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CHAPTER ONE

INTRODUCTION

1.1 MEANING

Laser is an acronym for light complication by stimulated emission

radiation. This is a relatively young and fascinating field. Since its inception

has witnessed a rapid growth. The laser field came into being via an expans

of stimulated amplification techniques that resulted from the microwave to

optical region of the electromagnetic spectrum.

1.2 HISTORICAL REVIEW

Laser existence was first experimentally observed in 1960. In the p

years, efforts had been made by renowned scientists to produce coherent li

which is amplified through stimulated emission. Stimulated emission w

introduced by Albert Einstein by showing that Plancks radiation form

results in thermal equilibrium from the interaction of spontaneous emissi

stimulated emission and absorption.

R. Ladenbery and A. Kopfermann (1925) observed a negative dispers

on gas discharge which actually resulted from stimulated emission non-therm

equilibrium, amplification and coherent generation of electromagnetic radiat

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are now possible by means of stimulated emission. E.M. Parcell and R

Pound (1950) showed also by experiment an inversion in the nuclear s

system of lithium fluoride leading to speculation about microwa

amplification by stimulated emission.

C.H. Townes had discussed the possibility of amplifying electromagne

radiation by passing it through a medium in which higher energy stales

dominant compared to thermal equilibrium produced by auxiliary radiation.

presented the ammonia-gas beam oscillator in 1954 with I.P. Gordon and H

Ziegor. He coined the term MASER for this type of amplifier. It is an acrony

for microwave amplification by stimulated emission .Many trials were made

invert the broader lines of the electro spin system of paramagnetic cryst

similar to the well-known procedure of the nuclear induction method to the f

pulsed solid-state maser oscillator with a two level stag system.

Bioembergen discussed the three level method in a solid by using pum

energy that is free of signal frequency to sustain a continuous inversion. Ba

and Prokhorov used this three level method for gas maser. As a traveling ma

Scovil (1958) investigated ruby maser and developed it into a technically a

extremely low-noise microwave amplifier.

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Schawlow and Townes (1958) showed how feasible it is to exc

individual mode in multi-mode resonators to coherent oscillations. Th

accounted for the excitation threshold of different level- systems of solid a

gasses. The inversion of fluorescent levels of ruby which had been discuss

appeared to lie beyond the experimental possibilities because of its three-le

character.

Maiman (1960) succeeded in overcoming this difficulty. The used

pulsed high-power flash tube as the pump source. He used a resonator in

inversion of a mirrored ruby. He could identify the starting off of a stimula

light called avalanche. This is a reduction of the lifetime of the fluorescen

and a decrease of the fluorescence band width, Maiman also brought a w

LASER which comes in as an analogy to MASER and LASER which

known as light amplification by stimulated emission by radiation. T

helium-neon gas was proposed later by Javan (1959) and this brought about

first continuous laser operation

In later years, advanced technology made laser to now become a c

study in most laboratories. The discovery of a large number of solid materi

that has laser properties especially rare earth and laser transition in gases.

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There came an expansion again for this field with the description

semiconductor laser. Liquid laser which is the nearest was found later and

resulted from the amalgamation of both gas and solid laser together.

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CHAPTER TWO

THE THEORY OF LASERS

We take a look at a two-level action, where the energy of the upper level is

and that of the lower E1. these levels are connected radiatively by a frequen

V. N2 and N1 corresponds to number of actions having their violence electro

state E2 and E1 respectively a thermal equilibrium.

dN2 = A21 N2 - B 21 N2 P(v) + B12 N1 P (v) = 0 (1)

dt

where P (v) radiation density association with temperature T, where T is

temperature at which the system is thermalized. B21 and B12 serves as

probability per unit time that radiation is absorbed. Therefore, B21 B12 = B, A

the probability per unit time that an excited atom emits spontaneously.

From 1

Bp (v) (N1 N2) = A21N2

A21 N1 N2 = N1 - 1 - (2)

B(p(v)) N2 N2

Number of atoms radiating spontaneously per second will be

nsp = A21N2 - - - - (3)

nsp = number of atom radiating spontaneously and number of stimula

emission per second is

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nst = BN2 P (v) - - - - (4)

Divide equation (3) by (4)

nsp = A21 N2 - - - - (5)

nst = B P(v) N/2

From equation (5) and (2)

nsp = N1u ----1

nst N2

N1 = exp (hv) -1 - ------ (6) (i)

N2 KT

In the optical region of the spectrum where v ~ 5 X 1014 H2 and at an avera

temperature of 4500K

exp hv

KT

Hence nsp


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While at optical frequencies, it corresponds to a high value

temperature. This brings the number of stimulated photons to only exceed

number emitted spontaneously. If N2>N1, then the system would not be

thermal equilibrium and nsp is not necessary greater than nst. We can theref

imply that the first requirement for laser action must be that the energy lev

concerned are not in thermal equilibrium and that the upper of the two levels

more populated than the lower. This is known as population inversion.

From equation (4), it is shown that a large amount of stimulated emiss

needs to be large and also the energy density of the radiation field at

appropriate frequency v is large. For laser action to occur, it is necessary

devise a mean of achieving N2>N1 and to build up the radiation field

frequency v. the chance of population inversion are thereby increased when i

supposed that the upper energy level has a fairly long spontaneous lifetime a

the lower a short life time.

Schadow, Towns and Prokhorov (1958) made an independent discuss

on conditions of achieving laser action. They each considered that although

was not possible to construct an optical resonator of the dimension of

wavelength concerned, as may be done in the microwave region of

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spectrum, the plain-parallel fabry-perot interferometer was in fact, a suita

resonator. That the fabry-perot interferometer is a more lossy type of cav

than a microwave cavity because it has no side walls, and the main sources

loss of energy would be due the fact that the reflection coefficient of the min

could never be unity.

The light in the cavity energy be considered to travel backwards a

forwards between the mirrors and the effective decay time of the cavity may

calculated by the simple expedient of considering that the energy of the w

falls to 1/e of its original value in n transverses, due to the imperf

reflectivity. Then R is the reflection coefficient of the minors

Rn = 1/e

and taking the logarithm and expanding

n = 1

1-R

8

Pump Light Active material

Emitted lig

g o o . . . . . . . . .

. . . . .

..

Mirrors

Fig. 2.2 Schematic diagram of laser device


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If the length of the cavity is d, the time tc, taken for the energy to decay to

of its previous value will be

tc = nd/C

tc = d second --------------(7)

C (1-R)

Supposing there are P-modes effective in producing spontaneous emiss

where P (v) dv, is the number of mode between V and V + dv then, for

excess population N = N2 N1, to sustain a quantium in each mode we m

have this equation;

Where;

Nhv hv ----------------- (8)

Pc tc

Where T could be said to be spontaneous lifetime of the upper level, wh

there are N quanta of energy hv radiating into P modes in T second. This r

per modes must be higher than the rate of decay energy in the mode hv/tc. T

made it possible in a steady state to say that the rate of stimulated emission i

single mode joint equal to the rate of spontaneous emission into the same sin

mode. Yariv and Gordon in (1963) showed that this is a special case that

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ratio of the induced transition rate to the rate of spontaneous emission int

given mode is equal to the number of quanta in the mode.

