nd:YAGlasers the efficiency is around 2%. For carbon dioxide lasers, it is often in the 15-20 % range. For fiber lasers it is usually in the 25 -30% range. Also there is lessenergy needed for purposes other than pumping the laser, like cooling andcirculating gas.

Let us study in detail:Fibre lasers are quasi-three-level systems. A pump photon excites a transitionfrom a ground state to an upper level; the laser transition is a drop from thelowest part of the upper level down into some of the split ground states. This isvery efficient: For example, ytterbium with a pump photon at 940 nm produces anemitted photon at 1030 nm-a quantum defect (lost energy) of only about 9%.


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In contrast, neodymium pumped on its standard 808 nm pump line has a quantumdefect of about 24%. So ytterbium has an inherently higher efficiency, although not allthat efficiency can be realized because some photons are lost. Ytterbium can bepumped in a number of bands. Erbium can be pumped at either 1480 or 980 nm; thelatter is not as efficient from a photon defect point of view, but is useful even so becausebetter pump sources are available at 980 nm.

Overall fiber-laser efficiency is the result of a two-stage process. First is the efficiency ofthe pump diode. Semiconductor lasers are very efficient, with on the order of 50%electrical-to-optical efficiency. Laboratory results are even better, with 70% or evenmore of the electrical pump energy being converted into light. When this output ismatched carefully to the fiber laser's absorption line, the result is the pump efficiency.The second is the optical-to-optical conversion efficiency. With a small photon defect,high excitation and extraction efficiency can be achieved, producing an optical-to-opticalconversion efficiency on the order of 60% to 70%. The result is a wall-plug efficiency in

the 25% to 35% range.

7.2. Configurations For Many Purposes:

Continuous-wave fiber lasers can be either single- or multimode (in terms of transversemodes). A single mode produces a high-quality beam for materials working or sending abeam through the atmosphere, while multimode industrial lasers can generate higherraw power. If an application does not require the extremely high intensities resultingfrom single mode operation, the higher total power from multimode operation is often an

advantage for example, for some kinds of cutting and welding, and particularly forheat-treating, where a large area is illuminated.Long-pulse fiber lasers are essentially quasi-CW lasers, typically producing millisecond-type pulses. Typically they have a 10% duty cycle (resulting from the pump diodemodulation). This results in higher peak powers than in CW operation typically on theorder of ten times higher. This can be an advantage for some kinds of materials workingsuch as pulse drilling. The repetition rate can range up to 500 Hz, depending on thepulse duration.Q -switching is possible in fiber lasers, with the principle being the same as for bulk Q -switched lasers. Typical pulse lengths range from low nanosecond up to the

microsecond range; the longer the fiber, the more time is needed to Q -switch the output,producing a longer pulse.Fiber properties impose some limitations on Q -switching. Nonlinearities are moresevere in a fiber laser due to the core's small cross-sectional area, so the peak powerhas to be somewhat limited. One can either use bulk Q -switches, giving higherperformance, or a fiber Q -switch, which is spliced to the ends of the active part of thefiber laser.
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The Q -switched pulses can be amplified in fiber or in bulk. An example of the latter isfound at the National Ignition Facility (NIF; Livermore, CA), where a fiber laser is themaster oscillator for the 192 beams of the NIF laser: Small pulses from the fiber laserare amplified up to megajoule size in large slabs of doped glass.In modelocked fiber lasers, the repetition rate depends on the length of the gainmaterial, as in any kind of modelocking scheme, while pulse duration depends on thegain bandwidth. The shortest achievable oscillator pulses are in the 50 fs range, withmore typical durations in the 100 fs range. Shorter pulses can be generated inoscillator-amplifier systems with external chirped-pulse amplification and subsequentpulse compression.

