Brijesh Laser Technology.pdf

Name :- Brijesh Yadav Roll No :- 257 ( Computer ) Std :- S.Y.B.Com Subject :- Foundation Course Topic Name :- Laser Technology College Name :- Khar Education Society’s College Of Commerce & Economics

INDEX Sr.no 1 2 3 4 5 6 7 8 9 10 11

CONTENTS Introduction The History Of Laser Technology Fundamentals Terminology Crystal Growth Development Of Compact Diode-Pumped Solid-State Laser Sources Lasers Applications Continuous Or Pulsed Modes Operation Types Of Operating Principles Conclusion Bibliography

INTRODUCTION The Lasers Technology Program of IPEN is committed to the development of new lasers based on the research of optical materials and new technologies, as well to laser applications in several areas: Nuclear, Medicine, Dentistry, Industry, Environment and Advanced Research. Additional goals of the Program are human resource development and innovation, in association with Brazilian Universities and commercial partners. The Program is basically divided into two main areas. “Material and Laser Development”, includes crystal growth of optical materials (laser crystals); characterization, modeling and optical spectroscopy of solids, plasmas and biological materials; development of compact diode pumped-solid state lasers and the development of a high power (TW) laser system, one of the main projects of the . High power ultrashort pulses lasers based on Chirped Pulse Amplification technologies and Ti:Sapphire gain media, allowed the generation of TW peak power on top of conventional optical tables and the realization of 18 2 relativistic intensities (10 W/cm ) at modest costs. The main area, “Laser Applications”, is concerned with technological laser uses such as laser processing, laser remote sensing, development of new diagnostic and therapeutic methods such as optical coherence tomography (OCT), laser Doppler flowmetry, photosensitization, prevention of dental caries, plus other advanced applications of high intensity lasers. Recent activities are highlighted bellow: Development of single crystal fibers for compact laser systems; -

Growth of a solid solution LiGd Lu Nd F :Nd crystal suitable to obtain a laser 0.232 0.75 0.018 4 medium for mode-locking purposes; - Characterization, modeling and optical spectroscopy of rare-earth doped crystals and glasses for the development of solid laser medium; - First single crystal Nd:YLF fiber laser; Evaluation of the performance of fs laser-induced Breakdown Spectroscopy (fs-LIBS) for the determination of elements in animal tissues. - New method for the evaluation of microvascular functionality using low-frequency fluctuations in the laser Doppler flow signal; - Construction of an automatized workstation with ultrashort laser pulses (femtoseconds) for the study of thermal and non-thermal processes in dielectrics, semiconductors and metals; - Study of a therapeutic method combining Nd:YAG laser and topical fluoride treatment for effective reduction of caries incidence in patient. - Development of studies showing that photodynamic antimicrobial therapy is able to reduce 99% of multi-resistant bacteria in burn wounds. Analysis of Optical Coherent Tomography applied to dermatology (research work winner of the Natura Campus 2010 Premium for Technological Innovation), - New LIDAR system for Industrial Emission and Detection installed in Cubatão/SP (collaboration in The National Institutes of Science and Technology Program /INCT). - Studies for isotope enrichment by ultrashort laser pulses.

THE HISTORY OF LASER TECHNOLOGY The origins of the laser can be traced back to the turn of the 20th Century. Since then lasers have increased in power and scale and are ubiquitous in our modern world. The laser or, light amplification by stimulated emission of radiation to give it its full name, has come a long way since its development in the 1960's. Today laser technology is ubiquitous in our modern world with applications from medical uses, telecommunications, and even weapon systems. In the following article, we'll take a very quick tour through the main events that led to the development of the laser and look at some future, in development, applications for lasers. What follows is a list of selected milestones in the fascinating and exciting development of laser technology. This list is far from exhaustive and is in chronological order. 1. Max Planck Kicks it All off The importance of the laser innovation or milestone: Max Planck, in 1900, deduced the relationship between energy and the frequency of radiation. He was the first to postulate that energy could be emitted or absorbed in discrete chunks or quanta. Year of Discovery/Development: 1900 Engineer or scientists behind the project: Max Planck Description of Milestone: Although Planck's theory was groundbreaking in its own right it had one very important effect. Planck's insight would inspire one of the most influential scientists of our age - Albert Einstein. Einstein would build on Planck's theory to release his paper on the photoelectric effect.

