LASER ENGEENERING 2 .pdf

Helium – Neon LASER
• First Continuous Wave laser ever constructed.
• The active medium is a mixture of the two gases He & Ne in
a glass tube.
• Partial pressure of helium is approximately 1 mbar and that
of neon is 0.1 mbar
• The initial excitation is provided by an electrical discharge
and serves primarily to excite helium atoms by electron
impact.
• Certain levels of helium and neon are very close in energy,
excited helium atoms subsequently undergo a process of
collisional energy transfer to neon atoms, very efficiently.
Laser : Fundamentals and Applications

• Because the levels of neon so populated lie above the lowest excited
states, a population inversion is created relative to these levels, enabling
laser emission to occur.
• Three wavelengths generated 632.8nm, 1.152 μm and 3.391 μm.
• Following emission, neon undergoes a two-step radiation less decay
back down to its ground state. This involves transition to a metastable
2p53s1 level, followed by collisional deactivation at the inner surface of
the tube.
• To assure that last step is rapid for efficient laser working,
surface/volume ratio of the laser tube has to be kept as large as possible,
which generally means keeping the tube diameter small.
• Generally tube diameters are in few millimeters.
• Narrow bandwidth, small and inexpensive.

Argon LASER
• Single component inert gas, Argon , act as active medium.
• Argon at a pressure of 0.5mbar is contained in plasma tube of 2 – 3 mm
bore.
• Excitation is achieved by continuous electric discharge.
• Atoms are ionized and further excited by electron impact.
• This pumping process produces a population of several ionic excited
states, and those responsible for laser action are on average populated
by two successive impacts.
• This results in emission at a series of discrete wavelengths over the
range 350-530 nm.
• The two strongest lines appear at 488.0 and 514.5 nm as a result of
transition from the singly ionised states with electron configuration
3s23p44p1 down to the 3s23p44s1 state. Further radiative decay to the
multiplet associated with the ionic ground configuration 3s23p5 then
occurs

• LASER cycle is completed by electron capture or further impact
excitation.
• As several wavelength is produced etalon or dispersing prism is
needed.
• By selecting a single longitudinal mode, an output linewidth of
only 0.0001 cm-1 is obtainable.
• Requires large and continuous flow of energy.
• The output power of a CW argon laser usually lies in the region
running from milliwatts up to about 25 W.
• Expensive and fragile.

Copper Vapor LASER
• The copper laser is essentially a three-level system.
• Electron impact on the ground state copper atoms results in
excitation to 2P states belonging to the electron configuration
3d10 4p1, from which transitions to lower-lying 3d94s2 2D levels
can take place.
• Laser emission thus occurs at wavelengths of 510.5 nm and
578.2 nm.

• Collisions of the excited atoms with electrons or the tube walls
subsequently result in decay back to the ground state.
• Operates in pulse mode with pulse repetition frequency of about
5 kHz.
• The physical design of the laser involves an alumina plasma tube
containing small beads or other sources of metallic copper at each
end.
• The tube also contains a low pressure of neon gas (approximately
5 mbar) to sustain an electrical discharge.
• The chief advantages of the copper vapour laser are that it emits
visible radiation at very high powers

Molecular Gas LASERs

Carbon dioxide LASER
• Energy levels involved in LASER action are vibration-rotation.
• The lasing medium consists of a mixture of CO2, N2 and He gas in varying
proportions.

Carbon dioxide LASER
• The first step is population of the first
vibrationaly excited level of nitrogen
by electron impact.
• Various rotational sub-levels belonging
to the vibrationaly excited state are
populated by the electron collision.
• These levels are all metastable, as
radiative decay back down to the
vibrational ground state is forbidden
by the normal selection rules for
emission.
Consequently, collision between the two molecules results in a very efficient transfer of
energy to the carbon dioxide.

• Laser emission in the CO2 then occurs
by two routes, involving radiative
decay to rotational sub-levels
belonging to the (100) and (020) states.
• They exist in a population inversion
with respect to the (001) levels.
• Transitions results in emission of
wavelengths of around 10.6 μm and
9.6 μm.
A small carbon dioxide laser, with a discharge tube about half a metre in length, may
have an efficiency rating as high as 30% and produce a continuous output of 20 W.

Nitrogen LASER
Similar to CO2 LASER, with following main differences:
• Electronic states are involved in LASER action, transition occurs between
C3πu to B3πg .
• Upper laser level has a lifetime of 40ns and hence cannot sustain
population inversion.
• all the excited nitrogen molecules undergo radiative decay together to
give Super Radiant Emission.
• 10 ns pulse of wavelength 337.1,with bandwidth 0.1nm is generated with
a repetition rate of 1-200Hz

Chemical LASERs

Iodine LASER
• Population inversion is created directly
through an exothermic chemical reaction
or other chemical means.
• The driving principle involved in the iodine
laser, is the photolysis of iodohydrocarbon
or iodofluorocarbon gas by ultraviolet light
from a flash lamp.
C3F7I + hνp C3F7 + I*
I* I + hνl
C3F7 + I + M C3F7I + M
hνp = pump photon
hνl = laser emission photon
C3F7I, is stored in an ampoule and introduced
into the silica laser tube at a pressure of
between 30 and 300 mbar.

• Laser action takes place between excited metastable 2P1/2 state
and the ground 2P3/2 state of atomic iodine this results in narrow
linewidth output at a wavelength of 1.315 μm.
• An important advantage of the iodine laser is the fact that the
active medium is comparatively cheap and, hence, available in
large quantities.

Excimer LASER
• The active medium is an exciplex, or excited diatomic complex.
• The crucial feature of an exciplex is that only when it is
electronically excited, it exists in a bound state with a well-
defined potential energy minimum.
• The exciplex is generally formed by chemical reaction between
inert gas and halide ions produced by an electrical discharge.

Kr + e Kr+ + 2e
F2 + e F- + F
F- + Kr+ + He KrF* + He
• Helium simply acts as a buffer.
KrF* is electronically excited and has a
Very short life time, it rapidly decays by
photon emission.

• Since this is an unbound state, hence the force between the atoms is always
repulsive, the exciplex molecule then immediately dissociates into its
constituent atoms.
• This state never attains a large population, and a population inversion,
therefore exists between it and the higher energy bound exciplex state.
In the case of KrF the krypton and fluorine gas is regenerated.
KrF* Kr + F + hνL
F + F F2
• The laser can be operated continuously without direct consumption of the
active medium.
• Excimer lasers are superradiant and produce pulsed radiation with pulse
durations of 10-20 ns and pulse repetition frequencies generally in the 1 to 500
Hz range.
• Pulse energies can be up to 1 J, with peak pulse power in the megawatt region
and average power between 20 and 100 W.