An important difference exists between erbium- and ytterbium-doped fibres, resultingfrom the fact that the two are operating in different dispersion modes. Erbium-dopedfibres emit at 1550 nm, which is in the anomalous-dispersion region; this allows theproduction of solutions . Ytterbium-doped fibres are in the positive or normal dispersionrealm; as a result, they generate strongly chirped pulses. As a result, a chirped fibre

Bragg grating may be needed to de chirp the pulses to compress the pulse length.There are a number of ways to modify fibre-laser pulses, particularly for things like picosecond ultrafast research. Photonic-crystal fibres can be made with extremely smallcores to produce strong nonlinear effects for applications such as super continuumgeneration. In contrast, photonic crystals also can be made with very large single modecores to avoid nonlinear effects at high power.Bendable large-core photonic-crystal fibers are being created for high-powerapplications; one of the tricks being looked at is intentionally bending such a fiberenough that any undesired higher-order modes will go away, leaving only thefundamental transverse mode. Nonlinearities allow harmonics to be generated; sum and

difference frequency mixing can create higher frequencies and shorter wavelengths.Nonlinear effects can also produce pulse compression, leading the way to theproduction of frequency combs.In a super continuum source, very short pulses produce a broad continuous spectrumvia self-phase modulation. In one example, initial 6 pulses at 1050 nm (from a ytterbiumfiber laser) produces a spectrum ranging from the ultraviolet to beyond 1600 nm, withsome irregularity across the spectrum. Another super continuum source, this oneworking in the IR, is pumped with an erbium fibre laser at 1550 nm. In this case, thespectrum of the super continuum source varies with pulse width. This design achieves arange of more than an octave, ranging out past 2200 nm.

7.3. Power:

Because fiber lasers can be coiled easily within a small volume, they can have greatlength, up to kilometers. This fact allows the power output to be scaled upwards in a
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simple manner. Fiber lasers have been developed with CW output power up to manykilowatts. In single mode operation, CW operation up to 10 kW has beendemonstrated. In CW multimode operation, 50 kW has been obtained. In pulsedoperation, megawatt values of peak power are possible.

7.4. High Brightness:

Brightness (power per unit area per unit solid angle, also called radiance) can be veryhigh in a single mode fiber laser. Brightness is usually given in units of watts per squarecentimeter per steradian, where a steradian is a unit of measurement of solid angles.Irradiance (power per unit area at a surface, also called intensity) is another commonlyused term to describe laser brightness..

The area of the beam as it emerges from the fiber is that of the fiber core, which is smalland in single transverse mode operation, the angular spread of the beam is at aminimum. This means that the brightness is very high.

7.5. Excellent Beam Quality:

In laser science, a parameter defining the quality of the laser beam is denoted M2. Thisparameter is defined as the ratio of the divergence angle of the beam to the beamdivergence angle of the lowest spatial mode (the Gaussian mode) with the sameaperture located at the same position. For a pure Gaussian beam, this ratio is obviouslyunity. If there is a small mixture of higher order modes, the value of M2 will be slightlygreater than unit and if there are many high order modes in the beam, the value of thisparameter will be much greater than unity.

Fiber lasers operating at relatively low power generally have small values of M2, oftenless than 1.1. This means that the quality of the beam is excellent. In general, as thepower of the

laser increases, the value of the beam quality factor also increases. But because of thenature of the fiber laser, with the beam generated in a fiber, the increase in M2 withpower is less than it is in most other lasers. Single mode fiber lasers have beenfabricated with values of M2 remaining near unity at fairly high values of power.Beam quality is important for many applications. In fiber-optic communications, beamsmust have M2 close to 1 in order to be coupled to a single-mode optical fiber. In laser

materials processing, small values of M2 are required in order to focus the beam to asmall spot. This gives fiber laser lasers an advantage over other lasers for theseapplications.

