Laser Types Lecture Notes PDF

Lecture 2: LASER TYPES Spectrum of available laser systems, their wavelength range and active media.  Basic Construction and Principles of Lasing Oscillating waves Gain/Active medium Pumping Process 100 % reflector...

Lecture 2: LASER TYPES Spectrum of available laser systems, their wavelength range and active media.  Basic Construction and Principles of Lasing Oscillating waves Gain/Active medium Pumping Process 100 % reflector Partial reflector  Types of Laser (A) By active media Solid state laser: crystal, or glass, doped with impurities. e.g. ruby laser, semiconductor laser. Gas laser - e.g. He-Ne laser, Ar+ laser, CO2 laser. Dye laser - active medium: dye molecules in liquid solvent Types of Laser (B) By mode of operation CW Pulsed (C) By pumping and laser levels 3-level laser 4-level laser  Solid state Laser 1. Doped Insulator Laser : Solid state lasers have active media obtained by embedding transition metals (Titanium(Ti+3), chromium (Cr+3) , cobalt(Co+2) , nickel(Ni+2), Iron(Fe+2)) 2. Rare earth ions (neodymium(Nd+3), ytterbium(Yb+3) Note: Different lasing wavelenght in the active is obtained by by dopping different host materials with same active ion.  Pumping Techniques in solid state laser Pulsed Pumping light sources Low-pressure quartz. Xenon lamps. Continuous Optical pumping Tungsten halogen lamps high-pressure mercury discharge lamp.  Pumping Techniques in solid state laser Optical pumping is the best and most efficient pumping method for solid state active media due to their broad optical absorption bands. Significant fraction of incident optical energy can be easily used for the pumping of ground state electrons using pulsed as well as continuous light sources. Excess light energy raises temperature of the laser materials; therefore pulsed light sources are more suitable for dissipation of heat through circulating water jackets.  Ruby Laser Host material: corundum( Aluminum Oxide) (Al2O3) crystal. Dopants: Cr ions. Lasing Wavelength: 694 nm. Pumping: Flash lamps and laser operates in pulsed regime. Construction of Ruby Laser  Ruby laser Active media in solid state lasers are cylindrical and rod shaped with few millimetres diameter and few centimetre lengths. Several arrangements of cylindrical flash lamp and rod-shaped active media are used for optical pumping to get laser radiation. The flash lamp and active medium assembly are placed inside gold-plated reflectors of circular or elliptical cross section. In the first practical operating laser, ruby rod was pumped by helical flash lamp inside the cylindrical reflecting cavity. Such arrangement has significant uniformity of irradiation inside the rod but exhibits poor optical coupling.  Ruby laser Side-by-side arrangement of flash lamp and laser rod inside the cylindrical gold-plated reflector or wrapping both together with a metal foil are simpler approaches having good optical coupling but poor uniformity of irradiation. An elliptical reflector having flash lamp at one focus and laser rod at the other focus is the most popular and best way of optical pumping in the solid state lasers. Light radiation leaving from the first focus gets focused close to the axis of laser rod placed at the second focus to make uniform energy distribution. Combination of a number of elliptical reflectors having laser rod at the common foci and several flash lamps at the other foci is used for better optical pumping with more uniform energy distribution.  Energy Levels Diagram This system is a three level laser with lasing transitions between E2 and E1. The excitation of the Chromium ions is done by light pulses from flash lamps (usually Xenon).  Energy Levels Diagram Action: The Chromium ions absorb light at wavelengths around 545 [nm] (500-600 [nm]). Result: The ions are transferred to the excited energy level E3.  Energy Levels Diagram From this level the ions are going down to the metastable energy level E2 in a nonradiative transition. The energy released in this nonradiative transition is transferred to the crystal vibrations and changed into heat that must be removed away from the system. The lifetime of the metastable level (E2) is about 5 [msec].  Advantages of Ruby Lasers  From cost point of view, the ruby lasers are economical.  Beam diameter of the ruby laser is comparatively less than CO2 gas lasers.  Since the ruby is in solid form therefore there is no chance of wasting material of active medium.  General Applications of Ruby Laser  One of the first applications for the ruby laser was in range finding.  