Dye LASERs
• The active medium is a solution of an organic dye.
• A wide range of over 200 dyes can be used for this purpose
the only general requirements are an absorption band in
the visible spectrum and a broad fluorescence spectrum.
• The most widely used example is the dye commonly
• known as Rhodamine 6G (C28H31N2O3CI)

In solution, the corresponding energy levels are broadened due to the strong
molecular interactions of the liquid state, and they overlap to such an extent
that an energy continuum is formed for each electronic state.
the absorption of visible light results in a
transition from the ground singlet state S0
to the energy continuum belonging to the
first excited singlet state S1
This is immediately followed by a rapid
radiationless decay to the lowest energy
level within the S1 continuum

• Fluorescent emission then results in a downward transition to
levels within the S0 continuum, followed by further radiationless
decay.
• It is the fluorescent emission process which can be made the
basis of laser action, provided a population inversion is set up
between the upper and lower levels involved in the transition.
• A dye laser based on a solution of Rhodamine 6G in methanol, for
example, is continuously tunable over the range 570-660 nm.

Applications of LASERs
• Non – Linear Optics
– Frequency Conversion
First it was observed in Ruby LASER where, 694.3
nm was converted to 347.15
BBO Crystal
1064 nm
355 nm
532 nm
1064 nm
266 nm

• Advantage of NLO -
LASER properties remain intact during the
frequency conversion viz. intense, coherent and
directional.
Material
Conventional light source
Interaction
Refraction Absorption
Reflection Scattering

• These interactions cannot significantly change
macro/microscopic properties of material as
the intensity of electric field of conventional
light sources (10 – 1000 V/cm) is very small as
compared to electric field within the material (
~109 V/cm)
• Comparable electric field ( 106 to 109 V/cm)
can be provided by a pulsed LASER.

• Electric induction (D) is given by
D = E + 4πP
P = polarization
For small E,
P α E
P = χ(1) E
χ(1) = Linear electrical susceptibility (It is a Tensor)

Origin of P = χ(1) E
P = Poreintation + Pinduced
= N
𝜇2
3𝑘𝑇
E + NαE
• If E is small then the displacement of these
electrons is periodical and can be modeled
with a harmonic oscillator
E
-
+
-
+
+
-
+
+
+ -
-
-

• When large electric field is used, then the
situation becomes similar to anharmonic
oscillator and we cannot express polarization in
a linear manner. Hence, it is expressed as
power series.
P = χ(1) E + χ(2) E2+ χ(3) E3 + …….
χ is a complex quantity and contains important
information about dielectric medium. Real part
contains information about refractive index and
imaginary part contains information about
absorption coefficient.

• Since χ is a tensorial quantity we rewrite
equation as,
P = χ(1)
ij Ej + χ(2)
ijk Ej Ek + χ(3)
ijkl EjEkEl + …….
• Under linear regime superposition principle is
followed.

Examples of NLO
• Refractive Index (n)
– When electric field strength is small, refractive
index (n) remains nearly constant. For very large
electric field refractive index varies with square of
electric field
n α E2

• Absorption coefficient
– When electric field is small Lambert – Beer law
stays valid and as electric field reaches high then
Lambert Beer law does not hold good.

2nd Order Optical Nonlinearity
• Second Harmonic Generation
• Sum Frequency Generation
• Difference Frequency Generation
• Parametric Conversion

• Electric field associated with a plane wave LASER
beam
E (r,t) = E (ω1,k1) ei(k1
r – ω1
t)+ E*(ω1,k1) e-i(k1
r – ω1
t)
= 2E (ω1,t) cos(k1r – ω1t)
= E0cos(k1r – ω1t) (induced polarization at frequency 2ω)
Now putting this value in
P(2) = χ(2)
ijk Ej Ek
= χ(2)
ijk (0) (E0
2/2) +(1/2) χ(2)
ijk (2ω1,2k)E0
2cos(k1r – ω1t)
First term is frequency independent DC effect and 2nd term
corresponds to 2nd order/ SHG wave.

• Fundamental and SHG wave can pass through in
and out of phase through the material, however
out of phase movement leads to a very low
intensity of SHG.
• We need to create a synchronization between
fundamental and SHG wave, this synchronization
is called as phase matching.

Interaction of radiation with matter
• When light of certain intensity is incident on the molecule it become
polarized and when it again returns to initial condition it emits light of
frequency that is characteristic of that molecule.
Symmetric Charge distribution
(equilibrium condition)
Polarized molecule
E
Incident photon
Emitted radiation

Non-Linear Optical Processes
• Polarization process can be considered as excitation of molecule to the
virtual state. Depending upon intensity of light used there can be linear
scattering, sum frequency generation(SFG), second harmonic
generation(SHG), third harmonic generation(THG) and so on…
Third Harmonic generation
𝜔2 = 𝜔1+𝜔1 + 𝜔1
Ground state
SFG
𝜔3 = 𝜔1+𝜔2
𝜔3
𝜔1
𝜔2
Ground state
𝜔1
𝜔1
𝜔2
Virtual states
SHG
𝜔2 = 𝜔1+𝜔1
Ground state
𝜔2
𝜔1
𝜔1
𝜔1
Virtual states

Phase Matching
• Phase matching can be done by synchronization of the phase velocity: 𝑣𝑝ℎ=
𝜔
𝑘
=
𝑐
𝜂
𝜔 = frequency, 𝑘 = wave vector, 𝑐 = velocity of light in vacuum,
𝜂 = refractive index of the medium which is a function of 𝜔.
Now,
𝑘2 2𝜔1 = 𝑘1 𝜔1 + 𝑘1 𝜔1
𝑘3 𝜔3 = 𝑘1 𝜔1 + 𝑘2 𝜔2
where, 𝜔1, 𝜔2, 𝜔3 = frequency of fundamental, second harmonic wave and a general wave
mixing case respectively.
𝑘1 , 𝑘2, 𝑘3 = wave vectors of fundamental, second harmonic wave and a general wave
mixing case respectively.

Condition for Phase Matching
Let us define:- Δ𝑘 = 𝑘3 𝜔3 − 𝑘1 𝜔1 − 𝑘2 𝜔2
For phase matching:- Δ𝑘 = 0
𝑘3 𝜔3 = 𝑘1 𝜔1 + 𝑘2 𝜔2
Using,
𝜔𝑎
𝑘𝑎
=
𝑐
𝜂𝑎
or 𝑘𝑎 =
𝜂𝑎𝜔𝑎
𝑐
𝜂3 2𝜔1 = 𝜂1𝜔1 + 𝜂1𝜔1 or 𝜂3 = 𝜂1
Since refractive index 𝜂 is a function of 𝜔 so using, 𝜂3 = 𝜂 2𝜔1 and 𝜂1= 𝜂 𝜔1
𝜂 2𝜔1 = 𝜂 𝜔1
This equation referred as the index matching condition .