From equation (8)

Where;

N Pc ----------- (9)

tc

Now for a Doppler-broadened line Schawlow and Townes showed that;

P = 8 2 V2VAN -------- (10)) in 2)1/2 C3

Where V is the cavity volume, hence;

N 8 2 V2 DV C (1- R) -------- (11)

1 n2)1/2 C2d

After substituting for P and tc from above equations, therefore, the populat

inversion per unit volume is then given by

n > II 8 2 T (1- R) V2 DV------(12)

/n2)11111/2) dc2

But life time of the upper level . (13)

i.e. C = 3hc2 (14)

64II2V2U2

By common and shortly, (1935)

Where U is the dipole matrix element.

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II >/ 8II 3hC2 (1-R) V2 DV . (15)

64 II4 V3 U2 (II2 n2)1/2 d1 C2

II = 3hC2 DV (1-R)

8VII2 (II/n2)1/2 U2d1 .. (16)

And for a Doppler broadened line

DV = V/C (2KT 1n-2)1/2

M

Then II = 3h = (2KT)1/2 (1-R) (17)

8II5/2 m U2d

Allen and Jones 1967 show that it makes good agreement for the value

the population inversion per unit volume obtained by lamb (1964) in

rigorous semi-classical treatment of the problem.

The Einstein coefficient for free space has been taken over here by

treatment and been applied for the case of a cavity. A more technology w

approach to the constant of the cavity is possible if the quality factor or Q,

the cavity is involved from this, it can be showed that given a medium wit

sufficiently high population inversion the electromagnetic field builds up so t

the stimulated emission may be appreciably greater than the total spontaneo

emission even in the case of a multimode cavity. Schawalow and Town

realised that the larger number of modes at infra-red or optical frequency wh

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are formed in any cavity or reasonable size become problems. The reason is t

of a need of high level of spontaneous emission to ensure that sufficient phot

go into any one particular mode, to maintain the rule of stimulated emissio

they now consider the possibility of selecting radiating from a single mode a

of making a multimode cavity sufficiently lossy to suppress the oscillation in

unwanted mode. They considered how this might be done using two pla

mirror, fox and Li (1961) established on the basis of diffraction theory that t

mirrors of finite extent may be said to sustain certain modes of oscillation.

Fig. (2.2) EXPECTED EMISSION PATTERN

It is worth noting, before this calculation one considered that in

terms of the simple semi-classical theory of Schawlow and Townes the

need for a resonant cavity can be considered in one of two possible ways.

It neither constitutes a way in which a large radiation density may be

12

Plane

Mirror

Where d1- mirror diam

and 4- angle gived

Diffraction


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made to grow at the appropriate frequency V, or known within the limits

of theory so far, as being a device of increasing the length of the active

medium.

By this, small amplification coefficient may be eroded and the

intensity is able to increase at such a rate that it over come other inherent

losses of light such as scattering or absorption. This theory will not allow

for mode of oscillation or for certain characteristics laser properties

which are essentially the effect of a feedback mechanism. Writing the

amplification coefficient in an analogous way to the more common

absorption coefficient. Considerable insight may be achieved concerning

the desirable degree of population inversion. The amplification

coefficient at the center of a Doppler-broadened line KO is given by

Limitchell and Zemansky, (1934).

KO = (1n2)1/2 A21 >4 N2 g2 N1 .. (18)

16 C2 3 DT0 g1

Where A21 is the transition probability from level 2 to 1 > the

wavelength, D > D the Doppler width and g1 and g2 the statistical weights

of the levels involved. If

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N2 < g2 N1 then there is an absorption coefficient but if {N2 >

g

1g2 N1}

g1

Then it is population inversion that occurs, it is an amplification

coefficient and may be associated with the gain/unit length of laser active

medium. It could be gathered from this discussion that the laser is a

noise-started oscillator where the noise is spontaneous emission. This

randomly phase light with its finite spread of frequency serves to build up

a radiation field at certain resonant frequency, which in turn act as the

stimulating field in the induced emission process enjoyed by the other

excited atoms.

Fox and L1 brought the first of a whole series of important papers

investigating the idea of resonant modes in an optical interferometer or

cavity. The cavity they consider was the passive type. It becomes

satisfactory if the interferometer is immersed in active medium and there

no side-wall discontinuities. A wave which propagated backwards and

forward between two parallel plane mirrors. An arbitrary initial field

distribution was assumed at the first mirror and the field at the second

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mirror was computed as the result of one transmission. This distribution

was then used to compute the distribution back at the original mirror as a

result of the second transit and so on. This assumed that the wavelength

of the light is small compared with the dimensions of the mirrors and that

the field is transverse and polarized in one direction only. The field at the

mirror after one transit may be expressed as:

Up = Up = ik ua

e

- IKR (1 + cos@) ds .. (19)

4II s R

Where:

Ua field at the first mirror, k wave number, R the distance

from a point on the first mirror to the point of observation and Q the

angle of R to the normal to the first mirror. After a transmits the field at

one mirror due to the reflected field from the other is given by replacing

Up by U9 + 1 and Ua by Uq. If a situation like a stationary mode of

oscillation exists, the distribution of a field would undergo negotiable

change from reflection to reflection and settle down after a certain

number of transits to a steady state. If such a situation exists, then the

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each transit, can be regarded on the propagation constant associated with

this normal mode.

Fox and Li discovered that numerical solution for many

geometrical configuration which include rectangular plane mirrors,

circular plane mirror and concave spherical mirrors. In each case, there is

a solution for v and such an interferometer be said to sustain various

modes of oscillation. This implies that if one of these contributions is

said to be introduced as an initial wave at one mirror. This sort of

distribution may be regarded as normal mode. That is the mode the

interferometer. The lowest order mode, that is the mode of simplest

symmetric is found to have the lowest diffraction loss it has a high

intensity at the middle of the mirror and low intensity at the edges. The

diffract or loss due to over-spill around the edges of the mirrors in

consequently much lower than would be predicted for a uniform plane

wave. The same is therefore essentially true, but to a lower extent for

high order modes, of more complex radial and angular symmetry. No

mirror can ever be perfectly reflecting so that some of laser light may be

release from the cavity to create a beam of laser light. It is necessary to

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have a finite transmission coefficient. The reflection loss is much more

serious than diffraction losses, and this has meant for low gain laser

systems very high reflectivity is necessary e.g.99.78% in all, the

reflection loss and medium gain determine whether or not the laser

oscillates, and the diffraction losses determine the mode of oscillation.

Schawlow and Townes considered the anticipated line width of the

laser oscillator. Gordon, Zieger and Townes in 1955 had previously

produced a formular for the band width of radiation in the microwave

region due to thermal radiation in the cavity; this was:

Vm = 4II KT (DV)2 ------- (21)

P

Where P is the power in the mode. The postulation that spontaneous

emission effects were equivalent to a thermal noise temperature T where;

T = hv

K

And DVl = 4IIhv (DV)2 ------- (22)

P

Where DV is the bandwidth of the passive cavity resonance. The ideal

theoretical line width for lasers is in the range 10-1 to 10-7 Hg. The

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problems of stabilizing the central oscillation frequency are such as to

render such values of line width relatively meaningless.

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CHAPTER THREE

CLASSES OF LASERS

There are different types of laser. Laser action can be exhibited and

produced through devices which are highly affected by new development

in technology and sciences. Classes of laser can be through under the

following: solid-state ionic, laser gasses lasers, liquid laser and semi-

conductor lasers.

