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UNIT-3 LEC-3 • • • • ESSENTIAL COMPONENTS OF A LASER; TYPES OF LASER, CO 2 LASER, Nd – YAG LASER (Doped Insulator laser), UNIT III Lecture 3 1 Essential components of a laser system : Active medium or Gain medium : It is the system in which population inversion and hence stimulated emission (laser action) is established. Active Medium Pumping Mechanism Optical resonator Pumping mechanism : It is the mechanism by which population inversion is achieved. i.e., it is the method for raising the atoms from lower energy state to higher energy state to achieve laser transition. UNIT III Lecture 3 2 DIFFERENT PUMPING MECHANISMS : i. Optical pumping : Exposure to electromagnetic radiation of frequency = (E2-E1)/h obtained from discharge flash tube results in pumping Suitable for solid state lasers. ii. Electrical discharge : By inelastic atom-atom collisions, population inversion is established. Suitable for Gas lasers iii. Chemical pumping : By suitable chemical reaction in the active medium, population of excited state is made higher compared to that of ground state Suitable for liquid lasers. iv. Optical resonator : A pair of mirrors placed on either side of the active medium is known as optical resonator. One mirror is completely silvered and the other is partially silvered. The laser beam comes out through the partially silvered mirror. UNIT III Lecture 3 3 Types of Lasers(Based on its pumping action) : •Optically pumped laser •Electrically pumped laser • Basis of the operation mode •Continuous wave Lasers •Pulsed Lasers According to their wavelength : •Visible Region, Infrared Region, Ultraviolet Region, Microwave Region, X-Ray Region and etc., According to the source : •Dye Lasers, Gas Lasers, Chemical Lasers, Metal vapour Lasers, Solid state Lasers, Semi conductor Lasers and other types. UNIT III Lecture 3 4 DYE LASERS Laser gain medium and type Dye lasers Operation wavelength(s) 390-435 nm (stilbene), 460-515 nm (coumarin 102), 570-640 nm (rhodamine 6G), many others Pump source Other laser, flash lamp UNIT III Lecture 3 Applications Research, spectroscopy, birthmark removal, isotope separation. The tuning range of the laser depends on which dye is used. 5 GAS LASERS LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S) Heliumneon laser 632.8 nm (543.5 nm, 593.9 nm, 611.8 nm, 1.1523 μm, 1.52 μm, 3.3913 μm) Argon laser 454.6 nm, 488.0 nm, 514.5 nm (351 nm,457.9 nm, 465.8 nm, 476.5 nm, 472.7 nm, 528.7 nm) Krypton laser Xenon ion laser PUMP SOURCE APPLICATIONS AND NOTES Electrical discharge Interferometry, holography, spectroscopy, barcode scanning, alignment, optical demonstrations. Electrical discharge Retinal phototherapy (for diabetes), lithography, confocal microscopy, pumping other lasers. 416 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm, 752.5 nm, 799.3 nm Electrical discharge Scientific research, mixed with argon to create "white-light" lasers, light shows. Many lines throughout visible spectrum extending into the UV and IR. Electrical discharge Scientific research. UNIT III Lecture 3 6 LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S) PUMP SOURCE APPLICATIONS AND NOTES 337.1 nm Electrical discharge Pumping of dye lasers, measuring air pollution, scientific research. Nitrogen lasers can operate superradiantly (without a resonator cavity). Carbon dioxide laser 10.6 μm, (9.4 μm) Transverse (high power) or longitudinal (low power) electrical discharge Material processing (cutting, welding, etc.), surgery. Carbon monoxide laser 2.6 to 4 μm, 4.8 to 8.3 μm Electrical discharge Material processing (engraving, welding, etc.), photoacoustic spectroscopy. Excimer laser 193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), 353 nm (XeF) Excimer recombination via electrical discharge Ultraviolet lithography for semiconductor manufacturing, laser surgery Nitrogen laser UNIT III Lecture 3 7 CHEMICAL LASERS LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGTH(S) PUMP SOURCE Hydrogen fluoride laser 2.7 to 2.9 μm for Hydrogen fluoride (<80% Atmospheric transmittance) Chemical reaction in a burning jet of ethylene and nitrogen trifluoride (NF3) Used in research for laser weaponry by the U.S. DOD, operated in continuous wave mode, can have power in the megawatt range. Deuterium fluoride laser ~3800 nm (3.6 to 4.2 μm) (~90% Atm. transmittance) chemical reaction MIRACL, Pulsed Energy Projectile & Tactical High Energy Laser Chemical reaction in a jet of singlet delta oxygen and iodine Laser