LASER ENGEENERING 1 .pdf

Unique properties of LASER
• Highly Monochromatic
• Highly Coherent
• Well Collimated Beam
• Directional
• Wide Tuneability
• High Power
Due to these unique properties LASER has found wider applications in
various field of commercial and research area.
Laser : Fundamentals and Applications

Applications:-
 Scientific
• Spectroscopy
• Non Linear Optics
• Raman Spectroscopy
• Laser Induced Breakdown Spectroscopy(LIBS)
• Super Resolution Spectroscopy
• Confocal Microscopy
• Optical Coherence Tomography(OCT)
• Space technology
• Nuclear Fusion Reactors
• Astronomy

 Industrial/Commercial
• Optical Storage (CD/DVD)
• Reading Barcodes
• LASER Printers
• Engineering
• Welding
• Cutting
• Peening
• Soldering
• Drilling
• Cladding
• Power Beaming

 Medical science
• Surgical Applications
• LASIK in eye surgery
• Soft tissue surgery
• Endoscopic surgery
• Dermatology
• Laparoscopy
• Photodynamic therapy
 Military
• As a Weapon
• Detection and Communication purposes

LASERS: A Walk Through History

Max Planck
• In his most important work, published in 1900, Planck
deduced the relationship between energy and the frequency
of radiation, essentially saying that energy could be emitted
or absorbed only in discrete chunks – which he called quanta.
• In 1905, Einstein released his paper on the photoelectric
effect, which proposed that light also delivers its energy in
chunks, in this case discrete quantum particles now called
photons.

Albert Einstein
• In 1917, Einstein proposed the process that makes lasers possible,
called stimulated emission. He theorized that, besides absorbing
and emitting light spontaneously, electrons could be stimulated to
emit light of a particular wavelength.
• But it would take nearly 40 years before scientists would be able
to amplify those emissions, proving Einstein correct and putting
up lasers.

Charles Hard Townes
• April 26, 1951: Charles Hard Townes of Columbia University in New
York conceives his maser idea while sitting on a park bench in
Washington.
• 1954: Working with Herbert J. Zeiger and James P. Gordon, Townes
demonstrates the first maser at Columbia University. The ammonia
maser, the first device based on Einstein’s predictions.

• 1955: At P.N. Lebedev Physical Institute in Moscow, Nikolai G.
Basov and Alexander M. Prokhorov attempt to design and build
oscillators. They propose a method for the production of a
negative absorption that was called the pumping method.
• 1956: Nicolaas Bloembergen of Harvard University develops the
microwave solid-state maser.

• 1958: Townes, and his brother-in-law, Arthur L. Schawlow, in a
joint paper published in Physical Review, show that masers could
be made to operate in the optical and infrared regions and
propose how it could be accomplished.
• At a conference in 1959, Gordon Gould published the term LASER
in the paper The LASER, Light Amplification by Stimulated Emission
of Radiation
This is the first page of Gordon
Gould's famous notebook, in
which he coined the acronym
LASER and described the essential
elements for constructing one.

• May 16, 1960: Theodore H. Maiman, a physicist, constructs the
first laser using a cylinder of synthetic ruby measuring 1 cm in
diameter and 2 cm long, with the ends silver-coated to make them
reflective and able to serve as a Fabry-Perot resonator. Maiman
uses photographic flash lamps as the laser’s pump source.
Theodore H. Maiman
Finally!!!!

Theodore H. Maiman
Ruby Laser

• November 1960: Peter P. Sorokin and Mirek J. Stevenson
demonstrate the uranium laser, a four-stage solid-state device.
• December 1960: Ali Javan, William Bennett Jr. and Donald Herriott
develop the helium-neon (HeNe) laser, the first to generate a
continuous beam of light at 1.15 μm.

• March 1961: At the second International Quantum Electronics
meeting, Robert W. Hellwarth presents theoretical work suggesting
that a dramatic improvement in the ruby laser could be made by
making its pulse more predictable and controllable.
• October 1961: Elias Snitzer reports the first operation of a
neodymium glass (Nd:glass) laser.

• December 1961: The first medical treatment using a laser on a
human patient is performed by Dr. Charles J. Campbell and Charles
J. Koester. An Optical ruby laser is used to destroy a retinal tumour.
• 1962: With Fred J. McClung, Hellwarth proves his laser theory,
generating peak powers 100 times that of ordinary ruby lasers by
using electrically switched Kerr cell shutters. The giant pulse
formation technique is dubbed Q-switching. Important first
applications include the welding of springs for watches.

• October 1962: Nick Holonyak Jr, publishes his work on the “visible
red” GaAsP (gallium arsenide phosphide) laser diode, a compact,
efficient source of visible coherent light that is the basis for today’s
red LEDs used in consumer products such as CDs, DVD players and
cell phones.
• June 1962: Bell Labs reports the first yttrium aluminium garnet
(YAG) laser.
• 1963: Logan E. Hargrove, Richard L. Fork and M.A. Pollack report
the first demonstration of a mode-locked laser. Mode locking is
fundamental for laser communication and is the basis for
femtosecond lasers.

• 1963: Herbert, and the team of Rudolf Kazarinov and Zhores Alferov,
independently propose ideas to build semiconductor lasers from
heterostructure devices. The work leads to Kroemer and Alferov winning
the 2000 Nobel Prize in physics.
• March 1964: William B. Bridges discovers the pulsed argon-ion laser,
which, although bulky and inefficient, could produce output at several
visible and UV wavelengths.