He proposed that light also delivers energy in chunks, or discrete quantum particles, called photons. 2. Einstein's Concept and Theory of Stimulated Light Emission The importance of the laser innovation or milestone: Einstein's theory would pave the way for the eventual development of the first practical lasers. Year of Discovery/Development: 1916-1917 Engineer or scientists behind the project: Albert Einstein Description of Milestone: Albert first theorized about the stimulation of light emission way back in 1917. In his paper, Zur Undertheorize der Strahlung (On the Quantum Theory of Radiation) he recorded his thoughts on this subject. He used Planck's law of radiation to describe probability coefficients (Einstein coefficients) for absorption and spontaneous and stimulated emission of EM radiation, including light. His theory proposed that electrons could be stimulated into emitting light of a particular wavelength. This would become the foundational principle of all lasers used today. It would take another 40 years or so before scientists were able to prove him right. 3. The Invention of Holography The importance of the laser innovation or milestone: Research into holography was stalled until the development of lasers in the 1960's. This would stimulate, in part, the development of both technologies thereafter. Holography is the means of producing a unique photographic image without the use of a lens. Holograms consist of a series of unrecognizable stripes and whorls that when illuminated by a

coherent light source, like a laser, become a 3D representation of the original image/object. Year of Discovery/Development: 1948 Engineer or scientists behind the project: Dennis Gabor Description of Milestone: Dennis Gabor, a Hungarian-born scientist, received the Nobel Prize for Physics for his invention in 1971. He was attempting to improve the resolution of electron microscopes by making holograms using the electron beam and then examining that with coherent light. At the time of discovery, it had little if any, practical use until the development of lasers in the 1960's. This would suddenly lead to an explosion in the use of holograms in the United States. Today this explosion has led to an enormous industry that includes HUD's. museum displays, VR, medical applications and solar panel efficiency. 4. The Rise of MASER (Microwave Amplification of Stimulated Emission of Radiation) The importance of the laser innovation or milestone: Microwave amplification by stimulated emission of radiation or MASER, was the first practical demonstration of Einstein's principles and used microwave radiation (instead of light in lasers). Year of Discovery/Development: 1954 Engineer or scientists behind the project: Charles Hard Townes, Arthur Schawlow, James P. Gordon, Herbert J. Zeiger Description of Milestone: MASERs are devices that produce and amplify EM radiation in the microwave part of the EM spectrum. In 1954 Townes and his research colleagues were able to demonstrate the first MASER at Columbia University. Their

Ammonia MASER would go down in history as the first device to demonstrate Einstein's prediction from 1917. It would successfully obtain the first amplification and generation of EM radiation through stimulated emission. The MASER radiates at a wavelength of a little more than 1 cm and generates approximately 10 nW of power. In March 1959 Townes and Schawlow were awarded the patent for their invention. MASER technology would go on to be used in to amplify radio signals and to be used as an ultra-sensitive detector. 5. The First Practical Laser is Patented The importance of the laser innovation or milestone: This was the first successful assembly of a complete laser device. It would be the first of many more to come. Theodore, a physicist at Hughes Research Laboratories in Malibu, California, built the first laser using a cylinder of mandmade ruby 1 cm in diameter and 2 cm long. Each end was coated with silver to make them reflective and help them serve as a Fabry-Perot resonator. His device used photographic flashlamps for the laser's pump source. Year of Discovery/Development: 1960 Engineer or scientists behind the project: Theodore H. Maiman Description of Milestone: After serving some time in the navy, Theodore earned his B.Sc. In Engineering Physics from the University of Colorado and then later earned his M.Sc. in Electrical Engineering and Ph.D. in Physics from Stanford University.

FUNDAMENTALS Lasers are distinguished from other light sources by their coherence. Spatial coherence is typically expressed through the output being a narrow beam, which is diffractionlimited. Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can have very low divergence in order to concentrate their power at a great distance. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency, whose phase is correlated over a relatively great distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase that vary randomly with respect to time and position, thus having a short coherence length Lasers are characterized according to their wavelength in a vacuum. Most "single wavelength" lasers actually produce radiation in several modes with slightly different wavelengths.