7.6. Low Operating Cost:

Because higher efficiency leads to less use of electricity and because there is littlemaintenance required and no use of gas in the laser, the operating cost of fiber lasers is


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lower than that of competing lasers. According to one estimate, at the 4 kW level, theoperating cost for a fiber laser is about $12 per hour for 8 years of use. This estimateincludes electricity, maintenance, replacement parts, gas, and depreciation and intereston the original purchase. In comparison, under the same conditions the operating costfor CO2 lasers was estimated to be about $24 per hour and about $38 per hour forNd:YAG lasers.

7.7. Low Maintenance:

There are no periodic maintenance tasks to be performed. There is no mirroradjustment or replacement and no regular pump source replacement. There are noblowers or gas supplies and there are no moving parts. In contrast to competing lasers,like carbon dioxide and Nd:YAG for materials processing applications, fiber lasers donot require preventative maintenance. Output optics and coolant may need to be

properly serviced by the user, but otherwise a fiber laser will perform consistentlywithout adjustment or other servicing. These low maintenance requirements are oneimportant factor in the low operating costs mentioned above.

7.8. Long Lifetime:

The operating lifetime is determined for fibre lasers by the lifetime of the diode laserpumps, which is estimated to be 100,000 hours. This is much greater than for Nd:YAGlasers and carbon dioxide lasers.

7.9. Reliability:

The manufacturers of commercial fibre lasers claim very high reliability for theirproducts. Their claim is that fibre lasers will operate continuously, 24 hours per day, fordecades. This claim is based on the simplicity of fibre lasers and on the fact that theyhave no components that degrade or require periodic maintenance.

7.10. Easy Coupling With Fibres:

Because the beam emerges from the end of the core of a fibre, it is very easy to direct itinto the end of another fibre. Simple bonding or end-to-end compression, well knowntechniques, can be used.


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7.11. Compact Size:

Fibre lasers can be contained in very small spaces. For example, one commercialytterbium fibre model emitting 100 W, capable of welding and cutting, is contained in arack mount 66 X 44.8 X 26.6 cm. This is much smaller than would be possible with aCO2 or Nd:YAG laser. Thus, for applications such as materials processing, use of a firerlaser can save valuable factory floor space reducing the amount of area that needs tobe heated, cooled and rented.

8. APPLICATIONS OF FIBRE LASERS :

Material processing is the area that has become the most developed application for thefibre lasers. This includes welding, cutting, micro-machining, marking, engraving and

wafer processing.8.1. Welding :

This is basically performed in two ways conduction welding and penetration welding.

a) Conduction Welding: Conduction welding is performed by delivering the laserbeam to a small area on the surface of the work piece. The laser energy isabsorbed at the surface. The energy heats the surface to a high temperature sothat a thin layer near the surface is melted. Energy is carried into the interior ofthe material by thermal conduction. This in turn melts the material to some depth.

The depth z to which the energy penetrates in time t is given approximately by:z=4 kt

where k is the thermal diffusivity of the material. Thermal diffusivity is a materialparameter which has dimensions of cm2/sec and describes how thermal energydiffuses through a material. Higher the value of thermal diffusivity, the greater the depththat can be melted in a given time.

The earliest uses of lasers for welding involved conduction welding and were carried outwith pulsed lasers or with continuous lasers having power below the 1000-1500 wattrange. Very early in the history of lasers it was demonstrated that conduction weldingcould produce high quality welds with small heat-affected zones and with full strength(that is tensile strength as high as that of the original material).


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b) Penetration Welding: In a different welding procedure, called penetration

welding, the laser delivers higher irradiance to the surface. The beam vaporizessome material, produces a hole in the material and energy is delivered to thebottom of the hole. This requires an irradiance of around 106 watts/cm2. Inpenetration welding the energy is delivered throughout the depth of the material.Thus, the depth is not limited by thermal conduction from the surface, and it ispossible to weld to much greater depths. Penetration welding is also sometimescalled deep penetration welding or keyhole welding. Usually multi-kilowatt lasersare used for penetration welding. Fiber lasers have been used for bothconduction welding and for penetration welding.

c) Welding Results: The use of fibre lasers is effective in welding applications. Mostof the welding applications of fibre lasers have involved ytterbium-doped fibresoperating near 1070 nm, because these lasers can emit the highest values ofpower. The figure shows a typical arrangement for welding applications. Themirrors for the fibre laser are shown as distributed Bragg reflectors. The figurebelow shows the the beam transmitted through a transmitting fibre to a remotework piece.