• 1964: Townes, Basov and Prokhorov are awarded the Nobel Prize
in physics for their “fundamental work in the field of quantum
electronics, which has led to the construction of oscillators and
amplifiers based on the maser-laser-principle.”
• 1964: The carbon dioxide laser is invented by Kumar Patel at Bell
Labs. The most powerful continuously operating laser of its time, it
is now used worldwide as a cutting tool in surgery and industry.

• 1964: The Nd:YAG (neodymium-doped YAG) laser is invented by
Joseph E. Geusic and Richard G. Smith. The laser later proves ideal
for cosmetic applications, such as laser-assisted in situ
keratomileusis (lasik) vision correction and skin resurfacing.
• 1965: Two lasers are phase-locked for the first time at Bell Labs, an
important step toward optical communications.

• 1966: Charles K. Kao, working with George Hockham, makes a discovery that
leads to a breakthrough in fibre optics. He calculates how to transmit light over
long distances via optical glass fibres, deciding that, with a fibre of purest glass,
it would be possible to transmit light signals over a distance of 100 km,
compared with only 20 m for the fibres available in the 1960s. Kao receives a
2009 Nobel Prize in physics for his work.
• 1966: French physicist Alfred Kastler wins the Nobel Prize in physics for his
method of stimulating atoms to higher energy states. The technique, known as
optical pumping, was an important step toward the creation of the maser and
the laser.
• 1966: Sorokin, P. and Lankard, J. - Demonstration of first Dye Laser action at IBM
Labs.
•

• 1970: Basov, V.A. Danilychev and Yu. M. Popov develop the
excimer laser.
• 1970: Arthur Ashkin invents optical trapping, the process by which
atoms are trapped by laser light. His work pioneers the field of
optical tweezing and trapping and leads to significant advances in
physics and biology.
• 1972: Charles H. Henry invents the quantum well laser, which
requires much less current to reach lasing threshold than
conventional diode laser and which is exceedingly more efficient.
Holonyak and students first demonstrate the quantum well laser in
1977.

• June 26, 1974: A pack of Wrigley’s chewing gum is the first product
read by a bar-code scanner in a grocery store.
• 1976: John M.J. Madey and his group at Stanford University in
California demonstrate the first free-electron laser (FEL).
• 1978: The LaserDisc hits the home video market, with little impact.
The earliest players use HeNe laser tubes to read the media, while
later players use infrared laser diodes.

• 1982: Peter F. Moulton develops the titanium-sapphire laser, used to
generate short pulses in the picosecond and femtosecond ranges.
• 1985: Steven Chu and his colleagues use laser light to slow and
manipulate atoms. Their laser cooling technique, also called “optical
molasses,” is used to investigate the behaviour of atoms, providing an
insight into quantum mechanics. Chu, Claude N. Cohen-Tannoudji and
William D. Phillips win a Nobel Prize for this work in 1997.

• 1994: The first semiconductor laser that can simultaneously emit
light at multiple widely separated wavelengths – the quantum
cascade (QC) laser – is invented at Bell Labs by Jérôme Faist,
Federico Capasso, Deborah L. Sivco, Carlo Sirtori, Albert L.
Hutchinson and Alfred Y. Cho.
• November 1996: The first pulsed atom laser, which uses matter
instead of light, is demonstrated at MIT by Wolfgang Ketterle.

• September 2003: A team of researchers from NASA’s Marshall
Space Flight Centre, from NASA’s Dryden Flight Research Centre
and from the University of Alabama successfully flies the first laser-
powered aircraft.
• 2004: Electronic switching in a Raman laser is demonstrated for the
first time by Ozdal Boyraz and Bahram Jalali . The first silicon
Raman laser operates at room temperature with 2.5-W peak
output power.

• September 2006: John Bowers and colleagues and Mario Paniccia,
announce that they have built the first electrically powered hybrid
silicon laser using standard silicon manufacturing processes.
• August 2007: Bowers and his doctoral student Brian Koch announce
that they have built the first mode-locked silicon evanescent laser,
providing a new way to integrate optical and electronic functions on
a single chip and enabling new types of integrated circuits.

• May 29, 2009: The largest and highest-energy laser in the world,
the National Ignition Facility (NIF) at Lawrence Livermore National
Laboratory is dedicated. In a few weeks, the system begins firing all
192 of its laser beams onto targets.
• June 2009: NASA launches the Lunar Reconnaissance Orbiter (LRO).
The Lunar Orbiter Laser Altimeter on the LRO will use a laser to
gather data about the high and low points on the moon.

• March 31, 2010: Rainer Blatt and Piet O. Schmidt and their team at
the University of Innsbruck in Austria demonstrate a single-atom
laser with and without threshold behavior by tuning the strength
of atom/light field coupling

Optical Transitions
Absorption Emission
E2-E1=hν12

Rate of Optical Transition
• Rate of absorption = -dN1/dt
here N1 = population of ground state.
-dN1/dt ∝ N1 ρ(hν12 )
-dN1/dt = B12N1 ρ(hν12 )
where B12 is constant of proportionality called as rate constant of absorption

• Rate of spontaneous emission = d N2/dt
where N2 = population of the excited state .
dN2/dt ∝ N2
dN2/dt = A21N2
where A21 is constant of proportionality called as rate constant for spontaneous
emission,21 indicates transition from state 2 to 1.

