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LASER SCIENCE & TECHNOLOGY An Overview Dr. BC Choudhary, Professor, Applied Physics NITTTR, Chandigarh-160019 Content Outlines Historical Developments Laser Types and Output Laser Beam Characteristics Major Application areas Laser Hazards and Safety Measures. LASER An Acronym for “ Light Amplification by Stimulated Emission of Radiations” One of the outstanding inventions of 20th century. A light source – but, very much different from traditional light sources. Not used for illumination purposes Widely used as a high power EM beam rather than a light beam. Common Light Source Vs Laser • Many wavelengths Monochromatic • Multidirectional Directional • Incoherent Coherent High Power IMPORTANCE Next to computers it is the laser that is bringing changes in our lives. Directly or indirectly it is helping us in living a better life. LASER: A generator of light – Store Energy It is a high technology device, used profitably in almost every field. Entertainment electronics, Industrial electronics, Consumer market, Communication, Mechanical industry, Metrology, Surveying, Surgery and related medical fields, Computers, Information processing, Sensing, Defense, Warfare etc. A HIGH TECHNOLOGY TOOL Drill bit: To drill holes in hard/soft materials A saw: To cut thick metal/non-metal sheets A phonograph needle: For compact discs A knife: During surgical operations A Target Designator: For military weapons Lasers in daily Life Military and Space aircraft are equipped with laser guns Airplanes are equipped with laser radar Bad eyesight can be corrected by optical surgery using lasers CD-Rom discs are read by lasers Dentists use laser drills Tattoo removal is done using lasers Laser tech. is used in printers, copiers, and scanners DVD players read DVD’s using lasers CD-Audio is read by a laser Laser pointers can enhance presentations Bar codes in grocery stores are scanned by lasers Video game systems such as PlayStation 2 utilize lasers Brief History of Laser 1917 - Einstein predicted the possibility of Stimulated radiations. 1952 - Charles H Townes, J. Gorden & H. Zeiger in USA and N. Basov & A. Prokhorov in USSR – independently suggested the principle of generating and amplifying microwave oscillations based on stimulated radiations. 1954 - Invention of MASER (Microwave Amplification by Stimulated Emission of Radiations). 1958 - Townes & Schawlow and Basov & Prokholov – independently extended the maser concept to optical frequencies i.e. LASER Townes, Basov and Prokhorov awarded Nobel Prizes for their work in this field. 1960 - Theodore Maimann – developed first laser using a Ruby crystal as amplifier and flash lamp as energy source. LASER HISTORY Sir Albert Einstein In 1917, the first foundation of laser was set in by Sir Albert Einstein with the concept of photons and stimulated emission of radiations. • In 1954, Charles Townes (Left) from US, Bosov (M) and Prokorov (R) from USSR put forwarded the details for the experimental set up for amplification of microwaves and the first MASER was discovered. Charles H. Townes (1915- 2015 ) Born in Greenville, South Carolina, Arthur L. Schawlow (1921-99) Born in Mount Vernon, N.Y. In 1958, Dr. Charles Townes (L) and Prof. Schawlow calculated the conditions for visible Laser light and theory of Stimulated Emission of radiations. At the same time, Basov and Prokhorov independently expressed their idea about extending the maser concept to optical frequencies i.e. Laser. Development of First Laser Theodore Maiman (1927-2007) Los Angeles, California In 1960, Dr.T. H. Maiman for the First time demonstrated the phenomenon of Laser Action using Ruby Crystal and the First Optical Laser was invented. Nobel Prize in Physics In 1964, Townes, along with two Russian laser Pioneers, Aleksander Prokhorov and Nikolai Basov, were awarded with The Nobel Prize in Physics. Major Landmarks in Development of Lasers Year Discoverer Type of Laser/Principle 1917 Albert Einstein Stimulated Emission 1952 N.G. Basov, A.M. Prokhorov and Townes Townes, Gorden, Zeiger Townes, Schawlow, Basov and Prokhorov Theodore Maiman A. Javan, W. Bennett and D. Harriott L.F. Johnson & K. Nassau R. Hall Maser Principle 1954 1958 1960 1961 1961 1962 Maser Laser Principle Ruby Laser Helium-Neon Laser Neodymium Laser Semiconductor Laser 1963 C.K.N. Patel Carbon Dioxide Laser 1964 W. Bridges Argon Ion Laser 1966 W. Silfvast, G.R. Fowles, and B.D. Hopkins He-Cd Laser 1966 P.P. Sorokin & J.R. Lankard