Your SlideShare is downloading. ×
The crypton laser:Description,Specificities and Applications
Upcoming SlideShare
Loading in...5
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×
Saving this for later? Get the SlideShare app to save on your phone or tablet. Read anywhere, anytime – even offline.
Text the download link to your phone
Standard text messaging rates apply

The crypton laser:Description,Specificities and Applications

495
views

Published on

THIS IS MY UNDERGRADUATE DISSERTATION! IN ORDER TO OBTAIN THE BACHELOR OF SCIENCE IN PHYSICS AT NATIONAL UNIVERSITY OF RWANDA.

THIS IS MY UNDERGRADUATE DISSERTATION! IN ORDER TO OBTAIN THE BACHELOR OF SCIENCE IN PHYSICS AT NATIONAL UNIVERSITY OF RWANDA.

Published in: Science, Technology, Business

0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total Views
495
On Slideshare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
Downloads
12
Comments
0
Likes
0
Embeds 0
No embeds

Report content
Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
No notes for slide

Transcript

  • 1. i DEDICATION To my parents For fostering and encouraging my interest in science. To my sisters and brothers To MUGISHA Keen Darren This work is dedicated
  • 2. ii ACKNOWLEDGMENT First and foremost, I thank Almighty God for his protection. This work was made possible by the support and contribution from many individuals to whom I am indebted and would like to express my gratitude. I would like to express my profound gratitude to my supervisor MSc Célestin MAGEZA, for his inspiring guidance and his assistance to accomplishing this research. I would like to extend my gratitude to the government of Rwanda for the granted bursary loan through Rwanda Education Board and National University of Rwanda. My special thanks are also expressed to all the teaching staff of the Faculty of Science, particularly in the Department of Physics and the Department of Applied Mathematics, for their knowledge package, favorable learning environment and cooperation during my stay at National University of Rwanda. I say thanks to NDINDABAHIZI Jean Félix for his constant encouragement during my studies and especially in the achievement of this research. I extend my thanks to my closed friends, brothers and sisters; the deepest gratitude for their encouragement and support. Lastly but not least, my appreciation and thanks are expressed to my colleagues NIYONSENGA Jean de Dieu, TUYISHIME Rose, INGABIRE Assumpta Berine and my fellow students for their support in one or other way; you have been nice to me and I wish you all the best and God bless you all. Kean Friend Manasseh MUHIRE
  • 3. iii TABLE OF CONTENTS DEDICATION ........................................................................................................................................ i ACKNOWLEDGMENT ........................................................................................................................ ii TABLE OF CONTENTS ...................................................................................................................... iii LIST OF SYMBOLS AND ABBREVIATIONS ................................................................................. vii LISTS OF FIGURES.............................................................................................................................. x LISTS OF TABLES ............................................................................................................................. xii ABSTRACT ......................................................................................................................................... xii GENERAL INTRODUCTION ........................................................................................................... 1 1. INTRODUCTION .......................................................................................................................... 1 2. PROBLEM STATEMENT ............................................................................................................. 1 3. CHOICE AND INTEREST OF THE STUDY ............................................................................... 1 4. HYPOTHESES OF STUDY .......................................................................................................... 2 5. OBJECTIVES OF THE STUDY.................................................................................................... 2 5.1. General objective ..................................................................................................................... 2 5.2. Specific objectives ................................................................................................................... 2 6. RESEACH METHODOLOGY ...................................................................................................... 2 7. SCOPE OF THE STUDY ............................................................................................................... 3 8. STRUCTURE OF THE STUDY .................................................................................................... 3 CHAPTER I. PHYSICAL PRINCIPLES OF GAS LASERS .......................................................... 4 I.1. INTRODUCTION ........................................................................................................................ 4 I.2. GAS LASER MEDIA 12 ............................................................................................................ 4 I.2.1. IONIZED GAS ...................................................................................................................... 5
  • 4. iv I.2.2. INTERACTIONS .................................................................................................................. 5 I.2.3. FREE ELECTRONS ............................................................................................................. 6 I.2.4. ELECTRON BEHAVIOR IN DISCHARGE EVENTS ....................................................... 6 I.3. GAS LASERS OPERATION MECHANISM ............................................................................. 7 I.4.1. POPULATION INVERSIONS IN GASES 13 .................................................................... 7 I.4.2. STIMULATED EMISSION 13 .......................................................................................... 8 I.4.3. AMPLIFICATION OF RADIATION 13 ............................................................................ 9 I.4. PUMPING TECHNIQUES FOR GAS LASERS 12 ............................................................... 11 I.4.1. DC DISCHARGE ................................................................................................................ 11 I.4.2. RF DISCHARGE EXCITATION ....................................................................................... 12 I.5. COOLING SYSTEMS FOR GAS LASERS 12 ...................................................................... 13 I.6. PROPERTIES OF GAS LASER RADIATION 18 ................................................................. 14 I.7. TYPES OF GAS LASERS ......................................................................................................... 14 I.7.1. GAS LASERS IN VISIBLE RANGE ................................................................................. 15 I.7.1.1. Helium-Neon lasers ...........................................................................................................15 I.7.1.2. Noble Gas Ion lasers ......................................................................................................... 17 I.7.2. UV GAS LASERS 11 ....................................................................................................... 18 I.7.2.1. Nitrogen Gas laser ............................................................................................................ 19 I.7.2.2. Excimer lasers…………………………...………………………...…..…………………20 I.7.3. INFRARED AND FAR INFRARED GAS LASERS [18] ................................................. 22 CHAPTER II. KRYPTON LASER .................................................................................................. 24 II.1. INTRODUCTION .................................................................................................................... 24 II.2. LASING MEDIUM .................................................................................................................. 24 II.3. OPTICS AND CAVITY OF THE KRYPTON LASER [9] ..................................................... 25 II.4. STRUCTURE OF THE KRYPTON LASERS 11 ................................................................. 26
  • 5. v II.5. KRYPTON LASERS CHARACTERISTICS AND SPECIFICITIES [4] ............................... 27 II.6. OPERATION OF THE KRYPTON LASER 5 ...................................................................... 28 II.6.1. SINGLE LINE OPERATION ............................................................................................ 28 II.6.2. MULTILINE OPERATION............................................................................................... 28 II.7. COMPARISON AND SPECIFICATIONS OF A KRYPTON LASER 11 ........................... 30 II.7.1. COMPARISON WITH ARGON ION LASERS ............................................................... 30 II.7.2. PERFORMANCE SPECIFICATIONS 5 ........................................................................ 31 CHAPTER III. APPLICATIONS OF THE KRYPTON LASER ................................................. 33 III.1. INTRODUCTION ................................................................................................................... 33 III.2. SCIENTIFIC APPLICATIONS .............................................................................................. 33 III.2.1. SPECTROSCOPY [15]..................................................................................................... 33 III.2.2. HOLOGRAPHY [3] ......................................................................................................... 34 III.3. INDUSTRIAL APPLICATIONS ............................................................................................ 35 III.3.1. NON DESTRUCTIVE TESTING (NDT) [4] .................................................................. 35 III.3.2. DATA STORAGE (Disc mastering) [4] ........................................................................... 36 III.4. MEDICAL APPLICATIONS .................................................................................................. 36 III.4.1. OPHTHALMOLOGY [14] ............................................................................................... 36 III.4.2. ADVANTAGES OF KRYPTON (RED) OVER ARGON LASER ................................. 37 III.4.3. DISADVANTAGES AND ERRORS WHEN USING THE KRYPTON LASER IN MEDICINE ................................................................................................................................... 37 III.4.3.1. Disadvantages [14] ......................................................................................................... 37 III.4.3.2. Sources of errors [2] ....................................................................................................... 38 CONCLUSION AND RECOMMENDATIONS ............................................................................. 39 1. CONCLUSION..............................................................................................................................39 2. RECOMMENDATIONS .............................................................................................................. 39
