Laser lecture 07

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Laser Types
Lecture 7
LASER AND ITS APPLICATIONS
421 Phys
 In 1960 Maiman demonstrated laser oscillation in the optical region of the spectrum
for the first time, using as the active medium a crystal of ruby.
Therefore, Ruby lasers have historical importance because they were the first
successful laser to operate
 The active ion in ruby is Cr3+ doped at a level of around 0.05% by weight
into sapphire (Al2O3).
 The Al2O3 host crystal is colorless. The light is emitted by transitions of the Cr3+
impurities. Ruby is a three- level laser
 The absorption bands of the ruby are in the green and violet parts of the
visible spectrum. that cause a ruby laser rod to appear pink.
 Ruby gemstones contain a much higher concentration of Cr3+, about 1%,
and consequently are a rich red color.
Ruby laserSolid State Lasers

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• Ruby is an aluminum oxide crystal in which some of the aluminum
atoms have been replaced with chromium atoms.
• Chromium gives ruby its characteristic red color and is responsible
for the lasing behavior of the crystal.
• Chromium atoms Absorb green and blue Light and emit or reflect
only red light.
• For a ruby laser, a crystal of ruby is formed into a cylinder.
• A fully reflecting mirror is placed on one end and a partially
reflecting mirror on the other.
• A high-intensity lamp is spiraled around the ruby cylinder to provide
a flash of white light that triggers the Laser action.

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Absorption Spectrum
Emission Spectrum
 The upper laser levels decay predominantly by emission of radiation on transitions to
the ground state. However, the radiative decay is electric dipole forbidden, and
consequently the lifetime of the upper levels is long (2 ≈ 3 ms).
 The long lifetime of the upper level means that it can act as a „storage‟ level, which
helps the formation of a population inversion.
 Excitation of Ruby rod is achieved by a powerful flashlamp.
 The excited electrons relax rapidly to the upper laser level by non-radiative
transitions in which phonons are emitted. This leads to a large population in the upper
laser level.
 If the flashlamp is powerful enough, it will be possible to pump more than half of
the atoms from the ground state to the upper laser level. In this case, there will then be
a population inversion between upper laser level and the lower laser level, and lasing
can occur if a suitable cavity is provided.
 The laser emission is in the red at 694.3nm.

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 The diagrams to the right show a typical
arrangements for a ruby laser.
 The crystal is inserted inside a powerful flash
lamp.
 Water-cooling prevents damage to the crystal
by the intense heat generated by the lamp.
 Mirrors at either end of the crystal define the
cavity.
 Reflective coatings can be applied directly to
the end of the rod or external mirrors can be used.
 The lamps are usually driven in pulsed mode
by discharge from a capacitor bank.
 The pulse energy can be as high as 100 J per
pulse. This is because the upper laser level has a
very long lifetime (3 ms) and can store a lot of
energy.
First Ruby Laser: (Maiman, 1960)

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Applications of ruby laser
• One of the first applications for the ruby laser was in rangefinders ( Military
applications).
• Ruby lasers were used mainly in research.
• The ruby laser was the first laser used to optically pump tunable dye lasers
• One of the main industrial uses is drilling holes through diamond.
• They are still used in a number of applications where short pulses of red light
are required.
• Can be used in Holography: Because of its high pulsed power and good
coherence length, the red 694 nm laser light is preferred for large holograms.
• Many non-destructive testing labs use ruby lasers to create holograms of
large objects such as aircraft tires to look for weaknesses in the lining. Ruby
lasers were used extensively in tattoo and hair removal, but are being
replaced by alexandrite and Nd:YAG lasers in this application
Advantages of Ruby Lasers
• From cost point of view, the ruby lasers are economical.
• Beam diameter of the ruby laser is comparatively less than CO2 gas lasers.
• Output power of Ruby laser is not as less as in He-Ne gas lasers.
• Since the ruby is in solid form therefore there is no chance of wasting
material of active medium.
• Construction and function of ruby laser is self explanatory.
Disadvantages of Ruby Laser
• In ruby lasers no significant stimulated emission occurs, until at least half of
the ground state electrons have been excited to the Meta stable state.
• Efficiency of ruby laser is comparatively low.
• Optical cavity of ruby laser is short as compared to other lasers, which may
be considered a disadvantage.