Tunable Dye Laser 1975 J.J. Ewing & C. Brau Excimer Laser 1976 J.M.J. Madey & coworkers Free- electron Laser 1979 Walling & coworkers Alexandrite Laser 1985 D. Mathews & coworkers X-ray Laser Types of Lasers Solid State (Ruby, Nd:YAG, Ti:Sapphire, Diode) Powered by light or electricity Gas (He-Ne, CO2, Argon, Krypton) Powered by electricity Liquid (Dye) Powered by light Chemical (HF) Powered by chemical energy Semiconductor or Diode Lasers Direct e-h transfer/injection currents Visible Light Wave Region More than 150 lasers have been developed over whole range of the optical spectrum (IR-Visible-UV). WAVELENGTHS OF MOST COMMON LASERS Laser Type Argon fluoride (Excimer-UV) Krypton chloride (Excimer-UV) Krypton fluoride (Excimer-UV) Xenon chloride (Excimer-UV) Xenon fluoride (Excimer-UV) Helium cadmium (UV) Nitrogen (UV) Helium cadmium (violet) Krypton (blue) Argon (blue) Copper vapor (green) Argon (green) Krypton (green) Frequency doubled Nd -YAG (green) Helium Neon (green) Krypton (yellow) Copper vapor (yellow) Key: Wavelength (mm) 0.193 0.222 0.248 0.308 0.351 0.325 0.337 0.441 0.476 0.488 0.510 0.514 0.528 0.532 0.543 0.568 0.570 UV = ultraviolet (0.200-0.400 µm) VIS = visible (0.400-0.700 µm) NIR = near infrared (0.700-1.400 µm) Helium Neon (yellow) Helium Neon (orange) Gold vapor (red) Helium Neon (red) Krypton (red) Rohodamine 6G dye (tunable) Ruby (CrAlO3) (red) Gallium arsenide (diode-NIR) Nd:YAG (NIR) Helium Neon (NIR) Erbium (NIR) Holmium (NIR) Helium Neon (NIR) Hydrogen fluoride (NIR) Carbon dioxide (FIR) Carbon dioxide (FIR) 0.594 0.610 0.627 0.633 0.647 0.570-0.650 0.694 0.840 1.064 1.15 1.504 2.10 3.39 2.70 9.6 10.6 Various Types of Lasers Laser Output Pulsed Output (P) Energy (Watts) Energy (Joules) Continuous Output (CW) Time Watt (W) - Unit of power or radiant flux (1 watt = 1 joule per second). Time Joule (J) - A unit of energy Energy (Q) - Energy content is commonly used to characterize the output from pulsed lasers and is generally expressed in Joules (J). Irradiance (E) - Power per unit area, expressed in watts per square centimeter. Laser Beam Characteristics Laser light differs from the light emitted by conventional light sources. Most striking features are; Directionality High Coherence High Intensity Mono-Chromaticity Laser light can be produced as Polarized light Can be generated as very short pulses, at High power Directionality Conventional light sources emit light in all directions. Lasers emit light only in one direction (along cavity axis). Directionality of a laser beam expressed in terms of “ Beam Divergence” Beam Divergence Light from a laser diverges very little. Upto certain distance, beam remains a bundle of parallel light rays; distance from the laser over which the light rays remain parallel is called “Rayleigh range”. The laser beam diverges beyond Rayleigh range Divergence of a laser beam Divergence angle is measured from the center of the beam to the edge of the beam, Edge: location in the beam where intensity decreases to 1/e2 of that at the center. Twice the angle of divergence is known as full angle beam divergence Spot size Measure of how much the beam will spread as it travels through the space. Two parameters, which cause beam divergence 1. Size of the beam waist 2. Diffraction Full angle divergence is given by 2 4 d 0 where d0 = 2W0 is the diameter of the beam waist Divergence is inversely proportional to „d0‟ Beam waist and divergence of laser beam Large for a beam of small waist. Beam divergence due to diffraction is determined from Rayleigh’s criterion; 1.22 D ; D is the diameter of laser’s aperture In case of gas lasers, the diffraction divergence is about twice as large as beam-waist divergence. A typical value of divergence for a He-Ne laser is; 10-3 rad. implies that the laser beam diameter increases by about 1 mm for every metre it travels. Beam divergence of large lasers is micro-degree (10-6). A laser beam of 5 cm diameter (divergence 10-6 degree) when focused from earth spread to a diameter of only about 10m on reaching the surface of the moon An Extreme Collimation Laser beam Targeting The Moon APOLLO 11 Expedition Intensity Power output of laser may vary from a few mWs to few kWs. This energy is concentrated in a beam of very small crosssection High intensity Intensity of a laser beam approximately given by 2 10 I P Wm 2 where P is the power radiated by the laser. In case of 1mW He-Ne laser of wavelength, = 632810-10 m 100 10 3 11 2 I 2 . 