TERMINOLOGY The word laser started as an acronym for "light amplification by stimulated emission of radiation". In this usage, the term "light" includes electromagnetic radiation of any frequency, not only visible light, hence the terms infrared laser, ultraviolet laser, X-ray laser and gamma-ray laser. Because the microwave predecessor of the laser, the maser, was developed first, devices of this sort operating at microwave and radio frequencies are referred to as "masers" rather than "microwave lasers" or "radio lasers". In the early technical literature, especially at Bell Telephone Laboratories, the laser was called an optical maser; this term is now obsolete. A laser that produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by the acronym. It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation", would have been more correct.

CRYSTAL GROWTH The Crystal Growth Laboratory works on bulk, micro and nanocrystals research for materials properties studies and development of new lasers including lamp and diode pumped systems from fluoride single crystals, as LiREF4 and KRE3F10 (where RE = rare earth ions), and double Tung states single crystal fibers, as AB(WO ) (where A = Na, 4 2 Li and B = La, Gd, Nd, Yb, Eu). The research on crystal growth performed in this period was supported by CNPq, Fapesp and CAPES including collaboration projects with Federal University of Sergipe - UFS (CNPq “casadinho” project) and Federal University of Pernambuco - UFPE (CNPq-INCT program in National Institute for Science and Technology in Photonics). Bulk single crystals growth 3+ Thulium ions (Tm ) are suitable for frequency upconversion of infrared to visible light, because n these ions have long-lived high excited 4f states that give rise to strong blue luminescence. In this period the effect of Nd and Yb used as sensitizers to pump Tm ions has been studied. Two matrices have been utilized as host for these ions: LiYF4 (YLF) and KY F (KY3F), which have the 3 10 scheelite and fluorite structures, respectively. YLF:Yb:Nd:Tm crystals were grown by the Czochralski method and doped with 0.5 mol% Tm and/or 20 mol% Yb, and/or 1.3 mol% Nd. KY3F samples of good quality were obtained using a simple synthesis method consisted by the slow cooling of liquid charges under a HF+Ar atmosphere. This method reduced the time to produce samples with sufficient transparency for spectroscopic studies. KY3F:Yb:Nd:Tm crystals were doped with 0.5 mol% Tm, and/or

1.3 mol% Nd and different concentrations of Yb (5, 10, 20 and 30 mol%). Spectroscopic studies determined the mechanisms of energy transfer that lead to the thulium upconversion emissions in the blue and ultraviolet regions. YLF:Er (15 mol%), YLF:Nd (1.3 mol%) and YLF:Yb (10 mol%):Tm(1 mol%) crystals grown by the Czochralski method were also utilized to obtain lasers with new features. A solid solution LiGd Lu Nd F :Nd crystal 0.232 0.75 0.018 4 was successfully grown and the spectroscopic studies showed that Nd presents larger bandwidth at 792 and 797 nm, which can be suitable to obtain a laser medium for mode-locking purposes. Inspired in the results obtained in the growth of solid solutions of LiGd Lu F by zone melting and 1-x x 4 by crystal growth from the melt, it is in course the study of the LiF - GdF LuF phase diagram by 3 3 computational methods, using the commercial simulation program FactSage. - Compact, diodeside-pumped Nd :YLF laser at 1053 nm with 45% efficiency and diffractionlimited quality by mode controlling. The highest efficiency obtained to date and 9.5 W of output 3+ power in a transversely diode-pumped Nd :YLF slab laser operating at 1053 nm were obtained. - 620 mW Single-Frequency Nd:YVO /BiB O Red 4 3 6 Laser. Using a type-I critically phase-matched bismuth borate crystal, a record 620 mW singlefrequency red laser at 671 nm is achieved from intra-cavity second harmonic generation (SHG) of a p-polarized single-end pumped Nd:YVO ring 4 laser oscillating on the lambda similar to 1342 nm transition. - Record 1.3 W single-frequency red laser at 661 nm. Achieved from intracavity second harmonic generation of a Nd:YLF ring laser oscillating on the p-polarized transition (lambda similar to 1321 nm).

