Numerical Study of Laser Interaction With Solid Materials

1

Chapter One

Basic concepts

11 Introduction

Laser is a mechanism for emitting light with in electromagnetic radiation

region of the spectrum with different output intensity Max Plank published

work in 1900 that provided the understanding that light is a form of

electromagnetic radiation without this understanding the laser would have

been invented The principle of the laser was first known in 1917 when Albert

Einstein describe the theory of stimulated emission and Theodor Maiman in

1960 invent the first laser using a lasing medium of ruby that was stimulated

by using high energy flash of intense light

We have four types of laser according to their gain medium which are

(solid liquid gas and plasma) such as (ruby dye He-Ne and X-ray lasers)

So laser is provided a controlled source of atomic and electronic excitations

involving non equilibrium phenomena that lend themselves to processing of

novel material and structure because laser used in wide range application in

our life such as welding cutting drilling industrial and medical field Maiman

and other developer of laser weapons sighting system and powerful laser for

use in surgery and other areas where moderated powerful pinpoint source of

heat was needed And today laser are used in corrective eye surgery and

providing apprecise source of heat for cutting and cauterizing tissue

12 Definition of the Laser

The word laser is an acronym for Light Amplification by Stimulated Emission

of Radiation The laser makes use of processes that increase or amplify light

signals after those signals have been generated by other means These

processes include (1) stimulated emission a natural effect that was deduced

2

by considerations relating to thermodynamic equilibrium and (2) optical

feedback (present in most lasers) that is usually provided by mirrors

Thus in its simplest form a laser consists of a gain or amplifying medium

(where stimulated emission occurs) and a set of mirrors to feed the light back

into the amplifier for continued growth of the developing beam as seen in

Fig(11) Laser light differs from ordinary light in four ways Briefly it is much

more intense directional monochromatic and coherent Most lasers consist

of a column of active material with a partly reflecting mirror at one end and a

fully reflecting mirror at the other The active material can be solid (ruby

crystal) liquid or gas (HeNe COR2R etc)

Fig(11) Simplified schematic of typical laser

13 Active laser medium or gain medium

Laser medium is the heart of the laser system and is responsible for

producing gain and subsequent generation of laser It can be a crystal solid

liquid semiconductor or gas medium and can be pumped to a higher energy

state The material should be of controlled purity size and shape and should

have the suitable energy levels to support population inversion In other

words it must have a metastable state to support stimulated emission Most

lasers are based on 3 or 4 level energy level systems which depends on the

lasing medium These systems are shown in Figs (12) and (13)