Stimulated Emission
• In 1917 Albert Einstein proposed another way of interaction of light
with matter which is called as stimulated emission. This process involves
decay of atom from excited state to ground state using a photon of light.

Rate Of Stimulated Emission
• Rate of stimulated emission = dN2/dt
where N2 = population of the excited state
-dN2/dt ∝ N2ρ(hν12)
-dN2/dt = B21 N2ρ(hν12)
where B21 is called as rate constant of stimulated emission ,21 indicates
that transition is from level 2 to 1.

Principle of Detail Balance
• According to principle of detail balance for a system rate of upward
transition is equal to the rate of downward transition at equilibrium.
Total rate of absorption = Total rate of emission
• Rate of absorption = B12N1ρ(hν12) --1
• Rate of spontaneous emission = A21N2
• Rate of stimulated emission = B21N2ρ(hν12)
• Total rate of emission = A21N2 + B21N2ρ(hν12) --2
From 1 & 2 -
B12N1ρ(hν12) = A21N2+ B21N2ρ(hν12)
B12, B21, A21 are collectively called as Einstein coefficients.

Calculation of Einstein Coefficients

• On solving the equation for ρ(hν12) we get:-
ρ(hν12)= -- 3
Ratio of population of two state at equilibrium is given by Boltzmann distribution law:-
N2/N1 = e- (hν12 /kT) -- 4
Here k is Boltzmann constant and T is temperature.
Using 4 & 3 :-
ρ(hν12) =
12
12
/
21
/
12 21
12
1
h kT
h kT
A e
B B
e
B




 
 
 
 
 

 
 
 
 
 
21 2
12 1 21 2
12 1
1
1
A N
B N B N
B N
 
 
 
 
 

 
 
 
 
 

Calculation Of Einstein Coefficient
• Using principle of detailed balance
ρ(hν12) =
This equation can be written as,
ρ(hν12)=
On comparing with Planck’s radiation law
B21 = B12
A21 / B21 =2hν12
3/c2
12
12
/
21
/
12 21
12
1
h kT
h kT
A e
B B
e
B




 
 
 
 
 

 
 
 
 
 
12
21
/
21 12
21
1
1
h kT
A
B B
e
B

 
 
 
 
 

 
 
 
 
 

Relation between Einstein Coefficients
• B21 = B12 ,which means
Rate constant of absorption = Rate constant of stimulated emission
So, stimulated emission and absorption are equal probable.
• A21 / B21 =2hν12
3/c2
Since h, c are constant so
Rate of spontaneous emission/Rate of stimulated emission ∝ ν 3
So at high frequencies spontaneous emission dominate than stimulated
emission. LASER action require stimulated emission so it is easier to achieve
laser action at lower frequencies.

Condition For LASER Action
Rate of Stimulated Emission / Rate of absorption = B21N2ρ/B12N1ρ = N2/N1
• Various cases are
• N2/N1 <1 Absorption dominates than emission
• N2/N1 =1 Saturation
• N2/N1 >1 Stimulated emission dominates
For LASER action, gain must be higher than the loss which in other
words means stimulated emission must dominate the absorption process.
From above relation it is clear that this condition is satisfied only when
population of excited state is more than ground state.

Properties of Stimulated Emission
• The stimulated emitted photon have
• Same frequency as incident photon
• Same direction
• Same phase
Due to this features of the stimulated photon LASER are Highly
Monochromatic, Directional, Coherent.

Population Inversion
• Condition for population inversion N2 > N1
N2= population of excited state, N1= population of ground state.
For a two level system :-
ⅆ𝑁2
ⅆ𝑡
= 𝐵12𝑁1𝜌 − 𝐵21𝑁2𝜌 − 𝐴21𝑁2
Solving for N2, using Einstein relations:- B12 = B21 = B, A21= A
𝑁2 =
𝐵𝜌𝑁𝑡
𝐴 + 2𝐵𝜌
ℎ𝑒𝑟𝑒, 𝑁 = 𝑁1 + 𝑁2

Since A > 0 always, so
𝑁2
𝑁
≤
1
2
or 𝑁2 ≤ 𝑁1
But for population inversion N2>N1, so in a two level system
population inversion can’t be achieved.

3 Level LASER System
E3
E2

• Pumping is required to create a population inversion in a LASER system.
The pumping may be optical, electrical etc.
• Metastable state is a excited state that is long lived(10-6 - 10-3s) than a typical
excited state(10-9s).
• Three level LASER gives burst of photons (pulses) which has a delay
between them.
Pulsing Of LASER

4 Level LASER System
E4
E3
E2
E1

Comparison between 4 level and 3 level LASER System
• Four level LASER System is better than three level system because
population inversion is easily achievable in a four level system, as
compared to three level system.
• Unlike 3 level system, which gives pulsed LASER, 4 level system gives
continuous LASER output and if required, by suitable technology, it can
be converted into pulsed LASER.

Components of LASER
• Active Medium:
Proper material capable of LASER action for example Ruby crystal.
• Pump Source:
A source which can provide sufficient amount of energy to excite the active
medium for example flash lamps.
• Cavity or Optical Resonator
An optical cavity or optical resonator is an arrangement of mirrors that is used for
the amplification of light.

Construction Of LASER
Typical Example Of LASER

• Spontaneous emission triggers the stimulated emission which leads to
avalanche. Ultimately it gives us the LASER beam:
LASER BEAM

Modes of LASER cavity
• Light confined in the cavity reflects multiple times producing standing
waves for certain resonance frequencies.
• The standing wave patterns produced are called modes.
• Longitudinal modes differ only in frequency.
• Transverse modes differ for different frequencies and have different
intensity patterns across the cross section of the beam.