Single crystal fibers growth Despite new developments for compact laser 3+ devices, the Nd ion continues to be the most widely used active laser ion. In 2004 we started a program on single fiber growth by the micropulling down (µ-PD) method aiming the development of laser materials. The objective was to obtain single crystal fibers with constant diameter and good optical quality for laser tests. This project that included the installation of two µPD furnaces started with fluorides fibers growth, resulted in 2009/2010 in the demonstration, for the first time, of laser action from Nd:LiYF (Nd:YLF) 4 single crystal fibers.At the present other materials are under investigation as fluoride fibers of Nd:BaY F and rare earth double tungstates fibers 2 8 also for laser tests.We have prepared Nd:NaLa(WO ) ,Nd:NaGd(WO ) and Eu,Yb and 4 2 4 2 Nd doped LiLa(WO ) .The structural and optical 4 2 characterizations of these materials are under investigation to improve their quality for future laser tests.

DEVELOPMENT OF COMPACT DIODE-PUMPED SOLID-STATE LASER SOURCES This activity comprises the development of new laser sources based on diode pumped solid state lasers (DPSSL) for applications in research, industry, medical and pollution control. Our investigations are focused mainly on controlling the temporal, spectral and spatial features of the laser beam. In some cases it also includes the production design of such systems including the reliability tests and application experiments. We have built several DPSSL systems emitting from the blue up to the far infrared. Some highlights of the last period are shown below: - A high power 2.3 mm Yb:Tm:YLF laser diodepumped simultaneously at 685 and 960 nm. The achieved output power of 620 mW is the highest reported so far. - First single crystal Nd:YLF fiber laser. Fiber orientation (Fig. 5) and the optical arrangement used in diode laser pumping Production and optical characterization of active laser media based on nanopowders and metamaterials This activity comprises the development of new laser sources based on diode pumped dispersive media. Lasing action in Nd: YVO nanopowder has 4 4 4 been analyzed by investigating the F → I 3/2 11/2 transition. A method to quantitatively determine the upconversion rate and the contribution of the spontaneous emission in the samples backscattering emission that includes

the random laser emission as a function of pump power has been created. Characterization, modeling and optical spectroscopy of rareearth doped solid laser media A luminescence spectroscopic system with spectral and temporal discrimination that uses a Box-car technique and tunable laser excitations of 4 ns (10 Hz) in the range of 420 to 2000 nm (10mJ), was used for lifetime measurements of rareearth ions in glasses and fluorides crystals. These measurements allowed determining the rate constant of the nonradiative energy transfer that happens due to dipole-dipole interactions between donor and acceptor ions in solids. Energy transfer mechanism involving two interacting erbium (and holmium) ions in the first (and second) excited state, energytransfer upconversion has been observed and the rate constant determined. The aim of this study is the development of solid laser medium emitting in the mid-infrared (~2750 nm) and to improve light signal amplifiers based on thulium-doped materials that operate in S-band of telecommunication (14701500 nm). A detailed investigation of the energy transfer process from the first excited state of 3+ holmium caused by PR ions in fluorozirconate (ZBLAN) glass was carried out. Solving the rate

LASERS APPLICATIONS Microvascular function evaluation by fluctuations in the laser Doppler flow signal

low

frequency

The laser Doppler flowmetry has been used to study microvascular dysfunctions, common in diabetics and chronic smokers. Low-frequency fluctuations in the laser Doppler flow signal (LDFS) from the skin are related to microvascular mechanisms of flow control. Wavelet spectral analysis has been used to correlate fluctuations in the LDFS with the endothelial, neurogenic and myogenic mechanisms of control in the frequency intervals 0.0095-0.02 Hz, 0.02-0.06 Hz and 0.06- 0.16 Hz, respectively. Generally the signal power, in each frequency interval, derived from the respective wavelet coefficients, is used as a measure of the activity of the related mechanism of microvascular control. However, the time-domain characteristics of the fluctuations in the LDFS in each frequency interval are poorly known. As a consequence, there is a lack of objective criteria to properly measure, in each frequency interval, the related hemodynamic parameters. A time-domain method was developed to analyze and quantify fluctuations in the LDFS in each frequency band. Baseline and thermally stimulated LDFS of forearms from healthy volunteers were collected and analyzed. The data obtained indicate that inappropriate time windows, frequently used for