weaponry, scientific and materials research, laser used in the U.S. military's Airborne laser, operated in continuous wave mode, can have power in the megawatt range. COIL (Chemical oxygen-iodine laser) 1.315 μm (<70% Atmospheric transmittance) UNIT III Lecture 3 APPLICATIONS 8 METAL-VAPOR LASERS LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGT H(S) Helium-cadmium (HeCd) metalvapor laser 441.563 nm, 325 nm Helium-mercury (HeHg) metalvapor laser 567 nm, 615 nm Helium-selenium (HeSe) metalvapor laser up to 24 wavelengths between red and UV Copper vapor laser 510.6 nm, 578.2 nm Gold vapor laser 627 nm PUMP SOURCE Electrical discharge in metal vapor mixed with helium buffer gas. APPLICATIONS Printing and typesetting applications, fluorescence excitation examination (ie. in U.S. paper currency printing), scientific research. Rare, scientific research, amateur laser construction. Rare, scientific research, amateur laser construction. Dermatological uses, high speed photography, pump for dye lasers. Electrical discharge Rare, dermatological and photodynamic therapy uses. UNIT III Lecture 3 9 SOLID STATE LASERS LASER GAIN MEDIUM AND TYPE Ruby laser Nd:YAG laser OPERATION WAVELEN GTH(S) 694.3 nm 1.064 μm, (1.32 μm) PUMP SOURCE APPLICATIONS Flashlamp Holography, tattoo removal. The first type of visible light laser invented; May 1960. Flashlamp, laser diode Material processing, rangefinding, laser target designation, surgery, research, pumping other lasers (combined with frequency doubling to produce a green 532 nm beam). One of the most common high power lasers. Usually pulsed (down to fractions of a nanosecond) UNIT III Lecture 3 10 LASER GAIN MEDIUM AND TYPE Er:YAG laser Neodymiu m doped Yttrium orthovanad ate (Nd:YVO4) laser OPERATION WAVELENGTH(S) 2.94 μm 1.064 μm PUMP SOURCE APPLICATIONS Flashlam p, laser diode Periodontal scaling, Dentistry laser diode Mostly used for continuous pumping of mode-locked Ti:sapphire or dye lasers, in combination with frequency doubling. Also used pulsed for marking and micromachining. UNIT III Lecture 3 11 LASER GAIN MEDIUM AND TYPE OPERATION WAVELENGT H(S) PUMP SOURCE APPLICATIONS Neodymium doped yttrium calcium oxoborate Nd:YCa4O(BO3 )3 or simply Nd:YCOB ~1.060 μm (~530 nm at second harmonic) Nd:YCOB is a so called "self-frequency doubling" or SFD laser material which is both capable of lasing and which has nonlinear characteristics suitable for laser diode second harmonic generation. Such materials have the potential to simplify the design of high brightness green lasers. Neodymium glass (Nd:Glass) laser ~1.062 μm (Silicate glasses), ~1.054 μm (Phosphate glasses) Used in extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial Flashlamp, confinement fusion. Nd:Glass lasers are laser diode usually frequency tripled to the third harmonic at 351 nm in laser fusion devices. UNIT III Lecture 3 12 LASER GAIN MEDIUM AND TYPE Titanium sapphire (Ti:sapphire) laser OPERATION WAVELENG TH(S) 650-1100 nm PUMP SOURCE Other laser APPLICATIONS Spectroscopy, LIDAR, research. This material is often used in highly-tunable mode-locked infrared lasers to produce ultrashort pulses and in amplifier lasers to produce ultrashort and ultra-intense pulses. UNIT III Lecture 3 13 Cerium doped lithium strontium(or calcium) aluminum fluoride (Ce:LiSAF, Ce:LiCAF) ~280 to 316 nm Chromium doped chrysoberyl (alexandrite) laser Typically tuned in Flashlamp, laser diode, the range of 700 mercury arc (for CW mode to 820 nm operation) Frequency quadrupled Nd:YAG laser pumped, excimer laser pumped, copper vapor laser pumped. UNIT III Lecture 3 Remote atmospheric sensing, LIDAR, optics research. Dermatological uses, LIDAR, laser machining. 14 SEMICONDUCTOR LASERS : Laser gain medium and type Operation wavelength(s) Semiconductor laser diode (general information) 0.4-20 μm, depending on active region material. GaN 0.4 μm Pump source Electrical current Applications Telecommunications, holography, printing, weapons, machining, welding, pump sources for other lasers. Optical discs. UNIT III Lecture 3 15 AlGaAs 0.63-0.9 μm Electrical current Optical discs, laser pointers, data communications. 780 nm Compact Disc player laser is the most common laser type in the world. Solidstate laser pumping, machining, medical. InGaAsP 1.0-2.1 μm Telecommunications, solid-state laser pumping, machining, medical.. Vertical cavity surface emitting laser (VCSEL) 850 - 1500 nm, depending on material Telecommunications Hybrid silicon laser Mid-infrared Research UNIT III Lecture 3 16 OTHER TYPES OF LASERS : Laser gain medium and type Free electron laser Operation wavelength(s ) Pump source A broad wavelength range (about 100 nm several mm); relativistic electron one free beam electron laser may be tunable over a wavelength range UNIT III Lecture 3 Applications atmospheric research, material science, medical applications. 