3

In case of a three-level laser the material is pumped from level 1 to level 3

which decays rapidly to level 2 through spontaneous emission Level 2 is a

metastable level and promotes stimulated emission from level 2 to level 1

Fig(12) Energy states of Three-level active medium

On the other hand in a four-level laser the material is pumped to level 4

which is a fast decaying level and the atoms decay rapidly to level 3 which is

a metastable level The stimulated emission takes place from level 3 to level 2

from where the atoms decay back to level 1 Four level lasers is an

improvement on a system based on three level systems In this case the laser

transition takes place between the third and second excited states Since

lower laser level 2 is a fast decaying level which ensures that it rapidly gets

empty and as such always supports the population inversion condition

Fig(13) Energy states of Four-level active medium

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14 A Survey of Laser Types

Laser technology is available to us since 1960rsquos and since then has been

quite well developed Currently there is a great variety of lasers of different

output power operating voltages sizes etc The major classes of lasers

currently used are Gas Solid Molecular and Free Electron lasers Below we

will cover some most popular representative types of lasers of each class and

describe specific principles of operation construction and main highlights

141 Gas Lasers

1 Helium-Neon Laser

The most common and inexpensive gas laser the helium-neon laser is

usually constructed to operate in the red at 6328 nm It can also be

constructed to produce laser action in the green at 5435 nm and in the

infrared at 1523 nm

One of the excited levels of helium at 2061 eV is very close to a level in

neon at 2066 eV so close in fact that upon collision of a helium and a neon

atom the energy can be transferred from the helium to the neon atom

Fig (14) The components of a Hilium-Neon Laser

5

Fig(15) The lasing action of He-Ne laser

Helium-Neon lasers are common in the introductory physics laboratories

but they can still be quite dangerous An unfocused 1-mW HeNe laser has a

brightness equal to sunshine on a clear day (01 wattcmP

2P) and is just as

dangerous to stare at directly

2- Carbon Dioxide Laser

The carbon dioxide gas laser is capable of continuous output powers above

10 kilowatts It is also capable of extremely high power pulse operation It

exhibits laser action at several infrared frequencies but none in the visible

spectrum Operating in a manner similar to the helium-neon laser it employs

an electric discharge for pumping using a percentage of nitrogen gas as a

pumping gas The COR2R laser is the most efficient laser capable of operating at

more than 30 efficiency

The carbon dioxide laser finds many applications in industry particularly for

welding and Cutting

6

3- Argon Laser

The argon ion laser can be operated as a continuous gas laser at about 25

different wavelengths in the visible between (4089 - 6861) nm but is best

known for its most efficient transitions in the green at 488 nm and 5145 nm

Operating at much higher powers than the Helium-Neon gas laser it is not

uncommon to achieve (30 ndash 100) watts of continuous power using several

transitions This output is produced in hot plasma and takes extremely high

power typically (9 ndash 12) kW so these are large and expensive devices

142 Solid Lasers

1 Ruby Laser

The ruby laser is the first type of laser actually constructed first

demonstrated in 1960 by T H Maiman The ruby mineral (corundum) is

aluminum oxide with a small amount (about 005) of Chromium which gives

it its characteristic pink or red color by absorbing green and blue light

The ruby laser is used as a pulsed laser producing red light at 6943 nm

After receiving a pumping flash from the flash tube the laser light emerges for

as long as the excited atoms persist in the ruby rod which is typically about a

millisecond

A pulsed ruby laser was used for the famous laser ranging experiment which

was conducted with a corner reflector placed on the Moon by the Apollo

astronauts This determined the distance to the Moon with an accuracy of

about 15 cm

7

Fig (16) Principle of operation of a Ruby laser

2- Neodymium-YAG Laser

An example of a solid-state laser the neodymium-YAG uses the NdP

3+P ion to

dope the yttrium-aluminum-garnet (YAG) host crystal to produce the triplet

geometry which makes population inversion possible Neodymium-YAG lasers

have become very important because they can be used to produce high

powers Such lasers have been constructed to produce over a kilowatt of

continuous laser power at 1065 nm and can achieve extremely high powers in

a pulsed mode

Neodymium-YAG lasers are used in pulse mode in laser oscillators for the

production of a series of very short pulses for research with femtosecond time

resolution

Fig(17) Construction of a Neodymium-YAG laser

8

3- Neodymium-Glass Lasers

Neodymium glass lasers have emerged as the design choice for research in

laser-initiated thermonuclear fusion These pulsed lasers generate pulses as

short as 10-12 seconds with peak powers of 109 kilowatts

143 Molecular Lasers

Eximer Lasers

Eximer is a shortened form of excited dimer denoting the fact that the

lasing medium in this type of laser is an excited diatomic molecule These

lasers typically produce ultraviolet pulses They are under investigation for use

in communicating with submarines by conversion to blue-green light and

pulsing from overhead satellites through sea water to submarines below

The eximers used are typically those formed by rare gases and halogens in

electron excited Gas discharges Molecules like XeF are stable only in their

excited states and quickly dissociate when they make the transition to their

ground state This makes possible large population inversions because the

ground state is depleted by this dissociation However the excited states are

very short-lived compared to other laser metastable states and lasers like the

XeF eximer laser require high pumping rates

Eximer lasers typically produce high power pulse outputs in the blue or

ultraviolet after excitation by fast electron-beam discharges

The rare-gas xenon and the highly active fluorine seem unlikely to form a

molecule but they do in the hot plasma environment of an electron-beam

initiated gas discharge They are only stable in their excited states if stable

can be used for molecules which undergo radioactive decay in 1 to 10

nanoseconds This is long enough to achieve pulsed laser action in the blue-

green over a band from 450 to 510 nm peaking at 486 nm Very high power