measurements, increase the variability of the measured signal power, diminishing the capability of the method when assessing microvascular dynamics and dysfunctions. Objective criteria were proposed to diminish the measured hemodynamic parameters variability, improving the method sensitivity. Potential applications are assessing endothelial, neurogenic and myogenic dysfunctions. Laser processing of special materials: thermal and non-thermal process Modern technological advances have demanded the development of new materials like high mechanical strength steels, superalloys, ceramics and composites, besides very small pieces with complex geometrical forms. Consequently, traditional milling and welding processes can no longer fulfill the requirements demanded by modern applications. Hence, laser processing comes as very useful and versatile alternative method, and has been used here for cutting welding, heat treating and ablating of some materials for important technological applications. In welding, a pulsed Nd:YAG laser has been used to join very thin foils of alloys highly resistant to corrosion with the purpose of using then as protective shields for sensors against harsh media . Pressure, flow and temperature sensors used in many industrial and nuclear plants must withstand extreme conditions of pressure, temperature and corrosive environments. This is done by covering these devices with foils of special alloys with 100prevention with laser was also

studied in vitro and a clinical trial was carried out to prove the safety and effectiveness of the method. A cream for topical use for the clinical photodynamic therapy of skin cancer was developed, tested and patented in Veterinary Medicine. We are also studying the potential treatment of burned skin with high intensity femtosecond laser as well as the potential use of erbium laser to cut bone during surgery High Intensity Lasers Operation and optimization of the TW peak power laser and applications High power ultrashort pulses lasers based on CPA (Chirped Pulse Amplification) technologies that allow the study, in conventional laboratory scales, of phenomena that only 10 years ago were restricted to national laboratories with annual budgets of billions of dollars. In the Center for Lasers and Applications at IPEN, a hybrid Ti:Sapphire/Cr:LiSAF TW peak power laser system is under continuous development. A flashlamp pumping cavity for a Cr:LiSAF gain medium in the shape of a rod was built. The pumping cavity was developed aiming to minimize the thermal load on the Cr:LiSAF crystal by the use of absorption filters between the filters and the gain medium, allowing the amplification of ultrashort pulses to the terawatt peak power region at high repetition rates.

CONTINUOUS OR PULSED MODES OPERATION A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. Of course even a laser whose output is normally continuous can be intentionally turned on and off at some rate in order to create pulses of light. When the modulation rate is on time scales much slower than the cavity lifetime and the time period over which energy can be stored in the lasing medium or pumping mechanism, then it is still classified as a "modulated" or "pulsed" continuous wave laser. Most laser diodes used in communication systems fall in that category. Continuous wave operation Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as continuous wave (CW). Many types of lasers can be made to operate in continuous wave mode to satisfy such an application. Many of these lasers actually lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will, in fact, produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the frequency spacing between modes), typically a few nanoseconds or less. In most cases, these lasers are still termed "continuous wave" as their output power is steady when

averaged over any longer time periods, with the very highfrequency power variations having little or no impact in the intended application. (However, the term is not applied to mode-locked lasers, where the intention is to create very short pulses at the rate of the round-trip time.) For continuous wave operation, it is required for the population inversion of the gain medium to be continually replenished by a steady pump source. In some lasing media, this is impossible. In some other lasers, it would require pumping the laser at a very high continuous power level which would be impractical or destroy the laser by producing excessive heat. Such lasers cannot be run in CW mode. Pulsed operation Pulsed operation of lasers refers to any laser not classified as continuous wave, so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing a number of different motivations. Some lasers are pulsed simply because they cannot be run in continuous mode. In other cases, the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up in between pulses. In laser ablation, for example, a small volume of material at the surface of a work piece can be evaporated if it is heated in a very short time, while supplying the energy gradually would allow for the

heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point. Other applications rely on the peak pulse power (rather than the energy in the pulse), especially in order to obtain nonlinear optical effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as Q-switching. The optical bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width. In the case of extremely short pulses, that implies lasing over a considerable bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers. The lasing medium in some dye lasers and vibrionic solid-state lasers produces optical gain over a wide bandwidth, making a laser possible which can thus generate pulses of light as short as a few femtoseconds (10−15 s).

