Lasers: Fundamentals, Advances, and Applications PDF

LASERS Fundamentals, advances & applications Dr. Amandeep Kaur Department of Applied Engineering Chitkara University Institute of Engineering & Technology Chitkara University  Introduction...

LASERS Fundamentals, advances & applications Dr. Amandeep Kaur Department of Applied Engineering Chitkara University Institute of Engineering & Technology Chitkara University  Introduction LASER stands for Light Amplification & Stimulated Emission of Radiation  Stimulates atoms or molecules to emit light at particular wavelengths and amplifies that light to produce a very narrow beam of radiation. The first laser, Ruby (Al2O3) laser was invented by Dr. T. H. Maiman in the year 1960 LASER action can be shown by solids, liquids Although invented in 1960, new applications are still being found. Lasers have either completely replaced existing processes and techniques or enhanced their performance efficacy. Lasers have a use in every conceivable area of application  Material processing applications Laser-based material processing is a versatile technology. Process a wide range of materials for various applications, such as cutting, welding, drilling, marking, rapid manufacturing, ablation and so on. Advantages of non-contact processing, high positioning accuracy, a narrow heat-affected zone and a high processing speed. Such applications range from routine operations to more complex processes such as cutting fine intricate cardiovascular stents, drilling guide vanes in the aerospace industry and welding thick steel in the ship-building industry. TYPES Requiring limited laser power/energy and Involving phase changes and requiring causing no significant change in phase or higher power/energy to induce those state of the material before and after changes from solid to liquid or solid to processing. vapor phase.  LASER Parameters Laser parameters of importance in material-processing applications include:  Wavelength: Important part in determining what fraction of incident laser energy is absorbed by a given material. If photon energy is high enough, it may even break an atomic bond The wavelength absorption spectrum of the material is a very important factor in laser-based material processing. For example, Metals are also poor absorbers of CO2 laser energy ceramics and glasses have good absorption Plastics absorb laser energy even better than ceramics and glasses, especially in the UV and CO2 regions.  Pulse width: Pulsed lasers are used in most material-processing applications. CW lasers are mainly used for cutting, welding and heat-treatment applications. For example, longer pulse widths can be used for micromachining operations on stainless steel compared to nickel, due to the poorer thermal conductivity of nickel  Pulse energy and spot diameter: Pulse energy needs to be sufficiently high to heat up a useful volume of the material to be processed in each pulse. pulse energy and pulse width together determine the quality of processing. While wavelength and pulse width determine the initial reaction, average power controls the process rate.  Beam quality or M2 value: Plays an important role in determining the laser spot diameter for a given pulse energy and therefore the power density. The closer the value of M2 to 1, the tighter the focus. For welding and heat-treatment applications, a larger value of M2 in the range 30–100 is generally used.  LASER applications for Mechanical Industry 1. Laser Marking Lasers are used to imprint unique identification (UID) numbers on parts and products, which allow them to be easily traced. Both human-readable and barcode information can be laser-marked on products with flat or curved part geometries. Lasers working in different wavelength region are used for different type of materials Benefits: Cost savings from no consumables: The consumable cost for ink or labels far exceeds the initial cost of a laser. Permanence: Stickers fall off and ink fades, but laser etching, will stand the test of time. Visibility: A laser marking is easy to read and vibrant, far superior to a printed mark. Speed: Laser engraving is fast and simple. Flexibility: Laser marking works on a variety of materials- metals, plastics or natural materials. Minimal maintenance: Avoid downtime associated with cleaning or unclogging a printer. 