Due to low output power they are class-I lasers and so may used as toys for children’s.  It can be used as decoration piece & artistic display.  Medical Applications of Ruby Laser  Targets pigment.  Along pulse is used for hair removal (by destroying hair follicle stem cells).  Ultra-short (nano-second) pulse for interfollicular and follicular melanocyte destruction.  Ophthalmic laser photocoagulator (operate at a constant coagulation or exposure time of about 500 μs). The drawback of the ruby laser was its pulsed and uneven output.  Nd:YAG Laser Pumping is optical : + Flash lamps for pulsed lasers. + Arc lamps for continuous wave lasers.  Nd: YAG Laser CW (continuous wave) lamps are gas discharge lamps that are mainly used in marking lasers and lasers for welding and cutting. Flash lamps are gas discharge lamps that are used in many applications. By far the most demanding of these is the optical pumping of lasers, the high optical power per pulse requires the best materials and designs to achieve long lifetimes.  Energy Level Diagram Nd-Yag lasers are four level lasers. Nd-Yag ions have two absorption band From these excited energy levels, the Nd ions are transferring into the upper laser level by a non radiative transition.  Energy Level Diagram The stimulated emission is happened from the upper laser level to the lower laser level, and the wavelengths of the emitted photons are around 1.06 [m]. From the lower laser level, a non-radiative transition to the ground level.  Advantages  Both continuous and pulsed output is possible.  Solid state lasers construction is comparatively simple.  Output power ranging from very low value of about 0.04 watts to high value of about 600 watts.  Cost of solid state lasers is economical.  Disadvantages  Low Efficiency of solid state laser is very low compared to CO2 lasers.  Divergence, which is not constant and ranges 1 milli radian to 20 milli radian.  Output power is also not very high as in CO2 lasers.  Due to thermal lasing in solid state lasers, the power loss occurs when the rod gets too hot. Summary of Nd_Yag lasers : Solid state laser. Emit in the Near-Infra-Red (NIR) spectrum range( 1060 nm ). Optically pumped. Operate in both pulsed and continuous mode. Four level laser.  Applications of Nd:YaG Lasers  As military application, Nd:YAG is used by target destination system.  Nd: YAG pulsed type solid state lasers can be used in medical applications such as in endoscopy.  Ophthalmology to correct posterior capsular opacification  Thermotherapy  Semiconductor Laser  The semiconductor laser is very small in size.  It is similar to a transistor and It has the operation like LED, but the output beam has the characteristics of laser light.  Construction and Working 1. laser diodes emit radiation by stimulated emission. 2. Operational current should be higher than the threshold value in order to attain the condition of population inversion. 3. The active medium in a semiconductor diode laser is in the form of junction region of 2 two-dimensional layers.  Advantages of Semiconductor Lasers  Smaller size and appearance make them good choice for many applications.  From cost point of view the semiconductor lasers are economical.  Semiconductor lasers construction is very simple.  Advantages of Semiconductor Lasers  No need of mirrors is in semiconductor lasers.  Semiconductor lasers have high efficiency.  The low power consumption is also its great advantage.  Disadvantages of Semiconductor Lasers  Beam divergence is much greater from 125 to 400 milli radians as compared to all other lasers.  The cooling system requirement in some cases may be considered its disadvantage.  Disadvantages of Semiconductor Lasers  Semiconductor laser is greatly dependent on temperature.  The temperature affects greatly the output of the laser.  The lasing medium of semiconductor lasers is too short and rectangular so the output beam profile has an unusual shape.  Application of Semiconductor Lasers  The semiconductor laser can be pulsed at varying rate and pulse widths. Therefore this laser is a natural transmitter of digital data.  Semiconductor laser is well suited for interface with fiber optic cables used in communication.  Gas Laser  Gas lasers are widely available in almost all power (milli watts to megawatts)  Wavelengths (Ultraviolet laser-Infra-Red laser)  It can be operated in pulsed and continuous modes.  Gas lasers use low density gaseous materials as active media.  Gas Laser  Electrical pumping (continuous, RF or pulsed)  The gas lasers can be made from neutral atoms (He-Ne, metal vapor etc), ions (e.g. Ar+) or molecules (e.g. CO2).  