  • 6. vi REFERENCES ................................................................................................................................... 40
  • 7. vii LIST OF SYMBOLS AND ABBREVIATIONS OEM: Opto-Electron Microscopy NUR: National University of Rwanda BSc: Bachelor’s degree of science CW: Continuous wave UV: Ultraviolet. DC: Direct current. RF: Radio frequency. ULL: Upper laser level. LLL: Lower laser level. CO2: Carbon dioxide. N2: Nitrogen. FIR: Far infrared. He-Ne: Helium-Neon NI: Near infrared MI: Medium infrared Ar: Argon Kr: Krypton LIDAR: Light detecting and ranging laser N2O: Nitrogen oxide CH3OH: Methanol (Alcohol)
  • 8. viii FM: Frequency modulation HR: High Reflector OC: Output coupler BeO: Ceramic (beryllium oxide) G: Gauss nm: Nanometer L: Cavity length E: Energy c: Celerity of the light in vacuum : Wavelength v: Frequency of oscillation h: Plank’s constant MHz: Megahertz Ti: Titanium NDT: Non-Destructive Testing W: Watt mW: Milliwatt V: Volt eV: Electron-volt LASER: Light Amplification by Stimulated Emission of Radiation P: Pressure
  • 9. ix V: Volume T: Temperature k: Boltzmann’s constant N: Avogadro’s number (6.0248x1023 molecules per mol) ni: Net charge density of free electrons   J : Vector current density in discharge. E: Electric field B: Magnetic field e : Specific power Ge: Germanium Nd:YAG: Neodymium-Yttrium-Aluminum Garnet GaAs: Gallium Arsenide ZnS: Zinc sulfide ZnSe: Zinc Selenium
  • 10. x LISTS OF FIGURES Figure 1.1: Inversion processes in gases [13]......................................................................................... 8 Figure 1.2: Stimulated emission of radiation [9] .................................................................................... 8 Figure 1.3: Schematic of amplification [13] .......................................................................................... 9 Figure 1.4: Various mirror configurations for resonant cavities [20] .................................................. 10 Figure 1.5: Discharge tube showing distribution of emitted light areas [12] ....................................... 12 Figure 1.6: Spectral map of popular gas laser radiation [12 ................................................................ 15 Figure 1.7: Energy levels diagram for He-Ne laser system [18] .......................................................... 16 Figure 1.8: Structure of helium-neon laser [17] ................................................................................... 16 Figure 1.9: (a) Illustration of ionization levels in atoms (b) Basic spectral diagram of ion laser action [12] ....................................................................................................................................................... 17 Figure 1.10: Typical ion laser discharge tube [12] ............................................................................... 18 Figure 1.11: Electrical schematic of a Blumlein laser [11] .................................................................. 19 Figure 1.12: Representative nitrogen laser energy levels [11] ............................................................. 20 Figure 1.13: Excimer laser energy-level [11] ....................................................................................... 21 Figure 1.14: Energy levels in the carbon dioxide laser [11] ................................................................ 23 Figure 2.1: Small yellow krypton ion laser [8] .................................................................................... 25 Figure 2.2: Basic krypton laser construction [4] .................................................................................. 27 Figure 2.3: Typical lasing wavelengths and relative power levels from 500 mW size kryptonLasers [5] ......................................................................................................................................................... 27 Figure 2.4: Single line operation [5] .................................................................................................... 28 Figure 2.5: Multiline ion lasers operation [5]....................................................................................... 29 Figure 2.6: Characteristic curves of a krypton laser operation ............................................................. 30 Figure 3.1: Raman microscope coupled to a krypton laser and a spectrometer [15] ........................... 34 Figure 3.2: Experimental configuration used to record color reflection holograms [3]....................... 35 Figure 3.3: Laser beam used to slow or stop the growth of abnormal blood vessels in the Retinal caused by diabetic retinopathy [10]...................................................................................................... 36
  • 11. xi LISTS OF TABLES Table I.1: Commercially wavelengths of He-Ne laser ......................................................................... 16 Table I.2: Excimer Species ................................................................................................................... 21 Table II.1:Ionization energies of krypton lasers and some representative transitions distinguished for pulsed and continuous wave (CW) operation [11] ............................................................................... 25 Table II.2: Representative wavelengths of a Krypton Ion Laser in the visible region [12] ................. 29 Table II.3: Comparison of Argon and Krypton Lasers Output [11] ..................................................... 31 Table II.4: Performance, Specifications of different models of Krypton and Argon ion lasers [5] ..... 32
  • 12. xii ABSTRACT In this study entitled «The krypton laser - Description, Specificities and Applications», the fundamental physical principles of gas lasers are discussed including gas laser media, gas lasers operation mechanism such as population inversion, stimulated emission and amplification of radiation. The pumping techniques, cooling systems for gas lasers and the properties of gas laser radiations are also developed progressively in the first chapter. At the end of this chapter we have classified the gas lasers according to their output wavelengths and their corresponding important applications. In order to achieve our objectives we have focused on krypton laser, its structure, output characteristics, specificities and its operation. We have shown that the krypton laser operates in single line operation rather than multiline operation; this permits suck a kind of laser to produce the strongest, red 647.1 nm line with 3.5 W output and the yellow 548.2 nm line which results in better performance. The comparison of argon laser and the krypton laser performance specifications of different models are described. At the end of present study, we discuss some main applications of krypton laser in science, in industry and in medicine especially in ophthalmology. Finally we describe some disadvantages and errors caused by the use of a krypton laser for retina photocoagulation. To conclude our study we prove that the use of krypton laser requires a professional judgment in order to obtain the better results.
  • 13. 1 GENERAL INTRODUCTION 1. INTRODUCTION The development of a country is due to the direct applications of modern physics in daily life. In order to provide the better solutions in short time for some problems, lasers were discovered. In this final graduate work we will refer to gas lasers where the krypton laser is included. After an introduction to fundamental physical principles of lasers focusing on gas lasers in first chapter, it will be possible to better understanding what a krypton laser is and how it operates. And finally, we will give and adumbrate the applications of a krypton laser in society. 2. PROBLEM STATEMENT The krypton laser is the one of the new implement in modern physics which is not widely used in many countries, but it is a very interesting laser; reason why its applications in science and technology are very important in the development of a country. This laser is designated for a variety of scientific, industrial and medical applications. These applications include different areas such as: Non-destructive testing, Semiconductor processing, Disc mastering, OEM medical applications, very high performing printing; typesetting, photo-plotting, image generation, forensic medicine, laser shows for entertainment, holography, spectroscopy, electro-optics research and optical pumping source for other lasers, etc… The krypton lasers are also used in medicine for photocoagulation of retina. It is a very performing laser, able to be a helpful research tool in particular for the NUR academic community in different domains of research; and for other higher institutions in general to contribute to the development of our country. 3. CHOICE AND INTEREST OF THE STUDY The interest of this work is to know and to understand the role of krypton lasers in society; because no other scientific discovery has been demonstrated during the 20th century and with so many exciting applications as laser. To know how a krypton laser is and how it differs from other lasers and how it is used, should help NUR community in different areas of research and the results of such a kind of research can be helpful to the Rwandan people.
  • 14. 2 4. HYPOTHESES OF STUDY  The krypton laser is a modern scientific tool in Research  A detailed description of krypton laser can help the reader to know how it operates.  The krypton laser has numerous applications in science, industry, medicine and may contribute to the development of our country. 5. OBJECTIVES OF THE STUDY 5.1. General objective The general objective of this work is to get enough skills on modern physics especially about lasers with emphasize on gas lasers common characteristics, operation and some applications. 5.2. Specific objectives In order to achieve the main objective of this research, the following specific objectives are addressed: 1. To describe and to show the specificities of a krypton laser in comparison to other lasers 2. To show the main applications of a krypton laser in different areas; in order to motivate the NUR students to have the curiosity of using such a kind of laser, for providing the better solutions to many problems in short time. 6. RESEACH METHODOLOGY In this final graduate project we used the methods below: Documentation: In this research a number of documents have been consulted during this research, focusing on publications, papers and scientific journals; and electronic websites have also been visited for related information. We visited the NUR main library in order to read the documents about our topics, use internet by visiting the scientific websites existing at the NUR E-library. Data management: We have summarized all collected information about the krypton laser-description, specificities and applications, to make them more understandable and written according to the NUR academic regulations on BSc dissertations.