3.1 SOLID-STATE IONIC LASERS

The first demonstration of laser action involved a solid-state ionic

laser. The ruby was achieved by Maimans in 1960. The report of other

pulsed solid-state lasers soon follow, U3t in Caf2 (Sopokin and Stevenson,

1960) Sm2+ in Caf2 in 1962 and Nd3+ in glass by Snitzer 1961. The first

continuously operating solid stale Laser, Nd3+ in CaN04, was found in

1962 by Johnson. A large number of solid stale laser emitting in the

visible and near infrared were discovered in the 60s based on transitions

in rare earth, transition metal and actinide ions in a variety of solid hosts.

The most highly developed solid stale laser are ruby, And in glass and

YAIG which to be discussed.

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LASER PROPERTIES OF IONS

3.1.2 RARE EARTHS

Laser action has been observed in trivalent and divalent states of

nine earth elements. These are P3t, Nd3t, E13t, 7m3t and the cso- electronic

pairs EU3t H03t, Vb3t and Sm2t, Dy2t, Tm2t respectively. The rare earth

electronic structure consists of a 52-electron xenon-like rare gas shell

with additional 4F electron 1 to 13 in number. Cso-electron pairs such as

FU3t and Sm2t have qualitative similar valent counter-part due to the

smaller nuclear change. The rare earths with the exception of the

radioactive element Pm are commercially available as high-purity oxides,

metals and salts. In rare earth ions in solids the 4F levels are well shelled

from the crystal field by filled 5s and 5p shells. This produces a result of

emission lines which are relatively narrow and the level structure varies

only slightly from one host to another.

(a.) TRIVALENT RARE EARTH

The ground stale electronic configurations for the trivalent rare

earths are given in the table below. A strong absorption due to transitions

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between differ configurations (the lowest energy is usually 4Fn-1 generally

occurs at near ultraviolet wavelengths and is not suitable for pumping).

Electronic Configurations and Radii of Trivalent Rare Earth

Trivalent Number Ground Stale Ionic Radius

Rare Earth 4F Electrons

Ce3t 1 F5/2 1.034

Pr3t 2 3H4 1.013

Md

3t

3 4I4/2 0.995

Pm3t 4 5/4 0.975

5m3t 5 6H5/2 0.964

EU3t 6 7F0 0.950

Gd3t 7 857/2 0.938

Tb3t 8 7F6 0.923

Dy3t 9 6H15/2 0.908

Ho3t 10 5I8 0.894

Er3t 11 4I15/2 0.881

Tm3t

123

H6 0.869

Yb3t 13 2F7/2 0.858

F.T. ARECHI, E.O. SCHULZ-DU BOIS (1972)

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Laser handbook where Pm is radioactive total configuration is 152, 252

2P63563p6 3d10 4s6 4p6 4d10 4Fn 5s2 5p6

There existing trivalent ion laser are pumped through the relatively week

4f -4f transitions. However, some non-radioactive transfer of excitation

from an added sensitizing agent to laser ion has been employed to

increase pump-light utilization.

(b.) DIVALENT RARE EARTHS

` In divalent rare earth, laser action has been observed in Sm2t, Dy2t

and Tm2t in CaF2, and Sm2t in SrF2, at low temperatures. Strong

absorption in the visible due to 4f-sd transitions provide the opportunity

for effective pumping, while transition within the shielded 4f shall are

suitable for laser emission. In some crystals grown under mildly reducing

conditions, Sm2t can be obtained. A variety of more drastic techniques

now exist for reducing rare earth ions present in the trivalent stale in

CaF2. These methods include exposure to B 8 or X rays. Metal diffusion,

electrolysis or photochemical reaction. The divalent rare-earth lasers that

have been noticed since, have had cubic site symmetry, which precludes

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electric dipole of 4f-4f transitions. They are of 4f-4f transition and

magnetic dipole in character except for Sm2+.

3.1.2 ACTINIDE

The partially filled 5F shell in the actinide series is qualitatively

similar to the 4f shell in the rare earth. The shelding of the 5f levels from

the crystalline field is much weaker. However, the only actinide ion to

exhibit laser action so far is U3t i.e. uranium which is cso-electronic with

Nd3t. The behavior of the uranium laser is not well understood mainly

because the observed spectral are varied and complex. This is because

there are many ways in which charge compensation of the U 3t, ions in the

fluoride hosts can be occurred. Explaining these observed features had

brought about propositions involving U3t sites of tetragonal,

orthorhombic and trigonal symmetries, divalent, quadrivalent and

tetravalent ion spectra, U3t pair spectra, U2t - U3t resonant transfer and

uranyloxide complex spectra.

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3.1.4 TRANSITION METALS

The transition metals have an electronic configuration 1s2 2s2 2p6

3s2 3p6 3dn where n= 1+ to 9 of the unfilled 3d shell is not effectively

shielded. The energy levels and transition probabilities are strongly

influenced by the host materials and the free-ion level designations

cannot be employed. Laser action was first observed in Cr3t (ruby). The

only other transition metal laser operated in a purely electronic transition

is Co2t MgF2 pulse pumped laser action which terminal level is an excited

vibrational state of the host lattice has been observed at reduced

temperatures in Ni2t Co2t and V2t.

3.1.5 SELECTED LASERS

RUBY

The ruby laser is a powerful and compact source of coherent red-

light pulses. A commonly employed pumping configuration is shown

below. Pumping is usually accomplished using xenon filled flash tubes.

The pumping light is produced by discharging a capacitor usually in the

range 50 F to 200 F through the lamp supply voltage between 1kv and

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4kv and lamp impedances on the order of ion are typical. The initial

breakdown voltage is usually or the order of 15kv.

(Fig. 3.1) Trigger Electrode Quartz Tube Ruby; Apparatus for

Pulsed Excitation of Ruby by Maiman

The output of the xenon flash tube has an equivalent black body

temperature of about 6500k. Ruby laser rods varying form 0.16cm-0.2cm

in diameter and 2cm-5cm in length have been employed. The rod exists

in perpendicular to the optic axis of the ruby, the user output is polarized

with the electric field perpendicular to the crystalline axis. The end of the

rod is usually polished flat and parallel to within one minute of arc.

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Laser output is extracted from the optical cavity by means of a

partially transmitting mirror (10% it 5% transmitivity is typical) or a

highly reflecting mirror with a small transmitting mirror. Mirror coatings

are sometimes deposited directly on the end of the rod. Silver coating has

been frequently employed even though absorption losses in these

coatings range from 2% to 40% depending upon the transmissivity.

The coherent laser output generally begins about 0.5ms after the

initiation of the pumping pulse and continues for the duration of the

pumping pulse usually a few milliseconds. The laser output consists of a

series of irregular pulses or spikes of approximately 1us durations.

The population inversion increases initially without the presence of

a coherent optical field. When the inversion exceeds the threshold value

for oscillation, a coherent optical field grows, along with the inversion

when the optical field becomes sufficiently large, the stimulated decay

rate exceeds the pumping rate, and inversion begins to decrease. When

the inversion decreases to the threshold value, the optical field has

reached its maximum. The optical field then decreases and the inversion

continues to decrease below the threshold value. Eventually, the optical

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field decreases to the point where the pumping re-established the

threshold value of the inversion.

At this, the optical field may be reduced to the level of quantum

noise, depending upon parameters such as the pumping rate. Cavity time

constant, if the optical field does not decreases to the noise level, the

conventional rate-equation analysis indication that regular, damped

oscillations of the optical field energy will occur.

