i

DEDICATION

This work is dedicated

To my parents

For fostering and encouraging

my interest in science.

To my sisters and brothers

To MUGISHA Keen Darren

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

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 MEDIA12.............................................................................................................4

I.2.1. IONIZED GAS.......................................................................................................................5

I.2.2. INTERACTIONS...................................................................................................................5

iv

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 GASES13.....................................................................7

I.4.2. STIMULATED EMISSION 13...........................................................................................8

I.4.3. AMPLIFICATION OF RADIATION13.............................................................................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 laser19

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

II.5. KRYPTON LASERS CHARACTERISTICS AND SPECIFICITIES [4]................................27

v

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

REFERENCES....................................................................................................................................40

vi

LIST OF SYMBOLS AND ABBREVIATIONS

vii

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)

FM: Frequency modulation

viii

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

V: Volume

ix

T: Temperature

k: Boltzmann’s constant

N: Avogadro’s number (6.0248x1023 molecules per mol)

ni: Net charge density of free electrons

: Vector current density in discharge.

E: Electric field

B: Magnetic field

: Specific power

Ge: Germanium

Nd:YAG: Neodymium-Yttrium-Aluminum Garnet

GaAs: Gallium Arsenide

ZnS: Zinc sulfide

ZnSe: Zinc Selenium

LISTS OF FIGURES

x

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

LISTS OF TABLES

Table I.1: Commercially wavelengths of He-Ne laser..........................................................................16

xi

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

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.

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.

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.

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

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 MEDIA12

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:

(1.1)

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 (ni), positive ¿¿ and negative ¿¿ions produced in the plasma tend to

zero:

(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:

6

- Charge transfer:

- Ion recombination:

- For elastic collision:

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:

(1.3)

where is the vector current density in discharge.

In the above equation we consider DC discharge, where drift velocity vc = const, and 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.

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 GASES13

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.

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:

(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

9

I.4.3. AMPLIFICATION OF RADIATION13

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.

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:

(1.5)

Where d is the separation between the two mirrors; R1 and R2 are respectively their radii of curvature.

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, in that case, the value is

preserved because .

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

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 , which is required for optimal laser performance, very high voltages are needed to keep

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:

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 « gas-

dynamic 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.

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.

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.

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

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

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

lasers are generally much larger than nitrogen lasers and have higher power outputs, producing

19

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.

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.

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.

I.7.3. INFRARED AND FAR INFRARED GAS LASERS [18]

22

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 20μm 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.

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.

CHAPTER II. KRYPTON LASER

24

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 ( ), double ( ) and triple ionized ( ). Let us now

consider singly ionized krypton (denoted ); 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.

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+(eV) pulsed CW

Kr++

(eV) pulsed

C

W

Kr3+

(eV) pulsed only

Kr4+

(eV)

atomic

mass

14 743.6 350.74

38.3

5 350.7 - 75.31 219.2 127.81 83.8

356.4 225.5

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 to allow only

certain wavelengths to oscillate, as is frequently done with krypton lasers to select only the red line.

26

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.

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]

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 elements as illustrated. The approximate

29

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) Multiline operation Single line operation(W)

  (relative power)  676.4 0.05 1.2647.1 0.14 3.5568.2 0.04 1.1530.9 0.06 1.5520.8 0.28 0.7482.5 0.02 0.4476.2 0.02 0.4468.2 0.02 0.5415.4 0.07 0.28413.1 0.04 1.8406.7 - 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 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

30

350 400 450 500 550 600 650 700

0

0.5

1

1.5

2

2.5

3

3.5

4

multiline operationsingle line operation(w)

wavelengths(nm)

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.

Table II.3: Comparison of Argon and Krypton Lasers Output [11]

Rela

tive

pow

er(W

)

31

Argon ion (Ar+) Krypton ion (Kr+)

Wavelength (nm)

Line power Wavelength (nm)

Line power

454.5 140 mW 406.7457.9 420 mW 413.1 150 mW465.8 180 mW 415.4 413.1 and 415.4 nm

(combined)472.7 240 mW 476.2 50 mW476.5 720 mW 482.5 30 mW488 1.8 W 520.8 70 mW

496.5 720 mW 530.9 200 mW501.7 480 mW 568.2 150 mW514.5 2.4 W 647.1 500 mW528.7 420 mW 676.4 120 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:

(2.1)

Where d is diameter (or divergence) at wavelength λ; is listed diameter (or divergence) at listed

wavelength .

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:

(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 laser current to maintain a

constant output. This feature also allows for the light level to be modulated externally with a 0-10V

32

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 35 o 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

Beam diameter (1/e2)    

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

Beam divergence (full angle)

514.5 nm TEM00 (Argon) 0.7 mrad 0.7 mrad

647.1 nm TEM00 (Krypton) 0.9 mrad 0.9 mrad

Beam polarization ratio cavity

Length (L)

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

Longitudinal mode spacing (c/2L)

With prism wavelength selector 188 MHz 150 MHz/122 MHz

With multiline mirror holder 197 MHz 156 MHz/126 MHz

Optical resonator Solid Invar® rod structure

Amplitude power stability (1 hour period after 30 min. warm-up)

In light control ≤ ± 0.2 % ≤ ± 0.2%

In current control ≤ ± 2 % ≤± 3%

Optical noise (10 Hz to 2 MHz)

Light control ≤ 0.5 % (rms) ≤ 0.2% (rms)

Current control ≤ 1.5(rms) ≤1.0(rms)

Electrical service requirements 220 V-AC single phase; 30A 208 V-AC, 3 phase ;

  50/60 Hz 35 A/50 A,50/60 Hz

Input voltage range 190-245 V 190-235 V

Cooling water requirements 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)

CHAPTER III. APPLICATIONS OF THE KRYPTON LASER

33

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.

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).

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.

III.3.2. DATA STORAGE (Disc mastering) [4]

36

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]

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

37

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.

III.4.3.2. Sources of errors [2]

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For krypton laser surgery, the probable sources of errors are the high power densities non- indicated,

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.

CONCLUSION AND RECOMMENDATIONS

1. CONCLUSION

39

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.

REFERENCES

40

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).

19. Walter Koechner Michael Bass: Solid-State Lasers: A Graduate Text, Springer, New York,

2003.

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20. WILLIAM S., C. CHANG: Principles of lasers and optics, Cambridge University Press, New

York, 2005.