  • 15. 3 7. SCOPE OF THE STUDY This research concerns only the krypton laser and its applications. Due to the limited time and the lack of equipments in general, the study is mainly based on the documentation. We emphasized on how a krypton laser selects the output wavelength which makes it to be useful in many applications. We have classified these applications into three categories: Scientific, industrial and medical applications. 8. STRUCTURE OF THE STUDY In addition to a general introduction, conclusion and recommendations, this work is divided into three chapters: 1. Physical principles of Gas lasers 2. Krypton laser 3. Applications of krypton laser
  • 16. 4 CHAPTER I. PHYSICAL PRINCIPLES OF GAS LASERS I.1. INTRODUCTION There have passed more than 50 years since the first laser was shown. Lasers are the unique coherent electromagnetic waves at the optical frequency which never existed till 1960 on the world, when T. H. Maiman demonstrated the first atomic lamp. Now lasers are indispensable tools in our modern life. Therefore its applications are so successful, especially in communication and in material processing, reason why much different kind of lasers is manufactured annually. As the first continuous-wave (CW) lasers, gas lasers laid the foundation for today’s laser industry. The red helium-neon laser was the first to be widely used in industry, and it was the standard demonstration laser for decades. Ion lasers pioneered important applications in ophthalmology, biomedical instruments and printing. CW gas lasers are giving way to diode and solid-state lasers for most visible and near-infrared applications, but the CO2 laser remains dominant for industrial applications at longer infrared wavelengths. This first chapter of this study deals with the lasers whose active medium is gaseous. Today the number of gas lasers manufactured is significantly greater than any other kind of lasers; however, the contribution of gas lasers to our life is just as important as that semiconductor lasers. Using different laser media makes it possible for gas lasers to reach their oscillating wavelength range from far infrared to ultraviolet. From this we consider the gas lasers are Visible Gas Lasers; UV Gas Lasers and Infrared Gas Lasers. Gas lasers media may be atomic, ionic and molecular. In this chapter, the basic physical principles of gas lasers are discussed; it also describes the theory, operating characteristics, and design features of gas lasers. We introduce the different common gas lasers, their common characteristics, operation and finally we give the main applications of each type of gas lasers. I.2. GAS LASER MEDIA 12 The lasers whose active medium is gas were the first and fastest developing devices at the beginning of their history. The gas media can be described by an ideal gas equation: pV NkT (1.1)
  • 17. 5 with p is pressure; V is volume; T is temperature; k is the Boltzmann constant and N is Avogadro’s number (6.02481023 molecules per mol). Gas medium, being treated as a chaotic assembly of species (atoms, molecules) that have no volume and interaction forces between them, the above equation describes the diluted gases; and in practice, all gaseous at atmospheric pressure are considered as diluted. Hence, the ideal gas equation can be applied for most gas laser media. The neutral gas considered here does not fulfill conditions for laser action. The medium has to be excited between the chosen internal energy levels of atoms or molecules for the appearance of population inversion. It can be achieved by different mechanisms of excitation. The main technique to obtain the population inversion in a gas medium is excitation by discharge. I.2.1. IONIZED GAS Gas laser discharge can be considered as the so-called weakly ionized plasma, which contains some charged species (free electrons, ions) necessary to obtain excitation of the gas medium. Ionized gas is described by its basic parameter: free electron density ne. A weakly ionized gas discharge can be still considered as a neutral gas. Such a gas discharge forms the so-called quasi neutral plasma, where strong electric fields do not appear. From a physical point of view, it means that the next net charge density of free electrons ( ), positive and negative ions produced in the plasma tend to zero: ne ni  ni (1.2) For ionized media, apart from free electrons there are several species of ions, which can give quite a complicated picture of discharge, particularly in the case of molecular gases. I.2.2. INTERACTIONS In every atoms or molecules there are two kinds of energy: kinetic and internal energy. The exchange of energy in the process of chaotic motions occurs via collision mechanisms, the collision can be elastic or inelastic according to kinetic or internal energy that was exchanged respectively. There are different processes in plasma to obtain population inversion, necessary to achieve the lasing condition. Electrical properties of plasma are mainly determined by inelastic collisions responsible for creating free electrons and ionized species, and the elastic collisions can also do that but in low scale. For example in ionic process we have:
  • 18. 6 - Charge transfer: X Y - Ion recombination: X X Y - For elastic collision: X Y XY Y X kinetic energy Y kinetic energy I.2.3. FREE ELECTRONS Electrons play the most important role in inelastic collisions. They are responsible for ionization and excitation of atoms and molecules. There are two basic parameters characterizing electrons: the electron density ne and electron temperature Te. The electron density is directly related to the electrical current discharge (DC or RF excitation). Free electrons in discharge as photons, are moving rapidly. The motion of a free electron in a gas discharge is determined by the local electric E and magnetic B fields and also by its collisions with ions and neutral atoms. From this condition, one can find that the electrical power consumed by heating is given by the following equation: e    E J nee2 E mvc (1.3)   where J is the vector current density in discharge. In the above equation we consider DC discharge, where drift velocity vc = const, and e is the power density lost in discharge, often called « the specific power ». I.2.4. ELECTRON BEHAVIOR IN DISCHARGE EVENTS The electron’s energy in an electric field of a discharge changes in time and space, and its behavior is determined in plasma. The electron in electric field increases its energy; consequently it gains energy from that field. However, in the meantime, it loses usually a small part of its kinetic energy in the process of an elastic collision; but much higher losses of the kinetic energy of electrons can occur in the case of inelastic collision with atoms or molecules. In that process internal quantum energy of atoms and molecules increases. The slower electron is again accelerated in the electric field.
  • 19. 7 I.3. GAS LASERS OPERATION MECHANISM For gas lasers as for other types of lasers, to produce the high-energy laser beam requires three main processes which are population inversion; stimulated emission and amplification. I.4.1. POPULATION INVERSIONS IN GASES 13 The necessary condition for stimulated emission is « population inversion ». Without population inversion, there will be net spontaneous absorption or emission instead of stimulated emission. Inversions in gas lasers are often produced by applying a voltage across a gas discharge tube which is made of a long, narrow glass or ceramic tube used to confine the gain medium, and with two electrodes installed at each end of the tube in order to allow a voltage to be applied across the length of the tube. The tube is then filled with a low-pressure gas or gas mixture that includes the species that will serve as the gain medium. The applied voltage produces an electric field within the laser tube that accelerates the electrons within the gas. Those electrons collide with the gas atoms and excite the atoms to excited energy levels; some of which serve as Upper Laser Levels (ULL) and others as Lower Laser Levels (LLL), which can be a transition consisting of typically decay to the ground state faster than the higher-laser levels; thereby establishing a population inversion between some of the higher and lower levels as indicated in (Figure 1.1). This inversion can be envisioned by considering that, if the lower levels drain out faster than the upper levels, there will be less population left in those lower levels than in the higher-lying levels. The laser light then occurs when the higher-laser levels decay to the lower levels while radiating photons at the wavelengths corresponding to the energy separation between the levels. In many instances, the excitation is a two-step process in which the electrons first excite a long-lived or metastable (storage) level or they ionize the atom, leaving an ion of that species and another electron. In either case, that level then transfers its stored energy to the upper laser level via a subsequent collision with the laser species. The laser transitions in gaseous laser media typically occur at relatively precise, discrete wavelengths that correspond to the energy difference of inherently narrow energy levels. There is minimum population inversion, referred to as threshold condition, required for lasing action.
  • 20. 8 Figure 1.1: Inversion processes in gases [13] I.4.2. STIMULATED EMISSION 13 Stimulated emission occurs when the incident photon (provided by spontaneous emission) of frequency v interacts with the excited atom of active laser medium with population inversion between the states 1 and 2, having energies E1 and E2 respectively; such that: E2 E1 h (1.4) Thus, the incoming photon (stimulating photon) starts the emission of radiation by bringing the atom to the lower energy state (Figure 1.2). The resulting radiations have the same frequency, phase and polarization as that of the incoming photon, giving rise to a stream of photons. Figure 1.2: Stimulated emission of radiation [9] The stimulated emission gives the special properties of laser, such as narrow spectral width and coherent output radiation
  • 21. 9 I.4.3. AMPLIFICATION OF RADIATION 13 The stimulated photons and the incoming photons are in the same phase and state of polarization, they add constructively to the incoming photon resulting in an increase in its amplitude. Thus, the amplification of the light can be achieved by stimulated emission of radiation. Amplification of laser light is accomplished in a resonant cavity consisting of a set of well-aligned highly reflecting mirrors at the ends, perpendicular to the cavity axis. Common to all laser amplifiers are at least two elements: a laser medium in which a population inversion among atoms, ions, or molecules can be achieved, and a pump process to supply energy to the system in order to maintain a non equilibrium state. For a laser oscillator, additionally a feedback mechanism is required for radiation to build up. Typically, two mirrors facing each other provide this feedback. A population inversion occurs within atoms, ions, or molecules, when the pump energy supplied to the medium is in the form of optical radiation, electrical current, kinetic energy due to electron impact in a gas discharge, or an exothermic reaction, depending on the type of laser and the type of active medium. The figure 1.3 presents the schematic of the amplification process. a) b) : Unexcited atom; : Excited atom Figure 1.3: Schematic of amplification [13] a) Amplification by stimulated emission and b) continued amplification due to repeated reflection from the end mirrors, resulting in subsequent laser output from one end of mirrors. The active laser material is placed in between the mirrors. Usually, one of the mirrors is fully reflective with reflectivity close to 100%, whereas the other mirror has some transmission to allow the laser output to appear. The preceding discussion on the amplification of stimulated emission assumes that the mirrors of the resonant cavity are flat (plane parallel). However, there are various other configurations which offer significant advantages over the flat mirrors. The common geometric configurations of resonant cavity are presented by the figure 1.4.
