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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.
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E3
E2
E1n1
n3
n31/ l2
1/ l3
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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
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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.
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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
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E0 (a) (b) (c) (d)
Conduction band
Donor band
Acceptor stale
Valence band
*Denotes electron0 Denotes hole
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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
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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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