17 "Nickel-like" Samarium laser First demonstration of efficient "saturated" operation Lasing in ultra-hot of a sub–10 nm Xsamarium plasma ray laser, possible formed by double applications in high pulse terawatt scale resolution X-rays at 7.3 irradiation fluences microscopy and nm created by holography, wavelength Rutherford operation is close to Appleton the "water window" Laboratory's at 2.2 to 4.4 nm Nd:glass Vulcan where observation of laser. DNA structure and the action of viruses and drugs on cells can be examined. UNIT III Lecture 3 18 Raman laser, uses inelastic stimulated Raman 1-2 μm for scattering in fiber version a nonlinear media, mostly fiber, for amplification Other laser, mostly Yb-glass fiber lasers UNIT III Lecture 3 Complete 1-2 μm wavelength coverage; distributed optical signal amplification for telecommunications ; optical solitons generation and amplification 19 CO2 LASER Introduction : CO2 lasers belong to the class of molecular gas lasers. In the case of atoms, electrons in molecules can be excited to higher energy levels, and the distribution of electrons in the levels define the electronic state of the molecule. Besides, these electronic levels, the molecules have other energy levels. C.K.N. Patel designed CO2 laser in the year 1964. UNIT III Lecture 3 20 Active medium : It consists of a mixture of CO2, N2 and helium or water vapour. The active centres are CO2 molecules lasing on the transition between the rotational levels of vibrational bands of the electronic ground state. . Optical resonators : A pair of concave mirrors placed on either side of the discharge tube, one completely polished and the other partially polished. UNIT III Lecture 3 21 Pumping : Population inversion is created by electric discharge of the mixture. When a discharge is passed in a tube containing CO2, electron impacts excite the molecules to higher electronic and vibrational-rotational levels. This level is also populated by radiationless transition from upper excited levels. The resonant transfer of energy from other molecules, such as, N2, added to the gas, increases the pumping efficiency. UNIT III Lecture 3 22 Contd. Nitrogen here plays the role that He plays in HeNe laser. A carbon dioxide (CO2) laser can produce a continuous laser beam with a power output of several kilowatts while, at the same time, can maintain high degree of spectral purity and spatial coherence. In comparison with atoms and ions, the energy level structure of molecules is more complicated and originates from three sources: electronic motions, vibrational motions and rotational motions. UNIT III Lecture 3 23 Fundamental Modes of vibration of CO2 : Three fundamental modes of vibration for CO2 Symmetric stretching mode (frequency 1), Bending mode (2) and Asymmetric stretching mode (3). In the symmetric stretching mode, the oxygen atoms oscillate along the axis of the molecule simultaneously departing or approaching the carbon atom, which is stationary. UNIT III Lecture 3 24 Contd. In the ‘bending mode’, the molecule ceases to be exactly linear as the atoms move perpendicular to the molecular axis. In ‘asymmetric stretching’, all the three atoms oscillate: but while both oxygen atoms move in one direction, carbon atoms move in the opposite direction. The ‘internal vibrations’ of carbon dioxide molecule can be represented approximately by linear combination of these three normal modes. UNIT III Lecture 3 25 CO2 LASER UNIT III Lecture 3 26 INDEPENDENT MODES OF VIBRATION OF CO2 MOLECULE UNIT III Lecture 3 27 The energy level diagram of vibrational – rotational energy levels with which the main physical processes taking place in this laser. As the electric discharge is passed through the tube, which contains a mixture of carbon dioxide, nitrogen and helium gases, the electrons striking nitrogen molecules impart sufficient energy to raise them to their first excited vibrational-rotational energy level. This energy level corresponds to one of the vibrational - rotational level of CO2 molecules, designated as level 4. UNIT III Lecture 3 28 Contd. collision with N2 molecules, the CO2 molecules are raised to level 4. The lifetime of CO2 molecules in level 4 is quiet