  • 22. 10 Figure 1.4: Various mirror configurations for resonant cavities [20] The use of different mirrors for laser cavity provides the well laser feedback mechanism. In order to reach lasing action, there is some condition of resonant cavity stability; the stability of the resonant cavity is determined by the radii of curvatures of the end mirrors and the length of cavity. Based on the ray transfer matrix analysis, the condition of the stability can be expressed as: 0 1 d R1 1 d R2 1 (1.5) Where d is the separation between the two mirrors; R1 and R2 are respectively their radii of curvature.
  • 23. 11 I.4. PUMPING TECHNIQUES FOR GAS LASERS 12 Gas lasers are usually excited by electrical current flowing through a gas medium. There are three basic techniques of electrical excitation: DC, RF, and pulse excitations. However, there are some lasers that can be pumped by using other mechanisms, such as gas dynamic expansion, chemical reaction, or optical pumping by another laser. The atom or molecule in the excited state can decay to the lower states by four main mechanisms: 1. Collision between an electron and the excited atom or molecule (super elastic collision) 2. Near-resonance collisions between excited species and the species in the ground state 3. Collision with the wall of the reservoir 4. Spontaneous emission Distribution of energy level population is the result of excitation process. The population inversion is determined by two basic conditions: the excitation rate should be greater for the upper energy level 2 than for the lower energy level 1, and the decay of the upper level 2 should be slower than for lower level 1. The rate of transition 2-1 has to be less than the decay rate of level 1 to obtain CW laser operation. When this condition is not fulfilled, the laser operation is still possible, but only in pulse regime. I.4.1. DC DISCHARGE DC gas discharge is usually described as the process of electron emission from the cathode as the result of collision of the cathode by ions, fast atoms, and photons from gas medium. The basic set up will consist of tube with two electrodes (anode and cathode) separated by a distance d and filled with gas under moderately high pressure P or neutral gas density N (Figure 1.5). The voltage developed across the laser gas is independent to the discharge current, which means that it cannot be increased just by changing the input, thus, Pd preserved because E cons tan t in that case, the value E is P V . d The DC discharge was first used to pump waveguide CO2 lasers in the early 1970’s, and produced a considerable increase in performance. Output power and gain were both increased, and the laser was
  • 24. 12 able to operate at a much higher pressure than typical CO2 lasers at the time. These results were due to a smaller d, and a higher molecular density. In a DC discharge the electrons are produced at one electrode and lost in the other, which requires a constant generation of electrons. In order to maintain constant E , which is required for optimal laser performance, very high voltages are needed to keep N up a DC discharge. These high voltages require very large power supplies and lasers which are less commercially valuable. The voltage–current curve can be divided into five basic regions: Townsend discharge; corona discharge; normal discharge; abnormal discharge and arc discharge. The normal discharge region is applied in continuous gas lasers. The positive column of the electrical discharge in a cylindrical tube forms the basic discharge configuration in many popular DC discharge lasers. DC discharge excitation can be applied to all CW gas lasers (atomic, ions and molecular gas lasers). Figure 1.5: Discharge tube showing distribution of emitted light areas [12] I.4.2. RF DISCHARGE EXCITATION The RF technique of laser plasma excitation became popular in the 1980s, when the idea of diffusion cooled molecular laser appeared in waveguide and slab configurations. Lower Voltages are needed to maintain a discharge using RF excitation, allowing smaller and more efficient power supplies. The RF excitation idea was applied mainly to molecular Gas lasers. If to obtain permanent population inversion by using DC or RF discharge methods is not possible for some gas media, there are other several methods which can be used. In that case one may use the followings:
  • 25. 13  Pulse discharge excitation: It is a best way to obtain the population inversion for high-pressure gas media (High-pressure CO2 lasers and Excimer lasers).  Microwave excitation: Up today, there are no practical ways of effective excitation of plasma. This kind of formation of laser plasma is quite attractive and prospective, but it requires some sophisticated and clever solutions which can push this idea ahead.  Gas-dynamic excitation: There is no need of electrical discharge in order to reach population inversion. It can happen for molecular CO2-N2 mixture called « gasdynamic lasers ».  Optical pumping: It is very popular in solid state laser technology, but also applied to one particular type of gas lasers-FIR lasers. For the molecules that have quite complicated vibration-rotational spectra like alcohols. I.5. COOLING SYSTEMS FOR GAS LASERS 12 During excitation processes, an important common problem that appears in gas lasers technology is the heat removal from laser discharge tube. Most gas lasers are not high-efficiency devices; only molecular lasers can reach 10%-15% efficiency. Most of the power delivered to the laser plasma has to be removed from the discharge volume; otherwise, it is difficult to keep the thermal conditions of discharge steady. Depending on the laser construction, overheated plasma can substantially decrease population inversion of the medium and can destroy the entire system with degradation of the population inversion by thermal population of the lower laser level (case of CO2 laser). Cooling mechanism was discussed in order to remove the heat from the system. High power water and air cooled systems are often useful for ion gas lasers (argon and krypton laser). Cooling systems can be divided into two categories: 1. Diffusion cooling systems: They are applicable when transverse dimensions of a laser discharge are relatively small (a few millimeters) like in He-Ne laser. 2. Gas flow system: It is used for very high power laser systems that require large volumes of gas media; and as result of this, larger transverse dimensions. This system is popular for molecular lasers or excimer lasers.
  • 26. 14 I.6. PROPERTIES OF GAS LASER RADIATION 18 The intense beam of light produced by the lasers have the number of characteristics which can never be obtained from any other natural source, which make them acceptable for a variety of scientific and technological applications. Their Monochromaticity, directionality, laser line width, brightness, and coherence make them highly important for various materials processing and characterization applications. I.7. TYPES OF GAS LASERS Gas lasers output covers all optical spectra from far infrared (FIR) radiation to ultraviolet radiation which make them to be useful in many industrial applications. Some representative examples are shown in Figure 1.6. In this section we classify the gas lasers in three categories according to the output wavelengths: a. Visible gas lasers (He-Ne lasers and ion lasers) b. Ultraviolet gas lasers (Nitrogen Lasers and Excimer Lasers) c. Infrared gas lasers (Carbon Dioxide lasers) And we give the major applications of those common gas lasers.
  • 27. 15 Figure 1.6: Spectral map of popular gas laser radiation [12] I.7.1. GAS LASERS IN VISIBLE RANGE I.7.1.1. Helium-Neon lasers The first and the popular gas laser, helium-neon laser, is still an important source of coherent red light (632.8 nm) beam, but multiple transitions are possible, allowing the laser to operate (with suitable optics) at wavelengths in the infrared, orange, yellow, and green. Commercially, four visible wavelengths of He-Ne laser are commonly available and presented in the table I.1. The lasing medium is the mixture of very pure helium and neon gases in the approximate ratio of 10: 1. This laser is pumped by electrical discharge (DC or RF discharge), the pressures depend on the diameter of the plasma tube and are between 1 and 3 torr. The excited helium energy level, so that a collision with an excited helium atom will result in the transfer of energy to neon atoms, raising them to an excited state. Helium-neon laser is a four-level laser with favorable dynamics; He-Ne lasers have low thresholds and operate in CW mode. The figure1.7 illustrates the energy levels and the general structure of helium-neon laser.