He-Ne Laser  The gain medium consists of a mixture of helium and neon (10:1) inside of a small bore capillary tube, usually excited by a DC electrical discharge.  He-Ne Laser Active medium: The He-Ne laser.  He-Ne laser is one type of atomic gas lasers.  The helium-neon laser operates at a wavelength of 632.8 nanometers (nm), in the red portion of the visible spectrum  Helium-neon laser construction The helium-neon laser consists of three essential components: Pump source (high voltage power supply). Gain medium (laser glass tube or discharge glass tube). Resonating cavity.  He-Ne Laser  The excitation of electrons in the He-Ne gas active medium is achieved by passing an electric current through the gas. Energy Level Diagram  Gain medium The gain medium of a helium-neon laser is made up of the mixture of helium and neon gas contained in a glass tube at low pressure. The partial pressure of helium is 1 mbar whereas that of neon is 0.1 mbar. The gas mixture is mostly comprised of helium gas. Therefore, in order to achieve population inversion, we need to excite primarily the lower energy state electrons of the helium atoms.  Gain medium (discharge glass tube or glass envelope) In He-Ne laser, neon atoms are the active centres and have energy levels suitable for laser transitions while helium atoms help in exciting neon atoms. Electrodes (anode and cathode) are provided in the glass tube to send the electric current through the gas mixture. These electrodes are connected to a DC power supply.  Advantages of He-Ne Laser  He-Ne laser has very good coherence property.  He-Ne laser can produce three wavelengths: that are 1152 nm, 3391 nm and 632.8nm, in which the 632.8nm is most common because it is visible usually in red color.  Advantages of He-Ne Laser  He-Ne laser tube has very small length approximately from 10 to 100cm and best life time of 20.000 hours.  Cost of He-Ne laser is less from most of other lasers.  Construction of He-Ne laser is also not very complex.  He-Ne Laser applications in Medical field He-Ne laser is used for a variety of therapeutic purposes such as promoting wound healing, encouraging healing of skin grafts, in skin diseases and in blood disorders.  Disadvantages of He-Ne Laser  It is relatively low power device means its output power is low.  He-Ne laser is low gain system/ device.  To obtain single wavelength laser light, the other two wavelengths of laser need suppression, which is done by many techniques and devices. So it requires extra technical skill and increases the cost also.  Disadvantages of He-Ne Laser  High voltage requirement.  Escaping of gas from laser plasma tube  Applications / Uses of He-Ne Laser  The Helium-Neon gas laser is one of the most commonly used laser today because of the following applications.  He-Ne lasers are produced in large quantities from many years.  He-Ne lasers also used in super market checkout counters to read bar codes and QR codes.  Applications of He-Ne Laser --Helium-neon lasers are used in industries. --Helium-neon lasers are used in scientific instruments. --Helium-neon lasers are used in the college laboratories.  Carbon dioxide laser 1. The carbon dioxide laser (CO2 laser) was one of the developed earliest gas lasers. 2. The discharge tube is filled with a mixture of carbon dioxide, nitrogen and helium gases in the ratio of 1:2:3 with water vapours.  Carbon dioxide laser application Pumping source: Electric discharge method is used for pumping and achieving population inversion. In this method, electrons will collide with CO2 molecules and pump them to excited states. Output: continuous wave lasers. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering on 9.4 and 10.6 (μm) Excimer laser It is a form of ultraviolet laser which is commonly used in eye surgery.  Excimer laser active medium An excimer laser typical uses a combination of a noble gas (argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation and high pressure. The wave length of an excimer laser depends on the molecules used, and is usually in the ultraviolet Excimer Wavelength Ar2* 126 nm Kr2* 146 nm F2* 157 nm Xe 2* 172 & 175 nm ArF 193 nm KrCl 222 nm KrF 248 nm XeBr 282 nm XeCl 308 nm XeF 351 nm  Excimer laser Pumping Discharge-pumped excimer lasers are usually operated with a pulse repetition rate of around 100 Hz and a pulse duration of ~10 ns.  Homework Summarize the laser types that used in the biomedical application only in one table which include laser type, Active medium, wave length, typical pulse duration, Pumping process.