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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 laser Solid 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

Pu

mp

ing

be

am

Le

ns

Mir

ro

r

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]

Ou

tpu

t [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 flash- lamp. 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:

22( )(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

qE 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

lN

Assume

2 2

0 (1 )2

qE m c

32THz