  • 28. 16 Figure 1.7: Energy levels diagram for He-Ne laser system [18] Figure 1.8: Structure of helium-neon laser [17] Table I.1: Commercially wavelengths of He-Ne laser Wavelengths (nm) Relative Gain (Compared to 632.8nm Output) 543.5 (Green) 0.06 594.1 (Yellow) 0.07 611.9 (Orange) 0.2 632.8 (Red) 1
  • 29. 17 He–Ne lasers are probably the most popular gas lasers in many university laboratories. Most students passing elementary courses in physics, optics, photonics, or optoelectronics are quite familiar with these lasers. Their nice red, green, orange, yellow beams (or some IR, as well) are applied to many elementary experiments: interferometers; modulators; holograms and scanners and soon… I.7.1.2. Noble Gas Ion lasers The term ion laser refers to a laser in which the lasing energy levels exist in the ionized atom of the species. In ion gas lasers, the gain medium is plasma, an electrically conducting gas consisting of electrons and ions produced by an electrical discharge. Argon and krypton are the most common ion lasers, Ion lasers are generally high-powered lasers (much higher powered than a He-Ne laser) emitting in the green-blue region of the spectrum (for argon) or in many lines across the entire spectrum (for krypton), and even in the UV. The laser action of ion gas lasers occurs between electronic levels, as other gas lasers; the only difference is that the ion lasers originate from preliminary ionization of the gas by electrical discharge. Atoms lose one or more electrons, becoming ions that are simultaneously pumped to their excited states. Lasing occurs between ground and excited states of the ions when population inversion is reached. The involved typical transitions in ion lasers action are shown in figure 1.9. (a) (b) Figure 1.9: (a) Illustration of ionization levels in atoms (b) Basic spectral diagram of ion laser action [12] Unlike a He-Ne laser, an ion laser is a complex beast with plasma tubes made of exotic ceramic materials (Figure1.10) and whereas the He-Ne laser operates at a relatively high voltage and low current, ion lasers operate at relatively low voltages but enormous currents causing the degradation of materials from which the optical cavity and tube are made; and therefore, the cooling mechanism is
  • 30. 18 needed for maintaining the thermal conditions. The basic construction of ion gas lasers are the same. A typical example of ion laser will be discussed in chapter 2. Although Ar and Kr ion lasers are popular, the other noble gases can be utilized. Figure 1.10: Typical ion laser discharge tube [12] The ion lasers play an important role in many sophisticated applications. They can be effective sources for: Doppler velocimetry; Doppler anemometry; Particle sizing devices; Laser interferometry. The Ar and Kr lasers work as pumps for other lasers such as: Dye laser; CW Ti: sapphire lasers. They found many medical applications in: Ophthalmology; Cytometric analysis (counting and sorting particles); Dermatology; Otolaryngology, and so on. The selected lines of Ar or Kr lasers are very good coherent sources for Raman spectroscopy experiments. The Ar and particularly mixed Ar-Kr lasers are very popular devices in all kinds of illumination performances, where visible laser beams can be scanned making patterns and pictures at discos and entertainment and advertising events. 16 I.7.2. UV GAS LASERS 11 The most important ultraviolet lasers are the excimer and the nitrogen lasers. These lasers are made under similar technology. Both lasers are molecular lasers in which lasing species are diatomic molecules. For nitrogen lasers, the active lasing species is nitrogen molecule (N2) and in an excimer lasers; the active medium is a transient molecule consisting of a halide and an inert gas. Excimer
  • 31. 19 lasers are generally much larger than nitrogen lasers and have higher power outputs, producing enormous power outputs in the ultraviolet region of the spectrum. The optics of these lasers must be designed for UV; so the coating of the high reflector must reflect UV (aluminum is frequently used), and windows on the laser tube must be made of quartz or some other transparent materials to UV radiation. I.7.2.1. Nitrogen Gas laser The basic requirement for a practical nitrogen laser is to supply a massive electrical current (i.e. a huge quantity of electrons) with a fast rise time and short pulse length to excite the gas. To achieve this, most nitrogen lasers use an electrical configuration called a Blumlein configuration (Figure 1.11). Figure 1.11: Electrical schematic of a Blumlein laser [11] Nitrogen lasers are different in construction with other lasers, because they can operate without mirrors; they constitute a type of lasers called superradiant. Lasing transition in N2 laser takes place between two electronic energy levels; therefore this laser operates in the ultraviolet region at 337 nm. Here, the upper electronic level has a shorter lifetime compared to the lower one; hence CW operation cannot be achieved, but pulsed operation with narrow pulse width is possible. The pulse width is narrow because as soon as lasing starts, population of the lower state increases, while that at upper state decreases, and rapidly a state at which no lasing is possible is rapidly achieved. Such a laser system is known as self-terminating. The energy levels of the nitrogen molecule as they apply to this laser are outlined in Figure 1.12.
  • 32. 20 Figure 1.12: Representative nitrogen laser energy levels [11] The N2 lasers found applications as dye pumping sources, in LIDAR investigations (remote sensing), in atomic and lifetime spectroscopy, and in medicine and in biology research. I.7.2.2. Excimer lasers Excimer lasers produce intense pulsed output in the ultraviolet. The excimer is unique because the lasing molecule is one consisting of a halogen and an inert gas. Modern excimer lasers produce pulses with energy ranging from 0.1 to 1 J and can (for a large industrial laser) produce these pulses at a rate of over 300 per second. Energy levels in an excimer laser are defined by the state of the atomic components. When unbound, the energy of the system depends purely on the separation between the individual atoms; as the atoms move closer together, energy rises. This is illustrated by the curve in Figure 1.13.
  • 33. 21 Table I.2: Excimer Species [11] Laser species wavelength(nm) Relative power output ArF 193 0.5 KrF 249 1 XeCl 308 0.7 XeF 350 0.6 Figure 1.13: Excimer laser energy-level [11] With high average powers (commonly over 100 W for many commercial lasers) and an output in the ultraviolet region of the spectrum, excimer lasers are useful for many applications, ranging from dye laser pumping to cutting and materials processing applications. The largest commercial applications for excimer are used in eye surgery to correct the shape of the cornea to reduce the need for corrective lenses; they are also used in lithography. Other applications for excimer lasers include wire stripping (especially for ultra-fine wires used in the microelectronics industry); surface-mount component marking; drilling inkjet printer; nozzle holes and marking wires.
  • 34. 22 I.7.3. INFRARED AND FAR INFRARED GAS LASERS [18] The carbon dioxide is the most commonly used in infrared gas lasers, with other gases, such as nitrous oxide (N2O) and carbon monoxide (CO), used less frequently. Most mid-IR molecular lasers operating in the wavelength range 2 to 20 m involve vibrational energy levels that result when bonds between atoms in these molecules bend or stretch. Longer wavelengths are possible in a molecular laser as well, but these involved purely rotational transitions corresponding with lower energy levels. Carbon dioxide is the most efficient molecular gas laser material that exhibits for a high power and high efficiency gas laser at infrared wavelength. Carbon dioxide is a symmetric molecule (O=C=O) having three modes of vibrations: symmetric stretching [i00], bending [0j0], and antisymmetric stretching [00k] (Figure 1.14), where i, j, and k are integers. For example, energy level [002] of molecules represents that it is in the pure asymmetric stretching mode with 2 units of energy. Very similar to the role of helium in He-Ne laser, N2 is used as intermediate in CO2 lasers. The first, V=1, vibrational level of N2 molecule lies close to the (001) vibrational level of CO2 molecules. The energy difference between vibrational levels of N2 and CO2 in a CO2 laser is much smaller (0.3 eV) as compared to the difference between the energy levels of He and Ne (20 eV) in He-Ne laser; therefore comparatively larger number of electrons in the discharge tube of CO2 laser having energies higher than 0.3 eV are present. The CO2 laser is pumped by electrical discharge and the water cooling is required, not just to remove discharge heat but also to reduce the thermal population of the lower energy levels, which are very close to ground level. In the far-IR region of 10 μm wavelength, the usual optical material of CO2 laser has large absorbance, and therefore cannot be used as windows and reflecting mirrors in the cavity. Materials such as Ge, GaAs, ZnS, ZnSe, and some alkali halides having transparency in the IR region are often used.