Longitudinal modes of LASER cavity
• In LASER Cavity only those modes can sustain which creates a standing
wave.
Condition for sustaining a mode: nλ = 2L n = 1,2,3,4 ……
n = order of mode, λ = wavelength of light, L = length of cavity.

Advantage of curved mirror cavity
• Low losses in comparison to plane mirror cavity , as it does not allow the
off axis photons to go out of the cavity.

Transverse Electromagnetic Mode (TEM)
• The transverse modes determine the
intensity distributions on the cross-
sections of the beam.
• A TEM mode is described as TEMpq,
where p and q are the indices of the
mode.
• We will use the convention where p
corresponds to number of nodes in
horizontal direction and q
corresponds to number of nodes in
vertical directions.
Transverse Modes

Threshold Condition
Rate of change of intensity of stimulated photons:
𝑑𝐼
𝑑𝑡
= 𝑁2 − 𝑁1 𝐵ℎ𝑣𝐼
N2 = population of excited state , N1= population of ground state
B = Einstein coefficient, 𝑣 = frequency of emitted photons.
Taking degeneracy in account:
𝑑𝐼
𝑑𝑡
= 𝑁2 −
𝑔2
𝑔1
𝑁1 𝐵ℎ𝑣𝐼
g1 = degeneracy of ground state, g2 = degeneracy of excited state.

Threshold Condition
• Rate of increase of stimulated photon intensity:
𝑑𝐼
𝑑𝑡
= 𝑁2 − 𝑁1
𝑔2
𝑔1
𝐵ℎ𝑣𝐼
On solving the equation we get: 𝐼 = 𝐼0𝑒𝑎𝑡 ; 𝑤ℎ𝑒𝑟𝑒 𝑎 = 𝑁2 − 𝑁1
𝑔2
𝑔1
𝐵ℎ𝑣
Now using: 𝑡 =
𝑥
𝑐′
, 𝑐′
=
𝑐
𝑛
n = refractive index of the medium
𝑐′, 𝑐 = velocity of light in medium and vacuum respectively.
𝑥 = position of wave at any time t
We get:
𝐼 = 𝐼0𝑒𝑘𝑥 ; where 𝑘 = 𝑁2 − 𝑁1
𝑔2
𝑔1
𝐵ℎ𝑣𝑛
𝑐

• Taking into account various losses with in cavity: 𝐼 = 𝐼0𝑒 𝑘−𝛾 𝑥R1R2
𝛾 = total losses with in cavity; R1,R2= reflectivity of mirrors
Gain(𝐺) is define as: 𝐺 =
𝐼
𝐼0
= 𝑅1𝑅2𝑒 𝑘−𝛾 𝑥
Taking 𝐺 =1 gives:
𝑘threhold = 𝛾 +
1
2𝐿
log
1
R1R2
Condition for LASER action(𝐺 >1 or 𝑘>𝑘threhold):
𝑁2 − 𝑁1
𝑔2
𝑔1
𝐵ℎ𝑣𝑛
𝑐
> 𝛾 +
1
2𝐿
log
1
R1R2

Directionality
Diameter of Laser beam is the position in the beam cross section at which:
𝐼 =
𝐼𝑐
𝑒2
𝐼𝑐
𝐈𝐧𝐭𝐞𝐧𝐬𝐢𝐭𝐲
Cross section
Axis
LASER Cavity

Spot dimension of Laser: 𝜔0 =
2𝜆
𝜋𝜃
where 𝜃 = angle of convergence
𝜆 = wavelength of light
Intensity
It is measured in terms of Irradiance(I) =
𝐸
𝑡𝐴
=
𝑃
𝐴
E = energy of beam at time(t), A = area of cross section of beam
P = power of Laser beam
𝜃

Coherence
Laser source gives photons having same phase so LASER are highly coherent. Quantitatively
coherence is given by:
Coherence time(tc) =
1
∆𝑣
Coherence length(Lc) =
𝑐
𝑛∆𝑣
where,
c = velocity of light ∆𝑣 = emission line width 𝑛 = refractive index of medium
If L < Lc than photon are correlated to each other in phase.
Holography is an interesting example which utilize this highly coherent nature of LASERs.

Monochromaticity
• Laser are highly monochromatic because of narrow line width which arises
due to various line broadening classified as:
• Homogenous broadening
• Heterogenous broadening
The resulting spectrum has distribution of frequency called as gain
bandwidth(∆𝑣).