TYPES OF OPERATING PRINCIPLES Gas lasers Following the invention of the He Ne gas laser, many other gas discharges have been found to amplify light coherently. Gas lasers using many different gases have been built and used for many purposes. The helium–neon laser (He Ne) is able to operate at a number of different wavelengths, however the vast majority are engineered to lase at 633 nm; these relatively low cost but highly coherent lasers are extremely common in optical research and educational laboratories. Commercial carbon dioxide (CO2) lasers can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot. This emission is in the thermal infrared at 10.6 µm; such lasers are regularly used in industry for cutting and welding. The efficiency of a CO2 laser is unusually high: over 30%. Argon-ion lasers can operate at a number of lasing transitions between 351 and 528.7 nm. Depending on the optical design one or more of these transitions can be lasing simultaneously; 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, often home-built by hobbyists, which produces rather incoherent UV light at 337.1 nm. Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (He Ag) 224 nm and neon-copper (Ne Cu) 248 nm are two examples. Like all lowpressure gas lasers, the gain media of these lasers have quite

narrow oscillation linewidths, less than 3 GHz (0.5 picometers),[30] making them candidates for use in fluorescence suppressed Raman spectroscopy. Chemical lasers Chemical lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of interest to the military, however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications. As examples, 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. Excimer lasers Excimer lasers are a special sort of gas laser powered by an electric discharge in which the lasing medium is an excimer, or more precisely an exciplex in existing designs. These are molecules which can only exist with one atom in an excited electronic state. Once the molecule transfers its excitation energy to a photon, its atoms are no longer bound to each other and the molecule disintegrates. This drastically reduces the population of the lower energy state thus greatly facilitating a population inversion. Excimers currently used are all noble gas compounds; noble gasses are chemically inert and can only form compounds while in an excited state. Excimer lasers typically

operate at ultraviolet wavelengths with major applications including semiconductor photolithography and LASIK eye surgery. Commonly used excimer molecules include ArF (emission at 193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm).[31] The molecular fluorine laser, emitting at 157 nm in the vacuum ultraviolet is sometimes referred to as an excimer laser, however this appears to be a misnomer inasmuch as F2 is a stable compound. Solid-state lasers Solid-state lasers use a crystalline or glass rod which is "doped" 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. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flashtube or from another laser. The usage of the term "solid-state" in laser physics is narrower than in typical use. Semiconductor lasers (laser diodes) are typically notreferred to as solid-state lasers. 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 and 266 nm (UV) beams, respectively.

Frequency-doubled diode-pumped solid-state (DPSS) lasers are used to make bright green laser pointers. Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers.[32] 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. It is also notable for use as a mode-locked laser producing ultrashort pulses of extremely high peak power. Thermal limitations in solid-state lasers arise from unconverted pump power that heats the medium. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can cause thermal lensing and reduce the quantum efficiency. Diode-pumped thin disk lasers overcome these issues by having a gain medium that is much thinner than the diameter of the pump beam. This allows for a more uniform temperature in the material. Thin disk lasers have been shown to produce beams of up to one kilowatt.

Fiber lasers Solid-state lasers or laser amplifiers where the light is guided due to the total internal reflection in a single mode optical fiber are instead 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 longliving color centers. Semiconductor lasers Semiconductor lasers are diodes which are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflection from the ends of the crystal form an optical resonator, although the resonator can be external to the semiconductor in some designs. Commercial laser diodes emit at wavelengths from 375 nm to 3500 nm.[35] Low to medium power laser diodes are used in laser pointers, laser printers and CD/DVD players. Laser diodes are also frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 20 kW, are used in industry for cutting and welding.[36] 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. In 2012, Nichia and OSRAM developed and manufactured commercial high-power green laser diodes (515/520 nm), which compete with traditional diode-pumped solid-state lasers.[37][38] 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. As of 2005, only 850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,[39] 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 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. Recent developments have also shown the use of monolithically integrated nanowire lasers directly on silicon for optical interconnects, paving the way for chip level applications.[40] These heterostructure nanowire lasers capable of optical interconnects in silicon are also capable of emitting pairs of phase-locked picosecond pulses with a repetition frequency up to 200 GHz, allowing for on-chip optical signal processing.[12] Another type is a Raman laser, which takes advantage of Raman scattering to produce a laser from materials such as silicon. Lasing without maintaining the medium excited into a population inversion was demonstrated in 1992 in sodium gas and again in 1995 in rubidium gas by various international