Second Harmonic intensity
Second harmonic intensity is given by: 𝐼2𝜔 = 𝜅(𝜒𝑒𝑓𝑓
2
)2 𝑙2
sin(
Δ𝑘𝑙
2
)
Δ𝑘𝑙
2
2
𝐼𝜔
2
𝜅 = constant, 𝜒𝑒𝑓𝑓
2
= second order susceptibility
𝑙 = path length of fundamental light through crystal
Wave vector difference Δ𝑘, 𝐼𝜔= intensity of fundamental light.
Ways to enhance SHG:-
1. Increasing incident power
2. Choosing material having larger second order susceptibility
3. Material fulfiling the condition of phase matching
4. Choosing longer crystal.

Practical Issues
• Polarization is given by: P = 𝜒𝑖𝑗
(1)
𝐸 + 𝜒𝑖𝑗𝑘
(2)
𝐸2 + 𝜒𝑖𝑗𝑘𝑙
(3)
𝐸 3 + ⋯
• Potential energy: V(E) = −
1
2
𝜒𝑖𝑗
(1)
𝐸2
+
1
3
𝜒𝑖𝑗𝑘
(2)
𝐸3
+
1
4
𝜒𝑖𝑗𝑘𝑙
(3)
𝐸 4
+ ⋯
V −E = −
1
2
𝜒𝑖𝑗
1
𝐸2 −
1
3
𝜒𝑖𝑗𝑘
2
𝐸3 +
1
4
𝜒𝑖𝑗𝑘𝑙
3
𝐸 4 − ⋯
For centrosymmetric molecules: V +E = V −E , this is possible only when all
even order susceptibility are zero ∶-
𝜒𝑒𝑓𝑓
2
= 𝜒𝑒𝑓𝑓
4
= ⋯ = 𝜒𝑒𝑓𝑓
2𝑛
= 0
By electric dipole approximation we can say for centrosymmetric molecule all
even order non linear process are not observed in particular ,SHG = 0

Autocorrelation
Pulse width of ultrashort pulses can be measured using autocorrelation
technique which is based upon non-linear optical phenomena. In this
technique two pulses from same source are allowed to interact as function
of time delay between them created using a movable mirror.

Lens
Detector
Filter
Beam Splitter
Mirror(M1)
Movable Mirror (M2)
Laser Beam
Autocorrelator
NLO

LIDAR
Light Detection and Ranging (LIDAR) is used to measure distance and
quantify particular analyte present far away from the observer.
This technique is based on scattering phenomena or other processes
like fluorescence, differential absorption, Raman spectroscopy in which
intensity of light from the analyte is monitored using a detection scheme.
Particularly, Raman spectroscopy has an advantage that we can probe
environment of the analyte.

Instrumentation
To have synchronization between Laser source and detector, pulse
trigger technique is used which couples the Laser source and the
detector through a box car integrator.
Schematic diagram of LIDAR
Pulse Trigger
Recorder
Beam Car
integrator
LASER
Detector
Analyte
Mirror (M1)
Mirror (M3)
Mirror (M2)

Laser spectroscopy
Various fields in which laser is applied:
• Absorption spectroscopy
• Fluorescence spectroscopy
• Laser induced breakdown spectroscopy (LIBS)
• Raman spectroscopy
• Isotopic enrichment

Absorption Spectroscopy
Conventional light source can’t be use to measure very low absorbance values.
Utilizing the highly convergent nature of laser very low absorbance values can be
measured by using a multipass cell. Effective path length is: 2f x n
where, n = no of round trip.
Multipass Cell
Path length
Concave
mirror
M
1
Concave
mirror
M
2

Raman Spectroscopy
• Raman spectroscopy is based upon Raman scattering which is observed due to shift in
frequency of scattered light. The shift can be Stokes shift or Anti Stokes shift depending
upon whether the frequency of scattered light is lower or greater than incident light
respectively. This is in contracts to the Rayleigh scattering in which the incident and
scattered light have same frequency. Various modes of Raman spectroscopy:
• Resonate Raman Spectroscopy
• Stimulated Raman
• Coherent Anti Stokes Raman Spectroscopy (CARS)
• Hyper Raman

Schematic diagram showing Raman and Rayleigh Scattering

Isotopic Separation
Laser can be used for isotopic enrichment by utilizing the selective
response of isotopes towards laser action. This is done using following
laser schemes:
• Selective ionization
• Selective photo-dissociation
• Photo chemical reactions
• Selective photo-deflection

Laser assisted isotopic separation
Enrichment of isotope can be done using laser schemes if:
• Isotopic shift in absorption frequency is well resolved.
• Laser linewidth is smaller than isotopic frequency shift.
• There is an efficient extraction stage.
To quantify the enrichment, we introduce enrichment factor:
𝛽 =
ൗ
𝑁(𝑃1)
𝑁(𝑃2)
ൗ
𝑁(𝑅1)
𝑁(𝑅1)
=
ൗ
𝑋(𝑃1)
1−𝑋(𝑃2)
ൗ
𝑋(𝑅1)
1−𝑋(𝑅2)
𝑁 𝑖 = indicates no. of mole of isotopes to be separated (𝑅1, 𝑅2) and
products after laser action (𝑃1, 𝑃2).
𝑋 𝑖 = indicates respective mole fractions.

Selective photoionization
This laser scheme is based upon fact that excited state of isotopes differs in
energy. By using proper combination of incident frequencies the isotope can
be selectively ionized. The condition is: ℎ𝑣1, ℎ𝑣2 < 𝐼 < ℎ𝑣1 + ℎ𝑣2
where, 𝐼 = ionization energy.
Continuum
Ground States
Excited States
I
ℎ𝑣1
ℎ𝑣2
Isotope A Isotope B
Selective Photoionization of isotope A

Selective dissociation
By bringing a suitable IR frequency we can selectively populate
vibration levels of one of the isotopes and than using a suitable UV
radiation the isotope can be dissociated selectively.
Energy levels of isotope A
Energy levels of isotope B
IR Transition
UV Transition
Electronic ground state
Electronic Excited state
Selective Photo-dissociation of isotope B

Photo-deflection
• When laser beam strikes a molecular beam than deflection of
molecules occurs depending upon molecular masses. As isotopes
differs in mass so they can be separated using this selective deflection
phenomena.
Molecular Beam
LASER Beam
Selective Deflection
Deflected molecules

Applications of Laser in Chemistry
Laser-Induced Chemistry

• Chemistry induced by optical excitation is by definition photochemistry, and
the whole of laser-induced chemistry can thus be regarded as one part of
this much wider field.
• Although lasers can replace other light sources in any conventional
photochemistry, there is a significant number of laser-induced reactions
that are not practicable with conventional light sources.
• Laser monochromaticity naturally lends itself to applications requiring the
selective excitation of particular sites within a heterogeneous system or of
one specific chemical species in a mixture of reactants.
• The generally high intensity of laser sources is significant both for increasing
excitation efficiency and also for thereby promoting multiphoton processes;
• Pulsed laser excitation offers a temporal selectivity that is now widely being
exploited for inducing and monitoring fast and ultrafast chemical processes.