Modes in a Laser output
Separation between various modes of Laser is given by: ∆𝑣𝑚 =
𝑐
2𝐿
c = velocity of light L = length of Laser cavity
No. of Modes in Laser output =
∆𝑣
∆𝑣𝑚
m

Type Of LASER
• Depending upon output, Laser are of two types:
• Continuous Wave(CW)
• Pulsed Laser
Pulse width
(∆t)
Time duration
(T)
time
E
Characteristic of Pulsed Laser
output
Time
output
Time
Continuous Wave LASER Pulse LASER

Peak Power and Average power
• Average Power(Pavg) =
𝐸
𝑇
E = Energy, T = time duration
• Peak Power(Ppeak) =
𝐸
∆𝑡
∆𝑡 = Pulse width
• Duty cycle of Laser is time for which Laser is on, over a period of
time(T) and it is define as:
∆𝑡
𝑇
=
𝑃𝑎𝑣𝑔
𝑃𝑝𝑒𝑎𝑘

Pulse Shape
Gaussian Pulse Hyper Secant Pulse
E
t
E
t
• Pulse shape is temporal profile of pulse.
• Time bandwidth product = ∆𝑡 × ∆𝑣
∆𝑡 = pulse width ∆𝑣 = spectral band width
This time bandwidth product sets a limit on minimum temporal width for
a given spectral width and vice versa, known as transformation limit. For
example hyper secant pulses have time bandwidth product = 0.31

Q 1. A helium-neon laser emitting at 633 nm makes a spot with a radius equal to 100
mm at 1/e2 at a distance of 500 m from the laser. What is the radius of the beam at the
waist (considering the waist and the laser are in the same plane)?
The problem can be solved by using the formula that links the divergence of
the beam and the waist size:
Θ = λ/ Π x w0
So
w0 = λ/ Π x Θ
Θ is expressed in radians and is equal to 100 x 10-3 m/500m which is equal to
2 x 10-4m
w0 = (633 x 10-9m)/ (3.14 x 2 x 10-4m)
w0= 1 mm

Q2 Calculate the gap in frequency between two longitudinal modes in a linear cavity whose
optic length, L =300 mm.
The gap between two consecutive longitudinal modes is defined by
c/2L ; where c = 3 x 108 ms-1 .
L = 0.3m
The gap is therefore equal to 500 MHz.

Q 3 . What is the rate of repetition of the pulses emitted by a mode-locked laser?
The optic length of the cavity, L, is 1 m.
The gap between two pulses from a mode-locked laser is defined by
2L/c.
The frequency is therefore equal to 1/(2L/c)
= 150MHz

Q 4 . A mode-locked laser emits an average power P equal to 1 W. The rate of
repetition of the pulses from this laser is equal to 100 MHz. Calculate the energy of
each pulse.
The period between each pulse is equal to
τ = 1/ repetition rate
During this period, only one pulse is emitted.
The energy (E) is therefore found from
E=Pτ
E= 1Js-1/ 108 Hz
= 10 nJ

Q5. Consider a lower energy level situated 200 cm-1 from the ground state.
There are no other energy levels nearby. Determine the fraction of the
population found in this level compared to the ground state population at a
temperature of 300 K.
Boltzmann's constant is equal to 1.38.10-23JK-1
The conversion from cm-1 to joules is given by:
E(J) =100hc E(cm-1) , where h is Planck's constant (6.62 x10-34Js) and c is the
speed of light in a vacuum (3x108ms-1)
Boltzmann's Law is used:
N2 = N1 e- (E
2
-E
1
)/kT)
By considering the energy of the ground state to be zero and calling 0 the ground
state and 1 the lower energy level:
N1 = N0 e- (E
1
)/kT) After converting cm-1->joules
E1=3.97 x 10-21J ;
E1/kT = 0.96
N1/N0 = 0.38

Q 6 . Consider an optical pump at 940 nm for a Yb:YAG crystal placed in a laser cavity.
The wavelength of ytterbium is 1030 nm. If all the photons emitted by the pump are
absorbed by the crystal and used for the lasing process, calculate the maximum
power output. The pump power is 1 W.
At best, a pump photon gives a laser photon. The maximum output power is
defined by
(P x hνl)/ hνp
νl, νp are laser and pump frequencyrespectively
P x λl/ λp = 912 mW

Q 7 .The amplifying medium of a helium-neon laser has an amplification spectral
band equal to Δν = 1GHz at 633 nm. For simplicity, the spectral profile is assumed to
be rectangular. The linear cavity is 30 cm long. Calculate the number of longitudinal
modes that can oscillate in this cavity.
The number of longitudinal modes is equal to the spectral band divided by the
interval between the two longitudinal modes
N = Δν/(c/2L) = 2

Q 8 . A Q-switched laser emits pulses of 10µJ of duration 1 ns. The repetition rate of
the pulses is equal to 10 kHz.
1. Calculate the peak power of the pulses.
2. Calculate the average output power of the laser.
1. The peak power is the energy of the pulse divided by its duration: = 10kW
2 . The average output power is determined by saying that for one period
(T=1/10kHz), one pulse is emitted so = 100mw

Pulsing Technique
• Pulsing device is a device that converts continuous wave(CW) input to
the pulsed output. Various pulsing techniques are:-
• Cavity Dumping
• Q-Switching
• Mode Locking

Cavity Dumping
• This technique involves dumping out of all stored energy in the cavity
at once. This can be done by either using switching mirror or by using
a acoustic optic modulator(AOM).
Active Medium
Mirror (M1) Mirror (M2)
Switchable Mirror
Output
Pumping

Cavity Dumping
• Acousto - optic modulator is device whose refractive index can be
modified by a sound wave.
Active Medium
Mirror Concave Mirror(CM1)
CM2
Acoustic Optical Modulator
(AOM)
Output
Pumping

Features Of Cavity Dumping
• Cavity dumped output has a sinusoidal temporal pattern.
• All the time resonator losses are kept at minimum.
• Intercavity photons can be extracted with in one round trip time.
Intensity
Time

Q-Switching
• In this technique quality factor(Q) of the cavity is decreased, where
𝑄 = 2𝜋
𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑡𝑜𝑟𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑐𝑎𝑣𝑖𝑡𝑦
𝐸𝑛𝑒𝑟𝑔𝑦 𝑙𝑜𝑠𝑠 𝑝𝑒𝑟 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑐𝑦𝑐𝑙𝑒
This is done by using an attenuator inside the laser cavity.
Active Medium
Highly Reflecting Mirror Partially Transmitting Mirror
(End Mirror)
Pumping
Schematic diagram illustrating basic principle of Q-Switching

Q-Switching
Quality of the cavity can be changed using an attenuator arrangement
which involves:-
• Pockels cell which is used to alter the polarization of the light. This is
based upon the Pockels’ effect according to which refractive index of
medium changes linearly with applied electric field.
• Quarter wave plate is used to convert the linear polarized into circular
polarized light.