2. Laser Cutting The focused laser beam is directed at the material, which then either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high-quality surface finish Benefits: It is versatile: The use is not limited to any one material-metals, ceramics, cardboards etc. Fast and accurate cutting: The kerf width in laser cutting is thin, less material is wasted, the cuts are more precise and efficiency is higher. Adaptable: Production can be changed mid process. Efficient: Multiple jobs can be done simultaneously. No need to clean materials: Laser cutting does not leave a mess and leaves smooth surfaces. Greener technology: Laser cutters are energy-intensive, but they are becoming vastly more efficient by fiber-optic lasers. Potential to combine with 3D printers: 3D printing is gaining steam in the same way laser cutting has 3. Laser Welding Laser welding is a process used to join together metals or thermoplastics using a laser beam to form a weld. Welding penetration up to 15 millimetre of steel or stainless steel can be achieved Benefits: Aesthetically better weld finishes More suited to high value items such as jewelery Great for inaccessible places Ideal for solenoids and machined components Better weld quality for a variety of metals and metal depths Overall improved productivity In Hybrid Laser-welding, the synergic effects of laser beam and electric arc results in an increase of welding speed and penetration depth along with the enhancement of gap bridging capability and process stability.  Other industrial applications Laser drilling  The process of creating thru-holes, referred to as “popped” holes or “percussion drilled” holes, by repeatedly pulsing focused laser energy on a material  Laser enables drilling of a diamond die in a few minutes as against 20 hours taken by conventional methods.  The plus point about laser drilling is that it does not cause any damage to the diamond or any other processed material. Laser Welding  To join pieces of metal or thermoplastics through the use of a LASER  The advantage of laser welding rests in the absence of physical contact with the electrode, in localized heating and cooling, in welding parts in a protective atmosphere or sealed into optically transparent material  Lasers can weld, e.g., air-tight shields of miniature relays, pacemakers, contacts in microelectronics, and metal sheets in car or aircraft industry.  Characteristics of LASER  High Directionality Ordinary light spreads in all directions, LASER is highly directional  High Intensity Due to high directionality, the intensity of laser beam reaching the target is very high The LASER spot can even appear brighter than Sun  Monochromatic The wavelength is single, whereas in ordinary light many wavelengths of light are emitted  Coherence In lasers the wave trains of same frequency are in phase, unlike ordinary light  Spontaneous Absorption, Spontaneous Emission Stimulated Emission (Einstein’s Theory)  Process 1: Stimulated Absorption An atom in the lower/ground energy level E1 absorbs the incident photon radiation of E2-E1=hυ and goes to the higher energy level E2 Rate of stimulated absorption= B12u(υ)N1  Process 2: Spontaneous Emission Excited atom returns to the ground state by emitting a photon of energy E2 – E1=hυ spontaneously This process is independent of incident radiation Rate of spontaneous emission=A21N2  Process 3: Stimulated Emission Excited atom return to the ground state by external triggering Results in emission of two photons of same energy, phase difference and of same directionality Rate of stimulated absorption= B21u(υ)N2 A and Bs are the Einstein’s coefficients; u(υ)= Energy density of incident radiation  Population inversion  Population Inversion Usually at thermal equilibrium, the number of atoms N2 at higher energy state is much lesser than the population of the atoms at lower energy state N1 The process of making N2> N1 is known as Population Inversion  Condition for Population inversion Normal distribution Population inversion 1. There must be at least two energy levels E2> E1 2. There must be a source to supply the energy to the medium. 3. The atoms must be continuously raised to the excited state.  