Exploitation of laser characteristics for chemical selectivity in energy, space and time

• Since, the input radiation is commonly pulsed and so has time-variable
intensity, and since both saturation and multiphoton absorption may further
complicate the dynamics of photoabsorption, it is dearly no longer
appropriate to employ the Beer-Lambert Law.
• The absorbance (optical density), defined through an obvious generalisation
as.
A = -log10(1-F), F being the fraction of energy absorbed
• One can then gauge the often complex dependence on a multitude of
factors such as: laser wavelength, fluence (energy density of the radiation),
pulse duration, optical path length, temperature, and the concentration or
pressure of both absorbing and non-absorbing species.
General feature of Laser excitation:

LASER Initiated Processes
• In polyatomic molecules, the initial photo-induced transition to an
electronically excited state is almost invariably followed by some degree of
intramolecular relaxation before any real chemistry takes place.
• Such unimolecular relaxation processes generally involve redistribution of
energy amongst vibrational states and take place typically over
nanosecond or sub-nanosecond timescales,
• The state directly populated by photon absorption may therefore have
little chemical significance. Relaxation may lead to ionisation,
isomerisation or dissociation.
For a polyatomic molecule ABC
Photoabsorption: ABC + nhν ABC*
Autoionisation: ABC* ABC+ + e
Isomerisation: ABC* ACB*
Dissociation: ABC* ABƚ + Cƚ

• While the previous considerations apply to unimolecular reactions, lasers
can also be used to induce bimolecular reactions in which either one or
both of the reactants are initially excited by the absorption of laser light.
• Often IR lasers are used instead if UV/Vis lasers.
• In principle, a wide range of reaction conditions can be obtained by
promoting each reactant to various energy levels.

Multiphoton Infra-red Excitation
• A very distinctive kind of laser photochemistry can be induced by powerful
infra-red sources, the carbon dioxide laser being by far the most widely
used.
• The multiphoton processes which can be induced by intense radiation
become particularly efficient if one or more resonance condition can be
satisfied by the molecular energy levels.
• Vibrational energy levels are more or less equally spaced, at least for the
lowest levels of excitation. Hence, with infra-red radiation of the
appropriate wavelength, multiphoton absorption can become highly
significant.

Diatomic Molecule
• To consider multiphoton infra-red absorption in more detail, we first take
the simple case of a diatomic molecule, where there is only one
vibrational frequency.
• The first thing to note is that as we move up the ladder of vibrational
states, although the spacing between adjacent levels starts off fairly
constant, it diminishes at an increasing rate.
• It also has to be borne in mind that each vibrational level has its own
manifold of much more closely spaced rotational levels.
• An asymptotic limit is reached, at which point there is no longer any
restoring force as the two atoms move apart, and dissociation occurs.
• The process of multiphoton absorption displays different characteristics
over different regions of the energy scale, and it has become common to
speak in terms of regions I, II and III, illustrated in figure..

• In region I, vibrational levels are quite widely spaced, and the spacing is
greater than the overall absorption bandwidth. Because the spacing is non-
uniform, however, the photon energy soon gets out of step, and multiphoton
processes occur.
• In the diagram, for example, the transitions v = 0  1, 1  2, 2  3, 3  4 and 4  5
all require energies close to that of a single photon and lying within the overall
bandwidth.
• These transitions therefore all take place by the process of single photon
absorption. The energy required for the 5  6 transition, however, is
sufficiently different that it lies outside the bandwidth and cannot take place
by absorption of one photon.
• Nonetheless, excitation can proceed up to the v = 10 level, as indicated, by a
direct 5  10 transition involving four-photon absorption.
• This necessitates a fairly intense flux of photons and, hence, a powerful source of
radiation. Generally achievable by mode-locked pulsed LASERs

• Region II is characterised by quasi-continuum behaviour resulting from the
fact that vibrational energy level spacing has become less than the
bandwidth. Here successive photons can be absorbed in a series of
energetically allowed single-photon transitions.
• Since energy conservation is satisfied at every step, the molecule can at each
point exist for a finite lifetime before absorbing the next photon; hence,
excitation through this region does not necessitate the enormously large
photon flux which might at first appear necessary.
• Finally, once the level of excitation has reached the dissociation threshold, a
true energy level continuum is encountered, and further photons can be
absorbed in the short time before the atoms separate; this is known as
region III behaviour.

Laser Photochemical Processes

Unimolecular Laser-Induced Reactions
• The largest number of laser-induced chemical reactions fall into the
category of unimolecular reactions, and the carbon dioxide laser,
producing powerful emission at numerous discrete wavelengths around
9.6 μm and 10.6 μm is the most commonly applied source.
• The simplest type of unimolecular reaction is isomerisation, and several
studies have shown how laser-induced photoisomerisation can modify the
relative proportions of different isomers in a mixture.
• The selective laser excitation of one isomer, using a wavelength which no
other isomer appreciably absorbs, can substantially modify the relative
proportions either towards or, indeed, in some cases away from
equilibrium.

• 1,2-dichloroethene, where the cis-isomer is more stable than the trans-
isomer by approximately 2 kJ mol-1. Pulsed irradiation of a mixture containing
an excess of the trans-compound at a frequency of 980.9 cm-1 results in
conversion to a mixture in which the cis-isomer predominates.
• Pulsed irradiation of hexafluorocyclobutene at 949.5 cm-1, however, results in
up to 60% conversion to its isomer hexafluoro-1,3-butadiene , which is
thermodynamically less stable by 50 kJ mol-1
• A classic case of laser-induced chemistry involves the conversion of 7 –
dehydrocholesterol (I) to previtamin D3 (II), which is, once again, an
isomerisation reaction.
The product (II) is reversibly convertible to vitamin D3 (III)

Molecular structures of (I) 7-dehydrocholesterol;(II) previtamin D3; (III) vitamin D3

• Most unimolecular laser-induced reactions involve multiphoton infra-red
dissociation
• Some good examples are provided by elimination reactions involving esters,
which proceed as follows
• Such reactions can be very effectively induced by laser irradiation at a
frequency of around 1050 cm-1, which produces excitation of the stretching
mode of the O-CH2 bond and ultimately results in its fission.