Fig.8.1.1. Typical arrangement for welding with a fiber laser


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8.2. Cutting:

Lasers have long been used for cutting, for example the cutting of metals in theautomotive industry. The laser of choice that has been used most often is the carbondioxide laser, often at a multi-kilowatt level. Now fiber lasers with multi-kilowatt

output are commercially available and are competing for metal cutting applications. As an example, a study using a fiber laser emitting 400 watts of power at awavelength of 1075 nm reported cutting rates around 2 m/minute for 2 mm thickstainless steel and about 6 m/minute for 2 mm thick mild steel. These cutting ratesincreased substantially as the sheet thickness decreased. The cut edges were ofhigh quality with little dross and small heat-affected zones.

Another study compared cutting rates at 4 kilowatts of power from a fiber laser and 4kilowatts from a carbon dioxide laser. The fiber laser cut relatively thin samples (1 - 2mm) at speeds about 5 times faster than the carbon dioxide laser. For greater thickness(10 mm) the fiber laser cut about 1.3 times faster. These results indicated that fiberlasers are valid candidates for sheet metal cutting applications.

The manufacturer of one type of fiber laser combined a fiber laser with linear-motor axisdrives to create a complete laser cutting system for sheet metal. They demonstratedthat their system cuts maintenance costs by up to 40% as compared to CO2 lasercutting systems. They have shown that their system can cut mild steel two to threetimes faster.Effective cutting with fiber lasers has also been demonstrated for non-metals, includingplastics, acrylics, polycarbonate and leather. Practical examples of cutting with pulsedfiber lasers include cutting silicon wafers for solar panels and stencil cutting. High powermultimode fiber lasers have been used for CW cutting of metals ranging from thinsheets to heavy plate for a variety of applications. The large depth of field and smallspot size of fiber lasers lead to small kerfs and straight walls -- even in thick metals.Common applications with high power multimode fiber lasers include cutting automotivebody parts like hydroform tubes.

8.3. Micro Machining:

Lasers have found many applications in micromaching. They have been used forapplications such as producing medical devices like stents, drilling holes for microvias incircuit boards, patterning of thin films, and repair of semiconductor memories. Theproperties that are needed for these applications include high peak power for rapidmaterial removal, short pulse length (nanosecond regime) for vaporization withoutproducing a large heat-affected zone, good beam quality for focusing to a small spot,and high pulse repetition rate for high volume production. Reasonably high averagepower is also desirable. Frequency-doubled or tripled Nd:YAG lasers have dominatedthese applications for many years. But fiber lasers have all the properties listed aboveand are now competing for these applications.


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8.4. Marking And Engraving:

Lasers have long been used for marking products. They have been used to imprintproduct identification, barcodes, serial numbers, logos, etc. Materials used in themarking process have included metals, plastics, glass, stone, wood, cardboard, jewelry,

etc. The laser used for a particular application is chosen with a wavelength that isabsorbed by the material to be marked. CO2 lasers have been used for materials likeplastics and wood, which have high absorption near 10 micrometers, while Nd:YAGlasers have often been used for metals, which are reflective in the far infrared.

Laser marking may be performed in several different ways. It may be in a dot matrixformat, in which the laser is repetitively pulsed and the beam is directed from pulse topulse to different spots on the target so as to form an alphanumeric character. Inanother technique, the laser beam may be spread over a broad area so as to strike areflecting mask in which the desired pattern is defined by vacant areas in the mask.

Using this technique, the entire pattern is formed in one laser pulse. Engraving isanother way to mark materials. Engraving involves t

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