  • 35. 23 Figure 1.14: Energy levels in the carbon dioxide laser [11] The idea of optical pumping was also well developed for FIR lasers. They are also called submillimeter-wave or terahertz lasers, and they establish a spectacular branch of molecular lasers. Most far-IR lasers use molecules such as alcohols (e.g., CH3OH) or other organic compounds. There is no doubt that millimeter and submillimeter wave radiations are getting rapidly to be extremely attractive spectral regions for interplanetary telecommunications. Stable FIR lasers with narrow line radiation are attractive carrier sources with potential application to free-space optical communication (FM telecommunication). The CO2 lasers are always high powered (compared to other gas lasers), mostly used for materials-processing applications, they are dominant in industrial domain such as: drilling and cutting materials such as cotton (used in making jeans) ; stainless steel and titanium, which are difficult to cut by any other means. It is also used in surgical applications since the wavelength is readily absorbed by flesh vaporizing it; the heat also serves to cauterize the cut, for reducing bleeding.
  • 36. 24 CHAPTER II. KRYPTON LASER II.1. INTRODUCTION Krypton is a chemical element which was discovered by William Ramsay and Morris Travers in 1898. Krypton is a colorless, odorless, tasteless gas about three times heavier than air; it occurs in nature as six stable isotopes. This kind of element must be used in many areas; here we are going to discus about a kind of laser whose active medium is krypton which is known as krypton laser. In this second chapter of the study we will describe the main features which make krypton laser to operate, we will give its output characteristics, operation and specifications of different models and make a comparison with other ion gas lasers. II.2. LASING MEDIUM The lasing medium in a krypton laser is a rare gas krypton that has been ionized; that is, it has one or more electrons removed from the outer shell. Ionized species exhibit different energy levels than neutral species do and the degree of ionization (the number of electrons removed) affects these levels. Krypton must be used as single ( Kr ), double ( Kr ) and triple ionized ( Kr 3 ). Let us now consider singly ionized krypton (denoted Kr ); ion is created by discharging a current of up to 40 A through low-pressure (1 torr = 1.013x105 Pa) krypton gas. The neutral (no ionized) configuration of the atom is 1s22s22p63s23p63d10 4s2 4p6 and when ionized which requires 14 eV of energy, the ground state for the ion (kr+) becomes 1s22s22p63s23p63d10 4s2 4p5. The more krypton is ionized, the more energy required to remove electrons from krypton nucleus. Discharges may be pulsed, as the earliest lasers were, but most krypton ion lasers are CW, so a continuous current of 40 A is required, which leads to complex tube and power supply designs, as we shall see. Ions are pumped to the ULL by a variety of methods, some by decay from a higher level (the expected route for a four-level laser) or directly to the ULL by electron impact in a process resembling that of a metal-vapor laser 12 . That decay process is fast a requirement to maintain a large population inversion of a krypton laser. Krypton can be doubly ionized as well but is of even lower efficiency than doubly ionized argon and not commonly available.
  • 37. 25 Table II.1: Ionization energies of krypton lasers and some representative Transitions distinguished for pulsed and continuous wave (CW) operation [11] Wavelengths (nm) Wavelengths (nm) Wavelengths (nm) Kr++ Kr+(eV) pulsed CW 14 743.6 350.74 38.35 (eV) Kr3+ pulsed 350.7 CW (eV) - 75.31 356.4 Kr4+ pulsed only 219.2 atomic (eV) mass 127.81 225.5 II.3. OPTICS AND CAVITY OF THE KRYPTON LASER [9] Some small krypton ion lasers have internal optics with extensive cooling system. In almost all cases, the laser tube has two Brewster windows protruding from the ends of the tube (on quartz stems sealed to the laser tube), so most ion lasers have a polarized output. Like a He-Ne, ion lasers have very low gain, so low-loss windows are necessary for operation. Cavity mirrors are mounted on a frame which keeps these aligned. For a longer laser the design of the frame becomes very important, since thermal expansion and mechanical movements can easily misalign the cavity. Figure 2.1: Small krypton ion laser [8] Cavities are frequently of plane-spherical mirrors type, the output coupler (OC) being spherical and having a radius of curvature slightly longer than the cavity length. This arrangement allows the use of interchangeable flat optics at the high reflector (HR) end (figure 2.2). For multiline use a broad-band reflector can be installed in the HR position. Selective reflectors may also bemuse 83.8
  • 38. 26 to allow only certain wavelengths to oscillate, as is frequently done with krypton lasers to select only the red line. Wavelength selectors using a prism and an HR are also an option for single line operation, and most tunable lasers also allow the addition of an intra cavity etalon, allowing single-frequency, and narrow spectral width operation. To reduce losses at the mirrors, mirrors are made from multiple layers of thin dielectric films. II.4. STRUCTURE OF THE KRYPTON LASERS 11 The temperature of the plasma is incredible hot, because of the high current densities exceeding 5000 K in the tube. Glass melts well below this point, so there are a limited number of materials available from which a plasma tube can be constructed: beryllium oxide (a ceramic) and a few high melting point (refractory) metals, including tungsten and graphite. In small lasers the bore is sometimes simply made from beryllium oxide, while larger lasers often use a beryllium oxide tube with graphite or tungsten disks inserted into the tube, holes drilled in the disks form the bore of the laser where the actual discharge takes place. Even with such exotic materials and construction techniques, the energetic plasma of a large ion laser (one with a discharge current of perhaps 30 to 40 A) can easily erode and destroy the tube material on contact. For this reason, magnetic confinement is invariably employed with large plasma tubes. The magnet is coaxial with the laser tube and is water cooled along with the plasma tube itself. Magnetic fields of about 1200 G are employed with visible lasers, which serve to confine the discharge to the center of the plasma tube. Whereas the use of a magnetic field enhances output power, too high a magnetic field can actually impair laser output. As the magnetic field is increased, the plasma becomes more confined to the center of the bore, increasing current density and hence output power. Heated cathodes are required in a krypton ion laser to prolong the life of the tube. By heating the cathode, electrons are emitted from the surface, which serves to reduce the voltage drop associated with the energy required to pull electrons off the surface of the cathode. The laser is powered from a separate power supply that consumes 45 A of 208 V three-phase power. Figure 2.2 shows the basic construction of krypton laser.