Pumping Process to achieve the population inversion in the medium is called Pumping action. S. No. Method for Pumping Definition Applied for Optical Pumping Atoms in the lower state are excited by photons Solid state lasers like Ruby 1 laser and Nd-YAG laser Chemical Pumping Excitation is attained by means of suitable exothermal Chemical Oxygen Iodine Laser 2 chemical reactions in the active medium (COIL), All Gas Phase Iodine Laser (AGIL) etc. Injection Current Pumping: Injection of current through the junction also results in Semi conductor lasers GaAs 3 population inversion among the minority charge carriers. and InP Inelastic atom-atom collisions A mixture of two gases is used, electrons of one atom are Helium-Neon (He-Ne) laser 4 excited by electric field, and these excited electrons further excite the electrons of second gas. 5 Electrical discharge method Atoms are excited by passing electric discharge/providing Gas lasers like CO2 and Argon high volateg laser  Pumping Scheme Principle: Due to stimulated emission, photons multiply in each step-giving rise to an intense beam of photons that are Coherent and Unidirectional. Two level pumping: Not suitable to attain population inversion Why? The excited electrons go down to a lower energy level spontaneously within a very short time (10-9 s or 1ns) For stimulated emission, longer time (10-6 – 10-3 s or 1μs -1ms) in excited state is required Another metastable state or energy level is necessary.  Main components of LASER  Active medium: Basic material in which atomic or molecular transitions take place. Depending upon the active medium, LASERs are classified as Solid, liquid/dye, gas or semi-conductor  Pumping Source: With the help of this, atoms/molecules can be raised to excited state, Helps in attaining population inversion.  Resonator: Specially designed cylindrical tube, fitted with set of mirrors at the ends. One mirror is completely silvered and another partially silvered. Photons are emitted parallel to the axis of the active medium undergo multiple reflections to amplify the intensity.  Three and Four level pumping  Three-level pumping: There are three levels in an atom with E2>E1>E0 The atoms are raised from E0 to E2 from where they rapidly decay to E1 (meta-stable state). Transition E2→E1 : non-radiative Transition E1 →E0 : radiative (Lasing action) Drawback: High pump powers are required & produce LASER beam in pulse mode  Four-Level pumping: There are four levels in an atom with E3>E2>E1>E0 The atoms in state E0 are pumped to E3 from where they rapidly decay to level E2(metastable state). Population inversion is achieved between levels E2 and E1 From E1, atoms go to ground state rapidly Transition E3→E2 : non-radiative Transition E2 →E1 : radiative (Lasing action) Transition E1→E0 : non-radiative Four-level pumping is better because E1 has zero population  Types of LASERs Gas lasers: Electric current is sent through a gas to generate light through population inversion. Eg. CO2 lasers, He-Ne lasers, Ar lasers. Applications: Holography, spectroscopy, barcode scanning, air pollution measurements, material processing and laser surgery, laser marking, laser cutting, and laser welding. Liquid lasers: Use an organic dye in liquid form as their gain medium. Advantage is that they can generate a much wider range of wavelengths. Applications: Laser medicine, spectroscopy, birthmark removal, and isotope separation. Solid state lasers: Use a solid (crystals or glasses) mixed with a rare earth element as their source of optical gain. Neodymium, Chromium, Erbium, Thulium, or Ytterbium are commonly used elements. Eg. Ruby Laser, Nd:YAG laser (neodymium-doped yttrium aluminum garnet). Applications: LIDAR technology, medical applications-tattoo and hair removal, tissue ablation, and kidney stone removal. Fiber Laser: Special type of solid-state laser where the gain medium is an optical fiber (silica glass) mixed with a rare- earth element. Have small footprint, good electrical efficiency, low maintenance and low operating costs. Applications: Material processing (laser cleaning, texturing, cutting, welding, marking), medicine, and directed energy weapons.  Operating Pumping LASER Applications Wavelength Mechanism Holography, tattoo removal. The first laser, invented by Theodore Ruby laser 694.3 nm Flashlamp Maiman in May 1960. Material processing, rangefinding, laser target designation, Flashlamp, laser Nd:YAG laser 1.064 μm, (1.32 μm) surgery, tattoo removal, hair removal, research, pumping other diode lasers, dental laser 390-435 nm (stilbene), 460-515 nm Research, laser medicine, spectroscopy, birthmark removal, isotope (coumarin 102), 570- Other laser, Dye lasers separation. The tuning range of the laser depends on which dye is 640 nm flashlamp used. (rhodamine 6G), many others 632.8 nm (543.5 nm, Helium–neon 593.9 nm, 611.8 nm, Interferometry, holography, spectroscopy, barcode scanning, Electrical discharge laser 1.1523 μm, 1.52 μm, alignment, optical demonstrations. 