3.1.6 Nd: GLASS (NEODYMIUM)

The glass laser currently provides pulses of higher power, energy

and radiance, and shorter duration than any other laser source. There are

several characteristics of the glass host which are important. Glass is

isotropic durable, can accept large dropping concentration uniformly and

can be fabricated in expensively in various shapes and large size with

diffraction limited optical quality. The index of refraction of the host

glass can be varied form 1.5 to almost 2 and the thermal properties can be

selected to minimize the optical abbreviations caused by temperature

variations in the laser rod. The longer in homogenous of broadening Nd3t

is advantageous for pulsed laser operation. It allows that storage of more

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inversion energy without serious depletion by amplified spontaneous

emission, thus making possible the Q-switched operation of large and

more powerful lasers. The comparatively large spectral width of the

glass, laser emission permits output pulse of short durations of the order

of a picoseconds.

A disadvantage of glass is its low conductively which limits the

maximum rod diameter and pulse repetition rate in And: YAIG system.

Time average power output is restricted to values considerably below

these output pulse energies of several hundred joules at repetition rates

exceeding one per second would be expected. The flexibility in

fabricating glass makes possible very large rods i.e. rod with various

types of cladding. The large rods produce the highest available output

energy and power and energy.

Glass laser pulses usually show random spiking in time, regular

undamped oscillations or dumped oscillations, in simplest system, the

duration of the output pulse is comparable to that of pump pulse which is

usually of several milliseconds.

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3.1.7 Nd YAIG (NEODYMIUM)

The And YAIG laser provides room temperature operation at

1.064cm with power output in the largest system approaching the

kilowatt level. Its final laser efficiencies vary from about 0.2% for small

single transverse mode laser to about 2% for the larger multimode lasers.

The YAIG host suitable mechanical properties, high thermal conductivity

and is available with high optical quality. Low threshold operation at

room temperature is facilitated by the relatively narrow line width

compared to And: (Glass) and the four-level operation laser rods are

commercially available in size up to 1cm in diameter by 15cm in length.

3.2 GAS LASER

Gas laser began with a simple example in 1961 when Javan,

Bennett and Herriot reported their initial work on the He-Ne system in

1961. Thereafter, oscillation was observed on several transitions in neon

including the celebrated 6328 a line in 1962. These inaugural discoveries

precipitated an avalanche of further work involving ionizing systems.

Neutral atoms, and molecules, these brought an advanced development

through the power output on efficiencies of gaseous devices were

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unusually low with typical value of a few mill watts and a few

hundredths of a percent. Now, continuous wave outputs in excess of ten

kilowatts have been achieved as well as efficiencies of over 30 percent.

There exist in this particular laser devices certain excitation and inversion

mechanism which provides the gain necessary for self-sustained

oscillation.

Excitation mechanism refers to the means of producing atoms in

excited states, and inversion mechanism refers to the process which

generates the inversion in a particular class or level among which

inversion exists.

3.2.1 EXCITATION MECHANISMS

There are different excitation mechanisms which are operative in

various gas laser systems. They are as follow:

1) Direct charged-particle excitation which are essentially electrons

2) Resonant energy transfer

3) Gas-dynamical processes

4) Chemical reaction

5) Penning effect

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All these processes generate excited stale population densities:

1) Direct Charged Particle Excitation: Gaseous discharge is the

most common medium by far which gas laser operates. This brings

about excitation by charged particle and more specifically electron

impact is utilized at some stages of the overall excitation and

inversion process for the majority of exciting system of laser.

2) Excitation through Resonant or Near-Resonant Energy

Transfer: It is very important in both molecular and atomic

systems of excitation mechanisms. It is excited through resonant or

near-resonant of energy exchange collisions. This form of

excitation present in a particular species is selectively transferred

to a particular stale in another system.

3) Excitation by Gas-Dynamical Processes: Population inversion

which has been described earlier can also occur due to the rapid

heating or cooling of a molecular gas. The transferent inversion

occurs only in time interval following change in temperature. This

explains the principles of operation of gas dynamical processes

using a simple case of population inversion.

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4) Excitation by Chemical Processes: The production of a

chemical comes in excited stales. Recently, this excited states are

used to generate population inversion and subsequent oscillation.

We can only establish a fact that talking a significant fraction of

the chemical energy released in a large class of exothermic

reaction. It goes into internal excitation of the molecule rather

than into the kinelinic mode.

5) Excitation by Penning Effect: The process is when the internal

energy of an excited atom causes the ionization of its collusion

partner during an encounter.

3.2.2 INVERSION MECHANISMS

Our own measure is a laser with two levels of which interact with a

plane light wave. The light wave is reflected back and forth between well

aligned mirrors. The lasers are pumped, the cavity resonant condition is

suppressed and standing wave effect is neglected.

(a.) Inversion with Respect to the Ground Stale: Basic level laser steady

stale inversion can be obtained in three-level systems. A medium

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consisting of atoms, molecules or ions having three unequally spaced

levels.

Fig. 3.2 Energy-Level Diagram for the Three-Level Oscillator System

1/ l2 is decay rate of the upper level 1/L3 relocation rate of third level.

This decay rate is much faster than either the pumping rate or the

relaxation rate of the third level (1/L3)

(b.) Inversion between Two Excited States: This three level laser by

inversion can be attained between an excited state and the ground state.

This can be based on the fact that the upper pump relaxes fas and the

bottle neck is an excited state, a population inversion between the excited

stale can be obtained. There are many examples of inversion in excited

levels of gas system.

34

E3

E2

E1n1

n3

n31/ l2

1/ l3

0


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DESCRIPTION OF OSCILLATION SYSTEMS

3.2.3 HELIUM-NEON ATOM SYSTEM

The first laser gas was proposed and made by Javen in 1959 and

1961 respectively using the helium neon system. A glass tube was taken

and it contains two 1 / 200 flat mirrors, dielectrically coated to have 99%

reflectively at 1.15cm wavelength and aligned to 5 of arc was filled with

a mixture of 10.1 He-Ne to a pressure of approximately 1 to ton. A

discharge in the tube that result in laser action at 1.15m on the 252j = 1

2p4 j = z transition was established by radio frequency transmitter with

electrodes around the tube. The gain was considerable achievement

particularly because it was predicted and the mechanisms well

understood, although it is the interplay of many excitation and de-

excitation process which makes it work.

The discharge thereby creates electrons to have a mean energy of a

few eV. He is therefore excited to the meta stable stale of 23 S and 21S by

the high-energy tail of the distribution. The meta stable he loses their

energy chiefly by energy transfer to neon. This is called a resonant

process and expands up to a cross section of 6 = 4 X 10-17cm2 when a

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collision occur between a Ne atom in ground stale with a He 235, there is

an excitation. The excitation brings the Ne to one of the 2s (2pS 4s) state.

It is a sort of resonant energy transfer that serves as a main, de-excitation

mechanism for He meta stables. This makes it to stand out and compared

more favourably with diffusion to the walls. It is also the main excitation

mechanism of the 2s stales of Ne compared to the direct electron

excitation (because of 10 He, Ne, ratio) and compared to the electron

excitation of the Ne meta stables. The upper laser level is 2s stales decay

at a rate of a = 10-7 see lifetime to the 2p stales. The 2p stales is i.e. the

lower laser level decay with a = 10-8 see lifetime to the ls stales. These

latter stales are also excited by the electrons and as they are quasi-meta

stables. Meta stable can be de-excited only at the walls. By quasi-meta

stable we mean that and two of the stales are forbidden to decay to

ground stale and the photon energy emitted by the other two are trapped

thereby making the number of excited atoms constant.