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Neodymium Lasers
 The first operating neodymium laser was developed in 1964 at Bell Labs,
not long after the invention of the ruby laser. Unlike the ruby laser,
however, the neodymium laser has continued to find new applications
and to grow in importance, right up to the present day.
 The reason for this difference can be understood by considering the nature
of the laser transition.
 Neodymium (Nd) is one of the rare earths, the group of atoms with atomic
number between 58 and 70.
 The triply ionized rare earths (Nd3+, for example) have optical transitions in the
visible and near infrared regions that are fairly well defined in energy, depending
only slightly on the host solid into which the ion is doped.
 This insensitivity of the transition energy to the ion‟s environment comes about
through a shielding effect unique to the rare earths.
 The shielding is not perfect, but to a first approximation the energy of the
various levels is not affected by the environment surrounding the rare earth ion
Nd:YAG Laser
 In Nd laser, Nd+3 ions (as impurities of up to a few percent by weight) are
replacing the atoms of the solid host in the active medium.
 Three important solid hosts are used for Nd laser where Nd+3 ions are
added as impurities:
 Glass - YAG (Y3Al5O12 (yttrium aluminum garnet ) Crystal - YLF (LiYF4)
Crystal.
 The choice between the three possible hosts is according to the intended
use of the laser:
 Glass is used as the host material when a pulsed laser is needed, with
each pulse at high power, and the pulse repetition rate is slow.
 The active medium of Nd-Glass Laser can be manufactured in a shape of
disk or rod, with diameters of up to 0.5 meter (!) and length of up
to several meters (!). Such dimensions are possible because glass is
isotropic material, cheap, and can be easily worked to the right
shape.
Neodymium Lasers

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 High percentage (up to about 6%) of Nd ions can be added to glass as impurity.
 The problem with glass as a host is its poor thermal conductivity. Thus cooling the
laser when it operates continuously or at high repetition rate is difficult.
 YAG crystal is used for high repetition rate pulses (more than one pulse per
second). In this case a large amount of heat need to be transferred away from the
laser, and the thermal conductivity of the YAG crystal is much higher than that of
glass.
 YAG crystal with the high quality needed for lasers can be made with diameters
of 2-15 [mm] and at lengths of 2-30 [cm].
 The price of a YAG laser rod is high, since growing crystals is a slow and
complicated process.
 The percentage of Nd ions in the YAG host is 1-4% by weight.
Absorption spectrum of Nd ions
Emission spectrum of Nd ions

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Energy levels and transitions in Nd ions
•The energies of the
lower-lying levels of
Nd3+ are shown in
the adjacent figure.
As can be seen from the energy level diagram of Nd:YAG Laser
• Nd lasers are four level lasers.
• Nd ions have two absorption band, and excitation is done by optical
pumping, either by flash lamps for pulsed lasers, or by arc lamps
for continuous wave lasers.
• From these excited energy levels, the Nd ions are transferring into the
upper laser level by a non radiative transition.
•The most important laser transition in Nd3+ is from the upper laser level to
lower laser level at wavelength of 1.064 μm.
• Since the lower laser level here is not the ground state, this constitutes a
four-level system.
• From the lower laser level, a non-radiative transition to the ground level.

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Note: In principle, lasing can occur between any pair of levels, but the
required population inversion is easily achieved only when the upper laser
level has a long lifetime.
•The lifetime of most of the Nd3+ levels is rather short, due to efficient
nonradiative relaxation to the next-lowest level.
•The required excitation rate is, therefore, much lower for the Nd3+ laser than
for the ruby laser, and this is a primary reason for the Nd3+ laser‟s initial and
continuing popularity.
• Other advantages of Nd3+ over ruby are an order of magnitude-higher peak
cross section (for Nd3+ in a crystalline host), and the ability to use higher ion
concentrations without significant lifetime quenching by ion–ion interactions.
• These both lead to a higher gain coefficient, which improves the lasing
threshold and lasing efficiency.
Rear Mirror
Adjustment Knobs
Safety Shutter Polarizer Assembly (optional)
Coolant
Beam
Tube
Adjustment
Knob
Output
Mirror
Beam
Beam Tube
Harmonic
Generator (optional)
Laser Cavity
Pump
Cavity
Flashlamps
Nd:YAG
Laser Rod
Q-switch
(optional)
Courtesy of Los Alamos National Laboratory