• There are certain cases, especially in comparatively small molecules, where
irradiation at different laser frequencies genuinely results in different products.
• Cyclopropane, where it is found that multiphoton excitation at around 3000
cm-1 corresponding to the C-H stretching frequency results in isomerisation to
propene. However, irradiation at around 1000 cm-1 , corresponding to the CH2
‘wagging’, produces both isomerisation and fragmentation in roughly equal
amounts.

Laser-Sensitised Reactions
• It involves the sensitisation of reactions by the excitation of a species
which does not itself undergo chemical change; this can be regarded as a
form of laser-assisted homogeneous catalysis.
• This kind of reaction generally proceeds as a result of the collisional
transfer of vibrational energy, often referred to as V-V transfer, from
molecules of the laser-excited species (the sensitizer) to reactant
molecules.
• The major advantage of laser sensitisation becomes apparent if the
reactants do not themselves strongly absorb in the emission region of a
particular laser.

• By choosing a strongly absorbing sensitizer to initiate the reaction, the rate of
reaction induced by laser stimulation can be greatly increased.
• Both sulphur hexafluoride and silicon tetrafluoride have been widely
employed as sensitizers
• In the presence of SiF4 , various types of sensitised gas-phase reaction have
been observed.
Isomerization
Condensation
Retro Diels – Alder Reaction

• Many such reactions which are normally carried out at high temperatures,
or even with CW laser heating, produce chemically cleaner products if they
are induced indirectly by laser sensitisation since the reaction vessel
remains cold.
• Such reactions may also be strongly influenced by the choice of
sensitizer and the pressure ratio of sensitizer to reagent.
• Because the reactants in a sensitised reaction do not need to possess
absorption bands in any particular infra-red region, then with a good
sensitizer like SiF4, the range of gas-phase reactions which can be laser-
induced is almost limitless.

• Another related topic is laser-catalysed reaction, a term which is a very
definite misnomer but is applied to a reaction in which the catalyst is
itself produced by laser chemistry.
• For example, the laser pyrolysis of OCS using 248 nm radiation from a KrF laser
produces ground state S2 molecules, which can catalyse the isomerisation of
cis-2-butene to trans-2-butene with an effective quantum yield of about 200.

CSO 202A : Atoms Molecules and Photons
Ultrafast Chemical Reaction Dynamics with Ultrashort-
Pulsed Lasers

Consider a chemical transformation
CH3I + Na  CH3---I----Na  CH3 + NaI
Transition
state
In any chemical reaction the motions of the electrons and
nuclei of atoms determine how the molecules interact, and
those interactions in turn create the forces that govern the
reaction's dynamics.
If one can determine how molecular motions change during
the critical transition phase, we can understand how new
chemical bonds form and old ones disappear.

Molecular structures for a reaction in progress involving two molecules (bimolecular).

Question
How can one study transition state(s) in real time?
Answer
Need ultrafast probe and detection technique

Ahmed
Zewail
The Nobel Prize in Chemistry 1999
“for his studies of the transition states of chemical reactions
using femtosecond spectroscopy"

Trotting Horse
There was a debate over the question of whether all four hooves of
a trotting horse are simultaneously out of contact with the ground
at any point in its stride.
Movie

Eadweard Muybridge resolved this!

Time sequence
images of a
falling apple
Q. How can you
get these time
sequence images?
A. Stroboscopy
What time
resolution is
needed to
capture sharp
images of the
falling apple????
We can work it
out

Let’s take a molecular model, rotating at high speed (say -2600 rpm)
10 mm

When static
View under room light
:when rotating
Freezing motion using
proper light pulse

What time resolution is needed to freeze this macroscopic molecular
model in motion???? Let’s find it out ~150 s !!! Need short
pulse!
What about a real molecule (say methane)?
~10-12 s !!! Need ultrashort pulse!
How can we generate light pulses???
We have already learnt about it.
Is just achieving the time resolution good enough for our goal???
No. Need synchronization as well !! Why????

J. Chem. Phys., 1987, 87(4), 2395

LASERs in Medical Sciences

Lasers have many varied applications in
dentistry,
cardiovascular medicine,
dermatology,
gastroenterology,
gynaecology,
neurosurgery,
ophthalmology
otolaryngology.
And ………

• In cardiovascular diseases, lasers are mainly used for laser angioplasty, laser
thrombolysis, photo chemotherapy, laser treatment of arrhythmias and trans
myocardial revascularization. The thermal interaction, photo ablation and
photochemical interactions are used in these treatments.
• For example, laser angioplasty uses thermal effects to vaporize the plaque
material, in contrast to balloon angioplasty where the plaque material is
fractured, compressed or displaced.

• One of the most obvious applications of lasers is removal of dental enamel,
dentin, bone or cementum, instead of using an uncomfortable drill.
• A CO2 laser is commonly used to ablate or vaporize superficially thin layers of
soft tissue or to perform excisional surgery.

• The most common imperfections of the skin, such as pigmented lesions
(port wine stains, haemangioma, lentigines) and tattoos, are usually
treated with visible lasers including dye, argon, diode and ruby lasers.
• Laser are applied in gastroenterology to treat gastrointestinal haemorrhage
from peptic ulcers (Nd:YAG) lithotripsy to fragment common duct stones in
humans (tuneable dye, Q-switched Nd:NAG, pulsed Nd:YAG) and many
other applications.

LASIK (laser-assisted in situ keratomileusis)
• LASIK surgery involves a suction ring that holds the eye steady while the platform
for the microkeratome, a cutting instrument, is put in place.
• The microkeratome glides across the surface of the cornea, cutting through
the outer layers. The instrument leaves an uncut part of the outer layer of
the cornea to act as a hinge.
• The microkeratome is removed, the attached corneal flap is lifted out of
the way, exposing the underlying layers of cornea to the laser beam, which
corrects the curvature of the surface by ablation.