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3.2.4 CARBON DIOXIDE MOLECULAR SYSTEM

Carbon dioxide system is basically used due to its high output

intensity. There is also a relative ease of construction and the general

scientific significance of the W2 molecule. The intense output has

enabled an examination of several non linear optical and relaxation

phenomenal. The C02 oscillator include the excitation and energy transfer

mechanisms that provide population of the upper levels as well as

relaxational processes that depopulate the lower carbon dioxide is a

linear and symmetric molecule. In this molecule interaction arises form

harmonic terms in the potential energy which is expressed in terms of the

inter-nuclear distances. Although, these terms are regarded as small but

they can generate.

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The partial energy level diagrams of helium and neon-appropriate to a

discussion of the He-Ne oscillator resonance is a good example present

for carbon dioxide molecules.

3.2.5 HELIUM-CARDMIUM IONIC SYSTEM

The frequencies of H-Cd system of oscillation are greater and the

operational efficiencies are less. This makes ionic system different a bit

from the neutral system. There is always a need for additional energy for

38

20.6

2150

-0.3

0

35

3P- 20.6 20.3

2S2

5

0.15

0.7

10

15

2P

2

516.6

15

0

15

He

21

20

19

18

17

16


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excitation in an ionic level. The high efficiency and consequent

simplified operation of He-Cd system are derived essentially from

excitation mechanism.

3.2.6 MOLECULE HYDROGEN SYSTEM

This represents the shortest wavelength source. The system

produce a distinct example of excitation by direct electron impact and the

manifestation of optical selection rules.

3.3.1 LIQUID LASERS

Liquid laser comprises of the advantages gathered from both solid

and gas laser system. They have unique properties which gave a wide

view to new dimensions of laser application. The high concentration of

active molecule in solid lasers give liquid laser high power capacities but

it remains adamant to solid laser irreversible radiation damage most

important, is the unlimited range of organic laser substance that can be

fully exploited only in laser substance that can be fully exploited only in

laser systems using liquid solutions of these substances. Gas laser using

organic molecules does not seem to exist so far. The first liquid laser

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systems used metallo-organic, namely rare earth, chelates and inorganic

system consist of neodymium in phosphoroxychloride.

3.3.2 INORGANIC AND METALLO-ORGANISMS SYSTEMS

(a) Neodymium in SeOcl2 (selemiumoxyenloride)

Absorption and emission spectra of the Nd3t ion in glass and liquid

solution show only minor differences. The quantum yield of florescence,

however, is less than 103 in most liquids, whereas in most glasses it is

approximately 0.3. The low quantum ion yield in liquids is due to the

transfer of electronic energy of the nld3t ion to over tones of the most

energetic vibrations of the solvent molecules. The most important case is

that of solvent containing hydrogen because it is the highest characteristic

frequency. The search for a solvent for Nd3t without hydrogen brought up

SeOcl2 a toxis and extremely corrosive liquid. Chloride is removed by

addition of an acid likes SnCl4. The design of the laser was much the

same as that of a glass laser.

(b) Rare-Earth Chelates

The metallo-organic system using rare-earth chelates were the first

system in which laser action in a true liquid solution was obtained. The

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rare earth ions has a complexity of an organic ligand. The first examples

of such a chelate laser involve an alcohol solution of a tetrakis chelate.

3.3.3 ORGANIC SYSTEMS

(a) Dyes with Spectroscopic Properties of Organic Compounds

with Conjugated double Bonds

In all organic molecules conjugated double bond are common

property used as active medium in liquid lasers. Dye may be used as a

term for substance containing conjugated double bonds. Though, the

basic mechanism used for light absorption is the same for all substances

including the dyes but these substances do not necessarily absorb in the

visible part of the double bond substance without conjugated double bond

usually absorbed at wavelength shorter than 200nm corresponding to a

photon energy of 150kcal/mole. Since this energy is higher than the

dissociation energy of most chemical bonds, photochemical

decomposition competes effectively with radioactive deactivation with

these substances are not very likely to exhibit laser action in solutions.

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(b) Oscillation Conditions for Dye Lasers

There are two possibilities in principles to use an organic solution

as active medium in a laser. They are either the fluorescence or the

phosphorescence emission the fluorescence bond of dye solution is

utilized in a dye laser, on the other hand, due to the highly transition, a

very high concentration of the active species is required to obtain

amplification factor large enough to overcome inevitable cavity losses. In

fact, for many dyes the concentration would be higher than the solubility

of these dyes in any solvent.

3.4.1 SEMI CONDUCTOR LASERS

Semi conductors laser, like other laser, have population inversions

which lead to stimulated emission of photons. The only difference is

primarily because the energy levels in semi conductor must be treated as

continuous of levels rather than as discrete level.

(b) Structures Excitation and Threshold

The original semi-conductor laser is P.n junctions prepared by

diffusion of acceptor impurities into n-types gas and it is at present. One

of the most common structures, semi-conductor without impurities are

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insulators at low temperature. All the p-n junction lasers are excited by

passing current through the p-n junction and the oscillation rate is

characterized by the current density. When a forward current flows,

electrons are injected into the p-type material and holes are injected into

the n-type material, the n-type holes is to a much smaller extent partly

because of the lower hole mobility in hetero-junction of carriers. The

excess of electrons and role concentration over their equilibrium values

creates a population inversion and leads to stimulated emission of photon

at sufficiently high excitation levels. The layer near the p-n junction

where this occurs is called the active region of the device.

43

102

3

3

3

3

104

10

-1530 0 15 30 45 60 75

FIG. 3.4


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Spatial distribution of the recommendation rate of electron and holes for

a simple model of the current flow of the p-n junction (Stern, 1967). The

above diagram shows that the effective of the active layer in graded

junction increase as the current density increases.

A second class of excitation methods involves excitation of the

semi-conductor with photons or with an electron beam. In optical

excitation, the active layer thickness will be of the order 1/* where * is

the absorption coefficient of the incident photons. The active layer

thickness will be a function of its energy which indicates the penetration

depth of the electron. Therefore in both cases, diffusion of carriers will

add a distance of the order of the diffusion length to the thickness given.

3.4.2 BASIC TECHNIQUES OF SAMPLE

Three basic techniques of sample configuration used in electron

beam electron pumping are as follows:

(a) A fabry perot cavity is perpendicular to the electron beam

which is a side pumped configuration and a thin surface

which is inverted.

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(b) This is parallel to the beam i.e. end-pumped

configuration

(c) The total internal reflection configuration is obtained in the

lowest threshold current density. The coherent light is

omitted in a 3600 disk-like beam centered on the crystal

with a divergence of about 50 perpendicular to the disk. The

end-pumped configuration is used for crystals with low

absorption coefficients such as cdus because the light must

propagate through a thick, non-inverted region.

(a.)

(b.)