significant to serve practically as a metastable state. Hence, population inversion of CO2 molecules is established between levels 4 and 3, and between levels 4 and 2. The transition of CO2 molecules between levels 4 and 3 produce lasers of wavelength 10.6 microns and that between levels 4 and 2 produce lasers of wavelength 9.6 microns . UNIT III Lecture 3 29 ENERGY LEVEL DIAGRAM UNIT III Lecture 3 30 The He molecules increase the population of level 4, and also help in emptying the lower laser levels. The molecules that arrive at the levels 3 and 2 decay to the ground state through radiative and collision induced transitions to the lower level 1, which in turn decays to the ground state. The power output of a CO2 laser increases linearly with length. Low power (upto 50W) continuous wave CO2 lasers are available in sealed tube configurations. UNIT III Lecture 3 31 Contd. •Some are available in sizes like torches for medical use, with 10-30 W power. • All high power systems use fast gas-floe designs. • Typical power per unit length is 200-600 W/m. • Some of these lasers are large room sized metal working lasers with output power 10-20 kW. • Recently CO2 lasers with continuous wave power output exceeding 100 kW. • The wavelength of radiation from these lasers is 10.6m. UNIT III Lecture 3 32 Nd: YAG Laser (Doped insulator laser) : Lasing medium : The host medium for this laser is Yttrium Aluminium Garnet (YAG = Y3 Al5 O12) with 1.5% trivalent neodymium ions (Nd3+) present as impurities. The (Nd3+) ions occupy the lattice sites of yttrium ions as substitutional impurities and provide the energy levels for both pumping and lasing transitions. UNIT III Lecture 3 33 Contd. When an (Nd3+) ion is placed in a host crystal lattice it is subjected to the electrostatic field of the surrounding ions, the so called crystal field. The crystal field modifies the transition probabilities between the various energy levels of the Nd3+ ion so that some transitions, which are forbidden in the free ion, become allowed. UNIT III Lecture 3 34 Nd: YAG laser UNIT III Lecture 3 35 The length of the Nd: YAG laser rod various from 5cm to 10cm depending on the power of the laser and its diameter is generally 6 to 9 mm. The laser rod and a linear flash lamp are housed in a elliptical reflector cavity Since the rod and the lamp are located at the foci of the ellipse, the light emitted by the lamp is effectively coupled to the rod. The ends of the rod are polished and made optically flat and parallel. UNIT III Lecture 3 36 Contd. •The optical cavity is formed either by silvering the two ends of the rod or by using two external reflecting mirrors. • One mirror is made hundred percent reflecting while the other mirror is left slightly transmitting to draw the output • The system is cooled by either air or water circulation. UNIT III Lecture 3 37 ENERGY LEVEL DIAGRAM Simplified energy level diagram for the Nd-ion in YAG showing the principal laser transitions UNIT III Lecture 3 38 This laser system has two absorption bands (0.73 m and 0.8 m) Optical pumping mechanism is employed. Laser transition takes place between two laser levels at 1.06 mm. UNIT III Lecture 3 39 OUTPUT CHARACTERISTICS : The laser output is in the form of pulses with higher repetition rate Xenon flash lamps are used for pulsed output. Nd: YAG laser can be operated in CW mode also using tungsten-halide incandescent lamp for optical pumping. Continuous output obtained. powers of over 1KW are UNIT III Lecture 3 40 Note: Nd: Glass laser : Glass acts as an excellent host material for neodymium. As in YAG, within the glass also local electric fields modify the Nd3+ ion energy levels. Since the line width is much broader in glass than in YAG for Nd3+ ions, the threshold pump power required for laser action is higher. Nd: Glass lasers are operated in the pulsed mode at wavelength 1.06 mUNIT III Lecture 3 41 Nd:YAG/ Nd: Glass laser applications : These lasers are used in many scientific applications which involve generation of other wavelengths of light. The important industrial uses of YAG and glass lasers have been in materials processing such as welding, cutting, drilling. Since 1.06 m wavelength radiation passes through optical fibre without absorption, fibre optic endoscopes with YAG lasers are used to treat gastrointestinal bleeding. UNIT III Lecture 3 42 Contd. •YAG beams penetrate the lens of the eye to perform intracular procedures. •YAG lasers are used in military as range finders and target designators. UNIT III Lecture 3 43