• In surgery, femtosecond pulses allow for much more precise cutting than do
nanosecond lasers.
• The biggest advantage of ultrashort pulsed lasers in surgical applications is limiting
biological tissue damage. The pulse interacts with the tissue faster than thermal
energy can diffuse to surrounding tissues. It simply means less, if any, burning and
destruction of neighbouring tissue.
• The radiation–biological tissue interaction is determined mainly by the laser
irradiance [W/cm2], which depends on the pulse energy, pulse duration, and the
spectral range of the laser light. The interaction depends also on thermal
properties of tissue – such as heat conduction, heat capacity and the coefficients
of reflection, scattering and absorption.
• The main components of biological tissue that contribute to the absorption are
melanin, haemoglobin, water and proteins.
A few important points

Absorption spectra of main absorbers in biological tissue..

• The absorption properties of the main biological absorbers determine the
depth of penetration of a laser beam.
• For example, the Nd:YAG laser can penetrate deeper and a cut made with the
Nd:YAG laser will not bleed due to tissue coagulation, in contrast to the CO2
laser which is a better ‘‘scalpel’’ for precise thermal cutting of tissue due to
vaporization by focusing on the tissue along a short optical path.

Categories of Interactions
There are five main categories of interaction:
• Photochemical interactions
• Thermal interactions,
• Photoablation,
• Plasma-induced ablation,
• Photodisruption.
Double logarithmic plot of the power
density as a function of exposure time. The
circles show the laser parameters required
from a given type of interaction with
biological tissue.

• With cw lasers or exposure time >1 s, only photochemical interaction can be
induced. Powers of only a few mW can be used for these purposes.
• For thermal interactions shorter exposure times (1 min–1 μs) and higher
energies must be used. Thermal effects can be induced both by cw or pulsed
lasers of 15–25 W power.
• Photoablation occurs at exposure time between 1 μs and 1 ns. In practice,
nanosecond pulses of 106–109 W/cm2 irradiance should be employed.
• Plasma-induced ablation and photodisruption occur for pulses shorter than
nanoseconds. In practice, pico- and femtosecond lasers with an irradiance of
1012 W/cm2 should be used.
• Both phenomena occur at a similar time exposure and irradiance, they
differ according to the energy densities that are significantly lower for
plasma-induced ablation.

PHOTOCHEMICAL INTERACTIONS
• Photochemical interactions do not need a high power density. Lasers of 1
W/cm2 power density and long exposure times ranging from seconds to
cw light are sufficient.
• For this category of interactions, a laser induces chemical effects by
initiating chemical reactions in tissue. For example, vision processes in
rhodopsin or proton pumping in bacteriorhodopsin are initiated by a laser
beam from the visible range.
• Photochemical interactions are used in photodynamic therapy (PDT)

Photodynamic Therapy
• Photodynamic therapy utilizes the laser light effect on various chemical
substances (e.g., some porphyrins) in an oxygen-rich environment. Light
induces a sequence of reactions that produce toxic substances such as
singlet oxygen or free radicals. These substances are very reactive and can
damage proteins, lipids, nucleic acids as well as other cell components.
• In the PDT method, a chemical substance known as a sensitizer is injected
intravenously. During the next several hours the sensitizer is distributed to
all of the organism’s soft tissues, both healthy and diseased.
• At first, the substance concentration is the same in healthy and diseased
cells, but after about 48–72 hours the sensitizer leaves the healthy cells in
contrast to cancer cells, where it remains accumulated for 7–10 days.

• After about 3 days post injection, the concentration of the sensitizers is
about 30 times higher in diseased cells than in healthy ones.
• About 3 days after the sensitizer injection, a patient is irradiated by a laser
light. The laser light induces a sequence of reactions with the excited singlet
state of oxygen 1O2* as a final product.
• The singlet oxygen 1O2* is very reactive, which makes it extremely toxic as it
reacts with components of biological cells and destroys them.
• To protect healthy cells carotene is injected. Carotene reacts with 1O2*
causing oxygen transfer to the harmless triplet oxygen state 3O2.
• The advantage of photodynamic therapy in cancer treatment over
commonly used radio– and chemotherapy is selective destruction of
diseased cells while saving healthy cells to a large extent.
• In most clinical applications haematoporphyrin derivatives (HPD) as well as
dihaematoporphyrinethers (DHE) are used. The commercial name for DHE is sodium
porfimer.

• The names of porphyrins contain also a number, e.g., uroporphyrin I.
The number I defines a regular substituent repetition, e.g., AP AP AP AP,
beginning with the pyrrole ring I. For porphyrins numbered with III, the
order in ring IV is reversed: AP AP AP PA, where,
– A acetic acid (–CH2COOH)
– P propionic acid (–CH2CH2COOH)
– M methyl group (–CH3)
– V vinyl group (–CH=CH2)

• Some porphyrins such as dihematoporphyrin have already found application in
photodynamic therapy or they have reached the III phase of clinical tests. HPD
and DHE belong to the first generation of sensitizers. Their main side effect is
skin photosensitivity.
• To reduce the side effects and increase efficacy, investigations have been made
to synthesize second- and third generation sensitizers, which absorb at longer
wavelengths (>650 nm). Examples- porphyrin, purpurin, benzoporphyrin,
phthalocyanine, and naphthalocyanine derivatives.
• For phthalocyanine or naphthalocyanine, which absorb at 670 nm and 770 nm, the
photosensitivity side effect disappears.
Dihematoporphyrin

Photochemistry of Sensitizers
• There are two main mechanisms of photochemical reactions in sensitizers-
I and II type photooxidation.
• In type I photooxidation, the sensitizer reacts directly with another
chemical entity by hydrogen or electron transfer to yield transient radicals,
which react further with oxygen.
• In type II photooxidation, the sensitizer triplet interacts with oxygen, most
commonly by energy transfer, to produce an electronically excited singlet
state of oxygen, which can react further with a chemical entity susceptible
to oxidation.

Type I Photooxidation
• The sensitizer in a singlet state, 1S, absorbs a photon of energy hν and is
promoted to the singlet excited state, 1S*.
• The excited singlet state 1S* emits the energy as fluorescence or in a
radiationless way, returning to the 1S state or crossing to the excited triplet
state 3S* as a result of intersystem crossing (ISC) with breaking of the
selection rule (spin change). The return from the triplet state to the
ground singlet state 1S may occur via emission of phosphorescence
• The triplet state 3S* can also vanish as a result of proton transfer or
electron transfer between the sensitizer and another chemical entity (RH)
(for example substances that are the components of a human cell)
3S* + RH SH + R
3S* + RH S- + RH +
(1)
(2)

• The reactions (1) or (2) induce further reactions with the oxygen triplet state
3O2 contained in a cell environment
SH + 3O2
1S + HO2
S - + 3O2
1S + O2
-