  • 39. 27 Figure 2.2: Basic krypton laser construction [4] II.5. KRYPTON LASERS CHARACTERISTICS AND SPECIFICITIES [4] Krypton lasers emit at several wavelengths through the visible spectrum: at 406.7 nm, 413.1 nm, 415.4 nm, 468.0 nm, 476.2 nm, 482.5 nm, 520.8 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm. The Krypton-ion lasers are almost identical in construction, reliability and operating life to argon lasers. Under some conditions, krypton lasers can produce wavelengths over the full visible spectrum with lines in the red, yellow, green and blue. The 647.1 nm and 676.4 nm red lines are the strongest and result in the best performance. Figure 2.3: Typical lasing wavelengths and relative power levels from 500 mW size Krypton Lasers [5]
  • 40. 28 Krypton lasers are normally rated by the power level produced at 647.1 nm. This wavelength is the most frequently used because it can produce an intense red laser light which is difficult to detect from other types of lasers. II.6. OPERATION OF THE KRYPTON LASER 5 II.6.1. SINGLE LINE OPERATION Most laser applications require only one laser wavelength to be produced at a time. Single line operation is achieved by replacing the multiline rear mirror with a prism wavelength selector as shown in the figure 2.4. This assembly consists of an internal prism aligned to properly deflect the intracavity optical path to the High Reflector. Because of the dispersive properties of the prism, only one wavelength at a time will be properly aligned and produce lasing. The wavelength selector thus allows easy tunability and selection of any of the individual lasing wavelengths. The power available from a single line using a prism wavelength selector is usually greater than the power that can be obtained from the same wavelength by splitting a multiline beam with an external prism. Figure 1: Single line operation [5] II.6.2. MULTILINE OPERATION In its simplest configuration, an ion laser is a multiline laser producing a number of simultaneously lasing wavelengths. The figure 2.5 shows the optical configuration of a basic multiline argon/krypton lasers. The mirror arrangement consists of a rear High Reflector and an output transmitter aligned with the plasma tube to produce lasing. With standard mirror coatings, the output beam of a krypton laser consists of ten discrete wavelengths emitted together. They can be separated into their individual lines by using an external prism or other dispersive
  • 41. 29 elements as illustrated. The approximate distributions of the output power among the ten over eleven wavelengths of a multiline and single line krypton laser operating at full rated power are shown in table II.2. Figure 2.5: Multiline ion lasers operation [5] Table II.2: Representative wavelengths of a Krypton Ion Laser in the visible region [12] Wavelengths(nm) 676.4 647.1 568.2 530.9 520.8 482.5 476.2 468.2 415.4 413.1 406.7 Multiline operation (relative power) 0.05 0.14 0.04 0.06 0.28 0.02 0.02 0.02 0.07 0.04 - Single line operation(W) 1.2 3.5 1.1 1.5 0.7 0.4 0.4 0.5 0.28 1.8 0.9 For highly ionized high-current regime, the UV lines can be obtained from doubly ionized Kr (Kr++): 356.4, 350.7, and 337.4 nm. The spectrum of typical Kr ion laser lines is given in the Figure 2.3 above. In practice, the Krypton laser is designated to operate in single line rather than multiline operation. The krypton based plasma is more unstable, the total power in multiline regime is twenty five times lower than for single line at 647.1nm. The large attraction of a Kr laser is a
  • 42. 30 strongest, red 647.1 nm line with 3.5 W output. This feature can find an interesting application, as will be described in the third chapter of this stud 4 3.5 3 2.5 multiline operation 2 1.5 single line operation(w) 1 0.5 0 0 200 400 600 800 Figure 2.6: Characteristic curves of a krypton laser operation Krypton lasers are generally not used in multiline mode but rather, with optics, to select the red (647.1 nm) line alone, both the red and yellow (568.2 nm) lines, or white-light mode, in which three or four lines are allowed to oscillate. By selecting only required lines, the output power of the already weak krypton laser is preserved. II.7. COMPARISON AND SPECIFICATIONS OF A KRYPTON LASER 11 II.7.1. COMPARISON WITH ARGON ION LASERS The basic construction of the Krypton lasers and Argon lasers are the same but there is a small difference in their operations. Argon lasers could operate in both multi and single line operations but the krypton laser frequently operates in single line. The table II.3 lists the common visible wavelengths of argon and krypton ion lasers and typical output power for a comparably sized single-line (wavelength-selected) laser using each gas.
  • 43. 31 Table II.3: Comparison of Argon and Krypton Lasers Output [11] Argon ion (Ar+) Wavelength (nm) 454.5 457.9 465.8 Line power 140 mW 420 mW 180 mW Krypton ion (Kr+) Wavelength (nm) 406.7 413.1 415.4 472.7 476.5 488 496.5 501.7 514.5 528.7 240 mW 720 mW 1.8 W 720 mW 480 mW 2.4 W 420 mW 476.2 482.5 520.8 530.9 568.2 647.1 676.4 Line power 150 mW 413.1 and 415.4 nm (combined) 50 mW 30 mW 70 mW 200 mW 150 mW 500 mW 120 mW II.7.2. PERFORMANCE SPECIFICATIONS 5 The listed specifications represent the general performance of standard models. Beam diameter and beam divergence increase slightly with increasing wavelength. If the mirror configuration is not changed, divergence values at other wavelengths will be: d (2.1) d0 0 Where d is diameter (or divergence) at wavelength λ; d 0 is listed diameter (or divergence) at listed wavelength 0 . The cavity length is the optical distance between the two mirrors making up the optical cavity. Due to the normal travel of the mirror tuning screws, this length can vary by ± 2mm. The resulting change in longitudinal mode spacing can be calculated from: t c (0.001) L (2.2) where c is 3 x 108 m/s and L is the listed cavity length Current regulation allows direct control of the current through the plasma tube. Light regulation provides the ultimate in laser output stabilization. A small portion of light is sampled within the laser and automatically adjusts the
  • 44. 32 laser current to maintain a constant output. This feature also allows for the light level to be modulated externally with a 0-10V signal. Krypton models have 5% lower input voltage range than that listed for the argon models. Filtered tap of water is used, the maximum temperature of filtered water is 35o C and the maximum static pressure is 70 psi (4.8 atm). TableII.4: Performance, Specifications of different models of Krypton and Argon ion lasers [5] Model 85 series Model 95 series 514.5 nm TEM00 (Argon) 1.1 mm ≤ 1.3 mm / ≤ 1.5 mm 647.1 nm TEM00 (Krypton) 1.2 mm ≤ 1.3 mm / ≤ 1.5 mm 514.5 nm TEM00 (Argon) 0.7 mrad 0.7 mrad 647.1 nm TEM00 (Krypton) 0.9 mrad 0.9 mrad With prism wavelength selector 0.8 m 1.0 m/124 m With multiline mirror holder 0.76 m 0.96 m/1.20 m With prism wavelength selector 188 MHz 150 MHz/122 MHz With multiline mirror holder 197 MHz 156 MHz/126 MHz Beam diameter (1/e2) Beam divergence (full angle) Beam polarization ratio cavity Length (L) Longitudinal mode spacing (c/2L) Optical resonator Solid Invar® rod structure Amplitude power stability (1 hour period after 30 min. warm-up) In current control ≤ ± 0.2 % ≤ ± 0.2% ≤±2% ≤± 3% ≤ 0.5 % (rms) ≤ 0.2% (rms) ≤ 1.5(rms) ≤1.0(rms) 220 V-AC single phase; 30A 208 V-AC, 3 phase ; 50/60 Hz In light control 35 A/50 A,50/60 Hz 190-245 V 190-235 V 1.5 gpm at 15 psi 2.0 gpm at 20 psi (5.6 liters/min at 1 atm) (7.5 liters/min at 1.4 atm) Optical noise (10 Hz to 2 MHz) Light control Current control Electrical service requirements Input voltage range Cooling water requirements
  • 45. 33 CHAPTER III. APPLICATIONS OF THE KRYPTON LASER III.1. INTRODUCTION We have just seen that the Krypton lasers are normally rated by the power level produced at 647.1 nm and 568 nm. Those wavelengths are the most frequently used because they can produce an intense red and yellow laser lights respectively, which are difficult to detect from other types of lasers. In this last chapter of the study we will give and adumbrate the applications of a krypton laser, its applications in science; in industry and in medicine; finally, we will highlight the disadvantages and error encountered when using the krypton laser in medicine. III.2. SCIENTIFIC APPLICATIONS III.2.1. SPECTROSCOPY [15] Spectroscopy is the study of the interaction of matter and radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelengths. Later the concept was expanded greatly to comprise any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is often represented by a spectrum, a lot of the response of interest as a function of wavelength or frequency. Krypton laser beam is introduced on Raman microscopy’s optical axis using a fiber optic, and the beam focused on the crystal can be viewed on a computer monitor. This configuration (figure 3.1) provides the degree of experimental control needed, because the crystals and focal spot are usually too small (on the tens of micrometers scale) to be viewed by the naked eye, in order to display its spectrum on the computer screen. Most systems, approximately100 seconds are required to collect a complete data set, using 100 mW of 647.1 nm Kr+ laser excitation at the sample.
  • 46. 34 Figure 3.1: Raman microscope coupled to a krypton laser and a spectrometer [15] Both video images and spectral data can be displayed in real time on the computer screen. Also shown is a magnified view of a protein crystal in a hanging drop under the microscope objective. III.2.2. HOLOGRAPHY [3] Holograms are images recorded by using laser light, which can be seen in three dimensions without special eyewear (with naked eyes). While many will be familiar with embossed holograms as security devices on credit cards, this kind of mass application hardly does justice to the quality of three-dimensional imaging that can now be achieved with holography. The red krypton laser light can be used with other lasers in order to produce the white light which is necessary to produce the holograms; here three or more lasers are required, that three primary recording laser wavelengths were: 476 nm, provided by an argon ion laser; 532 nm, provided by a continuous-wave frequency-doubled Nd: YAG laser; and 647.1 nm, provided by a krypton ion laser (figure 3.2).