Q-Switching
Active Medium
Highly Reflecting Concave Mirror
Partially Transmitting Mirror
(Output Coupler)
Pumping
Polarizer
Pockels cell
Quarter wave
plate
Schematic diagram of Active Q-Switching

Q-Switching
Q-Switching can be classified as :-
• Active Q-Switching involves Pockels cell and quarter wave plate as
discussed earlier. This arrangement need to be synchronized with the
population inversion.
• Passive Q-Switching is automatically synchronized technique. It involves
a saturable absorber which has strong absorption at the frequency of
emission of active medium.

Passive Q-Switching
Active Medium
Highly Reflecting Concave Mirror
Partially Transmitting Mirror
(Output Coupler)
Pumping
Saturable Absorber
Schematic diagram of Passive Q-Switching

Comparison of Cavity Dumping and Q-Switching
• Q-switching involves blocking the oscillations so cavity can’t work as optical
resonator while in case of cavity dumping oscillations are not blocked.
• Q-Switching involves storage of energy in active medium while cavity dumping
involves storage of energy with in the cavity.
Both the technique are limited to a minimum pulse width which is
of the order of nanosecond. So, we switch to another technique which is called
as Mode Locking. With this new technique we can get ultrashort pulses which
have time scale of the order of picosecond(10-12s) to femtosecond(10-15s).

Mode Locking
• As we know that the modes which satisfy the standing wave condition can
sustain in the laser cavity. This modes not necessarily have same phase
relationship. Due to interference there random fluctuations in the total
intensity of laser beam.
Total Output t
Various
Modes

• By Mode Locking technique we can lock the modes such that in certain
region we have constructive interference while in other region there is
destructive interference so we actually can create ultrashort pulses.
Output of Mode locked System
Constructive
Interference
Destructive
Interference

Parameter of Mode Locked Pulses
• Time gap(T) between the pulses: 𝑇 =
1
∆𝑣
• Pulse Width: 𝜏𝑝 =
1
𝑁∆𝑣
∆𝑣 =
𝑐
2𝐿
represents separation between two modes.
N = No. of modes taking part in interference.
Time gap
(T)
time
𝜏𝑝

modelocking
active modelocking passive modelocking
self-modelocking

The modulation of the optical resonator parameters with the frequency q can
be obtained in a variety of methods including:
a) acousto-optic devices which produce a sound-wave, modulating the laser
beam’s intensity propagating through a resonator;
b) electro-optical modulators driven at exactly the frequency separation of the
longitudinal modes, q ;
c) the saturable absorbers modulating the amplification factor of an active
medium.
The first two methods belong to the active modelocking methods,
whereas the last represents the passive modelocking.

What is the mechanism which causes the randomly oscillating
longitudinal modes to begin oscillating in synchronised phases, under
the influence of the modulating factor, at the frequency q?
This can be achieved only when the
longitudinal modes are coupled together.
When we modulate the amplitude or frequency of a given longitudinal
mode of frequency 0, with the modulation frequency , an additional
radiation component appears at 0  n.
If the modulation frequency  is equal to the frequency-separation,
q, of the longitudinal modes, these additional components overlap
with the neighboring modes, causing coupling of the modes and
stimulating oscillations in the same phase.

Basic aspects of frequency- or amplitude modulation.
In the first method, (a), an acousto-optic transducer generates a
sound wave that modulates the amplitude of the laser beam in the
optical resonator.
Understanding of the mechanisms governing the interactions
between light and sound waves is very important, since the
acousto-optic devices are often used in laser technologies—not
only for modelocking, but also in pulse-selection (cavity dumping)
and in the Q-switching amplification.

If a transducer emitting ultrasonic waves at
frequency  in the range of megahertz
is placed in a glass of water illuminated
with a laser beam of frequency , one
notices that the light passing through the
glass splits into several beams.
At each side of the fundamental beam,
which is unaffected in frequency  and
direction, one observes side beams having
frequencies   n.
Debye - Sears effect
similar to light diffraction by a slit

Sound wave is a longitudinal wave
its propagation occurs by creating regions of different density
the regions of dilation can be treated as the slits through which
more light passes than through the regions of greater density.
Nice similarity with diffraction of light! BUT
why do the frequencies    ,  2 ,   3,…. appear???

Let’s imagine that light of frequency  arrives at a medium characterised by
a refractive index n1
If n1> n0, the light in the medium travels n1/n0 times slower (since =c'= c/n)
Let’s assume that we have some way of modulating the refractive index, n1,
with frequency .
causes the light in the medium to propagate faster or slower, and the output
light from the medium is also modulated The output light is
characterised by the carrier frequency, , of the incident light and a side
frequency of  leading to the appearance of additional components at
frequencies of   n

The longer the optical path, l, in the material, the greater are
the amplitudes of the sidebands at the frequencies   n
.
The sidebands’ amplification is reached at the expense of the
amplitude of the fundamental beam at the carrier frequency, .
The optical pathlength, l, is the parameter defining when the Debye–
Sears effect can occur.