45

ELECTRON BEAM

PENETRATION

REGION

LASER BEAM

CRYSTALELECTRON BEAM

CRYSTAL

PENETRATION

REGION

LASER BEAM


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* *

`

0

0 0

Fig 3.6 Four Possible Radioactive Recombination Processes between

Holes and Electrons

An effective semi-conductor laser material must combine a fact

radioactive recombination path for the holes and electrons. Three basic

techniques exist to do this by injecting holes and electrons into an

insulating region by injecting electrons into and p-types materials so that

population inversion occurs. In n-types semi-conductor there are enough

electrons added by impurities to fill the conduction hand up to Fermi

47

E0 (a) (b) (c) (d)

Conduction band

Donor band

Acceptor stale

Valence band

*Denotes electron0 Denotes hole


48/74

level F. while in p-type, acceptor, impurities have been added which add

holes down to energy F (Fig. 3.7) shows an energy diagram of p-n

junction in a zero-voltage electron flow from the n-to the p-sides until

and electron potential barrier is provided to prevent further flow of

current. When a voltage is applied which raises the n-relative to the p-

side as shown in fig. 3.7 (b) electron can flow to the p-side where they

make transition to the p-side where they make transition to empty states

in the valence band and emit photons of energy which is EG.

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CHAPTER FOUR

TECHNICAL APPLICATION OF LASERS

4.1 LASER IN METEROLOGY

Laser has contributed to the field of meteorology in length

measurement field. Precise and accurate measurement can be obtained

using laser beams by three different methods which are convenient with

three different ranges as listed below:

(a.) Interferometer technique (up to 50cm in free air)

(b.) Telemetry with modulated beams (from 100m to 50km)

(c.) Optical radars (longer than 10km)

Both the upper and lower range limited depends on the required accuracy

stated or given.

4.1.1 INTERFERMETRIC TECHNIQUES

We can use a frequency stabilized laser whose wavelength is

compared with the distance to be measured the comparison is usually

performed with a michesa interferometer, where the phase of the EM

wave reflected by mirror m2 is compared with that reflected by mirror m1

in the diagram below.

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The phase difference Q between the two waves results from the

difference in their propation times

Q = 2IITC/ Vac = 2II l2 (n)2 L1 (n)1 Vac = 2IId1 Vac

Where L1 and l2 = geometrical lengths of the two arms of

Fig. 4.1 Interferometer set-up for distance measurements

Interferometer n(1,2) average value of the phase refractive index

along optical path 1 or 2 respectively and > Vac is the vacuum

wavelength of the reference source interference in plane waves and

spherical waves give rise to both rectilinear and concentric ring fringes

pattern respectively. The intensity at a point of the fringe pattern is

obtained by superposition of the two sinusoidal field n1(t) = E1 sinwt,

A2(t) = E2 sin(wt + Q) and subsequent squaring and time averaging

produces.

I = I1 + I2 + 2(I1 I2)1/2 Cos2IId1 Vac

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Where I1 and I2 are intensities of interfering beams A typical set up

for interferometer length measurement is shown in the above diagram.

The laser beam is sent to a beam expanding telescope which reduces the

beam divergence on the result of a wave front curvature correction. A

circular aperture is fixed in the focal plane P which performs spatial

filtering of the input beam to obtain a uniform and symmetrical intensity

distribution in the output beam. The fringe pattern is observed by a

couple of multipliers. The slit collects light each from regions of fringe

pattern where phase difference of the interfering beam differs by n.

The oscilloscope spot moves a full circle for a1/2> displacement of one of

the mirrors, where mirror m2 is used to simplify the alignment of the

interferometer. A logical circuit coupled with a reversible counter

processes the signals from the photomultiplier so that a count is added for

each > increase in the optical path difference. Interferometer distance

measurements are mainly used in the following fields:

(1.) Meteorology i.e. length standard calibrate it requires operation

of the interferometer and of the connected calibration bench in

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an antishock mounted platform in a thermally controlled

environment with acoustic isolation.

(2.) Mechanical tooling i.e. measurement of the displacement of

mechanical path in the workshop. It requires accuracies in the

range of few parts in 106.

(3.) Geodesy and seismology i.e. detection of earth strains induce

by earth or sea sides continental drift etc.

BEAM MODULATION THEORY

Polarization modulation can be used to obtain distance

measurement with light beams. A modulated beam that is with an

intensity I is transmitted from the sources S to the reflector R, RS = L

being the distance to be measured. A receiving system serves as a

collection point for the reflected beam where the intensity IR is

monitored. If t is the propagation time of the light beam through the

optical.

IR (t) = IT (t-t) where x-constant alternation coefficient.

Propagation gives rise to a time dependant and to a distortion of the

modulating waveform.

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4.1.3 OPTICAL RADAR SYSTEM

A light from source to the length and back to the receiver can be

measured to obtain the range information. The accuracy of the time

measure depends on the definition in terms of the light pulse and on the

time capability of the photo-detector and the timing systems.

Solid lasers in Q-switched operate and delivers pulse with a time

duration of the order of 20m seconds and peaks power of maximum 100

megawatts.

4.2 HOLOGRAPHY

4.2.1 DEFINITION

The complex light amplitude scattered from the illuminated object

is superposed upon a carrier wave by appropriate optical elements the

photograph recording such interference phenomenal is called

HOLOGRAPH. In a single scattering, there is a complete information on

the three dimensional location of the object point. Holograph requires a

coherent beam of radiation for illumination hologram may be viewed as

the photograph of a high complex interference pattern.

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4.2.2 APPLICATION OF HOLOGRAPHIC TECHNIQUES

(a) Measurement Technique

(i) Monochromatic and polychromatic of three-dimensional

wave front makes it possible to visualize relief i.e. surface

structure.

(ii) Holography is able to obtain, record, and reproduce

information on the location of object in three-dimensions

and possibly at more than once in a time.

(iii) Holography is used as optical filters.

(iv) Transmitting holograms through ordinary television

communication.

(b) Holographic Interferometer

(i) It is convenient to work with rough surface when sufficient

light is reflected towards holographic polate.

(ii) Simultaneous observation of event that happened in the past

at separate moment in time. This is done by superimposing

two different object waves at different times with the same

referent wave. The resulting two hologram are stored

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independently upon reconstruction both original object

waves are recreated and give rise to interference

phenomenal.

(iii) Changes in optical path lengths. Variations of refractive

index, deformation of surfaces are not needed when relative

measurement is required.

(d) Dynamic Holography

This is genuine sequence of high speed holograms. It is very useful

in the study of plasma and shockwave fronts generated by a high energy.

It falls into a solid object or thermal effects in solids due to the absorption

of high-energy light pulses alter the refractive index.

(e) Holography with Partially Coherent Light

It is important that angular size should not be large in order to have

adequate spatial coherence.

(f) Application of Holography to Microscopy

There are two advantages of holography in microscope

applications:

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(i) Magnification obtained by means other than a conversional

microscope imaginary system. A conversionary microscope

used observation is used to find magnification.

(ii) Holographic recording is accomplished by a short exposure.

Completing, three dimensional information on the object is

available in terms of the phase and amplitude details stored

in the plate.

(g) Holograms in Optical Information Processing

Holograms in optical information processing are used as memories

and it will be pointed out of the information capacity of a hologram and

be depended much in the light sensitive material from which it is made.

(h) Three Dimensional Television and Fourier Transform

Spectroscope

In television, intensity distribution to the holographic interference

pattern is scanned line by line and converted into an electric analog

signal. The signal passes various amplifiers and signal convertors. The

hologram is then displaced on a television picture tube. In Fourier

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transform, it is useful for the investigation of weak source for example in

astronomy.