  • 47. 35 Figure 3.2: Experimental configuration used to record color reflection holograms [3] Holography is being used for non-destructive testing, holographic information storage, display devices and pattern matching procedures for such tasks as credit card and identity card verification. Holographic methods can also be used for secret communication of information by recording the holograms of secret documents, maps and objects, and restructuring the images only at the receiver. Interference holography can be used to measure accurately how structures deform under the effect of mechanical stress or thermal gradient. Standard holograms may be used in industrial production processes to check high precision components with regard to their shape dimensional accuracy. [4] Krypton lasers have other numerous scientific applications that include: Laser Doppler velocimetry; Ti: Sapphire (Dye) laser pumping; Lithography; High laser printing; Cytofluorescence, etc… III.3. INDUSTRIAL APPLICATIONS III.3.1. NON DESTRUCTIVE TESTING (NDT) [4] The non destructive testing is the use of noninvasive techniques to determine the integrity of a material, component, structure or quantitatively measure some characteristics of an object (inspect or measure without doing harm). The different methods of NDT where lasers are contributing are Laser Interferometry; Radiography and in visual inspection. Frequently, the modern technologies are based on NDT techniques.
  • 48. 36 III.3.2. DATA STORAGE (Disc mastering) [4] The storage of higher density of data is possible by using optical techniques. The storage medium is generally a thin film of metal whose optical properties change when it is illuminated with a powerful write laser. The less powerful read laser reads the change in optical property as the required information. Since laser beam can be focused on the spots smaller than one micro diameter, it takes less than one square micro record one bit of information, i.e. 108 /cm2. The magnetic data storage vices like the present day video cassettes in market cannot have such high density data age. However, the main drawback of optical storage is that it is not erasable; such era’s optical discs are already into the market. The krypton lasers may be also used in semiconductors processing for developing integrated circuit. III.4. MEDICAL APPLICATIONS III.4.1. OPHTHALMOLOGY [14] Krypton laser is very often used in Ophthalmology because of its yellow and red wavelengths for photocoagulation of retina and other eye diseases. The red 647.1 nm krypton beam can be used advantageously for pan retinal photocoagulation when confronted with extensive retinal hemorrhages secondary to diabetic retinopathy, central retinal vein occlusion, branch retinal vein occlusion. The red and yellow krypton beams are excellent in the foveolar region for the photocoagulation and obliteration of sub pigment epithelial neo vascularisation due to minimal absorption by xanthophylls pigments, structural defects can be treated effectively. Figure3.3: Laser beam used to slow or stop the growth of abnormal blood vessels in the Retinal caused by diabetic retinopathy [10]
  • 49. 37 The energy of krypton at 647.1 nm would be absorbed by luteal pigment or by hemoglobin. It is possible to photocoagulate within the area containing luteal pigment (macular) without inner retinal coagulation, and it has been shown that irradiation of retinal vessels cause no focal damage and less attenuation of the incident energy by inner retinal layers (focal damage to the ganglion cell areas). Krypton laser may be used in the management of parafoveal disciform lesions. [1] The krypton lasers are more advantageous than argon lasers for photocoagulation of retina. III.4.2. ADVANTAGES OF KRYPTON (RED) OVER ARGON LASER The red beam of a krypton laser presents the following advantages compared to the argon laser:  Reduced risk in the inner retinal photocoagulation, especially at macular.  Less consequent hazard of foveal denervation and intra retinal fibrosis.  Lack of uptake in hemoglobin.  High uptake in choroid is an additional advantage in closing the choroidal vessels from which these neo-vascular tissues arise. III.4.3. DISADVANTAGES AND ERRORS WHEN USING THE KRYPTON LASER IN MEDICINE Even though krypton laser is advantageous in eye surgery, there are also disadvantages of using such kind of laser which may cause the common professional errors, if there is an even mistake during treatment. III.4.3.1. Disadvantages [14]  Inadequate absorption for focal retinal-vitreal neo-vascularisation coagulation.  Possible power density inadequacy.  Limited retinal layer application. Photocoagulation of surface neo-vascularisation and other retinal vascular anomalies is difficult because of foveal denervation and late inner retinal fibrosis. Less energy is available at the level of the pigment epithelium because of attenuation by luteal pigment.
  • 50. 38 III.4.3.2. Sources of errors [2] For krypton laser surgery, the probable sources of errors are the high power densities nonindicated, this causes atrophic scars after krypton laser treatment. In laser treatment, an incorrectly determined indication or one which is not determined at all has a lack of therapeutic success as its best case scenario. However, the consequences can be much more severe and even irreversible in some cases. Using lasers in surgery medicine requires for practitioners not only to have high levels of training and experience, but also to exercise sound professional judgment. Instead of all the precautions taken, the risk of complications and side effects can only be reduced, not only eliminated. Generally applicable quality guidelines should be created that will guarantee training, safety, and procedural quality in laser treatments.
  • 51. 39 CONCLUSION AND RECOMMENDATIONS 1. CONCLUSION Krypton laser has many applications due to its important wavelengths (red 647.1nm and yellow 548.2nm). Our aim was to make more understandable the physical principles of gas lasers, especially on the krypton laser, its output characteristics, operation and applications. A krypton laser is the laser whose active medium is the rare gas krypton in ionized species. We have shown that such a kind of laser finds widespread use in science, industry and is widely used in medicine especially for retinal photocoagulation. It has been also shown that the use of krypton laser in medicine has some disadvantages and these, may cause errors after krypton laser treatment then it requires professional judgment for obtaining better results. 2. RECOMMENDATIONS From the results of this work, the following recommendations are mentioned:  Krypton lasers are widely used in medicine: the government of Rwanda may try to improve the medical services with using krypton laser, especially in surgery service for treating the eye diseases.  In order to facilitate NUR students to improve their research in science, it is recommended that NUR department of physics laboratories be equipped by a krypton laser for its use in order to ameliorate our industrial development.  Because of the role of lasers in our everyday life, we suggest that the researchers should make an effort in lasers applications in order to find more advantages in their use.
  • 52. 40 REFERENCES 1. A. C. BIRD and R. H. B. GREY: Photocoagulation of disciform macular lesions with krypton laser, British Journal of Ophthalmology, UK, 1979, 63, 669-673. 2. BAERBEL GREVE, MD AND CHRISTIAN RAULIN, MD: Laser and IPL Errors, laserklinik karlsrube, Germany, February, 2002. 3. Hans Bjelkhagen and Jill Cook: Colour holography of the oldest known work of art from Wales, The British Museum Technical Research Bulletin, Vol.4 2010. 4. http://www.analytical-online.com/Application%20Notes/Lasers, visited on 20th march 2013. 5. http://www.lexellaser.com/techinfo-gas-ion.htm, visited on 6th September, 2012. 6. http://www.photonics.com, visited on 4th November, 2012. 7. http://www.ultrafast-innovations.com, visited on 7th November, 2012. 8. http://www.gizmag.com/spyder-3-krypton-laser/19747, (the newsletter for gizmag emergency technology magazine), visited on 7th November, 2012.. 9. http://www.lasers.coherent.com/lasers/krypton%20laser, visited on 7th November, 2012. 10. http://www.spacecoastlaser.com/lasers.html#krypton, visited on 10th November, 2012. 11. MARK Csele: Fundamentals of light sources and lasers; John Wiley & Sons, Inc., Hoboken, New Jersey, 2004. 12. Masamori Endo and Robert F. Walter: Gas lasers, CRC PRESS Taylor & Francis group, Boca Raton London, New York, 2007. 13. Narendra B. Dahotre, Sandip P. Harimkar: Laser Fabrication and Machining of Materials, Springer Science + Business Media, LLC, New York, 2008. 14. Orazio Svelto: Principles of Lasers, Fifth Edition, Springer New York Dordrecht Heidelberg, London, 2010. 15. Paul R. Carey: Annu. Rev. Phys. Chem. 2006. 57:527–54 16. R C K Loh: Use of krypton laser, SING MED J.1988; 29: 66-67. 17. Silfvast William T: Laser Fundamentals, New York, Cambridge University Press, 1996. 18. Subhash Chandra Singh, Haibo Zeng, Chunlei Guo, Weiping Cai: Nanomaterials: Processing and Characterization with Lasers, Copyright © 2012, Wiley-VCH Verlag & Co. KGaA (Published Online: 23th Aug 2012 06:11AM EST).
  • 53. 41 19. Walter Koechner Michael Bass: Solid-State Lasers: A Graduate Text, Springer, New York, 2003. 20. WILLIAM S., C. CHANG: Principles of lasers and optics, Cambridge University Press, New York, 2005.

×