3.3913 μm) Carbon dioxide Material processing (laser cutting, laser beam welding, 10.6 μm, (9.4 μm) Electrical discharge laser etc.), surgery, dental laser, military lasers. Solid Laser Liquid Laser Gas Laser  He-Ne Laser  A Helium-Neon laser, usually called He-Ne Laser, is a Gas Laser. He-Ne Lasers have many industrial and scientific uses and are often used in laboratories for experimental demonstration  He-Ne Laser is a four level Laser  Its usual operational wavelength is 632.8nm, lying in red-region of visible spectrum  It operates in CW mode  Long discharge tube, length of 50 cm and diameter 1cm  Mixture of He and ne in ratio 10:1 is used  Left side is fully silvered mirror and right side is partial silvered mirror  Contains RF generator also  He-Ne Laser: working  In He-Ne Laser, high voltage DC is used as pump source. A high voltage DC produces electrons, that travel through gas medium  The energetic electrons collide with and excite the HE-Ne atoms to metastable states 20.6 eV and 20.66 eV respectively above the ground state  He atoms help in achieving population inversion  When an excited Ne atom passes spontaneously from the meta-stable state at 20.66 eV to 18.70 eV state, it emits 632.8 nm photon  This photon moves back and forth between two reflecting ends and stimulates all other excited atoms  Finally we get a laser beam Applications The narrow beam of He-Ne laser is used in supermarkets to scan bar-codes Used in holography to produce 3D images of objects Advantages Laser light lies in visible portion High stability Low cost Operates without damage at high temperatures Disadvantages Low efficiency Low gain system Limited to low power tasks only  INTRODUCTION TO SEMICONDUCTORS  Semiconductor has conductivity between conductor and insulator  Doping a semiconductor with a small amount of impurity atoms greatly increases the number of charge carriers within it.  When a doped semiconductor contains excess holes it is called "p- type", and when it contains excess free electrons it is known as "n-type Principle and working of a semiconductor laser  When a p-n junction diode is forward biased, the electrons from n – region and the holes from the p- region cross the junction and recombine with each other.  During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As. This light radiation is known as recombination radiation.  The photon emitted during recombination stimulates other electrons and holes to recombine. As a result, stimulated emission takes place which produces laser.  When the PN junction is forward biased with large applied voltage, the electrons and holes are injected into junction region in considerable concentration. The region around the junction contains a large amount of electrons in the conduction band and a large amount of holes in the valence band. If the population density is high, a condition of population inversion is achieved. The electrons and holes recombine with each other and this recombination’s produce radiation in the form of light. When the forward – biased voltage is increased, more and more light photons are emitted and the light production instantly becomes stronger. These photons will trigger a chain of stimulated recombination resulting in the release of photons in phase. The photons moving at the plane of the junction travels back and forth by reflection between two sides placed parallel and opposite to each other and grow in strength. Characteristics 1. Type: It is a solid state semiconductor laser. 2. Active medium: A PN junction diode made from single crystal of gallium arsenide is used as an active medium. 3. Pumping method: The direct conversion method is used for pumping action 4. Power output: The power output from this laser is 1mW. 5. Nature of output: The nature of output is continuous wave or pulsed output. 6. Wavelength of Output: gallium arsenide laser gives infrared radiation in the wavelength 8300 to 85000 A. Advantages Disadvantages 1. It is very small in dimension. The arrangement is simple and compact. 1. It is difficult to control the mode pattern and mode structure of laser. 2. It exhibits high efficiency. 2. The output is usually from 5 degree to 15 degree i.e., laser 3. The laser output can be easily increased by controlling the beam has large divergence. junction current 3. The purity and monochromacity are power than other types 4. It is operated with lesser power than ruby and CO2 laser. of laser 5. It requires very little auxiliary equipment 4. Threshold current density is very large (400A/mm2). 6. It can have a continuous wave output or pulsed output. 5. It has poor coherence and poor stability. Application: 1. It is widely used in fibre optic communication 2. It is used to heal the wounds by infrared radiation 3. It is also used as a pain killer 4. It is used in laser printers and CD writing and reading.