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 Although the average energy of the sublevels in a manifold is fairly
independent of the host material, the position of the various sublevels
within the manifold varies considerably.
 When Nd3+ is doped in a glass, it can reside in any one of a great number
of different “sites,” each having a different local environment and
symmetry.
 This dependence on glass composition applies equally well to other rare
earth ions doped into glass, and has implications for optical
amplifiers as well as for lasers.
The Nd3+ laser is typically pumped with a lamp or with a diode laser. For
lamp pumping, the lamp and laser rod are often placed at the foci of
an elliptical reflector. The law of reflection applied to an elliptical
surface dictates that a light
 For diode laser pumping, in contrast, the excitation is at a single pump
wavelength. For example, Nd:YAG has a strong absorption peak at a
wavelength of 808 nm, which can be generated by an AlGaAs diode
laser. Absorption of a photon at this wavelength excites Nd3+ to E4
which decay rapidly to the upper laser level, E3 in a single
nonradiative step.
 An important advantage of diode laser pumping is its efficiency.
 In flashlamp pumping of a Nd:YAG laser: The lamp and laser rod are often
placed at the foci of an elliptical reflector to maximize the coupling of pump
light into the laser rod.
 In diode laser pumping: The pump light can be injected into the end of the
rod or as ribbon man elements from different directions.

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Laser
Diode
Pumping
beam
Lens
Mirror
Laser
cavity
 Ray emitted in any direction from one focus of the ellipse is reflected so that it
passes through the other focus. This geometry ensures optimal coupling of the
emitted lamp light into the laser rod.
 The pump light enters the laser rod from the side, and the laser is said to be “side
pumped.” In contrast to this, diode pumped lasers are often pumped from the end, or
“end-pumped,”. If the medium surrounding the laser rod is air, the pump light is
trapped in the rod by total internal reflection, and the Nd3+ ions are efficiently
excited by the pump.
 For lamp pumping, the pump spectrum is very broad, and there are many levels
above the upper laser level that simultaneously absorb the pump light and because of
the close energy spacing of these levels, they all decay rapidly (nanosecond time
scale) in a nonradiative cascade to the metastable upper laser level (0.23 ms) for
Nd:YAG.
 In this way, pump-light energy over a broad wavelength range is funneled into the
upper laser level.

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 The overall efficiency of a laser is often defined as the laser output power
divided by electrical input power. The overall laser efficiency depends not
only on how efficiently the laser medium converts absorbed pump power into
laser output, but also on how efficiently the laser medium absorbs the pump
light.
 This absorption efficiency is relatively low for lamp pumping, because the
lamp spectrum contains many photons with an energy that falls in
between the Nd3+ energy levels. However, the corresponding
efficiency for diode laser pumping is high, since all of the diode laser
power is concentrated at a wavelength at which the medium is
highly absorbing.
 Diode-pumped Nd:YAG lasers have a much higher efficiency (~ 30%)
than their lamp-pumped counterparts (~ 3%), due to the difference
in pump absorption efficiency.
 Neodymium lasers have been industrial workhorses ever since their
introduction.
 They can be operated efficiently in either continuous or pulsed mode, and
have found application in cutting and drilling and other types of
materials processing, as well as various medical applications (most of
which involve cutting tissue).
 Although YAG has been the most commonly used crystalline host, other
crystals such as YVO4 and YLiF4 have been used as well.
 Glass hosts have a much lower thermal conductivity than crystalline hosts,
and heat dissipation becomes a problem for Nd:glass lasers operated
at high average power. Also, the peak cross section for a glass host is
smaller. For these reasons, Nd:glass lasers are mostly operated in
pulsed mode.
Applications of Nd:YAG lasers

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 One of the more impressive applications of Nd:glass lasers is in the
generation of power by nuclear fusion.
 In nuclear fusion, two hydrogen nuclei (or a nucleus of hydrogen and one
of deuterium) are joined together to create a nucleus of helium,
thereby releasing considerable energy. To get the nuclei to come
together requires extraordinary conditions of temperature and
compression that are quite difficult to achieve. One proposed
scheme is to illuminate a small pellet of the hydrogen/deuterium
mixture from all sides with a high-power laser pulse, which will
then implode the pellet and create the necessary compression.
Titanium ion (Ti+3) embedded in a matrix of Sapphire (Al2O3) gives: Ti:Al2O3.
 This material is the active medium of the laser called Titanium doped Sapphire
laser.
 The amount of Titanium ions inside the host material is about 0.1%, and they
replace Aluminum atoms in the crystal.
 Ti:Saphire lasers belong to a family of lasers called Vibronic Lasers, in which
trivalent Chromium or Titanium are embedded in solid host.
 Ti: Sapphire laser was first demonstrated in 1982 by Peter Moulton MIT
Lincoln Laboratory.
 Commercial continuous wave systems entered the market in 1988. They
replace the Dye lasers in the Near-Infra-Red (NIR), because they are much
more reliable and easier to use.
 Titanium is a transition metal, thus Titanium Sapphire lasers belong to
transition metal lasers.
 Titanium doped Sapphire laser is an efficient, reliable tunable laser in the visible
and Near-Infra-Red (NIR) spectrum.
Titanium Sapphire Laser