4.3 LASERS IN HIGH SPEED PHOTOGRAPHY LASER AS

PHOTOGRAPHIC LIGHT SOURCE

The main advantages of lasers in comparison with other sources

consist of a much larger radiation brightness a high degree of mono

chromaticity and wherence of radiation. These characteristics give the

laser unique advantages as point light source. Laser used for high speed

photography such as gas or ruby are radiating in the visible or near

infrared spectral range.

The solid state pulse lasers permit the photographic device to the

investigation of fast processes with duration from 10-3 to 10-8sec. Gas

lasers have a good mono chromaticity of DV= 107 Hz and long.

Coherence time makes a good light source for special photographic

methods.

Despite the apparent simplicity of semi-conductor laser, their large

radiation divergence and the resulting small brightness make it difficult

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to utilize them in high speed photography. In that case, small brightness

makes it necessary to use image-converter tube as radiation receivers.

4.4 MATERIAL PROCESSING

4.4.1 LASER WELDING

The use of a laser as a heating source for welding and joining

materials was one of the first applications proposed for laser. These are

the feasibility and advantage of laser welding.

(a) The absence of physical contact with electrode

(b) Localized heating and rapid cooling due to the high heat flux and

small laser spot.

(c) The ability to weld many dissimilar metals and dissimilar

geometries.

(d) The ability to weld component in a controlled atmosphere or

sealed within optically transparent materials.

Laser welding is a fusion welding process with some of its

characteristics resulting directly from the short heating and cooling

times.

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LASER DRILLING MECHINING AND CUTTING

Drilling and other evaporative processes have their penetration

depend not only on the thermal penetration but primarily upon the energy

delivered to the work piece and certain geometrical factor. Laser pulse

duration is kept as short as possible when drilling or machining with the

volume of material to be removed.

For drilling with continuous lasers these result base on thermal

consideration may not apply since chemical reaction of the surface with

the working environment may contribute significantly to the material

removal mechanisms.

4.4.3 APPLICATION TO THIN FILMS

The small thermal penetration depth heat results from short

duration Q switching pulses makes such lasers useful as machining for

thin films structures. Thin film machining processes have found

application especially in a number of areas of microelectronics and

integrated circuit technology.

Interest in these processes is due to the fact that the non-contact

evaporation of the films eliminates the need for chemical etching of the

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thin processes. Special processes such as pattern generation of circuits

and marks and trimming of thin film resistors and other components,

depend primarily upon a single characteristic of the Q-switched laser, its

ability is to evaporate and optically absorbing film without damaging the

substrate beneath the film.

4.5 APPLICATION OF LASER TO COMPUTER MEMORIES

4.5.1

Information storage is a critical function in all digital computers.

Storage tasks include data storage diagram management, record keeping

and data magnetism and holding of large lists and tables. Modern

computers require safe storage of huge amounts of information typically

109 to 1013 bits. They require ability to access this information with

shortest possible delay. This information is usually stored in devices with

slow access and average access time for the actual data processing unit is

decrease by use of a hierarchy of storage devices ranging from magnetic

tapes, strip files and disks to magnetic cores and semi-conductor

memories.

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DISADVANTAGES OF THIS STORAGE DEVICES ARE

NUMEROUS

a) In the hierarchy of available memories, there is an unfulfilled need

for memories with moderately fast access to 108 1010 bits.

b) Large capacities (109 bits) are presently obtained only in devices

with mechanical motions and evolutionary extension of

conventional magnetic recording technology do not appear to hold

much promise, in terms of improvements in access times and data

mates.

c) The hierarchical organization is both complex and expensive.

4.5.2 OPTICAL MEMORIES

61

CACHE

4-32K WORDS

MAIN STORES200K-2K STORE

AUXILLIARY STORE

1- COM WORDS

FIXED HEAD DISKS

5-50m WORDS

MOVABLE HEAD DISKS

20-200m WORDS

ARCHIVAL STORE

1

5-20

5 x 102- 2 x 103

5 x 103 - 2 x 104

5 x 105 - 2 x 106

107 109

FIG. 4.3: Memory Hierarchy

Large System


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Optical memories appear to overcome these shortcomings to

some extent. They combine the large capacity of types or depth with the

short access times of cores or semi-conductor. They also promise a

substantial improvement in information density and consequent reduction

in size for a given capacity. There are two types of optical memories

which are:

(i) Point by Point Memories: each bit of information is stored in a

discrete point on the storage medium. The storage bits can be

written, read and erased individually.

(j) Holographic Memory: a whole page must be written, read or

erased at one time. Thus to change a single bit requires rewritten a

large amount of information.

4.5.3. KEY MEMORY ELEMENTS

Storage Media

Optical memories are diverse enough that a large number of

materials are:

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1) Read-only materials in which the information is written outside

the memory and can only be read during the operation in the

computer.

2) Write-read materials in which both writing and reading are

performed in real time in the memories but information once

written can not be erased.

3) Write-read-erasable material in which all these operations can be

performed in real time by the used of optical beams.

4.6 LASER RANGE FINDING

4.6.1 RANGE FINDING CONFIGURATIONS

(a) Basic Techniques

Lasers have been used in three modes of the purpose of ranging

and they are as follow:

1) The first method is the basis pulse techniques in which a

narrow pulse is transmitted and the transition to the target and

return to the receiver is proportional to the range. This types of

range finders was the first demonstrated application of the laser

and occurred shortly after the discovery of the ruby laser. More

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system of this first type has been fabricated than the other two

to be described. Pulse laser range finder has been used for

precise range tracking of satellites for geodetic applications for

meteorological applications, paramilitary application such as

fire control and under. Sea ranging and target recognition the

pulsed range finding is the most representative layer of laser

range finder and has definitely moved from laboratory and

development into field use and production.

2) The second technique utilized an amplitude modulated own

laser. The beam is directed at a target and the return signal will

have its phase shifted proportional to range. In many

applications where cw laser ranging and tracking is employed,

the target is made co-operative by attracting retro-reflector or

retro-reflective point to the target. The cw amplitude modulated

systems are typically used when automatic tracking of the

target is also desired e.g. ringing and tracking of a missile as it

leaves the launch pad. A cw system is usually dictated by the

requirement of high track rates. A sound typical application of

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cw systems is where high range accuracy is desired as in

surveying work.

3) The third technique is the interferometer method where the

frequency of the cw laser itself used. The displacement in

distance can be measured in the order of the wavelength of the

laser light used due to the fringe counting technique.

(b) Configuration of Pulsed Range Finder

The pulsed range finder contains a pulse laser transmitter, bore

sighted to an optical echo receiver and target viewing optics. There is a

timing circuit which measures the interval between the transmitted pulse

and received echo. The transmitter consists of a pulsed laser including

energy sources and beam collimation optics. The receiver consists of a

telescope acting as a light gathering appears and a detector followed by

an amplifier which provides stopping pulse to complete the timing. The

timing circuity can frequently select between targets at difference ranges

within the field of view.

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4.7 OPTICAL COMMUNNICATION THEORY

The subject is best explained by considering the communication

system model in the figure below. Information is to be transmitted

between a source and a user by the propagation of a modulated light

signal. The information signal will vary some attributes such as

amplitude, frequency or polarization, of the transmitted light. The optical

field at the receiver depends on the transmitted field, the effects of the

propagation medium (called the chained) and background radiation. The

receiver serves as to proceed this optical field in such a way as to

reproduce the information signal.