5 10 Wm (6328 10 10 ) 2 To obtain same intensity from a Tungsten bulb, temperature have to be raised to 4.6106 K (normal operating temp. of bulb ~2000K) Brightness: Power per unit area per unit solid angle Brightness of Sun T 4 2 Bsun = = 1000 W. cm-2. Sr 1mW He-Ne laser, = 632810-10 m B He-Ne =300,000 W.cm-2. Sr = 300 Bsun Due to high emittance laser beams are not allowed to see directly Coherence Light waves are coherent if they are in phase with each other. maintain crest-to-crest and trough-to-trough correspondence. Two conditions Necessary for Coherence They must start with same phase at the same position. Wavelengths must be same otherwise they will drift out of phase crests of higher frequency wave will arrive ahead of the crests of lower frequency wave. Conventional light sources : Incoherent- light that emerges is a combination of photons in random manner Lasers: Coherent – output that emerges is a resultant of large number of identical photons, which are in phase. Coherence requires - a connection between the amplitude and phase of the light at one point and time, and the amplitude and phase of the light at another point and time. Two classes of Coherence Temporal Coherence (Longitudinal): The constancy and predictability of phase as a function of time when the waves travel along the same path at slightly different times. Spatial Coherence (Transverse): The phase relationship between waves traveling side by side at the same time but at some distance from one another. Temporal Coherence: Same phase for any time interval of same duration. For, (t2-t1) = (t4-t3) ; if 2 = 1 Temporally coherent waves • Characteristic of a single beam. T.C. characterised by two parameters • Coherence length, lcoh • Coherence time, tcoh Both measure how long light waves remain in phase as they travel in space. 2 c L coh 2 • Fluorescent tubes, lcoh = 5040 Ao • Sodium lamp, lcoh = 0.29 mm • He-Ne laser, lcoh = 100 m Monochromaticity - a measure of temporal coherence. Spatial Coherence: Phase difference of waves remains same all times. • Phase difference between E1 and E2 remains same (zero) at t1 and t2. • Spatial coherence measures the area over which light is coherent. Spatial incoherence arises due to size of the light source. Interference – a manifestation of coherence. More number of fringes – longer T.C. Degree of contrast – measure of S.C. Laser is both Temporally & Spatially Coherent to a high degree Monochromaticity Light coming for a source has only one frequency of oscillation. Monochromatic light from a monochromatic source IN PRACTICE, NOT POSSIBLE TO PRODUCE LIGHT WITH ONLY ONE FREQUENCY Light form any source consists of a band of frequencies ‘’ closely spaced around the central frequency, 0 - linewidth or bandwidth. Conventional sources : 1010 Hz or more. Light from Lasers : 100 Hz Polarization Light Waves: Electric & Magnetic fields vibrating perpendicular to each other and to the direction of propagation. Light as an electromagnetic wave Polarization (P): Measure of alignment of electric and magnetic fields in a light wave. • Types: Linear, Circular & Elliptical Simplest is Linear or Plane polarization Linearly polarized light beam: Orientation of electric field remains in one plane while its magnitude changes with time. Any other type of polarized light: A result of superposition of two linearly polarized waves having electric fields perpendicular to each other. Unpolarized light can be divided into two components with linear polarization, one with a vertical field and other with a horizontal field. Conventional light sources: Unpolarized light Laser output: Unpolarized or Polarized Applications of Lasers Profitably used in almost every field. Broadly divided into two groups involving laser beams of high power involving laser beams of low power. High power Gas and Solid State lasers are used in: material processing, nuclear fusion, medical field, defence etc. Low power (semiconductor lasers) are used in: CD players, laser printers, optical floppy discs, optical memory cards, data processing and information processing devices, range finders, holograms, optical communication etc. Some Important and Well Established Applications of Lasers LASERS IN MECHANICAL INDUSTRY Drilling Cutting Welding Heat Treatment LASERS IN ELECTRONICS INDUSTRY Scribing Soldering Trimming LASERS IN NUCLEAR ENERGY Isotope Separation Nuclear Fusion LASERS IN MEDICINES Diagnostics, Alignments Surgery, Therapy LASERS IN DEFENCE Ranging Weapon Guide Weapon itself MEASUREMENT OF DISTANCE