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0 200 400 600 800 1000
Wavelength [nm]
Output[a.u.]
Absorption spectrum
Emission Spectrum
Transition energy levels of Ti:Sapphire laser

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 Usually optically pumped by another laser.
 Can be operated continuously or pulsed.
 Continuous power of a few watts can be achieved by pumping with Argon Ion
Laser.
 Has the broadest tuning range of all lasers known today, With
possible lasing wavelengths: 670 - 1100 [nm].
 Operate at room temperature.
 Very efficient (up to 80% quantum efficiency at room temperature)..
 The excited state lifetime of Titanium doped Sapphire is only 3.2
microseconds [msec], too short for pumping with flashlamp. Thus, the pumping
source is another laser.
 Absorption spectrum peaks near 500 [nm], so Argon ion lasers or copper
vapor lasers can be used as pumping sources.
Properties of Titanium Sapphire lasers:
 The main applications of Titanium doped Sapphire laser are in
research „laboratories, particular in spectroscopy.
 The large tuning range makes these lasers (with the appropriate non linear
crystal for multiplying frequencies) attractive for generating a tunable
sub-Pico-second pulses at short wavelengths.
 As an example, Titanium Sapphire laser is used in NASA project LASE (Lidar
Atmospheric Sensing Experiment) for measuring water vapor and
aerosols, and their effects on atmospheric processes.
 Titanium Sapphire amplifiers can produce:
 Tera-watt (1012 [W]) power levels.
 In femto-seconds (10-15 [sec]) pulses.
 At 10 [Hz] repetition rate.
 At wavelengths 760-840 [nm].
Applications of Ti:Saphire Lasers:

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Free electron laser (FEL)
concept : an electron beam with relativistic energy is forced by a magnetic
field („wiggler“), which varies periodically in space, to oscillate transverse to
the propagation direction z, the electrons emit synchrotron-radiation; consider
a plane wave introduced in propagation direction; depending on the position
along the wiggler, the radiation is either in or out of phase with the motion of
the electrons; the radiation will either accelerate or decelerate the electrons
(depending on the position in the wiggler); a stable situation is reached, when
the electrons form bunches (i.e. the slow electrons catch up the fast ones) :
the electrons are now synchronized with the formed plane wave and hence
the emitted radiation and electron motion are in phase

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A beam of relativistic electrons co-propagating with an optical field through a
spatially periodic magnetic field
What is an FEL?
• Undulator causes transverse electron oscillations
• Transverse e-velocity couples to E-component (transverse) of
optical field giving energy transfer.
• Interaction between electron beam and optical field causes
microbunching of electron beam on scale of radiation
wavelength leading to coherent emission
Output is radiation that is: tunable – powerful - coherent

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The required electron energy and emission line width of FEL
The wavelength of a FEL that is required to be amplified by the interaction of the
generated synchrotron radiation and electrons beam can be expressed as:
2
2
( )(1 )
2
q em c
E

  
Is the wavelength of FEL
Periodic length of the undulator
Mass of the electron
Energy of electron acceleration
Factor proportional to the undulator parameter
C: Velocity of light
q

em

E
The acceleration energy of electrons is then given by
2 2
0 (1 )
2
q
E m c



 
Hence the emission linewidth is expressed as
2 N

 
N: number of undulators

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Example
For a FEL laser with a wavelength of 46.9 nm, assume that the
undulator period is 10cm and its parameter is one and the length of
magnet array (or undulator) is 10m: find the acceleration energy of
the electrons and the emission linewidth
Sol.
E=747MeV
q
l
N

Assume
2 2
0 (1 )
2
q
E m c



 
32THz 

Laser lecture 07

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