We can distinguish two limiting cases,
where  is the optical wavelength, and  is
the length of an acoustic wave.
and,
This relationship defines the critical length of the optical path for which
the Debye–Sears effect can be observed. This relationship characterises the
conditions required for modelocking with acousto-optic devices. This is
Raman–Nath regime.
Pulse selection in cavity dumper

The simplest way to modulate the refractive index n1 is to make a periodic
change of a medium’s density, which can be achieved by passing the acoustic
wave through the medium.
The acoustic wave then creates regions of compression and dilation at
its frequency .
In real acousto-optic devices, a standing acoustic wave is generated
instead of a travelling wave whose forefront moves downward as shown
below

The standing wave shown below remains in place instead of moving down
the column, and the refractive index, n1, at each place in the column (e.g., the
dashed line) changes sinusoidally with the frequency .
Twice during the cycle the density is distributed uniformly along the whole
column (b and d), and twice it achieves a maximum at which the refractive index,
n1, is largest (a and e), as well as once when it achieves the minimum density at
which the refractive index, n1, is the smallest (c).

If this modulation is held at the frequency equal to the difference
between the longitudinal modes, q = c/2L, the Debye–Sears effect
leads to modelocking!!!
Thus, twice during the cycle T =1/ when the
density is distributed uniformly, the incident beam
passes unaffected and the frequency of the
transmitted beam is equal to , and the radiation
amplitude is equal to the amplitude of the incident
light.
At other times diffraction occurs, leading to the appearance of additional bands
at    , at the expense of weakening the amplitude of the carrier wave at
frequency  .
This is why an acousto-optical transducer modulates the amplitude of the light in
an optical resonator

In practical applications an acousto-optic modulator consists of a small fused
silica (SiO2) element (prism or plate) placed close to the optical resonator mirror
Model of piezoelectric transducer

The piezoelectric transducer at one end of a prism or a plate generates an
acoustic wave of frequency c /2L. The end walls of the prism are polished to
permit acoustic resonance to produce the standing acoustic wave inside. A
laser beam inside the optical resonator passes through the region of formation
of the standing acoustic wave, interacting with it in the manner described
earlier
As a consequence of this interaction, the laser beam with frequency  is
periodically modulated at the frequency  =c/2L by losses coming from the
sidebands at frequency   n .
Only the axial beam participates in the laser action: the sidebands which are
deflected from the main axis will be suppressed, since the length of the
optical path for the sidebands is different from L at which the condition n=2L
fulfilled.

Traditionally, the acousto-optic modulation is used in flash-lamp pumped
solid-state lasers such as Nd:YAG lasers.
A continuous-wave actively modelocked laser produces a train of pulses at
a repetition rate in the range of 80–250MHz and energy of a few nJ.
If more energy is required, a pulse selected from the train can be amplified
in a regenerative amplifier to reach a few mJ
If a more powerful pulse is needed, techniques that combine simultaneous
modelocking and Q-switching or cavity dumping are used.

Electrooptic devices can serve the same function as acousto-optic modulators
both for active modelocking
Other methods for active modelocking
A Pockels cell is a particular example of an electro-optic device

Passive modelocking
Uses saturable dye absorbers
There are various designs of passive modelocking, but a dye inside the
resonator is a major requirement
In the above configuration a dye cell and the rear mirror are combined to
reduce the number of reflective surfaces in the laser cavity, and to minimise
unwanted losses.

If the excited state lifetime , is of the order of magnitude of the cavity round-trip
time T= 2L/c , i.e., a few nanoseconds in typical resonators, the dye molecules
act like a passive Q-switching (We have already seen this!).
If the lifetime is comparable to the pulse duration of a modelocked pulse, i.e., a
few picoseconds, modelocking can occur.
Let us assume that the absorbing dye in a cell is characterised by the energy
levels E1 and E2 with E2 -E1 = h, where  is the frequency of one of many
longitudinal modes in the optical cavity.

We have assumed that the absorbing dye in a cell is characterised by the energy
levels E1 and E2 with E2 -E1 = h, where  is the frequency of one of many
longitudinal modes in the optical cavity.
light in the optical resonator arriving at the cell-mirror promotes some
molecules from the lower level, E1, to the upper level, E2, causing losses in the
light intensity as a result of absorption by the dye.
Initially, just at the beginning of pumping, the laser gain barely overcomes the
losses of the saturable dye. In the early stage of pulse generation, the longitudinal
modes are not synchronised in phase, and the laser output represents a chaotic
sequence of fluctuations. As a result, both the amplification and dye absorption are
not very efficient.
As the pump process continues to increase the intensity above a threshold, light-
amplification in the resonator approaches values of the saturation intensity in the
dye.
The gain in the laser medium is still linear, but the
absorption of the dye becomes non-linear.