Type II Photooxidation
• In type II photooxidation the triplet state of a sensitizer 3S* interacts
directly with the oxygen triplet state 3O2, leading to generation of the
singlet excited oxygen state 1O2*.
• Oxygen in the singlet excited state is very reactive. This leads to oxidation
of cell components such as proteins, lipids, and nucleic acids and
eventually to necrosis of the cell.
3S* + 3O2
1S H + 1O2
*

Thermal Interaction
• Thermal interactions are induced in a tissue by the increase in local
temperature caused by a laser beam.
• In contrast to photochemical interactions, thermal interaction may occur
without only specific reaction path and is highly non-selective and non-
specific.
Depending on the temperature achieved, the thermal effect on the tissue can be
classified as:
• Pyrolysis (T > 3000 C).
• Reversible hyperthermia (T > 310 C) – some functions of the tissue can be
perturbed but the effect is reversible.
• Irreversible hyperthermia (T > 420 C) – some fundamental functions of the
tissue can be destroyed irreversibly
• Coagulation (T > 600 C) – the tissue becomes necrotic,
• Vaporization (T ≥ 1000 C),
• Carbonization (T > 1500 C),

• In some cases all of these thermal effects can be observed as a result of
interaction with the laser.
• In most applications one effect usually dominates, depending on the goal of
the surgery. For example, an Nd:YAG laser beam traveling long path in the
tissue is used for coagulation, whereas CO2 lasers are more suitable for
vaporization.
The thermal effects on tissue.

PHOTOABLATION
• A molecule is promoted to the repulsive excited state (or to the Franck-
Condon vibrationally hot state) followed by dissociation.
• The chemical bond is broken, leading to the destruction of biological
tissue.
• As electronic transitions occur usually in the UV range, the photoablation
process is usually limited to UV lasers. Therefore, excimer lasers (ArF, KrF,
XeCl, XeF) are mainly employed but higher harmonics of other lasers can
also be applied.

Mechanism of photoablation (a) the excited state is repulsive, (b) the excited state is
a Franck-Condon state.

PLASMA-INDUCED ABLATION
• Typical lasers used for plasma-induced ablation are Nd:YAG, Nd:YLF,
Ti:sapphire with pico- or femtosecond pulses generating irradiance at
about 1012 W/cm2.
• Therefore, the Q-switched or modelocked lasers can ionize molecules in
biological tissue.
• An ultrashort pulse from a Q-switched or mode-locked laser ionizes biological
tissue and generates a very large density of free electrons in a very short
period of time with typical values of 1018 cm-3 due to an avalanche effect.
• Free electrons from ionization accelerate to high energies and collide with
molecules, leading to further ionization.
• Light electrons and heavy ions move at different velocities, leading to the
effect similar to that in the acoustic wave with areas of compression and
dilation.

Application of LASERs in Material
Science and Engineering
LASER welding, LASER cutting, LASER
cladding, LASER peening, LASER Surface
Chemistry, Purification of Materials and
LASER Induced Polymerization

LASER welding
• Laser beam welding is a technique in manufacturing whereby two or more
pieces of material (usually metal) are joined together by the use of a laser
beam.
• The weld is formed as the intense laser light rapidly heats the material –
typically calculated in Milli-seconds.
• Lasers are used for materials that are difficult to weld using other
methods, for hard to access areas and for extremely small components.
• In laser welding the absorption of energy by a material is affected by many
factors such as the type of laser, the incident power density and the base
metal's surface condition.
• The primary types of lasers used in welding and cutting are:
Gas LASERs, Solid state LASERs and Diode LASERs

• Laser output is not electrical in nature and does not require a flow of electrical
current. This eliminates any effect of magnetism, and does not limit the
process to electrically conductive materials.
• Lasers can interact with any material. It doesn't require a vacuum and it does
not produce x-rays.
• Laser output is not electrical in nature and does not require a flow of electrical
current. This eliminates any effect of magnetism, and does not limit the
process to electrically conductive materials.
• The laser beam has been used to weld carbon steels, high strength low alloy
steels, aluminium, stainless steel, titanium etc.
• Limitations are - Rapid cooling rate may cause cracking in some metals
High capital cost for equipment
Optical surfaces of the laser are easily damaged
High maintenance costs

LASER Cutting
• Laser cutting works by directing the output of a high-power laser most
commonly through optics.
• The laser optics and CNC (computer numerical control) are used to direct
the material or the laser beam generated.
• Piercing usually involves a high-power pulsed laser beam which slowly
makes a hole in the material, taking around 5–15 seconds for 0.5-inch-
thick (13 mm) stainless steel, for example.
• The focused laser beam is directed at the material, which then either
melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an
edge with a high-quality surface finish. Industrial laser cutters are used to
cut flat-sheet material as well as structural and piping materials.

There are three main types of lasers used in laser cutting.
 The CO2 laser is suited for cutting, boring, and engraving.
 Nd laser is used for boring and where high energy but low repetition are required.
 The Nd-YAG laser is used where very high power is needed and for boring and
engraving.
Advantages of laser cutting over mechanical cutting
 Easier work holding
 reduced contamination of work piece.
 Precision may be better, since the laser beam does not wear during the process.
 There is also a reduced chance of warping the material that is being cut, as laser
systems have a small heat – affected zone.
 Some materials are also very difficult or impossible to cut by more traditional
means.

Additionally,
Laser cutting for metals has the advantages over plasma cutting of being
more precise and using less energy when cutting sheet metal; however,
most industrial lasers cannot cut through the greater metal thickness that
plasma can.

LASER Cladding
Laser Cladding or Laser Deposition is a processing technique used for adding
one material to the surface of another in a controlled manner.
A stream of a desired powder is fed into a focused laser beam as it is scanned
across the target surface, leaving behind a deposited coating of the chosen
material.
• Additional material can be placed precisely where desired.
• This enables the applied material to be deposited selectively just where it
is required.
• A very wide choice of different materials can be both deposited and deposited
onto.
• Deposits are fully fused to the substrate with little or no porosity.
• Minimal heat input results in narrow HAZ (heat affected zone).
Advantages

• Minimal heat input also results in limited distortion of the substrate and
reduces the need for additional corrective machining.
• Easy to automate and integrate into CAD/CAM and CNC production
environments.