teams.[41][42] This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths, so that the likelihood for the ground electrons to absorb any energy has been cancelled. Dye lasers Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes, or mixtures of dyes, allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order of a few femtoseconds). Although these tunable lasers are mainly known in their liquid form, researchers have also demonstrated narrow-linewidth tunable emission in dispersive oscillator configurations incorporating solid-state dye gain media. In their most prevalent form these solid state dye lasers use dye-doped polymers as laser media. 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.

CONCLUSION Two-way laser communication in space has long been a goal for NASA because it would enable data transmission rates that are 10 to 1,000 times higher than traditional radio waves. While lasers and radio transmissions both travel at light-speed, lasers can pack more data. It's similar to moving from a dial-up Internet connection to broadband. If the technical hurdles can be overcome, lasers would benefit not only communications, but basic science as well. Astronomers could use lasers like very accurate rulers to measure the movement of planets with unprecedented precision. With microwaves, we're limited to numbers like a meter or two in distance, whereas [lasers have] a potential for getting down into well beyond the centimeter range. Lasers could also be used refine basic principles of fundamental physics. If you could make planetary scale measurements at the centimeter or millimeter level--which we can't at the moment-then we could understand some principles of relativistic physics which can only be tested at very extreme accuracies at very large distances.

As seen previously, the great efficiency and directionality of laser beams is the source of an extremely high hazard for the eye. Moreover, the wavelengths responsible for damages on the eye (cornea excepted) at the lowest intensities are in the domain 400 to 1400 nm. Indeed, if one considers that the image of a collimated beam on the retina has a width of about 10 mm, and that the diameter of a dilated pupil is of about 7mm, then a pulse carrying a few µJ or a continuous He-Ne laser beam of 1 m W are powerful enough to create permanent lesions on the retina. In order to find the spectral domain where the damage threshold on the eye is the highest, and therefore the hazard the lowest, one has to find a compromise between the absorption spectra of the different parts of the eye. Thus, concerning a laser emitting between 1.5 and 1.55 µm, the absorption is negligible in the crystalline lens, 75% occurs inside the cornea, and 25% inside the aqueous humor. We see that most of the energy is absorbed inside the cornea. The absorption is distributed along the relatively high thickness of this optical element, and thus the energy absorbed per volume unit is too weak to provoke important damages. The cornea is as resistant to radiations as the skin. Moreover, it has a very high regeneration ability. But outside of the previously defined wavelength range, the eye faces high hazard levels... Laser sources emitting in the so-called “ocular safety domain” (1.5 to 1.55 µm) can freely propagate without inducing laser hazard. These are so-called “ocular safety laser sources”.

BIBLIOGRAPHY • https://www.ipen.br/portal_por/conteudo/documentos/2 01205161524250.3.lasers_completo.pdf • https://interestingengineering.com/the-history-of-lasertechnology-and-what-it-can-do-today • https://en.wikipedia.org/wiki/Laser • https://www.google.com/search?q=laser+technology+pdf &rlz=1C1CHBF_enIN831IN831&oq=lazer+tecnology&aqs=c hrome.2.69i57j0l5.16617j0j8&sourceid=chrome&ie=UTF-8 • https://www.google.com/search?rlz=1C1CHBF_enIN831IN 831&ei=KCdUXLONIi8vwSkhZP4CQ&q=laser+technology&oq=lazer+&gs_l=ps yab.3.1.0i67j0i10i67l2j0i67j0i10l6.7916.17972..20093...1.0.. 5.311.4151.0j23j2j1......0....1..gwswiz.....6..0i71j35i39j0j0i22i30j0i131.fpnF3iFG2Fc