Interferometric Methods Laser Rangers Optical Radar or LIDAR Surveying VELOCITY MEASUREMENTS Doppler Velocimeters: measuring fluid flow rates Portable velocity measuring meters • Used by traffic police HOLOGRAPHY Generation of Viewing Holograms of Holograms ENVIRONMENT STUDIES For measurement of concentrations of various atmospheric pollutants: gases & particulate matter. CONSUMER ELECTRONICS INDUSTRY Super Market Scanners, Compact Discs Optical Data Storage Optical Communication Optical Computer Laser Hazards Lasers can be hazardous if necessary control measures are not followed. Types of Laser Hazards Eye : Acute exposure of the eye to lasers of certain wavelengths and power can cause corneal or retinal burns (or both). Chronic exposure to excessive levels may cause corneal or lenticular opacities (cataracts) or retinal injury. Skin : Acute exposure to high levels of optical radiation may cause skin burns; while carcinogenesis may occur for UV wavelengths (290-320 nm) Chemical : Some lasers require hazardous or toxic substances to operate (i.e., chemical dye, Excimer lasers). Electrical : Most lasers utilize high voltages that can be lethal. Fire : Solvents used in dye lasers are flammable. High voltage pulse or flash lamps may cause ignition. Flammable materials may be ignited by direct beams or specular reflections from high power continuous wave (CW) infrared lasers. Common Laser Signs and Labels Laser Safety Standards and Hazard Classification Lasers are classified by hazard potential based upon their optical emission. Necessary control measures are determined by these classifications. In this manner, unnecessary restrictions are not placed on the use of many lasers which are engineered to assure safety. Laser classifications are based on American National Standards Institute’s (ANSI) Z136.1-Safe Use of Lasers. Laser Class Criterion used to classify lasers: 1. Wavelength. If the laser is designed to emit multiple wavelengths the classification is based on the most hazardous wavelength. 2. For continuous wave (CW) or repetitively pulsed lasers the average power output (Watts) and limiting exposure time inherent in the design are considered. 3. For pulsed lasers the total energy per pulse (Joule), pulse duration, pulse repetition frequency and emergent beam radiant exposure are considered. ANSI Classifications Class 1 : Laser or laser systems that do not, under normal operating conditions, pose a hazard. Class 2 : Low-power visible lasers or laser systems which, because of the normal human aversion response (i.e., blinking, eye movement, etc.), do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods of time. Class 3a : Lasers or laser systems having a CAUTION label that normally would not injure the eye if viewed for only momentary periods with the unaided eye, but may present a greater hazard if viewed using collecting optics. Class 3a lasers have DANGER labels and are capable of exceeding permissible exposure levels. If operated with care Class 3a lasers pose a low risk of injury. Class 3b : Lasers or laser systems that can produce a hazard if viewed directly. This includes intrabeam viewing of specular reflections. Normally, Class 3b lasers will not produce a hazardous diffuse reflection. Class 4 : Lasers and laser systems that produce a hazard not only from direct or specular reflections, but may also produce significant skin hazards as well as fire hazards. CONTROL MEASURES Engineering Controls Interlocks Enclosed beam Administrative Controls Standard Operating Procedures (SOPs) Training Personnel Protective Equipment (PPE) Eye protection Concluding Thoughts Laser technology has already contributed to furthering the goals of humanistic advancements New ideas and applications are changing our every-day Life as we know it… The key to managing today‟s rapidly evolving technology is to constantly analyze how each advance affects us as individuals and as a society as a whole. As we advance towards the mid century, it is inevitable that laser technology will play an increasingly important role in the society. . . References: 1. LASERS: Theory and Applications; MN Avadhanulu, S. Chand & Company Ltd. 2. Lasers & Optical Instrumentation; S.Nagabhushana and N. Sathyanarayana, IK International Publishing House (P) Ltd. 3. Experiments with He-Ne Laser, RS Sirohi, 2nd Ed. New Age International Publishers 4. http://www.colorado.edu/physics/lasers/ 5. www.Google.co.in/Search engine CAUTION: Do not look a laser with remaining eye!