LASER Peening
Laser peening (LP), or laser shock peening (LSP), is a surface engineering process
used to impart beneficial residual stresses in materials.
The deep, high magnitude compressive residual stresses induced by laser
peening increase the resistance of materials to surface-related failures, such
as fatigue, fretting fatigue and stress corrosion cracking.
• Fundamentally, laser peening can be accomplished with only two
components: a transparent overlay and a high energy, pulsed laser system.
• This enables the applied material to be deposited selectively just where it
is required.
• A very wide choice of different materials can be both deposited and deposited
onto.
• The transparent overlay confines the plasma formed at the target surface
by the laser beam. It is also often beneficial to use a thin overlay, opaque
to the laser beam, between the water overlay and the target surface.
Advantages

• This opaque overlay can provide either one or each of three benefits:
– protect the target surface from potentially detrimental thermal effects
from the laser beam,
– provide a consistent surface for the laser beam-material interaction and,
– if the overlay impedance is less than that of the target surface, increase the
magnitude of the shock wave entering the target
• Laser pulses are generally applied sequentially on the target to treat areas
larger than the laser spot size. Laser beam shapes are customizable to circular,
elliptical, square, and other profiles to provide the most convenient and
efficient processing conditions. The spot size applied depends on a number of
factors that include material, laser system characteristics and other processing
factors.

Laser Surface Engineering
• Many of the most important topics in this field concern the treatment of
semiconductor surfaces and therein hold enormous potential for
application in the manufacture of microelectronic devices.
• It is worth noting that excimer lasers in particular produce emission in a
very useful wavelength range, where photon energies are sufficient to
break chemical bonds in a variety of compounds involving the Group IV
elements.
• For example, in the dissociation of propan-2-ol over CuO using 1070.5 cm-1
radiation from a CO2 laser , there are two competing reaction pathways
leading to different products

 The product ratio : propanone/propene can be varied from 0.02 to 6,
depending on the orientation of the catalytic surface relative to the laser
beam.
• Many laser-induced surface engineering involves the principle of
depositing a thin film covering onto a substrate surface by decomposition
of a gas. This method is known the laser field as laser chemical vapour
deposition.
• The mask-free writing of an adsorbate onto semi-conductor surfaces by
laser deposition provides a classic illustration of an application facilitated
by the distinctive properties of laser light.

• The principle involved in the process of deposition may be either pyrolytic or
photolytic by nature.
For both types of deposition, laser irradiances are typically of the order
1012 W m-2, and the partial vapour pressure of the vapour in the range
10-3-1 atm.
Under these conditions, rates of deposition with a scanning laser beam
are typically between 0.1 and 100 μm S-1.

Pyrolytic deposition involves thermal reaction and is, in general, an indirect
result of the surface heating produced by the laser radiation.
• For example, amorphous films of silicon can be pyrolytically deposited from
SiH4 vapour onto quartz or various other surfaces irradiated by 10.59 μm
radiation from a carbon dioxide laser.

Photolytic deposition (photodeposition), by contrast, results directly from the
absorption of laser light by molecules of the vapour.
Example : possibility of laying down an InP layer by co-deposition of indium and
phosphorus from a mixture of (CH3)3InP(CH3)3 and P(CH3)3
In this case, using 193 nm radiation from an ArF excimer laser, the
photodecomposition reactions are:

Purification of Materials
• The underlying principle is the specific excitation of a single chemical
component in a mixture, in this case usually the impurity.
• The removal of contaminants from silane, SiH4 can be done by using an
ArF laser operating at 193 nm, it has been shown that impurities of arsine
AsH3 , phosphine PH3 , and diborane B2H6 can all be photolysed and so
removed from silane gas very effectively.
• Another example based on the argon fluoride laser is the removal of H2S
from synthesis gas. This is particularly significant since H2S readily poisons
the catalysts used for hydrocarbon synthesis.
• The removal from BCl3 of carbonyl chloride, COCl2, which is often a fairly
troublesome contaminant, can be done by using the CO2 laser.

Laser-Initiated Polymerisation
• It is primarily pulsed UV radiation that is employed to produce radicals for
the process initiation.
• It generally proves that there are substantial differences in the character
of polymers obtained with laser radiation, compared to those produced
with radiation of the same wavelength and total energy from other
sources.

• One reason is that the high intensities associated with laser radiation can,
by increasing the transient concentrations of radical intermediates,
substantially increase the extent to which sequential absorption processes
enter into the reaction. A second reason is more directly connected with
the pulsed nature of the radiation.
• The mean chain length in the laser-produced polymer is then directly
proportional to the 'dark time' between pulses. So, the product is
characterised by a molecular weight distribution more directly amenable
to control and generally quite different from the polymer produced using
conventional photo initiation.

Lasers in communications

https://www.slideshare.net/asertseminar/laser-communications-33264562

LASER Safety

Classification of LASER Classes
• Class 1 - Safe under reasonably foreseeable operation
• Class 1M - Generally safe – some precautions may be required
• Class 2 - Visible light at low power, blink limits risk
• Class 2M - UV or IR light at low power, generally safe - some precautions
may be required
• Class 3R(A) - Safe for viewing with unaided eye, (i.e. not by telescope etc)
• Class 3B - Viewing beam hazardous, diffuse reflections safe
• Class 4 - Hazardous under all conditions, eyes and skin

Class Power Remarks Typical examples
I Very low
or beam
completely
enclosed
•Inherently safe,
•No possibility of exposure
CD, DVD drives, laser
printers…
II 1 mW
Visible only
•Staring into the beam is hazardous
•Eye protected by aversion response
Supermarket laser
scanners, some pointers
IIIa 1-5 mW •Aversion may not be adequate Laser pointers
IIIb 5-500 mW •Direct exposure is a hazard Ar laser
CF microscope
IV >500 mW •Exposure to direct beam and scattered
light is eye and skin hazard
•Fire hazard
Laser ablation setup

Labels on LASERs
Class II
Class IIIa with expanded beam
Class IIIa with small beam
Class IIIb
Class IV

Safety Measures
• Use minimum power/energy required for project
• Eyewear for classes IIIb, IV for everybody in the room.
• Beam paths above >200 mW should be guided through tubes.
• Highest risk is during alignment, optical setup modification.

• Reduce laser output with shutters/attenuators, if possible
• Terminate laser beam with beam trap
• Use diffuse reflective screens, remote viewing systems, etc, during
alignments, if possible
• Remove unnecessary objects from vicinity of laser
• Keep beam path away from eye level
• Be Informed
• Don’t put your body parts
(particularly your eyes) in the beam!!

Causes of Accidents
• Eye protection not used when needed.
• Unprotected eye exposure during alignment
• Badly aligned optics
• Equipment breakdown
• Covers not replaced after service/alignment
• Lack of operator training
• Altering beam path (e.g., adding optical components without regard to
beam path)
• Inserting reflective objects into beam path
• Bypassing interlock (particularly during servicing and alignment)
• Inappropriately turning on power supply
• Inappropriately firing of laser

Burning Injuries from CO2 LASER

LASER ENGEENERING 2 .pdf

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