With absorption of light at the high intensity the substance undergoes saturation
(bleaching), so the condition N1=N2 is fulfilled (where N1 and N2 indicate the
number of molecules at the levels E1 and E2).
The dye in the cell becomes transparent to the laser beam, which can arrive at
the reflective rear mirror and back to the active medium, which in turn causes
quick gain amplification.
Now the intensity is sufficiently high, and the amplification in the medium
becomes non-linear. The dye molecules return to the ground state, E1,
after time , and the process of light absorption is repeated.
Therefore, the transmission in the cavity is modulated by successive
passages of the high-intensity pulses resulting in a modelocked pulse
train appearing in the laser output
Finally, the population inversion is depleted, and the pulse decays.

To summarize, the mechanism of the passive modelocking with the saturable
dyes consists of three main steps:
1) linear amplification and linear dye absorption;
2) Non-linear absorption in the dye;
3) Non-linear amplification when the dye is entirely bleached.

Types of LASERs

Ruby LASER
• Solid State LASERS Nd : YAG LASER
Ti : Sapphire LASER
• Semiconductor LASERs
He-Ne LASER
• Atomic and Ionic Gas LASERs Argon LASER
Copper Vapor LASER
• Molecular Gas LASERs N2 LASER
• Chemical LASERs Iodine LASER
• Dye LASERs
ExcimerLASER
CO2 LASER

Solid State LASER
• Ruby LASER
 Ruby (0.05% Cr2O3 in an Al2O3 lattice), was used as active medium to
construct first ever LASER.
 The chromium (Cr3+) ions are excited by the broadband emission from a
flash lamp coiled around it, or placed alongside it within an elliptical
reflector.
 The energy level diagram may be regarded as pseudo – three level
system.

Solid State LASER
• Ruby LASER
 Ruby (0.05% Cr2O3 in an Al2O3 lattice), was used as active medium to
construct first ever LASER.
 The chromium (Cr3+) ions are excited by the broadband emission from a
flash lamp coiled around it, or placed alongside it within an elliptical
reflector.
 The energy level diagram may be regarded as pseudo – three level
system.
 Three Cr3+levels 4A2, 4T1 or 4T2, and 2E, are involved.
Ruby
Rod

 The initial flash lamp excitation takes the Cr3+ ions up from the ground state E1
(4A2) to one of the two E3 (4T) levels.
 Then they rapidly decay to E2 level.
 Population inversion is created between the E2 and E1 states, leading to laser
emission at a wavelength of 694.3 nm.
 Beam duration – 0.3 to 3ms
 Pulse delay – several seconds to a minute.
 Pulse energy - ~ 200 J

Nd:YAG LASER
• Yttrium Aluminium Garnet crystal (Y3AI5O2), act as a host for
neodymium ions.
• the energy levels of neodymium ions (Nd3+) which are naturally
degenerate in the free state, are split by interaction with the crystal
field.
• Transitions between components of the 4F3/2 and 4I11/2 states,
which are forbidden in the free state, become allowed due to
crystal field splitting and can give rise to laser emission.

4F9/2
4F7/2
4F5/2
4F3/2
4I15/2
4I13/2
4I11/2
4I9/2

• The 4F3/2 levels are initially populated following nonradiative
decay from higher energy levels.
• 4I11/2 laser level lies above the 4I9/2 ground state, we thus have a
pseudo-four-level system.
• The principal emission wavelength for neodymium laser is
around 1.064 μm, in the near-infra-red region.
• The output power of a Nd:YAG laser, in CW mode is several watts
and can exceed to 200 W.
• In pulse modes energy depends on method of pulsing but it can
vary from several to 100 J for single pulse.

Ti : Sapphire LASER
• Ti : sapphire lasers are tuneable lasers which emit red and
near-infrared light in the range from 650 to 1100 nm.
• Ti : Sapphire referred to active medium which comprises of a
crystal of Sapphire (Al2O3) doped with Titanium.
• The Ti3+ ion is responsible for the laser action.
• The electronic ground state of the Ti3+ ion is split into a pair of
vibrationally broadened levels.

• Absorption transitions occur over a range of 400nm to 600 nm
wavelengths, only one is shown.
• Fluorescence transitions occur from the lower vibrational levels of
the excited state to the upper vibrational levels of the ground state.
• Lasing action is only possible at wavelengths longer than 670 nm
because the long wavelength side of the absorption band overlaps
the short wavelength end of the fluorescence spectrum.

Semiconductor LASERs
• By applying an electrical potential across a simple diode junction
between p- and n-type crystal, electrons drop down from a
conduction band to a valence band, emitting radiation in the
process.

• Emission is mostly in Vis-Infrared region.
• Extremely small in size.
• Poor beam quality and poor collimation.
• Available as both tunable and fixed wavelength.
• Gallium arsenide lasers emit at a wavelength of around 0.904 μm
• The so-called 'lead salt' diode lasers, which are derived from non-
stoichiometric binary compounds of lead, cadmium and tin with
tellurium, selenium and sulphur, emit in the range 2.8-30 μm,
depending on the exact composition.
• Requires low operating temperature, and can be tuned by varying the
temperature.
• Modes in a diode lasers are typically separated by 1-2 cm-1
• Individual mode has a very narrow linewidth, of 10-3 cm-1or less.
• The output power of continuous semiconductor lasers is generally
measured in milliwatts, but can be increased upto 10 W.

Atomic and Ionic Gas LASERs
• Active medium is a gas which is either monatomic, or else
it is composed of very simple molecules.
• Laser emission occurs due to transitions in free atoms or
molecules, usually at low pressures, the emission line
width can be very small.
• The gas is often contained in a sealed tube, with the initial
excitation provided by an electrical discharge.

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 ENGEENERING 1 .pdf

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