INFORMATION SIGAL

Fig. 4.4 Optical Communication System Black Diagram Ur (t,r) Un

(t,r) and Ur (t,r) are respectively the transmitted background and

receiver field.

Optical communication application include situation for which the

channel effect are relatively unimportant from a system design viewpoint

66

OPTICAL

SOURCE

MODULATOR TRANSMITING

OPTICAL

CHANNEL RECEIVER

BACKGROUND RADIATION UN. 1 + r


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such as free space and wave guide channels and situations where channel

effects are very important or dominate such as the turbulent atmosphere

and scatter channels.

4.8 LASER APPLICATION TO BIOLOGY AND MEDICINE

4.8.1 APPLICATION OF THE LASER TO OPTHALOMOLOGY

A non-coherent polychromatic light source has become a proven

method of therapy, for the repair of retinal tears with photocoagulation of

ocular tissues. The light energy which is absorbed is converted to heat

which produced thermal coagulation of protein. A scar is formed in the

site of injury which strengthens the attachment between the neuronal

layer and choroids. Ruby laser are used when monochromatic light

source become available. The monochromatic nature of the ruby laser

also resulted in less transit absorption of the energy through the ocular

media before its absorption by the pigmented epithelium. Less damage

was produced in the peripheral tissues surrounding the light beam. The

second major area is where laser reduces edema in the macula by

injecting out the site of leakage of serum into the various humour. Laser

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destruction of areas which forms growth leading to hemorrhages to

destroy the retinal tissue would preserve the patient vision.

4.8.2 APPLICATION OF THE LASER TO DERMATOLOGY

The application of laser energy to dermatology has offered some

promising in roads for the treatment of anglomass tattoos and tumors. It

has been reported that strawberry angiomas in infants yielded favourable

to laser energy densities of 40-50/cm2. The point wine hamangiomas that

plastic surgery found difficult to treat has been treated with the ruby laser

with an energy density of 50-60 0/ cm2. it showed significant lightening

with very little associated scarring. The more intensity colored area

showed an immediate blue gray crust following laser treatment. This

progressed to a dark reddish-black crust after 24 hours. The crust

remained for two or three weeks and after shedding, the heated skin

showed a reddish-pink colouration and was smooth. During the next two

or three months the skin assumed an increasingly normal appearance.

4.8.3 APPLICATION OF THE LASER TO TURMORTHERAPY

One of the first tumors to be irradiated with the laser was the

pigmented melanoma. The melanin granules contained within the tumor

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cells served as chromophores for both the ruby and neodymium lasers.

The laser treatment of melanoma in mice range from complete regression

to accelerated deterioration of the tumor-bearing host. In man, the types

of tumors which seems to respond best to laser treatment include

melanomas, anglosarcomas, squamous cell epitheliums, lymphomas,

vascular tumor and glioblastoma multi-former. The laser effect could be

increased through the injection of dyes or copper slats. The tumors

depend upon the wavelength of the laser used.

An alternate approach to tumor therapy is that in which human

malignant cells were subjected to the combination of x-ray and ruby laser

energy. Using an electronic counter the analysis of the surviving cell

shows that the combined therapy offered a synergistic inhibitory

response. The energy levels require to eradicate tumors with the

combination therapy is not sufficient to produce tumor cell dissemination

in surrounding normal tissue.

4.8.4 APPLICATION OF THE LASER TO DENTISTRY

A focused laser (ruby) beam could produce a crater in either dental

enamel or metallic restorations. Feasibility studies conducted by (Stern

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and Sognnaes, 1964, Kinersly 1965, Goldmanin 1965, Lobene and Finc

1966) to determine if the new energy source could effectively remove

unwanted metallic fillings produced negative results. However, it has

been reported that laser energies ranging from 250-850 J/cm2 could

effectively alter the configuration of the enamel.

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CHAPTER FIVE

This work is basically to provide those who might make use of the

laser with authoritative accounts of its discovery its several forms and

their properties. It is also to indicate the most promising path of progress

in its application so far achieved.

Within a short period of its invention, the laser is already a

practical tool in several different ways. Though, the electron beam welder

was established first the laser as a welding tool gives it a competition.

The laser can be offered four distinct advantages which are:

(1) It generates no x-rays

(2) It requires no vacuum to operate

(3) If there are less heat lost by conduction it can be faster

(4) With ease and precision, its beam can be focused.

In communication, it matches highly with conventional

communication lines established at great expense. Laser unrivalry can be

found unparalleled in the application of laser in holography. Early work

on hologram was hampered by lack of sufficient sources of coherent light

the laser could supply. The computer gives a comfortable and effective

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tool for analyzing immense qualities of data to match its ability and

exploit its power to the full.

The scientist needs simple system for gathering very large amount

of data at minimum inconvenience to himself. Holography is a field that

promises this system mentioned. A simple means for gathering data that

can be analyzed by a complex system. Holography may yet emerge as the

most important laser technique of all.

It is a short step from medical applications to personal hazards.

Scientists over the years have been dreaming of a ray that might be

fried to kill an adversary or create a major destruction. The laser hardly

accomplishes this dream. This is so because the growing power of this

instrument is small unless focused by a lens placed very close to the

target.

Finally, to prevent disastrous occurrences and risks of the obvious,

only authorized and knowledgeable personnel should be allowed in the

vicinity of a working laser unless special precautions are taken.

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REFERENCES

Allen, L (1969): Essential ofLaser Pergamon Press, Oxford, London,

Edinburgh.

Arrech F.T. and Sona, A (1964): Symposium on Quasi Optics

Polytechnic Institute of Brooklyn.

Arrench F.T. and Schwlz-Dubois E.O. (1972): Laser Handbook North-

Holland Publishing Company, Amsterdam.

Asmus J.F and Ber F.S. (1969): Tenth Symposium on Electron Ion and

Laser Beam, Technology ( Francisco Press) Pg. 225

Basov, N.G. (1922): IEEEJ Quantum Electronics 4 Pg. 855

Beesley M.Y. (1972): Laser and their Applications, Taylor and Francis

Ltd., 10-14 Macklin Street, London WC. 285 NF

Chesler R.B., Larr, M and Geusie, J.E. (1969): IEEE J Quantum

Electronics 5 Pg. 345

Cohen, M.I. (1969): IEEE Conference on Laser Engineering and

Applications (Washington D.C.) Unpublished

Fishlock and David, (1967): A Guide to the Laser, Macdonald and

Company (Publishers) Ltd. Guld House, 2, Portman Street, London

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WI

Haken, H Laser Theory Journal in Atom, Molecules and Laser (Pg. 283)

Hoverston, E.V. (1970): International Symposium on Information

Theorem (Noordiwijk, The Netherlands)

Irving (1965): The Iron Ry, Pg. 51

Round, D.E. (1969): Biological Effect of Laser, Presented at the

International Conference on Laser Applications in Dentistry New

York, October, 15

Schnfer, F.P. (1968): Invited Paper International Quantum Electronics

Conference Maim

Sklizokov, G.V. (1971): Laser International and Related Plasma

Phenomenal Schwarz and H. AOVA

Stern, (1966): Physics Revision B.3/ pg. 3559

Stitech, M.L., E.J. Woodbury and J.H. Morse (1961): Electrons 34 Pg. 51

Viendit, J. Charles (1968): Symposium on Application Coherent Light

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